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GaN Devices for Motor Drive Applications
By Marco Palma
Preface
Brushless DC (BLDC) motors are popular and are finding increasing application in robotics, e-mobility and
drones. Such applications have special requirements such as lightweight, small size, low torque ripple, low
audible noise, and extreme precision control. To address these needs, the inverters powering the motors need
to operate at higher frequency but require advanced techniques to reduce the resultant higher power loss.
GaN transistors and integrated circuits offer the ability to operate at much higher frequencies without
incurring significant losses than silicon-based devices. This EPC eBook explores specific examples of the
benefits of using GaN devices to reduce cost, audible and electronic noise, size, and weight.
A comprehensive analysis of GaN-based solutions, reliability, and applications are available in EPC’s latest
text book, GaN Power Devices and Applications, released in October, 2021. The book is available for
purchase on EPC’s web site.
1.0 Introduction
Today, many electrical motors require variable frequency drives to adapt the speed and the torque of the
motor to the needs of the specific application operating point. Variable frequency drives are implemented
with a two-level voltage source inverter topology, consisting of three phases because most motors are wound
with three windings displaced by 120 electrical degrees. Windings are often star-connected with three
terminals available, while the internal connection (neutral point) is not available.
1.1 Motor Types
The permanent magnet motor, also known as DC brushless motor (BLDC), is widely used and offers
higher torque capability per cubic inch and higher dynamics when compared to induction motors and to
DC brushed motors. Permanent magnet motors require less current to generate torque because the flux is
generated by the magnets in the rotor. Induction motors, on the other hand, are widely used in the industrial
world because of their superior reliability and offer advantage if over-speed-constant-power field weakening
mode needs to be used [1]. DC brushed motors, despite the fact that they are simple to use, are more complex
to build and are being phased-out due to their lower performance and reliability when compared to the other
motor types.
In the last decade, the DC brushed motor phase-out and the subsequent brushless motor phase-in has
been aided by motor control algorithms that allow a precise and controlled orientation of the magnetic field
in the stator to get the optimum working point at every speed and at every torque. With these algorithms,
the torque and phase current have a linear relationship.
1.2 Inverter
So far, silicon-based power devices have been dominant in the inverter electronics, but today their
performance is nearing their theoretical limits [2]. There is an increasing need for higher power density, in
terms of amount of power, volume and weight the inverter can transfer to the motor from the DC source.
Gallium nitride transistors and ICs have the best attributes to satisfy these needs.
The relevant superior characteristics of GaN include: lower intrinsic carrier concentration, higher
electric breakdown field, higher thermal conductivity, and larger saturated electron drift velocity when
compared to silicon.
GaN’s superior switching behavior helps to remove dead time and increase PWM frequency to obtain
unmatched sinusoidal voltage and current waveforms for smoother, silent operation with higher system
efficiency. Power density increases with the substitution of electrolytic capacitors in the input filter with
smaller, cheaper, and more reliable ceramic capacitors.