Heat sinking an IRL520 Mosfet

[pjrebordao]

The board I'm building contains 3 IRL520's Mosfet, which I intend to heatsink.

However, the heatsink tab appears to be electrically connect to the Drain pin... How do I safely attach a heatsink (an aluminium strip) ? I can insert somethink to isolate it, but how to prevent the fixing screw and nut to make contact with the tab ?

 

[JimPerry]

http://www.ebay.co.uk/itm/TO-220-TRANSISTOR-INSULATING-KIT-Pack-of-20-/330835047313?pt=UK_BOI_Electrical_Components_Supplies_ET&hash=item4d074e3391 or similar

 

[srnet]

Ah .....

Mica washers and heatsing paste .........

 

[pjrebordao]

Thanks, but why do they do it in the first place ?

 

[JimPerry]

Would have been more logical to use the source - but that's life

 

[srnet]

Thanks, but why do they do it in the first place ?

What is it that 'they' are doing ?

 

[pjrebordao]

tying the tab to the drain

 

[srnet]

tying the tab to the drain

I would always start by assuming that those who design semiconductors on a lage scale have a basic understanding of what they are doing.

So, why do you think the designer(s) tied the tab to the drain ?

 

[pjrebordao]

I never doubted there was a reason, but as i'm not an EE i have to ask.

 

[Dippy]

It's always good to ask.

If buying that thin sheet insulation stuff don't forget those little plastic mounting bushes as shown in that Ebay image.

I'm sure someone can Google and provide instant expert advice. My guess; the drain is the most convenient/largest part of the semi blob to get the heat out.
Sometimes it's a pain, but often handy when devices are laid flat on pcbs.
The option, where available, is to get the plastic version.

 

[Jeremy Harris]

The reason is to do with thermal conductivity and the way that the chip inside is constructed, mounted and connected. The tab extends down into the package and the silicon of the chip is physically bonded to it, to ensure that the heat from the (very tiny) chip is conducted efficiently to the tab. It so happens that the back layer of the chip is the drain layer of the semiconductor, so that's the layer that ends up connected to the tab. The other connections are made by welding tiny wires from the semiconductor to the copper strips that form the pins, and have virtually no capacity to conduct heat at all.

The chip that makes up a power MOSFET like this is built up in layers, from the bottom (drain) silicon layer to the source layer at the top, with the insulated gate layer buried in the middle. The source layer isn't flat (because of the buried insulated gate), so would be difficult to connect to the tab and get good thermal conductivity, hence the reason they are made the way they are.