SWITCH MODE POWER SUPPLY (SMPS) is an electronic power supply unit which uses internal control circuit that switches load current t to stabilize the output voltage. They are used in military and avionics equipment. SWITCH MODE POWER SUPPLY (SMPS) can be opened with the help of screw driver. Switch off the AC power supply before opening the cabinet of SWITCH MODE POWER SUPPLY (SMPS).

Functional block diagram of SWITCH MODE POWER SUPPLY (SMPS) mainly consists of following sections:

Primary Rectifier - It is placed after the surge filter and consists of a combination of 4 diodes or rectifier device having 4 terminals. It converts the AC current into pulsating DC. They can be combined in pairs inside TO220 type packages. Take trail for shorted and open junctions.

Primary Filter - It is placed after the Primary Rectifier Section and consists of big size electrolytic capacitors of higher voltage capacity. The specifications of capacitors are C1 =330µ f/400 V, C2=330µ f/400 V and C3=10µ f/400 V. This unit is responsible for converting the pulsating DC current into almost DC current. This removes ripples from the input and provides pure DC.

Switching Transistor - Consists of a pair of switching transistors, which are rectangular power transistors. These are fixed on the heat sink. In the same section, there is another pair of small driver transistors which are general transistors. The driver transistors are used to drive the switching transistors. Driver transistors and switching transistors are coupled through a small transformer called driver transformer. Often BU or 2SC /2SD numbers transistors are used for this purpose.

Secondary Transformer - Secondary transformer is a main bigger transformer and is driven from the collector of the switching transistor. The transformer number is LPW050002800.

Secondary Rectifier - Consists of two groups of diodes, secondary positive rectifier and secondary negative rectifier. Positive rectifier generates +Vcc and negative rectifier generates -Vcc voltage.

Secondary Filter - Consists of inductors and electrolytic capacitors. The inductors are mounted on a single core or separate coils in the output section of the power supply. This component filters the minute ripples that are not filtered by the primary filter.

Pulse Width Modulator - Designed around the IC TL 494 and some other components. As this IC is used to generate the pulses of different widths, it requires some external components such as resistors, capacitors and ceramic capacitors for generating pulses. This component modulates the duty cycle of the output signal and directs it as an input to the Driver transistor and driver transformer.