This unit comes in with symptoms:
Will Not Charge
Will Not Work With Good Battery
Diagnosis begins by following the DC IN from the DC Jack. The voltage would drop to 3v and not enough to power laptop or close any gates on the MOSFETs.
Taking the MOSFET [FDS6675] source side out of circuit shows the direction of the shorted components. Since the power branches to the charge section or the run section with the other MOSFET shown below it. Checking to voltage at the source shows +19v. When in soldered on the board, the MOSFET DRAIN voltage is +3v. After replacing the FDS6675 the DRAIN side is still shorted.
Further testing indicates larger capacitor shown (in circle picture 1) is also shorted, removing it from circuit voltage on Drain side is now +19v. But the laptop would not turn on.
Still further testing and following the circuit path leads to the top side (facing technician when opened) to a series of three capacitors. Removing one side of this bank and testing each capacitor finds the shorted capacitor. As it turn out it was the topmost cap (marked with X in image), and when removed it was horizontally spit into 2 pieces.
Unit lights up!
Sorry for the picture orientation, but it appears blogger puts them in reverse sequence
You can read up on how MOSFET works in WIKIPEDIA
But in simpler terms –
There are 3 basic parts of a MOSFET. Source, Gate and Drain.
It acts as a doorway. When power is supplied to the GATE current or voltage crosses to the other side. In most laptops, common MOSFETs are 8 pins.
Pins 1,2,3 are SOURCE (power or current in)
Pin 4 is the gate
Pins 5,6,7,8 is the DRAIN (power or current out)
There are more complex MOSFETs such as Dual Channel with 2 gates, 2 sources, or 2 drain it is large matrix of numbers and combinations. P-Channel, N-Channel and others. Knowing what is what requires a good memory, manual or cross reference index.