High voltage and heat sinks

[pof@]

If I want to use a FET e.g.IRF840 for a high voltage regulator I will need a heat sink. But a heat sink inside the cabinet isn’t too good while the convection bad inside. But putting a heat sink with a FET connected to 400 VDC on the outside doesn’t sound like a good idea to me. How is this solved by others? Is good ventilation holes in the cabinet the only solution?

[stratus46]

Originally Posted by pof@

If I want to use a FET e.g.IRF840 for a high voltage regulator I will need a heat sink. But a heat sink inside the cabinet isn’t too good while the convection bad inside. But putting a heat sink with a FET connected to 400 VDC on the outside doesn’t sound like a good idea to me. How is this solved by others? Is good ventilation holes in the cabinet the only solution?

Typically the device is insulated from the heatsink and then a plastic cap covers any exposed terminals. Sometimes manufacturers have an alternate package that includes integral insulators. Is your device available in a different package?

G²

[pof@]

Originally Posted by stratus46

Typically the device is insulated from the heatsink and then a plastic cap covers any exposed terminals. Sometimes manufacturers have an alternate package that includes integral insulators. Is your device available in a different package?

G²

The only cover I can find is for TO3. TO220 cover is not possible at my favorite suppliers.

[HollowState]

Semiconductors mounted on heat sinks can be insulated with a mica or aluminum oxide spacer between the FET body and sink itself. Also use some heat sink compond on both sides of the insulator and a shoulder washer for the mounting screw. This method is used all the time and provides, for all practical purposes, just as good heat transfer as direct contact.

[pof@]

Originally Posted by HollowState

Semiconductors mounted on heat sinks can be insulated with a mica or aluminum oxide spacer between the FET body and sink itself. Also use some heat sink compond on both sides of the insulator and a shoulder washer for the mounting screw. This method is used all the time and provides, for all practical purposes, just as good heat transfer as direct contact.

But the terminals will be exposed

[HollowState]

So that's what they make heat shrink tubing for.

[pof@]

Originally Posted by HollowState

So that's what they make heat shrink tubing for.

I would prefer double isolation in this case if possible.

[M Gregg]

Hi,

This is what I would do!

The heatsink should be grounded so if a short takes place, it will blow a fuse on the unit "Kill the supply"!

The device should be insulated "mica" from the heatsink. Idealy the device should be mounted on the heatsink from inside the case. IE fins outside and device inside. Also if possible the screw should be "blind" (heat sink thick enough for the screw not to have to go through to the outside of the case).

Regards
M. Gregg

[taj]

I thought the question was about ventilation and removing the heat build up from the chassis cavity. If so, yes, just make sure there are holes in the chassis above and below the heatsink, and let convection take its course. Also make sure the fins are oriented vertically to permit proper convection.

I've also seen a nice chassis design where the heatsink was mounted on the back of the unit near the speaker connectors. More or less hidden from sight by the OP and power transformers.

Also, Kapton (polyimide) film works very well as an insulator. And YES, the heatsink should be attached (connected) to the chassis if it's on the outside.

..Todd

[pof@]

Originally Posted by M Gregg

Hi,

This is what I would do!

The heatsink should be grounded so if a short takes place, it will blow a fuse on the unit "Kill the supply"!

The device should be insulated "mica" from the heatsink. Idealy the device should be mounted on the heatsink from inside the case. IE fins outside and device inside. Also if possible the screw should be "blind" (heat sink thick enough for the screw not to have to go through to the outside of the case).

Regards
M. Gregg

It starts to make sence to me. I think mounting with a clip instead of a screw would be good.