The switch currents in Fig. 8-2 show that the switches in the full-bridge circuit
must be capable of carrying both positive and negative currents for RL loads.
However, real electronic devices may conduct current in one direction only. This
problem is solved by placing feedback diodes in parallel (anitparallel) with each
switch. During the time interval when the current in the switch must be negative,
the feedback diode carries the current. The diodes are reverse-biased when current
is positive in the switch. Figure 8-3a shows the full-bridge inverter with
switches implemented as insulated gate bipolar transistors (IGBTs) with feedback
diodes. Transistor and diode currents for a square wave voltage and an RL
load are indicated in Fig 8-3b. Power semiconductor modules often include feedback
diodes with the switches.