How a Coupled Inductor Works in a Flyback Converter

This article provides a concise explanation of how a coupled inductor operates within a flyback converter to regulate power. It covers the fundamental differences between standard transformers and coupled inductors, how magnetic energy is stored during the switch on-time, and the exact mechanism used to transfer that energy across a galvanic isolation barrier to the output load during the switch off-time.

The Role of the Coupled Inductor

In flyback converter electronics, the magnetic component is often called a "flyback transformer," but it is technically a coupled inductor. Unlike a true transformer, which transfers energy simultaneously from the primary to the secondary winding, a coupled inductor operates on a store-and-release cycle. It shares a single magnetic core between two or more electrically isolated windings, allowing energy to cross a physical isolation barrier solely through magnetic fields.

Energy Storage During the Switch On-Time

The energy storage phase begins when the primary-side switching transistor (typically a MOSFET) turns on.

  1. Current Flow: Closing the switch connects the primary winding directly across the input voltage source. Current begins to flow through the primary winding, ramping up linearly over time.
  2. Magnetic Flux Generation: According to Ampere’s Law, this rising current generates an expanding magnetic field, or magnetic flux, within the core of the coupled inductor.
  3. Blocking the Secondary Winding: The primary and secondary windings are wound with opposite polarities, indicated by dots on a schematic. During the on-time, the voltage induced in the secondary winding reverses the bias of the output rectifier diode. Because this diode acts as an open switch, no current can flow in the secondary circuit.
  4. Storing the Energy in the Air Gap: With the secondary circuit blocked, the coupled inductor acts like a simple single-winding inductor. All the input energy is stored in the magnetic field. Because standard ferrite cores saturate quickly under direct current, flyback cores include a physical "air gap." This gap increases the magnetic reluctance of the core, allowing it to store a substantial amount of magnetic energy without saturating.

Energy Transfer Across the Isolation Barrier (Switch Off-Time)

The energy transfer phase occurs the moment the primary-side switch turns off, breaking the primary circuit.

  1. Collapsing Magnetic Field: When the primary current abruptly stops, the magnetic field within the core begins to collapse rapidly.
  2. Voltage Reversal (Flyback Action): According to Lenz’s Law, an inductor resists changes in current. To maintain the magnetic flux, the collapsing field induces a voltage of opposite polarity across both windings. This sudden reversal of voltage is the "flyback" action.
  3. Crossing the Barrier: Because the windings share the same magnetic core, the collapsing magnetic field created by the primary winding cuts across the turns of the secondary winding. This transfers the energy across the galvanic isolation barrier purely via magnetism, with no physical electrical connection required.
  4. Current Delivery to the Load: The reversed polarity on the secondary winding now forward-biases the output diode. The diode conducts, allowing the stored magnetic energy to convert back into electrical current. This current ramps down as it flows into the output capacitor and powers the load.

Once the switch turns back on, the diode becomes reverse-biased again, the output capacitor powers the load temporarily, and the coupled inductor restarts its energy storage cycle.