Diagnosing & repairing SMPS

Discussion in 'General Electronics Chat' started by thavinator, Jan 26, 2012.

  1. thavinator

    Thread Starter New Member

    Jul 4, 2011
    20
    3
    I've got the driver from a laser 'light show' unit here I'm trying to repair, and could use some advice. The driver connects directly to 120VAC supply and supplies (presumably--I've only glanced at the secondary side) a constant current to the laser module. Some time ago, the whole unit got taken overseas and was plugged in to a 240VAC circuit. There may or may not have been an appropriate transformer between the 240VAC circuit and the 120VAC device--I've gotten conflicting stories on that count--but for the sake of troubleshooting, I've assumed there wasn't.

    And indeed, the input fuse on the driver board was thoroughly blown. Replacing the fuse revealed that the primary side switcher (On Semi NCP1014) was also blown--there's a tiny little pinhole through the top of the chip.

    But here's where it gets interesting: the datasheet for the NCP1014 indicates it should be good for 700V--so connecting the driver to a 240V supply shouldn't have directly caused the switcher IC to go poof, right? Plus the primary components all seem to be rated for at least 400V--so despite what the manufacturer of the overall unit says, it looks to me like the driver should be good for 240V.

    In any case, I can replace the switcher IC easily enough, but my concern is that the transformer might have been damaged as well. If that's the case, I'll probably have to toss the entire driver since, short of reverse engineering the entire thing, which I don't really have time for, I won't be able to spec a replacement. I'm measuring the DC resistance of the primary (when removed from the PCB) at 1.5Ω--Does that sound like a reasonable value, or low enough to possibly indicate a shorted winding?

    I've drawn up a schematic of the primary side of the supply that I'll try to post tomorrow, but I'd greatly appreciate any thoughts in the meantime.
     
  2. crutschow

    Expert

    Mar 14, 2008
    13,052
    3,244
    You can't usually tell from a resistance measurement of a transformer whether its shorted or not.

    It may be that the high voltage caused the transformer to saturate, generating a current spike that zapped the switcher IC.

    So you might just byte the bullet (pun intended) and install a new NCP1014, and see what happens.

    Good luck. :)
     
  3. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
    507
    Offline switchers are not designed to die with honor. usually, repairing them means replacing so much stuff it makes more sense just to replace them. At least replace all the switching and rectifier devices, any ICs and all the caps.
     
  4. thavinator

    Thread Starter New Member

    Jul 4, 2011
    20
    3
    Yeah, I'd love to just replace the whole board--the trouble is, I don't have enough information on the power supply or the laser diode to be able to identify an equivalent, let alone a direct replacement. None of the markings on the board or the diode have given me ANY info on either one. I have a few queries out there to hopefully help in that realm, but I'm trying to cover all my bases in the meantime. I guess I'll go ahead and order replacements for some of the other components along with the NCP1014, just in case.

    Thanks.
     
  5. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
    718
    Please post high resolution, clear/well focused pictures of the circuit board (both sides)? Avoid flash bounce, outside on a cloudy day is best.

    Click on the paperclip in advanced editor to add as an attachment to your post, no hosting needed.

    Max .JPG size is 300kB, and it may take a few minutes to have the attachment moderated if you have under 10 posts.

    The images are critical so that we can determine if there are problems with the board.

    If you don't have a camera, use a flatbed scanner to get the images if possible.

    Download Irfanview (freeware) which will allow you to "Save For Web" with the R|0T plugin and specify JPG size to be 300kB, it does a good job of keeping the high resolution while getting good compression.

    Do you have an LCR Meter? Checking the inductance and Q of the transformer is a better way to get an idea if it is bad or not.

    Also check for bad Electrolytic caps, bulged, sticking up from board, leaking, etc. Those could have allowed too much ripple to the line to blow up the IC as well.
     
  6. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
    507
    Most off-line switchers are designed around some kind of ASIC the manufacturer had ginned up by some pacific rim maker. It incorporates a power FET, control PWM IC, isolator stuff, etc. That's how they get them small in size. Good luck getting replacement parts, they usually don't sell them.
     
  7. thavinator

    Thread Starter New Member

    Jul 4, 2011
    20
    3
    This driver is all off-the-shelf parts, actually, with the possible exception of the transformer which has no markings, so who knows.

    Anyway, my other inquiries paid off, and I'm getting a new matched diode & driver to replace the ones I've got. It's nice to have a friend who has a "laser guy" :). I'll still probably play with this in my spare time, but I'm going to shelve this project for now so I can get to a couple of other pieces of kit that are waiting for bench time.

    Thanks for the input. I'll be back whenever I get a chance to get back to this.
     
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