TABLE OF CONTENTS The design of the switching power supply section of an AC/DC converter is more of an art than an electrical design. The schematic is only a portion of the whole design, since the physical design of all of the portions of the product also enter into consideration. In addition to the high voltage switching power supply design, switching noise and dielectric isolation are the primary concerns.
The AC mains is capable of delivering many kilowatts of power to any load, so many transformer-isolated topologies are available, depending on how much output power is required. The determining factor is the peak current that the power switches must sustain. A good maximum level is less than 30 A per semiconductor switch. This makes the discontinuous flyback topology the appropriate choice for loads less than 100 150 W. The one-transistor forward is a good choice for loads less than 300 W, the half-bridge topology until about 800 W and the full-bridge to beyond 1 kW.
The primary concern is the dielectric isolation between the primary circuits and the output circuits. This requires very special practices in the construction of the transformer, gaps in the printed circuit board layout, and isolated feedback circuits. There are many specifications as to how this should be done issued by the safety regulatory agencies around the world.
The transformer is larger in off-line switching power supplies due to the added requirements of spacing between the windings and the added insulating tape needed between winding layers within...
