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
The only cover I can find is for TO3. TO220 cover is not possible at my favorite suppliers.
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.
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
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
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
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
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It starts to make sence to me. I think mounting with a clip instead of a screw would be good.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
I would keep the device inside the unit and ground the heatsink!
The screw through the device should be isolated with a shoulder plastic washer, however if the screw comes into contact with the device and goes outside the unit it is a hazard! I would use mica with heat sink compound not the plastic pads you can get!
Then again the screw outside will be in contact with heat sink "this is grounded!".
The screw through the device should be isolated with a shoulder plastic washer, however if the screw comes into contact with the device and goes outside the unit it is a hazard! I would use mica with heat sink compound not the plastic pads you can get!
Then again the screw outside will be in contact with heat sink "this is grounded!".
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You should calculate the power disspation that device will need to contend with as a worst case. Voltage across * current through. That should be fairly easy if you have a schematic for the amp. Compare that result to the device datasheet to determine how much heatsinking you will need. It may be less than you think -- or not.
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It depends on size of your gear, dissipated power, and allowable overheating of the die. Dependence of thermal resistance between heatsink and atmosphere on temperature is non-linear. It depends on surface area, orientation of fins in respect to air flow, distance between them, form of them. The best I saw was a copper needle-type heatsink with round fins 8 mm apart.
It can be reduced also by a forced air flow. Forced air flow allows to reduce distance between fins below 8 mm, making heatsinks more compact. In tube amps forced cooling may be originated by hot tubes that heating surrounding surfaces generate forced air flow. It is hard to calculate, but modern infrared cameras can give pretty informative pictures where different temperatures color the picture in different colors. Amazing tool, I want to buy one...
Heat sink sandwich.
Put heat sink on outside of chassis and use thermal compound when mounting. Clamp semiconductor with appropriate thermal compound and electrical insulator to the chassis on the inside at the same location as the heat sink on the outside.
Yes, thermal resistance is slightly higher, but will work fine.
For better thermal performance, cut an opening in the chassis such that the semiconductor can mount directly on the heat-sink, but still exist on the inside of the chassis.
Put heat sink on outside of chassis and use thermal compound when mounting. Clamp semiconductor with appropriate thermal compound and electrical insulator to the chassis on the inside at the same location as the heat sink on the outside.
Yes, thermal resistance is slightly higher, but will work fine.
For better thermal performance, cut an opening in the chassis such that the semiconductor can mount directly on the heat-sink, but still exist on the inside of the chassis.
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