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X-Files... Power supply issue

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2PAC Mafia

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Hello guys,

I received some logic controller units to repair from boilers. All of them with problems on SMPS circuit, all of them solved except one.

On this one I can´t undestand the problem why power supply doesn´t work. I found the PWM IC (VIPer22A) destroyed and main filter capacitor at primary damaged also. After replacing them the unit didn´t work. I check all components in primary and secondary side, all of them correct.

To find the problem I replaced all primary components including transformator by known good components from another unit which worked properly, in fact all components had same values testing them with multimeter but as the power supply was not switching I took that way to solve it.

After replacing all components power supply didn´t switch on!!! I connect my power supplies to secondary side to see behavieur after secondary side, +5V, +12V and -12V, and unit worked properly.

If you check the drawing I made you can see it´s a really simple SMPS, where I didn´t put LC filter and NTC, directly AC on bridge. I have 310Vdc at drain from VIPer22A but it never switches not even for a second.

http://www.restoretronic.com/descargas/Esquema primario fuente de alimentacion.jpg
**broken link removed** dañado en SN0235.jpg

I have no answer about why PWM IC don´t start up with the high voltage there. I also tested the pin number 1 and 2 are connected to main groung at diode bridge.

May be one of you have some good idea...
 
You probably can't put your scope on the VIPer IC unless you have a isolation transformer.
I use a isolation transformer to keep things safer. OR I also have a 0-350V DC supply that will power the switcher. The current limit is important and getting off the power line is better.
You show a 2.2uF cap. At turn on the voltage should slope up to 14.5V where the switcher starts working. If something is wrong the voltage will slope down to 8V then the part turns off. The voltage heads back up to 14.5 and tries again. With out a scope a meter will read about 12 volts average.
If this voltage is below 8V then the part never tries to start. (leaky cap or diode)
 
Hi Ron,

I have an isolation transformer, I can measure on VIPer, no problem. On a good power supply I can see how VIPer switches. On this one I always see 310Vdc. That´s the problem, it never switch at start up so I never have voltage at pin 4 coming from auxiliar winding.
In fact, I connected my power supply (16V) to pin 4 simulating the voltage from auxiliar winding and it didn´t work. Any idea?
 
Try this:
Apply 50Vdc to the AC input. This will give the supply some current limited power.
Apply your 16V to pin 4. This should get the brains up and running.
What is on pin 3?
Pins 6-8 should be switching. (low voltage but working) It they are at 0V then maybe the primary winding is open. OR Maybe the IC is dead.
If the voltage is at the supply voltage then the IC is dead.
There could be a secondary short. (diode or cap) The current will be high for a very short time and the IC will back the duty cycle down to about 1%.
 
Pin 3 is Feedback from secondary. Transformator is checked, I removed and tested it working on a good power supply, L and Q values are the same as from transformator from a good power supply. Capacitors on secondary are replaced, new ones because they were out of range. Diodes are good.

As soon as I don´t see VIPer22A switching with scope, at least for a second or less seems that problem is on primary side. I would say on the IC itself but I put a new one again and it never starts up!!! It´s crazy...
 
Disconnect all power. Remove the loads.
Apply a signal generator across the primary of the transformer. 100khz as many volts as you can get. Look at the secondaries.
You could apply the generator across the highest voltage secondary. Look at the outputs.
I am trying to get a signal on the transformer and see what happens on the secondaries. (output)
 
OK, I´ll test these two options to see what happens and I come back to tell you. Thanks for your time.
 
Hmmm... the schematic doesn't look quite right to me. If it runs ok once you've started it, then I'd suspect the circuit that drops the 14v for the IC from the 350v main capacitor. I'd check round those two 180k resistors, although from the diagram you've drawn I can't see how they can feed the controller IC. Could you have accidentally put a rectifier or zener diode in back-to front when you swapped the components over?
 
I think the schematic is fine.
Inside the IC there is a current source from the Drain of the MOSFET to the power supply. During start up the current source pulls up the power supply capacitor. This current source is only on when the IC is not switching.
Years ago we would have added external resistors to do that job.
 
Bad news, I was doing tests and I saw finally I had some output at auxiliar primary winding to power the IC, but not enough and short time. Doing tests and powering the IC from external supply I disconnect the feedback because I put the IC on protoboard to use different ones. So doing all these things I forgot to disconnect the load!!! So I guess I damaged the rest of the circuit because power supply tried to give maximum...

No problem, I have repaired 10 of them and only this one was giving me a lot of problems. Strange thing was I never saw PWM at drain when I start up so I don´t know how I could see some signal at secondary or in this case at auxiliar primary winding. Probably diode was not good because I tested 2,2uF capacitor and it was OK.
 
Thanks ronsimpson, you're right - I stand corrected. That design's a new one on me...

Sorry you didn't get to the bottom of the problem in the end 2PAC, but nice work on the other 10 units anyway!
 
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