whinning of current mode full bridge 350V offline dc power supply

[happy2005]

Hi everyone,
I am designing an offline dc power supply from 3 phase grid. Its output voltage is 350Vdc and output current is max. 2A. I selected full bridge topology and current mode 1846 controller.Output inductor 1.5 mH and output filter capacitor is 10uH.

I have build a prototype. But there are some problems that I would like to ask you as follows.
1) There is whinning at no load. It is disappear at half and full load.
2) Output voltage decrease at full load very much circa %5. I hv connected a parallel capacitor across to the TL431 input nodes, and voltage regulation is better now. I think there is noise in feedback loop.

I would like to ask you that how can I find correct compensation at no load? What can be the problem at no load?

Output voltage is very high (300V dc) and TL431 control voltage is only 2.5V dc. I used only 2 resistor as voltage divider. Is is suitable for high voltage? is there anything that I missed?

Regards,

 

[dick_freebird]

Maybe at no load your duty cycle is so low that you get
into pulse-skipping (which the 18xx series will do if the
error amp voltage / pulse width gets too low) and your
designed switching frequency becomes some audible
subharmonic.

 

[happy2005]

Thank you for reply.
I reduced the output inductor to 400uH, and increased output capacitor to 50uH. After that there is small whinning.
Regards,