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Offline OpenCircuit

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ATX Power Supply Dead
« on: March 20, 2017, 03:45:29 PM »
Working on a power supply. Just can't toss it without completely destroying it first.  This PSU has almost no use and is out of warranty of course, so my lessons continue.  :) Upon close-magnified scrutiny of the components nothing indicates failure, i.e. bulging caps, burned FETS, broken wires, fuse, etc.

Primary Symptom: When green wire is grounded at main terminal none of the computer peripherals receive 12V, 5V and/or 3.3V. However , I do receive 5v on the standby rail. So the 168vDC being supplied to the +5vstandby rail is going through this smaller transformer and providing the correct voltage. I guess I am getting something to the secondary side.

The 400V cap is only showing 168V(dc) (waiting on cap tester now). Thinking this cap should be somewhere around 350-375V. This is the first discrepancy I find in-circuit. My second concern is the main transformer is not getting any power at all (red rectangle on photo).

Question: If cap is good what in the circuit would prevent it from charging to an estimated 350-375V?


I have added a few pictures indicating general observations from probing.

Image Notes (below):
Yellow arrows show general flow from introduction of 120AC.
Red lines on photos showing the top is where the board splits and goes to the secondary side.
Purple marks on photo showing the bottom-side of the PSU also show flow of voltage from introduction of 120AC.







Would greatly appreciate any diagnostic suggestions.
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Offline evb149

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Re: ATX Power Supply Dead
« Reply #1 on: March 20, 2017, 04:00:07 PM »
I haven't looked in detail, but AFAIK the mains should be rectified and fed into a boost / PFC type converter to generate the ~350VDC bus voltage without too much harmonic interference noise on the mains line.

Seems like some very old lower power PSUs had a physical switch that controlled whether a voltage doubler or just a full wave rectifier was active on the AC mains line.  220V got I guess the full peak to peak voltage and 120VAC/100VAC inputs got a doubler?  I don't recall the details.  If there is a voltage selection switch on the unit, it's old and that circuit should be checked and there may not be a PFC/boost converter (???) in there.

The +5V standby converter is usually pretty much entirely seperate from the main PSU so they can meet the standby power requirements not possible with the main PSU on and "idle".

 
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Offline evb149

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Re: ATX Power Supply Dead
« Reply #2 on: March 20, 2017, 04:06:43 PM »
http://www.onsemi.com/site/pdf/BoostingPowerSupply.pdf
Might help.  Depending on the wattage, quality, and year of manufacture of your PSU.
 
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Offline OpenCircuit

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Re: ATX Power Supply Dead
« Reply #3 on: March 20, 2017, 04:12:41 PM »
Thanks for the reply. This unit was manufactured in Nov. 2013. It doesn't have the 220/120 switch mentioned. Although it does accept 100*240v 47-63Hz input. It is a 850w. Thanks for the article, I'll check it out.
« Last Edit: March 21, 2017, 05:30:20 PM by OpenCircuit »
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Offline evb149

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Re: ATX Power Supply Dead
« Reply #4 on: March 20, 2017, 04:24:47 PM »
That's a pretty new unit and a pretty high wattage one, so it should have a decent PFC and also maybe even a full bridge type of converter from the HVDC bus to the isolated side's 12V rail.

I wonder if one of the diodes in a full wave rectifier near the mains input could be open circuit.  Might do it?

Maybe identify the IC manufacturer and model number from the markings on the PFC chip and see what the data sheet shows.
It may have some kind of "slow start" boost PFC mode where it gradually ramps the 350VDC bus up from a low voltage and due to a feedback problem or another fault causing the IC to stop running it may not be executing the PFC cycle.

Come to think of it since it is possibly a general sort of boost converter maybe the boost FET is just never being driven at all so you're just ending up with half wave rectified AC voltage on the 350DC bus voltage.  Boosting only happens after the boost FET shorts the inductor to ground to charge the inductor with a higher current flow than would occur at the input voltage going to the output circuit alone.


Thanks for the reply. This unit was manufactured in Nov. 2013. It doesn't have the 220/120 switch mentioned. It is a 850w. Thanks for the article, I'll check it out.
 
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Offline OpenCircuit

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Re: ATX Power Supply Dead
« Reply #5 on: March 20, 2017, 05:13:13 PM »
The main "transformer" is a "Viking E216944-W H-P CWT 1209", for which I am unable to find any datasheet. Transformer has 6 terminals on the primary side and 8 terminals on the secondary side (DC side). (see bottom of board photo red rectangles for the terminals)

You mentioned an "IC" chip that brought this to mind:
I did look into WT7502 ("PC Power Supply Supervisor"). My understanding is that it looks for the correct voltages on input terminals before it will initiate power to actually turn the PSU on. NOTE I MIGHT BE MISUNDERSTANDING THIS COMPONENT. The green wire (on/off) that switches the power supply on and off connects to terminal 4 of the Supervisor. I also checked and did not find voltage on either V33, V5 or VSS (all input signals) to this Supervisor. I assumed the lack of input voltages is preventing the "Supervisor" from pushing power to the transformer/converter (Viking E216944 previously mentioned) so, I turned my search elsewhere for now. Kinda hard to suspect this supervisor as it is situated near the 12vDC output (to peripherals). If you can see the top view photo it is located just between where the blue (-12v) and black wires go into the board at the bottom-left of the photo. I assumed I was way off and moved closer to the primary side of the main looking for voltage to transform for the secondary side.

It has two rectifiers that convert to DC just after the second choke>

I greatly appreciate your food for thought on this; thank you for taking the time to respond.
« Last Edit: March 21, 2017, 05:32:14 PM by OpenCircuit »
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Online blueskull

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Re: ATX Power Supply Dead
« Reply #6 on: March 20, 2017, 05:25:31 PM »
No visual damage indicates no serious failures.
No 375V indicates PFC not turning on.
Main converter not working is normal as it is designed to work after DC bus has reached a certain voltage, ~350V.
Therefore, check your opticouplers, one of them controls PFC on/off, you need to make sure it is sending the correct signal.
If it works, then the PFC control card or FETs are to be examined for damage.
If the optocoupler doesn't receive turn on signal, then the secondary control logic is fried, which can be much harder to diagnose.
SIGSEGV is inevitable if you try to talk more than you know. If I say gibberish, keep in mind that my license plate is SIGSEGV.
 
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Offline evb149

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Re: ATX Power Supply Dead
« Reply #7 on: March 20, 2017, 05:38:17 PM »
Please post the markings on the integrated circuit packages themselves.  The WT7502 is an IC that I am not immediately familiar with, but I understand the general function and and necessity of a supervisor.  Since it is on the "output side" of the converter and, as you say, monitors the V33, V5 nodes it is in part responsible to make sure that if an over voltage occurs on one of the monitored output rails +5V, +3.3V, maybe also +12V rails if it monitors any/all of those directly, etc. does not result in the PSU continuing to perpetuate that problem.  So typically such a supervisor will shut down the PSU somehow if there is overvoltage.  Maybe also over current and under voltage could be additional failures detected.  There is usually some time limit over which the conditions are monitored and a fault condition is decided if the out of bounds reading exists for more than the threshold time.

Your point in mentioning that IC is a good one in that if the PSU was ordered to shut down, or be in the "off" state it would possibly exhibit symptoms like you describe.  So yes, make sure the "soft on" wire that controls whether the PSU should be "on" or in stand by is correct and in the state it should be to activate the PSU.

PSUs usually require a minimum load on some of their primary output rails for proper regulation to occur.  Sometimes they even shut down or malfunction if a minimum load is not present on some rails like maybe one of the +12V and maybe also +5V and +3.3V, depends on the PSU.  So if you do not already have a "dummy load" attached while testing, see if you can hook up some power resistors or a motherboard or something that is safe to test with.

I don't know how the power supervisor IC(s) shut down the PSU.  There will be three converters at least, the AC mains side PFC boost, the AC mains side bridge or other type of converter that convets +350VDC to +12VDC and other outputs somehow, and the +5V standby converter you mentioned was working.

There must be high isolation from the AC mains / +350VDC side of the PSU and the +12V / other low voltage side of the PSU.  So only circuits like optocouplers or transformers should exist to connect signals other than earth GND between the hot side and low voltage side of the PSU.    So somewhere the "shutdown" signal from the "soft on/off" power supply activation wire as well as the shutdown signal from the voltage supervisor(s) must cross the isolation boundary by optocoupler or similar.

Anyway ultimately the full bridge converter IC must have a "start / stop" control and also the PFC boost converter must have a start/stop control.  They will be "off" when the PSU is "soft off" or in a fault condition (I am assuming that also applies to the PFC DC-DC boost converter system).

When I say converter I mean either generally a whole block of circuitry that does a particular kind of conversion function like the standby flyback ACDC converter, the main full bridge DCDC converter, or the DCDC PFC boost converter systems.  Each of those will have a transformer, some FETs, diodes, capacitors associated with them and also one of more integrated circuits ICs usually from 4 pins to maybe around 24 pins in size as well as smaller 3 pin transistors or whatever. 

Anyway identify the PFC boost converter section's control IC which should be near the AC mains input, the associated rectifiers, and the 400VDC capacitor bank and it should be a smaller / simpler IC than the main DCDC bridge converter control IC.

Get the data sheet download for the ICs that  are supervisors or optocouplers or DCDC converter controller / regulator ICs.  And then check to see if the "enable" or "fault" or other such control lines of then are in the expected state.

If you have an oscilloscope and can safely monitor the PSU during operations (it takes special safety procedures needless to say) then you can also see if you see any activity of the "enable" or "shutdown" or similar lines when the PSU is first plugged into the mains.  Sometimes PSUs with faults will actually try to start up for maybe a fraction of a second and then they will stop if they detect a fault.

I suppose with an advanced PSU the detected fault could even be something like the fan not spinning but my intuition has to do with another kind of control line or supervision or PFC boost circuit fault so far.


By converter, do you mean transformer? If so, the main "transformer/converter" is a "Viking E216944-W H-P CWT 1209", for which I am unable to find any datasheet. Transformer has 6 terminals on the primary side and 8 terminals on the secondary side (DC side).

You mentioned an "IC" chip that brought this to mind:
I did look into WT7502 ("PC Power Supply Supervisor"). My understanding is that it looks for the correct voltages on input terminals before it will initiate power to actually turn the PSU on. NOTE I MIGHT BE MISUNDERSTANDING THIS COMPONENT. The green wire (on/off) that switches the power supply on and off connects to terminal 4 of the Supervisor. I also checked and did not find voltage on either V33, V5 or VSS (all input signals) to this Supervisor. I assumed the lack of input voltages is preventing the "Supervisor" from pushing power to the transformer/converter (Viking E216944 previously mentioned) so, I turned my search elsewhere for now. Kinda hard to suspect this supervisor as it is situated near the 12vDC output (to peripherals). If you can see the top view photo it is located just between where the blue (-12v) and black wires go into the board at the bottom-left of the photo. I assumed I was way off and moved closer to the primary side of the main looking for voltage to transform for the secondary side.

I am puzzled by how I am getting DC readings before the main transformer actually "transforms" the 60hz wave into DC for the secondary side. [scratching head]. Sorry, I am trying to swallow all of this as quickly as I can and will put some more research into what you said in the morning.

I greatly appreciate your food for thought on this; thank you for taking the time to respond.
 
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Offline evb149

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Re: ATX Power Supply Dead
« Reply #8 on: March 20, 2017, 06:38:55 PM »
So you have two 8-pin SOIC package like ICs or FETs on the bottom of the main PCBs in the first post bottom photo.
Maybe another 8 or such pin IC on the first post bottom photo.
And a vertically mounted sub-PCBA on the top side of the PCB in the first post with a chip with a larger number of pins.
I wonder it that sub-board is a PFC controller module near where you have "inductor choke" written on the green tape and on the other side of the vertical heatsink from the main transformer.

The PFC unit should either be along the left side or along the right side of the top of the PCB in the first post photo.

Then there is the PCBA off to the left side of the photo of the bottom side of the main board in the first post.  Where did that come from?  That also has a controller IC of some substantial size on it.  Maybe there is the main SMPS controller?
No, the wiring to it looks all like the secondary outputs so that would likely be synchronous rectification / secondary regulation & supervision?


 

 
 
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Offline evb149

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Re: ATX Power Supply Dead
« Reply #9 on: March 20, 2017, 06:56:52 PM »
Ok as your top of PCBA photo shows the yellow arrow tracing from the AC mains input along the top edge of the top side photo going then down the right edge of the photo between the green tape inductor to the 400V   capacitor at the end.  So somewhere between the AC mains and the PFC output unless the problem is due to supervision shutting down the PFC the problem should be the PFC subsystem itself as blueskull also suggested.

 
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Offline OpenCircuit

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Re: ATX Power Supply Dead
« Reply #10 on: March 21, 2017, 11:34:27 AM »
Is there anyway I can get you some full-size photos (about 16MB)? I did have a little time to do some more work today, but need plenty more time to catch up on researching the things  mentioned.

Will try to answer all the questions this evening.
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Offline OpenCircuit

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Re: ATX Power Supply Dead
« Reply #11 on: March 21, 2017, 12:16:56 PM »
So these viking parts are transformers that lower voltage from 380vDC to working voltages of 12, 5 and 3.3 on the secondary side? Want to make sure I know what the components actually do.




« Last Edit: March 21, 2017, 05:21:33 PM by OpenCircuit »
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Offline OpenCircuit

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Re: ATX Power Supply Dead
« Reply #12 on: March 21, 2017, 12:42:48 PM »
?Optocouplers or Power regulators?:

I have identified three which straddle the primary and secondary sides of the board. Part number "L1213 817 B W" although I am trying to figure out what these are at this point.

Component photos and location (U10) below is situated next to +5vstdby ?PFC?:


Two next to main transformer:


The only things I see that crosses from the primary side to the secondary side are:
1. 3 optopcouplers;
2. PFCs (I will change this reference if I am mistaken on what it is, as I have been calling it a "transformer")
3. 1 Thin film cap (blue); and
4 One jumper bar (looks like s segment of paper clip).

Layout of optos (U3, U5 and U10) and PFCs:


Still working....
« Last Edit: March 21, 2017, 05:23:29 PM by OpenCircuit »
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Offline evb149

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Re: ATX Power Supply Dead
« Reply #13 on: March 21, 2017, 12:47:39 PM »
I think the PFC section is shown in the first post of the thread along the right side of the photo  between the medium-small orannge cap at the top edge, passing downward past the green tape wrapped coil, past a couple big vertical fets, and ending around the very large 470uF 400VDC capacitor.

For sending files you can try dropbox or maybe imgur or photobucket if you can get the full resolution ones on there.  I'd try dropbox first.
Or post on your web page if you have a web space area from your ISP.
 
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Offline evb149

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Re: ATX Power Supply Dead
« Reply #14 on: March 21, 2017, 12:57:41 PM »
BTW if you have a general make / model number try google search for teardown, schematic, diagnosis, etc.
In times past sometimes I'd see PC PSU teardowns with a little technical commentary from JohnnyGuru or HotHardware or some such site / reviewer.  Usually not with an EE llllevel of detail but at least some commentary as to major sections / components and maybe other details as to the OEM etc.

I think you had it right in the first post top side pictures with the yellow line and arrows, power comes in top left at the mains connector, flows along the top edge from left to right following the yellow line/arrows through the input choke and rectifier, turning downward at the top right corner offfffff the photo and proceeding down until the 400V big cylinder capacitor rated 400V then going leftward toward the main transformer's input side FETs etc.

So basically something is likely either keeping the PFC off or it is detecting some fault by some signal / means and is shutting down, or there is a component failure that keeps it from working.
Input rectifier,    PFC system, and any control / monitoring lines that affect the PFC operation would be suspect.
 
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Offline OpenCircuit

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Re: ATX Power Supply Dead
« Reply #15 on: March 21, 2017, 01:39:16 PM »
For the optos:
Can I probe the anode and diode terminals (pins 1 and 2) to determine if this side, at least, of the optos are good or bad?

I have made sure a fan was present (load) on the peripherals lines to ensure I would be alarmed at any changes while working.  Wanted to make sure since the PSU's fan was removed with the case.

Do believe I found a bad diode:
Neg to cathode and pos to anode:528
Reversed: 936
Faulty? Would need to remove it to get data.


Still working....
« Last Edit: March 22, 2017, 01:03:34 PM by OpenCircuit »
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Offline evb149

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Re: ATX Power Supply Dead
« Reply #16 on: March 21, 2017, 02:47:04 PM »
Could be a bad diode, coulld be just something that is supposed to beeeeee in parallel with it for some reason.

Try tracing the circuit from the   AC mains input line / neutral terminals through to the PFC fet and draw the schematic and you will bave better idea of what to expect if that is not too hard.

Does the PSU evennn try to start up when you apply power and activate the soft-ON signal?  I do not mean of course the 5V standby section, but the main converter and the PFC.

 
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Offline OpenCircuit

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Re: ATX Power Supply Dead
« Reply #17 on: March 21, 2017, 05:33:39 PM »
I am going back and correcting my posts as I learn.

So far:
1. 123AC going into the board through choke>>filtering cap>>choke>>filtering cap; then
2. 120AC  into 2 rectifier bridges and 168 DC coming out of them.

Question: Does the 168DC out of the 2 rectifiers appear correct? I can't get data from the rectifiers due to heat sink. Not ready to pull for testing yet. If you guys think the rectifiers are working properly then won't test them. Still studying on the "boost" you both mentioned.

Essentially, I  am getting 168VDC out of the two rectifiers. I don't understand the purpose of the cap "MEX JBN 105k450". (top right)

If you click the photo and then to the right at photobucket you can DL the image and zoom in to read the text.

« Last Edit: March 21, 2017, 06:23:08 PM by OpenCircuit »
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Offline Messtechniker

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Re: ATX Power Supply Dead
« Reply #18 on: March 21, 2017, 09:46:46 PM »

I have made sure a fan was present (load) on the peripherals lines to ensure I would be alarmed at any changes while working. 

One fan load may be not enough to satisfy the minimum load requirements of this rather large PSU. Or am I  :horse: ?
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Offline OpenCircuit

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Re: ATX Power Supply Dead
« Reply #19 on: March 22, 2017, 04:50:32 AM »
Mess....verified through manufacturer the single fan should suffice.

PFC refers to pre-rectifier bridge(full)/DC conversion?
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Offline OpenCircuit

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Re: ATX Power Supply Dead
« Reply #20 on: March 22, 2017, 10:10:41 AM »
Pulled the MOSFETS (below) and did a bench test here: http://www.eevblog.com/forum/beginners/proper-mosfet-bench-test/msg1166902/#msg1166902

Both are clearly working. Source terminals are connected to the large 400v Cap negative terminal.

Could rectifiers have anything to do with the cap not reaching the correct voltage? If, 123AC is going into the rectifiers (no id marks since they are pancaked into a heat sink) would the EXPECTED output value be 168DC?


« Last Edit: March 22, 2017, 11:35:45 AM by OpenCircuit »
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Offline OpenCircuit

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Re: ATX Power Supply Dead
« Reply #21 on: March 22, 2017, 11:14:32 AM »
After the ac input, two chokes, and two filtering caps, the AC hot and neutral come together through two SG1s. Can anyone explain what the EE's purpose was here?
Chip "GR8875R C2U01" (SOP 7), seems to be at the heart of this: www.grenergy-ic.com/attach/product/20120607130542_pic.pdf

Full size image:
https://postimg.org/image/smtd5jtz1/
« Last Edit: March 22, 2017, 12:03:02 PM by OpenCircuit »
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Offline evb149

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Re: ATX Power Supply Dead
« Reply #22 on: March 22, 2017, 12:05:52 PM »
Why not draw a schematic of what you have traced so far starting at the AC mains?
I haven't had time to look at all the updates from the last day or so so I don't know much about the PFC implementation from studying your information.
I speculated as to whether the PFC circuit if it is expecting a full wave rectified input signal at 120 peaks / second (for 60Hz power frequency) would not run if the input full wave rectifier had a failed diode so it would only be seeing a half wave rectified input signal, missing every other peak.  I could be wrong about the topology, I don't know how it senses the AC waveform exactly or what the input filter / rectification is supposed to deliver.

I would pull the data sheet on the PFC IC and see if you can read about what control and monitoring inputs it has and what its expected behavior is.
If you have gotten to the point of discovering that ICs model number and it has a public data sheet in a suitable language anyway.

If the input rectification of the mains was totally open you'd see no good rectification at the output of it.  If it was working and a half wave then you'd see a half wave rectified output.  If it works and is full wave you'd see full wave rectified output.  That article I mentioned on SMPS PSU topologies may give some hint about the generalities if not the specifics.

When you say you measure a certain voltage I am not sure if that is true RMS or non-RMS AC or peak or what.  If you had / have a waveform or more details than I have seen so far that might help.

Also we should know what your line voltage is in relation to the measured or displayed rectifier output.
A 100VRMS line sine wave voltage will have 100*sqrt(2) = 100 * 1.414 volt positive and negative peak magnitudes because the crest factor of a sine wave is sqrt(2) times the average value.  I am not 100% sure what the RMS value of a full wave rectified sine is, but logic tells me it should be the same as the line RMS value since energy / voltage is not being lost if the rectification is perfect.  Logic also tells me that the RMS of a sine that has half the peaks cut out by half wave rectification should be half of the RMS of the input voltage because half the signal is missing.  So depending on your line voltage and your DVM you can tell if your readings are as expected.  If your DMM measures frequency of mains signals safely then of course full wave has twice the nominal line frequency while half wave has half and unrectified mains has the nominal frequency.


Pulled the MOSFETS (below) and did a bench test here: http://www.eevblog.com/forum/beginners/proper-mosfet-bench-test/msg1166902/#msg1166902

Both are clearly working. Source terminals are connected to the large 400v Cap negative terminal.

Could rectifiers have anything to do with the cap not reaching the correct voltage? If, 123AC is going into the rectifiers (no id marks since they are pancaked into a heat sink) would the EXPECTED output value be 168DC?

 
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Online blueskull

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Re: ATX Power Supply Dead
« Reply #23 on: March 22, 2017, 12:14:48 PM »
PFC refers to pre-rectifier bridge(full)/DC conversion?

PFC stands for power factor correction. For most of the time, it is essentially a boost converter with current-voltage cascade control loop. It controls its real time current to track AC input's voltage, and it controls its average current based on output voltage.
The 168V clearly shows it is not working.
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Offline evb149

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Re: ATX Power Supply Dead
« Reply #24 on: March 22, 2017, 12:16:47 PM »
Well the primary side voltage would be somewhat  rippling DC value with a peak value somewhere in the 400VDC to 350VDC range roughly.  Depends on the PFC regulation and the load dynamics and how the DC bus filtering and capacitance is.  Basically something in the over 300V but under 400V range is usually some indication that the PFC is outputting a boosted value.  What is the peak of your actual mains voltage?  Do you know?
I don't know if you're in Japan or UK or whatever could be as low as 90VAC up to 240VAC.

The PFC input would be something like full wave rectified mains voltage, and the PFC would implement a boost converter to boost the PFC VIN FW sine rectified pulsating DC waveform up to the DC bus peak voltage around 350V.

The secondary of the main transformer is not 12VDC or 3.3VDC directly but probably something that is somewhat higher than 12VDC as an AC signal  and which is then synchronously rectified and filtered to make 12VDC from that.  Depending on if it is a full bridge or half bridge or LLC or forward converter or whatever determines the waveform of your primary and secondar.   There could be additional secondary and primary side windings as well.
Multiple secondaries if present may or may not be able to provide voltages that can be used to efficiently derive +12V, -12V, +5V, -5V, +3V,  etc. ratiometrically relative to the just over 12VDC AC equivalent most important seconary output.

Additional primary windings can be used as a bootstrap / power supply winding to feed back and supply a suitable voltage maybe in the range  of 5V to 40V to the PFC converter IC that needs to be powered by something, usually in the 15V-20V or so range maybe depending on implementation.
So a defective power supply for the PFC IC could also result in PFC failure.


So these viking parts are transformers that lower voltage from 380vDC to working voltages of 12, 5 and 3.3 on the secondary side? Want to make sure I know what the components actually do.


« Last Edit: March 22, 2017, 12:20:11 PM by evb149 »
 
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