Title of Invention	BALLAST, CIRCUIT FOR CONTROLLING OUTPUT CURRENT FORM BALLAST TO LAMP AND METHOD OF CONTROLLONG BALLAST
Abstract	The invention relates to an output current of a ballast is dynamically limited when an over-temperature condition is detected in the ballast according to one of (i) a step function or (ii) a combination of step and continuous functions, so as to reduce the temperature of the ballast while continuing to operate it. For example, an embodiment for dynamically limiting the output current of a ballast may include a circuit (300) that provides an indication (305) of a ballast temperature, Tb. Control circuitry, such as a foldback protection circuit (310) may provide a signal (315) responsive to the ballast temperature, Tb. The control circuitry (310) can reduce the output current from the ballast to the lamp (108) in response to the signal (315) according to one of (i) a step function or (ii) a combination of step and continuous functions, while continuing to operate the ballast.

Title of Invention

BALLAST, CIRCUIT FOR CONTROLLING OUTPUT CURRENT FORM BALLAST TO LAMP AND METHOD OF CONTROLLONG BALLAST

Abstract

The invention relates to an output current of a ballast is dynamically limited when an over-temperature condition is detected in the ballast according to one of (i) a step function or (ii) a combination of step and continuous functions, so as to reduce the temperature of the ballast while continuing to operate it. For example, an embodiment for dynamically limiting the output current of a ballast may include a circuit (300) that provides an indication (305) of a ballast temperature, Tb. Control circuitry, such as a foldback protection circuit (310) may provide a signal (315) responsive to the ballast temperature, Tb. The control circuitry (310) can reduce the output current from the ballast to the lamp (108) in response to the signal (315) according to one of (i) a step function or (ii) a combination of step and continuous functions, while continuing to operate the ballast.

Full Text	CROSS-REFERENCE TO RELATED APPLICATIONS [0001] This application claims priority to U.S. Application Serial No. 10/706,677, filed November 12, 2003, which is incorporated herein by reference in its entirety. FIELD OF THE INVENTION [0002] This invention relates to thermal protection for lamp ballasts. Specifically, this invention relates to a ballast having active thermal management and protection circuitry that allows the ballast to safely operate when a ballast over-temperature condition has been detected, allowing the ballast to safely continue to provide power to the lamp. BACKGROUND OF THE INVENTION [0003] Lamp ballasts are devices that convert standard line voltage and frequency to a voltage and frequency suitable for a specific lamp type. Usually, ballasts are one component of a lighting fixture that receives one or more fluorescent lamps. The lighting fixture may have more than one ballast. [0004] Ballasts are generally designed to operate within a specified operating temperature. The maximum operating temperature of the ballast can be exceeded as the result of a number of factors, including improper matching of the ballast to the lamp(s), improper heat sinking, and inadequate ventilation of the lighting fixture. If an over-temperature condition is not remedied, then the ballast and/or lamp(s) may be damaged or destroyed. [0005] Some prior art ballasts have circuitry that shuts down the ballast upon detecting an over-temperature condition. This is typically done by means of a thermal cut-out switch that senses the ballast temperature. When the switch detects an over-temperature condition, it shuts down the ballast by removing its supply voltage. If a normal ballast temperature is subsequently achieved, the switch may restore the supply voltage to the ballast. The result is lamp flickering and/or a prolonged loss of lighting. The flickering and loss of lighting can be annoying. In addition, the cause may not be apparent and might be mistaken for malfunctions in other electrical systems, such as the lighting control switches, circuit breakers, or even the wiring. US 6,621,239 discloses a method and apparatus for controlling the temperature of a multi-parameter light. DE 100 13 041 discloses a method of operating a light with a fluorescent lamp which involves setting a manufacturer's rated loading for a detected lamp type in normal operation and reducing/removing the load if a critical temperature is reached/exceeded. US 6,198,234 discloses a dimmable, backlight system for providing increased light output at low temperatures and which provides a full range of dimming. US 2003/031037 discloses a converter for converting an AC power main voltage to a voltage suitable for driving a lamp. DE 198 05 801 discloses a lamp control circuit for high pressure gas discharge lamps such as, for example, sodium, mercury, halogen and metal vapor lamps. DE 195 36 142 discloses a thermally-protected control apparatus containing electrical components, in particular for controlling high-pressure gas-discharge lamps in motor vehicle headlights. US 6,452,344 discloses an electronic dimming ballast which has a parallel loaded resonant output circuit plus a combination of pulse width modulation and frequency variation for use in the dimming of compact fluorescent lamps. US 4,800,974 discloses an electronic control system. US 5,869,969 discloses a temperature compensation module for use with battery charger/rectifier units. SUMMARY OF THE INVENTION [0006] A lamp ballast has temperature sensing circuitry and control circuitry responsive to the temperature sensor that limits the output current provided by the ballast when an over- temperature condition has been detected. The control circuitry actively adjusts the output current as long as the over-temperature condition is detected so as to attempt to restore an acceptable operating temperature while continuing to operate the ballast (i.e., without shutting down the ballast). The output current is maintained at a reduced level until the sensed temperature returns to the acceptable temperature. [0007] Various methods for adjusting the output current are disclosed. In one embodiment, the output current is linearly adjusted during an over-temperature condition. In another embodiment, the output current is adjusted in a step function during an over-temperature condition. In yet other embodiments, both linear and step function adjustments to output current are employed in differing combinations. In principle, the linear function may be replaced with any continuous decreasing function including linear and non-linear functions. Gradual, linear adjustment of the output current tends to provide a relatively imperceptible change in lighting intensity to a casual observer, whereas a stepwise adjustment may be used to create an obvious change so as to alert persons that a problem has been encountered and/or corrected. [0008] The invention has particular application to (but is not limited to) dimming ballasts of the type that are responsive to a dimming control to dim fluorescent lamps connected to the ballast. Typically, adjustment of the dimming control alters the output current delivered by the ballast. This is carried out by altering the duty cycle, frequency or pulse width of switching signals delivered to a one or more switching transistors in the output circuit of the ballast. These switching transistors may also be referred to as output switches. An output switch is a switch, such as a transistor, whose duty cycle and/or switching frequency is varied to control the output current of the ballast. A tank in the ballast's output circuit receives the output of the switches to provide a generally sinusoidal (AC) output voltage and current to the lamp(s). The duty cycle, frequency or pulse width is controlled by a control circuit that is responsive to the output of a phase to DC converter that receives a phase controlled AC dimming signal provided by the dimming control. The output of the phase to DC converter is a DC signal having a magnitude that varies in accordance with a duty cycle value of the dimming signal. Usually, a pair of voltage clamps (high and low end clamps) is disposed in the phase to DC converter for the purpose of establishing high end and low end intensity levels. The low end clamp sets the minimum output current level of the ballast, while the high end clamp sets its maximum output current level. [0009] According to one embodiment of the invention, a ballast temperature sensor is coupled to a foldback protection circuit that dynamically adjusts the high end clamping voltage in accordance with the sensed ballast temperature when the sensed ballast temperature exceeds a threshold. The amount by which the high end clamping voltage is adjusted depends upon the difference between the sensed ballast temperature and the threshold. According to another embodiment, the high and low end clamps need not be employed to implement the invention. Instead, the foldback protection circuit may communicate with a multiplier, that in turn communicates with the control circuit. In this embodiment, the control circuit is responsive to the output of the multiplier to adjust the duty cycle, pulse width or frequency of the switching signal. [0010] The invention may also be employed in connection with a non-dimming ballast in accordance with the foregoing. Particularly, a ballast temperature sensor and foldback protection are provided as above described, and the foldback protection circuit communicates with the control circuit to alter the duty cycle, pulse width or frequency of the one or more switching signals when the ballast temperature exceeds the threshold. [0011] In each of the embodiments, a temperature cutoff switch may also be employed to remove the supply voltage to shut down the ballast completely (as in the prior art) if the ballast temperature exceeds a maximum temperature threshold. [0012] Other features of the invention will be evident from the following detailed description of the preferred embodiments. BRIEF DESCRIPTION OF THEACCOMPANYING DRAWINGS [0013] Figure 1 is a functional block diagram of a prior art non-dimming ballast. [0014] Figure 2 is a functional block diagram of a prior art dimming ballast. [0015] Figure 3 is a functional block diagram of one embodiment of the present invention as employed in connection with a dimming ballast. [0016] Figure 4a graphically illustrates the phase controlled output of a typical dimming control. [0017] Figure 4b graphically illustrates the output of a typical phase to DC converter. [0018] Figure 4c graphically illustrates the effect of a high and low end clamp circuit on the output of a typical phase to DC converter. [0019] Figure 5a graphically illustrates operation of an embodiment of the present invention to linearly adjust the ballast output current when the ballast temperature is greater than threshold T1. [0020] Figure 5b graphically illustrates operation of an embodiment of the present invention to reduce the ballast output current in a step function to a level L1 when the ballast temperature is greater than threshold T2, and to increase the output current in a step function to 100% when the ballast temperature decreases to a normal temperature T3. [0021] Figure 5c graphically illustrates operation of an embodiment of the present invention to adjust the ballast output current linearly between temperature thresholds T4 and T5, to reduce the ballast output current in a step function from level L2 to level L3 if temperature threshold T5 is reached or exceeded, and to increase the output current in a step function to level L4 when the ballast temperature decreases to threshold T6. [0022] Figure 5d graphically illustrates operation of an embodiment of the present invention to adjust the ballast output current in various steps for various thresholds, and to further adjust ballast output current linearly between levels L6 and L7 if the stepwise reductions in output current are not sufficient to restore the ballast temperature to normal. [0023] Figure 6 illustrates one circuit level implementation for the embodiment of Figure 3 that exhibits the output current characteristics of Figure 5c. [0024] Figure 7 is a functional block diagram of another embodiment of the present invention for use in connection with a dimming ballast. [0025] Figure 8 is an output current versus temperature response for the embodiment of Figure 7. [0026] Figure 9 is a functional block diagram of an embodiment of the present invention that may be employed with a non-dimming ballast. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS [0027] Turning now to the drawings, wherein like numerals represent like elements there is shown in Figures 1 and 2 functional block diagrams of typical prior art non-dimming and dimming ballasts, respectively. Referring to Figure 1, a typical non-dimming ballast includes a front end AC to DC converter 102 that converts applied line voltage 100a, b, typically 120 volts AC, 60 Hz, to a higher voltage, typically 400 to 500 volts DC. Capacitor 104 stabilizes the high voltage output on 103a, b of AC to DC converter 102. The high voltage across capacitor 104 is presented to a back end DC to AC converter 106, which typically produces a 100 to 400 Volt AC output at 45 KHz to 80 KHz at terminals 107a, b to drive the load 108, typically one or more florescent lamps. Typically, the ballast includes a thermal cut-out switch 110. Upon detecting an over-temperature condition, the thermal cutout switch 110 removes the supply voltage at 100a to shut down the ballast. The supply voltage is restored if the switch detects that the ballast returns to a normal or acceptable temperature. [0028] The above description is applicable to Figure 2, except that Figure 2 shows additional details of the back end DC to AC converter 106, and includes circuitry 218, 220 and 222 that permits the ballast to respond to a dimming signal 217 from a dimming control 216. The dimming control 216 may be any phase controlled dimming device and may be wall mountable. An example of a commercially available dimming ballast of the type of Figure 2 is model number FDB-T554-120-2, available from Lutron Electronics, Co., Inc., Coopersburg, PA, the assignee of the present invention. As is known, the dimming signal is a phase controlled AC dimming signal, of the type shown in Figure 4a, such that the duty cycle of the dimming signal and hence the RMS voltage of the dimming signal varies with adjustment of the dimming actuator. Dimming signal 217 drives a phase to DC converter 218 that converts the phase Controlled dimming signal 217 to a DC voltage signal 219 having a magnitude that varies in accordance with a duty cycle value of the dimming signal, as graphically shown in Figure 4b. It will be seen that the signal 219 generally linearly tracks the dimming signal 217. However, clamping circuit 220 modifies this generally linear relationship as described hereinbelow. [0029] The signal 219 stimulates ballast drive circuit 222 to generate at least one switching control signal 223a, b. Note that the switching control signals 223a, b shown in Figure 2 are typical of those in the art that drive output switches in an inverter function (DC to AC) in the back-end converter 106. An output switch is a switch whose duty cycle and/or switching frequency is varied to control the output current of the ballast. The switching control signals control the opening and closing of output switches 210, 211 coupled to a tank circuit 212, 213. Although Figure 2 depicts a pair of switching control signals, 223a, b, an equivalent function that uses only one switching signal may be used. A current sense device 228 provides an output (load) current feedback signal 226 to the ballast drive circuit 222. The duty cycle, pulse width or frequency of the switching control signals is varied in accordance with the level of the signal 219 (subject to clamping by the circuit 220), and the feedback signal 226, to determine the output voltage and current delivered by the ballast. [0030] High and low end clamp circuit 220 in the phase to DC converter limits the output 219 of the phase to DC converter. The effect of the high and low end clamp circuit 220 on the phase to DC converter is graphically shown in the Figure 4c. It will be seen that the high and low clamp circuit 220 clamps the upper and lower ends of the otherwise linear signal 219 at levels 400 and 401, respectively. Thus, the high and low end clamp circuitry 220 establishes minimum and maximum dimming levels. [0031] A temperature cutoff switch 110 (Figure 1) is also usually employed. All that has been described thus far is prior art. [0032] Figure 3 is a block diagram of a dimming ballast employing the present invention. In particular, the dimming ballast of Figure 2 is modified to include a ballast temperature sensing circuit 300 that provides a ballast temperature signal 305 to a foldback protection circuit 310. As described below, the foldback protection circuit 310 provides an appropriate adjustment signal 315 to the high and low end clamp circuit 220' to adjust the high cutoff level 400. Functionally, clamp circuit 220' is similar to clamp circuit 220 of Figure 2, however, the clamp circuit 220' is further responsive to adjustment signal 315, which dynamically adjusts the high end clamp voltage (i.e. level 400). [0033] The ballast temperature sensing circuit 300 may comprise one or more thermistors with a defined resistance to temperature coefficient characteristic, or another type of temperature sensing thermostat device or circuit. Foldback protection circuit 310 generates an adjustment signal 315 in response to comparison of temperature signal 305 to a threshold. The foldback protection circuit may provide either a linear output (using a linear response generator) or a step function output (using a step response generator), or a combination of both, if the comparison determines that an over-temperature condition exists. In principle, the exemplary linear function . shown in Figure 3 may be replaced with any continuous function including linear and non-linear functions. For the purpose of simplicity and clarity, the linear continuous function example will be used. But, it can be appreciated that other continuous functions may equivalently be used. Regardless of the exact function used, the high end clamp level 400 is reduced from its normal operating level when the foldback protection circuit 310 indicates that an over-temperature condition exists. Reducing the high end clamp level 400 adjusts the drive signal 219' to the ballast drive circuit 222 so as to alter the duty cycle, pulse width or frequency of the switching control signals 223a, b and hence reduce the output current provided by the ballast to load 108. Reducing output current should, under normal circumstances, reduce the ballast temperature. Any decrease in ballast temperature is reflected in signal 315, and the high end clamp level 400 is increased and/or restored to normal, accordingly. [0034] Figures 5a - 5d graphically illustrate various examples of adjusting the output current during an over-temperature condition. These examples are not exhaustive and other functions or combinations of functions may be employed. [0035] In the example of Figure 5a, output current is adjusted linearly when the ballast temperature exceeds threshold Tl. If the ballast temperature exceeds T1, the foldback protection circuit 310 provides a limiting input to the high end clamp portion of the clamp circuit 220'so as to linearly reduce the high end clamp level 400, such that the output current may be reduced linearly from 100% to a preselected minimum. The temperature T1 may be preset by selecting the appropriate thresholds in the foldback protection circuit 310 as described in greater detail below. During the over-temperature condition, the output current can be dynamically adjusted in the linear region 510 until the ballast temperature stabilizes and is permitted to be restored to normal. Since fluorescent lamps are often operated in the saturation region of the lamp (where an incremental change in lamp current may not produce a corresponding change in light intensity), the linear adjustment of the output current may be such that the resulting change in intensity is relatively imperceptible to a casual observer. For example, a 40% reduction in output current (when the lamp is saturated) may produce only a 10% reduction in perceived intensity. [0036] The embodiment of the invention of Figure 3 limits the output current of the load to the linear region 510 even if the output current is less than the maximum (100%) value. For example, referring to Figure 5a, the dimming control signal 217 may be set to operate the lamp load 108 at, for example, 80% of the maximum load current. If the temperature rises to above a temperature value Tl, a linear limiting response is not activated until the temperature reaches a value of Tl*. At that value, linear current limiting may occur which will limit the output current to the linear region 510. This allows the maximum (100%) linear limiting profile to be utilized even if the original setting of the lamp was less than 100% load current. As the current limiting action of the invention allows the temperature to fall, the lamp load current will once again return to the originally set 80% level as long as the dimmer control signal 217 is unchanged. [0037] In the example of Figure 5b, output current may be reduced in a step function when the ballast temperature exceeds threshold T2. If the ballast temperature exceeds T2, then the foldback protection circuit 310 provides a limiting input to the high end portion of the clamp 220' so as to step down the high end clamp level 400; this results in an immediate step down in supplied output current from 100% to level L1. Once the ballast temperature returns to an acceptable operating temperature T3, the foldback protection circuit 310 allows the output current to immediately return to 100%, again as a step function. Notice that recovery temperature T3 is lower than T2. Thus, the foldback protection circuit 310 exhibits hysteresis. The use of hysteresis helps to prevent oscillation about T2 when the ballast is recovering from a higher temperature. The abrupt changes in output current may result in obvious changes in light intensity so as to alert persons that a problem has been encountered and/or corrected. [0038] In the example of Figure 5c, both linear and step function adjustments in output current are employed. For ballast temperatures between T4 and T5, there is linear adjustment of the output current between 100% and level L2. However, if the ballast temperature exceeds T5, then there is an immediate step down in supplied output current from level L2 to level L3. If the ballast temperature returns to an acceptable operating temperature T6, the foldback protection circuit 310 allows the output current to return to level L4, again as a step function, and the output current is again dynamically adjusted in a linear manner. Notice that recovery temperature T6 is lower than T5. Thus, the foldback protection circuit 310 exhibits hysteresis, again preventing oscillation about T5, The linear adjustment of the output current between 100% and L2 may be such that the resulting change in lamp intensity is relatively imperceptible to a casual observer, whereas the abrupt changes in output current between L2 and L3 may be such that they result in obvious changes in light intensity so as to alert persons that a problem has been encountered and/or corrected. [0039] In the example of Figure 5d, a series of step functions is employed to adjust the output current between temperatures T7 and T8. Particularly, there is a step-wise decrease in output current from 100% to level L5 at T7 and another step-wise decrease in output current from level L5 to level L6 at T8. Upon a temperature decrease and recovery, there is a step-wise increase in output current from level L6 to level L5 at T11, and another step-wise increase in output current from level L5 to .100% at T12 (each step function thus employing hysteresis to prevent oscillation about T7 and T8). Between ballast temperatures of T9 and T10, however, linear adjustment of the output current, between levels L6 and L7, is employed. Once again, step and linear response generators (described below) in the foldback protection circuitry 310 of Figure 3 allow the setting of thresholds for the various temperature settings. One or more of the step-wise adjustments in output current may result in obvious changes in light intensity, whereas the linear adjustment may be relatively imperceptible. [0040] In each of the examples, a thermal cutout switch may be employed, as illustrated at 110 in Figure 1, to remove the supply voltage and shut down the ballast if a substantial over- temperature condition is detected. [0041] Figure 6 illustrates one circuit level implementation of selected portions of the Figure 3 embodiment. The foldback protection circuit 310 includes a linear response generator 610 and a step response generator 620. The adjustment signal 315 drives the output stage 660 of the phase to DC converter 218' via the high end clamp 630 of the clamp circuit 220'. A low end clamp 640 is also shown. [0042] Temperature sensing circuit 300 may be an integrated circuit device that exhibits an increasing voltage output with increasing temperature. The temperature sensing circuit 300 feeds the linear response generator 610 and the step response generator 620. The step response generator 620 is in parallel with the linear response generator 610 and both act in a temperature dependent manner to produce the adjustment signal 315. [0043] The temperature threshold of the linear response generator 610 is set by voltage divider R3, R4, and the temperature threshold of the step response generator 620 is set by voltage divider R1, R2. The hysteresis characteristic of the step response generator 620 is achieved by means of feedback, as is well known in the art. [0044] The threshold of low end clamp 640 is set via a voltage divider labeled simply VDIV1. The phase controlled dimming signal 217 is provided to one input of a comparator 650. The other input of comparator 650 receives a voltage from a voltage divider labeled VDIV2. The output stage 660 of the phase to DC converter 218' provides the control signal 219'. [0045] Those skilled in the art will appreciate that the temperature thresholds of the linear and step response generators 610, 620 may be set such that the foldback protection circuit 310 exhibits either a linear function followed by a step function (See Figure 5c), or the reverse. Sequential step functions may be achieved by utilizing two step response generators 620 (See steps L5 and L6 of Figure 5d). Likewise, sequential linear responses may be achieved by replacing the step response generator 620 with another linear response generator 610. If only a linear function (Figure 5a) or only a step function (Figure 5b) is desired, only the appropriate response generator is employed. The foldback protection circuit 310 may be designed to produce more than two types of functions, e.g., with the addition of another parallel stage. For example the function of Figure 5d may be obtained with the introduction of another step response generator 62.0 to the foldback protection circuit, and by setting the proper temperature thresholds. [0046] Figure 7 is a block diagram of a dimming ballast according to another embodiment of the invention. Again, the dimming ballast of Figure 2 is modified to include a ballast temperature sensing circuit 300 that provides a ballast temperature signal 305 to a foldback protection circuit 310. The foldback protection circuit 310' produces, as before, an adjustment signal 315' to modify the response of the DC to AC back end 106 in an over- temperature condition. Nominally, the phase controlled dimming signal 217 from the dimming control 216, and the output of the high and low end clamps 220, act to produce the control signal 219 that is used, for example, in the dimming ballast of Figure 2. However, in the configuration of Figure 7, the control signal 219 and the adjustment signal 315' are combined via multiplier 700. The resulting product signal 701 is used to drive the ballast drive circuit 222' in conjunction with feedback signal 226. It should be noted that ballast drive circuit 222' performs the same function as the ballast drive circuit 222 of Figure 3 except that ballast drive circuit 222' may have a differently scaled input as described hereinbelow. [0047] As before, in normal operation, dimming control 216 acts to deliver a phase controlled dimming signal 217 to the phase to DC converter 218. The phase to DC converter 218 provides an input 219 to the multiplier 700. The other multiplier input is the adjustment signal 315'. [0048] Under normal temperature conditions, the multiplier 700 is influenced only by the signal 219 because the adjustment signal 315' is scaled to represent a multiplier of 1.0. Functionally, adjustment signal 315' is similar to 315 of Figure 3 except for the effect of scaling. Under over-temperature conditions, the foldback protection circuit 310' scales the adjustment signal 315' to represent a multiplier of less than 1.0. The product of the multiplication of the signal 219 and the adjustment signal 315' will therefore be less than 1.0 and will thus scale back the drive signal 701, thus decreasing the output current to load 108. [0049] Figure 8 illustrates the response of output current versus temperature for the embodiment of Figure 7. As in the response shown in Figure 5a, at 100% of load current, the current limiting function may be linearly decreasing beyond a temperature Tl. However, in contrast to Figure 5a, the response of the embodiment of Figure 7 at lower initial current settings is more immediate. In the multiplier embodiment of Figure 7, current limiting begins once the threshold temperature of Tl is reached. For example, the operating current of the lamp 108 may be set to be at a level lower than maximum, say at 80%, via dimmer control signal 217 which results in an input signal 219 to multiplier 700. Assuming that the temperature rises to a level of Tl, the multiplier input signal 315' would immediately begin to decrease to a level below 1.0 thus producing a reduced output for the drive signal 701. Therefore, the 100% current limiting response profile 810 is different from the 80% current limiting response profile 820 beyond . threshold temperature T1. [0050] It can be appreciated by one of skill in the art that the multiplier 700 may be implemented as either an analog or a digital multiplier. Accordingly, the drive signals for the multiplier input would be correspondingly analog or digital in nature to accommodate the type of multiplier 700 utilized. [0051] Figure 9 illustrates application of the invention to a non-dimming ballast, e.g., of the type of Figure 2, which does not employ high end and low end clamp circuitry or a phase to DC converter. As before, there is provided a ballast temperature sensing circuit 300 that provides a ballast temperature signal 305 to a foldback protection circuit 310". The foldback protection circuit 310' provides an adjustment signal 315" to ballast drive circuit 222. Instead of adjusting the level of a high end clamp, the adjustment signal 315" is provided directly to ballast drive circuit 222. Otherwise the foregoing description of the function and operation of Figure 3, and the examples of Figures 5a - 5d, are applicable. [0052] The circuitry described herein for implementing the invention is preferably packaged with, or encapsulated within, the ballast itself, although such circuitry could be separately packaged from, or remote from, the ballast. [0053] It will be apparent to those skilled in the art that various modifications and variations may be made in the apparatus and method of the present invention without departing from the spirit or scope of the invention. For example, although a linearly decreasing function is disclosed as one possible embodiment for implementation of current limiting, other continuously decreasing functions, even non-linear decreasing functions, may be used as a current limiting mechanism without departing from the spirit of the invention. Thus, it is intended that the present invention encompass modifications and variations of this invention provided those modifications and variations come within the scope of the appended claims and equivalents thereof. WE CLAIM: 1. A circuit for controlling output current from a ballast to a lamp (108) comprising: a) a temperature sensing circuit (300) thermally coupled to the ballast to provide a temperature signal (305) having a magnitude indicative of ballast temperature, Tb; and, b) control circuitry (218', 220', 222) capable of causing the ballast to enter a current limiting mode when the magnitude of the temperature signal indicates that Tb has exceeded a predetermined maximum desired ballast temperature, T1; characterised in that: the control circuitry is arranged to reduce the output current in response to the temperature signal according to one of (i) a step function or (ii) a combination of step and continuous functions, while continuing to operate the ballast. 2. The circuit as claimed in claim 1, wherein the continuous function is a linear function. 3. The circuit as claimed in claim 1, wherein the control circuitry (218', 220', 222), when operating the ballast in the current limiting mode, is responsive to a determination that Tb is equal to or less than a threshold temperature T2 to increase the output current, wherein T2 is less than Tl, such that the output current profile exhibits hysteresis in the current limited mode. 4. The circuit as claimed in claim 3, comprising circuitry that provides a first threshold signal having a magnitude indicative of T1 , and at least another, second, threshold signal having a magnitude indicative of T2. 5. The circuit as claimed in claim 3, wherein the control circuitry (218', 220', 222) is arranged to increase the output current in a step function. 6. The circuit as claimed in claim 3, wherein the control circuitry (218', 220', 222) is arranged to both reduce and increase the output current in step functions. 7. The circuit as claimed in claim 1, wherein the current limiting mode has a first state arranged to reduce the output current in a linear function and a second state, following the first state, arranged to further reduce the output current in a step function. 8. The circuit as claimed in claim 7, wherein the control circuitry (218', 220', 222) is arranged to cause the ballast to enter the first state of current limiting mode when the magnitude of the temperature signal (305) indicates that Tb has exceeded T1 and to enter the second state when the magnitude of the temperature signal indicates that Tb has exceeded a temperature T2, that is greater than Tl. 9. The circuit as claimed in claim 8, wherein the control circuitry (218', 220', 222), when operating the ballast in the second state of the current limiting mode, is responsive to a determination that Tb has decreased to a temperature T3, that is between T1 and T2, to increase the output current in a step function. 10. The circuit as claimed in claim 1, wherein the current limiting mode has a first state that reduces the output current in successive step functions. 11. The circuit as claimed in claim 10, comprising circuitry arranged to provide a first threshold signal indicative of the magnitude of T1 and a second threshold signal indicative of the magnitude of a temperature T2 that is greater than Tl, wherein the control circuitry (218', 220', 222), when operating the ballast in the first state of the current limiting mode, is responsive to a determination that Tb has reached T1 to decrease the output current in a first step function, and to a determination that Tb has reached T2 to further decrease the output current in a second step function. 12. The circuit as claimed in claim 11, wherein the circuitry is arranged to provide a third threshold signal indicative of the magnitude of a temperature T3 that is less than T1 and a fourth threshold signal indicative of the magnitude of a temperature T4 that is between T2 and Tl, and wherein the control circuitry (218', 220', 222), when operating the ballast in the first state of the current limiting mode, is responsive to a determination that Tb has decreased to T4 to increase the output current in a third step function, and to a determination that Tb has further decreased to T3 to further increase the output current in a fourth step function. 13. The circuit as claimed in claim 10, wherein the current limiting mode has a second state, following a last one of the step functions, that further reduces the output current in a linear function. 14. The circuit as claimed in claim 1, having a temperature cutoff circuit (110) for shutting down the ballast if Tb reaches or exceeds an unsafe maximum temperature that is greater than Tl. 15. The circuit as claimed in claim 1, wherein the control circuitry (218', 220', 222) is arranged to generate at least one switching signal (223a, 223b) for driving at least one output switch (210, 211) of the ballast, and is responsive to a difference between Tb and Tl to alter one of duty cycle, pulse width or frequency of the at least one switching signal. 16. The circuit as claimed in claim 14, wherein the ballast is a dimming ballast responsive to a phase controlled AC dimming signal (217) produced by a dimming control (216), and the control circuitry comprises: a phase to DC converter (218') arranged to convert the dimming signal to a DC signal (219') having a magnitude that varies in accordance with a duty cycle value of the dimming signal, and a drive circuit (222) arranged to generate at least one switching signal (223a, 223b) for driving at least one output switch (210, 211) of the ballast; and wherein the drive circuit is responsive to the DC signal and to a feedback signal (226) indicative of the output current to alter the at least one switching signal. 17. The circuit as claimed in claim 16, wherein the control circuitry comprises a clamp circuit (220') arranged to prevent the magnitude of the DC signal (219') from exceeding a pre-selected upper level (400), and wherein the pre-selected upper level is adjusted in accordance with the difference between Tb and Tl. 18. The circuit as claimed in claim 14, wherein the ballast is a dimming ballast responsive to a phase controlled AC dimming signal (217) produced by a dimming control (216), and the control circuitry comprises: a phase to DC converter (218') arranged to convert the dimming signal to a DC signal (219') having a magnitude that varies in accordance with a duty cycle value of the dimming signal, a multiplier circuit (700) providing an output (701) in accordance with the DC signal and a scaled difference between Tb and Tl, and a drive circuit (222) arranged to generate at least one switching signal (223a, 223b) for driving at least one output switch of the ballast; and wherein the drive circuit is responsive to the output of the multiplier and to a feedback signal (226) indicative of the output current, to alter the at least one switching signal. 19. The circuit as claimed in claim 1 wherein reductions and increases in output current cause reductions and increases in illumination provided by the lamp (108), and wherein the reductions are abrupt and perceptible to a human. 20. A method of controlling a ballast comprising the steps of: a)measuring ballast temperature, Tb; b)comparing Tb to a first reference, Tl; c)providing an indication of difference between Tb and Tl ; and characterised by: d)adjusting an output current provided by the ballast according to one of (i) a step function or (ii) a combination of step and continuous functions, while continuing to operate the ballast, in accordance with the result of step (c). 21. The method as claimed in claim 20, wherein step (d) comprises altering one of duty cycle, pulse width or frequency of at least one switching signal (223a, 223b) provided to at least one switch (210, 211) in an output circuit of the ballast in accordance with the difference. 22. The method as claimed in claim 20, which involves shutting down the ballast if the ballast temperature reaches or exceeds an unsafe maximum temperature. 23. The method as claimed in claim 20, wherein step (d) comprises reducing the output current linearly when Tb is between T1 and a second reference T2, where T2 is greater than T1, and reducing the output current in a step function when Tb is equal to or greater than T2. 24. The method as claimed in claim 23, wherein step (d) comprises increasing the output current, after Tb reaches T2, in a step function at a third reference T3 that is between Tl and T2. 25. The method as claimed in claim 20, wherein step (d) comprises reducing the output current in successive step functions. 26. The method as claimed in claim 25, wherein step (b) comprises comparing Tb to a second reference T2, greater than T1; step (c) further comprises providing an indication of the difference between Tb and T2; and step (d) comprises reducing the output current in a step function when Tb is between T1 and T2, and further reducing current in a step function when Tb is equal to or greater than T2. 27. The method as claimed in claim 26, comprising the steps of: (e)after Tb has equaled or exceeded Tl , but before Tb has equaled or exceeded T2, comparing Tb to a third threshold T3, less than Tl; (f)providing an indication of the difference between Tb and T3; (g)increasing the output current in a third step function responsive to the indication of step (f); (h)after Tb has equaled or exceeded T2, comparing Tb to a third threshold T4, between T1 and T2; (i)providing an indication of the difference between Tb and T4; and (j)increasing the output current in a fourth step function responsive to the indication of step (i). 28. The method as claimed in claim 20, wherein the ballast is responsive to a phase controlled AC dimming signal (217) produced by a dimming control (216) and the output current is controlled by at least one output switch (210, 211); and wherein step (d) comprises converting the dimming signal to a DC signal (219') having a magnitude that varies in accordance with a duty cycle value of the dimming signal; and controlling the at least one output switch in response to the DC signal and to a feedback signal (226) indicative of the output current. 29. The method as claimed in claim 28, wherein step (d) comprises clamping the magnitude of the DC signal (219') from exceeding a pre-selected upper level (400), and wherein the preselected upper level is adjusted in accordance with the difference between Tb and Tl. 30. The method as claimed in claim 20, wherein the ballast is responsive to a phase controlled AC dimming signal (217) produced by a dimming control (216) and the output current is controlled by at least one output switch (210, 211); and wherein step (d) comprises the steps of (l)scaling the indication of the difference between Tb and Tl; (2)converting the dimming signal to a DC signal (219') having a magnitude that varies in accordance with a duty cycle value of the dimming signal; (3)multiplying the DC signal and the scaled indication of the difference between Tb and Tl from step (1); and (4)controlling the at least one output switch in response to the result of step (3) and to a feedback signal (226) indicative of the output current. 31. The method as claimed in claim 20, wherein controlling the output current causes reductions and increases in the illumination provided by a lamp (108) connected to the ballast, and wherein the reductions are abrupt and perceptible to a human. ABSTRACT BALLAST, CIRCUIT FOR CONTROLLING OUTPUT CURRENT FROM BALLAST TO LAMP AND METHOD OF CONTROLLING BALLAST. The invention relates to an output current of a ballast is dynamically limited when an over-temperature condition is detected in the ballast according to one of (i) a step function or (ii) a combination of step and continuous functions, so as to reduce the temperature of the ballast while continuing to operate it. For example, an embodiment for dynamically limiting the output current of a ballast may include a circuit (300) that provides an indication (305) of a ballast temperature, Tb. Control circuitry, such as a foldback protection circuit (310) may provide a signal (315) responsive to the ballast temperature, Tb. The control circuitry (310) can reduce the output current from the ballast to the lamp (108) in response to the signal (315) according to one of (i) a step function or (ii) a combination of step and continuous functions, while continuing to operate the ballast.

Full Text

CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority to U.S. Application Serial No. 10/706,677, filed
November 12, 2003, which is incorporated herein by reference in its entirety.
FIELD OF THE INVENTION
[0002] This invention relates to thermal protection for lamp ballasts. Specifically, this
invention relates to a ballast having active thermal management and protection circuitry that
allows the ballast to safely operate when a ballast over-temperature condition has been detected,
allowing the ballast to safely continue to provide power to the lamp.
BACKGROUND OF THE INVENTION
[0003] Lamp ballasts are devices that convert standard line voltage and frequency to a
voltage and frequency suitable for a specific lamp type. Usually, ballasts are one component of a
lighting fixture that receives one or more fluorescent lamps. The lighting fixture may have more
than one ballast.
[0004] Ballasts are generally designed to operate within a specified operating
temperature. The maximum operating temperature of the ballast can be exceeded as the result of
a number of factors, including improper matching of the ballast to the lamp(s), improper heat
sinking, and inadequate ventilation of the lighting fixture. If an over-temperature condition is
not remedied, then the ballast and/or lamp(s) may be damaged or destroyed.
[0005] Some prior art ballasts have circuitry that shuts down the ballast upon detecting
an over-temperature condition. This is typically done by means of a thermal cut-out switch that
senses the ballast temperature. When the switch detects an over-temperature condition, it shuts
down the ballast by removing its supply voltage. If a normal ballast temperature is subsequently
achieved, the switch may restore the supply voltage to the ballast. The result is lamp flickering
and/or a prolonged loss of lighting. The flickering and loss of lighting can be annoying. In
addition, the cause may not be apparent and might be mistaken for malfunctions in other
electrical systems, such as the lighting control switches, circuit breakers, or even the wiring.

US 6,621,239 discloses a method and apparatus for controlling the temperature of a
multi-parameter light.
DE 100 13 041 discloses a method of operating a light with a fluorescent lamp which
involves setting a manufacturer's rated loading for a detected lamp type in normal
operation and reducing/removing the load if a critical temperature is
reached/exceeded.
US 6,198,234 discloses a dimmable, backlight system for providing increased light
output at low temperatures and which provides a full range of dimming.
US 2003/031037 discloses a converter for converting an AC power main voltage to a
voltage suitable for driving a lamp.
DE 198 05 801 discloses a lamp control circuit for high pressure gas discharge lamps
such as, for example, sodium, mercury, halogen and metal vapor lamps.
DE 195 36 142 discloses a thermally-protected control apparatus containing electrical
components, in particular for controlling high-pressure gas-discharge lamps in motor
vehicle headlights.
US 6,452,344 discloses an electronic dimming ballast which has a parallel loaded
resonant output circuit plus a combination of pulse width modulation and frequency
variation for use in the dimming of compact fluorescent lamps.
US 4,800,974 discloses an electronic control system.
US 5,869,969 discloses a temperature compensation module for use with battery
charger/rectifier units.

SUMMARY OF THE INVENTION
[0006] A lamp ballast has temperature sensing circuitry and control circuitry responsive
to the temperature sensor that limits the output current provided by the ballast when an over-
temperature condition has been detected. The control circuitry actively adjusts the output current
as long as the over-temperature condition is detected so as to attempt to restore an acceptable
operating temperature while continuing to operate the ballast (i.e., without shutting down the
ballast). The output current is maintained at a reduced level until the sensed temperature returns
to the acceptable temperature.
[0007] Various methods for adjusting the output current are disclosed. In one
embodiment, the output current is linearly adjusted during an over-temperature condition. In
another embodiment, the output current is adjusted in a step function during an over-temperature
condition. In yet other embodiments, both linear and step function adjustments to output current
are employed in differing combinations. In principle, the linear function may be replaced with
any continuous decreasing function including linear and non-linear functions. Gradual, linear
adjustment of the output current tends to provide a relatively imperceptible change in lighting
intensity to a casual observer, whereas a stepwise adjustment may be used to create an obvious
change so as to alert persons that a problem has been encountered and/or corrected.
[0008] The invention has particular application to (but is not limited to) dimming ballasts
of the type that are responsive to a dimming control to dim fluorescent lamps connected to the
ballast. Typically, adjustment of the dimming control alters the output current delivered by the
ballast. This is carried out by altering the duty cycle, frequency or pulse width of switching
signals delivered to a one or more switching transistors in the output circuit of the ballast. These
switching transistors may also be referred to as output switches. An output switch is a switch,
such as a transistor, whose duty cycle and/or switching frequency is varied to control the output
current of the ballast. A tank in the ballast's output circuit receives the output of the switches to
provide a generally sinusoidal (AC) output voltage and current to the lamp(s). The duty cycle,
frequency or pulse width is controlled by a control circuit that is responsive to the output of a
phase to DC converter that receives a phase controlled AC dimming signal provided by the
dimming control. The output of the phase to DC converter is a DC signal having a magnitude
that varies in accordance with a duty cycle value of the dimming signal. Usually, a pair of
voltage clamps (high and low end clamps) is disposed in the phase to DC converter for the
purpose of establishing high end and low end intensity levels. The low end clamp sets the
minimum output current level of the ballast, while the high end clamp sets its maximum output
current level.

[0009] According to one embodiment of the invention, a ballast temperature sensor is
coupled to a foldback protection circuit that dynamically adjusts the high end clamping voltage
in accordance with the sensed ballast temperature when the sensed ballast temperature exceeds a
threshold. The amount by which the high end clamping voltage is adjusted depends upon the
difference between the sensed ballast temperature and the threshold. According to another
embodiment, the high and low end clamps need not be employed to implement the invention.
Instead, the foldback protection circuit may communicate with a multiplier, that in turn
communicates with the control circuit. In this embodiment, the control circuit is responsive to
the output of the multiplier to adjust the duty cycle, pulse width or frequency of the switching
signal.
[0010] The invention may also be employed in connection with a non-dimming ballast in
accordance with the foregoing. Particularly, a ballast temperature sensor and foldback protection
are provided as above described, and the foldback protection circuit communicates with the
control circuit to alter the duty cycle, pulse width or frequency of the one or more switching
signals when the ballast temperature exceeds the threshold.
[0011] In each of the embodiments, a temperature cutoff switch may also be employed to
remove the supply voltage to shut down the ballast completely (as in the prior art) if the ballast
temperature exceeds a maximum temperature threshold.
[0012] Other features of the invention will be evident from the following detailed
description of the preferred embodiments.

BRIEF DESCRIPTION OF THEACCOMPANYING DRAWINGS
[0013] Figure 1 is a functional block diagram of a prior art non-dimming ballast.
[0014] Figure 2 is a functional block diagram of a prior art dimming ballast.
[0015] Figure 3 is a functional block diagram of one embodiment of the present
invention as employed in connection with a dimming ballast.
[0016] Figure 4a graphically illustrates the phase controlled output of a typical dimming
control.
[0017] Figure 4b graphically illustrates the output of a typical phase to DC converter.
[0018] Figure 4c graphically illustrates the effect of a high and low end clamp circuit on
the output of a typical phase to DC converter.
[0019] Figure 5a graphically illustrates operation of an embodiment of the present
invention to linearly adjust the ballast output current when the ballast temperature is greater than
threshold T1.

[0020] Figure 5b graphically illustrates operation of an embodiment of the present
invention to reduce the ballast output current in a step function to a level L1 when the ballast
temperature is greater than threshold T2, and to increase the output current in a step function to
100% when the ballast temperature decreases to a normal temperature T3.
[0021] Figure 5c graphically illustrates operation of an embodiment of the present
invention to adjust the ballast output current linearly between temperature thresholds T4 and T5,
to reduce the ballast output current in a step function from level L2 to level L3 if temperature
threshold T5 is reached or exceeded, and to increase the output current in a step function to level
L4 when the ballast temperature decreases to threshold T6.
[0022] Figure 5d graphically illustrates operation of an embodiment of the present
invention to adjust the ballast output current in various steps for various thresholds, and to
further adjust ballast output current linearly between levels L6 and L7 if the stepwise reductions
in output current are not sufficient to restore the ballast temperature to normal.
[0023] Figure 6 illustrates one circuit level implementation for the embodiment of Figure
3 that exhibits the output current characteristics of Figure 5c.
[0024] Figure 7 is a functional block diagram of another embodiment of the present
invention for use in connection with a dimming ballast.
[0025] Figure 8 is an output current versus temperature response for the embodiment of
Figure 7.
[0026] Figure 9 is a functional block diagram of an embodiment of the present invention
that may be employed with a non-dimming ballast.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0027] Turning now to the drawings, wherein like numerals represent like elements there
is shown in Figures 1 and 2 functional block diagrams of typical prior art non-dimming and
dimming ballasts, respectively. Referring to Figure 1, a typical non-dimming ballast includes a
front end AC to DC converter 102 that converts applied line voltage 100a, b, typically 120 volts
AC, 60 Hz, to a higher voltage, typically 400 to 500 volts DC. Capacitor 104 stabilizes the high
voltage output on 103a, b of AC to DC converter 102. The high voltage across capacitor 104 is
presented to a back end DC to AC converter 106, which typically produces a 100 to 400 Volt AC
output at 45 KHz to 80 KHz at terminals 107a, b to drive the load 108, typically one or more
florescent lamps. Typically, the ballast includes a thermal cut-out switch 110. Upon detecting an
over-temperature condition, the thermal cutout switch 110 removes the supply voltage at 100a to

shut down the ballast. The supply voltage is restored if the switch detects that the ballast returns
to a normal or acceptable temperature.
[0028] The above description is applicable to Figure 2, except that Figure 2 shows
additional details of the back end DC to AC converter 106, and includes circuitry 218, 220 and
222 that permits the ballast to respond to a dimming signal 217 from a dimming control 216. The
dimming control 216 may be any phase controlled dimming device and may be wall mountable.
An example of a commercially available dimming ballast of the type of Figure 2 is model
number FDB-T554-120-2, available from Lutron Electronics, Co., Inc., Coopersburg, PA, the
assignee of the present invention. As is known, the dimming signal is a phase controlled AC
dimming signal, of the type shown in Figure 4a, such that the duty cycle of the dimming signal
and hence the RMS voltage of the dimming signal varies with adjustment of the dimming
actuator. Dimming signal 217 drives a phase to DC converter 218 that converts the phase
Controlled dimming signal 217 to a DC voltage signal 219 having a magnitude that varies in
accordance with a duty cycle value of the dimming signal, as graphically shown in Figure 4b. It
will be seen that the signal 219 generally linearly tracks the dimming signal 217. However,
clamping circuit 220 modifies this generally linear relationship as described hereinbelow.
[0029] The signal 219 stimulates ballast drive circuit 222 to generate at least one
switching control signal 223a, b. Note that the switching control signals 223a, b shown in Figure
2 are typical of those in the art that drive output switches in an inverter function (DC to AC) in
the back-end converter 106. An output switch is a switch whose duty cycle and/or switching
frequency is varied to control the output current of the ballast. The switching control signals
control the opening and closing of output switches 210, 211 coupled to a tank circuit 212, 213.
Although Figure 2 depicts a pair of switching control signals, 223a, b, an equivalent function that
uses only one switching signal may be used. A current sense device 228 provides an output
(load) current feedback signal 226 to the ballast drive circuit 222. The duty cycle, pulse width or
frequency of the switching control signals is varied in accordance with the level of the signal 219
(subject to clamping by the circuit 220), and the feedback signal 226, to determine the output
voltage and current delivered by the ballast.
[0030] High and low end clamp circuit 220 in the phase to DC converter limits the output
219 of the phase to DC converter. The effect of the high and low end clamp circuit 220 on the
phase to DC converter is graphically shown in the Figure 4c. It will be seen that the high and
low clamp circuit 220 clamps the upper and lower ends of the otherwise linear signal 219 at
levels 400 and 401, respectively. Thus, the high and low end clamp circuitry 220 establishes
minimum and maximum dimming levels.

[0031] A temperature cutoff switch 110 (Figure 1) is also usually employed. All that has
been described thus far is prior art.
[0032] Figure 3 is a block diagram of a dimming ballast employing the present invention.
In particular, the dimming ballast of Figure 2 is modified to include a ballast temperature sensing
circuit 300 that provides a ballast temperature signal 305 to a foldback protection circuit 310. As
described below, the foldback protection circuit 310 provides an appropriate adjustment signal
315 to the high and low end clamp circuit 220' to adjust the high cutoff level 400. Functionally,
clamp circuit 220' is similar to clamp circuit 220 of Figure 2, however, the clamp circuit 220' is
further responsive to adjustment signal 315, which dynamically adjusts the high end clamp
voltage (i.e. level 400).
[0033] The ballast temperature sensing circuit 300 may comprise one or more thermistors
with a defined resistance to temperature coefficient characteristic, or another type of temperature
sensing thermostat device or circuit. Foldback protection circuit 310 generates an adjustment
signal 315 in response to comparison of temperature signal 305 to a threshold. The foldback
protection circuit may provide either a linear output (using a linear response generator) or a step
function output (using a step response generator), or a combination of both, if the comparison
determines that an over-temperature condition exists. In principle, the exemplary linear function .
shown in Figure 3 may be replaced with any continuous function including linear and non-linear
functions. For the purpose of simplicity and clarity, the linear continuous function example will
be used. But, it can be appreciated that other continuous functions may equivalently be used.
Regardless of the exact function used, the high end clamp level 400 is reduced from its normal
operating level when the foldback protection circuit 310 indicates that an over-temperature
condition exists. Reducing the high end clamp level 400 adjusts the drive signal 219' to the
ballast drive circuit 222 so as to alter the duty cycle, pulse width or frequency of the switching
control signals 223a, b and hence reduce the output current provided by the ballast to load 108.
Reducing output current should, under normal circumstances, reduce the ballast temperature.
Any decrease in ballast temperature is reflected in signal 315, and the high end clamp level 400
is increased and/or restored to normal, accordingly.
[0034] Figures 5a - 5d graphically illustrate various examples of adjusting the output
current during an over-temperature condition. These examples are not exhaustive and other
functions or combinations of functions may be employed.
[0035] In the example of Figure 5a, output current is adjusted linearly when the ballast
temperature exceeds threshold Tl. If the ballast temperature exceeds T1, the foldback protection
circuit 310 provides a limiting input to the high end clamp portion of the clamp circuit 220'so as

to linearly reduce the high end clamp level 400, such that the output current may be reduced
linearly from 100% to a preselected minimum. The temperature T1 may be preset by selecting
the appropriate thresholds in the foldback protection circuit 310 as described in greater detail
below. During the over-temperature condition, the output current can be dynamically adjusted
in the linear region 510 until the ballast temperature stabilizes and is permitted to be restored to
normal. Since fluorescent lamps are often operated in the saturation region of the lamp (where
an incremental change in lamp current may not produce a corresponding change in light
intensity), the linear adjustment of the output current may be such that the resulting change in
intensity is relatively imperceptible to a casual observer. For example, a 40% reduction in output
current (when the lamp is saturated) may produce only a 10% reduction in perceived intensity.
[0036] The embodiment of the invention of Figure 3 limits the output current of the load
to the linear region 510 even if the output current is less than the maximum (100%) value. For
example, referring to Figure 5a, the dimming control signal 217 may be set to operate the lamp
load 108 at, for example, 80% of the maximum load current. If the temperature rises to above a
temperature value Tl, a linear limiting response is not activated until the temperature reaches a
value of Tl*. At that value, linear current limiting may occur which will limit the output current
to the linear region 510. This allows the maximum (100%) linear limiting profile to be utilized
even if the original setting of the lamp was less than 100% load current. As the current limiting
action of the invention allows the temperature to fall, the lamp load current will once again
return to the originally set 80% level as long as the dimmer control signal 217 is unchanged.
[0037] In the example of Figure 5b, output current may be reduced in a step function
when the ballast temperature exceeds threshold T2. If the ballast temperature exceeds T2, then
the foldback protection circuit 310 provides a limiting input to the high end portion of the clamp
220' so as to step down the high end clamp level 400; this results in an immediate step down in
supplied output current from 100% to level L1. Once the ballast temperature returns to an
acceptable operating temperature T3, the foldback protection circuit 310 allows the output
current to immediately return to 100%, again as a step function. Notice that recovery temperature
T3 is lower than T2. Thus, the foldback protection circuit 310 exhibits hysteresis. The use of
hysteresis helps to prevent oscillation about T2 when the ballast is recovering from a higher
temperature. The abrupt changes in output current may result in obvious changes in light
intensity so as to alert persons that a problem has been encountered and/or corrected.
[0038] In the example of Figure 5c, both linear and step function adjustments in output
current are employed. For ballast temperatures between T4 and T5, there is linear adjustment of
the output current between 100% and level L2. However, if the ballast temperature exceeds T5,

then there is an immediate step down in supplied output current from level L2 to level L3. If the
ballast temperature returns to an acceptable operating temperature T6, the foldback protection
circuit 310 allows the output current to return to level L4, again as a step function, and the output
current is again dynamically adjusted in a linear manner. Notice that recovery temperature T6 is
lower than T5. Thus, the foldback protection circuit 310 exhibits hysteresis, again preventing
oscillation about T5, The linear adjustment of the output current between 100% and L2 may be
such that the resulting change in lamp intensity is relatively imperceptible to a casual observer,
whereas the abrupt changes in output current between L2 and L3 may be such that they result in
obvious changes in light intensity so as to alert persons that a problem has been encountered
and/or corrected.
[0039] In the example of Figure 5d, a series of step functions is employed to adjust the
output current between temperatures T7 and T8. Particularly, there is a step-wise decrease in
output current from 100% to level L5 at T7 and another step-wise decrease in output current
from level L5 to level L6 at T8. Upon a temperature decrease and recovery, there is a step-wise
increase in output current from level L6 to level L5 at T11, and another step-wise increase in
output current from level L5 to .100% at T12 (each step function thus employing hysteresis to
prevent oscillation about T7 and T8). Between ballast temperatures of T9 and T10, however,
linear adjustment of the output current, between levels L6 and L7, is employed. Once again, step
and linear response generators (described below) in the foldback protection circuitry 310 of
Figure 3 allow the setting of thresholds for the various temperature settings. One or more of the
step-wise adjustments in output current may result in obvious changes in light intensity, whereas
the linear adjustment may be relatively imperceptible.
[0040] In each of the examples, a thermal cutout switch may be employed, as illustrated
at 110 in Figure 1, to remove the supply voltage and shut down the ballast if a substantial over-
temperature condition is detected.
[0041] Figure 6 illustrates one circuit level implementation of selected portions of the
Figure 3 embodiment. The foldback protection circuit 310 includes a linear response generator
610 and a step response generator 620. The adjustment signal 315 drives the output stage 660 of
the phase to DC converter 218' via the high end clamp 630 of the clamp circuit 220'. A low end
clamp 640 is also shown.
[0042] Temperature sensing circuit 300 may be an integrated circuit device that exhibits
an increasing voltage output with increasing temperature. The temperature sensing circuit 300
feeds the linear response generator 610 and the step response generator 620. The step response

generator 620 is in parallel with the linear response generator 610 and both act in a temperature
dependent manner to produce the adjustment signal 315.
[0043] The temperature threshold of the linear response generator 610 is set by voltage
divider R3, R4, and the temperature threshold of the step response generator 620 is set by voltage
divider R1, R2. The hysteresis characteristic of the step response generator 620 is achieved by
means of feedback, as is well known in the art.
[0044] The threshold of low end clamp 640 is set via a voltage divider labeled simply
VDIV1. The phase controlled dimming signal 217 is provided to one input of a comparator 650.
The other input of comparator 650 receives a voltage from a voltage divider labeled VDIV2. The
output stage 660 of the phase to DC converter 218' provides the control signal 219'.
[0045] Those skilled in the art will appreciate that the temperature thresholds of the
linear and step response generators 610, 620 may be set such that the foldback protection circuit
310 exhibits either a linear function followed by a step function (See Figure 5c), or the reverse.
Sequential step functions may be achieved by utilizing two step response generators 620 (See
steps L5 and L6 of Figure 5d). Likewise, sequential linear responses may be achieved by
replacing the step response generator 620 with another linear response generator 610. If only a
linear function (Figure 5a) or only a step function (Figure 5b) is desired, only the appropriate
response generator is employed. The foldback protection circuit 310 may be designed to produce
more than two types of functions, e.g., with the addition of another parallel stage. For example
the function of Figure 5d may be obtained with the introduction of another step response
generator 62.0 to the foldback protection circuit, and by setting the proper temperature thresholds.
[0046] Figure 7 is a block diagram of a dimming ballast according to another
embodiment of the invention. Again, the dimming ballast of Figure 2 is modified to include a ballast temperature sensing circuit 300 that provides a ballast temperature signal 305 to a
foldback protection circuit 310. The foldback protection circuit 310' produces, as before, an
adjustment signal 315' to modify the response of the DC to AC back end 106 in an over-
temperature condition. Nominally, the phase controlled dimming signal 217 from the dimming
control 216, and the output of the high and low end clamps 220, act to produce the control signal
219 that is used, for example, in the dimming ballast of Figure 2. However, in the configuration
of Figure 7, the control signal 219 and the adjustment signal 315' are combined via multiplier
700. The resulting product signal 701 is used to drive the ballast drive circuit 222' in conjunction
with feedback signal 226. It should be noted that ballast drive circuit 222' performs the same
function as the ballast drive circuit 222 of Figure 3 except that ballast drive circuit 222' may
have a differently scaled input as described hereinbelow.

[0047] As before, in normal operation, dimming control 216 acts to deliver a phase
controlled dimming signal 217 to the phase to DC converter 218. The phase to DC converter 218
provides an input 219 to the multiplier 700. The other multiplier input is the adjustment signal
315'.
[0048] Under normal temperature conditions, the multiplier 700 is influenced only by the
signal 219 because the adjustment signal 315' is scaled to represent a multiplier of 1.0.
Functionally, adjustment signal 315' is similar to 315 of Figure 3 except for the effect of scaling.
Under over-temperature conditions, the foldback protection circuit 310' scales the adjustment
signal 315' to represent a multiplier of less than 1.0. The product of the multiplication of the
signal 219 and the adjustment signal 315' will therefore be less than 1.0 and will thus scale back
the drive signal 701, thus decreasing the output current to load 108.
[0049] Figure 8 illustrates the response of output current versus temperature for the
embodiment of Figure 7. As in the response shown in Figure 5a, at 100% of load current, the
current limiting function may be linearly decreasing beyond a temperature Tl. However, in
contrast to Figure 5a, the response of the embodiment of Figure 7 at lower initial current settings
is more immediate. In the multiplier embodiment of Figure 7, current limiting begins once the
threshold temperature of Tl is reached. For example, the operating current of the lamp 108 may
be set to be at a level lower than maximum, say at 80%, via dimmer control signal 217 which
results in an input signal 219 to multiplier 700. Assuming that the temperature rises to a level of
Tl, the multiplier input signal 315' would immediately begin to decrease to a level below 1.0
thus producing a reduced output for the drive signal 701. Therefore, the 100% current limiting
response profile 810 is different from the 80% current limiting response profile 820 beyond
. threshold temperature T1.
[0050] It can be appreciated by one of skill in the art that the multiplier 700 may be
implemented as either an analog or a digital multiplier. Accordingly, the drive signals for the
multiplier input would be correspondingly analog or digital in nature to accommodate the type of
multiplier 700 utilized.
[0051] Figure 9 illustrates application of the invention to a non-dimming ballast, e.g., of
the type of Figure 2, which does not employ high end and low end clamp circuitry or a phase to
DC converter. As before, there is provided a ballast temperature sensing circuit 300 that
provides a ballast temperature signal 305 to a foldback protection circuit 310". The foldback
protection circuit 310' provides an adjustment signal 315" to ballast drive circuit 222. Instead of
adjusting the level of a high end clamp, the adjustment signal 315" is provided directly to ballast

drive circuit 222. Otherwise the foregoing description of the function and operation of Figure 3,
and the examples of Figures 5a - 5d, are applicable.
[0052] The circuitry described herein for implementing the invention is preferably
packaged with, or encapsulated within, the ballast itself, although such circuitry could be
separately packaged from, or remote from, the ballast.
[0053] It will be apparent to those skilled in the art that various modifications and
variations may be made in the apparatus and method of the present invention without departing
from the spirit or scope of the invention. For example, although a linearly decreasing function is
disclosed as one possible embodiment for implementation of current limiting, other continuously decreasing functions, even non-linear decreasing functions, may be used as a current limiting mechanism without departing from the spirit of the invention. Thus, it is intended that the present
invention encompass modifications and variations of this invention provided those modifications
and variations come within the scope of the appended claims and equivalents thereof.

WE CLAIM:
1. A circuit for controlling output current from a ballast to a lamp (108)
comprising:
a) a temperature sensing circuit (300) thermally coupled to the ballast to provide a
temperature signal (305) having a magnitude indicative of ballast temperature, Tb; and,
b) control circuitry (218', 220', 222) capable of causing the ballast to enter a current limiting
mode when the magnitude of the temperature signal indicates that Tb has exceeded a predetermined
maximum desired ballast temperature, T1;
characterised in that: the control circuitry is arranged to reduce the output current in response to the
temperature signal according to one of (i) a step function or (ii) a combination of step and
continuous functions, while continuing to operate the ballast.
2. The circuit as claimed in claim 1, wherein the continuous function is a linear function.
3. The circuit as claimed in claim 1, wherein the control circuitry (218', 220', 222), when
operating the ballast in the current limiting mode, is responsive to a determination that Tb is equal
to or less than a threshold temperature T2 to increase the output current, wherein T2 is less than Tl,
such that the output current profile exhibits hysteresis in the current limited mode.
4. The circuit as claimed in claim 3, comprising circuitry that provides a first threshold signal
having a magnitude indicative of T1 , and at least another, second, threshold signal having a
magnitude indicative of T2.
5. The circuit as claimed in claim 3, wherein the control circuitry (218', 220', 222) is arranged
to increase the output current in a step function.
6. The circuit as claimed in claim 3, wherein the control circuitry (218', 220', 222) is arranged
to both reduce and increase the output current in step functions.
7. The circuit as claimed in claim 1, wherein the current limiting mode has a first state
arranged to reduce the output current in a linear function and a second state, following the first
state, arranged to further reduce the output current in a step function.

8. The circuit as claimed in claim 7, wherein the control circuitry (218', 220', 222) is arranged
to cause the ballast to enter the first state of current limiting mode when the magnitude of the
temperature signal (305) indicates that Tb has exceeded T1 and to enter the second state when the
magnitude of the temperature signal indicates that Tb has exceeded a temperature T2, that is greater
than Tl.
9. The circuit as claimed in claim 8, wherein the control circuitry (218', 220', 222), when
operating the ballast in the second state of the current limiting mode, is responsive to a
determination that Tb has decreased to a temperature T3, that is between T1 and T2, to increase the
output current in a step function.
10. The circuit as claimed in claim 1, wherein the current limiting mode has a first state that
reduces the output current in successive step functions.
11. The circuit as claimed in claim 10, comprising circuitry arranged to provide a first threshold
signal indicative of the magnitude of T1 and a second threshold signal indicative of the magnitude
of a temperature T2 that is greater than Tl, wherein the control circuitry (218', 220', 222), when
operating the ballast in the first state of the current limiting mode, is responsive to a determination
that Tb has reached T1 to decrease the output current in a first step function, and to a determination
that Tb has reached T2 to further decrease the output current in a second step function.
12. The circuit as claimed in claim 11, wherein the circuitry is arranged to provide a third
threshold signal indicative of the magnitude of a temperature T3 that is less than T1 and a fourth
threshold signal indicative of the magnitude of a temperature T4 that is between T2 and Tl, and
wherein the control circuitry (218', 220', 222), when operating the ballast in the first state of the
current limiting mode, is responsive to a determination that Tb has decreased to T4 to increase the
output current in a third step function, and to a determination that Tb has further decreased to T3 to
further increase the output current in a fourth step function.
13. The circuit as claimed in claim 10, wherein the current limiting mode has a second state,
following a last one of the step functions, that further reduces the output current in a linear function.

14. The circuit as claimed in claim 1, having a temperature cutoff circuit (110) for shutting
down the ballast if Tb reaches or exceeds an unsafe maximum temperature that is greater than Tl.
15. The circuit as claimed in claim 1, wherein the control circuitry (218', 220', 222) is arranged
to generate at least one switching signal (223a, 223b) for driving at least one output switch (210,
211) of the ballast, and is responsive to a difference between Tb and Tl to alter one of duty cycle,
pulse width or frequency of the at least one switching signal.
16. The circuit as claimed in claim 14, wherein the ballast is a dimming ballast responsive to a
phase controlled AC dimming signal (217) produced by a dimming control (216), and the control
circuitry comprises:
a phase to DC converter (218') arranged to convert the dimming signal to a DC signal (219')
having a magnitude that varies in accordance with a duty cycle value of the dimming signal, and
a drive circuit (222) arranged to generate at least one switching signal (223a, 223b) for driving at
least one output switch (210, 211) of the ballast; and
wherein the drive circuit is responsive to the DC signal and to a feedback signal (226)
indicative of the output current to alter the at least one switching signal.
17. The circuit as claimed in claim 16, wherein the control circuitry comprises a clamp circuit
(220') arranged to prevent the magnitude of the DC signal (219') from exceeding a pre-selected
upper level (400), and wherein the pre-selected upper level is adjusted in accordance with the
difference between Tb and Tl.
18. The circuit as claimed in claim 14, wherein the ballast is a dimming ballast responsive to a
phase controlled AC dimming signal (217) produced by a dimming control (216), and the control
circuitry comprises:
a phase to DC converter (218') arranged to convert the dimming signal to a DC signal (219')
having a magnitude that varies in accordance with a duty cycle value of the dimming signal,
a multiplier circuit (700) providing an output (701) in accordance with the DC signal and a scaled
difference between Tb and Tl, and
a drive circuit (222) arranged to generate at least one switching signal (223a, 223b) for

driving at least one output switch of the ballast; and
wherein the drive circuit is responsive to the output of the multiplier and to a feedback
signal (226) indicative of the output current, to alter the at least one switching signal.
19. The circuit as claimed in claim 1 wherein reductions and increases in output current cause
reductions and increases in illumination provided by the lamp (108), and wherein the reductions are
abrupt and perceptible to a human.
20. A method of controlling a ballast comprising the steps of:
a)measuring ballast temperature, Tb;
b)comparing Tb to a first reference, Tl;
c)providing an indication of difference between Tb and Tl ; and characterised by:
d)adjusting an output current provided by the ballast according to one of (i) a step function
or (ii) a combination of step and continuous functions, while continuing to operate the ballast, in
accordance with the result of step (c).
21. The method as claimed in claim 20, wherein step (d) comprises altering one of duty cycle,
pulse width or frequency of at least one switching signal (223a, 223b) provided to at least one
switch (210, 211) in an output circuit of the ballast in accordance with the difference.
22. The method as claimed in claim 20, which involves shutting down the ballast if the ballast
temperature reaches or exceeds an unsafe maximum temperature.
23. The method as claimed in claim 20, wherein step (d) comprises reducing the output current
linearly when Tb is between T1 and a second reference T2, where T2 is greater than T1, and
reducing the output current in a step function when Tb is equal to or greater than T2.
24. The method as claimed in claim 23, wherein step (d) comprises increasing the output
current, after Tb reaches T2, in a step function at a third reference T3 that is between Tl and T2.
25. The method as claimed in claim 20, wherein step (d) comprises reducing the output current
in successive step functions.

26. The method as claimed in claim 25, wherein step (b) comprises comparing Tb to a second
reference T2, greater than T1; step (c) further comprises providing an indication of the difference
between Tb and T2; and step (d) comprises reducing the output current in a step function when Tb
is between T1 and T2, and further reducing current in a step function when Tb is equal to or greater
than T2.
27. The method as claimed in claim 26, comprising the steps of:
(e)after Tb has equaled or exceeded Tl , but before Tb has equaled or exceeded T2,
comparing Tb to a third threshold T3, less than Tl;
(f)providing an indication of the difference between Tb and T3;
(g)increasing the output current in a third step function responsive to the indication of step
(f);
(h)after Tb has equaled or exceeded T2, comparing Tb to a third threshold T4, between T1
and T2;
(i)providing an indication of the difference between Tb and T4; and
(j)increasing the output current in a fourth step function responsive to the indication of step
(i).
28. The method as claimed in claim 20, wherein the ballast is responsive to a phase controlled
AC dimming signal (217) produced by a dimming control (216) and the output current is controlled
by at least one output switch (210, 211); and
wherein step (d) comprises converting the dimming signal to a DC signal (219') having a
magnitude that varies in accordance with a duty cycle value of the dimming signal; and
controlling the at least one output switch in response to the DC signal and to a feedback
signal (226) indicative of the output current.
29. The method as claimed in claim 28, wherein step (d) comprises clamping the magnitude of
the DC signal (219') from exceeding a pre-selected upper level (400), and wherein the preselected
upper level is adjusted in accordance with the difference between Tb and Tl.
30. The method as claimed in claim 20, wherein the ballast is responsive to a phase controlled

AC dimming signal (217) produced by a dimming control (216) and the output current is controlled
by at least one output switch (210, 211); and wherein step (d) comprises the steps of
(l)scaling the indication of the difference between Tb and Tl;
(2)converting the dimming signal to a DC signal (219') having a magnitude that varies in
accordance with a duty cycle value of the dimming signal;
(3)multiplying the DC signal and the scaled indication of the difference between Tb and Tl
from step (1); and
(4)controlling the at least one output switch in response to the result of step (3) and to a
feedback signal (226) indicative of the output current.
31. The method as claimed in claim 20, wherein controlling the output current causes
reductions and increases in the illumination provided by a lamp (108) connected to the ballast, and
wherein the reductions are abrupt and perceptible to a human.

ABSTRACT

BALLAST, CIRCUIT FOR CONTROLLING OUTPUT CURRENT FROM BALLAST TO
LAMP AND METHOD OF CONTROLLING BALLAST.
The invention relates to an output current of a ballast is dynamically limited when an over-temperature
condition is detected in the ballast according to one of (i) a step function or (ii) a combination of step
and continuous functions, so as to reduce the temperature of the ballast while continuing to operate it.
For example, an embodiment for dynamically limiting the output current of a ballast may include a
circuit (300) that provides an indication (305) of a ballast temperature, Tb. Control circuitry, such as a
foldback protection circuit (310) may provide a signal (315) responsive to the ballast temperature, Tb.
The control circuitry (310) can reduce the output current from the ballast to the lamp (108) in response
to the signal (315) according to one of (i) a step function or (ii) a combination of step and continuous
functions, while continuing to operate the ballast.

BALLAST, CIRCUIT FOR CONTROLLING OUTPUT CURRENT FORM BALLAST TO LAMP AND METHOD OF CONTROLLONG BALLAST

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