SMPS Primary Side Short?

[dicky96]

Hmmmm.  First thing that makes me wonder, is gate of Q3 has as much as 13V on it.  If that is 13V more than the source then that does not look right.  Are you measuring Q3 gate with other meter probe on negative of the main smoothing cap?  Remind me which one is Q3, is it the power FET from SMPS transformer primary to 0V?

The fact the 160V discharges quickly on the main cap when you disconnect power says something is drawing current like the the SMPS is actually running. 

The DSO138 is actually what we need here because it is running from a 9V supply so it is isolated from the mains as long as it is not conneccted to anything else like a PC which is grounded.  This means we can safely connect the ground of your scope to the -ve end of the main smoothing capacitor :-)  With a 'proper' scope you would need a mains isolation transformer.

OK so on your toy scope, ground connected to main smoothing capacitor negative, what do you see on gate of Q3?  We already know from your multimeter it is less than 50V.   

Rich

[dicky96]

Actually are you sure the power supply isn't working now?

If you have no load then the light bulbs may not light at all, especially as this type of supply has a PFC (power factor correction) circuit, so there is a large inductor between the bridge rectifier and the main smoothing capacitor, plus the smoothing cap will be a relatively low value.  This would reduce power on surge. 

Have you checked the output voltage?

It's just that the discharge pattern on the 160V capacitor looks correct for a working power supply with little or no load.  Nice quick discharge down to 40V at power off, then it goes to a slower discharge at about 40 Volts - which would be when the smps oscillator cut off.

Rich

[OpenCircuit]

1. Are you measuring Q3 gate with other meter probe on negative of the main smoothing cap? 

2. Remind me which one is Q3, is it the power FET from SMPS transformer primary to 0V?

3. OK so on your toy scope, ground connected to main smoothing capacitor negative, what do you see on gate of Q3? 

4. Actually are you sure the power supply isn't working now? Have you checked the output voltage?

Rich

1. I believe I probed the neg on the main cap and Q3 gate.

2. Q3 drain connects directly to the + terminal of the main cap.

3. Pulled the PWM/PFC controller to see if I could conduct any tests to confirm operation: https://www.eevblog.com/forum/beginners/testing-pfcpwm-controller-(cm6800)-out-of-circuit/msg1212880/#msg1212880 Ordered some a few weeks ago just waiting for the 6800s to arrive. Impossible to do any work with the IC daughter board installed. Not sure the primary side is boosting as it should.

4. Was getting .6v from  3.3, 5 and 12v rails.

[dicky96]

Hi
I don't think CM6800 is your problem here 

From the discharge pattern on the 160V main smoothing cap It's pretty obvious to me that the SMPS is actually running to some extent, in other words the CM6800 is driving the chopper FET Q3 and you are getting some voltage (albeit a low amount) on the output of the PSU.  To get that the CM6800 has got to be driving PWM to Q3.

Now if we just refer to page 15 of the CM6800 datasheet and the typical application, which is probably very similar to your PSU:

http://www.champion-micro.com/datasheet/Analog%20Device/CM6800.pdf

The top section of the schematic, that is the PFC circuit. This is where we had Q2 short circuit causing your lamp to light full.  Now we have 160V on the main smoothing cap this is sorted.  Actually even if the PFC section was not oscillating you would still get charge to the main smoothing cap via the coil L1 and the PSU would run.  The whole point of PCF is to make the PSU appear as a resistive load and increase efficiency.  You would still have the PSU running without this section working.

Now you said Q3 drain is connected directly to +ve of the main cap.  I don't think this is actually the case, if we are referring to Q3 in the bottom section of the schematic on CM6800 page 15 then you will see the drain connects to V+ of the main cap via a primary winding on the SMPS transformer.  On your DMM this would read near to zero ohms to main cap +ve but if you look at the PCB and am fairly certain you will find Drain goes to V+ via the winding.

I believe you said way back on this thread that Q3 was short circuit and you replaced it?  If that is correct did you fit the same part number as a replacement?

Now here is the interesting thing.... you were saying you had like 12-15V varying voltage on gate of Q3 with respect to -ve of the main smoothing cap.  That had me puzzled for a while as it can't actually happen, unless Q3 was faulty maybe and if the SMPS is basically running and generating some output Q3 must be working.   But.......

If you look again on Page 15 schematic bottom section, you will see Q3 source connects to -ve main smoothing cap via R31.  This will be the current sense resistor and very low value.  It may even be 4 or 8 SMD resistors in a series/parallel arrangement.   

It seems likely that what is happening here is when Q3 was short circuit it blew some of these resistors so they went high value.

This would cause a couple things to happen - first the source of Q3 would probably be floating at some volts above 0V.  Then of course the gate could read 12V or 15V or whatever with respect to 0V. The PowerFET would probably run but the current through the SMPS transformer primary would be very limited and you would get minimal output.  Secondly the CM6800 would think it had an overcurrent fault on the SMPS output and reduce the PWM duty cycle to minimum giving you even less output.  And thirdly your light bulb would not even flicker.  Hmmm sounds a bit familiar?

OK so...
1. Verify Q3 Drain goes to cap +ve via primary of SMPS transformer, not directly.

2. Measure resistance Q3 source to -ve main smoothing cap.  It should be very low maybe 0.1R or less.  If it is high, try to find R31 which may be an array of SMD or a hefty through hole resistor and see what value it should read.

3. With the PSU running measure voltage Source to Gate of Q3 or even better put you scope Ground on Q3 source and probe on Q3 gate.  This is safe to do as your scope runs from a low voltage supply and is not grounded, as long as it is not connected to anything else other than the device under test at the same time.  You should see voltage no more than 3V or pulses about 3V high

Right.... let me know what you find?

Rich

PS there is another scenario could be the problem - that your PSU actually has a high side and low side FET driving the SMPS transformer and Q3 is OK but the other one is not.  But do the above checks first before we worry about that possibility.  Anyway it should be fairly obvious if this is the case as you would have two large power FETS (both same type) around the primary of SMPS transformer

[OpenCircuit]

...did you fit the same part number as a replacement?

This is essential. Same part number, but package type.  FET body isolated v. non-isolated

Q2 was shorted (right-middle; see below) marked "Q2".

"IN5406" was shorted middle of picture.

Q3 (top-left; see below) click on image then when the page loads click the image again. This will get you high-resolution of the traces. When looking at the FETs (markings side) the left leg is the gate, then middle drain and finally source right. Just double-checked this to make sure diagram is correct.

Thinking of running wires for remote mounting of the CM6800 daughter card allowing me to probe the 6800 IC terminals directly while in-circuit.

I will look into the other things you mentioned as soon as I can get this back together. I agree, I think it is running since I was getting .6v on all rails on the secondary side The block diagrams are roughly the same from 6800 block diagram and my application, but certainly operation differences. This is a 600w power supply for a desktop computer. Thanks a ton for all the ideas and information. This is the entire purpose of this project-learning.

[dicky96]

No worries nmate that is what the forum is for and also it is an interesting challenge for me to try and diagnose something via forum posts only.

If it is a 600W PSU it is likely to have high side and low side FETs driving the primary.    Are Q3 and Q4 both the same type of device?  If so pull Q4 and test it with a bench supply and fan for example

Are there just three terminals on the primary side of the SMPS transformer? If this is a single sided board  then that looks to be the case - 3 transformer primary connections to the left of Q3 Q4, and one unsolderd terminal.  In that case, if these three terminals all read pretty much zero ohms to each other I think we have a two FET output stage and a centre tapped primary in which case you have a push-pull output stage (see below).

So we probably need to work out the topology (type) of smps design you have

Take a look
Could be half bridge type design or push pull design
http://www.smps.us/topologies.html

Half Bridge LLC
http://www.infineon.com/dgdl/Infineon-ApplicationNote_600WHalfbridgeLLC_Evaluation_Board_with_600V_CoolMOS_C7-AN-v01_00-EN.pdf?fileId=5546d4624e24005f014e2ef4761132a3
but then we would see MosFETS rather than rectifiers on the secondary side

There is an interesting link to SMPS topology here as well.
http://ww1.microchip.com/downloads/en/DeviceDoc/dsPICSMPS%20AC_DC%20Users%20Guide.pdf

Regards CM6800 daughter board: exending it so you can work on it would be helpful but the only really important thing now is what do you see on your scope on pins 11 and 12 with respect to -ve of main capacitor.  These are probably connected direct to pins where the daughter board fits the main board, or maybe via a resistor so once you trace that out you can test from the daughter board pins with your scope.  I know I have said this before but do not ground your scope - if it is just connected to it's PSU then you will be fine.

[OpenCircuit]

Are Q3 and Q4 both the same type of device?

Are there just three terminals on the primary side of the SMPS transformer? If this is a single sided board  then that looks to be the case - 3 transformer primary connections to the left of Q3 Q4, and one unsolderd terminal.  In that case, if these three terminals all read pretty much zero ohms to each other I think we have a two FET output stage and a centre tapped primary in which case you have a push-pull output stage (see below).

Q3 and Q4 are the same.

Power supply has three transformers, but one really small transformer is situated (physically) between Q3 and Q4. If you see the green marks on the image you can see the pins to that transformer and connectivity to Q3 and Q4. The secondary side (4 pins total) of this transformer is connected to Q3 gate and Q4 gates and appears to controlthem. Removed both Q3 and Q4 and tested them with a fan (12V).

Got the CM6800s in the mail and was tracing (see below) circuitry...found an open 75ohm SMD that runs to the 6800 Isense terminal. Nothing in my scrap bin.  More waiting.
 
I will have to look at the various topologies mentioned, but I think push/pull is correct.

[dicky96]

Q3 and Q4 are the same.

Power supply has three transformers, but one really small transformer is situated (physically) between Q3 and Q4. If you see the green marks on the image you can see the pins to that transformer and connectivity to Q3 and Q4. The secondary side (4 pins total) of this transformer is connected to Q3 gate and Q4 gates and appears to controlthem. Removed both Q3 and Q4 and tested them with a fan (12V).

Got the CM6800s in the mail and was tracing (see below) circuitry...found an open 75ohm SMD that runs to the 6800 Isense terminal. Nothing in my scrap bin.  More waiting.
 
I will have to look at the various topologies mentioned, but I think push/pull is correct.

Hmm o/c resistor in current sense circuit.... well isn't that an interesting thing   Certainly could explain quite a lot seeing as both power FETs test as good. 

The very small transformer between Q3 and Q4 would be part of the gate drive circuit.  Most likely either
1. the source of one of the FETs is connected to the low potential (0V) side of the main SMPS transformer primary and the gate is driven direct by CM8400 and the other FET is on the hot side with Drain to main cap V+ and the gate is driven by a seperate winding on that little transformer thus isolating the CM8400 from the high voltage. 

Or 2. as you describe 4 pins on the secondary side, you have CM8400 driving a primary on the little transformer and each power FET gate drive is on a separate secondary.

You may well have gate drive (and maybe even the entire PSU) something like this schenmatic http://www.smpspowersupply.com/atx-power-supply.html   

This schematic I think is half bridge not push pull as there is no centre tap on main primary. I'm not an expert in any way on these dual FET drive PSU but that sounds right to me, I'll bet someone more knowledgable than me could confirm or refute that.

Anyway we do seem to be getting somewhere, happy fault finding my friend... and this I think is is a journey of discovery and education for both of us 

Rich

[OpenCircuit]

Got the new CM6800 installed and the resistor replaced. No change.