4 diodes are needed (across each of the 4 driver transistors)

If its a PCB level design use schottky devices so the path of current is thru the diodes and not thru the transistors. (slightly lower forward biased on voltage)

The 4 diodes limit your range of operation to a diode junction above-below the power-ground rails.

Plus a series RC snubber across the motor itself. (R equals motor winding resistance, C depends on application need)

H bridge drivers, since you reverse polarity, doesn't allow the easy use of a flyback diode.

Word of caution - watch the control of your top-bottom drivers to avoid shoot-thru, when top and bottom drivers are both on - on one side of the H-bridge. Great potential for a smoke show.

sheldon wrote on Jan 14^th, 2013, 3:25am:

Good point - motor drivers tend to be pretty slow things however.

One other reason for using external diodes is that often the characteristics of the transistor body diodes is often not well specified.