Overview of drive circuit and trigger circuit of thyristor

Overview of drive circuit and trigger circuit of thyristor

Overview of Power Electronic Device Drive Circuit

The basic task of the drive circuit is to convert the signal output by the information electronic circuit into the turn-on and turn-off signal of the control end of the power electronic device in accordance with the control requirements of the system, so as to realize the weak current control of the strong current. Different power electronic devices have different driving requirements. For semi-controlled devices, only the turn-on control signal needs to be provided; for a fully-controlled device, it is necessary to provide both a turn-on control signal and a turn-off control signal to ensure the reliable turn-on and turn-off of the device. According to the different trigger signals required by the device, the drive circuit can be divided into a current drive circuit and a voltage drive circuit. Thyristors, GTO and GTR are current-driven devices, and power MOSFETs and IGBTs are voltage-driven devices.

Compared with the high voltage of the main circuit, the control circuit is a low voltage circuit. Therefore, the drive circuit must provide electrical isolation between the control circuit and the main circuit. Electrical isolation generally uses optical isolation or magnetic isolation. Optical isolation uses optocouplers, and magnetic isolation devices are usually pulse transformers.

Trigger circuit of thyristor

After the positive voltage is applied to the anode of the thyristor, a trigger voltage must be applied between the gate and the cathode at the same time, so that the thyristor can go from blocking to conducting, which is usually called trigger control. The circuit that provides the required trigger voltage pulse is called the trigger circuit of the thyristor.

The thyristor trigger circuit should meet the following requirements:

(1) The width of the trigger pulse should ensure reliable conduction of the thyristor. For example, the trigger pulse used to trigger the three-phase fully-controlled bridge rectifier circuit should be a single-wide pulse with a width greater than 60° and less than 120°, or a double narrow pulse with an interval of 60°.

(2) The trigger pulse should have enough power, but it must be within the reliable trigger area of the device’s gate volt-ampere characteristics. Since the gate parameters of the thyristor device are highly dispersed and the parameters change with temperature, in order to ensure reliable triggering, it is required that the trigger pulse should have sufficient voltage and current amplitude. For example, for thyristors used in outdoor cold places, the amplitude of the trigger pulse current should be increased to 3-5 times of the maximum trigger current of the device, and the steepness of the pulse front should also be increased to 1~2A/μs.

(3) The trigger pulse must be synchronized with the anode voltage of the thyristor, and the phase shift range of the pulse must meet the circuit requirements.

(4) The trigger circuit should have good anti-interference, temperature stability and good electrical isolation from the main circuit. The misconduction of QT is often caused by disturbing signals entering the gate circuit. Therefore, it is necessary to take anti-interference measures such as shielding and isolation in the trigger circuit.

The ideal thyristor trigger current waveform is shown in Figure 1. In order to trigger the thyristor quickly and reliably, a strong pulse is often superimposed on the leading edge of the trigger pulse, and the amplitude of the strong trigger current can reach 5 times the trigger current. In the figure, t1~t4 are pulse widths, among which t1~t2 are the rise time of the pulse front edge (<1μs), t1~t3 are the strong pulse width, and IM is the strong pulse amplitude.

Overview of drive circuit and trigger circuit of thyristor
Figure 1 – Ideal Trigger Pulse Current Waveform

Figure 2 shows a common thyristor trigger circuit. V1 and V2 form the pulse amplification link, and the pulse transformer TP and VD2, VD3, R4 form the pulse output link. When a pulse output is required, V1 is turned on to provide a base current for V2 and make it conductive, so the pulse is output through the pulse transformer TP. VD1 and R3 form a freewheeling circuit. When V1 and V2 change from on to off, TP releases its stored energy. R2 is a current-limiting resistor, C1 is an accelerating capacitor, and VD2, VD3, and R4 form a thyristor gate protection circuit to protect the gate from the impact of reverse current and reverse voltage and limit current. If you want to get a strong pulse, you need to add other circuit links.

Overview of drive circuit and trigger circuit of thyristor
Figure 2 – Common thyristor trigger circuit