hi Can anyone tell me how to check mosfet shorting on laptop mobo.
Thanks sop-8 type
Thanks sop-8 type
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Last edited by scooby01; 01-28-2012, 11:34 AM. -
Re: How to check mosfet
There is a section in the guide at (this is how I test them)Originally posted by scooby01 View Post
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Re: How to check mosfet
I successfully repaired an Acer laptop with a shorted top MOSFET and 2 SMD capacitors in the CPU core supply, caused by failure of the controller i.c.
A few words of warning here, taken from my experience with this one. You can probably find the faulty FET in circuit, mine was shorted and tripping the power supply (brick). Removing it is not easy, the one I had the tab was soldered to the board for heat dissipation, it has to be pretty damn hot to get it off (I used a hot air reflow station at 450C for a good while, shielding the nearby areas with foil).
Out of circuit the MOSFET is extremely vulnerable to ESD, I destroyed one by accidentally touching the gate. Clearly a wrist strap is a good idea here. During a meter test I also thought I had another shorted one, as it gave a reading of around 30 ohms between 2 pins after testing twice (first test had been ok). It did return to normal though, I assume my meter biased the device to saturation and narrowed the depletion layer, which took time to recover.
I don't know much about FET's to be honest, but you can usually tell a shorted one. By the way, remove the CPU before doing in-circuit tests as it presents a very low resistance and could easily have you drawing the wrong conclusion.Comment
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Re: How to check mosfet
450*C is fine for defective part removal and board destruction but don't put parts back on with that high of temperature. I dilute all the pads with lead solder from my iron to drop the melting point. For TO-263 and TO-252 I heat the tab with the iron at 350*C and heat the pins with the heat gun at 300*C.
With some practice MOSFETs come off and go on quick and easy even in confined areas.
With the MOSFET out of the circuit it is possible to charge the gate with the DMM such that the MOSFET drain to source appears to be a short circuit.Comment
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