Results 1 to 8 of 8

Thread: APFC problem

  1. #1
    Join Date
    Jan 2010
    Location
    Greece
    Posts
    1,528
    Thanks Thanks Given 
    185
    Thanks Thanks Received 
    7
    Thanked in
    4 Posts

    Default APFC problem

    I have a dead PSU that blows the PFC caps. I measured voltage in APFC right before the caps. At standby is 325VDC but once I switch on the PSU it goes over 450-500VDC (this is why it broke the first caps). Any ideas on why this is happening. I am thinking of the PFC's boost diode but since my time is limited this period I would like to hear some ideas, before I start de-soldering components.

    I measured the boost diode (without desoldering it) and it looks normal.

  2. #2
    Join Date
    Nov 2010
    Location
    Serbia
    Posts
    2,245
    Thanks Thanks Given 
    129
    Thanks Thanks Received 
    12
    Thanked in
    6 Posts

    Default

    I'd say it's more likely that the PFC PWM controller is missing the timing, so that PFC MOSFETs are held on a higher duty cycle than normal. Which PFC controller does your PSU use? How is it biased? Small caps, resistors? Follow it's sense/return route, and see if something's wrong there.
    Careful what you wish for... You just might get it.

  3. #3
    Join Date
    Jan 2010
    Location
    Greece
    Posts
    1,528
    Thanks Thanks Given 
    185
    Thanks Thanks Received 
    7
    Thanked in
    4 Posts

    Default

    it is a CM6802. The strange thing is that I just removed the hold-up caps to take some shots and didn't desolder any other component. Once I installed again back the caps the PSU worked flawlessly outside its case.

    Once I installed again into the casing and switch it on one of the primary caps almost exploded and both of them got really hot. And we are talking about Rubycon caps!

    Then I installed some new caps and measured the APFC DC bus and find out that it was way over the spec. I assumed the boost diode but found it ok. The APFC coil there is no way to break! It is a really strange prob this and I don't want to lose another pair of fine caps for this unit.

  4. #4
    Join Date
    Nov 2010
    Location
    Serbia
    Posts
    2,245
    Thanks Thanks Given 
    129
    Thanks Thanks Received 
    12
    Thanked in
    6 Posts

    Default

    That would indicate a grounding problem, wouldn't it?

    Can you measure the potential of the PSU casing while it's connected, with respect to a neutral line inside the PSU? Also, measure it against a grounding point of your choice that has no connection to the PSU.

    Perhaps an EMI filter component is to blame? Or maybe an unwanted solder joint somewhere?
    Careful what you wish for... You just might get it.

  5. #5
    Join Date
    Jul 2009
    Location
    Germany
    Posts
    3,641
    Thanks Thanks Given 
    6
    Thanks Thanks Received 
    66
    Thanked in
    60 Posts

    Default

    Quote Originally Posted by crmaris View Post
    Once I installed again back the caps the PSU worked flawlessly outside its case.

    Once I installed again into the casing and switch it on one of the primary caps almost exploded and both of them got really hot. And we are talking about Rubycon caps!
    What's the difference between the operation outside the case and inside the case?

    Are there any shorts?? Any room?

    Have you checked the earth??

  6. #6
    Join Date
    Jan 2010
    Location
    Greece
    Posts
    1,528
    Thanks Thanks Given 
    185
    Thanks Thanks Received 
    7
    Thanked in
    4 Posts

    Default

    forgot to mention. After I changed the caps the problem persists even outside the case.

    Nope there are no shorts of any kind. I searched the entire PCB. Everything is as it should be and no visual damage is shown anywhere.

    This is a very strange problem..

  7. #7
    Join Date
    Nov 2010
    Location
    Serbia
    Posts
    2,245
    Thanks Thanks Given 
    129
    Thanks Thanks Received 
    12
    Thanked in
    6 Posts

    Default

    Perhaps you could try adding a 1 MOhm (5%, 1-5W) resistor, connecting pins 1 and 16 on the CM6802. It will switch it to "non-green" mode of operation, limiting it's output duty cycle. I'm curious to see if this will make a significant difference...
    Careful what you wish for... You just might get it.

  8. #8
    Join Date
    Jan 2010
    Location
    Greece
    Posts
    1,528
    Thanks Thanks Given 
    185
    Thanks Thanks Received 
    7
    Thanked in
    4 Posts

    Default

    Thanks! I will try it once I find some free time. I can't understand what the hell happened to the CM6802 since I didn't mess with it at all.

Similar Threads

  1. How Does APFC Work?
    By Magic Smoke in forum Electronic Theory and Principle
    Replies: 9
    Last Post: 12-30-2018, 10:59 PM
  2. APFC
    By ashiekh in forum PC Power Supply Discussion
    Replies: 0
    Last Post: 09-08-2018, 09:49 PM
  3. UPS and aPFC PSU compatibility?
    By MikeN in forum PC Power Supply Discussion
    Replies: 5
    Last Post: 11-17-2015, 07:06 PM
  4. APFC power supplies and UPS's
    By jamason56 in forum PC Power Supply Discussion
    Replies: 2
    Last Post: 08-28-2009, 02:39 PM
  5. APFC and UPS
    By Killy in forum PC Power Supply Discussion
    Replies: 0
    Last Post: 03-11-2008, 11:10 PM

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •