LNK305 open feedback protection does not work

Posted by: Hans Günter on 01/16/2014

Hello,

in my attached design, if the feedback loop ist open (for example R3 is missing) the output voltage rises inadmissibly.

What could be the reason? How can I get the open loop protection to work?

Many thanks!

Comments

I do not know why C10 is used in your circuit? Could you open C10 first？ And PLS try a fast diode for D2. Hopefully your output is stable with these tow modifications.

PLS let me know if they work or not work.

Regards

Thanks for the hints.

C10 is used for softstart.

I think the open loop protections works, but not when the Drain voltage is below 70V.

When the drain voltage ramps up, is it correct that the autostart doesn't work until 70V drain voltage is reached? (in case of open loop)

And is it correct that the Source voltage = Drain voltage, when the Drain voltage ramps up?

C10 and D2 doesn't change anything.

Best regards

Datasheet claims that the part should be designed with at least 70Vdc at input, so the internal logic and auto restart may not work for input below 70V.

For the question about source voltage during start up, I think the part you were tested already have some problem. For any power supply, the output need to be controlled even during start up. If the source voltage is same as the drain voltage, the output can not be controlled.

regards