Heatsink Mosfet

[seandepagnier]

I have mosfets in to220 package for motor controller.

I need to electrically isolate the mosfet tab from the metal enclosure. So far I have just used a silicon pad for this.

I decided to mount them under the pcb and use bolts to clamp the pcb to the aluminum heat sink, so I don't put bolts through the mosfet tab or need a special washer.

Can I use silicon tube/hose instead of the pad? I have some which fits perfectly over mosfets. I like this because it will stay in place, but I'm concerned about the heat transfer characteristics.

Are mica pads really much better than silicon? Is it worth dealing with the thermal adhesive?

 

[kellys_eye]

Do you know what heat transfer characteristic you need to meet?? Obviously a thicker insulation creates a poorer path for heat but WHAT that 'poor path' really is and how it affects your device depends on how much heat you are trying to dissipate.

 

[dave9]

I need drawings and pictures or something more than described. Generally no you can't just put a fet in a silicone tube and assume it will heatsink okay. The minimal thickness is very important.

 

[(*steve*)]

You have a number of options.

The simplest is to get a mosfet which is in a plastic TO-220 package, or maybe one with an insulated tab (these details are typically visible on the sellers web sites or can be found from the datasheet.

Another option is to use plastic bolts.

And finally the traditional option is to use special insulating washers which prevent the metallic bolt from contacting the tab of the mosfet.

My preference would be the first of these options, followed by the last. I'm not overly enthused in using plastic bolts.

 

[shrtrnd]

I use nylon bolts, they stand-up to heat better than a lot of the cheap non-descript ones I sometimes find in overseas factory installations where they save money by installing the cheapest parts they can find.