No voltage on the source pins of PNY280pn switcher

[eagle1109]

Hi,

I started working on this Dell PSU after purchasing a new 220V/100W current limiter lamp.

My start inspection is that the PNY280pn has 385V on Drain pin but 0V on source side. Why it's not working ?

 

[barry]

"Why it's not working?" Because it's broken.

 

[betwixt]

That is normal. The Source pin refers to the internal switching MOSFET's source connection which is the negative supply to the IC. You can probably measure a direct link between the Source and negative side of the capacitor you marked on the photograph. If you have no output, either the IC is faulty or a voltage on one of the other pins is preventing it running.

Brian.

 

[eagle1109]

I'm sorry I forgot to mention that I have some PNY280pn chips, so I replaced it with a new one but the same result.

And this is the way I was checking the voltages on this chip, putting the red probe on the chip pins and the black on the -ve of the capacitor.

--- Updated Mar 12, 2024 ---

voltage on one of the other pins is preventing it running.

This what I'm thinking of, checking the chip pin description:

 

[eagle1109]

Hi,

I did a quick check today and found that there are components shorted to the GND.

1. All the MOSFETs in the yellow boxes have the gate and source shorted to ground, but the drain isn't.
2. The MOSFET in the blue box isn't shorted.
3. The switcher source is also shorted to ground.

What could be the problem ?

 

[eagle1109]

That is normal. The Source pin refers to the internal switching MOSFET's source connection which is the negative supply to the IC. You can probably measure a direct link between the Source and negative side of the capacitor you marked on the photograph. If you have no output, either the IC is faulty or a voltage on one of the other pins is preventing it running.

Brian.

I measured the voltages:

1. EN/UV - it's less than 0.5V

2. BP/M - I'm getting more than 5V:

+ the short circuit issues I posted in #5.

 

[eagle1109]

I guess I have to remove some components until the short is gone, then identify the faulty element.

 

[FvM]

I guess, you need to understand the circuit first As far as I see, the switcher involves an auxiliary and a main converter. PNY280 is the auxiliary switcher, main converter uses a full bridge formed by four MOSFET. But how are the MOSFETs driven? Seeing low gate-source resistance can be due to driver circuit.

Measuring 6.2 V at BP/M pin indicates that the pin is externally driven to force shutdown state. You should check what is driving the pin.

 

[betwixt]

Also note that a DVM will only show approximate average voltages but the pins may have substantial signal levels on them.
Be very careful taking measurements, you are working on the line side of the supply, not the isolated output so dangerous voltages will be present.

Brian.

 

[eagle1109]

You should check what is driving the pin.

The problem is that I don't have the schematic. I tried to search for it on google but no result.

 

[eagle1109]

Be very careful taking measurements, you are working on the line side of the supply, not the isolated output so dangerous voltages will be present.

Even if I'm using a current limiter lamp ? I just received this 100W/220V heat lamp, and it's working.

I tested a working PSU and the lamp didn't interrupt the launch of the PSU. But didn't encounter a short circuit one until now, but hopefully it will glow when there's a short in the PSU.

 

[betwixt]

Yes, it is still dangerous. These PSUs have a 'line' side and an isolated DC output side. You are working on the line side which has direct connections to the wall socket. You will notice a distinct line across the PCB where there are no tracks, the bottom half is the non-isolated side, the top is isolated by the transformer and row of opto-isolators. The transformer couples power to the output, the opto-isolators carry feedback from the output back to the switching circuits to stabilize the voltages.

You can't measure from one side to the other or connect any grounded measurement equipment to the 'line' side as it could cause a short to earth.

Basically, if you touched any part of the line side and ground, the only thing between you and wall socket voltages is the heat lamp and I can guess which will get hottest!

Brian.

 

[eagle1109]

Yes, it is still dangerous. These PSUs have a 'line' side and an isolated DC output side. You are working on the line side which has direct connections to the wall socket. You will notice a distinct line across the PCB where there are no tracks, the bottom half is the non-isolated side, the top is isolated by the transformer and row of opto-isolators. The transformer couples power to the output, the opto-isolators carry feedback from the output back to the switching circuits to stabilize the voltages.

Yep, I know this concept as the line side carries +320V, and I'm very careful with using the DMM probes. I know that any mistake will cause a short between a two lines with different potentials.

As I encountered a incident happened to me, where I forgot to move the screwdriver a way + I wasn't using a lamp limiter at that time, and there was a series of current strikes within some milliseconds ! The result, that part I was attempting to fix got more damaged

You can't measure from one side to the other or connect any grounded measurement equipment to the 'line' side as it could cause a short to earth.

Absolutely, no way I would do that.

Basically, if you touched any part of the line side and ground, the only thing between you and wall socket voltages is the heat lamp and I can guess which will get hottest!

Brian.

I remember measuring the lamp resistance which didn't exceed couple hundreds of ohms, where human resistance is over 1000 ohms in worst cases.

I have wondered about this many times, as why even when I'm wearing shoes with rubber base, there's a high chance I might get electrocuted ? which happened to me one time ago.

But I believed in one thing which is reality isn't like theory, because the situations are different, I don't know what is my resistance to the ground even when wearing shoes.

 

[eagle1109]

Hi,

Today I'm checking without powering up the PSU, I want to find the problem using the ohmmeter, continuity or diode meter.

My main suspect now is that it could be either the main MOSFETs or the optocouplers.

First let me investigate the optocouplers if the measurements I'm getting are ok or not.

The optocoupler used are 817:

and according to this photo:

So my question:
According to the pinout and the example circuit: must the emitter pin no. 3 be connected to the GND through all the 4 optocouplers ? because the 2nd optocoupler measures +400kohms, then it decrements gradually then loops again.

 

[betwixt]

Without a schematic it is impossible to confirm whether that is correct or not. The transistor side of the opto-coupler can be wired in any configuration so the 400K you measure may not be indicting a fault.

I have had opto-couplers fail before but it is fairly rare. By far the biggest cause of failure of this type of PSU is the electrolytic capacitors. Look for obvious signs like bulging at the tops but usually it is the smaller ones on the line side that give problems. When the power is turned on, the charge current into some of the smaller capacitors is used to provide the initial supply voltage to start the main circuits working. Even though they may look good, it is worthwhile replacing the smaller ones if the PSU is more than a few years old. Typically the ones that give most trouble are < 10uF and usually rated at 50V or more.

Brian.

 

[eagle1109]

Good morning,

I tested the caps with the capacitance meter, there are only 2 electrolytic caps on the line side. They measure the exact as rated value.

Luckily, I found a working PSU with the exact model number and PCB. Now I hope to do some comparative measurements and find the issue and fix it hopefully.

 

[eagle1109]

Hi,

Latest updates, well, in regard to the main issue I declared in the thread title. Which is there's no voltage on the source pin, and actually that's how the circuit should work because the drain could be connected to the auxiliary transformer.

One of these PSUs is working with now problems, and the second one is the faulty one.

Also, I forgot to short the green pin to the ground. So good news, the faulty PSU has only 12V rail broken, rest of voltages are OK.

So now I guess either the 12V output mosfet or the 12V output capacitor might be broken.

 

[Gaber Mohamed Boraey]

Hello eagle

Also, I forgot to short the green pin to the ground. So good news, the faulty PSU has only 12V rail broken, rest of voltages are OK.

are you joke? , for what vault you look for if the PSU not working already?,
First thing to check the fault is to operate the psu by adding jumper between the green wire and the black , then yucas check the voltages as you discovered lately
Now the 12v line is the problem? Okay now check this line mosfets without electricity, I mean turn off the psu , be careful if any capacitors have charge, discharge if so, then take every mosfet on the 12v line out of the circuit and check, and look for short in that line , maybe zener doide shorted, mosfet shorted, big resistor , etc
And tell us , now you going in the right direction for repair, before no.
Regards

 

[eagle1109]

Strange thing, which is the faulty PSU is now working and all the voltages are present. Maybe the -12V is -11.36V I don't know if it's a problem or not.

But there's something affecting the stability of this PSU and cause it to shutdown, and then it could boot up without problems.

 

[Gaber Mohamed Boraey]

Try to look for dry points in the board and resolder, maybe bad contact cause the psu not stable
also connect the psu with current limiter lamp you have and see if it will shut down or no

For the -12 v reading I think normal
Do these trails and tell us