SMPS troubleshooting

Discussion in 'The Projects Forum' started by blowhard7, Dec 4, 2008.

  1. blowhard7

    Thread Starter New Member

    Dec 4, 2008
    3
    0
    I'm relatively new at electronics repair with little experience, and have a difficult problem. Posted at a couple other sites and have not had much help.

    I have an Akia 200watt SMPS that I have partially repaired. The supply powers a 27 inch lcd tv display. Originally it had 5v standby power only and would not turn on. Almost all of the low voltage output capacitors were domed and leaking. I replaced all of the output caps with new low esr, high frequency caps of the same voltage and capacitance. Both main filter caps were replaced also as a precaution.

    The display now functions properly. 5v, 12v and 24v outputs appear good.

    Unfortunately, the supply has a noticeable buzz coming from both PQ3220-1000C transformers when turned on. Annoying to the point that the set is not usable. I do not know if the supply buzzed prior to the cap failure since the unit was given to me dead.

    Perhaps the bad caps caused damage to other components?? I see no other visible damage. I have checked almost all other components in circuit (to the best of my ability) with a digital VOM and cannot find any bad components.

    Suggestions are appreciated. Not sure what to look for next. I'll attach a picture of the supply. Schematic is avialable here: http://www.turuta.md/PSU/LCT-2016.pdf
     
  2. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    Check if there are loose parts on the transformers which cause the annoying sound because they vibrate due to interaction with the transformers magnetic field.
     
  3. IT_Guru

    Member

    Nov 20, 2008
    17
    0
    Ahhhhhhh.... Nothing like a beautiful schematic to get the blood going ;-)

    OK... 4 things can cause this 'buzzing' in an SMPS

    It isn't oscillating fast enough, so although the DC out V appear OK, they'll buckle under mild loading. This can be caused by a bad diode in the bridge, leaky capacitors in the osc or feedback sections, bad optocouplers, bad protection diodes (D2, D6)on the MOSFETS (or other out of spec timing or control cktry. In 90% of cases where I had buzzing or vibration after repair, I'd replace these and the problem would go away.

    2. Loose parts or de-laminated cores, chokes or coils (see above)

    3. Bad AC to DC filter caps on the input side - Hum or buzz may be a resonant freq of the 50/60hz you are running on, or superimposed on a good but 'fuzzy' DC Input voltage (typically 330-380vdc) Put a DVM on ACV and measure it. My bet is you have 10-20vac on the DC and those transistions are causing problems

    4. Bad Hi AMP diodes and/or filter capacitors or a feedback capacitor on the output side. Use the DVM and check the outputs for superimposed ACV

    Hope that helps
     
  4. John Luciani

    Active Member

    Apr 3, 2007
    477
    0
    Unless you have reviewed the design it is not a good to replace the caps with
    different part numbers. A number of SMPS designs require a minimum ESR cap
    for stability (especially the older and lower cost ones). You need to look at manufacturer
    specifications for the controller.

    The buzz is caused by the magnetostriction in the transformer. The transformer expands
    and contracts as the magnetic field changes. Unfortunately you don't if it had buzzed
    before. If it buzzed before I would change the transformer. It if didn't buzz before
    I would change the caps back to original part numbers and see if the buzz goes away.

    Since the supply is working (other than the buzz) I wouldn't suspect the caps caused other problems.

    (* jcl *)
     
  5. blowhard7

    Thread Starter New Member

    Dec 4, 2008
    3
    0
    Well, unfortunately I don't see any appreciable AC on any of the low votage 5,12 and 24VDC outputs. 125 millivolts DC was the highest reading on any of them.

    Bridge outputs 109.6VAC/52.5VDC. Voltage across the two main filter caps shows 381VDC no AC. Input to the two buzzing PQ3220-1000C transformers is 32.3VAC/380millivolt DC.

    Poking and proding the transformers, I don't see anything moving and does not quiet them down at all. It's a substantial amount of buzz. Not just a little hum.

    I had some correspondence with a repair facility that claimed they had repaired close to 50 of these same supplies and had never run into a buzzing problem after recapping. These supplies have a history of bad output caps. He said he always uses Nichicon high freg/low esr caps. Tends to make me believe it's not an ESR problem??

    Interesting though, I recapped another one of these same supplies about a year ago with cheap no name off the shelf caps. It buzzed too, output voltages again good. Ended up trashing the unit because the display was bad.
     
  6. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    I have never heart of that, its an interesting concept.
     
  7. IT_Guru

    Member

    Nov 20, 2008
    17
    0
    Here's an idea... Visit a Chinese or Japanese take-away... Grab an extra chopstick and a paper cup on your way out. When you get home (not in car please) Make a tiny (1/4") slit with a sharp knife in the bottom center of the cup and poke the chopstik thru it about 1/2" and hot glue in place (not permanent)

    Power up the SMPS, touch the free end of the chopstick to various parts of the board and put your ear in the cup (drinking end) to 'zero-in' on the fault.

    I chose chopstick because it is Non-conductive (unless wet), cheap, stiff enough that U can push hard, small size to get into and back out of tight places.

    I'm still betting that the problem lies in the feedback (Opto-Coupler) area as when your old caps died (dried up) and let AC in, it toasted the LED or Phototrans in there ;-)

    Another poss. is that one or more components are no longer bonded to the boadr (Silicone or Hot glue) and may need a cardboard or PCB scrap 'shim' to quiet down.

    Good luck!

    Replace it and see
     
    Last edited: Jan 11, 2009
  8. AchMED

    Active Member

    Aug 5, 2008
    41
    0
    From the l5991 controller datasheet.

    Code ( (Unknown Language)):
    1. STANDBY FUNCTION
    2. The standby function, optimized for flyback topology,
    3. automatically detects a light load condition
    4. for the converter and decreases the oscillator frequency
    5. on that occurrence. The normal oscillation
    6. frequency is automatically resumed when the output
    7. load builds up and exceeds a defined threshold.
    If you are just testing this with no load or a load below when the converter is in standby this may be the source of your noise. The oscillator frequency maybe low enough to be audible.

    I remember hearing this when building an SMPS it turned out to be my poorly mounted Torrid (prototype).
     
  9. IT_Guru

    Member

    Nov 20, 2008
    17
    0
    Another thing I just recalled... I used to repair these SMPS (Astec, others..) for the government and we HAD to use replacement filter Lyticaps of the same 'physical' dimension (and value), although a wide variey of them 'fit' in the mounting holes and would work, b/c the short fat ones would ring and the high-frequency whine drove the users nuts. Odd, I agree.. but tall and skinny worked better and were much quieter. Had to do with the proximity of metal-cased Lyticaps to the Coils and Xfmrs ;-)

    Don't forget to LOAD the SMPS +5 and +12v outputs whenever testing it!
     
  10. z1k

    New Member

    Mar 23, 2009
    5
    0

    I have the same problem and trying to figure it out. HAve you resolved this?
    Thanks
     
  11. Peter_England

    Member

    Jun 29, 2013
    73
    1
    I need help with 300 watt atx smps.. The problem is when starting this smps with load or without load with shorting green and black wires it stay on for only 5 to 10 minutes and shutdown itself automatically but when it is in on condition it give proper outputs nothing is burnt or giving physical sign of failure please help in this matter.
     
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