Insulated and bare tabs of power devices

Examples? I don't think I've ever seen a power device data sheet that didn't list individual junction-to-case thermal resistances for each and every package option.

I wouldn't make any assumptions -- other than if the data sheet doesn't go into much detail, then that's an indication that the part is designed with insufficient care, and should be avoided if possible.

--
My liberal friends think I'm a conservative kook.
My conservative friends think I'm a liberal kook.
Why am I not happy that they have found common ground?

Tim Wescott, Communications, Control, Circuits & Software
http://www.wescottdesign.com

Did you have any specfic parts/data sheets in mind?

The insulation they add has got to increase theta, usually a lot.

The best way to heatsink a power device is to bolt it to a bare, very flat copper heat spreader block, with silicone grease, and then bolt the block to the main heat sink, with an insulator if necessary. The best practical insulator is hard anodize on very flat aluminum.

TO-220s are not the best package. Thermal footprint is small, half of that (the wing part with the hole in it) is not very effective, and the bolt hole is off center.

Here's 32 power fets, clamp mounted on copper heat spreaders, then the main aluminum heat sink. The only insulator is between the heat sink and the chassis.

ftp://jjlarkin.lmi.net/Amp.jpg

Actually, it's the case-to-heatsink Rth, not j-c, that I'm concerned about.

One example is the popular LM3886 audio amplifier. It comes with T and TF suffixes, the latter being the insulated version.

Oi. (Sound of forehead slapping). My reading comprehension is down today.

Case to heatsink is your responsibility. There are ap notes to help, but

-- it's still your responsibility.

You can figure, though, that the case to heatsink thermal resistance with an insulated package will be less than the case to mica tab to heatsink thermal resistance of a non-insulated package, because there's only one thermal interface, and no thermal insulator, however thin.

I'd go on an ap note hunt (and, alas, I can't help you with guidance on where to look, other than I know that the Motorola RF transistor ones tended to be very detailed and informative).

--
My liberal friends think I'm a conservative kook.
My conservative friends think I'm a liberal kook.
Why am I not happy that they have found common ground?

Tim Wescott, Communications, Control, Circuits & Software
http://www.wescottdesign.com

An example is the popular LM3886 audio amplifier. It comes with T and TF suffixes, the latter being the insulated version. I've come across several other devices although I can't recall a specific type number off the top of my head.

Of course, but it's not always practicable to use that mounting method.

Unfortunately, I've never been able to connect to your ftp links, either with IE8 or FF. Maybe it has something to do with my ISP. But I'm familiar with the mounting method you described.

I agree, at least to some extent, that it's up to the end-user to provide a good device-to-heatsink interface. And a mica or other external insulator is certainly not an integral part of the device and therefore outside the control of the device manufacturer. However, an insulated tab _is_ a part of the device and it would be nice if the manufacturer gave us some idea of what to expect.

Once again, your confusion confuses me. If the manufacturer is selling something in an insulated case and publishes a junction to case thermal resistance, they mean the junction to the _outside_ of the case, not the junction to some internal feature of the case.

Then it's your job to figure out how to get the heat out of the case.

--
My liberal friends think I'm a conservative kook.
My conservative friends think I'm a liberal kook.
Why am I not happy that they have found common ground?

Tim Wescott, Communications, Control, Circuits & Software
http://www.wescottdesign.com

That's assuming that the thermal resistance from junction to the outside of the case is the same for both insulated and bare cases. Is that a safe assumption? The mounting tab of an insulated type is considerably thicker than that of a bare-metal type, indicating that the metal is covered with a not-insignificant layer of plastic. I would have thought that this would make at least a small difference even if the datasheet doesn't say so. With a device dissipating tens of watts, a difference of 0.1 C/W could mean a difference of several degrees.

Let's review: You gave a long spiel about junction to heatsink resistances, I assumed it was about junction to case, and said I didn't think that a data sheet would be inaccurate about this.

You said "no, no, it's about junction to _heatsink_". So I went and blattered on about that.

Now you're saying "no, no, it's about junction to case after all".

Make Up Your Mind.

The fact that they only list one junction to case number means one of two things: they're screwed up and you can't trust them, or that number is correct for both cases. If they're screwed up and you can't trust them, then, well, they're screwed up, you can't trust them, and you can't use that number for _either_ case. If that number is correct for both cases, well, they're not screwed up, you can trust them, and proceed.

I'd see if I could get one in the insulated case, test it into a resistor (I like my ears), and see how it does. If it does OK -- OK. If it doesn't -- I might experiment with the non-insulated version.

--
My liberal friends think I'm a conservative kook.
My conservative friends think I'm a liberal kook.
Why am I not happy that they have found common ground?

Tim Wescott, Communications, Control, Circuits & Software
http://www.wescottdesign.com

For a moment, you made me think that I'd screwed up and actually started typing an admission. Then I went over my posts again and saw that I didn't. You say "You gave a long spiel about junction to heatsink resistances......". I didn't mention J-HS at all in my opening post. It was about case-to-heatsink. Where I might have been unclear was that I was thinking about the thermal resistance from the metal tab (which I assumed to be inside the plastic) to the heatsink. I assumed that the metal tab is the same in both versions and that the plastic coating introduces an additional theta.

I mentioned the junction-to-ambient resistance to point out that the case-heatsink Rth can be a significant part of it. I repeat, I never mentioned J-C at all in my initial post.

I realise now that assuming that there's a metal tab inside the plastic type could be wrong. Maybe it's all plastic. Tomorrow, I'll see if I can find an old insulated TO-220 and break it open (it's well past midnight here now).

If I'm NOT wrong and both types have the same metal tab inside, with an extra layer of plastic in the insulated type, then that must introduce an additional Rth. My original question was whether that additional Rth is about the same as that of a mica insulator.

Just copy and paste the address into the browser address bar to see them :-)

Rheilly P

"Pimpom"

** Natsemi seem not to have published additional data for the thermal resistance of the TF pack for the LM3886TF.

Using 2 thou mica and thermal grease on a smooth, flat heatsink results in

0.2 degrees C per watt (mounting base to heatsink) - according to Natsemi's notes on the LM3886T. As this is a low voltage device, the plastic coating on the rear tab of the TF version is likely to be very thin but possibly not as thin as a 2 thou mica.

Comments on web forums suggest a figure of 1 degree C per watt.

... Phil

If it is not stated in the electricals of the datasheet, then you are on your own.

Chips in plastic might have a slug in the package. Or they just use a heavier bonding wire. [Heat transfer through the bonding wire is proportional to the square of the radius, so a little helps a lot.] Sometime they are put in the package upside down to get the die away from the PCB.

You can measure the junction temperature by forward biasing a diode on the chip. I can post how to do this if you are actually going to do the work.

"miso"

** Read the OP's question - f*****ad !!

Fuck you very much.

I've done that before. Doesn't work for me, and I remember some others saying that it doesn't work for them either. It works with other links that don't open directly from my newsreader, but not wih JL's. Tinkering with or turning off my firewall makes no difference.

Thanks. That about agrees with the ballpark estimate I had in mind. The difference between that and a bare metal + mica mounting is quite significant at high dissipation levels.

MJE340 / MJE350 has been a pain in this sense - nice to have multiple sources, but you never know if you get them with or without plastic insulation. I think ST or OnSemi datasheet looked like metal visible on the cover sheet, but the thickness in the last page indicated a plastic covered version. Also, most of the datasheets do not list thermal resistance..

--
Mikko

Data sheets assume best possible metal to metal contact (short of = welding) if they discuss Theta(c-hs) at all.

Absolutely not. Electrical insulators are always thermal insulators as well. The material properties go hand in hand.

?-)