GB2455754A

GB2455754A - A rapid gate discharge circuit for a switch in a rectifier or inverter

Info

Publication number: GB2455754A
Application number: GB0724825A
Authority: GB
Inventors: Lesley Chisenga; Andrabadu Viraj
Original assignee: Enecsys Ltd
Current assignee: Enecsys Ltd
Priority date: 2007-12-20
Filing date: 2007-12-20
Publication date: 2009-06-24
Anticipated expiration: 2027-12-20
Also published as: GB2455754B; GB0724825D0

Abstract

A switch T in a rectifier or inverter is driven by a signal derived from the AC line 1,2. In order to achieve rapid gate discharge of T, and thus prevent shoot-through in the converter, a low value shunt resistor R3 is provided. The switch may be a MOSFET, IGBT, or thyristor.

Description

Driver Circuits and Techniques

FIELD OF THE INVENTION

This invention relates to driver circuits, in particular for power semiconductor switching devices of the type that are employed in ac (alternating current) inverters. More particularly the invention relates to techniques for rapid removal of charge from a control terminal of a power switching device such as a MOSFET (Metal Oxide Semiconductor Field Effect Transistor) IGBT (Insulated Gate Bipolar Transistor) or Thyristor.

BACKGROUND TO THE INVENTION

Attempts have been made previously to directly couple switching semiconductor devices to the grid in order to maintain phase synchronisation and simp1i1r system design. One of the key problems is how to rapidly remove gate charge and at the same time minimise power loss in the driving circuit. If charge is not removed rapidly from the gate terminal a shoot-through problem results and a ground fault develops on the grid. One possible solution is to use small ohmic resistors. However, the use of small ohmic resistors to form potential dividing circuit often results in huge losses in the resistors and therefore reduced lifetime and reliability for the system.

We describe techniques to address these problems.

SUMMARY OF THE INVENTION

According to the present invention there is therefore provided a driver circuit for switching on and off a semiconductor switching device connected to an alternating current (ac) power supply, said semiconductor switching device having first and second terminals, and a switching control terminal to control switching between said first and second terminals, the driver circuit being configured to derive from said ac power supply a control signal for application to said switching control terminal of said semiconductor switching device to control said switching, said driver circuit comprising: an input to receive a voltage derived from said ac power supply; a reference line for coupling to one of said first and second terminals of said semiconductor switching device; a rectifier having an input coupled to said input and an output; and a resistive element coupled between said output of said rectifier and said reference line; and a drive output for driving said switching control terminal of said semiconductor switching device, said drive output being coupled to a circuit node between said resistive element and said output of said rectifier.

In embodiments the resistive element comprises a resistor although the skilled person will appreciate that an FET may also be used as a resistive element. Preferred embodiments of the circuit include a voltage limiting element such as zener diode coupled between the circuit node and the reference line. This is particularly important when driving grid mains. Preferred embodiments also include a potential divider coupled to the input, the rectifier being coupled to an output of the potential divider. In embodiments the resistive element described above has a resistance of less than 1/5, more preferably less than 1/10 or less than 1/20 of a resistance value of an arm of the potential divider.

In embodiments of the circuit the semiconductor switching device comprises a MOSFET, IGBT, or Thyristor, more particularly a power device (that is a device with an operating or switching voltage capability of greater than 100 volts and/or a power rating of greater 1 watt).

In some preferred embodiments the ac power supply comprises a grid mains power supply and the semiconductor switching device has a breakdown voltage of at least 100 volts. The grid mains power supply may either be a domestic mains power supply such as a 110 volt or 230 volt power supply or a three phase power supply, typically operating at 415 volts.

The invention further provides a full-bridge or hail-bridge rectifier circuit including one or more semiconductor switching devices and respective driver circuits as described above. 3.

The invention thither provides a power conditioning circuit with a dc input and an ac output for connection to an ac grid mains power supply. Then embodiments of the above-described driver circuit may be employed to drive a semiconductor switching device chopping a power supply derived from the dc input to provide an ac output to the grid mains supply. Some preferred embodiments of such a power conditioning circuit have two semiconductor switching devices driven by respective driver circuits, switching in alternate half cycles of the ac grid mains power supply.

Thus one or more driver circuits and switching devices as described above may be employed as one or more switches in a dc-to-ac power converter of a type described below: A dc-to-ac power converter, the converter including a transfonner having a primary and a secondary winding, the primary winding of said Iransformer being coupled to a dc input of said power converter and the secondary winding of said transformer being coupled to an ac output of said converter, and wherein the converter further comprises: a first pair of switches on said primary side of said converter, coupled between said dc input and said primary winding, to convert a dc supply from said dc input to an ac current for driving said transformer, a second pair of switches on said secondary side of said converter coupled between said secondary winding and said ac output, one in a forward path to said ac output and one in a return path from said ac output; a diode coupled across each of said secondary side switches; and a controller configured to control said primary and secondary side switches to convert a dc supply at said dc input to an ac supply at said ac output.

A DC-to-AC power converter, the converter including a transformer having a priinaiy and a secondary winding, the primary winding of the transformer being coupled to a dc input of the power converter and the secondary winding of the transformer being coupled to an ac output of the converter, and wherein: a first and second switch connected to the primary winding of the transformer to convert a dc supply from the dc input to an ac current for driving the transformer; a first and second switch connected to the secondary winding of the transformer such that the first switch is in a forward path to the ac output and the second switch is in a return path to the ac output; a first and Li-.

second diode coupled across the respective first and second switches connected to the secondary winding; and wherein the first switch connected to the piimaiy winding of the transformer is controlled to provide a first half cycle of an ac voltage to the primaiy winding of the transformer; the second switch connected to the primary winding of the transformer is controlled to provide a second half cycle of an ac voltage to the primary winding of the transformer; and the first and second switches connected to the secondary winding of the Iransfonner as switched to alternately conduct the first and second half cycles of the signal coupled from the primary winding of the transformer to the secondary winding of the transformer.

Further details of such circuits can be found in the applicant's co-pending UK and US patent applications GB 0612859.9 filed 29 June 2006 and US 11/771,593 filed 29 June 2007 (both of which are hereby incorporated by reference in their entirety).

In a related aspect there is further provided a method of removing control terminal charge from a power semiconductor switching device, the method comprising supplying a drive signal to said control terminal of said power semiconductor switching device via a rectifier, and leaking current from said control terminal to a reference line whilst said power switching device is turned on.

In embodiments of the method the power semiconductor switching device comprises a MOSFET, IGBT or Thyristor; in embodiments the reference line comprises a ground line.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will now be further described, by way of example only, with reference to the accompanying figures in which: Figure 1 shows a circuit diagram of a driver circuit according to an embodiment of the invention; Figure 2 shows a circuit diagram of a driver circuit omitting a rectifier, illustrating operation of the circuit of Figure 1; Figure 3 shows graphs of effective input voltage to the driver circuit over a half-cycle of ac grid mains for circuits with and without a rectifier and lacking a zener diode (upper) and with a zener diode (lower); Figure 4 shows, schematically, leakage capacitances of an MOSFET; Figure 5 shows measured waveforms from a circuit similar to that illustrated in Figure 2; Figure 6 shows waveforms from circuits similar to that illustrated in Figure 1; Figure 7 shows a circuit of a dc-to-ac power convener comprising 4 switches (to on a transformer primary side, two on a transformer secondary side) incorporating secondary-side driver circuits according to an embodiment of the invention; Figure 8 illustrates an example of a hill-bridge rectifier circuit incorporating switching devices with switching controlled by an ac grid mains supply to which the circuit is connected, for either receiving power from a mains supply and delivering power to a load or for receiving a dc power input and providing an ac power output to a mains supply; and Figure 9 shows waveforms illustrating the operation of Figure 8.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Broadly speaking, we will describe a driver circuit that switches on and off a semiconductor device connected to the utility grid by using the grid voltage as the switching signal. The new driver circuit solves the previous problem associated with removing charge quickly from the gate tenninal of the semiconductor device when the grid voltage changes polarity. The rapid removal of charge from the gate terminal enables grid connection of devices with high gate charge density that are therefore slow switching. This in turn leads to the transfer of high power densities through the switching device. The driver circuit can be used in energy conversion systems such as solar photovoltaic and wind and in rectifier circuits connected to the utility grid or similar alternating current supplies.

Figure 1 shows the circuit solution. The circuit uses high value ohmic resistor RI and therefore affords low potential divider losses. A low value resistor R3 is used to enable rapid removal of gate charge as the grid voltage drops to zero. The semiconductor switch T can be implemented as a MOSFET device, IGBT or Thyristor.

Principle of Operation Assume the grid voltage Vgrid is zero, implying that the potential difference between point 1 and 2 in figure 1 is zero, and the potential difference between 2 and 5 is zero.

Also assume the zener diode Z has a value Vz. The resistors Rl and R2 are chosen to have high value, example 1 mega ohm each. When the grid voltage rises from zero, that is a positive potential difference develops between 1 and 2, the voltage at point 4, V4, also increases. The diode D becomes forward biased and begins to conduct as V4 rises.

The gate voltage Vgate, that is the potential between 3 and 5, therefore increases due to diode charge. As the grid voltage continues to rise, so does Vgate. The transistor T turns on when Vgate equals the turn on threshold of 1. Vgate stays constant at Vz even when Vgrid rises further.

Vgate stays constant until V4 drops below Vz as Vgrid drops. There are two possibilities to the state of D when V4 drops below Vz. If the gate charge is removed rapidly, D would remain forward biased until Vgrid becomes zero. In this case T tunis off before the Vgrid reverses polarity. On the other hand if the Vgate remains higher than V4, D is reverse biased. In this case there is a possibility that the MOSFET is on when Vgrid reverses. If this happens in, say, a half or full-bridge rectifier, the result is a short circuit in the power circuit.

To remove charge rapidly, the value of resistor R3 is chosen to have a low value, for example 2Okilo ohm to lOOkilo ohm. This would enable the removal of charge rapidly and therefore enable high gate charge switches to be used. It is possible to have a low value of R3 across Vgate because of the blocking diode D. If D is short-circuited (as shown in Figure 2, which is included to illustrate this) Vgate may not attain Vz for all or part of the half grid cycle and therefore the switch would not work properly. An example for the value of Vz that would result in normal operation of the switch is say 15V (this varies with the type of switching device). In mains driven circuits the zener diode is important. Figure 2 illustrates a circuit with D replaced by a conducting wire.

In this case the resistors R2 and R3 form a parallel network with value equal to R2 * R3/(R2 + R3). Figure 3 shows the resulting gate drive signals with and without the diode D connected.

Gate Charge and Shoot-through The gate drive circuit has the ability to discharge the gate terminal of the connected switching device rapidly, therefore preventing short-circuiting the grid when the grid voltage reverses. Switching devices such as MOSFET have parasitic gate capacitances that store charge. Figure 5 shows a representation of the MOSFET with drain-to-gate and gate-to-source parasitic capacitors.

The charge stored in the combined capacitance Cl and C2 is discharged through R3 and through some leakage current in the MOSFET and the zener diode. The time constant for the discharge assuming that the diode D stays reverse biased is given by equation 1.

T=lI(CR3 + CRL) were C is the overall gate capacitance and RL is the leakage resistance due to the MOSFET and zener diode. This equation also indicates that a small R3 reduces T. Figure 5 shows some experimental results obtained when the gate circuit is designed without the diode D and the resistor R3. In the figure, it can be observed that the falling gate voltage overshoots the zero-crossing point of the grid by a significant amount to cause short circuit when the other half cycle rises (see the double-ended arrow). Figure 6 shows the results obtained when D and R3 arc included in the design. In this case the gate signals fall rapidly enough to avoid any significant overshoot.

Application examples

The driver circuits can be used in applications where synchronised switching of the grid is used for power transfer in either direction. One example is as used in the circuit diagram of figure 7. The principle of operation of this circuit is described in our earlier patent application (ibia). In this circuit the drivers switches the two IGBTs in alternate half cycles to allow power transfer from a source such as solar photovoltaic energy.

Figure 8 shows another application of the proposed driver circuit. In this case power can be transferred from the grid to the load or the load can supply power to the grid. Figure 9 illustrates the waveforms appearing across the load. The amplitude difference between Yg and VL are for illustration clarity.

No doubt many other effective alternatives will occur to the skilled person. It will be understood that the invention is not limited to the described embodiments and encompasses modifications apparent to those skilled in the art lying within the spirit and scope of the claims appended hereto. 9.

Claims

CLAIMS: 1. A driver circuit for switching on and off a semiconductor switching device connected to an alternating current (ac) power supply, said semiconductor switching device having first and second terminals, and a switching control terminal to control switching between said first and second terminals, the driver circuit being configured to derive from said ac power supply a control signal for application to said switching control terminal of said semiconductor switching device to control said switching, said driver circuit comprising: an input to receive a voltage derived from said ac power supply; a reference line for coupling to one of said first and second terminals of said semiconductor switching device; a rectifier having an input coupled to said input and an output; and a resistive element coupled between said output of said rectifier and said reference line; and a drive output for driving said switching control terminal of said semiconductor switching device, said drive output being coupled to a circuit node between said resistive element and said output of said rectifier.
2. A driver circuit as claimed in claim 1 further comprising a voltage limiting element coupled between said circuit node and said reference line.
3. A driver circuit as claimed in claim 1 or 2 further comprising a potential divider coupled to said input, and wherein said rectifier is coupled to an output of said potential divider.
4. A driver circuit as claimed in claim 3 wherein said resistive element has a resistance value of less than one fifth a resistance value of an arm of said potential divider.
5. A driver circuit as claimed in claim 3 wherein said resistive element has a resistance value of less than one tenth a resistance value of an arm of said potential divider. 10.
6. A driver circuit as claimed in any preceding claim wherein said semiconductor switching device comprises a MOSFET, IGBT or Thyristor.
7. A driver circuit as claimed in any preceding claim wherein said ac power supply comprises a grid mains power supply, and wherein said semiconductor switching device comprises a power semiconductor switching device with a breakdown voltage of at least volts.
8. A full-bridge or half-bridge rectifier circuit including a plurality of semiconductor switching devices each configured as a rectifier, each of said semiconductor switching devices having a respective driver circuit as claimed in any preceding claim.
9. A power conditioning circuit having a dc input and an ac output for connection to an ac grid mains power supply to provide power into said ac grid mains power supply, said power conditioning circuit including at least one semiconductor switching device and a driver circuit as claimed in any one of claims 1 to 8 for switching power derived from said dc input to provide said ac output to said ac grid mains power supply.
10. A power conditioning circuit as claimed in claim 9 comprising two of said semiconductor switching devices and each having a respective said driver circuit for switching said semiconductor switching devices in alternate half cycles of said ac grid mains power supply.
11. A method of removing control terminal charge from a power semiconductor switching device, the method comprising supplying a drive signal to said control terminal of said power semiconductor switching device via a rectifier, and leaking current from said control terminal to a reference line whilst said power switching device is turned on.
12. A method as claimed in claim!! wherein said power semiconductor switching device comprises a MOSFET, IGBT or Thyristor.
13. A driver circuit substantially as herein described and/or illustrated.