There are many advantages of using high frequency PWM (in the range of 50 to 100 kHz) in motor drive applications. High motor efficiency, fast control response, lower motor torque ripple, close to ideal sinusoidal motor current waveform, smaller filter size, lower cost filter, etc. are a few of the advantages. However, higher frequency PWM is also associated with severe voltage reflection and motor insulation breakdown issues at the motor terminals. If standard Si IGBT based inverters are employed, losses in the switches make it difficult to overcome significant drop in efficiency of converting electrical power to mechanical power. Work on SiC and GaN based inverter has progressed and variable frequency drives (VFDs) can now be operated efficiently at carrier frequencies in the 50 to 200 kHz range, using these devices. Using soft magnetic material, the overall efficiency of filtering can be improved. The switching characteristics of SiC and GaN devices are such that even at high switching frequency, the turn on and turn off losses are minimal. Hence, there is not much penalty in increasing the carrier frequency of the VFD. Losses in AC motors due to PWM waveform are significantly reduced. All the above features put together improves system efficiency. This paper presents results obtained on using a 6-in-1 GaN module for VFD application, operating at a carrier frequency of 100 kHz with an output sine wave filter. Experimental results show the improvement in motor efficiency and system efficiency on using a GaN based VFD in comparison to the standard Si IGBT based VFD.

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GaN devices have superior performance over Sibased devices, and high voltage normally-off GaN HEMTs with cascode structure have been available for industry application as they can be easily driven by mature commercial Si-MOSFET drivers. Due to the very fast switching speed, the PCB layout and driver circuit design should be very careful to keep the parasitic parameters as small as possible. Paralleling semiconductor devices is an effective and simple way for higher power application. It is very challenging to parallel GaN HEMTs in hard-switching bridge power converter application, especially for discrete leaded package devices. However, leaded packages are still dominant in industrial applications because of their simplicity for PCB assembly and capability for a wide variety of heat-sinking techniques. In this paper, a solution to paralleling GaN HEMTs for diode-free bridge power converters is proposed, and GaN device driver design is discussed. The partial phase method is also suitable for the low power application to improve the efficiency. Simulations of driving circuit and experimental results on a 5 kW half bridge operating in synchronous boost mode with 4 paralleling GaN HEMTs are provided for validation.

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