E-Business
Issue No. 11 - June/July 2003
When You're Hot, You're Hot
by David Twiss
A big problem for many electronic systems is heat and how to get rid of it. Inside the integrated circuits of a digital system are transistors, and those transistors operate like switches. Much of that heat comes from the energy used to turn transistors on and off. Like a light switch on the wall it only takes a small amount of effort to flick the switch, but a single switch can turn on lots of lights, or control lots of energy.
Try switching the light on and off as fast as you can for a ten minutes. You’ll have a sore hand and a hot light switch, assuming it’s not broken! So it is with the millions of transistors that comprise the integrated circuits in our computers. The faster they are switched, the hotter they get.
So why not just let them get hot? Well, for each 10oC rise in temperature the rate of a chemical reaction doubles. In electronic devices, this creates “leakage currents” that roughly double with each 10oC rise. (Leakage currents are the wasted current flow through each transistor; a bit like water leaking through a tap that is turned off).
At the normal operating point of electronic devices, leakage currents are only a very small proportion of the total current, but doubling every 10oC means that a 50oC rise increases leakage current by a factor of around 32 times.
The extra leakage current raises temperature further, in a cycle that eventually becomes a self-sustaining thermal runaway which will damage or destroy the device. (In particular devices there can also be several other factors at work to make the situation worse.)
Even if thermal runaway does not result, operating electronic devices at elevated temperatures severely reduces their Mean Time Between Failure (MTBF) and can often lead to unstable operation.
MTBF is usually an estimate of how many operating hours you get before a failure. This estimate is a statistical construct, and is not an indication of average component life. For example, a...
