Everything You Know Is Wrong February 2004

Answers to those Doggone Thermal Design Questions

By Tony Kordyban

Copyright by Tony Kordyban 2004

Dear Tony,

The chip engineer tells me our new ASIC dissipates 30W, but I want to measure it.  The ASIC die has a special diode which gives the junction temperature directly.  So I can measure the junction temperature, but apparently it is very tricky to measure the electrical power of the entire chip.

In one of your previous columns, you suggested that component power could be estimated by measuring case temperature, then comparing that to a CFD model of the component on its board.  You modify the component power in the CFD software until the case temperature in CFD matches the measurement.

But isn’t there another way of getting to the power number? Here is an idea I have:

Put on top of the component package a layer of material of known thermal conductivity and known thickness, and measure the temperature on top of that layer.

Then take that off, and do the same thing with another, different material with different thermal conductivity.

Then I have the following equations,

Power = (Tj – Tmaterial1)/(Rjc + Rmaterial1)

Power = (Tj – Tmaterial2)/(Rjc + Rmaterial2)

Combining these equations to get rid of Rjc I get:

Power = (Tmaterial2 – Tmaterial1)/(Rmaterial1-Rmaterial2)

Tmaterial1 and Tmaterial2 are measured and Rmaterial1 and Rmaterial2 are known, and so Power can be calculated.

What do you think about this approach?

A Newbie in the thermal jungle

Dear Newbie,

It is a good thing that new people are entering the field of electronics cooling.  I welcome you and your fresh ideas.  No one has suggested this idea to me before, and it has some merit.

Notice I didn’t say it would work.  Just that it has merit.

I like your idea of using two different materials to cancel out the unknown and unreliable Rjc, (thermal resistance between the junction and case.)

There is one serious problem with your method, the same problem that Rjc has:  you assume that all the power conducts out of the die via a single path — out through the top of the case to the air.  In reality, heat conducts out of the die through multiple paths.  They can be summarized by saying some flows into the air through the top of the case, and the rest conducts through the leads into the printed circuit board and spreads out in the copper traces and planes.  After that, it goes from the board into the surrounding air.

Given that there are multiple paths for the heat, and each path has its own temperature drop, just where do you propose to put your materials of different conductivity?  Just putting a layer on top of the component won’t do, because that will just force more of the power to conduct into the board.

I have an idea that might make your method practical:  force all the component power to go out through the top of the package, and none into the board.  How do you do that?

Place your material on top of the component package.  The rest of the board has to be covered with insulating material, the best insulator you can find.  If you perfectly insulate the board from the environment somehow, then all the component power has to go through your layer of material to the air.  Then apply your two-material method, and you should get a reasonably good estimate of the component power.

Actually, you will get not just the ASIC power, but the power of all the components on the entire board.  Because if you perfectly insulate the board, then the heat from all the other components will be forced to flow through the board, then through your ASIC to get to the air.  That ASIC would certainly get quite warm.  Another drawback is that you could probably measure the total board power more easily by just measuring the total current draw of the board at its input connector.

So my idea is not so great either, unless your ASIC is the only component on the board.  But if it were the only component on the board, you could just measure the current directly, so you wouldn’t need this thermal method.

Sorry, this method just can’t work.

Maybe with three different materials …

Dear Thermal Explainer,

I don’t know who is dumber, my buddy Jeff for asking this question, or me, because I couldn’t give him a good answer.

Here goes:  To cool something off you blow air on it with a fan, right?  If you put your hand in the air blowing out of a fan, it feels cooler than the surrounding air.  But if you put a temperature probe at the inlet of the fan and in the exhaust, they read exactly the same temperature.

How do I explain to Jeff how the fan can cool something down without making the temperature lower?

Mutt the Confused One

Dear Mutt,

Here are some facts to start with:

1.  Fans don’t make air colder.  Your temperature probes are correct.  A fan makes the air move faster.  It doesn’t change the air temperature.  (OK, you wise guys — yeah, the fan can make the air a little hotter by causing friction and from the heat given off by the fan motor.  That’s not what we’re talking about here!)

2.  Your hand does not measure temperature.  Your sense of cold and hot is strictly a feeling of whether heat is going into or out of your skin.  If your hands are freezing cold, and you stick them under the cold water tap, doesn’t the cold water feel hot?  That’s because your skin is colder than the cold water, and heat is going into your skin.  Don’t rely on your skin to measure temperature or you could end up in real hot water.

3.  A fan can’t cool something off unless the air temperature is lower than the temperature of the object you want to cool.  A fundamental law of physics is that heat flows from high temperature to low temperature.  For heat to flow from your object (like your hand) to the air, the air has to be at a lower temperature.  The fan just makes the heat flow faster in the direction it already wants to go.

Sit down with Jeff and memorize these facts.  They should not be that foreign to you or anybody else.  Put out of your mind the myth that fans cause cooling.  Fans push air.  They are little paddles on a wheel that push air along as the wheel spins.  That is a concept that is easy to grasp.

Once you have that in mind, we can talk about a cherry pie on the kitchen table.  It has just finished baking, and it will take an hour to cool before you can eat it.  But you and Jeff are hungry now.  You can’t wait an hour.  You want it to cool faster.

It is already cooling, because the pie is hotter than the surrounding air.  Chunks of air make contact with the pie, heat up, and move along with natural currents, carrying some heat away from the pie with them.

You and Jeff find an electric fan on the back porch.  You plug it in and point it at the pie.  Now more chunks of air per minute are hitting the surface of the pie (the technical term is crust).  Each chunk carries away some heat, so now more heat per minute is being carried away from the pie.  More heat per minute means you get to eat the pie sooner.

The fan did not reduce the air temperature.  It did cool the pie faster by throwing more chunks of air at the pie per minute than it was getting before.  Does that make sense?

Then you touch the crust, and decide the pie is cool enough to eat.  But when you bite into pie, the inside is still piping hot and you burn your mouth.  That will teach you another lesson, something about the Biot Number of cherry pie filling and transient heat transfer.  But that lesson will wait for another day.  I’d write about it now, but all this talk about pie has made me hungry.


Isn’t Everything He Knows Wrong, Too?

The straight dope on Tony Kordyban

Tony Kordyban has been an engineer in the field of electronics cooling for different telecom and power supply companies (who can keep track when they change names so frequently?) for the last twenty years.  Maybe that doesn’t make him an expert in heat transfer theory, but it has certainly gained him a lot of experience in the ways NOT to cool electronics.  He does have some book-learnin’, with a BS in Mechanical Engineering from the University of Detroit (motto:Detroit— no place for wimps) and a Masters in Mechanical Engineering from Stanford (motto: shouldn’t Nobels count more than Rose Bowls?)

In those twenty years Tony has come to the conclusion that a lot of the common practices of electronics cooling are full of baloney.  He has run into so much nonsense in the field that he has found it easier to just assume “everything you know is wrong” (from the comedy album by Firesign Theatre), and to question everything against the basic principles of heat transfer theory.

Tony has been collecting case studies of the wrong way to cool electronics, using them to educate the cooling masses, applying humor as the sugar to help the medicine go down.  These have been published recently by the ASME Press in a book called, “Hot Air Rises and Heat Sinks:  Everything You Know About Cooling Electronics Is Wrong.”  It is available direct from ASME Press at 1-800-843-2763 or at their web site at http://www.asme.org/pubs/asmepress,  Order Number 800741.