Xbox 360 PSU failure diagnosis

[nick122]

Hi everyone!

I recently got a bunch of 360 accessories for a good price, among which was also this PSU. It's a unit is rated for 175W (14.2A). OEM is Liteon.

The unit briefly powers on for a few seconds, the green light comes on, and powers back off shortly after the green light changes to red. There is no output voltage, I can't hear the fan running either.

Upon opening it up, there is nothing that would indicate why the unit failed. No burn marks, everything looks good visually. Primary cap is 130uf 450v NCC KMG, secondary caps are 1500uf 16v TAICON, all of which look good too. It appears to be using a continuous topology, judging by the inductor on the secondary. It uses a single switching MOSFET, which is a Toshiba K3569. I appears to be using a single forward topology, as I couldn't spot a capacitor in series with the primary transformer (assuming the transformer acts as an inductor). Somebody please correct me if I'm wrong.

On the secondary we have 2 N-channel MOSFETS for asynchronous rectification, an inductor and output filter caps.

The single switching transistor is driven by a surface mount PWM controller at the underside of the board. I used a multimeter to check the zero fuse resistors, and have found 2 to be blown (I replaced them with jumper wire). They connect the PWM controller through an optocoupler, which are probably part of some voltage feedback circuit. It suggests that something might be shorted, maybe the PWM controller? I can't quite tell the markings on the PWM chip, but it appears to be an IR part.

Any suggestions are highly appreciated.

[CapLeaker]

Re: Xbox 360 PSU failure diagnosis

To me there shouldn’t be anything majorly wrong on the primary side of the PSU. The mosfet can’t be shorted otherwise the main fuse would be blown. As far as I understood, the PSU turns on, then off… which means it tries to start, but then fails. So the primary must work, because you get a green light for a few seconds. There can be a few reasons why it does this. I’d check if there is a short on the secondary first, check the output caps, then check the main filter capacitor on the primary and the starter cap.

[nick122]

Re: Xbox 360 PSU failure diagnosis

Thanks for the reply! I just checked the secondary side, there's no short. The caps test fine and the MOSFETs aren't shorted either. They're wired in parallel. I also took some more pictures of the whole board.
As you said, the primary switching MOSFET appears to be good too. I'm still confused about this PWM controller chip, as I can't find anything online. It appears to be an IR part though.

[nick122]

Re: Xbox 360 PSU failure diagnosis

Update: I found this strange (to me) inductor(?) that sits in between the transformer output and the output filter, it seems to just be a copper bridge running through a ferrite core, which is wound with thin wire. The small coil seems to be controlled by some SMD MOSFETs. Is that supposed to improve efficiency compared to a regular coil or is it something completely else?

Update 2: I tested the optocouplers, and they appear to be good too.

Update 3: The PWM controller is NCP1377, not sure why i kept thinking it war an IR part.

[lotas]

Re: Xbox 360 PSU failure diagnosis

This is not an inductor, this is a current sensor, a current flows through a thick wire, and a coil with a thin wire reads this current and when the current increases (load or short circuit at the output), the circuit blocks the output power.

[nick122]

Re: Xbox 360 PSU failure diagnosis

Thanks for letting me know! In that case, this PSU definitely uses a discontinuous design.

[CapLeaker]

Re: Xbox 360 PSU failure diagnosis

If you didn’t find anything wrong yet, look at the feedback from the secondary back to the primary. The PWM in the primary also has some special features like undervoltage cutout, OVP etc… As you see there could be tons of reasons why this PSU isnt working.

[stj]

Re: Xbox 360 PSU failure diagnosis

i dont want to spoil the fun, but how was it tested?
the 360 psu has a main SWITCHED output and a seperate standby supply,
is the standby running?
how did you try to activate the main output?
what load did you use?

[nick122]

Re: Xbox 360 PSU failure diagnosis

Thanks everyone!
@CapLeaker I have checked everything on the primary side. The only clue that I have are the 2 blown zero fuse resistors in series with the PWM chip VCC. NCP1377 VCC to ground measures 300k, so it's not a dead short. What else could cause these resistors to fail? I've replaced them with some wire, but that only made the third resistor in series blow. 5vsb, which uses the same power source, works just fine. This leads me to believe that there's something wrong with the NCP1377 PWM chip. What do you think?
Here's a quick and dirty schematic I drew:

@stj I get standby voltage, but when I try to power the 12V rail by shorting PWRGD to 5VSB, the light turns green, and shortly changes to red. Even when it's green, there's no 12V output.

P.S. The main 12V power supply is definitely a flyback.

[lotas]

Re: Xbox 360 PSU failure diagnosis

Check the resistance at these points.

[stj]

Re: Xbox 360 PSU failure diagnosis

i have a feeling the power on signal should be 3.3v - use a resistor

[CapLeaker]

Re: Xbox 360 PSU failure diagnosis

Pin 8 of the NCP1377 PWM should have high voltage and that is where the 0 ohm resistors come in play. I am thinking the PWM is bad. I’d make sure that the mosfet and the gate drive is o.k., replace the 0 ohm resistors and the PWM and pray nothing else is bad.
What caused it? Could be a few scenarios starting from a lightning storm and ending by dirt accumulation and dampness between the HV pin and a different pin of the PWM.