The maximum voltage stress that will be seen by the synchronous rectifier switch on the secondary side can be calculated using Equation 55.

Equation 55.

Equation 56.

Note that the maximum expected voltage is approximately 20 V, but a higher rating should be selected to allow for transient spikes. For the design, an 80-V rated GaN FET was conservatively chosen for the synchronous rectifier. The current rating should be sufficient to handle the maximum secondary current as calculated in Section 9.2.2.6. In order to reduce the current through GaN FET during the soft-start period, when the controller SRA and SRB signals are off, a Schottky diode can be used in parallel with the synchronous rectifier GaN FETs. This diode would also mitigate the reverse conduction losses attributed to the GaN FET during the dead time and boost the overall efficiency of the system.