Flyback Converter

The single-ended flyback converter is the simplest isolated converter. The transformer is actually a close-coupled inductor and is energized by means of a unipolar pulse.

This circuit can be configured to operate as a boost, buck, or inverting converter. The output circuit is also isolated from the input.

This technique can be used to create multiple output power supplies since load changes on one winding have little effect on the others. A disadvantage of this approach is the need for large filter capacitors since the entire load current is ultimately supplied during the transistor OFF period. The capacitor should have a low ESR.

Flyback converters are relatively inexpensive and are widely used in high voltage monitor supplies.

Initial State (simplified circuit)

Step 1 – Close the switch.

When the switch closes:

  • Current begins flowing in the primary inductor, but not in the secondary.

Step 2 – Open the switch.

When the switch opens:

  • An initial high inductive kickback voltage is generated across the primary. (The forward converter removes this high voltage spike by adding a diode and clamp winding across the primary)
  • Current begins flowing in the secondary inductor, but not in the primary.
  • The secondary winding voltage is reflected across the primary.

Two-Transistor Flyback Converter

The very high inductive kickback which occurs when the switch turns OFF places a great deal of strain on the transistor. This can be reduced by using two transistors and a pair of steering diodes. This prevents the voltage across either switch from exceeding the input voltage level.

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