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Old 26th September 2010, 06:42 PM   #1
pof@ is offline pof@  Denmark
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Default High voltage and heat sinks

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 isnít too good while the convection bad inside. But putting a heat sink with a FET connected to 400 VDC on the outside doesnít sound like a good idea to me. How is this solved by others? Is good ventilation holes in the cabinet the only solution?
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Old 26th September 2010, 06:50 PM   #2
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Originally Posted by pof@ View Post
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?

G≤
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Old 26th September 2010, 06:55 PM   #3
pof@ is offline pof@  Denmark
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Originally Posted by stratus46 View Post
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?

G≤
The only cover I can find is for TO3. TO220 cover is not possible at my favorite suppliers.
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Old 26th September 2010, 06:57 PM   #4
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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.
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Old 26th September 2010, 07:00 PM   #5
pof@ is offline pof@  Denmark
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Originally Posted by HollowState View Post
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
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Old 26th September 2010, 07:02 PM   #6
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So that's what they make heat shrink tubing for.
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Old 26th September 2010, 07:05 PM   #7
pof@ is offline pof@  Denmark
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So that's what they make heat shrink tubing for.
I would prefer double isolation in this case if possible.
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Old 26th September 2010, 07:05 PM   #8
M Gregg is offline M Gregg  United Kingdom
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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).

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M. Gregg
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Old 26th September 2010, 07:06 PM   #9
taj is offline taj
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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

Last edited by taj; 26th September 2010 at 07:18 PM.
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Old 26th September 2010, 07:08 PM   #10
pof@ is offline pof@  Denmark
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Originally Posted by M Gregg View Post
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.
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