SMPS WORKING

*** Explain principle, construction, working of switch mode power supply (SMPS)

            An ac to dc rectifier operates at 50Hz. For a negligible ripple in d.c. o/p, the filter circuits physical size is quite large.

Due to it, the d.c. power supply becomes quite bulky and weighty.

           But  for IC , digital ckts and advanced electronic ckts, the circuit is in smaller size and lighter weight d.c. power supply such a power supply was first developed by “NASA”,  It is called SMPS.

 PRINCIPLE:

    A switch mode power supply is based on the chopper principle.

       SMPS works like a d.c. chopper and  It operates as an ON/OFF switch very rapidly.

                Due to it, a.c. ripple frequency raises and is easily filtered by L and C filter circuits

               These are small in size and weight of the power supply.  Thus SMPS has small physical size and weight.

               A switched-mode power supply switches a power transistor between saturation and cut off with a variable duty cycle of a  desired output voltage.

CIRCUIT DIAGRAM:

 BLOCK DIAGRAM OF  MAINS OPERATED AC-DC SMPS WITH OUTPUT VOLTAGE REGULATION

OPERATION:

The various parts in block diagram are

a)    INPUT RECTIFIER AND FILTER:

              If the input of  SMPS is AC, then the first stage rectifier converts A.C. to D.C.

            The Rectifier circuit  operate as a voltage doubler by adding a switch.

             This switch is operated either manually or automatically, at low voltages(i.e., ~120V A.C.).

             At high voltages i.e.,(~240v A.C.), the input switch is not used and a full wave rectifier is used.

            The rectifier produces an unregulated dc voltage.

            Therefore, the output of the rectifier is sent to a large capacitor filter to remove ripples.

             The input current of such basic SMPS has high harmonic content and relatively low power factor.

            Harmonics are removed by filter banks.

An SMPS with a DC input does not require this stage.

b)   INVERTER STAGE:

            The ripple free DC obtained from the rectifier stage(or directly from the input) is  converted into AC by inverter stage.

            It consists of a power oscillator with a small transformer having a few windings.

           The frequency of the oscillator is generally kept above 20KHz to make it inaudible to humans.

c)    OUTPUT TRANSFORMER:

           Since in a power supply, the o/p is required to be isolated from the i/p ,

          The Inverted AC is fed to the primary windings of a high frequency transformer.  It converts the voltage up or down on its secondary windings.

          SMPS transformers run at high frequency (above 20 KHz).

          Their size is quite small as compared to low frequency (50 Hz) transformers.

           The high frequency transformers do not allow much leakage through their ferrite core.

           Thus SMPS are space saving and cost saving as compared to general power supply.

4.OUTPUT RECTIFIER AND FILTER:

           When a DC o/p is required, the AC O/P from the transformer is rectified,

          Ordinary silicon diodes are used as the rectifier elements, for output voltages above ten volts or so.

          For lower voltages, shocky diodes are used for faster recovery time than silicon diodes.

          They allow a low loss operation at high frequencies and a lower voltage drop at conducting.

          MOSFETS are used for synchronous the rectifiers in  lower output voltages.

         The rectified output is then filtered  with lower capacitance and inductance .

5.CHOPPER CONTROLLER:

        The output voltage is coupled to the input and very tightly controlled.

        A feed back circuit monitors the output voltage and compares it with a reference voltage.

         It is set manually or electronically to the desired output.

         If there is an error in the output voltage, the feed back circuit compensates by adjusting the timing with the MOSFETS are switched ON and OFF.

         This part of the power supply is called the switching regulator and this purpose is served by the chopper controller.

          Switching supplies are provided by opto-couplers in computers, TVS and VCR s to tightly control the output voltage.