The circuit arrangement for a full-bridge SMPS is shown in Fig. It consists of an uncontrolled rectif‌ier, four power MOSFETs, transformer with mid- tap secondary, two diodes and LC f‌ilter circuit. As in all the previous circuits, the function of control circuit is to sense the output load voltage and to decide about the duty ratio of MOSFETs.

When power MOSFETs M1 and M2 are turned on together, voltage $V_{s}$ appears across transformer primary, i.e. $v_{1}=V_{s}$ and secondary voltage $v_{2}=\frac{V_{s}}{N_{1}} \cdot N_{2}=\alpha V_{s}$ . Diode D 1 gets forward biased and $V_{0}=\alpha V_{s}$ . When $\mathrm{M} 3$ and $\mathrm{M} 4$ are turned on together, the primary voltage is reversed, i.e. $v_{1}=-V_{s}$ and $v_{2}=-\frac{V_{s}}{N_{1}} N_{2}=-\alpha V_{s}$ . Therefore, diode D2 now begins to conduct and the output voltage is again $V_{0}=\alpha V_{s}$ .

The open circuit voltage across each MOSFET is $V_{o c}=V_{s}$ . Of all the four configurations of SMPSs, full-bridge converter operates with minimum voltage and current stress on the power MOSFET. It is therefore very popular for high power applications above 750 W.

The overall size of SMPSs is dependent on its operating frequency. Use of power transistors is limited to approximately 40 to 50 kHz. Above this operating frequency, power MOSFETs are used up to about 200 $\mathrm{kHz}$ .