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SMPS Block Diagran, Basic Concepts, Theory of operation

SMPS Block Diagran, Basic Concepts, Theory of operation

Power supply (or power supply unit, PSU) is the device that transfers electric power from a source to a load using electronic circuits.
A typical application of power supplies is to convert utility’s AC input power into regulated DC voltage(s) required for electronic equipment. Depending on the mode of operation of power semiconductors PSU can be linear or switching (SMPS).

What is SMPS? SMPS stands for switch mode power supply. In such a device power handling electronic components are continuously switching on and off with high frequency in order to provide the transfer of electric power via energy storage components (inductors and capacitors). By varying duty cycle, frequency or a relative phase of these transitions average value of output voltage or current is controlled . The frequency range of an SMPS is from 20 kHz to several MHz.

Below is the block diagram of a typical off-line switching power supply.

The AC input voltage first passes through fuses and a line filter and is rectified by a full-wave bridge rectifier. The rectified input voltage is next applied to PFC (power factor correction) pre-regulator followed by output DC-DC converter(s).


F1 and F2 shown on the left of the diagram are input fuses. Fuse is a safety device designed to physically open the circuit when the current being drawn through it exceeds its rating for a certain period of time. The fusing time depends on the degree of overload. Due to this time delay fuses will not always protect power supply circuit from a catastrophic failure caused by abnormal conditions. Their main purpose is to protect input line from overheating, prevent tripping of circuit breaker and prevent fire inside the PSU that may be triggered by failed components.

Lowpass EMI filter reduces high frequency currents getting from PSU into the AC line to an acceptable level.

It is necessary to prevent the PSU from causing interference on the mains wiring. There is a number of standards (such as EN55022 for Information technology equipment) that govern the maximum level of EMI caused by PSU.

The filter is followed by the bridge rectifier- the circuit that converts bipolar AC voltage to unipolar pulsating voltage. It uses four diodes in a bridge arrangement to provide the same polarity of output voltage for both polarities of input voltage.

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