Figure 2 shows that the six pulses are symmetric about the
x-axis. The negative six pulses are due to the conduction of
the other switches of that same leg that are conducting in real.
The magnified output waveform of converter cum inverter
with resistive load is shown in Figure 3. From this figure it is
clear that the switching cycle is 40μS. For each 20μS
switching action takes place in a leg and no two switches of
same leg will not conduct.