dell 7720 dual N mosfet keep burn, cpu VCC Core

[casti3l]

hello, i need help, i recived this laptop with a short, after a few min i identify the problem PQ43, i replaced the mosfet with normal N mosfet and laptop work for few day, after burned again. Now i tried many types of mosfet's, now it burn after a few min.
In this moment i replaced PQ45, PQ44, PQ43 with BSC0924NDI, same thing 10 min working fine then die. PQ43 keep burn.

[zpi]

Re: dell 7720 dual N mosfet keep burn, cpu VCC Core

Hello, i think you should change PQ43 and PU10 at the same time. PU10 drives PQ43 so it can be culprit.

[piernov]

Re: dell 7720 dual N mosfet keep burn, cpu VCC Core

You have to use the same MOSFETs as what was on the board originally and replace the controller at the same time indeed as @zpi said.

[casti3l]

Re: dell 7720 dual N mosfet keep burn, cpu VCC Core

thank you, i will try to find identical mosfet, and also replace PU10. I will replay after that.

[zpi]

Re: dell 7720 dual N mosfet keep burn, cpu VCC Core

@piernov, why they have to be same as originally on board? Can't they be different, just with same or better specs? I think you can, but have to make sure you look closely at datasheets.

[emestee]

Re: dell 7720 dual N mosfet keep burn, cpu VCC Core

@piernov, why they have to be same as originally on board? Can't they be different, just with same or better specs? I think you can, but have to make sure you look closely at datasheets.

They can be if you understand what the important parameters are in a given circuit. Sadly, we cannot expect that everyone who does this kind of thing understands deeply how e.g. MOSFETs work internally, and what physical properties have side effects that are meaningful for the operation of the boards. Frankly, I don't claim that I do. Experience shows that even the board engineers often don't, and that's how you end up with boards which have the same burned out transistors in the same place every time. So very often you get an answer that is technically correct, but errs on the safe side. "Better" in a load bearing circuit is different from "better" in a switching power supply.

[piernov]

Re: dell 7720 dual N mosfet keep burn, cpu VCC Core

Yes you could replace them with a different part number but there are a lot of parameters to be taken into account so it may not be easy to find a matching one. Also they must have the same part number across all phases in a multiphase buck converter.

Some parameters to take into account:
— Package and pinout (must be the same of course)
— Vgs and Vds breakdown voltage (must be same or higher)
— Id current rating (must be same or higher)
— Rdson resistance (must be same or lower)
— Ci input capacitance (similar or lower)
— Qg total gate charge (similar or lower)
— Tdon turn-on delay time (similar or lower)
— Tdoff turn-off delay time (similar or lower)
— Body diode forward voltage (similar or lower)

Vds is a hard requirement but it's rarely a problem, you just have to make sure it's at least 30V.

MOSFETs with higher current rating often have a lower Rdson, that's a good thing (less losses when turned on), but they conversely are often slower and harder to drive (higher input capacitance/total gate charge and slower turn-on/off delay times), so you can't got crazy on the current rating because it'll be counterproductive: the MOSFET will be much less efficient when switching and the MOSFET driver will have a hard time driving it.

Some MOSFETs (very often the low-side on synchronous buck converters) have a schottky-type body diode, that has a lower voltage drop than a "regular" body diode. (otherwise it'd be less efficient)

Thermal characteristics also play a small role in this so it's better to have a MOSFET with low thermal resistance overall. (this is often dictated by the package, but there are still variations)

These are especially critical in multiphase buck converters like CPU VCore and GPU VCore since the MOSFETs are stressed a lot, these buck converters must suddenly deliver dozens of amps when there's a load spike. Even up to a few hundred amps on high-performance desktop parts.

[zpi]

Re: dell 7720 dual N mosfet keep burn, cpu VCC Core

Thanks for good explanation