Electronics > Projects, Designs, and Technical Stuff
Adding heatsink with electrical isolation
T3sl4co1l:
Thermal pads can be used in those enclosures, with moderate success, but yes, you've found one good reason why they are to be avoided where much power dissipation is required.
Tim
jbb:
SIL pads and similar are great. Their conductivity seems to be a bit lower than mica + grease, but mica + grease is delicate and messy. I’m guessing you’re not after maximum performance given you’re using an enclosure heatsink.
Could you put the TO220 right at the end of the board so that you can slip a clip over it once the PCB has slid all the way into the enclosure?
How much voltage does the insulator need to stand off? If it short circuits, would you expose a user to dangerous voltages?
BravoV:
--- Quote from: EEEnthusiast on December 11, 2019, 02:09:41 pm ---The electrical isolation is guaranteed by the precision in the assembly process only. The TO-220 package height above the PCB is fixed and so is the heat sink height. The other heat sink on the case is also fixed at a constant location by marking it.
--- End quote ---
--- Quote from: EEEnthusiast on December 11, 2019, 02:30:00 pm ---My issue is with the case style. The case I am using is a aluminium extrusion with open ends on two sides. The PCB slides into the case from the sides. So there is no easy way for me to access the package once the PCB is inside.
--- End quote ---
As you said you can do precision assembly process, although not used at extrusion style case, below are examples used a lot at common switching psus, this is from Meanwell.
I'm guessing this technique can be implemented in your case, all you need is just to precisely drill a hole at the case for the screw, and provide a metal insert sandwiched between the case and the TO-220 body as below.
The gray box is a TO-220 encased inside a box style thermal pad (cap) as above Wraper's example post, as this is a HV transistor, its secured using a strong "L" shaped steel with a hole (threaded) for the screw, when fastened will make the L steel pressing the TO-220 body, and its secured/fastened from "outside" of the case.
Same as above, this is the schottky diode using the same securing style. You see clearly a big piece of metal insert behind.
andy3055:
Rather than complicating the process by precision engineering the “heat exchange” type h/sinks, why not mount the transistor (with proper electrical insulation-I would use mica/plastic washer/thermal compound) in a convenient position that does not interfere with the PCB and have the leads soldered to 3 wires that end up in a plug? This is a common practice in a lot of things. Examples can be seen in most Tek scopes. As long as you have a tight fitting plug, I don’t see any issue.
See attached image where you can see the transistor on the left of the EHT transformer on a 465M scope.
EEEnthusiast:
--- Quote from: jbb on December 11, 2019, 11:50:04 pm ---SIL pads and similar are great. Their conductivity seems to be a bit lower than mica + grease, but mica + grease is delicate and messy. I’m guessing you’re not after maximum performance given you’re using an enclosure heatsink.
Could you put the TO220 right at the end of the board so that you can slip a clip over it once the PCB has slid all the way into the enclosure?
How much voltage does the insulator need to stand off? If it short circuits, would you expose a user to dangerous voltages?
--- End quote ---
The voltage is well below 24V. I think the idea to move the TO-220 to the edge of the board would work. I would still be able to access the TO-220 after the PCB is slid into the slot.
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