Consider the hard-switched VSI topology. Based on the
current direction shown in Figure 1. , S1 and S4 experience
both turn-on and turn-off losses. However, S3 and S4 do not
have any switching losses as diodes conduct for these
devices. If the current direction is reversed, a similar
argument holds and S1-S4 have no switching losses, while
S1 and S3 witness both turn-on and turn-off losses.
Therefore, at any switching instant, the VSI has both turn-on
and turn-off losses in two switches. As for the conduction
loss, at any time either two switches are conducting or two
diodes are conducting. Without loss of generality, it can be
assumed that the diode drops are nearly the same as the
voltage drop across the switches. Hence, at any time,
conduction losses equal to two devices are observed.