Computer Power Unit Failure

[KrisBlueNZ]

If you can get the STF25NM60ND then yes, I would use it instead of the STF34NM60ND that I suggested.

The STF25NM60ND is an "FDmesh II" device, compared to the original which is "MDmesh II". (In my previous post, I originally said that the original MOSFET was "FDmesh" but that's wrong; the original device is "MDmesh II". I've corrected that post.)

The significant difference between the ...N and the ...ND seems to be the gate threshold voltage (the ND has a narrower gate threshold range and should be suitable as a replacement) and the speed of the parasitic drain-source diode (the ND has a faster diode, which should be fine).

Re the MTP20N10 and MTP20N10E, I wouldn't use them. I don't know what the special features of the MDmesh devices are, but I would assume there are some, and it would be better to use a similar STMicroelectronics part.

 

[VirajKoth]

ah sure thank you again dude....then i will go for this "STF25NM60ND"

 

[KrisBlueNZ]

There could also be other components that either failed first, and caused the main switching MOSFET to fail, or failed as a result of the heavy current that flowed when the MOSFET went short.

I would check the rectifier diode(s) on the secondary side, and check all diodes on the primary side. Look for any resistors that have overheated, or large-ish resistors that may have failed instantaneously. If there's a resistor between the MOSFET's source and the primary negative rail, check it as well.

 

[Rleo6965]

One source of shorting Mosfet was defective PWM ic. Can you give the part number of IC?

 

[KrisBlueNZ]

Good point rleo6965. Even if the PWM chip didn't cause the MOSFET to fail, it could have been damaged when the MOSFET died. It would be hard to test; perhaps the best option would be to replace it just in case.

 

[VirajKoth]

guys BTW sorry for the late reply as i was out this weekend.

There could also be other components that either failed first, and caused the main switching MOSFET to fail, or failed as a result of the heavy current that flowed when the MOSFET went short.

I would check the rectifier diode(s) on the secondary side, and check all diodes on the primary side. Look for any resistors that have overheated, or large-ish resistors that may have failed instantaneously. If there's a resistor between the MOSFET's source and the primary negative rail, check it as well.

As you said i checked all the remaining diodes and transistors. But neither of them is gone.
I figured out a big resistor is gone, which coming from the two parallel bridge rectifier`s negative rail to MOSFET`s source.
I think nothing has gone except those big resistor.

why those resistors are covered with a rubber shield?
seriously what is the resistance of this resistor ? that`s 84.2ohoms ryt?

One source of shorting Mosfet was defective PWM ic. Can you give the part number of IC?

oh seriously..had that same issue with my PSU too before the PSU got completely defected. So i wired the fan from one of the 12V output directly and used the PSU for few months. Then it came up with a short circuit(that faulty MOSFET) recently. So as u said that may be due that defective PWM circuit.

The PWM circuit is attached to the PSU`s board that located right near to those output rails from the PSU ryt?
I`ll try to figure out that IC and will tell u soon.



I got some questions guys...
Will it again burn that MOSFETs if i replace them without repairing the PWM circuit ?
And i thought this will be a waste if um going to repair this whole unit? (replacing resistors, mosfets, may be that IC and manymore !! )
And how the hell did it happen to burn the MOSFETs in primary side due to a tiny fault(PWM circuit) in secondary side ?

 

[KrisBlueNZ]

I figured out a big resistor is gone, which coming from the two parallel bridge rectifier`s negative rail to MOSFET`s source. I think nothing has gone except those big resistor.

why those resistors are covered with a rubber shield? seriously what is the resistance of this resistor ? that`s 84.2ohoms ryt?

It's covered with a rubber shield because it can dissipate a huge amount of power if the MOSFET fails short, and you don't want it exploding and splattering all the other nearby components!

As for the value, I don't know. I've never seen a resistor marked 5 gold 3 4 silver before. It doesn't match the standard colour code.

(re controller chip failing) oh seriously..had that same issue with my PSU too before the PSU got completely defected. So i wired the fan from one of the 12V output directly and used the PSU for few months. Then it came up with a short circuit(that faulty MOSFET) recently. So as u said that may be due that defective PWM circuit.

It sounds like you've been having a lot of trouble with this power supply...

The PWM circuit is attached to the PSU`s board that located right near to those output rails from the PSU ryt?

It's hard to say. I suspect it's on the circuit board on the left side of the picture, just beside the long thin heatsink.

Will it again burn that MOSFETs if i replace them without repairing the PWM circuit?

If the PWM controller is faulty, then yes, the MOSFET will probably blow up immediately.

And i thought this will be a waste if um going to repair this whole unit? (replacing resistors, mosfets, may be that IC and manymore !! )

I agree. That power supply looks like an explosion in an inductor factory! It's pretty tightly packed and messy, which isn't a good sign. I couldn't say that it's an unreliable design, or a poor quality product, though, just from the pictures.

And how the hell did it happen to burn the MOSFETs in primary side due to a tiny fault(PWM circuit) in secondary side ?

That's the mystery of the "PWM gremlin" It's an imaginary evil spirit that plagues switching supplies and causes them to randomly fail for no clear reason! Seriously, there could be many causes, including poor design, dry joints, complete or partial failure of almost ANY component in the circuit, and so on. Switching supplies like this one are dealing with extremely large amounts of energy, and if ANYTHING doesn't work exactly as it should, the usual result is immediate failure of the main switching device.

 

[Rleo6965]

And how the hell did it happen to burn the MOSFETs in primary side due to a tiny fault(PWM circuit) in secondary side ?

The secondary side was the load side of power supply. An accidental short on its load tends to drop output voltage on secondary side. The PWM circuit will sense this low voltage and and it will compensate it by increasing pulse width waveform of inverter.. Driving more current for the MOSFET. Until MOSFET short circuit itself. PWM IC usually destroyed when Drain and Gate of MOSFET was shorted. Because the primary voltage of 300V will pass the shorted MOSFET and destroy Gate resistor and PWM IC..

A good designed pwm circuit will shutdown the power supply to avoid further damage of its circuit.

 

[VirajKoth]

Thank you all so much for helping me to figure out this fault...And i thought of this will be a waste of replacing so much components to get this PSU back to working order. And also here i couldn`t find those PWM IC and exact FETs too.
Also one of my friends will be here from China in next yr beginning. So i`ll manage to get these components for cheapo, u know.

One source of shorting Mosfet was defective PWM ic. Can you give the part number of IC?

here`s the Part Number of that PWM IC "CM6800G" (image is attached)


And um gonna buy this a "CoolerMaster GX550W PSU" http://www.coolermaster.com/product.php?product_id=6641


but for the same price there have a RX650W CoolerMaster PSU(http://www.coolermaster.com/product.php?product_id=5924). Everyone says that PSU is crappy. So not going to buy it.
gonna buy that previous mentioned one.


Thank you

 

[KrisBlueNZ]

CM6800: http://www.hardwaresecrets.com/datasheets/download/1201
"Champion" brand.