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Tube❄️Time

i'm really hoping that a simple FET replacement will get this thing going reliably. the motherboard is technically off the shelf (Adlink M-880-Nitro) but people want lots of money for them.

14 comments
Dantali0n :arch: :i3: replied to Tube❄️Time

@tubetime seems a wonder it didn't die with that mosfet but maybe CPUs have features against mosfet latch up or the pmic detects it and shuts off

Tube❄️Time replied to Tube❄️Time

the MOSFETs arrived. who knows if this will work...
but hope it does

Tube❄️Time replied to Tube❄️Time

well it boots but now the frame isn't detected. so there's something wrong with the PCI card, the cable, or the backplane itself.

Tube❄️Time replied to Tube❄️Time

BTW, this is why the CPU didn't fry--the multiphase buck controller (the ADP3168) has crowbar protection in case of a shorted high-side MOSFET!

OUTPUT CROWBAR

As part of the protection for the load and output components of the supply, the PWM outputs are driven low (turning on the low-side MOSFETs) when the output voltage exceeds the upper Power-Good threshold. This crowbar action stops once the output voltage has fallen below the release threshold of approximately 450 mV.

Turning on the low-side MOSFETs pulls down the output as the reverse current builds up in the inductors. If the output over- voltage is due to a short of the high-side MOSFET, this action current limits the input supply or blows its fuse, protecting the microprocessor from destruction.
Tube❄️Time replied to Tube❄️Time

so the sequence of events in this case went:
Buck MOSFET failed
Buck controller crowbarred
12V power supply went overcurrent and shut down
Sequencer CPLD detected that and shut everything else down.

Tube❄️Time replied to Tube❄️Time

it would be nice to have a second probe cable. this is the less common 60-pin cable, not the stubbier 50-pin one used on the 1650 series. they're super hard to find for some reason.

Tube❄️Time replied to Tube❄️Time

thinking about the failure a bit more. here's the precise cause with a nice diagram. a high-side MOSFET failed in a short circuit, which would put 12V on the sensitive CPU core rail. however, the controller chip detected the overvoltage and turned on all the low side FETs (circled in green).

Andy replied to Tube❄️Time

@tubetime Do you know whe there are only three high-side FETs but six on the low side?

Dantali0n :arch: :i3: replied to Tube❄️Time

@tubetime ah so the pmic did protect the CPU, as expected, nice to see that even in this era such protections where already in placw, can't even begin to imagine the complicity of a pmic powering a modern Intel or AMD CPU.

pcbeard replied to Tube❄️Time

@tubetime I was making popcorn. Great troubleshooting!

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