3. Switches. The switches in the full-bridge circuit must be capable of
carrying current in either direction for pulse-width modulation just as they
did for square wave operation. Feedback diodes across the switching
devices are necessary, as was done in the inverter in Fig. 8-3a. Another
consequence of real switches is that they do not turn on or off instantly.
Therefore, it is necessary to allow for switching times in the control of the
switches just as it was for the square-wave inverter.