Fanless Power Supply: Marko's Homebrew

Do-It-Yourself Systems | Power
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Unsoldering the MOSFETs

It would be easiest to unbolt the MOSFETs, and unsolder them separately, but often you won't be able to do that, because the transformers and capacitors are in the way. They were for me. By unsoldering the two big capacitors, I could easily unbolt 4 of the 6 MOSFETs, which really helped, as I didn’t have a de-soldering "sucker" tool when I did this. Don’t forget to note down where each MOSFET was!

The other heatsink shown in the 2 pictures above is a bit more interesting. Two MOSFETs were screwed to the heatsink with mica shims in-between, so they could be transferred to the new HS and mounted the same way.

But the other two MOSFETs were directly screwed to the HS without insulating shims. The metal part of the MOSFET with the mounting hole is connected to its middle pin. So the middle pins of these MOSFETs are connected to the heatsink. One anchor of this heatsink was also soldered to a live trace. I checked the traces on the PCB, and saw that the middle pins of these two MOSFETs and the heatsink are all soldered to the same trace. In other words, the heatsink is not the only electrical path for the connection between the middle pins and the anchor point. Even after removal of the HS, the electrical circuit would be unchanged. Whew!

Extension Wires

The next task was to solder extension wires to the PCB mounting points for the MOSFETs. I used wires from a old blown-up PSU that was kicking around. They were the cables to the Molex connectors, which explains the colors. They seemed to be thick enough so the electrical resistance is not too high. (Editor's note: Usually 18 AWG -- gauge) They shouldn't be any bigger, otherwise it will be difficult to solder cleanly to the PCB. Now you should plan how you want to mount the HS, so you don’t use wires that are too long, like I did! Double-check that you didn’t short any of the wires when soldering on the PCB.

Before soldering the wires to the MOSFETs, you should maybe first shorten the pins a bit. Check the right MOSFET on the photo above.

Make sure you wires are soldered to the correct pins. Messing this up could be very dangerous! Color coding the wires for the 3 pin types might be a good idea to help you with this. Don’t forget to insert the heat shrink tubing over the wires before soldering, and solder as cleanly as possible!!! Apply heat from a strong hot hair dryer (or heat gun) to ensure the heatsink tubing provides insulation protection for the MOSFET pin solder joints.

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