Well if the machine was being used and then just shut off in the middle of something never to come back on, or maybe a short "burst" of it trying to come on but shutting down is the result of a dead short.
99% of the time it is a MOSFET somewhere, as ive seen cases where these things have actually caught fire before because of a shorted mosfet exploding. Its been a few years, but ive seen some pictures and cases online where some of those machines had popped and caught fire in a MOSFET cluster area.
Maybe the same FET failed, but less destructively. What you want to do is locate all your power rail clusters and read them to ground. the CPU v-core will read low ohms, this is normal. but it should NEVER read dead short. if any power rail reads dead short, you have found the faulty circuit.
Also pay attention to every semiconductor component on the motherboard. MOSFETs are in several different package types on the motherboard, some are in the higher power package types as you picture, but ive seen lower power MOSFETs hiding in your standard SOIC 8-pin or 14-pin style chips in totem-pole configurations. so it looks like a regular old integrated circuit chip but is actually a transistor. You can usually pick these apart because the most common MOSFET IC chips start with 4XXX like 4420 or 4430 etc being its only markings.
the 14-pin style are mostly FDSXXXX series MOSFET totem-pole ICs. There are shitloads of different ones, those are examples of what to look for. These little buggers drive me nuts becuase they are hard to locate and once you do, its smooth sailing. Ive fixed many of motherboards in my day, and some LCD TV mainboards with those little buggers shorted.
its rare, but another thing to check too once you do locate the shorted mosfet, is sometimes it can damage the gate drive resistor and speed-up shaping capacitor/diode. at worse, it could also damage the Maxim PWM control IC as well. But this is extremely rare.
Dont know about that mac, but i know on PC mobos they use an ENE chip that runs the entire power management and is the boss over all the buck/boost converters on the entire board. So i always start from there and work my way out.
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