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.
Permanent-magnet synchronous motors designed for high speed or motors with a lower inductance often need a high PWM frequency to reduce current ripple and achieve optimum motor performance. End-equipment examples include hair dryers, air blowers and pumps.
Higher current ripple in the motor winding can cause unwanted torque ripple, increased copper and core losses, and inaccuracies in the average motor current sensed during switching.
MOSFET- or IGBT-based IPMs are typically rated for usage at 20kHz; however, they are normally used at a lower switching frequency (6kHz to 16kHz) because of high switching losses. With GaN offering much lower switching losses even at a lower dv/dt, it is possible to switch at a much higher frequency to improve motor efficiency and performance.
Figure 3 shows the functional block diagram of the DRV7308, which integrates predrivers for all GaN FETs with slew-rate control of phase-node voltages. The DRV7308 helps achieve more than 99% inverter efficiency for a three phase-modulated, field oriented control-driven 250W motor-drive application in a quad flat no-lead (QFN) 12mm-by-12mm package, eliminating the need for a heat sink.
Designers often have to consider how dv/dt affects motor insulation, bearing lifetime, electromagnetic interference (EMI) and reliability.
The DRV7308 incorporates an integrated predriver slew-rate control circuit that controls dv/dt at the phase node. It is possible to control the slew-rate settings down to 5V/ns and to configure the slew rate as a trade-off between the motor winding insulation and switching-loss optimization. The lower slew-rate settings of the DRV7308 cover the ranges offered by existing IGBTs, while higher slew rates help hold switching losses to much lower values.
Figure 4 and Figure 5 show the phase-node switching voltage of the DRV7308 at a 1A load, at 300V, with a 10V/ns slew-rate setting and a 2m motor cable. The zero reverse recovery of the GaN FET with lower parasitics and predriver slew-rate control help achieve a clean voltage switching waveform.