According to four-switch combination, three output voltage levels, +V, -V, and 0, can be synthesized for the voltage across A and B. During inverter operation shown in Fig. 2.1, switch of S1 and S4 are closed at the same time to provide VAB a positive value
and a current path for Io. Switch S2 and S4 are turned on to provide VAB a negative value
with a path for Io. Depending on the load current angle, the current may flow through the
main switch or the freewheeling diodes. When all switches are turned off, the current will flow through the freewheeling diodes.