13.25.1 MOSFET Voltage Suppression

When a motor shaft is rotating and power is removed, the magnetism of the motor components will cause the motor to act like a generator. The current that was flowing into the motor will now flow out of the motor. As the motor magnetic field decays, the generator output will also decay. The voltage across the generator terminals will be proportional to the generator current and circuit impedance of the generator circuit. If the power supply is part of the return path for the current and the power supply is disconnected, then the voltage at the generator terminals will increase until the current flows. This voltage increase must be handled externally to the driver. A voltage suppression device may be used to clamp the motor terminal voltage to a level that will not exceed the maximum system operating voltage during the high-voltage transients. A voltage suppressor circuit may be connected from power ground to the motor power supply rail to create a path for the motor current when the supply is disconnected (Figure 13-8). The PCB traces must be capable of carrying the motor current with minimum voltage and temperature rise.

An additional method is to inactivate the high-side drivers and to activate the low-side drivers. This allows current to flow through the low-side external MOSFETs and prevents the voltage from increasing at the power supply terminals.