"Freewheeling diode" is generally used in circuits to protect components from being broken down or burned by induced voltage.

In the H-bridge circuit, it is connected in parallel with each MOS switch and across the inductor (load).

So what is its role in the H-bridge circuit?

1. Preventing sudden current changes

When the power switch device in the circuit is switched, according to Lenz's law, the motor winding will generate reverse electromotive force to maintain the original current direction.

When Q1 and Q4 are turned off, the reverse electromotive force generated by the motor winding will cause the current to continue to flow from Q1 and Q4. At this time, the two freewheeling diodes D2 and D3 will be turned on.

D2 and D3 provide a loop for the current in the motor winding, allowing the current to continue to flow and avoiding a sharp change in current. Without these two freewheeling diodes, the reverse electromotive force generated by the motor winding may generate extremely high voltage spikes across Q1 and Q4, causing Q1 and Q4 to be broken down and damaged.

2. Providing a circuit

In the working cycle of the H-bridge circuit, when the two diagonal switches are turned on or off at the same time, for example, Q1 and Q4 are turned on at the same time (the motor rotates forward), the current flows from the power supply Vbat through Q1, the motor M and Q4 to form a loop.

When Q1 and Q4 are turned off at the same time, the current in the motor winding needs a return path to maintain continuity. At this time, D2 and D3 are turned on to provide a return path for the current.

This loop is essential to maintain current continuity in the inductor (motor winding), especially in applications where a smooth DC voltage is required at the output.

In summary, the role of the freewheeling diode in the H-bridge circuit is to prevent current mutations and provide a loop to ensure smooth changes in the current in the inductor, thereby achieving effective power control and motor drive.