Top227 fails to start up

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NickWilliams
Top227 fails to start up

I have a problem with a TOP227 which is behaving strangely in a piece of medical equipment.

Hi All, 

After being life tested for a considerable period with no problems the unit was left switched off for about a month. When it was then switched on the PSU failed to start. The supply rail was measured as 320V. The TOPSWITCH was changed and the unit operated normally. To investigate this I fitted the failed device into another power supply, where it started up and seemed to behave without fault.As it is important to understand any failures I fitted the device back into the original equipment; where it again operates correctly. The rise on the control pin appears normal, so I can see no cause for the original failure.I wondered if anyone had any ideas on this? The capacitor on the control pin is a 47u tantalum, rated at 16V.

NickWilliams
Futher info

I caught it in the non-starting condition and found a high voltage on the control pin, in the region of 30V. I was probing with a meter and the PSU started up, and returned to normal operation.

PI - Traveler
Failure to Start-Up

It sounds more like a platform related failure and not an IC related failure because the IC is continuing to work but it has problems with the life-tested platform.

 

A few things come to mind:

Life cycle testing is going to stress your electrolytic caps heavily.  If there are bias winding components with damaged electrolytic caps, this could cause the device to stop operating correctly.

 

You mentioned that removing the IC and then resoldering it back to the life-tested platform allowed the power supply to function again.  It's possible you might have had an IC pin with a cold solder joint.  The repeated heat cycling during life testing could have worsened the bad solder joint.

 

If you were doing any shake/shock/vibe during your life testing, this could have also damaged some solder joints.

 

You also mentioned that you're seeing 30V or so on the control pin.  This me thinking that something in your bias circuitry didn't make it all the way through life-cycle testing.   A bad capacitor in your bias circuitry could cause something like this.

 

 

As a further aside, when you say that the power supply was failing to start up, was it entering auto-restart or was it not switching at all?

 

Regards,

The Traveler