MP4001 LED string dimmable

I made a board based on the MP4001 circuit datasheet. This circuit is used to supply a LED array with a current of 3A. The circuit works well for voltages below 80Vac. The circuit stops working as soon as the voltage exceeds 80Vdc (voltage taken after the diode bridge). The MP4001 circuit’s Vcc voltage collapses. However, the circuit’s IN input can withstand 500V. Why does the circuit break?
The circuit consists of an isolation transformer followed by an autotransformer to obtain a voltage of 110Vac at the circuit input.

Hi @marcel.souliers

If you’re comfortable sharing your schematic here, please do or you can reach out using MPS NOW Technical Support form: MPS NOW Technical Support - Contact and we can help you.

Thank you