A block diagram of Switch Mode Power Supply is shown in figure. The first block is rectifier and filter that converts the A.C. supply voltage to pulsating D.C. which is then filtered out to reduce the amount of ripple content. This section uses the power diodes in bridge configuration to obtain the pulsating d.c. and the capacitor is used as a filter element. The second block is the high frequency switching section and it uses either MOSFETs or BJTs to convert the D.C. voltage to a high frequency ac\.c. square wave. This high frequency a.c. square waves ranges from 20 KHz to 100 KHz. Since the power transistors are not operated in their active region, their operation results in low power dissipation. Thus it is a two stage conversion i.e. the input a.c. supply voltage is first rectified to d.c. and then the high frequency switching section changes it back to A.C.
The next block of SMPS is high frequency power transformer that isolate the circuit and step up or step down the voltage to the desired voltage level. The output of the transformer is the input of the second rectifier section, called the output rectifier section. This rectifier section is different fron the first block of the rectifier in that the frequency of the voltage is very high. Therefore, the bridge configuration of this rectifier uses a high frequency diodes such as schottky diode and the output ripple is naturally filtered becouse of the number of the overlaps between each individual output pulse. Since the ripple is very small in output voltage of the rectifier, a small capacitance is required in the filter section
The last section of the SMPS is the control and feedback block, which contains circuitry that provides Pulse Width Modulation (PWM) output signal. The PWM controller provides duty cycle that varies pulse by pulse to provide an accurate d.c. output voltage.