Why EPC is slowing the edges on its new GaN motor boards
EPC has set the switching edges on its new 100 V GaN motor drive boards at about 10 V/ns, deliberately holding back some of the speed available from the transistors. In a motor drive, that can be a sensible use of GaN: a little more switching loss may be an acceptable price for less interference in the rest of the system.
The EPC91124 and EPC91125 accept 18–80 V and share a 79 × 80 mm footprint. EPC specifies continuous phase currents of 35 A RMS and 30 A RMS respectively, with PWM operation up to 100 kHz and 150 kHz listed as the maximum. The boards include current and voltage sensing, temperature monitoring and motor position interfaces.
The motor decides how much frequency is worth having
Higher PWM frequency gives the winding current less time to depart from its commanded value between switching events. With the same voltage across the same inductance, shortening that interval reduces ripple. A compact motor with low winding inductance can benefit considerably, especially if the current controller is struggling to keep torque smooth.
But every extra switching event consumes energy. The inverter’s thermal budget has to accommodate the increased number of transitions, and the reduction in energy per transition has to be large enough to make the higher frequency affordable. A motor whose losses are already dominated by conduction offers a different bargain from one that needs the shorter switching interval to control its current properly.
Switching frequency and edge speed can then be chosen separately. Running more PWM cycles does not require the voltage to move as quickly as the transistor permits on every edge.
The cable carries the edge too
The phase voltage also appears across parasitic capacitances in the motor, cable and chassis, producing displacement current according to i = C × dv/dt. For a hypothetical 100 pF path, an edge of 10 V/ns produces a 1 A pulse during the transition. Even a modest capacitance can therefore carry an appreciable transient.
Faster edges increase that pulse and can alter ringing or couple more interference into position and current sensing. Slowing the gate drive gives up some switching efficiency, but it can reduce the filtering and layout work elsewhere. Whether that saves more than it costs depends on the complete drive, including the motor cable.
EPC’s Hall current sensors have a stated 250 kHz bandwidth. At 100 kHz PWM, the controller still has to find a suitable sampling window after each transition. Sensor response and ADC timing affect how well the sample represents phase current, while blanking and reconstruction determine what happens in the parts of the cycle that cannot be measured cleanly.
Having the sensing and gate drive on the same evaluation platform allows those interactions to be examined together. A clean transistor waveform alone would reveal much less about the quality of the current feedback reaching the control loop.
EPC lists the boards at $690 and $680 through DigiKey and Mouser. Schematics, bills of materials and board files are available with the platforms, including the gate drive and sensing arrangements used to obtain the stated performance.
