In contrast to the half bridge, the full bridge (or H-Bridge) converter uses four transistors as shown in Figure 6-8.
In a full bridge circuit the diagonally opposite transistors (Q1/Q2 or Q3/Q4) are turned on during alternate half cycles. The highest voltage any transistor is subjected to is VI, rather than 2 x VI as is the case in the push-pull converter circuit. The full bridge circuit offers increased reliability because less voltage and current stress is placed on the transistors.
The disadvantage of this circuit is the space required by the four transistors and the cost of the two additional transistors.