Driver circuit for giant transistor (GTR)

Driver circuit for giant transistor (GTR)

1. GTR requirements for the base drive circuit

The ideal base drive current waveform of GTR is shown in Figure 1. Usually the requirements for the GTR base drive circuit are:

(1) When the GTR is turned on, a strong drive should be used, the leading edge should be steep, and there should be a certain oversaturation drive current to shorten the turn-on time and reduce the turn-on loss.

(2) After the GTR is turned on, the driving current is correspondingly reduced, so that the device is in a critical saturation state, the driving power is reduced, and the storage time is shortened.

(3) When the GTR is turned off, a large reverse base current should be provided to quickly extract the remaining carriers in the base region and shorten the turn-off time.

(4) Realize the electrical isolation between the main circuit and the control circuit to ensure the safety of the circuit and improve the anti-interference ability.

(5) With fast protection function. When the main circuit is overheated, overvoltage, overcurrent, short circuit and other faults, the base circuit must be able to quickly and automatically cut off the driving signal.

Driver circuit for giant transistor (GTR)
Figure 1 – Ideal base drive current waveform

2. Example of GTR base drive circuit

Figure 2 is a GTR driver circuit with negative bias to prevent oversaturation. In the figure, V is the driven power transistor GTR. When the input signal ui is high level, the transistors V1, V2 and the photocoupler are all turned on, the transistor V3 is turned off, V4 and V5 are turned on, V6 is turned off, the power supply voltage E provides the base current to V through V5, the acceleration capacitor C2, and the resistor R3, and V is turned on. At the end of charging, the voltage on C2 is positive on the left and negative on the right, and its magnitude is determined by the ratio of the power supply voltage E to R4 and R5. When ui is low level, V1, V2 and optocoupler are all turned off, V3 is turned on, V4 and V5 are turned off, and V6 is turned on. The discharge path of C2 is: ①C2→V6→V3→VS→VD5→VD4→C2, providing base current for V6; ②C2→V6→V→VD4→C2, provide reverse base current for V, accelerate V turn-off, this process is very short, once V is completely turned off, its current is zero; ③C2→V6→VS→VD5→VD4→C2, because VS is turned on, the base-emitter junction of V is subjected to the reverse bias voltage to ensure its reliable cut-off.

Driver circuit for giant transistor  (GTR)
Figure 2 – GTR driver circuit with negative bias to prevent oversaturation

The circuit consists of diodes VD2, VD3 and V to form an anti-saturation circuit, also known as a Baker clamp circuit. When IC decreases at light load, the saturation depth of V increases, the diode VD2 is turned on, the base current is shunted, and the saturation depth of V is reduced; when the overload or DC gain decreases, the IC value increases, the E of V increases, and the current bypassed by VD2 will automatically return to the base to ensure that V will not exit saturation. This allows the saturation depth to remain substantially constant under load changes. The function of transistors V6, R5, C2, diodes VD4, VD3 and voltage regulator VS is to make the base-emitter withstand reverse bias voltage when V is turned off, and the voltage regulator value of VS is 2~3V. Capacitor C1 eliminates high frequency parasitic oscillations generated by transistors V4 and V5.

Among the widely used GTR integrated drive circuits, UAA402 of THOMSON and M57215BL of Mitsubishi are more common. (Extended reading: The structure and working principle of GTR, the basic characteristics and main parameters of GTR.)