Dynamic characteristics of thyristor

Dynamic characteristics of thyristor

When analyzing power electronic circuits, thyristors are often regarded as ideal devices, that is, the switching on and off of the device is considered instantaneous. However, in actual operation, due to the change of carriers inside the device, it takes a certain amount of time to turn on and off the device.

The curve of current and voltage changing with time in the dynamic process of turning on and turning off the thyristor is called the dynamic characteristics of the thyristor. Figure 1 shows the waveforms of the turn-on and turn-off process of the thyristor.

Dynamic characteristics of thyristor
Figure 1 – Turn-on and turn-off process waveforms of thyristor

12Figure 1 – Turn-on and turn-off process waveforms of thyristor

(1) Start and pass

After the gate is triggered by the ideal step current, it takes time and the influence of the external circuit inductance to establish the positive feedback process inside the thyristor. The increase of the anode current and the decrease of the voltage between the anode and the cathode require a process. From the moment of the gate current step until the anode current rises to 10% of the steady state value, this period of time is called the delay time td, and at the same time the forward voltage drop of the thyristor is also decreasing. The time required for the anode current to rise from 10% to 90% of the steady-state value is called the rise time tr, and the turn-on time ton of the thyristor is defined as the sum of the two, namely

                               ton=td+tr                                             (1-1)

The delay time of ordinary thyristor is 0.5~1.5μs, and the rise time is 0.5~3μs. The delay time decreases as the gate current increases. In addition to reflecting the characteristics of the thyristor itself, the rise time is also affected by the inductance of the external circuit. The delay time and rise time are also related to the anode voltage.

In order to ensure the reliable turn-on of the thyristor, the trigger pulse width is usually 20~50μs.

(2) Shut-off process

For the thyristor that was originally on, when the voltage applied to the external circuit suddenly changes from forward to reverse, due to the inductance of the external circuit, there will be a transitional process in the attenuation of the anode current. It can be seen from the waveform in Figure 1 that the anode current will gradually decrease to zero. After the reverse recovery current flows in the opposite direction and the maximum IRM is passed, it will decrease to zero again. At this time, the thyristor resumes its blocking ability. When the reverse recovery current decays rapidly, due to the effect of the external circuit inductance, a reverse peak voltage URRM will be caused at both ends of the thyristor.

The time from when the forward current drops to zero to when the reverse recovery current decays to close to zero is the reverse blocking recovery time trr of the thyristor. After the reverse recovery process is over, because the carrier recombination process is relatively slow, it will take some time for the thyristor to recover its blocking ability to the forward voltage, which is called the forward blocking recovery time tgr. The turn-off time toff of the thyristor is defined as the sum of trr and tgr, namely

                                toff=trr+tgr                                          (1-2)

The turn-off time of ordinary thyristors is several hundred microseconds. Due to the existence of the turn-on and turn-off time of the thyristor, the improvement of its operating frequency is limited.

During the forward blocking recovery time tgr, if a forward voltage is applied to the thyristor, the thyristor will be turned on again without being controlled by the gate current. Therefore, in practical applications, the reverse voltage should be applied to the thyristor for a long enough time, so that the thyristor can fully recover its blocking ability against the forward voltage, and the circuit can work reliably.