LiteON PA-2311-02A blown fuse solved (iMac 27")

[Francisco.s]

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.

[sam_sam_sam]

Re: LiteON PA-2311-02A blown fuse solved (iMac 27")

Is it possible to post some pictures of this power supply

[PeteS in CA]

Re: LiteON PA-2311-02A blown fuse solved (iMac 27")

Gate resistors control how fast a MOSFET turns on and off. Too fast can cause noise problems. Turning off too fast can turn the MOSFET back on again due to the Miller capacitance.

The gate resistor controls how fast the gate capacitance is charged (and maybe discharged). That means the gate resistor sees current spikes every time the MOSFET is turned on (and maybe off). Calculating the dissipated power can be pretty complex. It may be that that gate resistor was dissipating too much power for the temperature of the environment in which it was operating.

[REpairAT]

Re: LiteON PA-2311-02A blown fuse solved (iMac 27")

Seems to be a standard issue on these.
For everyone else, if fuse blows up, check following components first:

- Q050 & Q051 "K10A60D" MOSFETs
- Primary fuse T6,3A
- 2x 10Ohm SMD Resistor backside

Cheers!

[JSalvadorTRZ]

Re: LiteON PA-2311-02A blown fuse solved (iMac 27")

Hello people,
Logged to share my issue with this PSU:

Failure: suddenly my Mac did not start anymore. Seems like some kind of over voltage at home...

I disassembled the PSU and found:

• Input fuse (6.3A blown)
• Diode bridge with 2 out of 4 diodes shorted
• MOSFETs from LLC stage short-circuited
• One power diode short-circuited
• 105k450VHF polyester capacitors blown (swollen, exploded)

I changed every single component. Also checked the MOSFET Q2 (sharing heatsink with one diode) and it is OK.
I assembled back the PSU on my Mac, no more fuse getting blown, but no longer turning ON of my Mac so far.
I guess there's still sth wrong in my PSU I did not realize yet.

Did you guys found sth similar to mine???

Thanks in advance

[lotas]

Re: LiteON PA-2311-02A blown fuse solved (iMac 27")

Read here, maybe you will find something useful.
https://forums.macrumors.com/threads...iteon.2243174/