Switch Mode Power Supply(SMPS)

* Like a linear power supply,the switch mode power supply too converts the available unregulated ac or dc input voltage to a regulated dc output voltage.

* In case of smps with input suply drawn from the ac mains, the input voltage is first rectified and filtered using a capacitor at the retified and filtered using a capacitor at the rectifier output.

* The unregulated dc voltage across the the capacitor is then fed to high frequency converter.

* The high frequency transformer used in a SMPS circuit is much smaller in size and weight cimpared to the low frequency transformer of the linear power supply circuit.

* A switching mode power supply provides the power supply function through low loss components such as capacitors.inductors,Transformers and by the use of switches that are in one of two states,on or off.

Heater

One of the simpler switching mode power suplly is used to control a heater.

When the temperature is less than rated value simple thermostat turns the ac power on and temperature is above the set point it turns the ac power off.The heater turn on and off by keeping temperature constant.for this circuit we only need to use the switch.

The waveform for the on and off situation

1. The time period of T of the wave form is Ton and Toff and by definition,the reciprocal of the period is the frequency

Duty cycle(D)

The duty cycle is given by below ratio

                   D=Ton /(Ton+Toff)

And the average value of the wave form(Vrms) is given by,

                  Vrms=Vin*SQRT(D)

Switched Capacitor Converter

By adding a capacitor to the heater circuit we get the schematic shown in below.

If we control the switch to maintain 5 v dc on the capacitor we have the 100% efficient converter,Bot, this is not case since charging a capacitor directly from a voltage source or since the capacitor dissipates as mutch energy as it is transfered to the capacitor.

Switched Inductor Converter

By adding an inductor  and diode to our heater circuit we get the schematic as shown figure below.

Unlike our capacitor example,this circuit is 100% efficient with ideal components.

It is major problem is terrible load trnasient response. If the load is  removed,the current must continue to flow in the inductor and output voltage tends to go to infinity.If the load is incresed by two,the output voltage is halved,since the inductor current remains the same until adjusted by the L/R constant.Because of the terrible load transient response this circuit is rarely used as a voltage regulator.

 Note that diode is necessary to provide a current path for the inductor current when the switch opens.Otherwise the voltage across the inductor would build up to several hundred volts until the energy stored in the inductor would dissipates as an arc in the switch, charge the parastic capacitance of the inductor winding,or results in some other undesired phenomenon.

Example:

For a 12v input, v out put,20 A load ,and 20 KHz switching frequency,the inductor can be calculated from the formula.

 

       V=L*(di/dt)