Self supplied gate drive

Abstract

A self-supplied thyristor gate driver circuit includes a thyristor 20, a driver unit 18 for turning ON the thyristor, a rechargeable energy storage device, e.g. capacitor 12, and an insulated-gate bip

Description

This invention relates to circuitry for controlling the firing of a thyristor and more particularly to a self-powered circuitry for providing power to thyristor gate drivers. To drive power semiconductors, or any other three-terminal triggerable switch device, some kind of gate driving circuit must be provided to have means of switching the device on and off. A gating signal delivers a small amount of energy to the associated power semiconductors gate electrode to change its conduction state (turn on or turn off). In general thyristors are turned on by a current signal to its gate electrode whereas IGBT's and MOSFET's require a voltage signal to their control gate. Usually, the associated gate driving circuits need to be operated with power that is isolated from the main system ground since the input and output voltages may be several thousand volts or even more. The general approach to this problem is to employ separate isolated DC power supplies that provide galvanic isolation between their own source of power and the associated DC output circuitry which, along with the associated power semiconductor drive circuitry, must float at the high potential of the power semiconductor. However, these driver power sources occupy valuable space and add significant cost and complexity to the power semiconductor driven system. Further, the supply to the isolated DC power sources for the gate drivers is sensitive to electromagnetic interference, which can cause additional problems. Hence, there remains a necessity for improved gate driver power systems by which isolated gate driver power can be provided in a cost-efficient manner without adding large external power sources and more complexity. The present invention removes the need for such supplies, using a circuit, isolated from ground and arranged around the high potential terminals of a power semiconductor, which harvests energy from the circuit of the associated power semiconductor that is used to trigger the power semiconductor when required. Various aspects of the present invention are now summarized to facilitate a basic understanding of the invention. With the present invention, instead of a separate power supply for the driving circuit of the power semiconductor, a rechargeable storing device, preferably a capacitor is provided for the driving circuit of the power semiconductor. The capacitor is charged via a converter system from the main power circuit of the power semiconductor. When the power semiconductor, which is connected to some kind of power circuit, is in the off state, a significant voltage will be established between the anode and the cathode of the power semiconductor. Via a semiconductor switch, which is connected between the anode of the power semiconductor and the positive voltage terminal of the capacitor, a current can be drawn from the main power circuit to charge the capacitor. When the capacitor has reached an appropriate voltage to maintain power to the gate driving circuit, the semiconductor switch turns off and isolates the voltage between the anode of the power semiconductor and the capacitor. Hence, the capacitor will store a certain amount of energy which can be provided to the gate driving circuitry. If the capacitor discharges down to a certain voltage due to the power dissipated by the gate driving circuitry, the semiconductor switch turns back on again to recharge the capacitor via the main power circuit. Hence, a controllable link is provided between the main power circuit and the gate driving circuit of the power semiconductor switch, providing energy drawn from the main power circuit to the gate driving circuit. Because of this there is no need for an isolated DC power supply. The invention will now be described solely by way of example and with reference to the accompanying drawings in which: Figure 1 shows the complete schematic of the self-supplied gate driver including a thyristor, being, without the loss of generality, an example for a three-terminal triggerable switch device, and the associated power circuit. Figure 2 shows the circuit schematic of a constant current supply used in the self-supplied gate driver. 08 11 24 In figure 1, if the main thyristor 20 is in the off state the associated power circuit 25 will establish a voltage between the anode 21 and the cathode 19 of the thyristor 20. Further, the cathode 19 of the thyristor 20 is connected to local reference potential 22. Via an anti surge resistor 4 and the reverse blocking diodes 3 and 2 a constant current source 6 will draw a small amount of current 17 from the main power circuit 25 to provide a zener diode 8 with a constant current 17 and establish the associated zener voltage across the zener diode 8. An adjustable resistor 7 can provide means of fine adjusting the voltage of a small storage capacitor 9 which is connected to the gate 27 of an IGBT switch 1 via a gate resistor 5. The combination of the constant current source 6, the Zener diode 8 and the fine adjustment network 7 and 9 will establish a constant voltage at the gate electrode 27 of the IGBT switch 1 independent of the voltage across the main SCR 20. The collector electrode 28 of the IGBT switch 1 is connected in series to the reverse blocking diode 3 and the anti-surge resistor 4, which is connected to the anode 21 of the thyristor 20. The emitter electrode 26 of the IGBT 1 is connected to the gate drive energy storage capacitor 12, which provides energy to the gate driving unit 18, which is connected to the gate 23 of the main thyristor 20 and the cathode 19 of the main thyristor 20. If the gate electrode 23 of the thyristor 20 is fired, the capacitor 12 will discharge its energy via the gate driving circuit 18 into the gate 23 of the thyristor 20 and provided the zener voltage of the zener diode 8 is well above the gate emitter threshold voltage of the IGBT 1, the gate emitter voltage will then be positive and the IGBT 1 will be in a conductive state. Hence, a low impedance current path between the anode 21 of the thyristor 20 and the energy storage capacitor 12 of the gate driving circuit 18 will be established via the anti surge resistor 4, the reverse blocking diode 3 and the IGBT switch 1. An overvoltage protection scheme consisting of the transient voltage suppressors 10 and 11 makes sure that during a fast discharge of the gate drive energy storage capacitor 12 the gate emitter voltage of the IGBT 1 remains below the maximum permissible gate emitter voltage of the IGBT 1. When the thyristor 20 turns off again due to current commutation in the main power circuit 25 the voltage between anode 21 and cathode 19 will start to rise by a rate dictated by the properties of the main power circuit. Due to the IGBT 1 being in a conductive state the anti surge resistor 4 and the gate drive energy storage capacitor 12 will function as a snubbing device and draw charge from the main power circuit 25 to capacitor 12. Capacitor 12 will be charged in a short timescale which is determined by the value of the anti surge resistor 4, the capacitance of capacitor 12 and the voltage rise time across the main thyristor 20. The maximum charging voltage of the capacitor is determined by the zener voltage of the zener diode 8 plus the voltage drop across the adjustable resistor 7 minus the gate emitter threshold voltage of the IGBT 1. When this voltage is reached, the IGBT 1 will turn off again and the thyristor 20 anode voltage in 21 will be insulated from the positive terminal of the capacitor 12. Because of this the gate drive circuit 16 can be powered up by the turn off cycle of the thyristor 20 and is ready for firing the thyristor 20 when a firing command is received. The power drawn from the main power circuit 25 will be proportional to the number of on / off cycling periods of the thyristor 20. If due to the power requirements of the gate driving unit 18 the voltage across the capacitor 12 starts to drop while the main thyristor 20 remains in the off state, the gate emitter voltage of the IGBT 1 will start to increase again. As a consequence of this, a current will be drawn from the main power circuit via the anti surge resistor 4, the reverse blocking diode 3 and the IGBT 1, charging the capacitor 12 until an equilibrium is reached between the supply current requirement of the gate driving unit 18 and the current drawn from the main power circuit 25. One of the main advantages of this self-supplied gate driving scheme is the fact that the power supplied to the gate drive 18 is independent of the voltage across the thyristor 20. The energy storage capacitor 12 will always charge up to the same voltage provided the voltage across the thyristor 20 is above the minimum voltage requirement dictated by the zener diode 8 voltage, the voltage drop across the adjustable resistor 7 and the gate emitter threshold voltage of the IGBT 1. Hence, the capacitor 12 will operate as a rechargeable battery drawing a small amount of current of the power circuit 25 to provide power to the gate driver 18. The IGBT switch 1 and the associated 08 11 24 components function as a charging device with the charging energy drawn from the main power circuit 25. Further, the charging voltage of the capacitor 12 can be easily tailored according to the requirements of the gate driving unit 18 by simply choosing an appropriate zener diode 8. The combination of constant voltage at the gate 27 of the IGBT 1 and the gate emitter threshold voltage of IGBT 1 will keep the capacitor 12 from being overcharged by the power circuit 25. There is no requirement for bulky voltage insulation transformers to supply the gate driving circuit 18 with power. Hence, the power circuit could be controlled with complete galvanic insulation via a fibre optic link between some control circuit and the gate driver 18 of the thyristor 20. In figure 2 a close-up circuit schematic of the constant current source 6 is shown. The constant current source 6 can be made of a combination of pnp transistors 13 and 16 and resistors 14 and 15 to provide a constant current 17 to the zener diode 8 via the adjustable resistor 7. The value of the resistors 14 and 15 can be tailored according to the required current value 17, which can be well below 1 mA, and the number of constant current sources switched in series can be tailored according to the maximum voltage which will be established across the constant current source 6. It should be noted here that the method and apparatus described above are only exemplary and do not limit the scope of the invention, and that various modifications could be made by those skilled in the art that would fall under the scope of the invention. For example, instead of driving a current controlled thyristor the self-supplied driver mentioned above could also drive a power IGBT, which is a voltage-controlled device and therefore would require less driving energy than a thyristor. Further, the IGBT 1 in figure 1 can be replaced by a MOSFET with a drain source voltage similar to the collector emitter voltage of IGBT 1. A difference in the threshold voltage values of the MOSFET can be easily compensated by either choosing an appropriate zener diode 8 or adjusting the voltage across the resistance 7. Then again, the anti surge resistor 4 in figure 1 can be replaced by a carefully matched inductor which would reduce the dissipated losses in the circuit each time capacitor 12 is charged. This inductor can be tailored according to the capacitance of the energy storage capacitor 12, the electrical properties of the IGBT 1 and the expected voltage rise time across the power semiconductor 20. 18 03 25

Claims

1. A circuit for turning on at least one three-terminal triggerable switch device, the three-terminal triggerable switch device having an anode, a cathode and a gate electrode, the three-terminal triggerable switch device anode connected to a voltage source providing a substantial voltage and the three-terminal triggerable switch device cathode connected to a power circuit, the circuit comprising:a driver linked to the three-terminal triggerable switch device gate electrode for turning on the three-terminal triggerable switch device, the driver including a power lead for receiving power required for the driver; a rechargeable energy storage device linked to the power lead for the driver; a voltage controlled three terminal switch having a collector electrode, a emitter electrode and a gate electrode, with gate electrode of the voltage controlled three terminal switch connected to a voltage source and with the emitter electrode of the voltage controlled three terminal switch connected to the rechargeable energy storage device and with the collector electrode of the voltage controlled three terminal switch connected in series with an impedance to the anode of the three-terminal triggerable switch device in order to provide power from the voltage source connected to the anode of the three-terminal triggerable switch device to the driver when a positive voltage is applied between the three-terminal triggerable switch device anode and cathode.

2. The circuit of claim 1 wherein the three-terminal triggerable switch device is replaced by an IGBT, the IGBT having a collector electrode, an emitter electrode and a gate electrode, with the IGBT gate electrode connected to the gate driver of claim 1 and with the IGBT collector electrode taking the part of the anode of the three-terminal triggerable switch device and with the IGBT emitter electrode taking the part of the cathode of the three terminal triggerable switch device.

3. The circuit of claim 1 wherein the three-terminal triggerable switch device is replaced by a MOSFET, the MOSFET having a drain electrode, a source electrode and a gate electrode, with the MOSFET gate electrode connected to the gate driver of claim 1 and with the MOSFET drain electrode taking the part of the anode of the three-terminal triggerable switch device and with the MOSFET source electrode taking the part cathode of the three-terminal triggerable switch device.

4. The circuit of claim 1 wherein the three-terminal triggerable switch device is replaced by a thyristor having an anode and a cathode and with the gate driver of claim 1 driving the gate terminal of the thyristor.

5. The circuit of claim 1 wherein the rechargeable energy storage device includes a capacitor.

6. The circuit of claim 1 wherein the voltage source connected to the gate of the voltage controlled three terminal switch consists of a voltage regulating diode and a constant current source connected between the anode of the three-terminal triggerable switch device and the voltage regulating diode to provide a constant voltage to the gate of the voltage controlled three terminal switch.

7. The circuit of claim 1 wherein the voltage controlled three terminal switch of claim 1 is a IGBT, the IGBT having a collector electrode, an emitter electrode and a gate electrode, with the IGBT gate electrode taking the part of gate electrode of the voltage controlled three terminal switch and with the IGBT collector electrode taking the part of the collector electrode of the voltage controlled three terminal switch and with the IGBT emitter electrode taking the part of the emitter electrode of the voltage controlled three terminal switch.

8. The circuit of claim 1 wherein the voltage controlled three terminal switch of claim 1 is replaced by a MOSFET, the MOSFET having a drain electrode, a source electrode and a gate electrode, with the MOSFET gate electrode taking the part of the gate electrode of the voltage controlled three terminal switch and with the MOSFET drain electrode taking the part of the collector electrode of the voltage controlled three terminal switch and with the MOSFET source electrode taking the part of the emitter electrode of the voltage controlled three terminal switch.

9. The circuit of claim 1 wherein the voltage controlled three terminal switch of claim 1 is replaced by a bipolar junction transistor, the bipolar junction transistor having a collector electrode, a emitter electrode and a base electrode, with the bipolar junction transistor base electrode taking the part of the gate electrode of the voltage controlled three terminal switch and with the bipolar junction transistor collector electrode taking the part of the collector electrode of the voltage controlled three terminal switch and with the bipolar emitter electrode taking the part of the emitter electrode of the voltage controlled three terminal switch .LDCM

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