When a thyristor fires, the current and voltage of the firing pulse must lie within a safe firing region (BSZ in figure 1). This of course includes the requirement that the maximum permissible gate power (PG) specified for various pulse times tp is not exeeded.
Figure 1 shows the regions of firing current and firing voltage defining safe thyristor firing (BSZ) for temperatures of -40°C, 25 °C, and 130°C. The boundary of the safe firing region determined by the maximum permissible peak gate power PG(t) is shown for 0.1 ms, 0.5 ms and 8 ms long firing current pulses. The actual firing current used should be as far above the specified minimum values of gate current required for firing (IGT) as possible, to ensure that firing still occurs reliably even at low temperatures.
As already mentioned, the PG(t) values must not be exceeded. Furthermore, the leading edge (slew rate) of the firing current pulses should be as steep as possible. Using five times IGT and a slew rate of at least 1 A/μs is often recommended. For firing pulses that lie below these values, the critical rate of rise of current (di/dt) drops drastically.
For resistive load, a short, single pulse lasting at least 10 μs will generally suffice since the current in the main circuit increases rapidly. For inductive load, the current can only increase slowly. Normally, however, an RC element is connected in parallel to the thyristor, and the discharge current of this protective component facilitates thyristor firing. If this is not the case, the firing pulses have to be longer or sequences of brief pulses are to be used.
It should be noted that the required firing pulse times for a thyristor always depend on the load in the main circuit.
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