Attaching and Isolating TO-220 devices from heatsink

[luvdunhill]

Hey everyone! I have a quick question about the best way to attach four TO-220 devices to this heatsink:

I was planning on tapping the back of the heatsink and then attaching the devices to the heatsink using screws. However, since the devices need to be isolated from each other I wasn't sure what people use... it seems that plastic screws will not be strong enough, and I cannot run a bolt on the other side. Can anyone recommend a mounting method? The devices are IRF610 and will be dissipating around 3.5W each.

Also, I was thinking about attaching my PCB to the heatsink as well, any recommendations on standoffs and screws for this application as well?

Thanks!

 

[r221b]

There are shouldered washers tha fit into the TO-220 mounting holes that isolate the screws from the transistor mounting tab. You'll also need an insulating thermal pad underneath. You CAN use nylon screws as an alternative, but you'll still need the thermal pads. The trick with nylon is not to tighten them too much. The idea is to get a snug fit against the heatsink. Look at Digi-Key (www.digikey.com) or Mouser (www.mouser.com) to find the hardware. They also have a large variety of standoffs. You can usually get those in brass, steel, aluminum, or nylon, depending upon your needs.

sherlock

 

[luvdunhill]

hm, okay... I found three kits that might fit the bill... any recommendations?

532-4880M 4880MG Mica Insulator Mounting Kit for TO-220. Includes:
1 Flat Washer #4, 1 No. 4-40 UNC-2B Hex Nut, 1 No. 4-40, UNC-2A X 1/2 Long Phillips Pan Head Screw, 1 Lockwasher, No. 4

532-4880 4880G Thermalfilm™ Mounting Kit for TO-220. Includes:
1 Shoulder Washer, 1 Flat Washer #4, 1 No. 4-40 UNC-2B Hex Nut,
1 No. 4-40 UNC-2A X 1/2 Long Phillips Pan Head Screw, 1 Lockwasher, No. 4

532-4880SG 4880SG Thermalsil™ III Mounting Kit for TO-220. Includes
1 Shoulder Washer, 1 Flat Washer #4, 1 No. 4-40 UNC-2B Hex Nut,
1 No. 4-40 UNC-2A X 1/2 Long Phillips Pan Head Screw, 1 Lockwasher, No. 4

So, can I use these with devices like IRFP9240 and IRFP240 that will dissipate more heat, like in the Pass F4?

As for the standoffs, would something like ceramic be a good choice, so it doesn't wick heat up to the PCB from the heatsinks? Any recommendation on how far the PCB should be off the heatsink?

Thanks!

 

[unclejed613]

use the thermasil if you don't want to mess with heat sink goop. sil pads don't need any goop on them, but the mica pads do. when i'm doing repairs, however, if a transistor using sil pads seems to have failed thermally (sometimes the heatsinks aren't in the best locations for proper cooling), i will add some goop to the sil pad. with the heatsink you're using, go with the sil pads. make sure there's no burrs around the holes, or any dings in the surface under the transistors. i like to lightly sand the surface to flatten it. a stray piece of solder or aluminum caught under the transistor will poke through the pad and short, even with mica pads. while to most, the size of heat sink you are using seems like overkill, remember that the lower the temperature, the lower on the derating curve your transistor is.

 

[AndrewT]

Hi,
instead of bolting the tab direct to the sink, one can clamp the body of the device to the sink.

The "clamping bar" can be any solid metal strip stiff enough to carry the clamping loads without bending.

6mm thick by 10mm wide aluminium strip would do nicely.

Then one only needs electrical insulation between the back of the FET and the sink. No isolated screws or other hardware.

 

[Eva]

Clamping may be achieved with the help of the own PCB by sandwitching the transistors between it and the heatsink and placing screws in suitable places. This works better when transistors are mounted in pairs (with a screw inbetween).