Title of Invention	"A NEW GATE DRIVE CIRCUIT FOR IGBTS AND MOSFETS FOR INDUSTRIAL ELECTRONICS APPLICATIONS"
Abstract	The invention relates to an improved switched mode power supply (SMPS) circuit of fly-back and/or forward type for industrial electronic appiication being connectable to an input supply voltage (E), comprising an unisolated driver (2,3) connected to a first solid state switching device of MOSFET or IGBT switches via two halves (RG1, RG2) of a gate circuit resistance, said first switching device being operably connected to a second solid state switching device of MOSFET or IGBT switches; a fly-back inductor and/or forward converter transformer (4) disposed between said first and second switching devices. An isolated gate driver (5,6) is interposed between said switching devices for generating isolated gate pulses capable to withstand a voltage potential difference which the source terminals of said sol id state switches being subjected to, the isolated gate driver (5,6) comprising a capacitor (C), plurality of resistances (Rl, R2, R3), and at least one voltage clamping device (zl).

Title of Invention

"A NEW GATE DRIVE CIRCUIT FOR IGBTS AND MOSFETS FOR INDUSTRIAL ELECTRONICS APPLICATIONS"

Abstract

The invention relates to an improved switched mode power supply (SMPS) circuit of fly-back and/or forward type for industrial electronic appiication being connectable to an input supply voltage (E), comprising an unisolated driver (2,3) connected to a first solid state switching device of MOSFET or IGBT switches via two halves (RG1, RG2) of a gate circuit resistance, said first switching device being operably connected to a second solid state switching device of MOSFET or IGBT switches; a fly-back inductor and/or forward converter transformer (4) disposed between said first and second switching devices. An isolated gate driver (5,6) is interposed between said switching devices for generating isolated gate pulses capable to withstand a voltage potential difference which the source terminals of said sol id state switches being subjected to, the isolated gate driver (5,6) comprising a capacitor (C), plurality of resistances (Rl, R2, R3), and at least one voltage clamping device (zl).

Full Text	1A. This invention relates to an improved switched mode powers supply circuit for IGBTS and MDSFETS for industrial electronics applications-Fast solid-state switches like IGBTS (Insulated Gate Bipolar Transistor) and MOSFETS (Metal Oxide Semi-Conductor Field Effect Transistor) Fig .1 (a ),1(b) are the most commonly used devices in modern power-electronic circuits. Many such circuits require more than one solid state switch connected at different voltage potentials. The gate control signals required for turning-on and turning- off of these switches often need to withstand large voltage isolation of the order of several hundred volts and are required to deliver considerable amount of peak current (2 to 5 amps). PRIOR ART Presently the gate drive ciruits which meet the voltage isolation and the current requirement appear to be too expensive although being used in many power electronic circuits. Some examples of such circuits are: Fly-back and Forward converters using two switches and diodes. The Fly-back and Forward converters are the most widely used circuits in several industrial applications requiring regulated DC output voltages, such as regulated DC power supplies, magnet power supplies, etc.. Currently used techniques for isolated gate drives of MOSFETs and IGBTs can be broadly divided in two categories: 2 (a) The ones that use at least one stage of optical isolation using opto-isolators to provide ohimc (or electrical isolation between any two isolated gate pulses. These often require isolated control power supplies, one on the input side 'of the opto-isolator and one on the output side of the opto-isolator. Such opto-isolators need to have very fast response for high frequency operation. Most often the output stage of the fast acting opto-isolators are not directly capable of supplying (or receiving ) the required magnitude of gate current and hence are followed by an ouput amplifier (or buffer stage). (b) The second category of isolated gate drivers use at least one stage of transformer type isolation using magnetic circuits adding to the cost and weight. Such isolated driver circuits need to be specially designed to minimize leakage inductance of the isolated windings so that they can supply (or receive) the required magnitude of gate current at a fast rate and often call for the use of more cumbersome torroidal type of core for coil winding. It also often calls for the use of bi-filar type windings requiring higher electrical insualtion of the winding wire. Yet another problem with the magnetic type isolation is the special (and often patented) techniques required to see that the magnetic core does not saturate for the required gate voltage pattern and the gate current magnitude, while keeping the size of the core small. 3- There are disadvantaoes associated with the present system of tho gate drive circuit (as described above) which requires a costly isolated gate driver for switches like MOSEFTs and ISBTs. In many applications, particularly in low power applications, the cost of the isolated gate drive circuit exceeds several- times the cost of the solid state switch itself. SUMMARY OF THE INVENTION Therefore, the main object of the present invention is to propose a novel and reliable switched mode power supply circuit that is cost-effective and efficient and applicable to some important category of power-electronic converters. Another object of the present invention is to propose an improved switched mode power supply circuit which costs significantly lower than the presently used isolated gate drive circuits. Yet another object of the present invention is to propose an improved switch mode power supply circuit which is effective in withstanding the required gate voltage isolation without causing any significant power loss or without compromising in the gate driving performance. Still another object of the present invention is to propose an improved switched mode power supply circuit which can supply or absorb the desired magnitude of gate current for fast switching operation with low switching losses. 4- Further objects of the present invention is to propose an improved switched mode power supply circuit which, though not a generic type but is applicable to a very important high—tech power electronic converters. This includes some very papular switched-mode power supply (SMPS) circuits for magnet current control. According to the invention there is provided an improved switched mode power supply (SMPS) circuit of fly-back and/or forward type for industrial electronic application being connectable to an input supply voltage (E), comprising unisolated drive (2,3) connected to a first solid state switching device of MOFSET or IGBT switches (01) via two halves (RG1, RG2) of a gate circuit resistance, said first switching device being operably connected to a second solid state switching device of MOFSET or ISBT switches (02); a fly-back inductor and/or forward converter transformer (4) disposed between said first and second switching devices, characterized in that an isolated gate driver (5,6) is interposed between said switching devices for generating isolated gate pulses capable to withstand a voltage potential difference which the source terminals of the said sol id state switches being subjected to, the isolated gate driver (5,6) comprising a capacitor (C), plurality of resistances (R1,R2,R3), and at least one voltage clamping device (zl). 5 following description made with reference to non-1imiting exmplary embodiments of the invention represented in the accompanying drawings. Fig.1(a) - insulated gate bipolar transistor; Fig.l(b) — metal oxide semi-conductor field effect transistor (n-channel); Fig.2(a) - an IGBT based circuit requiring isolated gate drive; Fig.2(b) - a MOSFET based circuit requiring isolated gate drive? Fig.3(a) - a single switch, fly-back type switched mode power supply; Fig.3(b) — a two switch, fly-back typed switched mode power supply; Fig.4(a) - a two switch, fly-back type switched mode power supply with the proposed isolated gate drive (using n-channel switches); Fig.4(b) - proposed isolated gate drive circuit using p-channel MOSFET DETAILED DESCRIPTION OF THE INVENTION Insulated Gate Bipolar Transistor (IGBT) and Metal Oxide Semiconductor Field Effect Transistor (MOSFET) are presently the most popular switching devices for power-electronics applications. They are of different types but the most commonly used ones are shown by their circuit symbols in Fig. l(a) and Fig.l)b). These are voltage-control led devices and are switched (turned-on or turned-off) by applying suitable magnitude of voltage signals (often referred as gate pulses, gate signals or gate voltages) 6 across their Gate and Source (Emitter) terminals,. These gate voltages are of smal1 magnitudes (often less than twenty volts) and need to be below the maximum magnitudes specified by the device manufacturers. The above-mentioned devices (ISBTs) andi MOSFETs) are often required to switch very fast to reduce the switching related losses in the device (often known as switching lasses) . This calls for a fast rising (or fast falling,) gate pulse capable of supplying (or receiving) the required gate current quickly. These gate voltage signals are referenced with respect to the source terminal of the above-mentioned switching devices. In many power—electronic applications more than one of these switches are used and the source ,terminals of these may be at different voltage potentials (often this potential difference may be of several hundred volts) and in such cases the respective contrail ing gate pulses must also be able to withstand the voltage difference that the device source terminals are subjected to. Any two such gate pulses that are required to withstand the said voltage potential difference are referred here as isolated gate pulses and the circuit that generates these pulses are called as isolated gate drive circuits Fig.2(a) and Fig.2(b) show some representative power—electronic circuits that may use any one or both of the above mentioned swtiches and require gate pulse isolation - For example, in Fig.2(a) gate pulses for switches Ql, Q2or Q3 need to be isolated from each other 1 - similarly in Fig.2(b) gate pulses for switches Q1 and Q2 need to be isolated by at least thei.r input voltage magnitude (marked as 'E' in the figures). One illustrative example is the very popular switch fly-back type switched mode power supply circuit which has been schematically shown in Fig.3(a). This circuit is presently the most popular one in low power ranges (20 watts to 100 watts). The main advantage of this circuit is that it requires a single switch and hence there is no need for any isolated gate driver. The other advantages are its simple output filter and the ability of the circuit to work over a wide range of input voltage (with some compromise on the output power capability ). However, the circuit of Fig.3(a) has the following drawbacks: It requires high voltage rated switch (significantly higher than the maximum working input voltage) requires snubber circuits and has considerable snubber losses and has low efficiency. A two-switch version of the same fly-back circuit is shown in Fig.3(b) and is technically much superior to the one-switch version of Fig.3(a). It has almost all positive features of the one-switch version but without its drawbacks just mentioned (except that some minor re-design of the fly-back inductor transformer may be required to match the large operating input voltage range of the one-switch version). The voltage rating of the switch in the two-switch version need not be % signifieanly higher than the maximum input DC voltage and in most of the designs the combined cost of the two switches is likely to be less than the cost of the single switch of the circuit in Fig.3(a). The two-switch version need not have the snubber circuit and the related losses. Inspite of its relative merits the two-switch version is not the industry favourite (at least. for low power ranges) because of two gate pulses need to have isolation between them and calls for an isolated gate driver. The gate drive circuit is shown within dotted lines marked (5,6) in V the illustrative power-electronic circuits of Fig.4 (a) and Fig.4(b). The proposed driver circuit is well suited to circuit topologies such as the one discussed in the previous section and shown in Fig,3(b). In Fig.3(b) (ie a two-switch fly-back power supply ) the switches Ql and Q2 need to be turned-on and turned— off together. Fig.4(a) and Fig.4(b) essentially reproduce the circuit of Fig.3(b) with the proposed invention. Fig.4(a) is shown to use n-channel power MOSEFTs whereas Fig.4(b) uses p-channel power MOSEFTs. The invention is applicable to gate controlled devices like MOSFETs and IGBTs of many different types and their combinatons. WORKING OF THE PROPOSED CIRCUIT The working of the proposed circuit is explained here with the illustrative circuits of Fig.4(a) and Fig.4(b). 9 In the circuit of Fig.4(a), a regular non-isolated type driver circuit is used for the upper switch Ql. For proper circuit operation Ql and Q2 need to be turned on and off without much time delay between them. In the proposed circuit, switching of Q1 itself is used for fast and reliable switching of Q2. After the turning-on of Q1, Q2 turns—on almost instantly and similarly turning-off of Q1 is almost immediately followed by the turning off of Q2- With the proper choice of the components shown in the proposed circuit, the time delay in the switching of Ql and Q2 can be kept within one micro—second or even lower and will suit most of the intended applications. Let us consider in detail the working of the dotted portion of the proposed circuit in Fig.4(a). As 01 turns on the voltage between the drain and source terminals of Q1 collapses to a very Low value determined by the on-state voltage drop of the switch. As a result, the source potential of 01 is pulled close to the positive end of the input supply voltage (E). A sharp charging current flow through the capacitor 'C', R2 and R3 to charge the effective gate to source capacitor of the switch Q2. Value of resistor Rl is very high and as a result, current through resistor Rl is negligibly smal1 and is used mainly to compensate for lost charge due to leakage etc. As a result, the power loss in Rl is also negligibly small. Zl and Z2 are voltage—clamping devices such as a zener diode connected 10- as a signle diode or a combination of several of them together with any other kind of diode. These may be connected in parallel, anti-parallel, in series or in anti-series and may provide voltage clamping of desired magnitude in either direction. Further, in some designs, it may be sufficient to use just one of Zl and Z2. Finally, the clamping devices denoted by Zl and Z2 keep the applied source to gate voltage of switch Q2 under the desired limit. The choice of resistors R2 and R3 together with the choice of capacitance 'C' decide the peak gate charging current and also decide the rate at which the gate capacitance is charged and need to be suitably chosen based on the given input voltage range and on the switching device characteristics. The relative position of 'C' , R2 and R3 may be altered and few more resistors and capacitors may be added but the basic circuit operation remains essentially same. Once the gate to source capacitor of switch Q2 is charged to the required voltage, switch Q2 turns-on and starts conducting along with switch Ql. A positive current starts flowing through the dotted end of the induetor-transformer connected between switches Ql and Q2 (called the primary winding). The charging of capacitor 'C' will stop once it is charged to its full magnitude, this voltage magnigude being decided by the input voltage and the clamping circuit denoted by Zl and Z2) voltage. The charge on the capacitor 'C' is 11- maintained as long as Ql is; on (with Ql end of capacitor plate kept positive) . Once switch Ql is turned-off due to control section of its driver, the primary winding current of the transformer-inductor, mentioned above, flows through the capacitor 'C', R2 , R3 and through the gate to source capacitance of switch Q2 but the current direction now is reversed and the. action is to discharge the gate to source capacitor of Q2 and also to discharge the capacitor 'C' . The gate to source capacitor of Q2 may be discharged to zero or to a clamped negative value depdnding on the choice of the clampinct circuit denoted by Zl and Z2. The gate discharge current is once again limited by the choice of the proposed circuit components 'C', R2 and R3. If the load (denoted by the primary winding current of fly-back transformer) is in excess of this gate charge current diode Dl turns on to conduct the remaining magnitude of the inductive load current. With the turning on of diode Dl, any remaining charge on the gate circuit capacitance of Q2 and on the capacitor 'C' will discharge even faster and quickly conclude the turning-off process of Q2 . The next turning—on and turning-off cycle repeats as described before. A similar charging/discharging cycle repeats for the p-channel MOSEFT Ql shown in cicuit of Fig.4(b). Here, the lower switch Q2 is driven directly and the proposed circuit is able to 12- effactively switch the upper switch (Q1). The lower switch Q2 could be a p-channel switch or alternatively a n-channel switch. In most of the fly-back type power supplies (as shown in Fig.4(a) and Fig.4(b), the gate power of the directly driven switch (such as switch Ql in Fig.4(a) and switch Q2 in Fig.4(b)) is derived from one of the floating outputs of the switched mode power supply itself (the starting power required for few initial Switchings is derived from the input DC supply 'E' through a clamped low rating resistive-capacitive circuit) and as such it does not make much difference if the directly driven switch' is upper switch or lower switch. However, some times, especially when a separate independent supply is used for the directly driven switch, there may be common mode noise present in the driving power supply. This problem is greatly reduced by dividing the gate circuit resistance in two halves and putting one half in series with the gate and other half in series with the source end of the driver. The arrangement is being shown in Fig.4(a) where RG1 and RG2 are the two halves of the gate circuit series resistor. The arrangement of dividing the gate circuit series resistor in the above manner is claimed as yet another related invention by the inventors- 13 The invention relates not only to the fly-back circuit described above but is equally applicable to many other circuits, such as two switch forward converters and the two-switch magnet power supplies as they essentially have the same circuit topology. The invention may also be fully or partly applicable to some other circuit topologies not mentioned here. The invention described here inabove is in relation to a non-limiting embodiment used as defined by the accompanying claims. 14. WE CLAIM: ,1. An improved switched mode power supply (SMPS)circui t of flyback and/or forward type for industrial electronic application being connectable to an input supply voltage (E), comorising an unisalated driver (2,3) connected to a first solid state switching device of MOFSET or IGBT switches (Q1) via two halves (RG1, RG2) of a gate circuit resistance, said first switching device being operably connected to a second solid state switching device of MOFSET or IGBT swtiches (Q2): a fly-back inductor and/or forward converter transformer (4) disposed between said first and second switching devices, characterized in that an isolated gate driver (5,6) is interposed between said switching device for generating isolated gate oulses capable to withstand a voltage potential difference which the source terminals of said solid state switches being subjected to, the isolated gate drive (5,6) comprising a capacitor (C),plurallty of resistances (R1,R2,R3) . and at least one voltage clamping device (Ql). 2. The switched mode power supply circuit as claimed in claim 1 wherein the switching devices are n-channel MOSFET having gate and device© source terminal for input of voltage signal and a drain terminal. 15. 3. The switched mode power supply circuit as claimed in claim 1 wherain the switching devices are p-channel MOSFET devices at least one device being connected to the driver circuit (2,3) 4. The switched mode power supply circuit as claimed in claim 1 wherein charging current flows through the capacitor "C" resistor 'R2 , and 'R3 ' to charge the effective gate to source capacitor of the switching device. 3. The switched mode power supply circuit as claimed in claim 4 wherein the value of the resistor 'R1' being high. the Currant flow iss small causing lower power loss and primarily used to compensate for leakage of charge in gate capacitance of the switching device. 6. The switched mode power supply circuit as claimed in claims 4 and 5, wherein between the switching devices two diodes (D1 D2 are provided. 7. The switched mode cower supply circuit as claimed in claim 1 wherein one half of the Gate circuit series resistor is disposed in series with the gate and the other half in series with the source of the uni-isolated driver (2,3). 16. 8. An improved switched mode power supply (SMPS) circuit for industrial electronic application as herein described and illustrated with the accompanying drawings. The invention relates to an improved switched mode power supply (SMPS) circuit of fly-back and/or forward type for industrial electronic appiication being connectable to an input supply voltage (E), comprising an unisolated driver (2,3) connected to a first solid state switching device of MOSFET or IGBT switches via two halves (RG1, RG2) of a gate circuit resistance, said first switching device being operably connected to a second solid state switching device of MOSFET or IGBT switches; a fly-back inductor and/or forward converter transformer (4) disposed between said first and second switching devices. An isolated gate driver (5,6) is interposed between said switching devices for generating isolated gate pulses capable to withstand a voltage potential difference which the source terminals of said sol id state switches being subjected to, the isolated gate driver (5,6) comprising a capacitor (C), plurality of resistances (Rl, R2, R3), and at least one voltage clamping device (zl).

Full Text

1A.
This invention relates to an improved switched mode powers supply circuit for
IGBTS and MDSFETS for industrial electronics applications-Fast solid-state switches like IGBTS (Insulated Gate Bipolar Transistor) and MOSFETS (Metal Oxide Semi-Conductor Field Effect Transistor) Fig .1 (a ),1(b) are the most commonly used devices in modern power-electronic circuits. Many such circuits require more than one solid state switch connected at different voltage potentials. The gate control signals required for turning-on and turning- off of these switches often need to withstand large voltage isolation of the order of several hundred volts and are required to deliver considerable amount of peak current (2 to 5 amps). PRIOR ART
Presently the gate drive ciruits which meet the voltage isolation and the current requirement appear to be too expensive although being used in many power electronic circuits.
Some examples of such circuits are: Fly-back and Forward converters using two switches and diodes. The Fly-back and Forward converters are the most widely used circuits in several industrial applications requiring regulated DC output voltages, such as regulated DC power supplies, magnet power supplies, etc..
Currently used techniques for isolated gate drives of MOSFETs and IGBTs can be broadly divided in two categories:

2
(a) The ones that use at least one stage of optical isolation
using opto-isolators to provide ohimc (or electrical isolation
between any two isolated gate pulses. These often require
isolated control power supplies, one on the input side 'of the
opto-isolator and one on the output side of the opto-isolator.
Such opto-isolators need to have very fast response for high
frequency operation. Most often the output stage of the fast
acting opto-isolators are not directly capable of supplying (or
receiving ) the required magnitude of gate current and hence are
followed by an ouput amplifier (or buffer stage).
(b) The second category of isolated gate drivers use at least
one stage of transformer type isolation using magnetic circuits
adding to the cost and weight. Such isolated driver circuits need
to be specially designed to minimize leakage inductance of the
isolated windings so that they can supply (or receive) the
required magnitude of gate current at a fast rate and often call
for the use of more cumbersome torroidal type of core for coil
winding. It also often calls for the use of bi-filar type
windings requiring higher electrical insualtion of the winding
wire. Yet another problem with the magnetic type isolation is the
special (and often patented) techniques required to see that the
magnetic core does not saturate for the required gate voltage
pattern and the gate current magnitude, while keeping the size of
the core small.

3-
There are disadvantaoes associated with the present system of
tho gate drive circuit (as described above) which requires a

costly isolated gate driver for switches like MOSEFTs and ISBTs. In many applications, particularly in low power applications, the cost of the isolated gate drive circuit exceeds several- times
the cost of the solid state switch itself.
SUMMARY OF THE INVENTION

Therefore, the main object of the present invention is to propose

a novel and reliable switched mode power supply circuit that is cost-effective and efficient and applicable to some important category
of power-electronic converters.

Another object of the present invention is to propose an improved switched mode power supply circuit which costs significantly lower than the
presently used isolated gate drive circuits.
Yet another object of the present invention is to propose an
improved switch mode power supply circuit which is effective in withstanding
the required gate voltage isolation without causing any significant power loss or without compromising in the gate driving performance.
Still another object of the present invention is to propose an
improved switched mode power supply circuit which can supply or absorb the

desired magnitude of gate current for fast switching operation with low switching losses.

4-
Further objects of the present invention is to propose an improved switched mode power supply circuit which, though not a generic type but is applicable to a very important high—tech power electronic converters. This includes some very papular switched-mode power supply (SMPS) circuits for magnet current control.
According to the invention there is provided an improved switched mode power supply (SMPS) circuit of fly-back and/or forward type for industrial electronic application being connectable to an input supply voltage (E), comprising unisolated drive (2,3) connected to a first solid state switching device of MOFSET or IGBT switches (01) via two halves (RG1, RG2) of a gate circuit resistance, said first switching device being operably connected to a second solid state switching device of MOFSET or ISBT switches (02); a fly-back inductor and/or forward converter transformer (4) disposed between said first and second switching devices, characterized in that an isolated gate driver (5,6) is interposed between said switching devices for generating isolated gate pulses capable to withstand a voltage potential difference which the source terminals of the said sol id state switches being subjected to, the isolated gate driver (5,6) comprising a capacitor (C), plurality of resistances (R1,R2,R3), and at least one voltage clamping device (zl).

5
following description made with reference to non-1imiting exmplary embodiments of the invention represented in the accompanying drawings. Fig.1(a) - insulated gate bipolar transistor;
Fig.l(b) — metal oxide semi-conductor field effect transistor (n-channel);
Fig.2(a) - an IGBT based circuit requiring isolated gate drive;
Fig.2(b) - a MOSFET based circuit requiring isolated gate drive?
Fig.3(a) - a single switch, fly-back type switched mode power supply;
Fig.3(b) — a two switch, fly-back typed switched mode power supply;
Fig.4(a) - a two switch, fly-back type switched mode power supply with the proposed isolated gate drive (using n-channel switches);
Fig.4(b) - proposed isolated gate drive circuit using p-channel MOSFET
DETAILED DESCRIPTION OF THE INVENTION
Insulated Gate Bipolar Transistor (IGBT) and Metal Oxide Semiconductor Field Effect Transistor (MOSFET) are presently the most popular switching devices for power-electronics applications. They are of different types but the most commonly used ones are shown by their circuit symbols in Fig. l(a) and Fig.l)b). These are voltage-control led devices and are switched (turned-on or turned-off) by applying suitable magnitude of voltage signals (often referred as gate pulses, gate signals or gate voltages)

6
across their Gate and Source (Emitter) terminals,. These gate voltages are of smal1 magnitudes (often less than twenty volts) and need to be below the maximum magnitudes specified by the device manufacturers. The above-mentioned devices (ISBTs) andi MOSFETs) are often required to switch very fast to reduce the switching related losses in the device (often known as switching lasses) . This calls for a fast rising (or fast falling,) gate pulse capable of supplying (or receiving) the required gate current quickly. These gate voltage signals are referenced with respect to the source terminal of the above-mentioned switching
devices. In many power—electronic applications more than one of
these switches are used and the source ,terminals of these may be
at different voltage potentials (often this potential difference may be of several hundred volts) and in such cases the respective contrail ing gate pulses must also be able to withstand the voltage difference that the device source terminals are subjected to. Any two such gate pulses that are required to withstand the said voltage potential difference are referred here as isolated gate pulses and the circuit that generates these pulses are called as isolated gate drive circuits Fig.2(a) and Fig.2(b) show some representative power—electronic circuits that may use any one or both of the above mentioned swtiches and require gate pulse isolation - For example, in Fig.2(a) gate pulses for
switches Ql, Q2or Q3 need to be isolated from each other

1 -
similarly in Fig.2(b) gate pulses for switches Q1 and Q2 need to be isolated by at least thei.r input voltage magnitude (marked as 'E' in the figures).
One illustrative example is the very popular switch fly-back type switched mode power supply circuit which has been schematically shown in Fig.3(a).
This circuit is presently the most popular one in low power ranges (20 watts to 100 watts). The main advantage of this circuit is that it requires a single switch and hence there is no need for any isolated gate driver. The other advantages are its simple output filter and the ability of the circuit to work over a wide range of input voltage (with some compromise on the output power capability ). However, the circuit of Fig.3(a) has the following drawbacks: It requires high voltage rated switch (significantly higher than the maximum working input voltage) requires snubber circuits and has considerable snubber losses and has low efficiency. A two-switch version of the same fly-back circuit is shown in Fig.3(b) and is technically much superior to the one-switch version of Fig.3(a). It has almost all positive features of the one-switch version but without its drawbacks just mentioned (except that some minor re-design of the fly-back inductor transformer may be required to match the large operating input voltage range of the one-switch version). The voltage rating of the switch in the two-switch version need not be

%
signifieanly higher than the maximum input DC voltage and in most of the designs the combined cost of the two switches is likely to be less than the cost of the single switch of the circuit in Fig.3(a). The two-switch version need not have the snubber circuit and the related losses. Inspite of its relative merits the two-switch version is not the industry favourite (at least. for low power ranges) because of two gate pulses need to have
isolation between them and calls for an isolated gate driver.
The gate drive circuit is shown within dotted lines marked (5,6) in
V
the illustrative power-electronic circuits of Fig.4 (a) and Fig.4(b). The proposed driver circuit is well suited to circuit topologies such as the one discussed in the previous section and shown in Fig,3(b). In Fig.3(b) (ie a two-switch fly-back power supply ) the switches Ql and Q2 need to be turned-on and turned— off together. Fig.4(a) and Fig.4(b) essentially reproduce the circuit of Fig.3(b) with the proposed invention. Fig.4(a) is shown to use n-channel power MOSEFTs whereas Fig.4(b) uses p-channel power MOSEFTs. The invention is applicable to gate controlled devices like MOSFETs and IGBTs of many different types and their combinatons. WORKING OF THE PROPOSED CIRCUIT
The working of the proposed circuit is explained here with the illustrative circuits of Fig.4(a) and Fig.4(b).

9
In the circuit of Fig.4(a), a regular non-isolated type driver circuit is used for the upper switch Ql. For proper circuit
operation Ql and Q2 need to be turned on and off without much

time delay between them. In the proposed circuit, switching of Q1

itself is used for fast and reliable switching of Q2. After the

turning-on of Q1, Q2 turns—on almost instantly and similarly

turning-off of Q1 is almost immediately followed by the turning off of Q2- With the proper choice of the components shown in the proposed circuit, the time delay in the switching of Ql and Q2 can be kept within one micro—second or even lower and will suit most of the intended applications. Let us consider in detail the working of the dotted portion of the proposed circuit in Fig.4(a). As 01 turns on the voltage between the drain and source terminals of Q1 collapses to a very Low value determined by the on-state voltage drop of the switch. As a result, the source potential of 01 is pulled close to the positive end of the input supply voltage (E). A sharp charging current flow through the capacitor 'C', R2 and R3 to charge the effective gate to source capacitor of the switch Q2. Value of resistor Rl is very high and as a result, current through resistor Rl is negligibly smal1 and is used mainly to compensate for lost charge due to leakage etc. As a result, the power loss in Rl is also negligibly small. Zl and Z2 are voltage—clamping devices such as a zener diode connected

10-
as a signle diode or a combination of several of them together with any other kind of diode. These may be connected in parallel, anti-parallel, in series or in anti-series and may provide voltage clamping of desired magnitude in either direction. Further, in some designs, it may be sufficient to use just one of Zl and Z2. Finally, the clamping devices denoted by Zl and Z2 keep the applied source to gate voltage of switch Q2 under the desired limit. The choice of resistors R2 and R3 together with the choice of capacitance 'C' decide the peak gate charging current and also decide the rate at which the gate capacitance is charged and need to be suitably chosen based on the given input voltage range and on the switching device characteristics. The
relative position of 'C' , R2 and R3 may be altered and few more
resistors and capacitors may be added but the basic circuit
operation remains essentially same. Once the gate to source capacitor of switch Q2 is charged to the required voltage, switch Q2 turns-on and starts conducting along with switch Ql. A positive current starts flowing through the dotted end of the induetor-transformer connected between switches Ql and Q2 (called the primary winding). The charging of capacitor 'C' will stop once it is charged to its full magnitude, this voltage magnigude being decided by the input voltage and the clamping circuit denoted by Zl and Z2) voltage. The charge on the capacitor 'C' is

11-
maintained as long as Ql is; on (with Ql end of capacitor plate kept positive) . Once switch Ql is turned-off due to control section of its driver, the primary winding current of the transformer-inductor, mentioned above, flows through the capacitor 'C', R2 , R3 and through the gate to source capacitance of switch Q2 but the current direction now is reversed and the. action is to discharge the gate to source capacitor of Q2 and also to discharge the capacitor 'C' . The gate to source capacitor of Q2 may be discharged to zero or to a clamped negative value depdnding on the choice of the clampinct circuit denoted by Zl and Z2. The gate discharge current is once again limited by the choice of the proposed circuit components 'C', R2 and R3. If the load (denoted by the primary winding current of fly-back transformer) is in excess of this gate charge current diode Dl turns on to conduct the remaining magnitude of the inductive load current. With the turning on of diode Dl, any remaining charge on the gate circuit capacitance of Q2 and on the capacitor 'C' will discharge even faster and quickly conclude the turning-off process of Q2 . The next turning—on and turning-off cycle repeats as described before.
A similar charging/discharging cycle repeats for the p-channel MOSEFT Ql shown in cicuit of Fig.4(b). Here, the lower switch Q2 is driven directly and the proposed circuit is able to

12-
effactively switch the upper switch (Q1). The lower switch Q2 could be a p-channel switch or alternatively a n-channel switch. In most of the fly-back type power supplies (as shown in Fig.4(a) and Fig.4(b), the gate power of the directly driven switch (such as switch Ql in Fig.4(a) and switch Q2 in Fig.4(b)) is derived from one of the floating outputs of the switched mode power supply itself (the starting power required for few initial Switchings is derived from the input DC supply 'E' through a clamped low rating resistive-capacitive circuit) and as such it does not make much difference if the directly driven switch' is upper switch or lower switch. However, some times, especially when a separate independent supply is used for the directly driven switch, there may be common mode noise present in the driving power supply. This problem is greatly reduced by dividing the gate circuit resistance in two halves and putting one half in series with the gate and other half in series with the source end of the driver. The arrangement is being shown in Fig.4(a) where RG1 and RG2 are the two halves of the gate circuit series resistor. The arrangement of dividing the gate circuit series resistor in the above manner is claimed as yet another related invention by the inventors-

13
The invention relates not only to the fly-back circuit described above but is equally applicable to many other circuits, such as two switch forward converters and the two-switch magnet power supplies as they essentially have the same circuit topology. The invention may also be fully or partly applicable to some other circuit topologies not mentioned here.
The invention described here inabove is in relation to a non-limiting embodiment used as defined by the accompanying claims.

14.
WE CLAIM:
,1. An improved switched mode power supply (SMPS)circui t of flyback and/or forward type for industrial electronic application being connectable to an input supply voltage (E), comorising an unisalated driver (2,3) connected to a first solid state switching device of MOFSET or IGBT switches (Q1) via two halves (RG1, RG2) of a gate circuit resistance, said first switching device being operably connected to a second solid state switching device of MOFSET or IGBT swtiches (Q2): a fly-back inductor and/or forward converter transformer (4) disposed between said first and second switching devices, characterized in that an isolated gate driver (5,6) is interposed between said switching device for generating isolated gate oulses capable to withstand a voltage potential difference which the source terminals of said solid state switches being subjected to, the isolated gate drive (5,6) comprising a capacitor (C),plurallty of resistances (R1,R2,R3) . and at least one voltage clamping device (Ql). 2. The switched mode power supply circuit as claimed in claim 1 wherein the switching devices are n-channel MOSFET having gate and device© source terminal for input of voltage signal and a drain terminal.

15.
3. The switched mode power supply circuit as claimed in claim 1
wherain the switching devices are p-channel MOSFET devices at
least one device being connected to the driver circuit (2,3)
4. The switched mode power supply circuit as claimed in
claim 1 wherein charging current flows through the capacitor "C"
resistor 'R2 , and 'R3 ' to charge the effective gate to source
capacitor of the switching device.
3. The switched mode power supply circuit as claimed in claim 4 wherein the value of the resistor 'R1' being high. the Currant flow iss small causing lower power loss and primarily used to compensate for leakage of charge in gate capacitance of the switching device.
6. The switched mode power supply circuit as claimed in claims
4 and 5, wherein between the switching devices two diodes (D1 D2
are provided.
7. The switched mode cower supply circuit as claimed in claim 1
wherein one half of the Gate circuit series resistor is disposed
in series with the gate and the other half in series with the
source of the uni-isolated driver (2,3).

16.
8. An improved switched mode power supply (SMPS) circuit for industrial electronic application as herein described and illustrated with the accompanying drawings.
The invention relates to an improved switched mode power supply (SMPS) circuit of fly-back and/or forward type for industrial electronic appiication being connectable to an input supply voltage (E), comprising an unisolated driver (2,3) connected to a first solid state switching device of MOSFET or IGBT switches via two halves (RG1, RG2) of a gate circuit resistance, said first switching device being operably connected to a second solid state switching device of MOSFET or IGBT switches; a fly-back inductor and/or forward converter transformer (4) disposed between said first and second switching devices. An isolated gate driver (5,6) is interposed between said switching devices for generating isolated gate pulses capable to withstand a voltage potential difference which the source terminals of said sol id state switches being subjected to, the isolated gate driver (5,6) comprising a capacitor (C), plurality of resistances (Rl, R2, R3), and at least one voltage clamping device (zl).

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Patent Number

201682

Indian Patent Application Number

542/CAL/2001

PG Journal Number

08/2007

Publication Date

23-Feb-2007

Grant Date

23-Feb-2007

Date of Filing

20-Sep-2001

Name of Patentee

INDIAN INSTITUTE OF TECHNOLOGY

Applicant Address

AN INDIAN INSTITUTE OF KHARAGPUR 721 302.

Inventors:

#	Inventor's Name	Inventor's Address
1	PRASAD DR. DINKAR	INDIAN NATIONALS OF DEPARTMENT OF ELECTRICAL ENGINEERING, INDIAN INSTITUTE OF TECHNOLOGY, KHARAGPUR-721 302.
2	VENKATARATNAM PROF. K.	INDIAN NATIONALS OF DEPARTMENT OF ELECTRICAL ENGINEERING, INDIAN INSTITUTE OF TECHNOLOGY, KHARAGPUR-721 302.

PCT International Classification Number

H 01 H 47/00

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA