How GaN increases inverter efficiency

The conduction losses attributable to GaN FETs are proportional to the on-state resistance of the GaN, similar to a MOSFET. For an IGBT, however, conduction losses depend on the knee voltage and dynamic on-state resistance, which are typically higher than GaN FETs or MOSFETs.

As for switching losses, GaN FETs offer much lower losses compared to MOSFETs and IGBTs because of these reasons:

• GaN offers zero reverse recovery. With zero reverse recovery, it is possible to switch a GaN FET at a very high current slew rate (di/dt) and voltage slew rate (dv/dt). In MOSFETs, the body diode suffers from high zero reverse recovery, limiting the switching di/dt and dv/dt and causing additional losses and phase-node voltage ringing. With an IGBT, even the addition of an optimized antiparallel diode can still cause challenges related to reverse recovery.
• When switching off, IGBTs suffer from minority carrier recombination current, commonly known as tail current, which increases turnoff losses. GaN doesn’t have any tail current.
• GaN offers lower capacitance compared to IGBTs and MOSFETs, resulting in lower capacitive switching losses.
• Controlled and faster di/dt and controlled dv/dt help optimize voltage-current overlap losses during switching.

Figure 1 shows a theoretical inverter efficiency comparison between GaN-, IGBT- and MOSFET-based solutions with a 20kHz switching frequency, the phase-node voltage slew rate for the GaN-based inverter limited to 5V/ns, and an ambient temperature of 55°C. You can see that the GaN solution helps reduce power losses by at least half.

Figure 2 compares the efficiency of the Texas Instruments (TI) DRV7308 three-phase GaN intelligent power module (IPM) to a 5A peak-current-rated IGBT IPM with a 300V_DC supply at a 20kHz switching frequency with a fan motor that has 2m of cable at a 25°C ambient temperature, delivering 0.85A of root-mean-square winding current and 250W of inverter output power. The slew rate of the GaN IPM is configured for 5V/ns.