High-Frequency, GaN Diode-Free Motor Drive Inverter with Pure Sine Wave Output

Silicon-based power switching devices have improved dramatically since their inception decades ago. An impressive example: Si super-junction MOSFETs, such as CoolMOS FETs, broke the theoretical Si limit in the tradeoff between blocking voltage and resistance (Ref. 1). However, the large amount of minority charge in the body diode results in large reverse recovery losses in hardswitched bridge circuits—typical for motor-drive inverters. Thus insulatedgate-bipolar-transistors (IGBTs) are preferred in applications also requiring additional reverse-connected diodes for freewheeling current. With the relatively low-frequency bandwidth, IGBT-based inverters typically operate at 4–20 kHz and directly apply pulsewidth-modulation (PWM) power to the motor with the intention of utilizing the motor windings as energy storage inductances. The resultant voltage transients induce high stresses between motor windings and cause current spikes through motor bearings, leading to issues such as insulation breakdown and excessive bearing wear (Refs. 2–3). This paper demonstrates the use of wide-band-gap semiconductor GaN for hard-switched motor drive applications. The functionalities and frequency capabilities of the GaN high-electron mobility transistor (HEMT) eliminate the need for freewheeling diodes and allow a much higher PWM frequency, in turn enabling compact filters and resultant, spike-free, pure sine wave power output. Devices and Module The unit cell device used in this study is a GaN hybrid HEMT (Transphorm, Inc.). It incorporates a normally “off” low-voltage Si device at the input and a normally “on” high-voltage GaN HEMT at the output in cascode form (Fig. 1). The combined device is normally “off” with a gate threshold of + 2.1 V—typical at 1 mA drain current—and a drain leakage of 10 μA—typical at Vgs = 0 V and Vd = 600 V. The “on” resistance is