Switch-mode power supplies or SMPSs are used in various applications as an efficient & effective source of power. SMPS offers various advantages in terms of size, weight, cost, efficiency & overall performance. These have become an accepted part of electronic gadgets. Basically, it's a device where energy conversion & regulation is provided by power semiconductors that are continuously switching "on" & "off" with high frequency.
Different Kinds of SMPS
DC to DC converter
The primary power received from the AC main is rectified & filtered as high voltage DC. It is then switched at the huge rate of speed & fed to the primary side of the step-down transformer. The step-down transformer is just a fraction of the size of a comparable 50 Hz unit, thus relieving the size & weight problems.
In a forward converter, it is the choke that carries the current when the transistor is conducting as well as when it's not. The diode here carries the current during the OFF period of the transistor. Thus, energy flows into the load during both these periods. The choke stores the energy during the ON period & also passes some energy into the output load.
In a flyback converter, it is the magnetic field of the inductor that stores energy during the ON period of the switch. The energy gets emptied into the output voltage circuit while the switch is in its open state. The duty cycle determines the output voltage.
Self-Oscillating Flyback Converter
This one is the most simple & basic converter which is based on the flyback principle. During the conduction time of the switching transistor, the current starts ramping up linearly primarily through the transformer with the slope equal to Vin/Lp.
The voltage induced in the secondary winding & the feedback winding makes the fast recovery rectifier reverse-biased & holds the conducting transistor ON. When the primary current reaches the peak value Ip, where the core starts saturating, the current tends to rise very sharply. This sharp rise in the current can't be supported by the fixed base drive provided by the feedback winding. As a result, the switching begins to come out of saturation.