An open-loop flyback with its load removed

Symptom

A flyback that runs at a fixed duty ratio, without feedback, drives its output far above its normal value when the load is disconnected, during a test or through a loose connector. A part on the output side fails, or the primary clamp overheats.

Why

Each switching period stores energy in the magnetizing inductance and delivers it to the output. Without a load the output capacitor keeps that energy, and the output voltage rises from period to period. It stops rising only when the output’s voltage, reflected to the primary, reaches the clamp’s voltage: from then on the clamp takes the energy instead (open-load output voltage) [Erickson & Maksimović 2020, Ch. 6 (Converter Circuits)]. The clamp then dissipates all of the input power. Take a TVS’s clamping voltage at the peak primary current, and at the temperature that power gives it: its breakdown voltage rises with temperature, and the ceiling with it [STMicroelectronics 2022]. An RCD clamp sets no fixed ceiling. Its voltage settles where its resistor dissipates the power it takes, the power balance behind the RCD clamp voltage [Kollman 2024], which in normal operation is the leakage energy alone. Without a load that power is the whole input power, so the clamp’s, the output’s and the switch’s voltages all rise far above their loaded values.

In the clamp tool’s example, the output of 12 V rises to 16 V without its load, before the TVS warms.

How to confirm

Compute the ceiling from the clamp’s voltage, the turns ratio and the diode’s drop (open-load output voltage), and compare it with the ratings of every part on the output side. Remove the load on the bench only if every one of those parts is rated above the ceiling, the switch is rated above the input voltage plus the clamp’s voltage at the TVS’s temperature (switch voltage while the clamp conducts), and the primary clamp is a TVS or a Zener rated for the whole input power; then watch the output rise towards the ceiling. A lower input voltage lowers the power the clamp takes, but the ceiling only a little, through the peak current and the TVS’s temperature.

Fix

Keep the output from being left without a load: regulate it with feedback, keep a minimum load the converter cannot lose, or clamp the output below the ratings of what it feeds. If the primary clamp must survive an open load, rate it for the whole input power; an RCD clamp needs a fixed-voltage clamp beside it, or an input power limited at no load, as the clamp check warns.

References

[Erickson & Maksimović 2020, Ch. 6 (Converter Circuits)]: the flyback and its transformer model, from which the ceiling follows.
[STMicroelectronics 2022]: a TVS’s clamping voltage at the actual current, and its breakdown voltage rising with temperature.
[Kollman 2024]: the primary clamp, and an RCD clamp’s voltage against its losses.
Snubbers and clamps and the clamp check, which gives the ceiling.

The numbers in the examples, presets and exercises on this site are synthetic: round teaching values, not data from any real design.