Schematic and firmware for a very simple shunt regulator based on the ODrive one. This shunt regulator was designed to be used with a 24V or 48V supply powering brushless motors. When doing negative work the motors will back-feed the power supply, raising the voltage and potentially triggering the over voltage protection. The shunt regulator will detect this rise in voltage and dissipate power through a power resistor, essentially acting as a variable load and regulating the voltage.
The amount of power dissipated in the brake resistor is controlled via the 20kHz PWM signal going to the MOSFET driver. The duty cycle is ramped up from 0% for any voltages above overvoltage_ramp_start and maxes out at 100% for voltages at or above overvoltage_ramp_end. This provides a smoother fade in or out, as opposed to suddenly dumping the maximum possible power throught the resistor.
overvoltage_ramp_start and overvoltage_ramp_end must be set in the firmware based on the power supply voltage. overvoltage_ramp_start should be at least 2V to 4V above the power supply voltage. Setting this value too close or below the power supply voltage will result in a fried MOSFET or brake resistor. If the resistor gets warm without the motors braking, increase this value. Set overvoltage_ramp_end to a value which is below the power supplies overvoltage protection and the motor's maximum rated voltage, but not too close to overvoltage_ramp_start.
The circuit in the lab has a maximum rated voltage of 65V.