Unclamped inductive loads are difficult to handle for solid-state relays. The inductive energy is usually clamped by transient voltage suppression (TVS) diodes. However, the commutation path of the inductive load current from the drain of the switching power transistor to the clamping TVS diodes has a stray inductance. It adds an inductive voltage overshoot on-top of the supply voltage to the drain voltage. This is particularly critical if MOSFETs are turned off very slowly within several tens of microseconds as the switching energy itself is heating up the switch and the inductive overshoot increases the losses further more. Therefore, the current capability is often limited for solid-state relays and a fast turn-off is required to stay inside the switched transistor’s safe operating area (SOA). This presentation explains first a suitable test up for testing solid-state relays with an unclamped inductive load. Then , the commutation path is identified and helpful layout guidelines for this case are derived. Finally, the talk compares various solid-state isolators having different turn-off current and their effect during turn-off is shown using the turn-off I-V-trajectory.
