I was playing around disassembling an Asus laptop power brick (19V, 4.74A) and came across this strange arrangement in the circuit, where just after the bridge rectifier, there is a MOSFET (Q1, N channel) that can make a <1 Ohm connection from the + supply (330Vdc) to the negative. This seems to be its only function, and seems it can only result in a blown input fuse. Further down the line as you can see in my rough schematic is of course the switching MOSFET (Q2) feeding the SMPS transformer.
Any of you engineers out there, what the heck is the purpose of this ludicrous circuit, if it is there to protect from overvoltage/spikes, wouldn't a MOV be much cheaper than the MOSFET, and wouldn't result in a bricked device after such an incident?
I'm quite curious to hear your thoughts! Thanks in advance!
EDIT I kind of lied in the schematic, L4 is actually some kind of transformer, whose secondary goes towards the MCU, it is seen on the board as the yellow tape clad transformer with the wide copper foil strip over top. FYI