The circuit for half-bridge SMPS configuration is shown in Fig . It consists of an uncontrolled rectifier, two capacitors $\mathrm{Cl}$ and $\mathrm{C} 2,$ two power MOSFETs $\mathrm{M} 1$ and $\mathrm{M} 2,$ one transformer with mid-tap on the secondary side, two diodes D1 and D2 and filter components L and C.

Two capacitors $\mathrm{Cl}$ and $\mathrm{C} 2$ have equal capacitance, therefore voltage across each of the two is $\frac{V_{s}}{2}$ . When $\mathrm{M} 1$ is turned on, voltage of $\mathrm{Cl}$ appears across transformer primary, i.e. $v_{1}=\frac{V_{s}}{2}$ and voltage induced in secondary is $v_{2}=\frac{V_{s}}{2 N_{1}} \cdot N_{2}$ and diode D1 gets forward biased.

When $M 2$ is turned on, a reverse voltage of $\frac{V_{s}}{2}$ appears across transformer primary from $C 2$ i.e. $v_{1}=-\frac{V_{s}}{2}$ and voltage induced in secondary winding is $v_{2}=-\frac{V_{s}}{2 N_{1}} N_{2}$ , therefore diode D2 gets forward biased. This means that transformer primary voltage swings from $-\frac{V_{s}}{2}$ to $+\frac{V_{s}}{2}$ . Average output voltage, however, is

$$V_{0}=\frac{V_{s}}{2 N_{1}} \cdot N_{2}=0.5 \alpha V_{s}$$

When $M 1$ is off, open circuit voltage across $M 1$ terminals is $V_{o c}=V_{s}$ . When $M 2$ is off, as before $V_{o c}=V_{s}$ . For h.v d.c applications, half-bridge converter is, therefore, preferred over push-pull converters. For l . v . d.c. applications, push-pull SMPS is preferred due to low MOSFET currents.