Title of Invention

BATTERY RECHARGING CIRCUIT

Abstract

A battery recharging circuit capable of operating stably against a variation of a supply voltage. The battery recharging circuit includes a voltage source; an AC-to-DC converter for converting an AC supply voltage from the voltage source into a DC supply voltage, to generate a charging voltage; a voltage sensor for comparing the charging voltage with a reference voltage to generate a voltage control signal when the charging voltage is equal to or higher than the reference voltage; a current sensor for comparing a charging current with a reference current to generate a current control signal when the charging current reaches the reference current; and controller including a switching element connected between the voltage source and the AC-to-DC converter, to connect and disconnect a power path between the voltage source and the AC-to-DC converter so as to maintain the constant charging voltage.

Full Text

BATTERY RECHARGING CIRCUIT Background of the Invention 1. Field of the Invention The present invention relates to a circuit for recharging a rechargeable battery, and in particular, to a battery recharging circuit capable of operating stably against a variation of a supply voltage. 2. Description of the Related Art A common battery recharging circuit includes an AC-to-DC converter to convert an AC supply voltage into a DC charging voltage, and provides a rechargeable battery with the DC charging voltage. Further, in order to prevent overcharge of the rechargeable battery, the battery recharging circuit controls a supply of the charging voltage according to a charging status of the rechargeable battery. A known battery recharging circuit can be widely divided into three types. A first type of the known battery recharging circuit provides the rechargeable battery with a constant current regardless of a voltage level of the charging voltage. Since this prior art battery recharging circuit has a great power loss, there is a need of a separate solution for the heat radiation in the circuit. A second type of the known battery recharging circuit has a temperature sensing function and an overcharge prevention function, to protect the circuit and the rechargeable battery from damages. Though this prior art battery recharging circuit is preferable, it is very costly and difficult to precisely control the circuit. A third type of the known battery recharging circuit maintains a constant output voltage until an output current reaches a predetermined value. In the interim, if the output current reaches the predetermined value, the battery recharging circuit varies the output voltage level while maintaining the constant current value. Fig. 1 illustrates the third type of the known battery recharging circuit. As illustrated, a voltage converter 112 composed of a transformer converts an AC supply voltage from a voltage source 111 into the charging voltage. A rectifier 113 rectifies and smoothes the charging voltage output from the voltage converter 112, and provides a rechargeable battery 100 with the rectified charging voltage output thereof. The voltage converter 112 and the rectifier 113 constitute an AC-to-DC converter for converting the AC supply voltage into a DC voltage having the charging voltage level. A current sensor 114 senses a current of the charging voltage output from the rectifier 113, and generates a control signal when the current exceeds a predetermined value. A switching circuit 115 composed of a photocoupler is turned on and off in response to the control signal output from the current sensor 114. A controller 116 composed of a - 2 - switching element connected between the voltage source 111 and the voltage converter 112 controls the supply of the AC supply voltage to the voltage converter 112 according to a switching status of the switching circuit 115. In operation, the current sensor 114 compares the charging current of the rechargeable battery 100 with a reference current, to generate the control signal if the charging current is identical to or greater than the reference current. Then, the switching circuit 115 and the controller 116 operate to cut off the AC supply voltage being applied to the voltage converter 112, in response to the control signal output from the current sensor 114. In this manner, the battery recharging circuit maintains the constant output voltage before the output current reaches the predetermined value, and varies the output voltage level while maintaining the constant output current, if the output current reaches the predetermined value. However, this prior art battery recharging circuit is very sensitive against the variation of the supply voltage. Thus, it is difficult to precisely control the constant charging voltage and current. Further, the prior art device has a great loss of the charging voltage, which results in an obstacle to minimization of the device. Summary of the Invention It is therefore an object of the present invention to - 3- provide a battery recharging circuit capable of operating stably against a variation of a supply voltage. It is another object of the present invention to provide a compact battery recharging circuit capable of reducing a loss of a charging voltage. To achieve theses and other objects, a battery recharging circuit according to the present invention includes a voltage source; an AC-to-DC converter for converting an AC supply voltage from the voltage source into a DC supply voltage, to generate a charging voltage; a voltage sensor for comparing the charging voltage with a reference voltage to generate a voltage control signal when the charging voltage is equal to or higher than the reference voltage; a current sensor for comparing a charging current with a reference current to generate a current control signal when the charging current reaches the reference current; and controller including a switching element connected between the voltage source and the AC-to-DC converter, to connect and disconnect a power path between the voltage source and the AC-to-DC converter so as to maintain the constant charging voltage. Brief Description of the Drawings The above and other objects, features and advantages of the present invention will become more apparent in the light of the following detailed description of an exemplary - 4 - embodiment thereof taken with the attached drawings in which: Fig. 1 is a schematic block diagram of a battery recharging circuit according to the prior art; Fig. 2 is a schematic block diagram of a battery recharging circuit according to an embodiment of the present invention; Figs. 3a and 3b are characteristic curves of the battery recharging circuit shown in Fig. 2; Figs. 4a and 4b are timing diagrams for showing variation of a duty cycle of a switching pulse signal according to an embodiment of the present invention; and Fig. 5 is a detailed circuit diagram of the battery recharging circuit shown in Fig. 2. Detailed Description of the Preferred Embodiment A preferred embodiment of the present invention will be described in detail referring to the attached drawings. Though the specific embodiment will be exemplarily defined and described in detail to clarify the subject matter of the present invention, the present invention may be implemented with the description of the present invention by those skilled in the art even without the details. In addition, an unnecessary detailed description of widely known functions and constructions may be avoided here. Referring to Fig. 2, a battery recharging circuit - 5 - according to the present invention includes a voltage source 211 for outputting an AC supply voltage. The AC supply-voltage from the voltage source 211 is rectified and smoothed by a first rectifier 212 composed of a bridge diode BD and a capacitor Cs. A voltage converter 213 composed of a transformer converts the rectified voltage output from the first rectifier 212 into a charging voltage level. A second rectifier 214 rectifies and smoothes the voltage output from the voltage converter 213, and provides a rechargeable battery 200 with the charging voltage output. Here, the first rectifier 212, the voltage converter 213, and the second rectifier 214 constitute an AC-to-DC converter for converting the AC supply voltage into the DC charging voltage. A sub-voltage generator 215 connected to the voltage converter 213 generates a sub-voltage. A reference voltage generator 216 divides the sub-voltage to generate a reference voltage. A current sensor 217 compares a reference current (i.e., a current associated with the reference voltage) with the charging current from the second rectifier 214, to generate a current control signal when the charging current is equal to or greater than the reference current. A voltage sensor 218 compares the reference voltage with the charging voltage output from the second rectifier 214, to generate a voltage control signal when the charging voltage is equal to or higher than the reference voltage. - 6 - A photocoupler 219 generates a switching control signal in response to the current control signal and the voltage control signal. A controller 220 is composed of a control circuit 221 and a switching element Q2 connected between the first rectifier 212 and the voltage converter 213. The switching element Q2 in the controller 22 0 cuts off the supply voltage output being provided to the voltage converter 213 from the first rectifier 212, in response to the switching control signal output from the photocoupler 219. The photocoupler 219 and the controller 220 constitute a switching controller for controlling the supply voltage in a switching mode of operation. In other words, the current sensor 217 and the voltage sensor 218 sense the charging current and the charging voltage supplied to the rechargeable battery 200, to generate the current control signal and the voltage control signal respectively, when the charging current and the charging voltage are equal to or higher than the predetermined value (i.e., the reference current and voltage) . The current control signal and the voltage control signal are generated in the form of a pulse signal according to the charging current and the charging voltage supplied to the rechargeable battery 200, and these pulse signals turn the switching element Q2 on and off, so that the controller 220 may control the supply voltage in the switching mode. Figs. 3a and 3b are characteristic curves of the - 7- battery recharging circuit shown in Fig. 2, and Figs. 4a and 4b are timing diagram showing variation of a duty cycle of the switching control signal, for controlling the charging voltage of the rechargeable battery 200 in the switching mode. Now, in operation, the AC supply voltage provided from the voltage source 211 is rectified by the first rectifier 212 and converted into the charging voltage level by the voltage converter 213. Then, the second rectifier 214 rectifies and smoothes the charging voltage output from the voltage converter 213 and provides the rechargeable battery 2 00 with the rectified voltage output thereof as the charging voltage. As mentioned above, the first rectifier 212, the voltage converter 213, and the second rectifier 214 constitute the AC-to-DC converter. In this AC-to-DC converter, the voltage converter 213 changes a magnetic flux thereof according to a duty cycle of the switching pulse signal generated by the switching element Q2 in the controller 22 0, thereby to convert an energy of a primary winding into an energy of a secondary winding. The following Equation (1) expresses a input-output relation of a flyback transformer, in which it can be understood that the duty cycle should be properly controlled to stabilize the output voltage with respect to variation of the input voltage. - 8 - where d represents the duty cycle, and D=ton* (1/f) . Further, in case of a resistive load, since v0=i0*R0, Equation (1) can be rewritten as the following Equation (2) • where R0 represents a load resistance, and I0 represents an output current (i.e, load current or charging current). It is also apparent from Equation (2) that the output current I0 may be stabilized against variation of the load resistance R0 by properly controlling the duty cycle of the switching pulse signal. In order to satisfy the characteristic curve shown in Fig, 3a, the battery recharging circuit should be controlled so as to satisfy Equation (1) before the output current I0 of the rechargeable battery 2 00 reaches a target current lb, and should be controlled so as to satisfy Equation (2) after the output current I0 of the rechargeable battery 200 has reached the target current lb. In the light of the foregoing idea, operation of the battery recharging circuit according to the present invention will be described in detail hereinbelow. The AC-to~DC converter composed of the first rectifier 312, the voltage converter 213, and the second rectifier 214 - 9 - generates a desired charging voltage level. The voltage sensor 218 senses the charging voltage supplied to the rechargeable battery 200, in order to stabilize (or keep) the output voltage until the output current (i.e., load current) l0 reaches the target current lb. In the meantime, if the load current increases, the voltage sensor 218 generates the voltage control signal and the photocoupler 219 generates the switching control signal in response to the voltage control signal. Consequently, the switching element Q2 of the controller 220 operates to form a power path through which the supply voltage from the first rectifier 212 is transferred to the voltage converter 213, in response to the switching control signal generated from the photocoupler 219. Accordingly, if the load current I0 increases, a turn-on time ton also increases as shown in Fig. 4a. At this moment, the current sensor 217 is disabled. Thereafter, if the load current I0 reaches the target current lb, the voltage sensor 218 is disabled and the current sensor 217 is enabled. The current sensor 217 connected in series to the charging voltage output from the second rectifier 214 senses variation of the charging current of the rechargeable battery 200. As the result, if the charging current increases, the current sensor 217 generates the current control signal and the photocoupler 219 inactivates the switching control signal in response to the current control signal. Consequently, the switching - 10 - element Q2 of the controller 220 cuts of the power path through which the supply voltage from the first rectifier 212 is transferred to the voltage converter 213. Accordingly, if the load current I0 increases, the turn-on time ton decreases as shown in Fig. 4b. That is, though an output impedance decreases, it is possible to maintain the constant output current l0 by properly controlling the duty cycle. In accordance with Equation (1) , the voltage sensor 218 feeds the output voltage V0 back to the input voltage Vi to control the duty cycle, and after the control of the duty cycle, the current sensor 217 switches to a constant-current control mode at the point of the target current lb. In the meantime, if the duty cycle decreases, the output voltage V0 will drop as shown in Fig. 3b. The battery recharging circuit according to the present invention includes the sub-voltage generator 215 and the reference voltage generator 216, in order to secure the above mentioned feedback function. That is, with use of the sub-voltage generator 215 and the reference voltage generator 216, the battery recharging circuit can maintain the constant charging voltage, though the voltage drops in a current restriction mode shown in Fig. 3b. Fig. 5 illustrates a detailed circuit diagram of the battery recharging circuit shown in Fig. 2. As illustrated, the first rectifier 212 for converting the AC supply voltage - 11 - includes a bridge diode BD, and capacitors C2 and C3. The voltage converter 213 includes a primary winding Tl and a secondary winding T21, to convert the AC supply voltage into the charging voltage level. The second rectifier 214 includes a diode D2, and capacitors C7 and C8, to rectify and smooth the charging voltage induced at the secondary winding T21 and provide the rechargeable battery 200 with the output voltage thereof. The primary winding Tl and a secondary winding T22 constitute the sub-voltage generator 215. The sub-voltage is independent of the charging voltage and used for generating the reference voltage. The reference voltage generator 216 includes a resistor R13 and a detector (i.e., diode) U2, to divides the sub-voltage and generate the reference voltage to the current sensor 217 and the voltage sensor 218. The voltage sensor 217 has a transistor Q3 with a collector connected to a cathode of a light emitting diode (LED), an emitter connected to the reference voltage, and a base connected to a node of resistors RIO and Rll. The voltage sensor 217 stabilizes the output voltage V0 until the output current I0 reaches the target current lb. Accordingly, when the voltage output from the second rectifier 214 reaches a target voltage (i.e., the reference voltage), the voltage sensor 217 is triggered to form a current path of the light emitting diode (of a photocoupler PCI) connected to a node Nl. - 12 - Further, a current detecting resistor R16, resistors R14 and R15, and a detector (i.e., diode) Ul constitute the current sensor 218. A cathode of the detector Ul is connected to the node Nl and a reference voltage terminal is formed at a node of the resistors RIO and Rll. The sum of voltage drops by the resistors R14, R15, and R16 is compared with the reference voltage at the reference voltage terminal. Accordingly, if the load current of the charging voltage reaches the target current lb, the detector Ul and the photocoupler PC1 generates the current control signal according to the current value of the charging voltage. If the current of the charging voltage increases, the controller 220 is disabled in response to the current control signal, thereby decreasing the duty cycle. As the result, it is possible to maintain the constant output current. The voltage control signal and the current control signal are applied to a base of an NPN transistor Q2, the switching element, of the controller 220 via the photocoupler PC1. The NPN transistor Q2 is turned on and off according to the voltage control signal and the current control signal to provide the AC-to-DC converter with the switching voltage. As can be appreciated from the foregoing descriptions, the battery recharging circuit of the invention maintains the constant output voltage until the output current - 13 - supplied to the rechargeable battery reaches a predetermined value, and varies the voltage level while maintaining the constant current value, if the output current reaches the predetermined value. In addition, the magnetic flux of the transformer is controlled to minimize the loss of the charging voltage. Although a preferred embodiment of the present invention has been described in detail hereinabove, it should be clearly understood that many variations and/or modifications of the basic inventive concepts herein taught which may appear to those skilled in the art will still fall within the spirit and scope of the present invention as defined in the appended claims. - 14 - WE CLAIM 1. A battery recharging circuit comprising: a voltage source; an AC-to-DC converter for converting an AC supply-voltage from said voltage source into a DC supply voltage, to generate a charging voltage; a voltage sensor for comparing said charging voltage with a reference voltage to generate a voltage control signal when said charging voltage is equal to or higher than said reference voltage; a current sensor for comparing a charging current with a reference current to generate a current control signal when said charging current reaches said reference current; and controller including a switching element connected between said voltage source and said AC-to-DC converter, to connect and disconnect a power path between said voltage source and said AC-to-DC converter so as to maintain the constant charging voltage. 2. A battery recharging circuit according to claim 1, further comprising: a sub-voltage generator for generating a sub-voltage being independent of said charging voltage; and a reference voltage generator for generating said - 15 - reference voltage according to said sub-voltage. 3. A battery recharging circuit according to claim 1, wherein said AC-to-DC converter comprises: a first rectifier for rectifying and smoothing the AC supply voltage from said voltage source; a voltage converter for converting the rectified voltage output from said first rectifier into the charging voltage; and a second rectifier for rectifying and smoothing the charging voltage output from said voltage converter. A battery recharging circuit capable of operating stably against a variation of a supply voltage. The battery recharging circuit includes a voltage source; an AC-to-DC converter for converting an AC supply voltage from the voltage source into a DC supply voltage, to generate a charging voltage; a voltage sensor for comparing the charging voltage with a reference voltage to generate a voltage control signal when the charging voltage is equal to or higher than the reference voltage; a current sensor for comparing a charging current with a reference current to generate a current control signal when the charging current reaches the reference current; and controller including a switching element connected between the voltage source and the AC-to-DC converter, to connect and disconnect a power path between the voltage source and the AC-to-DC converter so as to maintain the constant charging voltage.

Documents:

02358-cal-1997-abstract.pdf

02358-cal-1997-claims.pdf

02358-cal-1997-correspondence.pdf

02358-cal-1997-description (complete).pdf

02358-cal-1997-drawings.pdf

02358-cal-1997-form-1.pdf

02358-cal-1997-form-2.pdf

02358-cal-1997-form-3.pdf

02358-cal-1997-form-5.pdf

02358-cal-1997-gpa.pdf

02358-cal-1997-pa.pdf

02358-cal-1997-priority document others.pdf

02358-cal-1997-priority document.pdf

2358-CAL-1997-FORM-27-1.1.pdf

2358-CAL-1997-FORM-27.pdf