1. Field of the Invention
The present invention relates to a gate driver for driving a thyristor, such as a GCT (gate commutated turn-off) thyristor, a GTO (gate turn-off) thyristor, a static inductive thyristor (SITH), or a power transistor, in a stable condition.
2. Prior Art
GCT and GTO have similar turn-on and on-state gate current requirements. The gate driver provides a turn-on pulse with high dI/dt and peak current, and thereafter provides a steady state DC on-state sustaining current. The gate driver sometimes receives negative gate voltage during on-state of the GCT/GTO.
A new situation, unlike in the case of the GTO, is introduced for driving the GCT. The gate driver for the GCT has its feature in the amount of turn-on peak current and its shape. The turn-on current increase (dI/dt) for a GCT is 10 times to 50 times higher than that of typical GTOs, and trigger peak currents also are selected 5 to 10 times higher in order to reduce turn-on loss. A 6 inch GCT thus may require dI/dt=500A/us and a peak current of 300A.
While such a sharp high pulse safely turns on the GCT accompanying a high rate of rise of anode current, a long pulse duration must be provided after the sharp high pulse to maintain the GCT in on-state, if GCT operation accompanies a small rate of rise of anode current. Thereafter, on-state gate current similar to the GTO""s (approximately 10A for a 4 inch or 6 inch device) is to be applied.
If no protection circuit, such as a current limiter, is provided, the gate driver may be damaged when the GCT""s or GTO""s load current changes its flow direction from a forward direction to a reverse direction. When the load current flows in the reverse direction, the load current will flow through a freewheel diode provided in parallel to the GCT. Thus, the GCT""s anode turns negative with respect to its cathode. A parasitic diode in the GCT allows a negative potential to appear at the gate of the GCT.
Thereafter when the load current changes its flow direction again from the reverse direction to the forward direction, the load current may flow again through the GCT. Then, a safe GCT on-condition has to be established again.
Basically such requirements also are well known from GTO operation. GCT circuits are designed for higher switching frequency, and, as a consequence, freewheel diodes must respond to a considerably higher forward voltage drop, resulting in GCT gate voltage as low as xe2x88x925V during normal converter operation.
The peak current is created from a voltage source by a resistor and capacitor circuit. In order to produce a high shooting pulse with a steep and rapid rising (dI/dt=500A/us), and a relatively long down slope pulse (10 us to 40 us) after the shooting pulse, requires several RC combinations with complicated adjustments. In such a design the total loss may exceed 50W for a 6 inch GCT drive.
A German Patent Laid-open publication No. DE3709150 and a PCT International Publication No. WO9407309 disclose the GTO driver using switched current sources. Inductors are fed to create the sources, and excessive energy is fed back to the power supply.
Basically in such way, very low loss can be achieved. But four high current switching devices and three diodes are required for generating a turn-on pulse and an on-state current. And the high turn-off peak current has to pass through a series connection of a switching element and an additional diode. Such a circuit cannot be used for a gate driver of GCT.
U.S. Pat. No. 4,791,350 discloses a gate driver which uses a switch-mode step down current regulator as a source for the GTO""s steady state gate current. The regulator incorporates a switch and a freewheel diode, it""s output is directly coupled to the GTO""s gate, and a high current pulse is generated by a separate circuit having a switch and a resistor.
In this way, U.S. Pat. No. 4,791,350 suggests reducing gate driver losses. But if a negative GCT gate voltage should appear, the regulator""s output current will increase without control. Moreover considerable losses are generated in the regulator""s freewheel diode, when gate currents exceeding 5A are required. Also, high losses will result from the high current pulse resistor.
Japanese Patent laid-open publication No. H3-97315 and EP Patent publication No. 0 416 933 discloses a circuit to solve the problem with negative gate potential. The freewheel diode is connected to the negative supply line. The inductor is charged by the positive supply source. Upon freewheel, the charge in the inductor will be discharged to the negative supply. In this way the circuit can operate stably under all positive and negative GTO gate voltage conditions.
Such circuit is applicable for the generation of a small gate current. With high gate current, however, a lot of energy is transferred from the positive supply source to the negative supply source, and it must be transferred back to the positive supply by an appropriate power returning circuit. In the case of GCT, for example, a gate current Ig may be about 10A and a gate voltage Vg is about xe2x88x9220V. Then, a circulating power would be approximately 200W. In contrast to this, the active gate power according to this reference may be as small as Vgxc3x97Ig 0.6Vxc3x9710A=6W. As a result, a tremendous over design of gate current generator and power supply is required, and a loss (approximately 20W-40W) is generated even with high-performance switch-mode circuits.
EP Patent publication 893883 discloses a gate driver which handles the GCT""s freewheel situation in another way. A bipolar transistor, implemented as emitter follower, is to limit the GCT""s gate current at negative gate voltage. The current is generated from voltage pulses with high efficiency.
For gate current Ig up to 2A, a circuit can be designed with appropriate components. At a higher gate current, the bipolar transistor gain will decrease below 20. Then, high base current is required, and loss will increase due to higher VCEsat (saturation of voltage Vce). At Ig=10A, Ib greater than 0.5A will be required, and VCEsat will amount to approximately 1V with the known PNP transistors.
Moreover, a high base current must be maintained with a GCT gate voltage being maintained not lower than 0.4V. Then, for higher GCT gate voltage and for open circuit condition (no GCT installed), a severe trade-off must be handled, complicating design and limiting the performance of the gate driver.
A circuit must be found which is appropriate to realize high current GCT drive. It has to generate a sharp, high trigger current pulse for GCT turn-on, a long trigger current tail and high gate current with low loss, and it has to be safe under all gate voltage operating conditions.
Moreover the circuit has to be such that it can be realized mainly with SMD components and technology in order to allow very compact and cost effective solutions.
One aspect of the invention includes a gate driver for driving a thyristor having an anode, a cathode, and a gate by providing a gate current to the gate of the thyristor during an on-command signal, the gate driver comprising:
a turn-on pulse generator for generating a turn-on pulse in response to a leading edge of the on-command signal;
a down converter for producing a down slope current immediately following the firing pulse; and
a current limiter having a MOSFET connected to the gate of the thyristor for supplying current from the down converter to the gate of the thyristor, the current limiter monitoring the gate voltage at the gate of the thyristor and increasing an internal resistance of the MOSFET relative to the negative voltage increase of the gate voltage.
According to the invention, the down converter comprises:
a pattern generator for generating a pattern of a current to be produced from the down converter;
a conduction pulse generator for generating conduction pulses in accordance with the pattern;
a switching element for conducting a current from a power source in response to the conduction pulses and producing a pulse current; and
an inductor for smoothing the pulse current;
the pattern generator generating a pattern having a rising edge and down slope portion after the rising edge, so that the down slope current produced from the down converter decreases relative to the pattern.
In a gate driver, the down converter further comprises:
another conduction pulse generator for generating other conduction pulses in accordance with the pattern;
another switching element for conducting a current from the power source in response to the other conduction pulses and producing another pulse current; and
another inductor for smoothing the other another pulse current so that the down slope current is increased.
In a gate driver, the down converter further comprises:
an additional inductor in parallel with the inductor; and
an additional switching element in series with the additional inductor to produce a greater down slope current.
In a gate driver, the down converter further comprises:
an additional inductor in parallel with the inductor; and
a saturating reactor element in series with the additional inductor to produce a greater down slope current.
In a gate driver, the current limiter comprises:
a comparator for comparing an output voltage at the output of the down converter with a predetermined voltage and producing an adjust signal relative to a difference between the output voltage and the predetermined voltage when the output voltage falls below the predetermined voltage, and
the MOSFET receiving the adjust signal to change its internal resistance relative to the adjust signal.
A gate driver according to the invention has as the comparator an operational amplifier.
In a gate driver, the comparator comprises bipolar transistors.
A gate driver according to the invention includes in the current limiter a freewheel diode connected parallel to the MOSFET.
In the gate driver, the turn-on pulse generator comprises a capacitor, a diode connected parallel to the capacitor, a switching element, and a reactor to produce a sharp high pulse.
The gate driver may comprise a bias current generator to provide a low level bias current to the gate of the thyristor.
A gate driver according to the invention is a GCT.