Help figuring out buck converter

[P3tras]

Hello everyone,

New to the forum. I recently bought a faulty xbox one s and trying to track the fault and got stuck a little bit.

Xbox powers on and then immediately powers off. I figured that main processor is not getting power. Checked MOSFETS, no short circuits. But found something odd in Mosfet control circuit.

In all repair videos they check voltage drop (diode mode) between low side mosfet gate and source which are likely to fail. Normal readings are 0.6V with negative on gate and 1.9V with positive on gate. Video with timecode as an example: https://youtu.be/HEceb1Qxyec?t=5m40s
On my console I am getting 1.1V (1.1kOmh) with negative on gate and 38kOmh with positive on gate, which is much higher than it supposed to be. I am having issues trying to figure out where these values are coming from. Definitely not mosfet, as I got confirmation that values stay the same even with mosfet completely removed. I suspect maybe mosfet controller, but don't want to start removing parts randomly. Found datasheet for same/similar mosfet controller. Pinout did match and also internal resistance values between gate output and SW/GND pins.

Extra information that might help. It is 3 lines buck converter (1 powers CPU and 2 power GPU). Readings are the same on all 3 lines. Each line has its own mosfet controller and all 3 mosfet controllers are controlled by larger NCP4205 chip. I found datasheet for NCP4200 chip, which is similar, however pinout does not match the board. However, Mosfet side of circuit seems to be very similar as on the board itself.

Circuit diagram:


Tried to do the same test on other power mosfets on the board. There is one more buck converter powering CPU northbridge. Values read out fine on it, even though circuit is not the same (same mosfets, but different controller).

Photo of area that I am talking about: https://photos.app.goo.gl/Z6boeB8P3X3VmKnR9


Any help is greatly appreciated. Not sure if such gate-source readings are unique to this xbox or other buck converters are similar, but it would be great to have any insights before I start removing components randomly trying to figure it out.

[MathWizard]

Do you have an oscilloscope to see what's on any of the power-good, fault signals, current sense signals IDK enough about fets or contorler chips to say, but comparing them against each other, out of circuit or in the same circuit maight be worth it.

[P3tras]

Used to have an old school oscilloscope one from like 70's but it recently broke 

Anyhow, I desoldered mosfet controller and now gate pin is completely open circuit. So gate goes straight to mosfet controller and nowhere else. New mosfet controllers ordered. But it is strange that it is same fault on all 3 phases, I wonder what caused it.

I also unsoldered low/high side fets. Source>drain diode check is OK, but I am unable to initialize FET using multimeter. Basically using multimeter to charge the gate (to close circuit on mosfet) and discharging it with finger. Now I wonder if mosfets are faulty as well or such method does not work on all Fets.

[P3tras]

OK, so I ordered new mosfet controllers. Replaced one and readings are exactly the same as before. Then figured out that these readings are coming from SW and mosfet driver pins, as switch is connected straight to mosfet source pins. Block diagram of the regulator attached.

When measuring in 200komh range, multimeter is outputting 0.3V and I get same reading both ways (both polarities). That is almost 40komh, which matches datasheet. When I measure in lower range, my multimeter outputs 2.6V and I get 1.1komh with negative on mosfet gate (drv pin), but nothing when I flip terminals. Higher voltage is enough to trigger internal semiconductor, but I was thinking maybe my multimeter supplies too low voltage or current, thus I do not get same results as others. Not really sure what is happening inside.

I've taken same measurement with brand new mosfet regulator (outside the circuit) and I get exact same readings. So it's either something happening outside this circuit that affects all 3 mosfet regulators or simply an issue with my readings.