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LT3753 with Vin 400Vin, auxiliary winding supply

Category: Hardware
Product Number: LT3753

Hello,

I'd like to design a forward converter with specs: 220-425Vin, 28Vout, 150W.

To save space, I woudn't use LT3752-1 with integrated auxiliary flyback, but LT3753, I would supply it with the "classical" auxiliary winding on the main transformer (I don't use synchronous rectification, so I need only one auxiliary supply voltage).

The problem is the auxiliary voltage (INTVCC): it doesn't rise above Vin when the forward converter is not loaded (like the image below where the load is 10k), the kick-start circuit supplies the IC with big power dissipation.

Instead, when the load is present, the INTVCC rises at about 13V and the kick-start circuit is turned-off.

Con someone suggest one solution?