GATE UNIT FOR A GATE-COMMUTATED THYRISTOR AND INTEGRATED GATE-COMMUTATED THYRISTOR

Abstract

The invention relates to a gate unit (22) for controlling a gate commutated thyristor (21), comprising: a voltage selector (26) for selectively applying a high supply potential (V.sub.pos), a middle supply potential (V.sub.mid), and a low supply potential (V.sub.neg); a nonlinear inductor (27) serially coupled between the output of the voltage selector (26); a gate control unit (23) configured to control the voltage selector (26) to control switching of the gate commutated thyristor (21) in its turn-on state comprising a turn-on pulse generation, a positive-gate-voltage backporch operation, a negative-gate-voltage backporch operation and a retrigger pulse generation; wherein the nonlinear inductor (27) has a nonlinearity to have a high inductance during any of the backporch operations and to have a low inductance during the turn-on pulse generation and retrigger pulse generation.

Claims

1. Gate unit (22) for controlling a gate commutated thyristor (21), comprising: a voltage selector (26) for selectively applying a high supply potential (V.sub.pos), a middle supply potential (V.sub.mid), and a low supply potential (V.sub.neg); a nonlinear inductor (27) serially coupled between the output of the voltage selector (26); a gate control unit (23) configured to control the voltage selector (26) to control switching of the gate commutated thyristor (21) in its turn-on state comprising a tum-on pulse generation, a positive-gate-voltage backporch operation, a negative-gate-voltage backporch operation and a retrigger pulse generation; wherein the nonlinear inductor (27) has a nonlinearity to have a high inductance during any of the backporch operations and to have a low inductance during the turn-on pulse generation and retrigger pulse generation.

2. Gate unit (22) according to claim 1, wherein the gate control unit (23) is configured to operate switching of the gate commutated thyristor (21) in soft-switching mode, particularly in a zero voltage switching mode or a zero current switching mode.

3. Gate unit (22) according to claim 1, wherein the voltage selector (26) is configured as a transistor switched voltage selector such as a three level NPC circuit.

4. Gate unit (22) according to claim 1, wherein the gate control unit (23) is configured to control the voltage selector (26) for the positive-gate-voltage backporch operation by controlling an inductor current through the nonlinear inductor (27) via pulses generated by applying two supply potentials in an alternating manner as long as a measured gate-to-cathode voltage of the gate commutated thyristor (21) is positive.

5. Gate unit (22) according to claim 1, wherein the gate control unit (23) is configured to control the voltage selector (26) for the negative-gate-voltage backporch operation by controlling an inductor current through the nonlinear inductor (27) via pulses generated by applying two supply potentials in an alternating manner as long as a measured gate-to-cathode voltage of the gate commutated thyristor (21) is negative.

6. Gate unit (22) according to claim 1, wherein a turn-off stage (28) is provided configured to apply the negative supply potential (V.sub.neg) to selectively turn off the gate commutated thyristor (21).

7. Gate unit (22) according to claim 6, wherein the gate control unit is configured to, in the turn-on operation, keeping the negative supply potential (V.sub.neg) applied by means of the turn off stage while applying the high supply potential (V.sub.pos) or the middle supply potential (V.sub.mid) to increase an inductor current of the nonlinear inductor, while after a given time delay, the turn off stage (28) is deactivated so that the inductor current is fully commutated as gate current i.sub.g into a gate terminal (G) of the gate commutated thyristor (21), wherein after an inductor current (i.sub.on) has been commutated into the gate terminal (G), the middle supply potential (V.sub.mid) is applied.

8. Gate unit (22) according to claim 1, wherein nonlinearity of the nonlinear inductor (27) is selected so that the low inductance for the turn-on pulse generation has a value at least 50% lower than the high inductance during the backporch operation.

9. Gate unit (22) according to claim 1, comprising a communication channel output to interlink with a control input of another gate unit (22) wherein the gate control unit (23) is configured to propagate a control signal received via the control input through the communication channel output, wherein the control signal includes switching commands for the gate commutated thyristors (21).

10. Gate unit (22) according to claim 1, comprising a communication channel output to interlink with a control input of another gate unit (22) wherein the gate control unit (23) is configured to propagate a control signal received via the control input through the communication channel output, wherein the control signal includes an error signal indicating the occurrence of an error in one of the gate units (22) wherein the gate control unit (23) is further configured to halt operation of the gate unit once an error signal has been received.

11. Integrated gate commutated thyristor comprising a gate commutated thyristor (21) and the gate unit (22) according to claim 1.

12. Converter stage (2) for use in a converter system (1), comprising a number of integrated gate commutated thyristors each including a gate commutated thyristor (21) and the gate unit (22) according to claim 9, wherein one of the gate units (22) is coupled with a central controller (5) to receive the control signal and at least a part of the other gate units (22) are respectively coupled via its communication channel output with the control input of a further one of the other gate units (22), so that the control signal is available in all gate units (22).

13. Use of the integrated gate commutated thyristor according to claim 11 in a converter stage (2) of a converter system (1).

14. Method for operating a gate commutated thyristor (21) using a gate unit (22) according to claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Embodiments are described in more detail in conjunction with the accompanying drawings, in which:

[0041] FIG. 1 schematically shows an LLC resonant converter configured for zero-voltage switching during turn-on.

[0042] FIG. 2 shows a circuitry for illustrating the design of the gate unit for the gate-commutated thyristors.

[0043] FIG. 3 shows an example characteristics of the nonlinear inductor used in the power stage of the gate unit.

[0044] FIG. 4 shows a system of gate units with an interlock function for error handling.

[0045] FIG. 5 shows a further system of gate units with an interlink function for switching control.

DESCRIPTION OF EMBODIMENTS

[0046] FIG. 1 schematically shows circuitry for an LLC resonant converter 1 with an active inverter stage 2, a resonant tank 4, and a rectifier stage 3. The inverter stage 2 comprises a bridge circuit of four gate-commutated thyristors 21 (GCT), each controlled by an individual gate unit 22 for operating or controlling the switching the GCT 21.

[0047] The gate units 22 are controlled by a central controller 5 for converter operation by commanding switching on or switching off of each of the GCTs. Switching on and off of the GCTs 21 is handled by the corresponding gate units 22.

[0048] The inverter stage 2 is made of two separate inverter series connections of GCTs 21 which are coupled with the resonant tank 4 via their middle nodes. The resonant tank 4 comprises a series connection of a first capacitor 41 and a first inductance 42 at a first of the middle nodes wherein a second inductance 43 connects the series connection with a second of the middle nodes of the inverter stage 2. Furthermore, the resonant tank 4 is coupled with the rectifier 3, e.g. implemented as a passive diode rectifier with diodes 31.

[0049] The central controller 5 is capable of operating the LLC resonant converter 1 in a zero-voltage switching or zero-current switching mode to improve the efficiency of converter 1. In the following, it is assumed that converter 1 is configured to be operated in the zero-voltage switching mode or zero-current switching mode, as in these operation modes, the value of the current slope di/dt of the anode current after turn-on pulse is relatively low. For the implementation of the zero-voltage switching mode or the zero-current switching mode appropriate voltage and/or current measurement units for measuring cathode-anode voltage/current are implemented in the IGCT formed by the respective GCT 21 and gate unit 22.

[0050] FIG. 2 schematically shows the circuitry of one of the gate units 22 in more detail. Each gate unit 22 has a similar or identical configuration.

[0051] The gate unit 22 uses a power supply 24 which generally provides three supply voltage potential levels, such as a high supply potential V.sub.pos, a middle supply potential V.sub.mid and a negative supply potential V.sub.neg. The power supply 24 of the gate unit may be powered by an (not shown) external medium-voltage-isolating power supply.

[0052] A power stage 25 is connected with the power supply 24 basically comprises a voltage selector 26 comprising switching transistors, such as MOSFETs, to connect one of the supply potentials V.sub.pos, V.sub.mid, V.sub.neg to a selector output node N. The selector output node N is coupled with a nonlinear inductor 27 to the gate unit output G to be applied to a respective gate terminal of the GCT 21.

[0053] In detail, a first switching transistor S.sub.T is coupled between the high supply potential V.sub.pos and the selector output node N, the middle supply potential V.sub.mid is coupled via two switching transistors S.sub.M1, S.sub.M2 with the selector output node N, and the low supply potential V.sub.neg is coupled via a fourth switching transistor S.sub.B with the selector output node N. The selector output node N is coupled via the nonlinear inductance 27 with the gate unit output G.

[0054] For the correct operation, measurement units for measuring the gate-to-cathode voltage polarity (e.g. by comparators) and the inductor current have to be provided.

[0055] When controlling the switching transistors, the first and second switching transistors S.sub.T, S.sub.M1 are operated complementary and the third and fourth switching transistors S.sub.M2, S.sub.B are operated complementary. Control of the switching transistors S.sub.T, S.sub.M1, S.sub.M2, S.sub.B is made by a gate control unit 23. To connect the high supply potential V.sub.pos with the selector output node N, only the first switching transistor S.sub.T is switched on, while the others are switched off, to connect the middle supply potential V.sub.mid with the selector output node N, the second and third switching transistors S.sub.M1, S.sub.M2 are switched on, while the others are switched off, and to connect the low supply voltage V.sub.neg with the selector output node N, only the fourth switching transistor S.sub.B is switched on, while the other switching transistors S.sub.T, S.sub.M1, S.sub.M2 are switched off. This allows having complete control over the turn-on current at the gate unit output G, which is applied to the gate of the GCT 21.

[0056] Hence, the voltage selector 26 of the power stage has a three-level T-type NPC topology and is configured to provide three voltage levels at the selector output node N.

[0057] Furthermore, there is a turn-off stage 28 configured to turn off the GCT 21 and to keep it off by applying the negative supply potential V.sub.neg between its gate terminal and its cathode terminal. This is made with a turn-off transistor S.sub.off also controlled by the gate control unit 23. So, the turn-off stage 28 has the task to connect the gate terminal of the GCT 21 with the low supply potential V.sub.neg to discard the charge from the gate terminal in order to turn off the GCT 21. During the turn-off process, the inductor current i.sub.on is completely commutated into gate unit 22 for the GCT 21 to restore its blocking capability. Therefore, the low supply potential V.sub.neg is buffered by a sufficiently high capacitance C.sub.off.

[0058] The control method applied by the gate control unit 23 is as follows: [0059] Turn off: During turn-off (turn off transistor S.sub.off is conductive), the voltage selector 26 is controlled to select the low supply potential V.sub.neg to ensure that an inductor current i.sub.on does not increase. This current slowly decreases via parasitic resistances. [0060] Turn on: In the turn-on operation, the turn-off state (i.e. the conductive S.sub.off) is not deactivated immediately, but first it is actively used to build up the current in the nonlinear inductor 27 together with the activated power stage. In other words, while the GCT 21 is still in the turn off state (transistor S.sub.off is conductive), the voltage selector 26 is controlled to select the high supply potential V.sub.pos or the middle supply potential V.sub.mid to increase the inductor current i.sub.on of the inductor 27 to the required value. The required value is defined by the application and GCT technologye.g. between 20-80 A. After a certain time delay, the turn-off transistor S.sub.off is deactivated and the inductor current i.sub.on is fully commutated as gate current i.sub.g into the gate terminal. This generates a high current pulse. After the inductor current i.sub.on has been commutated into gate, the middle supply potential V.sub.mid is kept to ensure that that the gate current i.sub.g declines if a gate-to-cathode voltage V.sub.GC is identified as positive. If gate-to-cathode voltage is identified as negative, the low supply potential is applied to actively decrease the gate current. Once the gate current i.sub.g has decayed to a defined threshold the corresponding backporch operation is activated (according to an identified/measured gate-to-cathode voltage V.sub.GC of the GCT 21). [0061] positive-gate-voltage backporch operation: This operation is applied after turning on operation has been finished as long as the identified gate-to-cathode voltage V.sub.GC is positive. The gate current i.sub.g is controlled via pulses generated by two switching states of the voltage selector V.sub.pos, V.sub.mid. A possible control is a hysteresis control or a fixed turn-on time control. [0062] Negative-gate-voltage backporch operation: This operation is applied as long as the identified gate-to-cathode voltage V.sub.GC is negative. It can be provided that the backporch gate current i.sub.g is increased, when the gate voltage v.sub.g becomes negative in order to accelerate the build-up of the retrigger pulse. The gate current i.sub.g is controlled via pulses generated by two switching states of the voltage selector V.sub.neg, V.sub.mid. This is possible because the excessive energy during this kind of operation is not dissipated, but it is recuperated into the capacitor C.sub.off. A possible control is a hysteresis control or a fixed turn-on time control. [0063] retrigger pulse generation: When in the negative-gate voltage backporch operation the gate-to-cathode voltage V.sub.GC becomes positive again, a retrigger pulse is generated by applying a high supply potential V.sub.pos by the voltage selector 26 to ramp up the inductor/gate current i.sub.g in the nonlinear inductor 27. Once the required value of the current is reached, the middle supply potential V.sub.mid is applied to let the gate current i.sub.g decay. The required value is defined as above by the application and GCT technologye.g. between 20-80 A.

[0064] The voltage selector 26 of the power stage has a three-level T-type NPC topology and is configured to provide three voltage levels at the selector output node N, thereby avoiding an operation state where a low gate voltage v.sub.g of the backporch operation has to be formed by a close to 50-percent duty cycle operation which would in turn require a relatively high switching frequency to keep the gate current i.sub.g ripple low. The gate control unit 23 controls the operation of the GCT 21 basically based on a comparator value of the gate voltage v.sub.g and the measurement of the power stage 25 current e.g. based on a shunt and a current sense amplifier.

[0065] By applying the high supply potential V.sub.pos to the selector output node N, a voltage drop over the nonlinear inductor 27 is generated. This is made for a delay time of several microseconds to build up an inductor current i.sub.on in nonlinear inductor 27. After the turn-on delay, the turn-off stage is finally deactivated, and the inductor current i.sub.on is rapidly commutated into the gate terminal generating the turn-on pulse. After that, the middle supply voltage V.sub.mid is applied to decrease the gate current i.sub.g to its backporch value when the gate voltage v.sub.g is measured positive. When the gate voltage v.sub.g is measured negative, the low supply voltage is applied to ensure that the gate current decreases after the initial pulse as this can be understood as a sign that the antiparallel diode of the gate-commutated thyristor is conducting. After the gate current i.sub.g has reached the desired backporch current value, the backporch operation state is activated depending on the polarity of the gate voltage v.sub.g.

[0066] When the nonlinear inductor 27 has a nonlinearity to have an inductance sufficiently high during the backporch operation to ensure that an acceptable low current ripple can be achieved at a switching frequency appropriately low and that the inductance is sufficiently low to ensure a fast ramp-up of the gate current i.sub.g to generate and propagate the high slope current at turn-on pulse generation and retrigger pulse generation. As these conditions are associated with different current regimes of the inductor current, the nonlinear saturable inductor is used that decreases its inductance with increasing currents.

[0067] An exemplary characteristics of the nonlinear inductor is shown in FIG. 3 wherein inductor 27 has a current dependency where the inductance decreases as the current rises.

[0068] Preferably, the nonlinearity is selected so that the low inductance for the turn-on pulse generation has a value at least 50% lower than the high inductance during the backporch operation.

[0069] With reference to FIG. 4, in a switching system such as a converter system 1 as shown in FIG. 1 gate units 22 may be provided with an interlock function, which may be implemented as an independent function in each of the gate units 22. The idea is to provide an additional communication channel 29, such as fiber optics, on the gate units 22 that allows for interlocking the gate units in the case of an error detection in one of the gate units 22. The idea is to make an unidirectional ring structure capable of propagating the error state through the gate units 22 without involvement of the central controller 5.

[0070] As shown in FIG. 4 information about error is propagated via all gate units 22. By default all gate units 22 are in no error state and send a corresponding signal but when an error in some gate unit 22 is recognized, this gate unit 22 starts to transmit an error signal. The gate unit 22 that receives an error signal also goes into an error state. Hence, the error signal is propagated rapidly through all units effectively interlocking the converter and reducing a change of a follow-up failure.

[0071] The advantage of this approach is that central controller 5 does not require to possess many fiber optic inputs to get an error state from every single gate unit 22 to disable the application. Moreover, the length of the utilized optical fiber could be shorter, considering that the fiber optic interconnections are only within the IGCT stack.

[0072] Furthermore, as shown in FIG. 5 an interlink between the series-connected IGCTs 21, 22 is provided to simplify the cabling of optical fibers within the application. This approach is configured by an additional communication channel output on each gate unit 22 to enable an option for an interlink. Here, gate units 22 propagate the control signal which controls the switching of the IGCTs and only one connection to the central controller 5 is necessary. This significantly simplifies the cabling of the communication channels and also requires lower number of communication outputs at the central controller 5.

[0073] Since the turn-on and turn-off need to happen in all IGCTs at the same time, it is recommended to wait for a certain amount of time in each device to compensate for the propagation delays due to communication transmitters and receivers (e.g. 30 ns in first device, 20 ns in second and 10 ns in third, assuming the propagation delay of 10 ns per gate unit). The particular waiting time for each IGCT can either be configured manually or it can be programmed to happen automatically during converter power up. This could be implemented by propagating the number via the interlink chain, which would be increased in each gate unit. This would provide sufficient information on how much time has to be waited in the particular IGCT.