Given the energy consumption of consumer appliances; building heating, ventilation and air-conditioning (HVAC) systems; and industrial drives, efforts are underway to establish system efficiency ratings through programs such as the seasonal energy efficiency ratio (SEER), minimum energy performance standards (MEPS), Energy Star and Top Runner.
Variable frequency drives (VFDs) offer the best system efficiency in heating and cooling systems, especially if they have an accurate and very wide range of speed control. VFDs use an inverter to control motor speeds, along with high-frequency pulse-width modulation (PWM) switching to obtain truly variable speed control.
Although these inverters are currently realized using insulated-gate bipolar transistors (IGBTs) and metal-oxide semiconductor field-effect transistors (MOSFETs) as the power switches, the switching frequency and power delivery are limited given high overall losses. With advancements in wide band-gap technology, however, gallium nitride (GaN)-based power switches in motor drives can help increase power density, power delivery and efficiency.
Manu Balakrishnan
Systems engineer
Motor drivers
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:
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 300VDC 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.