By referring to figure 3.5, three switch conducts for 180 degree
mode of operation. The conduction sequence can be written as follows
T6T1T2, T1T2T3, T2T3T4, T3T4T5, T4T5T6, and T5T6T1. For consider
one switching sequence T1T2T3, T1T3 are upper group and T2 is lower
group.