What maybe causing this Chopper to short ?

[SM-Piyes90]

Hello everyone here .

Am right now trying to repair a switching style power supply , this one converts low input current to pretty much 160 amps to be mainly used as a MMA welder .

Anyways i checked it step by step , started with the primary side , the primary caps got 300 Vdc on them , now arriving to the chopper circuit witch consists of 2 power IGBTs , that are making the input signal chopped at a very high frequency with the command of a pulse width modulation chip " UC3842 " .

So i checked the power transistors with a continuity tester , and they are both shorted , also found 2 shorted A3120 optoisolators , four 18 v zeners .

I then changed them all with new parts , in the hope to save this unit but again all the parts that i' ve replaced have again fried .

I guess something else is causing them to burn , i hope i get some ideas from people who encountered similar issue .

Here is a Service manual of the SMPS welder :

[selldoor]

Re: What maybe causing this Chopper to short ?

perhaps pictures of the board would help , take it you have ~220vac mains.
Perhaps use the dim lamp trick - a 75watt filament bulb in line wth the + mains
saves blowing up components

[SM-Piyes90]

Re: What maybe causing this Chopper to short ?

perhaps pictures of the board would help , take it you have ~220vac mains.
Perhaps use the dim lamp trick - a 75watt filament bulb in line wth the + mains
saves blowing up components

Hi thanks for the comment , ireally don't know about the light bulb trick , but i got this in mind

You can take a look at the PDF service manual , it contains schematic and picture of the PCB .

[sam_sam_sam]

Re: What maybe causing this Chopper to short ?

That very good manual most of them are not very good

[budm]

Re: What maybe causing this Chopper to short ?

One thing can be that the U3845 (as shown in page 9) DR+ has the output stuck HI instead of turning those two Optos on and off.

[ben7]

Re: What maybe causing this Chopper to short ?

Check the secondary side rectifiers?

[SM-Piyes90]

Re: What maybe causing this Chopper to short ?

That very good manual most of them are not very good

Thanks ths one is for professional servicing.

[SM-Piyes90]

Re: What maybe causing this Chopper to short ?

One thing can be that the U3845 (as shown in page 9) DR+ has the output stuck HI instead of turning those two Optos on and off.

I have changed that too , but still the boom occurs

[SM-Piyes90]

Re: What maybe causing this Chopper to short ?

Check the secondary side rectifiers?

ALL secondary side rectifiers were good and still good after chopper short .

I see that you guys think in most of the cases capacitors are bad , but how about the transformer how can i check if its good or bad ???

also how about ESR check for small value caps like plastic non polarized ones ??

[budm]

Re: What maybe causing this Chopper to short ?

You need the ringer to test that power transformer, it may have shorted turn. The DCR of the transformer will be very low (<1 Ohm).

[SM-Piyes90]

Re: What maybe causing this Chopper to short ?

One thing can be that the U3845 (as shown in page 9) DR+ has the output stuck HI instead of turning those two Optos on and off.

Hi again am back topic .

Thanks BUDM for the assistance there also all the other members , thanks for the suggestions .

I have a few questions about how the PWM ic performs .
- Do you know an adequate method to check the output signal of that PWM IC ?
- And what is the nature of the signal at the IC output , is is square or sine ???
- Also what is the minimum frequency that needs to be applied to the IC in order to monitor the output .

[kc8adu]

Re: What maybe causing this Chopper to short ?

where do you buy your parts?
seen many repeat failures where counterfiet parts were the issue.

[senz_90]

Re: What maybe causing this Chopper to short ?

squarewave signal on the output pin of 3842 IC

[SM-Piyes90]

Re: What maybe causing this Chopper to short ?

where do you buy your parts?
seen many repeat failures where counterfiet parts were the issue.

I buy them from a local electronics shop , but the transistor i used is a FAIRCHILD one took from another device .

After analysing the card more i found two bad plastic capacitors ( 0.1 uF 250 V-AC) that are directly connected in paralell to input mains , i was measuring only 3 nF on them , the main filter cap is a 68 uF , 400 V , measuring 56 uF and having a dissipation factor of 1.05 (DF) , these measurements were took with an LCR meter .

Any ideas ? could this be causing the chopper transistor to short ?

squarewave signal on the output pin of 3842 IC

Thanks

[SM-Piyes90]

Re: What maybe causing this Chopper to short ?

UPDATE :

I have changed the two main's 0.1 uF capacitors , and the unit got working again , but i get small problem on the two output sections :

First section is 5 v and 3.3 v given via voltage regulators, and they are outputing the right voltages .
Second sections is outputing 5.8 , 12 , 14 , 38 V dc , but i get higher results , for example the 12 and 14 v outputs gives me 22 v , 38 v output gives 65 .

Any one familiar with this scenario ??

[Agent24]

Re: What maybe causing this Chopper to short ?

I can't see 14v or 38v rails on the schematic - do those rails have a feedback circuit? Perhaps there is something wrong with it.

[budm]

Re: What maybe causing this Chopper to short ?

" i found two bad plastic capacitors ( 0.1 uF 250 V-AC)" What are the caps designator as shown in the diagram, I see C1 (1uF 275V X type) which is across the line. CI is the line filter cap, the unit will still function without it but it will inject the noise into the AC line.
"the main filter cap is a 68 uF , 400 V" the schematic shows two (C3, C4) 680uF 400VDC caps.

[SM-Piyes90]

Re: What maybe causing this Chopper to short ?

Sorry if i have mislead you , but am talking about another card witch i use for tryouts , It has the same problem as the one i mentioned earlier ; So when i replace the switching transistor , it shorts again.

what i did next is checked the two interference suppression capacitors , and changed the main filter cap with another one but i doubt it was bad , what am sure of is that those two 0.1 uf suppressor caps were giving only 3 nF and after re[placing them the unit started to work , but i only get two correct outputs : 5v & 3.3 v , the other ( 12,14,5.8,38) voltages give respectively ( 22,22,7.6,64) , is there a possibility those outputs were meant to be regulated on another terminal ?? this card BTW belongs to an old satellite receiver .


Here is the second card :

[Agent24]

Re: What maybe causing this Chopper to short ?

Probably better if you start a new thread for this other PSU. But have you checked the output capacitors? If they are dead, bad regulation problems can occur.

[SM-Piyes90]

Re: What maybe causing this Chopper to short ?

Probably better if you start a new thread for this other PSU. But have you checked the output capacitors? If they are dead, bad regulation problems can occur.

I need ESR meter for that .

I guess there is no need to start another thread .

Thread can be closed .