Working Principle of SMPS Power Plant

What is the working Principle of SMPS Power Plant?

Working Principle of SMPS Power Plant:

What is SMPS ?

SMPS means Switch Mode Power Supply. This is 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 the switching is done at very high rate in the order of several KHz to MHz.

Working Principle of SMPS Power Plant


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

Principle of Switching Regulator

Switching Regulator

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, 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 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.

Regulation is achieved by  modifying the Duty cycle. Duty cycle depends on onetime of transistor, which in turn depends on the width of the pulse applied to the base of the transistor, which is controlled by ‘Pulse width modulation’ by regulator circuit.

Principle of Regulation

Principle of 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 feed back 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 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 larger time and more voltage comes out of the L.C.filter. This cancels out the original decrease in output voltage.

Pulse width modulation

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 minimum as it conducts only for a fraction of a cycle.

Now a days SMPS technology is extended to power plants also. Power plants upto 2000A capacity have been developed using SMPS principle.

Specification of 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.

 ITI’s 50V – 2000A POWER PLANT (Multi Rack Type) Suitable for VRLA Batteries with 100A SMPS Rectifier Modules


The power system is intended primarily to provide uninterrupted DC power to Telecom equipments and current for charging the batteries in the presence of AC Mains. The system works from commercial AC mains which is rectified and regulated to –50V DC and is fed to the equipment (exchange). The system has provision to connect three sets of VRLA batteries and facility to charge them simultaneously to ensure that uninterrupted DC power supply is always available to the exchange.

The power system –50V, 2000A has the following features :

  • Multi-rack configuration.
  • Facility to parallel a maximum of 21 nos. (or 22 nos.) of 100A (5600W) Rectifier modules operation from three phase, 400V, 50Hz AC input.
  • Termination for three sets of VRLA batteries and exchange.
  • System input : Three phase, 4-wire, 50 Hz supply.

The power system has a single DC bus called auto float/charge bus. Depending upon the status of the batteries, the output DC voltage is maintained at 54.0 + 0.5 V under auto float condition. During auto charge the maximum DC voltage reached across the bus is 55.2 volts. The exchange battery and rectifier modules are connected in parallel.

The system employ natural convection cooling and has AC input distribution, DC output distribution, protection and alarm circuitry for rectifiers, battery and equipment.

Technical Specification

For Module

(1) Input Voltage :

  • 320V to 480V r m s three phase (Nominal Voltage – 400V).
  • Frequency : 45 Hz …. 65 Hz.

(2) Output Voltage :

Float mode :

Nominal voltage  :  -54.0 + 0.5V,

Adjustment range :  -48.0 to –56.0 V

Charge mode Voltage :  -55.2 + 0.5 V

(3)        Rated current : 100 Amps.

(4)        Psophometric noise   :

Less than 4 mV without battery floated.

Less than 2 mV with battery floated.

(5)  Input power factor: Greater than 0.95 lag with 25% to 100% load at nominal input.

(6) Efficiency :Greater than 90% at full Load and nominal input.

(7) Protection                               :

  • Short circuit protection.
  • Input over/under voltage protection.
  • Output over voltage protection.
  • Constant current features settable from 80 Amps. to 110 Amps. In auto float/charge mode.

(8) Alarms and indicating lamps           :

  • FR/BC on Auto Float/Charge : Green LED
  • Rectifier module over voltage :  Red LED
  • DC output fail/Under voltage : Red LED
  • FR/BC Over Load (Voltage Drop): Amber/Yellow LED
  • Mains Available : Green LED

For System

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