Thyristors and diodes can be operated in parallel. Since this is so-called bipolar devices with negative temperature coefficient, it makes sense to use selected classes of thyristors and diodes to create a uniform current distribution obtained by them. In a hard parallel circuit it must be possible for the thyristors to ignite after. A selection by ignition starting voltage may prove helpful. The current lead should be constructed that the parallel lines have the same length.
The series circuit of diodes avalanche diodes should preferably be used. Avalanche diodes are terminated by the steep blocking characteristic particularly in this application. At a certain output size, it is also useful to wire avalanche diodes with a resistor as a static potential divider. The even distribution of reverse bias is enforced within the series circuit. For balancing the voltage peak when switching off the diodes each diode must be connected additionally with an RC circuit. Voltage distribution due to different parasitic earth capacitances must be taken separately. Physical measures, or an additional capacitor circuit can remedy this. Through the use of specially selected diodes the wiring complexity can be significantly minimized.
Basically thyristors must be provided as well as diodes, with a static and dynamic circuit. In addition, it should be noted that thyristors can be selectively turned on by a firing pulse to the gate unlike diodes. It is of utmost importance that all thyristors within the series circuit ignite and switch simultaneously. If one (or more) of thyristors in series connection do not ignite, they would be damaged. An emergency firing circuit using break over diods can avoid this danger. The break over diod is connected between the anode of the thyristor and the gate. In the supply and control of the underlying at a high potential for actuation, make sure that the required isolation voltage and partial discharge strength are ensured. The latter also applies to the entire mechanical structure of the series circuit.
Thyristors are ignited by an ignition current at the gate of the thyristor and switches on. The need for a reliable ignition firing current is far higher than the firing current Igt shown in the data sheets. The latter merely shows that the thyristor ignites and compliance with the specification values. With the technology required in the application of trigger current Ig, however, this value has to do nothing. The need for a reliable ignition trigger current Ig can be seen in the data sheet under the parameters di/dtcr, it has about a five times higher value than the IGT. Such an ignited thyristor can be operated safely even at -45 °C ambient temperature and a high current rate of rise (di/dt).
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