Reduce Audible Noise in Motor Drive Designs Using eGaN FETs and ICs
Jan 15, 2021
Brushless DC (BLDC) motors are popular and 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. Enhancement-mode gallium nitride (eGaN ®) transistors and integrated circuits offer the ability to operate at much higher frequencies without incurring significant losses.
In some motor applications, drones for example, quiet operation is a crucial requirement. The noise generated by the drone’s motors and propellers can be particularly troublesome when it is being used for video, as the noise can drown out the sound the user is trying to capture. Additionally, consider a possible military application using flocks of drones to perform intelligence, surveillance, and reconnaissance. In this situation, where several drones are flying together, the quietest operation possible is desired.
To achieve the quiet motor operation required for these applications, the high switching frequency capability of GaN is essential. eGaN devices allow motor drive inverter designs to reach higher frequencies in the 100 kHz+ range compared to silicon designs which typically operate around 10 – 20 kHz. At this higher frequency, it is possible to significantly reduce the dead time, resulting in a significant reduction in the audible noise of the motor.
The design shown in the video below is a three-phase inverter operated from a 48 V DC supply voltage while switching at 40 kHz to run an unloaded 150 W NEMA we motor. It can operate from 20 kHz through 1 MHz switching frequency and deliver a peak current of 15 A into each phase of the motor when a heat sink is attached. The board measures just 45 mm x 55 mm.
The inverter design demonstrated uses the EPC2152 monolithic ePower™ Stage, which integrates two 70 V, 10 mΩ main FETs and a self-contained half-bridge gate driver optimized for driving the FETs. In the video, the difference in the acoustics of the motor, when the deadtime for the design is set at 1 microsecond versus when it is set at 50 ns, can clearly be heard. With a deadtime of 50 ns and below,w hich is only achievable with GaN, it is possible to obtain very silent operation.