Dell 255W PSU 8.3V on source power MOSFET

Hello,

I'm trying to fix this Dell 255W PSU, I think the fuse was broken and I replaced it, then I tried to power up the unit but didn't work on output rails.

So, I measured the voltage on the high power side and I think the power MOSFET isn't working, the drain receives 316V from the high voltage capacitor, the gate is 8.3V from the switcher IC and on the source I found 8.3V, so I thought the MOSFET is broken.

Is my guess right?

Usually the source has a low value resistor to ground (less than 1 Ohm) so it is likely that resistor has gone open circuit.

Brian.

I didn't see any resistor, the source is going directly to the high frequency transformer, which is connected to another high frequency transformer.

I don't know exactly, but I think it's maybe the MOSFET is broken, I didn't measure a short between gate-drain or drain-source. In this PSU most of the MOSFETs measure short between gate-source.

The only problem is that there's a low voltage on the MOSFET.

The green wire is shorted all the time during the troubleshooting process. So, I know this wire should run the power supply.

There's 3.1V on the green rail.

Also, the voltage on the big capacitor drops very fast after unplugging the power cable.

There's 316V on the big capacitor, moving to the power MOSFET, where the power transformation stops there! There's 8.3V on the source, so I think there's the problem, but I don't know if it's the MOSFET or the power switcher or something else. But the main sign here is that there's no power on the source pin of the MOSFET.

Below is shown a typical Dell type PSU.
As mentioned, there is normally a low-value resistor in the source return of the MOSFET for current sensing. In this case, it is 2 x 0.68 ohm resistors. Things you should look at also include the controller IC, any snubber diodes and leaky HV capacitors.

My current inspection.

I think what you're looking at is a dual-switch forward converter with active PFC (power factor correction). The low-resistance reading between gate and source is likely the gate drive transformer winding for each switch with a small gate resistor in series like I show in the attached example diagram. The additional transistor and diodes to the gate may not be used in your particular design. The dual switch is clearly recognizable with their corresponding diodes connected between B+ and GND.

Can you check which controller chip is used on your board as that will tell us if we are making the correct assumptions?

Note R14 on the bottom MOSFET source pin to GND, which is used for current sensing and feedback. This will be a low value.

Showing video or high res pictures of the component side will be very helpful as well.

My last inspection process, Enjoy I know it's not a fix at the end but I'm learning how to fix and appreciate the help.

Make sure your standby/bias supply is working. This is using the TNY280 chip. Without this giving the correct power, the rest of the circuits may not start up correctly. Also identify the PFC/switcher controller chip/s. You want to check the two clamping diodes connected to the MOSFET's that I marked in blue. The first coil on the main board you show must be the PFC coil, marked red on my diagram. Have you located the current sense resistor (R14 in example)?

Refer to the data sheet of the TNY280. Look at a typical application circuit. Manufacturers often copy them exactly, apart from a few voltages setting resistor values that may differ. Furthermore, make sure that all the power components in the PFC stage are good.

Do you have a scope? That always helps. Then I can suggest some safe initial testing procedures.

I have a scope but it's not mine it's for the college. I can bring it to my office, but there are trainers told me that using the scope probe into a high voltage would cause a problem; like, explosion or burning the probe. Because I think there're no high voltage probes.

Well, we can substitute from an external power supply the voltage that the bias supply would normally supply, and measure if there are any activity around the PFC/switching chip once we can look up the chip data. This can be done without applying mains to the PSU itself. We might be able to determine if the gate drive signals are ok, etc.

You can test a lot of things with a DVM, but at some stage, you need to look at waveforms if you could not find all the faults with meter testing.

Hello eagle1109,
I need to comment in regards to the second PSU you have with those shorted MOSFET's.

You need to check the main fuse to see if its open circuit.
You need to also check for a short across the main filter capacitors.

Going by E-design's attached schematic in Post#7, you need to check D27 for a short,
and reverse leakage.
Also check Q1 for any shorts as well.

When ever you get ANY appliance that shows no signs of life, here are the steps you should follow:
(1) When the unit is open, immediately check the area for any burn smells. If there is something
burned out, you can usually smell it as soon as you open the unit up.
(2) Even if there is no smell of burned components, do a close visual check to see if there is any
evidence of split open semiconductors, burned or overheated resistors, as well as overheating on
the PCB itself. Use a magnifying glass with good area lighting.
(3) Check visually for any electrolytic capacitors that may have bulges at the top, as well as the
bottoms of them to see if the rubber bung has been partially forced out.
(4) Flip the PCB over and check the solder side for any faulty solder joints, especially with any
through-hole components and high current areas. You can usually tell of high current sections
by the track widths. The wider the track and/or the track/s themselves have solder on top of them
will determine high current spots.
(5) Grab your multi-meter and set it to its highest ohms setting, then measure the main fuse to
see if it has gone open circuit.
(6) Measure across the main filter capacitor/s for any shorts. This will determine if your B+ rail
is shorted. If it is, you need to check the bridge rectifier, or individual diodes within the rectifier
circuit for the short.
(7) Measure any diodes for shorts, as well as doing a reverse bias punch-through tests for any
reverse leakage. One leg of the diode MUST be lifted from the circuit to perform an accurate
punch-through test.
(8) Check any transistors or MOSFET's for shorts.

If any one or more of the above is confirmed, then more intensive tests need to be performed.
Any shorted components that has support circuitry attached to it has to be checked.
i.e. Resistors, electrolytic and non-electrolytic capacitors MUST be tested.
In regards to resistors, one leg MUST be lifted from circuit, and for SMD types, they have
to be removed entirely and measured.
You would only need to lift a leg or remove the SMD component entirely only if the in circuit
readings you're getting are a little ambiguous. eg. A 10k resistor is reading 173k.
If they read close to the value indicated on the component, then more than likely they
won't be faulty.
Non-polarized capacitors can be measured for their capacitance values, but electrolytic and
tantalum caps need to be tested for their ESR values as well as capacitance.

Your main goal in the above is to determine if just one component is the culprit for the failure
or there are other components that are responsible, as well as causing a part to go short
circuit.

If you find nothing in the list above, then the only course is to look on the secondary side
of the PSU to see if any components are causing the shut-down.

If there's nothing obvious in the secondary side, then the only way to determine what may
be wrong, is to fire the unit up and perform voltage checks, preferably with a load.
Doing the above is dangerous, and unless you know exactly what you're doing, you risk
a nasty electric shock, perhaps even a fatal one.
If you are unsure, then you need to give the unit to a qualified technician.

Sorry for the story, but I felt that doing the above via a structured procedure should help
you or anyone else to diagnose the problem in a short time. Without having to poke around
measuring components and the like that have nothing to do with the actual fault.
Regards,
Relayer