SMPS

 ·         Basic Idea of SMPS 

The regulated power supply, studied in the earlier section uses series regulators which are linear designed power supplies. Most of the energy supplied to them is wasted as heat. For small appliances it is acceptable, but for a PC power supply it is unacceptable.

Thus, instead of using series pass element, switching regulator which produces large load currents with much less power dissipation is used.

A switched-mode power supply (SMPS) is an electronic circuit that converts power using switching devices that are turned on and off at high frequencies, and storage components such as inductors or capacitors to supply power when the switching device is in its non-conduction state.

SMPS uses a transistor switch and a closed feedback loop to produce D.C. output that is properly regulated.

The basic idea of SMPS is as shown in fig.

Working: Trains of pulses from pulse generator drives the base of the transistor. When base voltage is high, the transistor is in saturation, i.e. it is “ON” and when the base voltage is low, it will be in cut-off,i.e. it is “OFF” Thus transistor is working like a switch.

The important characteristics of switch is that it does not dissipate heat when it is ON or OFF.

The diode connected between emitter and ground protects the transistor from inductive kickback voltage.

The inductor L will keep the current through it constant. If the diode is not connected then inductive voltage across L will produce high reverse voltage to damage the transistor.

The switching of transistor is controlled by as duty cycle of pulse.

The duty cycle of the pulse is the ratio of ON time to total time of the pulse.

The input voltage varies from zero to Vinput due to the pulse generator.

Due to high frequency smaller value of L and C are selected.

The output of LC filter is D.C. voltage with oly smaller ripple across it.

If D is duty cycle, then D.C. output voltage is given by Vout= DVin

The output voltage is fed back to the pulse generator.

If the output voltage tries to increase, the duty cycle will decrease. This means that, narrower pulses will drive LC filter and hence output will decrease.

The reverse will be the action when output voltage decreases.

There is very less power dissipation across switching element, which is the most advantageous property of SMPS.

·         Block Diagram of SMPS

  

                   Practical SMPS images

         

Advantages Of SMPS:

1. The main advantage of a witching power supply is that it is far more efficient than a linear design.          while dealing with hundreds of watts of power, this issue is important.

2. All the energy wasted in the power supply as heat has to be removed by the PC's cooling system.

3. Therefore, more efficient power supplies produce less heat that the system has to exhaust.

4. The power efficiency of SMPS is high, the weight and size is smaller.

5. It can operate under low A.C. input voltage.

6. It has a relatively long hold up period if input power is lost momentarily.

7. This is because more energy can be stored in its input filter capacitors.

Disadvantages Of SMPS

1. The disadvantages of SMPS are, they are more complex than series regulators.

2. It has higher output ripple voltages; response to load variation is not fast as dissipative regulators.