Stopping or coasting a rotor already in motion is a typical use case that can cause problems for a high-power design. In this specific definition, coasting is when all of the high and low sides are turned off, which can be understood as floating the motor phases. Motors are partially inductive loads, so inductors attempt to resist changes in current by producing voltage to maintain current to flow in addition to the back EMF generated from the magnetic material of the rotor passing by the stator coils. As a result, the voltage at the motor phase rises higher than the voltage seen at the drain of the FET, which causes current to flow from the motor, through the body diodes of the FETs and into the supply over time during this coasting condition.

These spikes in voltage and the increased current out of the motor phases goes into the supply and increases the equivalent voltage at the FET drain to a higher value. As mentioned previously, the bulk capacitors absorb some or all of this energy, but the resulting rise in voltage can easily exceed absolute maximums for the gate driver if left unchecked as the bulk capacitors increase in voltage.

This actually occurs during the dead time of every PWM cycle, but the FETs stay in the coast state for such a short amount of time that the resulting energy usually does not move to the supply fast enough to cause damage. However, the increased voltage on the high-side source can be detected.

Luckily, this is avoided through motor control methods or external circuits, and the best practice is to have a plan to manage the energy stored up in the coils. Instead of coasting, implementing a braking control method or adding an external circuit is preferred.