Publisher Summary This chapter deals with the designs of transformers isolated converters. Feedback Mechanism is applied to transformer circuit applications where the transformer is used for isolation, such as in off-line supplies. The output can be connected directly to the control integrated circuit (IC) in applications where isolation is not required. Most transformer circuits use the magnetic circuit of the transformer to provide electrical isolation of the secondary circuit from the primary circuit. Putting the control IC on the input side of the supply requires that feedback of the output voltage to the control IC has to cross an isolation barrier. If the IC is powered from an isolated supply, then the switch control must cross the isolation barrier. Using an optocoupler is the easiest way to transfer output voltage information across the isolation barrier to a control IC on the primary side. The usual method of compensating for the low optocoupler gain and the capacitance of the optocoupler is to use an amplifier and voltage reference on the isolated side of the supply. Another method of feedback isolation is to use a small power line transformer to generate an isolated auxiliary supply for the IC. The chapter also discusses flyback circuits; a flyback converter works in a fashion similar to a boost converter, where energy is stored in the inductor while the switch is on and the energy is delivered to the load when the switch turns off.