Asus X705mb - not turning on

[reformatt]

Re: Asus X705mb - not turning on

Power MOSFET's used in buck converters usually have inbuilt diodes from drain to source. Saves the designer from having to add them externally. So you should see in diode test a reading in one direction from drain to source, and open the other. The diode is there to protect the FET from the back EMF of the coil when it switches off.

Take note when metering these out of circuit that diode mode on G-S will switch the FET on and it will then appear shorted D-S.

[lofaszjancsi]

Re: Asus X705mb - not turning on

When measuring resistance between drain and source of a MOSFET and the value is small (ohms rather than k ohms), then the MOSFET is bad.
If you measure resistance of a capacitor and it is infinite, then most likely, the capacitor is good. When capacitor fails, it goes short, when resistor fails it goes to infinite resistance. Keep that in mind.

Thank you, I'll recheck everything.
Edit: the board had no power all night (it is 6 am here, last powerup was yesterday 10pm), and the resistance between the MOSFET's drain and source is 28 ohms or if I switch sides ~80 ohms, there's another MOSFET of exactly the same type right beside it, that show infinite resistance measuring the same points. The only thing that seems interesting with the bad one is that the gate doesn't look to be shorted.

[Adenitz]

Re: Asus X705mb - not turning on

A question to Adenitz:

Is this measurement method also possible in circuit?
Is a mosfet not charge-driven? So it can have both a low and a high resistance between drain and source? When I tested the P channel mosfet today in diode mode a saw both open line and a small voltage between source and drain.
Could you please explain this to me?

Yes, the measurements are possible in the circuits. I usually do like this:
1. Switch multimeter in diode mode and measure between drain and source and source and drain (or just reverse leads) In one direction, you should read values in range (0.2-0.5) and in other direction OL or some high value. This is because of the built in body diode.
2. Swicth multimeter to resistance mode and measure resistance between D-S and S-D. This value should be in kohm and not in ohms.
3. Measure in diode mode or resistance mode each pin (D,G and S) to ground. Depending of the schematics, on leg can indeed be connected to ground (for example source).

It is possible that you have a conducting MOSFET when measuring, but when you practice measurements and immediately reverse leads and measure again, this should give you pretty accurate picture whether the MOSFET is faulty or not.

For example, if you measure 30 ohms between D and S and similar value between S and D, then MOSFET is for sure short circuited.
Go ahead and apply the described way of measurement on your board (with all power removed) and you'll see.

[lofaszjancsi]

Re: Asus X705mb - not turning on

So measuring <100ohm between s and d from both directions is 100% short right?
Edit: it's still strange that the gate isn't shorted anywhere, i mean due to physical damage (bent PCB) is it possible that the gate survived?

[Adenitz]

Re: Asus X705mb - not turning on

So measuring <100ohm between s and d from both directions is 100% short right?
Edit: it's still strange that the gate isn't shorted anywhere, i mean due to physical damage (bent PCB) is it possible that the gate survived?

Measuring 100 ohm between S and D in both directions means that MOSFET is bad 100%. I really wouldn't be curious about Gate, this is irrelevant since you have a faulty MOSFET.

[lofaszjancsi]

Re: Asus X705mb - not turning on

Measuring 100 ohm between S and D in both directions means that MOSFET is bad 100%. I really wouldn't be curious about Gate, this is irrelevant since you have a faulty MOSFET.

Well if the gate would be shorted to s/d that would mean that the whatever connected to it (that drives it) might be shot too

[Adenitz]

Re: Asus X705mb - not turning on

Yes, that is true. If Gate is shorted to D-S that it is most probably hit by some higher voltage. This means that IC chip (driver) could be bad. ON the other hand. D-S short on high side MOSFETS are very dangerous for the rest of the circuit, because it can send much higher voltages on low voltage power rails.
Please have a look at the attached pictures. In one case 1.35V is produced from the PWR_SRC (can be 12V or higher) and in other case 1.05V is produced.
High side MOSFET D-S short can cause 10 times higher voltage to hit that rail and all components on it.

[lofaszjancsi]

Re: Asus X705mb - not turning on

Yes, that is true. If Gate is shorted to D-S that it is most probably hit by some higher voltage. This means that IC chip (driver) could be bad. ON the other hand. D-S short on high side MOSFETS are very dangerous for the rest of the circuit, because it can send much higher voltages on low voltage power rails.
Please have a look at the attached pictures. In one case 1.35V is produced from the PWR_SRC (can be 12V or higher) and in other case 1.05V is produced.
High side MOSFET D-S short can cause 10 times higher voltage to hit that rail and all components on it.

Well i measured 19v around the MOSFET I found bad, so that's not good news. Now it seems that I have many dead mosfets, found at least 3 on different positions that show really low resistance between s&d. I tried to disable them by touching dmm's positive to gate and negative to source and even to the ground.
Sorry for all these dumb things, I'm not really experienced.
Edit: i see the issue on the pictures you sent

[lofaszjancsi]

Re: Asus X705mb - not turning on

Another dumb question:
Am i right, that the M3056M (MOSFET QFN-8 N-CHANNEL 30V 103A QM3056M6 QM3056M M3056M) cannot be replaced with an AON6520 (30V 50A) (https://cdn.badcaps-static.com/pdfs/...85e49fbd9d.pdf)

[EineWildeStehlampe]

Re: Asus X705mb - not turning on

Firmware from another device where the TXE region hasn't been cleaned may stop this one from POSTing iirc. I had a very similar one of these ASUS boards in last week, X705MA I think, which wouldn't power on. Turned out to be a part of the ROM was wiped for some reason (when compared to a "known working" image), TXE region had to be cleaned before it would POST.

[EineWildeStehlampe]

Re: Asus X705mb - not turning on

TXE Region of the "known working" dump had to be cleaned before it would POST. My bad, that wasn't very clear.

[Adenitz]

Re: Asus X705mb - not turning on

Another dumb question:
Am i right, that the M3056M (MOSFET QFN-8 N-CHANNEL 30V 103A QM3056M6 QM3056M M3056M) cannot be replaced with an AON6520 (30V 50A) (https://cdn.badcaps-static.com/pdfs/...85e49fbd9d.pdf)

Please read this:

https://alltransistors.com/mosfet/ho...for_mosfet.php

The biggest difference is Rdson resistance. 6x greater.if thatis my only option, I would try it, but only as last option

[lofaszjancsi]

Re: Asus X705mb - not turning on

TXE Region of the "known working" dump had to be cleaned before it would POST. My bad, that wasn't very clear.

Thank you, i'll check that.

[lofaszjancsi]

Re: Asus X705mb - not turning on

Please read this:

https://alltransistors.com/mosfet/ho...for_mosfet.php

The biggest difference is Rdson resistance. 6x greater.if thatis my only option, I would try it, but only as last option

Thanks, i think i'll order a few of the original to be sure.