TNY 279 PN

Posted by: Murat SEZER on 01/25/2010

Hi,

I designed a power supply with PFC with tny 279pn. there are five 15V 150mA and one 5V 1A outputs. But my design does not work.when I checked with osiloscope. I saw that It is always at auto restart mode. I checked BP/M pin. The voltage is 6V and EN/UV pin voltage is 2.2V. from here ı understood that there is an open loop at feedback. but when ı checked ı found nothing. So would you help me to find where the problem is??

thanks.

best regards.

Murat Sezer

Comments

Hello Murat,

Auto-restart mode in your case is a sure hint for something really bad with your design. Most of the time the transformer windings are wrong; also, check the feedback loop. Make sure you have the right polarity for the optocoupler for example.

Cheers,
PI_Crusher

Hello Crusher,

ı checked feedback circuit again. there is no problem. Previously ı prepared an almost similar design which is working without any problem. I compared the waveforms with it. As you said the problem is coming from transformer windings.
ı winded 2 times acording to PI expert desing program. but ı couldnt solve it.
ı attached my design. ı wind transformer according to PI expert design program. so would you check why there is a problem at the windings?

thanks

Murat SEZER

Hello Murat,
There are two reasons this circuit could go into auto restart. One condition is because output (or transformer) short circuit or overload. You can identify this one looking at the drain current, check for the normal current slope and peak current to be under maximum current. The second auto-restart condition could appear because of input voltage drops under minimum allowed (see page 4 from http://www.powerint.com/sites/default/files/product-docs/tny274-280.pdf). Just make sure your input voltage is stable.

Cheers,
PI_Crusher

hello Crusher,

ı found the problem. ı winded transformer again but this time ı changed number of turns. ı do not understand why previous transformer did not work. now my design is working. but there is one more problem. when ı connected a load to one of the outputs, especially 15V, it decreased to 10V. what should I do??

thanks

Murat SEZER

Hello Murat,

Your problem now is cross regulation. Check the multiple examples you have here:
http://www.powerint.com/en/design-support/reference-designs/design-examples
Also, search for cross regulation in this forum and select the best match for your case. Try coupled output filter inductor, resistive (also with zenner) preloads, and combined feedback.

Cheers,
Adi