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Old 29th April 2009, 11:09 AM   #1
dognut is offline dognut  United States
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Default why insulate transistors????

Hello to all. On the smaller transistors like the 220s why do we use insulator pads when were trying to do is get rid of the heat into a heatsink? Why not just apply heat compound to the transistor and bolt it right to the heatsink? Most of the time the heatsink justs sit on top of the circut board anyway.Thanks, tom
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Old 29th April 2009, 11:32 AM   #2
AndrewT is online now AndrewT  Scotland
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if your heatsink cannot accidentally be shorted to supply or output or ground, then direct mounting with thermal paste is the best way to keep the device cool.

However, due to vibration, the extra weight of the sink will cause fatigue of the device connections to the PCB. If this is serious then it will eventually break the device leg/s.
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Old 29th April 2009, 12:10 PM   #3
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another consideration, one i've always wondered about. if you bolt a transistor directly to the heatsink, the heatsink now becomes an extension of the transistor collector. it's also a huge stray capacitance. as long as the collector is at rail voltage, this isn't much of a problem, but if the collector is the output element (in a common emitter amp for instance) it's a huge problem. with an insulator, you now have a very small capacitance in series with the larger capacitance, and the total capacitance is smaller than either capacitance. also, live heatsinks are a safety hazard. we recently got a couple of large Pioneer receivers in for repair. the output devices were mounted on two heat sinks on insulated plastic brackets. there were no warning stickers or indication otherwise that these heatsinks were live. sure enough one was at +60V and the other at -60V. those are large heatsinks and it's difficult to work around them. one tech got bit by brushing up against one. i found out they were live by clipping the -lead of my meter to one of them and starting to take voltage measurements in the amp section. the voltages i was seeing didn't make any sense the speaker wires coming off the amp board appeared to be at +60V as well as the ground wire to the amp board. it was then i figured out that the heatsink wasn't grounded. good thing i didn't connect a scope ground to it.... SMPSs are another place to beware of live heatsinks, i.ve been bit by live heatsinks on those a couple of times.

if you connect a heatsink directly to a rail voltage. put a sticker or something on it..... there have been some pro-audio manufacturers that used those little box heatsinks for TO-3's and they mark them with stickers
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Old 29th April 2009, 06:43 PM   #4
dognut is offline dognut  United States
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Default heatsink heat

Thanks guys.I didn't think about the potential voltage that would be there.Thanks again, tom
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Old 30th April 2009, 04:05 AM   #5
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there are a lot of manufacturers making TO-220-iso devices where the whole package is plastic and can be mounted directly to a heat sink. there are a lot of TO-3P packaged devices with the heat sink tab insulated as well. you still need heat sink grease or sil pads to fill gaps between the device and heat sink. the sil pads aren't just there as electrical insulators, but to fill gaps and actually transfer more heat than you would if you just bolted the device directly to the heat sink.
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Old 30th April 2009, 11:14 PM   #6
Minion is offline Minion  Canada
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Also if you are useing both NPN and PNP transistors on the same heatsink with a dual rail supply the transitors will short out ...
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Old 30th April 2009, 11:35 PM   #7
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Quote:
Originally posted by unclejed613
there are a lot of manufacturers making TO-220-iso devices where the whole package is plastic and can be mounted directly to a heat sink. there are a lot of TO-3P packaged devices with the heat sink tab insulated as well. you still need heat sink grease or sil pads to fill gaps between the device and heat sink. the sil pads aren't just there as electrical insulators, but to fill gaps and actually transfer more heat than you would if you just bolted the device directly to the heat sink.
Any conformability that the silpads have that might help thermal conductivity should be more than outweighed by the silpads' thickness (i.e. resistance to conductivity). I wouldn't use an insulator unless insulation was actually needed. Always use thermal compound, though.
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Old 1st May 2009, 10:45 AM   #8
jitter is offline jitter  Netherlands
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Default Re: why insulate transistors????

Quote:
Originally posted by dognut
Hello to all. On the smaller transistors like the 220s why do we use insulator pads when were trying to do is get rid of the heat into a heatsink? Why not just apply heat compound to the transistor and bolt it right to the heatsink? Most of the time the heatsink justs sit on top of the circut board anyway.Thanks, tom
Bolting the component straight to the heatsink will cause it to get the same potential (voltage) as the metal part of that component. If the heatsink is positioned in a way that there is no electrical connection to other parts with different potentials, then you can leave out the insulation pad. You may want to consider the remark unclejed613 made about parasitic capacitance: in some cases you might be better off with the pad.

Often more components with different potentials are bolted to one big heatsink. Then insulating them is mandatory, otherwise you will have one big shortcut, obviously.

Yes, the pad has some thermal resistance, but that's small enough not to be of importance, UNLESS the chosen size of the heatsink is barely able to keep things cool. Undersizing is never a good idea here (just ask Microsoft about the Red Ring of Death with their XBOX's), overheating is one of the most common causes of shortened component life.
Thermal paste will only be beneficial if used sparingly, otherwise it will actually make things worse, not better. If you take a look at guys assembling a PC (the ones that know what they're doing), you will notice they use no more thermal paste than the size of a grain of rice between CPU and heatsink. The pressure of the heatsink will spread this tiny bit out over the whole surface.
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Old 1st May 2009, 11:43 AM   #9
wwenze is offline wwenze  Singapore
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IMHO the thermal resistance of the small heatsink inside a non-active-cooled case is too high for the insulating pad's thermal resistance to be of any significance. We're not talking about 130W CPUs that need to be kept below 60 degrees by a huge active cooler.

And personally I hate anything live. Imagine poking around while the thing is running, and... zap. I've also been bitten by live heatsinks of SMPS b4.
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Old 1st May 2009, 05:27 PM   #10
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IMO depends on your application. Many times, you want the HS to be isolated, but other times, heat transfer is priority.

On a large class AB CFP amp I built, the collectors go to the leads of the output resistors, so there's only a few mV between the sinks, and they are not grounded. Being there's no thermal insulator at all, the transistors never feel hotter than the heatsinks, and the heatsinks are also fan cooled as well. I've had no problems with stability, and I use a good zobel network also.

However, most of the time, it's best to use a thermal insulator.
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