Easy Way to Make Mosfet Insulation Pads
- #1
I am sure most of you must have experience with mounting Class A mosfet on the heatsink. The way it is mounted is of utmost important in order to transfer heat generated to the heat sink.
a. I assume all the mosfets must have a ceramics insulation between the heatsink and mosfet. Both sides of the insulation must be coated with thermal paste. Computer heatsink paste suitable?
b. How do I ensure when screwing the mosfet on the heatsink, the mosfet is flat?
c. I believe I need an insulation screw slot(to hold the scew) to isolate the mosfet backpak from the heatsink.
Can those who have more inputs to this please comment.
- #3
Yes apply thermal paste to both sides. Usually the insulator is mica. I dunno about computer grease / paste - but I would think it would be the same as 'standard'.
>b. How do I ensure when screwing the mosfet on the heatsink, the mosfet >is flat?
If the heatsink is flat, the mosfet will lay on it flat - nothing difficult.
>c. I believe I need an insulation screw slot(to hold the scew) to isolate the >mosfet backpak from the heatsink.
That sleeve / insulated bushing IS needed for TO-220 cases - but NOT for TO-247cases. If the mounting screw makes contact with a surface of the case which is electrically connnected to one of the leads, yes, use a bushing.
A fairly good-sized washer might be used on the face of the transistor to make sure the tightening force is applied evenly. Sometimes a flat bar is used for one or more transistor.
Some threads about insulators and sil-pads, etc. :
here
and here
A little more info
- #4
Many thanks to chipco and particularly to JCM........
- #6
dw8083,
Thermal pad will work but it is better or worse than mica with thermal paste(gool)? that is the question.. What is the best way to transfer heat away from the heat sink.
You see many computer DIY's remove their thermal pad between the CPU and heatsink and replace it with thermal paste. This way the transfer of heat is more efficient.
During amp testing period it is cleaner and easier to use thermal pad. For final installation, remove the thermal pad and replace it with mica and thermal paste. The latter has better heat transfer..If you think otherwise let me know.
- #8
If the heatsink is anodized and free from deep scratches on the surface where the mosfet touches, then you can dispense of the silpads, and mount the mosfet directly on the heatsink, with paste. This lowers the thermal resistance by a factor of 5 or so.
- #9
Lars Clausen said:
That works well - I do it on the F3 with the power JFET, where
the drain voltage is only a few volts above ground.
- #10
Lars Clausen said:
Are you sure that won't cause a short ?
I use the ceramic pad's. WOrk well and will never ever cause a short 
- #11
The isolation voltage of Aluminium Oxide (which is the surface layer after anodization) is 16 kV per mm, so with a layer thickness of 10 um, you have 160 V usable isolation. The actual isolation is usually better though. 10 um is the minimum layer thickness, and the paste also adds a little distance, and thus maybe 100V more of isolation.
I would use it up to 160 V, and sleep safely at night.
Watch out for scratches, and metal splinters!
- #12
Lars Clausen said:
The isolation voltage of Aluminium Oxide (which is the surface layer after anodization) is 16 kV per mm, so with a layer thickness of 10 um, you have 160 V usable isolation. The actual isolation is usually better though. 10 um is the minimum layer thickness, and the paste also adds a little distance, and thus maybe 100V more of isolation.
I would use it up to 160 V, and sleep safely at night.Watch out for scratches, and metal splinters!
True.. but to be safe id alway use my ceramic pad's.
Source: https://www.diyaudio.com/community/threads/mounting-mosfet-on-heat-sink.77572/
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