To protect against the ±250-V voltage spike event outlined by MIL-STD-1275E, there are three main components of the circuits that help suppress the voltage spike. The first component is the main MOSFET or Q1 in the schematic. Select a MOSFET with a voltage rating greater than 250 V to help with the spike voltage suppression and ensure that the MOSFET is not damaged due to the voltage spike event. The other two components to help suppress the two TVS diodes are D3 and D5. The TVS diode, D3, suppresses the main +250-V voltage spike event. Set the breakdown voltage of D3 greater than the maximum voltage of the surge event (100 V) but less than the maximum voltage spike event to ensure that the TVS diode does not get damaged during the surge event. In this circuit, the 5.0SMDJ120A was used which has a breakdown voltage of 120 V.

D5 is used to suppress the main –250-V voltage spike event. To select the proper diode, there are a couple of parameters that must be considered. The first is that during the –250-V body diode of Q1 makes it so that the spike event is seen completely by Q2. This means that D5 should have a breakdown voltage that is less than the voltage rating of Q2. For this design, the 5.0SMDJ33CA is a bidirectional TVS diode that has a reverse breakdown voltage of –33 V which is lower than the voltage rating of Q2 (60 V).