Very high voltage spikes occur when transformers are turned on and off under low load or no load at all. In addition to these occasional non-periodic switching operations, periodic switching operations caused by the thyristor firing also occur in controllable converter circuits. This is particularly true for diode assemblies which have to work in connection with an AC converter on the primary side of the rectifier transformer. For these reasons, particular attention must always be paid to the snubber on the AC side .
It may be possible for a single-switch snubber to be rated such that it provides sufficient protection from high energy voltage spikes. Normally, however, even in thyristor assemblies with single-switch snubbers, an additional snubber circuit is needed on the AC side. In diode assemblies, on the other hand, a DC side snubber is often sufficient.
In 3-phase circuits, in particular, it is often favorable to use a single RC element connected via an auxiliary bridge rather than the three RC elements otherwise needed. A circuit such as this can be seen in Figure 1.
Figure 1. AC side snubber circuit using auxiliary bridge and auxiliary diode
The power rating of the resistor R, however, only needs to be a few Watts. The additional diode D7 is not needed in every case. It is responsible for reducing the load on the resistor R and the capacitor C caused by harmonics (especially in circuits with phase control). The discharge resistance R1 ensures that the capacitor discharges quickly after device turn-off. The auxiliary diodes D1 . . .D7 are to be selected such that the maximum rated surge current for the related conduction time t ≈ R×C is double the size of the peak value ILM of the load current which flows into the capacitor C at turn-on. Heat sinks or heat plates for the auxiliary diodes are not necessary since the continuous load is very low.
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