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Low saturation voltage output transistors

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  1. I was planning to use this part in a 2 mm sq package with thermal pads. Rds in the teens of mOhms.

Digikey is better than Google for this sort of search.

The thermal heat spreading pad is 1 square inch of PCB copper as clearly st ated. Why do you think it is anything different?

A DPAK foot print is 10x7 mm and must all be on the same side of the circui t board. The copper for this 2x2 package can be split between the top and bottom of the board and can be various shapes as long as any impact on the thermal conductivity is accounted for.

I love the way you think this is some sort of problem when the part is used every day in production in many, many products. I guess that the problems you get when you don't understand how to do the math.

--

  Rick C. 

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Reply to
Ricketty C
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I guess you could do your own Class D. Many years ago I did, but I have not be following what the integrated solutions offer (probably low THD)

Cheers

Klaus

Reply to
Klaus Kragelund

such chips

It doesn't get ruled out if you can put a thermoelectric refrigerator into the mix (low internal dissipation of a class D amp means you needn't pump much heat). I'm not sure, though, what TEC materials are useful in that temperature range.

Reply to
whit3rd

They have been used - though not by me and I don't know any detail - to cool a specific part, an image sensor. AIUI, a ziggurat of a few TECs was used. Generally, they're not useful, too inefficient.

Sometimes a Dewar flask is used with a lump of cold copper, but obviously, that's time-limited mostly by the dissipation of the contents, and the smallest OD of such an instrument is probably >60mm which limits the usefulness.

--
Cheers 
Clive
Reply to
Clive Arthur

One should remember that the "hot" side must get rid of not only the heat power from the "cold" side (that is, the dissipation of electronics to be cooled) but also the electric power fed into the Peltier element itself.

To dump the heat from the hot side into the environment, the hot side must be warmer than the ambient. If the ambient is air at 180 C, the Peltier hot side must be at least 200-220 C if a small heatsink is used.

OTOH, the Peltier effectiveness depends on delta_T (T_hot-T_cold), thus the hot side should be only slightly warmer than the ambient temperature, thus a big heatsink should be used on the hot side, to minimize delta_T.

Select a Peltier element rated for a few times larger heat than the heat power to be removed from the cold side. Adjust Peltier current for required delta_T

At least some Peltiers do not survive well in very hard vibration environment.

Reply to
upsidedown

And I think on most 'normal' devices the solder used is a low temperature type and they'd fall apart at 180'C.

It's something I've often wondered about - in this environment, even keeping things down to 150'C in 180'C ambient greatly improves lifetime and the availability of components.

As I understand it - and that's not well - a fundamental issue with Peltier devices is that you need an electrical conductor which is a thermal insulator, and that's tricky. Could you make a 150'C from 180'C ambient TEC using gas discharge tubes?

--
Cheers 
Clive
Reply to
Clive Arthur

So you have a device that tolerates 150 C and assume it internally dissipates say 5 W (based on your initial question). You have to attach your device to the cold side and use thermal insulation around the device and cold side to avoid side leakage.

As a minimum, you would have to transfer 5 W from 150 C to at least

180 C. However, this is not enough, since no power would flow into the 180 C environment. You need a higher hot side temperature to transfer the 5 W from the hot plate into the environment.

This is not the whole picture, since also the electric power fed into the Peltier needs also be transferred from the hot plate through the heatsink into the 180 C environment. Assuming the Peltier needs 20 W to transfer the initial 5 W into the environment, thus a total of 25 W needs to be handled by the heatsink. The heatsink temperature might be over 200 C, thus with 150 C cold side temperature, the delta_T is 50 C, which is quite a lot, requiring considerable Peltier power.

Check delta_T vs. power curves for a specific Peltier element to find how much Peltier power is needed, it can be quite significant.

Reply to
upsidedown

Sometimes I use several small transistors in parallel, to spread the heat out, especially on a PCB where the heat spreading is high-theta copper planes, or when I need a lot of insulation that adds thermal resistance.

--

John Larkin         Highland Technology, Inc 

Science teaches us to doubt. 

  Claude Bernard
Reply to
jlarkin

Back in 2011-12 I did an ultrastable laser for downhole applications. I tried very hard to get my customer's customer (Baker Hughes) to use a thermoacoustic fridge. I had a paper design (based on published TA fridge code) that would have got us about 50W of cooling of a 25C cold plate in a 180C ambient.

BH wouldn't go for it--apparently there's lore in the oil business about Stirling coolers being junk, so nobody wanted to hear about the TA approach. Stirlings are super fragile because they have a free piston inside, so it's easy to picture them not surviving long when they're getting chucked into the back of a truck and bouncing down a washboard road to the next well.

TA fridges have no moving parts--just a stainless steel tube with some copper and close-packed ceramic tubes inside. You dump a few hundred watts into a Chromalox element at one end, take the heat out of the middle, and the other end gets cold. They could easily be made to survive rough handling, but they weren't having any of it.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC / Hobbs ElectroOptics 
Optics, Electro-optics, Photonics, Analog Electronics 
Briarcliff Manor NY 10510 

http://electrooptical.net 
http://hobbs-eo.com
Reply to
Phil Hobbs

The Melcor/Laird ones use soft solder, but the Marlow / Ferrotec / Tellurex ones use hard solder. I routinely attach small TECs using InSn eutectic solder (138 C).

Don't think so. You need solid state band structure for the thermoelectric effect to work.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC / Hobbs ElectroOptics 
Optics, Electro-optics, Photonics, Analog Electronics 
Briarcliff Manor NY 10510 

http://electrooptical.net 
http://hobbs-eo.com
Reply to
Phil Hobbs

Yes, that's often done when the derating figures show that you can only dissipate negative power at the required temperature ;-)

--
Cheers 
Clive
Reply to
Clive Arthur

I tested some big n-channel mosfets. Gate threshold voltage declined with temperature, hitting zero, hard turnon, at about 300C. At 320, they failed irrecoverably.

I should have tried negative gate voltage. Maybe they could be turned off at 300C.

--

John Larkin         Highland Technology, Inc 

Science teaches us to doubt. 

  Claude Bernard
Reply to
jlarkin

Yes, you mentioned that before. Often multiple instruments (tools) are used and size is important - length can be a problem as the toolstring needs to be assembled and hoisted, and while small diameter tools can go into big holes, bigger diameter tools have a more restricted range of use.

Anything which limits the application area of a tool makes it harder to pay for.

However, some of the fancy imaging tools are necessarily big and usually deployed on their own, so you might try punting the idea to, for example, Archer at their Bergen Technology Centre, who have an ultrasonic imaging tool which I'm pretty sure won't run at 180'C and I'm pretty sure they'd like it to.

--
Cheers 
Clive
Reply to
Clive Arthur

This was for use down a 2-inch cased hole (38 mm max OD). It would have been about 3 feet long iirc. Last time I brought it up, Joe Gwinn accused me of trying to take food from his children's mouths by making the instruments last longer. ;)

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC / Hobbs ElectroOptics 
Optics, Electro-optics, Photonics, Analog Electronics 
Briarcliff Manor NY 10510 

http://electrooptical.net 
http://hobbs-eo.com
Reply to
Phil Hobbs

Might have been Syd Rumpo.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC / Hobbs ElectroOptics 
Optics, Electro-optics, Photonics, Analog Electronics 
Briarcliff Manor NY 10510 

http://electrooptical.net 
http://hobbs-eo.com
Reply to
Phil Hobbs

The several-stage solutions to get to cryogenic temperatures may not be good examples; you maybe need 30 to 80 C of delta-T, and we can adjust the 'load' heat output with high-efficiency class D operation.

Conclusions depend on those numbers.

Reply to
whit3rd

I don't recall this, but it certainly has possibilities. I like it.

Aww ...

Joe Gwinn

Reply to
Joe Gwinn

It might have been, I used to be Syd before it became tiresome.

--
Cheers 
Clive
Reply to
Clive Arthur

?C

Fun. Can you tell us one of the high T parts that 'everyone in the biz' knows about?

I used a to-220 npn darlington (TIP 120? I think) and a to-220 resistor as a heater. Typical operation was to keep a little aluminum cube (with lamp inside) at 120 C. But I ran one up to 150 C for several hours without any 'loss of function' that I could notice.

150 C was almost running this class A heater flat out so most of the heat was made in the resistor.

Are bipolars better than fets at high temperature?

George H.

Reply to
George Herold

I've tried a few power opamps. And for some unknown load. I found the LM675 is still pretty good.

Some managers idea, ?

George H.

Reply to
George Herold

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