Due to the silicon FET’s long reign as the dominant power-switch technology, many designers are unaware that the nameplate drain-source voltage cannot be used as an equivalent point to compare devices across technologies. The nameplate drain-source voltage of a silicon FET is set by the avalanche breakdown voltage. The nameplate drain-source voltage of a GaN FET is set by the long term compliance to data sheet specifications.

Exceeding the nameplate drain-source voltage of a silicon FET can lead to immediate and permanent damage. Meanwhile, the breakdown voltage of a GaN FET is much higher than the nameplate drain-source voltage. For example, the breakdown drain-source voltage of the LMG3626 GaN power FET is more than 800 V which allows the LMG3626 to operate at conditions beyond an identically nameplate rated silicon FET.

The LMG3626 GaN power FET switching capability is explained with the assistance of Figure 7-1. The figure shows the drain-source voltage versus time for the LMG3626 GaN power FET for four distinct switch cycles in a switching application. No claim is made about the switching frequency or duty cycle. The first two cycles show normal operation and the second two cycles show operation during a rare input voltage surge. The LMG3626 GaN power FETs are intended to be turned on in either zero-voltage switching (ZVS) or discontinuous-conduction mode (DCM) switching conditions.

Figure 7-1 GaN Power FET Switching Capability

Each cycle starts before t₀ with the FET in the on state. At t₀ the GaN FET turns off and parasitic elements cause the drain-source voltage to ring at a high frequency. The high frequency ringing has damped out by t₁. Between t₁ and t₂ the FET drain-source voltage is set by the characteristic response of the switching application. The characteristic is shown as a flat line (plateau), but other responses are possible. At t₂ the GaN FET turns on. For normal operation, the transient ring voltage is limited to 650 V and the plateau voltage is limited to 520 V. For rare surge events, the transient ring voltage is limited to 800 V and the plateau voltage is limited to 720 V.