Method for testing thermal coupling

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ilya
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Method for testing thermal coupling

Post by ilya » Sat Jun 20, 2020 6:13 pm

I'm making a device that requires thermal coupling of transistors. I can couple devices mechanically in several ways. I'd like to test and find out which of the coupling method works best.
Is there any recommended procedure or, maybe, paper that describes how to measure such a thing?

terkio
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Re: Method for testing thermal coupling

Post by terkio » Sun Jun 21, 2020 3:12 am

A google search on
"Measure thermal resistance"
might give you some results.

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mediatechnology
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Re: Method for testing thermal coupling

Post by mediatechnology » Sun Jun 21, 2020 5:51 am

Expanding on terkio's response you might visit the old National Semiconductor Voltage Regulator Handbook.
Though it was written for power devices it may have some useful information.
I recall it had a thorough discussion on thermal resistance.
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ilya
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Re: Method for testing thermal coupling

Post by ilya » Sun Jun 21, 2020 5:54 am

Thanks!
Sometimes the correct search phrase is important. I tried searching for "thermal coupling" and got no useful information. That's why I posted here.
I'll check out the Netional stuff and see what the "thermal resistance" brings.

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JR.
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Re: Method for testing thermal coupling

Post by JR. » Sun Jun 21, 2020 9:38 am

The obvious way is empirically... perhaps drive an active device with constant power dissipation and use an IR temperature detector to determine experimentally which HS gets hottest, an indication of lowest thermal resistance heat sink to junction. Back several decades ago we could buy temperature sensing dots that we stuck on heatsinks that changed color when they exceeded some nominal temperatures. These days we have better instrumentation.

Another even cheaper way is to measure the voltage drop of say a base-emitter junction. IIRC the Vbe voltage drops something like 3mV for every degree C increase (all else equal). Normalized for ambient temp, the highest Vbe voltage at constant power dissipation indicates the lowest junction temperature and most effective heat sink.

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mediatechnology
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Re: Method for testing thermal coupling

Post by mediatechnology » Sun Jun 21, 2020 9:57 am

One thing that I've found to be beneficial for equalizing the temperature differences in differential pairs is thermal mass.

Air current thermal effects tend to look like 1/f noise.
By making the mass larger, the thermal "1/f" corner frequency lowers.
I have some examples of that shown in the ZTX851 preamp thread.

I like JR's B-E junction measurement approach.
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Re: Method for testing thermal coupling

Post by terkio » Mon Jun 22, 2020 5:21 pm

Can you tell more about what you want to thermal couple and why ?

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JR.
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Re: Method for testing thermal coupling

Post by JR. » Mon Jun 22, 2020 5:59 pm

terkio wrote:
Mon Jun 22, 2020 5:21 pm
Can you tell more about what you want to thermal couple and why ?
Just guessing but I speculate he is trying to link two or more semi junctions together....

For example in a LTP you want both devices to track each other...

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Re: Method for testing thermal coupling

Post by ilya » Tue Jun 23, 2020 5:37 am

terkio wrote:
Mon Jun 22, 2020 5:21 pm
Can you tell more about what you want to thermal couple and why ?
JR nailed it. Yes, that's for coupling transistors in LTP, current mirrors etc. Maybe I'll get to the discrete VCA (JR will probably laugh at that).
It's still not clear to me how to measure the amount of tracking between the coupled components. I don't need to measure the effectiveness of the heatsink. Rather, how good is thermal transfer between the parts in different scenarios.
Thinking about that, maybe putting a fixture to the warm plate (or oven) and log the THD profile while it warms up to the plate temperature. Then do it with another fixture and compare the profiles.... Hmmmm...

When companies do potted blocks (when they pot for thermal reasons), do they measure actual thermal performance? How do they choose the exact potting method? That, probabply, is my main question.

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JR.
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Re: Method for testing thermal coupling

Post by JR. » Tue Jun 23, 2020 9:40 am

ilya wrote:
Tue Jun 23, 2020 5:37 am
terkio wrote:
Mon Jun 22, 2020 5:21 pm
Can you tell more about what you want to thermal couple and why ?
JR nailed it. Yes, that's for coupling transistors in LTP, current mirrors etc. Maybe I'll get to the discrete VCA (JR will probably laugh at that).
:lol:
It's still not clear to me how to measure the amount of tracking between the coupled components. I don't need to measure the effectiveness of the heatsink. Rather, how good is thermal transfer between the parts in different scenarios.
same way.... measure the Vbe drop while running at constant current... Maybe measure over time to ignore short term fluctuations.
Thinking about that, maybe putting a fixture to the warm plate (or oven) and log the THD profile while it warms up to the plate temperature. Then do it with another fixture and compare the profiles.... Hmmmm...
not sure how thd tracks with temp, Vbe is more literal.
When companies do potted blocks (when they pot for thermal reasons), do they measure actual thermal performance? How do they choose the exact potting method? That, probabply, is my main question.
They do the best they can...

I recall trying to use transistor arrays when possible, because the common silicon delivers better thermal tracking. FWIW one aspect of IC design involves temperature tracking across the silicon substrate, In some precision IC designs thermal management is a major factor.

The latest generation THAT corp VCAs are superior because of thermal tracking, and other solid state device improvements (behavior of NPNs vs PNPs, etc).

JR
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