ZHCU763 March 2021
Figure 3-19 shows a 6-channel SSR topology powered from two transformers, for example from the cooling transformer (RC) and heating transformer (RH) in an HVAC thermostat. The common wires (C – wire) of both the transformers are connected together. The single-isolation gate-drive circuit in the reference design can be used to drive all the six SSR channels. However, in this case the turn on PNP transistor (Q1, Q2 in Figure 2-6) in the gate-drive circuit and the blocking diodes (D1, D2 in Figure 2-6) has to be rated for two times the peak AC voltage plus the gate-drive voltage (this is by considering a use case where the transformers are mis-wired to create opposite polarity). For example, with a 24-VAC ±20% AC supply, having a peak instantaneous value of 41 V, the gate-drive PNP transistors and the blocking diodes has to be rated for more than 100 V.