A fixed-duty flyback that pins its source's voltage
Symptom
Section titled “Symptom”A flyback sized to present a high-impedance source’s resistance takes the expected share of the source’s power at small amplitudes. Above some amplitude its input voltage stops following the source, and it takes less than the source’s available power.
In DCM a fixed-duty-ratio flyback presents a resistance at its input (loss-free resistor) [Erickson & Maksimović 2020, Ch. 15 (AC and DC Equivalent Circuit Modeling of the Discontinuous Conduction Mode)]. It stays in DCM only below the critical input voltage. A source strong enough to raise the input to it drives the flyback into CCM, where, with the output fixed, the conversion ratio allows only that one input voltage [Erickson & Maksimović 2020, Ch. 6 (Converter Circuits)]. The input stays there: the flyback has become a constant-voltage sink, and its share of the source’s power depends on the source’s amplitude (constant-voltage extraction).
In the example, a source of 1 kV behind 10 kΩ can give 25 W to a matched resistance. The flyback’s input is held at its critical voltage, 220 V, where it takes a share of 0.686 of that power.
How to confirm
Section titled “How to confirm”Plot the input voltage against the source’s amplitude: it follows the amplitude in DCM and flattens at the critical voltage. Or watch the magnetizing current: in CCM it no longer falls to zero in each period. The simulator’s current-limited source shows the input settling at the critical voltage:
▶ Open in the simulator: Flyback
Preset from the example flyback-source
Choose the duty ratio and the turns ratio so that the critical voltage lies above the highest input voltage the source can drive (bus voltage of a loss-free resistor), then rate the switch for the voltage that follows (switch voltage). A smaller turns ratio raises the critical voltage. A different duty ratio also changes the input resistance (input resistance), so re-size the magnetizing inductance with it (magnetizing inductance for a match).
References
Section titled “References”- [Erickson & Maksimović 2020, Ch. 15 (AC and DC Equivalent Circuit Modeling of the Discontinuous Conduction Mode)]: the loss-free resistor.
- [Erickson & Maksimović 2020, Ch. 6 (Converter Circuits)]: the flyback’s CCM conversion ratio.
- The DCM flyback as a loss-free resistor and the source matcher.
The numbers in the examples, presets and exercises on this site are synthetic: round teaching values, not data from any real design.