Growing GaN Ecosystem for BLDC Motor Drives

­So far, silicon-based power devices have been dominant in inverter electronics, but today their performance is nearing its theoretical limits. There is an increasing need for higher power density.

Gallium nitride (GaN) transistors and ICs have the best attributes to satisfy BLDC inverter needs. The superior switching capability of GaN 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, less expensive, and more reliable ceramic capacitors. GaN transistors and ICs allow the design of motor drives that operate smoother, while reducing size and weight. These advantages are critical for the motor drives used in applications such as warehousing and logistical robots, servo drives, e-bikes and e-scooters, collaborative robots and medical robotics, industrial drones, and automotive motors.

EPC has developed a series of reference designs to ease the design of motor drive control applications and to help reduce time to market.

GaN Motor Drives Outlook

Many applications are becoming battery operated with the advent of high-capacity and high-current cells for batteries, such as lithium-ion cells. Some applications are being converted from thermal engines to electric engines, and others that were originally connected to the power grid, are now becoming wireless. Application examples include forklifts, automatic guided vehicles (AGV) in warehouses or agricultural fields, urban mobility electric vehicles, drones for cargo transportation, and in the near future, drones to transport people.

The battery voltages for these applications range from 36 V to 96 V. There is a trade-off between the system weight and the duration of the machine operation with a single battery charge. In fact, the longer the operation time with one single charge, the higher the battery weight. This compromise is serious and tough with flying machines, where the weight must be optimized to the last gram.

Today, all these applications are based on a two-level inverter with the power size dependent on the application. The motor’s phase current may range from 25 A_rms for pedal-assisted bikes to 200 A_rms for a more demanding system requiring the lifting of heavy weight. A designer who wants to design an inverter using GaN technology must quickly learn how to take advantage of the superior attributes of GaN , including how to layout the printed circuit board avoiding common pitfalls when the phase current is high, which requires more devices to be placed in parallel.

Released Reference Designs

EPC has released several reference designs to help motor drive system designers quickly evaluate GaN FET and GaN IC performance within the specific motor drive application. These EPC motor drive reference designs include both power and controller boards. The system designer makes the decision of which power board to test and the choice of the controller to use. The available reference designs of power boards are shown in Figure 1, while the controller boards are shown in Figure 2.

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Figure 2: EPC Motor Drive Reference Design Controller Adapter boards

Figure 1 shows two miniature power boards (EPC9146 and EPC9176) based on EPC GaN integrated circuits. The EPC9146 (with EPC2152 IC) is capable of 10 A_rms with a heatsink, while EPC9176 (with EPC23102 IC) is capable of 15 A_rms with a heatsink. The EPC9167HC and the EPC9173 are slightly larger and can run up to 25 A_rms with a heatsink.

All the boards are tested without air convection, and the maximum current capability is defined when the temperature rises 50°C above the ambient temperature. All power boards share a similar block diagram, with a common connector which allows interchanging of the controller boards.

The example in Figure 3 shows the EPC9173 building blocks. All reference designs have on-board auxiliary power supply, current sensing, over-current protection, voltage sensing, and temperature sensing circuits. Some reference designs provide two methods to sense the current, either in the leg shunts or in the phase shunts.

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Figure 3: Block diagram of EPC9173 board in BLDC drive example application

Upcoming Power Board Reference Designs

EPC plans to release additional power boards (see Figure 4) that include the latest GaN FETs, so that customers can quickly evaluate the EPC2302 within the specific application. The EPC2302 is the lowest 100 V, R_DS(ON) GaN FET in the market, and the EPC2619, which is based on the recently released EPC Gen 6 technology. These upcoming boards are shown in Figure 4. The EPC9193, shown on the left, has two EPC2619 Gen 6 FETs in parallel, while the EPC9194, shown on the right, features the EPC2302, which is the first EPC GaN FET released in QFN package.These new boards are expanding the EPC motor inverter family and can be driven by the existing controller boards.

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Figure 4: EPC9193, on the left, has two EPC2619 Gen 6 FETs in parallel. EPC9194, on the right, features the EPC2302

High Current Power Board Reference Design

Motor drive applications, such as forklifts and drones, require high current in the range of hundreds of amperes. Power switches are usually put in parallel to increase the current capability, but this approach poses serious challenges to the power designer because poor layout inductances cause unwanted ringing, excessive power dissipation, and limited functionality. With GaN FETs, paralleling switches becomes more difficult due to their faster switching speed and their logic level gate threshold. One approach to paralleling is shown in Figure 5, where the upcoming EPC9186 demonstrates how to parallel four EPC2302 per each inverter switch.  The EPC9186 is equipped with a minimum number of ceramic DC bus capacitors. In fact, it is possible to drastically reduce the DC bus capacitance by increasing the PWM frequency.

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Figure 5: EPC9186 is a three-phase inverter with four EPC2302 in parallel per switch

Controller Boards and Firmware

When it is time to choose the motor controller IC, each customer has a preferred solution. EPC’s philosophy is to allow testing with the preferred controller by supplying specific adapter boards for existing controller reference designs, as shown in Figure 2. There is also a generic adapter board, the EPC9147E, that has been developed for those customers that want to re-use their personal controller cards. In this case, the customer can wire the necessary signals from their board to the EPC9147E and immediately start testing the EPC GaN-based inverters.

For the existing specific adapter boards (for Microchip^TM, ST^TM and Texas Instruments^TM), EPC has adapted the original firmware to the EPC power boards and has made the firmware available in a GitHub repository. The user can download the controller vendor original programming tools and then can download the specific projects in the EPC Github repository (see Figure 6). In the root directory, an Excel file allows the customer to quickly find the proper firmware by choosing the correct combination of controller card and power board. The firmware is generated by the original vendor tools, and when available, includes all source code and all necessary configuration files.

Summary

DC and battery-powered motor applications are moving from conventional Si MOSFETs and low PWM frequency inverters to GaN-based, high-frequency PWM inverters. The advantages are a higher system efficiency and the elimination of the electrolytic capacitors and input inductor. EPC has developed a series of application-specific reference designs for motor drives that can be used to evaluate the GaN technology and copied or adapted to generate the customer’s schematic.

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