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December 2005
library  >  Application Notes  >  Tony Kordyban

Everything You Know is Wrong -- PART VI

answers to those doggone thermal design questions
by tony kordyban



dear thermal pessimist,


isn't your job actually getting easier day by day? it seems like every year the voltage level of digital logic devices goes down another notch. years ago everything ran on 12v and 5v. now you got your 3.3v logic family, your 1.8v stuff, and 0.5v is just around the corner. isn't the heat generated proportional to the square of the voltage drop? pretty soon the power you deal with will be only 1/100th as much as your grandpappy handled back in the good ol' days of ttl (twentieth thentury logic). why be so pessimistic, except about your job security? 


pollyanna, cape of good hope


dear polly,


i am pessimistic about everything, including job security for electronics cooling specialists. but not for the reason you give.


you are right that power (heat generation) is related to the square of the voltage drop. power per gate has gone down dramatically, and thermal issues for run-of-the-mill logic devices like flip-flops, buffers, and inverters have been eradicated like smallpox. but gate sizes have shrunk at an even faster rate. in the time that power per gate has been divided by a factor of 100, the number of gates on a single die has gone up by a factor of something like 1 million. there have been lots of other changes that have prevented the average processor power from being 10,000 watts. the point is that power per area is still going up faster than power per gate is going down.


thermal challenges continue to get worse with time. power per chip is growing exponentially. there are plans to continue shrinking die features, and clock speed appears to be able to increase without limit. power is directly proportional to clock speed for technologies like cmos.


one of these days they will figure out how to make integrated circuits in three dimensions. that will increase gates per volume, while reducing the surface area. and surface area is the only tool we thermal engineers have to pull out the heat.


optical computing and optical communications have just entered the game. lasers produce much more heat than they do light, and are very sensitive to temperature. lasers with tight wavelength specs actually work best when cooled below normal room temperatures. they are some of my biggest headaches (opportunities!).


and in case you think our jobs are much easier today because of all the advances in computer analysis tools, like desktop cfd, i agree, up to a point. ever try to find flash memory that is rated to 125 degrees c ambient? if my "grandpappy" couldn't cool his 8 khz micro by slapping on a heat sink, he'd just bump up the device temperature rating from commercial to industrial grade, or even military grade if he had to, for a few extra bucks. thanks to the end of the cold war, nobody makes military grade components anymore.


all that should point to better thermal job security, right? not when you take a closer look at this minimally researched timeline of electronics cooling history: 


vacuum tube computers

thermal engineers are created to add heat sinks, fans, and water cooling to keep tubes from burning out every five minutes


transistors replace tubes

thermal guys become tv repairmen


ttl integrated circuits

re-hire thermal guys


cmos replaces ttl

thermal specialists re-trained to write c code


high frequency cmos, introduction of the 100w microprocessor

mechanical designers and analog circuit gurus take 3-day courses in cfd


worst thermal problems ever

electronics cooling viewed as a legitimate engineering discipline


it is pretty obvious, at least to me, that we are overdue for a huge, unforeseen breakthrough in technology which will wipe out all our thermal problems for the next 5 to 10 years. what will it be? i haven't got a clue.


could the person plumbing cold water into eniac have foreseen the invention of the transistor?


i saw a paper presented at semitherm a few years ago that may provide a hint (v. guruprasad, synchronous coherent extraction of heat, proceedings of ieee semiconductor thermal measurement and management symposium, 1998). i couldn't tell if the guy was a genius or a fruitcake, or both. his idea was that cmos devices all generate heat synchronously -- all gate transitions occur on a clock signal. that makes them predictable. so you could add some other device (exactly what it was, he was somewhat vague about), acting out of phase with the clock, that could either cancel out the heat, or recapture the energy back into electricity, or laser light. maybe it was perpetual motion. but some outlandish notion like that is bound to put me out of a job soon.


that is my kind of pessimism.



dear mr. temperature,


my grandpappy always told me that the electronics cooling business was cyclical. do you have a favorite anecdote about it from the world of tv, or maybe science fiction?


z. cochrane, talos iv


dear zeph,


i'll do you one better -- a true story about electronics cooling from sci-fi and tv.


in 1966 they were filming the original star trek series. the show was set a few hundred years in the future -- when thermal problems were so important that an engineer was a major character. scotty was always warning the captain, i forget his name, that his engines were about to overheat, or that the dilithium crystals were on the verge of cracking from channeling too much energy.


the show's writers must have become familiar with these kinds of problems by just hanging around the enterprise bridge set. robert justman, the associate producer of the series, described the situation in inside star trek, the real story, (herbert f. solow and robert h. justman, simon and schuster, 1996.) 

"it was the equipment, specifically that which powered all the console read-outs and light displays in the bridge set. this was 1966. there were no computer chips or integrated circuits yet. left on for any length of time, all the old-fashioned vacuum tubes, flickering lights, and wiring built up heat inside the cramped consoles until the overheated circuits blew out.


we shut down the displays during rehearsal, but that didn't solve the overheating problem. heat built up continuously, and by afternoon the circuits overloaded within minutes of being activated. our only solution was to rent portable air conditioning units. large flexible ducts snaked across the stage floor and pumped cold air into special effects boss jim rugg's complicated electronics."


so cooling electronics was critical in 1966, and will be again when the star date is 4213.8. the trouble is convincing the bosses how important it is in between.






about 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 20 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 b.s. in mechanical engineering from the university of detroit and a master’s in mechanical engineering from stanford. in those 20 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 at https://www.amazon.com/hot-air-rises-heat-sinks/dp/0791800741. this advice column is an extension of that educational effort. 

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