Top227 fails to start up II

Posted by: NickWilliams on 11/19/2012

I am still investigating the non-starting of a TOP227. It only happens intermittently.

However I managed to catch most of the control pin start up on a failure. As can be seen the voltage rises way above the normal 5.8V, up to about 12V. It then falls rapidly below the proper voltage and the process repeats and the PSU starts.

Can anything external cause this? Just looks like a chip fault to me.

Nick

What's all this about Ugg boots?

Comments

Hi,

A copy of the schematic would help in debug. Any load or input line conditions that makes it worse or better?

rgds..

Here is the schematic, its pretty standard. Since posting I have caught several more failures, in some cases the control voltage rose as high as 30V before it sharply fell and the PSU starts. Also sometimes after falling it enters auto restart mode. I am slightly worried about the inclusion of the ferrite bead (multi turn) in series with the main contol capacitor.

Hi,

Not sure yet the root cause of the problem. Can you remove the bead and see if things change? I will ask around to see if there is a possible explanation for such behavoiur.

If possible can you connect a 6V zener to the Control pin and see if the failure to start-up happens.

rgds

Hi Scotty,

Thanks for your reply.

I was looking at the layout yesterday and think I have found something significant. There is a fairly short length of track to the main control capacitor(C24 R29) which is shared with a connection to one of the snubbers (D30 R70) . As this is meant to be a Kelvin connection it does not seem good, and is capable of injecting noise into the control pin.

I tired shorting out the bead yesterday and no failures occured, however on removing the shorts I also could not get failure - so no result. Its an intermittent thing.

Hi,

Did the layout changes help in fixing the problem?

rgds.

Hi,

Also is it possible to send me how the output looks like during the failure to power-up. What are the output voltages of this design and load conditions?

Rgds..

HI Scotty,

I just tried repeating some testing but forgot to use an isolation transformer, so there was a big bang and I have probably destroyed the TOP227.

Before this I was also having no luck in otaining the failure any more. I soldered a wire from the control pin to the far side of the bead to see the effect: no failures occured. However when I removed the wire there were also no failures, and nothing seemed to bring them back. The only thing I could concluse was the heat of the soldering iron on the control pin close the chip body, in some way 'fixed' the problem. Seems improbably, I know.

There are number of outputs, which can be seen on the schematic, basically the 5V is regulated and there are several other outputs, 12V and 5V also taken off. Although the design will support output powers in the region of 100W, only about 25W is taken. The line condition is about 320V - a voltage doubler is used so it is not used at lower input voltages.

Nick