Diode Sizing for DC Motor H-Bridge
This is not a proper answer. The post is to give some schematics to help generate some comments based on @gbulmer's answer.







simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. At t = 0 the H-bridge and motor is in conduction.







simulate this circuit

Figure 2. At t = 0+ all H-bridge switches are open. Note the voltage reversal on the motor.


At the instant before the H-bridge opens a current I is flowing in the motor.
At the instant after the H-bridge opens the same current is flowing in the motor due to its inductance. Diodes D2 and D3 provide the path for the current.
To forward bias the diodes we must get a voltage reversal on the motor coil.
This circuit relies on the PSU being able to sink the current from the motor somehow.


As discussed in gbulmer's answer comments I suspect that the diodes should only have to be rated for supply voltage (plus safety margin) and maximum motor current on the basis of Figure 2.

You may treat this "answer" as a wiki post and feel free to improve it.