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Switched Mode Power Supply/Switching Power Supply (SMPS)

Discussion in 'Power Electronics' started by H2814D, Sep 21, 2018.

  1. H2814D

    H2814D

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    Nov 4, 2017
    Hi all. I have a question about a power supply. Input is 120vac. There are two outputs. One is a four wire 12vdc (two pos, two neg, hooked to a single pos and single neg output on the board) and the other is 52vdc (one pos and one neg). This thing runs a surveillance system (cameras and a DV recorder).

    My question is this, should both outputs (12, 52) always have the rated voltage when the AC voltage is supplied to the unit, or is it possible one of them (the 52) would not show any (other than a small mv) until something triggers it and that is why it is called a SMPS? I wonder this because I do not see any other wires coming from or going into the power supply that would logically return a signal from the rest of the DVR to trigger anything. There is another PCB (you can see it in pic 3) on the side of the PS with another bunch of components. Part of it runs a 12 volt cooling fan, which is working properly. As I am reading this for errors prior to posting, I am wondering now if that secondary PCB may have wi-fi components, which would turn on the power supply (52V) to activate the recording?

    The 12 volt side is producing 11-11.5 vdc. So I'm good there. The 52v side is not producing anything from the wire outputs. However, when I take a reading from the heat sink (which is attached to some mosfets/transistors and a diode) in the middle of the board (it has the number "2" on it), I am getting +51.5vdc to the chassis ground. I also get a steady 64.7vac (either probe) doing the same thing with the DVM set on AC. The DC voltage is not steady and goes up and down a fraction of one volt. Oddly though, the negative lead on the DVM has to be on the heatsink and the positive on the ground to get a positive DC reading. To my non-electronic educated self, that should make no sense. Based on the power supply box design, however, I do think some voltage should be there (on the heat sink), because the inside of the enclosure has insulation material positioned above it. By the way, don't ask me how I found out about the voltage coming off the heat sink. :)

    Note that the original power mosfet, all other mosfets, and all of the transistors have been replaced, as have all of the caps. The diode was removed and checked. It is operating properly. All components were replaced with the same as what was removed, and before you ask, they were put back in the same way as they came out. I don't have the rest of the DVR to plug it in to test it, so I'm hoping that is the only reason I am not getting the higher voltage reading at the wire outputs.

    Does anyone know if it is probably operating correctly, or what should I check next? I see places on the bottom of the PCB that may be test points, but unless a/the "switch" has to be activated for the 52 volt side, I don't know how to do that. I do not have a schematic. It is a Delta DPS-200PS-185B power supply.

    Thanks in advance for your reply.
     

    Attached Files:

  2. Harald Kapp

    Harald Kapp Moderator Moderator

    9,413
    1,924
    Nov 17, 2011
    No, impossible. As both outputs are wired to the same terminals on the pcb, both should show the same voltage (apart from a few mV difference coming from voltage drop due to output currrent and wire resistance).
    The name SMPS (switched mode power supply) is an acronym derived from teh internal opeartion of the power suply as opposed to a linear power supply.
     
  3. H2814D

    H2814D

    75
    9
    Nov 4, 2017
    Thanks for the reply, Mr Kapp, however, this is a dual output voltage power supply. One side is 12vdc and the other is supposed to be 52vdc.

    Maybe I didn't explain well enough. The 52 volt side only has two wires on it (red and white) and a two pin plug. The 12 volt has 4 wires (black and yellow) and a 4 pin plug, but both positives and both negatives in that plug are only attached to a single positive and a single negative output on the board. See the pic.

    The 12vdc side is working. I am trying to get the 52vdc side figured out. I am wondering if there has to be a load on the output of the 52vdc side to get it to supply the current? And is there a workaround to get that current moving just so I can see if that side is operational or not. I don't have the rest of the DVR to attach it back into.
     

    Attached Files:

  4. Harald Kapp

    Harald Kapp Moderator Moderator

    9,413
    1,924
    Nov 17, 2011
    All is fine, a misunderstanding on my side, sorry.

    That is one possiblity. Simply connect a resistor >= 10 kΩ across the 52 V output to check this.
    Also try a resistor (1 kΩ) across the 12 V output. Current draw from the 12 V output might be used internally by the PSU to activate the 52 V output.

    Most unlikely. If the 52 V can be controlled a separate control signal (wire) is what I'd expect to find. It doesn't look like there is one, however.
     
  5. H2814D

    H2814D

    75
    9
    Nov 4, 2017
    Thanks again, Mr. Kapp. I will try our suggestion in the morning. If it doesn't work, and I mean activate that 52volt side, I won't be able to sleep. Just better to be hopeful and think it will, so I can. :)
     
  6. kellys_eye

    kellys_eye

    4,281
    1,144
    Jun 25, 2010
    The board looks to have two distinctly separate SMPS circuits (paths) to generate the voltages - given the two lots of input filters and two 'transformers' - so losing ONE supply is quite possible without losing the other.

    I can see output current limiting (detection) resistors and two lots of opto-couplers for feedback/regulation etc. What I can't see are the devices on the main heatsink in the middle of the board. Not sure whether these are the rectifiers or switching devices - possibly both - but the 'stand-up' circuit board seems to have all the gubbins needed for the controlling part of the system.

    What's the device on the separate heatsink close to the 52V output end of the board?
     
  7. H2814D

    H2814D

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    Nov 4, 2017
    Ok. I took photos of the components attached to the center heat sink and the stand alone heat sink. For the center, number 1-13NM60N (Q801), 2- a diode-checks good, 3-F9NK90Z (Q501). Attached on the other side of component number 3, is a W9NK90Z (Q351). Q351 was definitely bad when I started troubleshooting this. The BR tests ok, as do all of the other resistors. The transistors and mosfets have been replaced with new components, because it was faster to order all of them new and just replace them from the beginning. The original .56 Ohm resistor next to Q351 had also failed and was replaced.

    The component on the stand alone heat sink by the 52V output is a FML24S. It is new as well.

    Just for piece of mind, the voltage (51.5vdc and 60vac) coming off of the heat sink is nothing to be concerned about? And the fact that I place the neg DVM probe on heat sink and the pos probe to chassis ground provides a positive DC voltage, is that normal?

    And then there is this. I posted a photo of the bottom of the PCB. There are two contacts that, when out of the enclosure, have no conductivity, however, these would be grounded together when it is back in the enclosure. Would that have anything to do with my polarity concerns? I would think, if they were ground contacts, that they should be connected on the PCB as well.
     

    Attached Files:

  8. kellys_eye

    kellys_eye

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    Jun 25, 2010
    Of no concern at this time.

    Working 'backwards' the terminals marked 8 and 9 on the bottom of the PCB are the secondary windings of the switching transformer and is applied to the anodes of the large rectifier package at the bottom right of the image (on its own heatsink) which then produces the DC across the smoothing capacitor.

    If there is no DC at the capacitor then either the rectifier is dud or there is nothing on the secondary of the transformer which means the primary switching component isn't doing its job.

    This means the primary switching device is either dud or isn't getting its switching signal from the actual controller which is mounted on the vertical PCB.

    Can we see a close up of that PCB?
     
  9. H2814D

    H2814D

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    Nov 4, 2017
    Photos of the vertical PCB. Vertical PCB 1.JPG Vertical PCB and Bottom of PCB.JPG Vertical PCB.JPG
     
  10. H2814D

    H2814D

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    Nov 4, 2017
    On the transformer where points 3 and 4 are (bottom of main pcb), I get 166vdc. I get nothing at numbers 8-9 and nothing at the smoothing capacitor. I should get some voltage at 8-9, shouldn't I?
     
  11. H2814D

    H2814D

    75
    9
    Nov 4, 2017
    Placing resistors across the outputs, either the 12V only, the 52 only, or both, didn't change anything.
     
  12. kellys_eye

    kellys_eye

    4,281
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    Jun 25, 2010
    Your meter 'might' show a reading but the output at 8/9 is at a high frequency that most meters can't read.

    It's points 1,2 and 3 for the primary of the transformer for the 52V supply, again at high AC frequencies so probably not properly measurable using your test meter.

    If it isn't the main switching transistor (MOSFET etc) or it isn't the main SMPS controller chip (possibly the 16-pin device on the sub board) then it's a fault that would be too complicated to trace via a thread like this due to lack of technical details - schematic etc.
     
  13. H2814D

    H2814D

    75
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    Nov 4, 2017
    Thank you, kellys eye. This is the controller chip. It is a CM6800UBX. I can find it, however, not in the US. Do I have to have this exact same number?

    Another question. I had removed and tested the bridge rectifier earlier. It tested good at the time. While doing a closer inspection of the board, I found what appeared to be leakage from the "-" pin. I removed and tested it again and it still tested ok. I have ordered another one anyway. Could this have been part of the issue? Low voltage on the 12 volt side and no voltage on the 52. CM6800UBX.JPG
     
  14. kellys_eye

    kellys_eye

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    Jun 25, 2010
    The bridge rectifier won't be the problem.

    We can go on changing parts 'at random' to no good effect. Try downloading the data sheet for the CM6800 and see if there is an application schematic for it - this could give some pointers on where the fault may exist as many designs are built around the original manufacturers 'prototype' designs

    http://pdf1.alldatasheet.com/datasheet-pdf/view/85304/ETC/CM6800.html

    (see pages 16 and 17)
     
  15. H2814D

    H2814D

    75
    9
    Nov 4, 2017
    As I was reviewing other's posts, someone had advised that the 52 volt side was the power supply for the IR lighting (surveillance system). That side would only come on when another component (light sensitive) was activated due to darkness. If that is the case, I am wondering if, quite possibly, that this PS is actually working as it should and that the nighttime trigger would electronically and then actually activate the 52 volt side.

    The only problem with that is, unless the trigger is on the other part of the system, it would have to be done so by placing a load on the 52v output, because there are no other inputs to this power supply and only another 12vdc output that is showing only 10.9vdc. And I have already tested the output by placing an 18Kohm resistor (advised to use something greater than 10kOhm) and nothing changed as far as voltage. Actually I have loaded both sides to check them.

    As far as the component replacement, most of them have been rather inexpensive, and this is only taking my time, of which I have plenty. I think I will do the CM6800 review, and will order the IC from China in the meantime. I am not done. :)

    Thanks again, sir.
     
  16. kellys_eye

    kellys_eye

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    Jun 25, 2010
    The only 'feedback' to the PSU to indicate (or activate) the outputs is the output itself. If you've already loaded the output to check for operation then it's unlikely that load-detection is the method being used.

    This assumes you're loading it sufficiently...... you could try to load it by connecting four 12V 5W light bulbs in series across the 52V output - such bubs are commonly used as side lights on cars.

    I note a set of 'current detection' resistors on the 52V side but these may be (are usually) used to detect short circuits and protect the output.
     
  17. H2814D

    H2814D

    75
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    Nov 4, 2017
    OK a new discovery. When testing the 12 volt side, I am not getting quite 12 vdc. It is low and jumping sporadically and constantly up and down between 10.4-11.9vdc.

    I understand that an octocoupler between the main components of the board and the final circuit output should act like a diode and a transistor of some sorts. Additionally, they are partly responsible for maintaining a steady output voltage. I see the same octocouplers between both the 12 volt and 52 volt output sides and the main board components. If the octo on the 12 volts side is malfunctioning (non steady voltage) and the current draw and/or octocoupler activation on that side is the trigger for the other side (52vdc), could that quite possibly be my problem?

    I've attached a close up of the octo on the 12v side (52 has the same one). I can find EL1013 octos, but not with the "445V." Looking at the datasheet below, there may be a correlation between the octos and the activation of the 52 volt circuit, because it appears they may "communicate" with each other somehow?

    http://www.everlight.com/file/ProductFile/201407061745083848.pdf

    I have tested both of them with the DVM and they do not react the same. Realizing that testing them in the circuit may give false readings, this is what I found. The 12volt side diode end (the side with the dot) is allowing current in both directions, and normally it shouldn't, if acting as a diode. The other end is showing open (no change on the DVM) from pin 4 to 3, but is allowing current from 3-4. The 52 volt side diode end is only allowing current from pin 1 to 2, like I would expect it would out of circuit. The other end is allowing current in both directions.

    The ones on this board are glued down. I am afraid I may damage them if I try and remove them, but if your inclination is to remove and check them, I will. Is there something I should use on the glue to soften it without messing up the rest of the board?

    Once again, your help is very much appreciated.
    EL1013 445V Octocoupler Close Up.JPG
     
  18. kellys_eye

    kellys_eye

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    Jun 25, 2010
    Opto couplers provide the isolation necessary between the output (low) voltage and input (high) voltage sides of the system. Their input - the diode side - is a 'sample' of the output voltage which is then fed back to the controller chip to self-regulate the output voltage.

    If the output is 'cycling' then the circuitry is detecting a fault with the output voltage or current - either of which may be monitored and any of which, if out-of-range, will cause the system to lower the output to protect it. Once the output falls to an 'under-danger' level the system tries to restore normal operation.

    Did you check/replace the output filter capacitors? and if so did you put them back correctly?

    Testing the devices in-circuit can be problematic so removing them would be the best way - if you apply sufficient heat to the solder pads it's often enough to soften the glue to afford removal but don't apply too much heat or you could damage the opto coupler.
     
  19. H2814D

    H2814D

    75
    9
    Nov 4, 2017
    Whoops. I see I spelled them "octo." :)

    All of the capacitors have been replaced with the same type and size and they have also been replaced correctly.

    I will attempt at least partial removal of the "optos" and let you know. Prior to, I think I will do a precautionary localized schematic and define the current paths.

    By your description, and functionality, it does sound as if they could be partly responsible.
     
  20. XYZ_123

    XYZ_123

    1
    0
    Sep 28, 2018
    Hi all,
    I have a question. I am simulating a 20V to 5V buck converter at 600KHz using voltage controlled closed loop. I have used type3 compensation for the same. The problem is the PWM waveform at high switching frequency is a little improper. However the pWM waveform at 100KHZ frequency is almost perfect. Can you tell the reason. And ways to improve it?
     
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