May 2020

20 POWER GaN Issue 2 2020 Power Electronics Europe Integrated GaN Power Stage for eMobility Brushless DC (BLDC) motors are a popular choice and are finding increasing application in robotics, drones, electric bicycles, and electric scooters. All these applications are particularly sensitive to size, weight, cost, and efficiency. To address these needs, inverters powering the motors need to operate at higher frequency, but require additional filtering to prevent excessive losses, EMI generation, and unwanted mechanical wear related to high frequency common mode and induced current flow. Alex Lidow, CEO and Michael de Rooij, VP of Applications, Efficient Power Conversion (EPC), USA GaN FETs and ICs offer the ability to operate at much higher frequencies in hard-switching topologies without incurring significant losses [1]. In March, EPC introduced a new monolithic GaN half- bridge ePower TM Stage IC [2] that is capable of 1 MHz switching and up to 15 A RMS load current per phase. This tiny IC greatly reduces PCB size. The system size, however, is further diminished because the very high frequency reduces filtering requirements, thus reducing size and weight. One application example that will be discussed is an eScooter (Figure 1) with a 400 W BLDC motor. High-frequency switching drives for BLDC motors Due to the structure of the brushless DC (BLDC) motor, most will be powered by a three-phase inverter also known as a motor drive. Different applications place different requirements on the motor and drive system; for example, drones require efficiency, speed, and must be lightweight; while e-bikes require high efficiency and high torque capability. Many motor drives operate at 20 kHz but can go as high as 60 kHz, and selection for a given application is driven by cost constraints, power losses, audible limitations, EMI regulations and maximizing mechanical life for the motor. Some other application requirements, such as precision, may be difficult to achieve when limited to using 20 kHz, necessitating a higher switching frequency. Additional benefits of operating at a higher switching frequency include: Lower AC component magnetic losses due to lower ripple current magnitude and thus higher motor efficiency and lower operating temperature, lower filtering requirements thus reducing filter size, volume, and weight of the inverter, lower THD at higher motor frequencies that keeps audible emissions low, supports newer class of low inductance motors such as slot-less motors [3]. Inverter switching with GaN FETs and ICs Higher inverter switching frequency does have the disadvantage of higher inverter operating losses, particularly when using MOSFETs. This is in large part due to reverse recovery charge (Q RR ) and other dynamic characteristics of MOSFETs. GaN FETs, with zero reverse recovery and low hard-switching losses, can overcome these high frequency inverter limitations and have already been demonstrated in several motor drives [4 – 8]. EPC is now taking it to the next level by using a monolithically integrated GaN half-bridge power stage that can reduce the inverter size, weight and increase operating frequency further. The next evolution for GaN FETs is monolithic integration of the power FETs and complete half-bridge gate driver. There are many benefits to monolithic integration of the power stage such as: It virtually eliminates common source inductance (CSI), and reduces the power loop and gate loop inductances [1]. The gate drivers are matched to the FETs and can be designed to optimize switching speed against EMI, voltage spikes, and efficiency resulting in the shortest practical transition times. It improves thermal power dissipation distribution allowing optimized FET scaling that yield higher efficiencies. This feature is more useful for high step- down ratio converters. Figure 1: eScooter powered by a 400 W BLDC motor driven by EPC2152 ePower Stage ICs