Random blow up of smps at startup

[mord]

I repaired a power supply (laptop adaptor) and it was working fine, next day i tried testing and it blew even after working fine. it utilizes UC3842 and a FET. lately i also repaired a dvd player power and it was okay. during testing after a short time of power off, it blew up while trying to power it up again. What could be causing random blowup at power up? i have experienced a lot of this problem.

 

[pradeep.g.belchada]

check your mains supply voltage. if its normal then check the mosfet mounting heatsink. their may be short in smps trafo.

 

[avoidme]

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[mord]

thanks. mains supply is ok (220).can shortd trafo cause blowup even after working for a while? the problem is always at startup.

 

[pradeep.g.belchada]

if their is a short ckt in primary side then mosfet will blow. also check the feedback voltage to the pwm ic

 

[BradtheRad]

Uncharged capacitors can draw a lot of current in the first milliseconds of power-up.

If the Farad value is excessive then the cap will draw large current for a longer time, and may overload something.

 

[mord]

The mains cap is 47uf. besides if the capacitance is much, it can only destroy the fuse not the FET and it driver. Maybe my problem is dv/dt related, thought these ICs are designed to tackle that.

 

[schmitt trigger]

First question, what blew up the first time?
2nd question, what blew up the 2nd time?

In other words, are the failures related?

 

[pradeep.g.belchada]

my question is every smps you repair works for some time & fails or only one smps had this problem?

 

[mord]

some of them(smps) had this problem. The first time just the fuse, second time the FET. And in another case, the IC (FET inbuilt) blew up, changing the IC, it blew again after pluging in the next day. its ICE3BR... from infenion.

 

[Venkadesh_M]

some of them(smps) had this problem. The first time just the fuse, second time the FET. And in another case, the IC (FET inbuilt) blew up, changing the IC, it blew again after pluging in the next day. its ICE3BR... from infenion.

After posting your schematic only any one can answer it...............

 

[mord]

hello everybody, i discovered something, i dont usually scrape off the flux after soldering which can form little capacitance, it might be nothing, but.between drain to gate can be catastrophic. I have corrected that and it(the power supply) has been ok since den.

 

[schmitt trigger]

Glad that you found it!
Flux dissolves the metal oxides, and the remaining residue is slightly conductive.
I always fully clean my boards with Isopropil Alcohol