What is a freewheeling diode good for?
The motor winding is inductive and the current flowing through it cannot be cut abruptly. If this current is not given a path when the switch turns off, a reverse voltage that can reach hundreds of volts appears at the winding terminals. The freewheeling diode offers this current a closed circuit, dissipates the energy safely and protects the switch from overvoltage.
The basic behaviour of an inductance is to produce a voltage opposing the change in its current. Trying to cut the current in a very short time means a very large voltage spike. This is how contact arcing forms in relay coils and breakdown in semiconductor switches.
The diode carries this current:
- While the switch is conducting, the diode is reverse biased and out of the circuit.
- When the switch turns off, the winding current wants to keep its direction; the diode conducts and the current circulates in a closed loop, dissipating in the winding resistance.
In H-bridge drivers this function is usually taken on by the body diodes of the MOSFETs; during the dead time the current flows exactly along this path. For this reason the reverse recovery speed of the body diode directly affects the circuit's switching loss and the risk of shoot-through.
The point for the user to watch is fitting their own suppression device to the relays and contactor coils added between the driver and the motor. In installations where the driver output is to be switched by a relay, adding a diode or an RC to the coils both extends contact life and reduces the noise leaking into sensitive circuits such as the RF receiver. Capacitors and ferrite rings fitted at the motor terminals are also used to suppress brush arcing noise.
The full topic: DC Motor Driver Selection Guide