Problem with L6566B in flyback power supply

MIInge_8 · ‎2025-10-16

Greetings.

I am building a 24V, 240W power supply with the help of the ST eDesign Suite website. The problem I have is that before starting, it goes into a blinking mode. To start it, I have to measure with a multimeter on the ZCD pin. Then, suddenly, it enters the steady state and outputs a continuous voltage value. Before this, the Vcc voltage has a triangular shape in the power supply, reaching the minimum supply value of 10.4V and 12.5V.

Would anyone know what this is due to and how I can fix it?

I was wondering if it would be possible to obtain the Spice model to simulate the component.

Attached is the link to view the power supply design in eDesign Suit: L6566B QR FLYBACK AC/DC Converter | eDesignSuite | STMicroelectronics

I have also used two different transformers in case the problem was this. I am attaching them as well.

Greetings and thank you very much.

Peter BENSCH · ‎2025-10-17

Welcome @MIInge_8, to the community!

The Turn-On threshold of the L6566B is min 13V, max 15V. If you want to achieve a safe start, you should therefore ensure that the voltage reaches at least 15V.

Regards
/Peter

In order to give better visibility on the answered topics, please click on Accept as Solution on the reply which solved your issue or answered your question.

MIInge_8 · ‎2025-11-14

Hello Peter,

I’m sorry I wasn’t able to get back to you sooner.

I have been testing other transformers to obtain 15 V at the Vcc input, but the voltage at this node continues to oscillate in a triangular pattern between 15 V and 10.4 V at a frequency of about 1 Hz. I also tested lowering the Rsense resistor in case the controller was hitting an overcurrent peak, but the behavior remains the same.

Do you have any other ideas about what might be causing this?

Peter BENSCH · ‎2025-11-14

As I already wrote - if the necessary value for VCC is not reached, the L6566B will constantly try to restart. This could also have other reasons, for example that the driving of the MOSFET is insufficient or the windings are reversed.

Without further oscilloscope measurements of the voltages at pin GD, the gate, the transformer winding from which VCC is derived, or the output windding, we cannot proceed here.

Regards
/Peter

In order to give better visibility on the answered topics, please click on Accept as Solution on the reply which solved your issue or answered your question.