SMPS OVERVIEW

WHAT IS SMPS?

The full form of SMPS is switched mode power supply. As it name implies, SMPS is a electronics circuitry that converts ac main supply to required output voltage at the load using switching devices that is turned on and off at high frequency, and energy storage devices such as capacitor , inductor etc.

WHY SWITCHED MODE POWER SUPPLY IS USED?

In every electronics component there is requirement of a dedicated power supply as per the requirement of the loads.

In linear mode power supplies, The useful power is lost in the form of heat in order to keep the output voltage constant as per the requirement. The efficiency of linear power supply is very less for a high power applications due to such loses. Despites efficiency and power loses, linear mode power supplies requires big size transformer and filter so the entire system looks bulky.

Taking all these things in consideration the concept of SMPS was brought in action which is capable of solving the limitations found in linear mode power supply.

IMPORTANT POINTS

• The main working cycle of SMPS is first it converts AC to high voltage DC then this high voltage DC is converted to high voltage,high frequency square wave and finally this high voltage, high frequency AC is converted to regulated DC.
• Square wave generator or high speed electronics switch (i.e mosfet) are responsible for converting High voltage DC to high frequency AC signal.
• The main propose of converting low frequency AC to high frequency AC is to minimize the size and cost of inductor,capacitor and transformer.
• The high frequency signal at the switch is a square wave so the output voltage can be regulated with the concept of Pulse Width Modulation (PWM). he duty cycle of the PWM from the oscillator can be varied and hence the output is perfectly regulated without any over voltages.
• The DC output is completely isolated from the AC mains even the feedback signal is provided from secondary side to primary side with the help of opto coupler.
• Since there is a High Frequency AC Signal in the SMPS, there is a chance of high frequency harmonics and as a result, SMPS is more susceptible to RF Interference.

WORKING PRINCIPLE OF SMPS

BLOCK DIAGRAM OF SMPS

The working of smps is clearly explained in the above block diagram and each block is explained properly in the following section

INPUT SECTION

The input supply of our system is generally 220 volt ac and 50hz frequency. In smps this supply is directly rectified and filtered without using any step down transformer in order to receive high volt dc. Diodes are used as rectifier and capacitors are used as filter.

SWITCHING SECTION

A fast switching device such as a power mosfet is used as a switch which is capable of operating at high frequency. It is switched at very high frequency nearly about 15khz – 50khz and fed to the primary side of the transformer .The transformer here will be light and small then the standard main transformer. The primary and secondary side of the transformer is magnetically coupled and electrically isolated so transformer also serves as galvanic isolation.

OUTPUT SECTION

The output of the transformer is high frequency ac so we can use low value of capacitor to filter our signal and rectifier is used to convert that ac voltage to required dc voltage. so finally we can receive a regulated output voltage.

CONTROL SECTION

The regulated output voltage is given to the control unit which task is to maintain the required voltage at output.

we use optocoupler as output sensor which senses the output voltage and transfer the reference voltage to primary side in form of light which help to isolate that signal from other so that any sudden spikes do not affect the circuitry. The reference voltage and the output voltage is given to the error amplifier based upon the output of the error amplifier the duty cycle of the pwm signal is changed in order to obtain the required output.

ADVANTAGES OF SMPS

• High efficiency nearly about (80%-90%).
• Less heat generation
• less power wastage
• small size
• cost efficient
• useful for more voltage output

DISADVANTGES OF SMPS

• Complex circuit
• produce electro magnetic interference