1 kW GaN reference design for pedal-assisted e-bikes

Marco Palma, Director of Motor Drive Systems and Applications at Efficient Power Conversion

Introduction

Many electronics applications, such as pedal-assisted bicycles, require that inverters driving electric motors to be able to handle even very high currents while remaining compact and lightweight. This poses a great challenge for designers, as power, size and weight have always been opposing attributes. In battery-powered applications, every cubic centimetre of space occupied and every gram of weight saved enables longer operating time between battery recharges. The Efficient Power Conversion gallium nitride (GaN) FET transistors help designers increase power density and overcome this demanding challenge.

EPC9167 and EPC9167HC e-bike motor drive power evaluation board

The typical EPC2065 80V 2.7mΩ eGaN® FET transistor is an optimal candidate for power applications where the DC bus voltage is less than 70 VDC. In e-bike motor drives, the PWM modulation frequency is usually kept at 20 kHz and dead times are greater than 500 nanoseconds. In these cases, the conduction resistance (RDSON) of the device used as a switch is the main parameter that designers look at most carefully. The thermal dissipation capacity, in particular, the junction-to-case thermal resistance of the device, Rθjc, is the second most important parameter to be taken into account. Conventional MOS-based transistor solutions use one or more devices in parallel for each switch inserted in 3 x 3 mm packages mounted on PCBs, letting the latter act as a vehicle for the heat to be dissipated.

The EPC9167 evaluation board houses the EPC2065 transistors and everything needed to drive a motor, except the microcontroller. It can operate with a maximum DC bus voltage of 70 VDC and a maximum phase current of 30 ARMS. The motor controller can be chosen from those available on the market and can be connected using the special coupling board provided by EPC. In figure 1, from left to right, you can see the control signal connector, the conditioning circuit for voltage and current feedback signals to the external microcontroller, the ceramic capacitor bank, the three-phase inverter with the shunt resistors placed on the outputs of the phases (in-phase shunt) or on the lower branches of the half-bridge switching circuit (leg shunt) and, finally, the motor connector.

Reference design for rapid prototyping

The printed circuit board (PCB) of the EPC9167 reference board has been laid out following the rules for optimal layout specified by EPC, which ensure minimum inductance in the power loop. The main criterion to be followed is symmetry in the component layout and confining the entire high-frequency path in the top face and first inner layer of the circuit board.

When using eGaN FET transistors or an integrated power stage GaN ePowerTM in an inverter for motor drive, it is very common to use an in-phase shunt together with an isolated (functionally or galvanically) integrated circuit that extracts the small differential signal at the ends of the shunt resistor overlapped on the common mode voltage of the output switching phase, to measure the instantaneous current. This approach has the advantage that the phase current signal can be accessed continuously over the entire PWM period. However, compared to the solution of inserting shunt resistors in the lower branch of the half-bridge switching circuit (leg shunt), it has a higher cost and lower bandwidth, so it could be an obstacle to the adoption of inverters with GaN semiconductors in motor drives.

The EPC9167 gives you the opportunity to evaluate both solutions to decide which one is best suited to your application. In fact, it houses both 1 mΩ in-phase shunt resistors and 1 mΩ leg shunt resistors for each switching cell. The amplification gain, offset and polarity are the same for both circuits, so either of the two current measurement schemes can be connected to the external microcontroller without having to make any changes to the firmware.

There are two versions of the EPC9167 board: the EPC9167 has six EPC2065 devices for the inverter, the EPC9167HC has 12 EPC2065 devices. The choice of version depends on the e-bike you want to design and the acceleration you want to provide to the end user. Both boards are sold with a heatsink. The designer can decide whether or not to remove it. GaNFET EPC devices are more efficient than MOS, especially if you put them in contact with a metal chassis on the top side.

The EPC9167 inverter with EPC2065 GaN transistor can easily be used with 100 kHz switching frequency and 50 ns dead time. The advantage is that the input voltage and the residual ripple of the current decrease, which makes it possible to remove the electrolytic capacitors and use only ceramic capacitors, which are smaller, lighter and more reliable. At this frequency, it is no longer necessary to filter the battery cables for electromagnetic compatibility.

For more information

Many battery-powered motor applications are switching from the use of conventional silicon MOSFET transistors, which operate at low PWM frequencies, to inverters with GaN transistors that can run at much higher PWM frequencies, with the advantage of being able to reduce the size and weight of the inverter without sacrificing overall system efficiency. With proper gate drive circuitry and optimal layout, the switching waveforms remain clean and the dv/dt change rate is easily manageable.

More information on EPC, GaN transistors and the EPC9167 board design can be found here.

For commercial information contact us.