Harnessing the Power of GaN for Motor Drives – Servo drives, robotics, drones
Sep 12, 2019
With advancements in motor technology, power densities have increased; motors are built in smaller form factors and designed for higher speeds, and higher precision, which requires higher electrical frequencies.
3-phase brushless DC (BLDC) motors are compact for their power ratings, can be precisely controlled, offer high electro-mechanical efficiency, and can operate with minimal vibration when properly controlled. These motors are increasingly or exclusively used in precision applications like servo drives, robotics, such as surgical robots, and drones, such as quadcopters. To keep current ripple within a reasonable range, these motors – given their low inductance – require switching frequencies up to 100kHz. A FET that can operate efficiently at high frequency is required to minimize losses and offset the torque ripple in the motor which creates vibrations, reduces drive precision and decreases efficiency.
Gallium nitride (GaN) high-electron-mobility transistors (HEMTs) can switch much faster than silicon MOSFETs, significantly reducing switching losses. Another advantage of GaN transistors is the lack of a reverse-recovery charge, which causes switch-node ringing with traditional silicon MOSFET designs(1).
A GaN FET power stage device, such as the LMG5200, is an 80 V GaN half-bridge power module. This device integrates the driver and two 80 V GaN FETs in a 6 mm x 8 mm QFN package optimized for extremely low-gate loop and power loop impedance(2). The inputs are 5 V TTL and 3.3 V CMOS logic-compatible and have a typical propagation delay mismatch of 2 ns. This enables a very short dead time, which reduces losses and output current distortions. The LMG5200 extends the advantages of discrete GaN FETs by offering a user-friendly package, which is easy to layout and assemble into the final product.
For motor drive applications, Texas Instruments offers a 48 V/10 A High Frequency PWM 3-Phase GaN Inverter Reference Design using three LMG5200 half-bridge GaN power modules. Figure 1 shows the block diagram of this reference design.
Gallium nitride (GaN) transistors can switch much faster than silicon FETs and integrating the GaN FET and driver in the same package reduces parasitic inductances and optimizes switching performance reducing losses, thus allowing the down-sizing or complete elimination of the need for a heatsink.
Efficiency testing of this reference design was performed at a 27 ֯C lab temperature using a Tektronix PA4000 Power Analyzer. The reference design board was powered with a 48-V DC and a Teknic low-voltage servo motor was used as the load. The reference design board’s board power losses at the maximum load current of 7 ARMS were 4.95 W at a 40 kHz PWM and 5.65 W at a 100 kHz PWM. Figure 2 shows power dissipation as a function of output current.
The efficiency of this reference design with an output power of 83 W while driving the Teknic servo motor was 94.3% at a 40 kHz PWM and 93.6% at a 100 kHz PWM. The theoretical maximum efficiency from the 48 V bus is reached at full power, 400 W. This gives a phase-to-phase voltage of 34 VRMS (space vector PWM) at a 7 ARMS phase current and an inverter efficiency of 98.5% at 100 kHz(3).
BLDC motors are the best choice for applications such as surgical robotics and drones due to their small size to power ratio. GaN FETs and ICs are ideal devices for sinusoidally modulated motor drives given their higher efficiency and higher operating frequency. GaN devices used in the motor control circuit yield higher precision and results in a more compact drive for the motors. This combination allows designers to create systems that are more compact and with superior dexterity over equivalent MOSFET solutions. GaN power stages, such as the LMG5200, extend the advantages of discrete GaN FETs by offering a user-friendly package, which is easy to layout and assemble into the final product.