Dell Inspiron 5520 - PQ101

[caspian]

Dear Members,

I have a Dell Inspiron 5520 Laptop.
A capacitor on the B+ rail was shorted and I replaced it. But I noticed that the source and the drain of PQ101 are always shorted together. Its gate is not shorted to source or drain.
Is this normal?
Do I have to replace PQ101 or I should leave it as is?
What harm will happen in the future if I use a PQ101 that is always shorted from source to drain?

Thanks

[Sephir0th]

Re: Dell Inspiron 5520 - PQ101

No, that's not normal

You're Sure about that? Discharged the Mosfet by touching with Finger? Short disappears when you Remove PQ101?

However, you don't want a shorted DC-IN mosfet even If the Board is still working. They're there for a reason Like protecting from Reverse Current and so on. So Better to replace this one, If you're Sure it's shorted.

[caspian]

Re: Dell Inspiron 5520 - PQ101

Thanks for your recommendation.
I removed PQ101 and discharged it. but It is always shorted.

[caspian]

Re: Dell Inspiron 5520 - PQ101

Is AO4409L compatible if I replace it by AO4423 on PQ101?

[caspian]

Re: Dell Inspiron 5520 - PQ101

https://www.badcaps.net/forum/showpo...&postcount=275

[caspian]

Re: Dell Inspiron 5520 - PQ101

I have another laptop and its second DC-IN Mosfet is shorted too.
Is it usual if the second DC-IN Mosfet such as PQ101 become permanently shorted from source to drain in Laptops, in general?
Is it OK if we leave them shorted?

[mcplslg123]

Re: Dell Inspiron 5520 - PQ101

@caspian, its not advised to leave that mosfet in shorted condition. Replace it. AO4409 can be replaced by 4407A/4435/4835D. Ao4423 will work but not advisable.

[caspian]

Re: Dell Inspiron 5520 - PQ101

thank you, @mcplslg123.
AO4423 has a higher amperage tolerance than AO4409. The other parameters of AO4423 are better than AO4409. But I realized that AO4423 may be a weak Mosfet. But I am not sure.
Why is AO4423 not advisable to work as PQ101?

[caspian]

Re: Dell Inspiron 5520 - PQ101

In this Dell Inspiron 5520, the AC adapter is not original but it has short-circuit protection. If B+ is already shorted to GND, when I connect the AC adapter, the adapter turns off.
However, while the AC adapter is connected and B+ is not shorted, if suddenly a B+ capacitor gets shorted to GND, then too much current flows through that capacitor and through PQ101 so that PQ101 gets damaged by becoming permanently shorted from source to drain.
This is strange to me. Please explain the reason.
Why do not the AC adapter turn off fast to prevent damaging PQ101?
Why do not PU100 (with current sensing using PR102) turn PQ100 off fast to prevent damaging PQ101?
Is some protection circuit defective in this laptop that cannot prevent damaging PQ101?
Are all laptops acting the same when a B+ capacitor gets shorted?

note:
I used healthy Mosfet for PQ101, once AO4423 and once AO4407A. Shorting capacitor happen three different times in this laptop and in all the three times, PQ101 got damaged.
PQ100 never got damaged in the three times and seems to work well.

[Sametbey]

Re: Dell Inspiron 5520 - PQ101

When the circuit diagram is examined, in case of a short circuit at +B, it should switch to protection in 0.2ms. However, both the ISL88731 charge control and the PQ101 input control have used many transistors in succession. protection cannot be activated in real time. maybe more powerful mosfet can be used. The PQ100 will not fail. There is a diode addition in the conduction direction.

[caspian]

Re: Dell Inspiron 5520 - PQ101

thanks for your complete explanation.
Is it common in all laptops that the second DC-in Mosfet gets damaged when a B+ cap gets shorted?