Switched mode power supply (SMPS): Working Principle of SMPS  ITI’s 50V – 2000A Power Plant?

From AC to DC: Exploring the Advantages of Switched Mode Power Supply
 for High-Efficiency Power Conversion

Working Principle of Switched mode power supply Power Plant:

What is SMPS?

SMPS means Switch Mode Power Supply, used for D.C to D.C conversion. This works on the principle of switching regulation. The SMPS system is highly reliable, efficient, noiseless, and compact because it did the switching at a very high rate in the order of several kHz to MHz.


The SMPS regulators are used in B.S.N.L at various locations like CDOT, E10B Transmission systems, etc.

Principle of Switching Regulator

Switched Mode Power Supply (SMPS)

A pulse train drives the base of ‘switching or pass transistor’. When the voltage to the base is high, the transistor saturates, when the voltage is low, the transistor turns off. Here the transistor functions as a switch. When the transistor is ON, the load current is drawn through the transistor and choke L. When the transistor is OFF the load current is maintained by the energy stored in the choke L. The current flows through the earth, Diode D, choke, load and earth. Hence this diode is called ‘Retrieval Diode’.

Duty cycle of the Transistor =        On-Time                = D

On Time + Off Time       (one cycle time)

The output voltage = Input voltage x D

For example

If I/P voltage is 200 volts and D=0.25

O/P voltage     = 200 x 0.25   = 50V.

It achieves regulation by modifying the Duty cycle. The duty cycle depends on the onetime transistor, which depends on the width of the pulse applied to the base of the transistor which is controlled by ‘Pulse width modulation’ by the regulator circuit.

Principle of Regulation

Principle of switching Regulation

The relaxation oscillator produces a square wave. The square wave is integrated to get a triangular wave, which drives the non-inverting input of a triangular to pulse converter. The pulse train out of this circuit then drives the Pass Transistor. The output is sampled by a voltage divider and fed to a comparator. The feedback voltage is compared with a reference voltage. The output of the comparator then drives the input of the triangular to pulse converter.

If the output voltage tries to increase the comparator produces a higher output voltage which raises the reference voltage of the triangular- to pulse converter. This makes the pulse that drives the base of the switching transistor narrower. That means the duty cycle is reduced. Since the duty cycle is lower the output becomes less which tries to cancel almost all the original increase in output voltage.

Conversely, if the regulated output voltage tries to decrease, the output of the comparator decreases the reference voltage of the triangular -to pulse converter. This makes the pulse wider and the transistor conducts for a larger time and more voltage comes out of the L.C.filter. This cancels out the original decrease in output voltage.


For maximum efficiency, the duty cycle should be less than 0.5. As long as the triangular voltage exceeds the reference voltage, the output is high. Since Vref is adjustable, we can vary the width of the output pulse and hence the duty cycle.

Switching regulators are more efficient than conventional regulators as the power loss in the switching element is reduced to a minimum as it conducts only for a fraction of a cycle.

Nowadays SMPS technology is extended to power plants also. Power plants up to 2000A capacity have been developed using the SMPS principle.

Specification of Switched mode power supply (SMPS) Power Plant

1) Input Voltage 320 V to 480 V

Frequency   45 Hz TO 65 Hz

2) Output Voltage

in Float Mode  -54.0 ± 0.5 V. adj range -48 V to -56V

in charge mode  : -55.2 V ± 0.5V

3) Input power factor >0.95 Lag with 25% to 100% load at nominal input.

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