I want to share how I solved a weird issue in a power supply from an iMac 27", that presents a blown fuse (6.3A 250v)
There were no mosfet or capacitor short-circuited, in fact I placed a bulb instead of the blown fuse, and the bulb light for a moment, having 320 volts at the output of the pfc circuit.
I have replaced the fuse with a new one and the pfc output doesn't reach its normal working value. (the resonant part of the psu didn't work at all, no control pulses from NCP1396A)
I can measure some burst pulses at the drain of the pfc power mosfet.
There were pulses in the gate of the pfc power mosfet, but only 4 v. peak to peak.
Pin 10 of NCP1605A outputs the pfc control pulses, and these pulses had a good shape and amplitude. (about 8 volts)
That difference was certainly the problem of the psu, and it was due to a 10 ohm resistor damaged. (it measured 3 megaohms)
This resistor is mounted in series with the gate of the power mosfet.
After replacing this 10 ohms resistor the psu works like a charm.
I hope this information can help somebody* with this super complicated psu.
There were no mosfet or capacitor short-circuited, in fact I placed a bulb instead of the blown fuse, and the bulb light for a moment, having 320 volts at the output of the pfc circuit.
I have replaced the fuse with a new one and the pfc output doesn't reach its normal working value. (the resonant part of the psu didn't work at all, no control pulses from NCP1396A)
I can measure some burst pulses at the drain of the pfc power mosfet.
There were pulses in the gate of the pfc power mosfet, but only 4 v. peak to peak.
Pin 10 of NCP1605A outputs the pfc control pulses, and these pulses had a good shape and amplitude. (about 8 volts)
That difference was certainly the problem of the psu, and it was due to a 10 ohm resistor damaged. (it measured 3 megaohms)
This resistor is mounted in series with the gate of the power mosfet.
After replacing this 10 ohms resistor the psu works like a charm.
I hope this information can help somebody* with this super complicated psu.
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