A flyback regulator can produce single or multiple output voltages that are lower or greater than the input voltage. Its operation is similar to the step-up regulator, except the output switch controls the primary current of a flyback transformer.
The primary and secondary windings are out of phase, therefore no current flows through secondary when current flows through the primary. This allows the primary to charge up the transformer core when the switch is on. When the switch turns off, the core discharges by sending current through the secondary, and this produces the output voltage. This voltage is controlled by adjusting the peak primary current, similar to the operation of the step-up regulator.
The following simple flyback regulator circuit uses the LM2577T-15 IC from National Semiconductor producing ±15V (at 225mA each) from a single 5V input:
The LM1577/LM2577 is available in different packages: 5-Lead TO-220 (T), 16-Lead DIP (N), 24-Lead Surface Mount (M), TO-263 (S) 5-Lead Surface-Mount and 4-Lead TO-3.
Pin numbers shown in the schematic are for the TO-220 package (LM2577T-15).
The 12V fixed voltage version and the ADJ version of the IC can be used too, for different output voltages. In case of the ADJ version if you must disconnect pin 2 from +Vout and connect it to the middle of a divisor made with 2 resistors in series. Calculate the resistors with the formula:
VOUT = 1.23V (1 + R1/R2)
Example circuit: 5V to ±15V flyback regulator with LM2577T-ADJ (PNG).