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Recently I was doing a little research about SFP/SFP+ modules by reading the SFF-8472 spec.

In there, I learned that SFP modules use TECs (Thermoelectric Coolers) to "pump" heat away from the load.

My question; how come this technology is not deployed on common Desktop and Laptop (x86) machines that we use every day? From disassembling an ordinary Desktop, it is clear that the only thing between the heatsink and the CPU is thermal paste.

Andrew

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  • \$\begingroup\$ Because physics. \$\endgroup\$ Commented Jun 4, 2015 at 4:11
  • \$\begingroup\$ Could you be a little more clear? I saw on this Application Note (assets.newport.com/webDocuments-EN/images/…) on pg.3 that a TEC can be used between the load and heatsink. So why not on Desktop machines? \$\endgroup\$
    – Andy J
    Commented Jun 4, 2015 at 4:19
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    \$\begingroup\$ They take lots of power, and desktop machines run cool enough already. \$\endgroup\$ Commented Jun 4, 2015 at 4:23
  • \$\begingroup\$ In order to completely get rid of the heat generated by X power, you need to expend X+Y power in the TEC. So in the end, you're actually expending *2*X+Y power. \$\endgroup\$ Commented Jun 4, 2015 at 4:23
  • \$\begingroup\$ In SFP modules, the TEC is generally not used for cooling so much as for holding a constant temperature. This temperature may actually be significantly above room temperature. The reason for this is the wavelength of the laser is temperature dependent, so the wavelength stability depends on the temperature stability. This is very important for wavelength division multiplexing where the wavelengths have to be muxed and demuxed with narrow filters. A CPU, on the other hand, just needs to be kept at a reasonable temperature. \$\endgroup\$ Commented Jun 4, 2015 at 7:30

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A TEC is known in physics jargon as a "Peltier element" because it uses the Peltier effect. The peltier effect is semiconductor magic that cools one side of an array of p/n junctions while simultaneously heating the other side of the it. You should think of it more as a heat pump (thermodynamically) and not direct electric cooling.

A peltier is a curious beast. You can use it to heat something or cool something just by reversing current, with a proper control loop and a temperature sensor you can also keep something at a precise temperature. It also works in reverse, you can generate modest amounts of current into a low-z load by heating one side and cooling (or insulating from the heat) the other. There are a few consumer devices that use this to generate small amounts of power directly from a heatsource.

To answer your question. A CPU produces some amount of heat energy every second (Thermal Power), this heat needs to dissipate into the atmosphere at a rate equal to whatever the cpu is putting out. Cooling fins (heatsink) spreads that energy over a larger volume and more surface area for cooling and fans to create airflow increase the rate that heat escapes from the heatsink into the air. This is done to increase the amount of power a cpu can generate and still stay within operating temperatures. A TEC can be used to directly keep the CPU cool, but you need to use quite a bit of extra power to do so, and you have to keep the hot side of the TEC at a reasonable operating temperature so you will need an even bigger heatsink than you already have. If you let the TEC freerun as a peltier off the CPU heat you can divert a modest amount of heat as electric current to dissipate elsewhere, but this generally would be less effective than just using a larger heatsink.

In mission critical situations its easier to underclock and run the cpu conservatively than use active cooling, in hot systems or those driving them to extremese its more effective to use liquid cooling or just dunk the whole thing in non-conducting fluid. Otherwise CPU manufacturers are good at making CPU's that keep themselves cool with modest heatsinks, and a TEC is not a solution. The only situation I can think of where a peltier is a good design decision is if a constant or below room-temp operating temperature is a requirement.

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In no particular order:

  1. A Peltier element would cost money.
  2. A Peltier element would add extra weight. The kind of Peltier element used would probably weigh 10-20 grams. Given that high-end ultra-light laptops are marketed as being "20 grams lighter than the Macbook Air!", the extra weight would be harmful to the marketing message.
  3. The main problem is dissipating heat into the air. This is done by the heatsink and fan. A Peltier element would help move heat from the CPU into the heatsink, but doesn't help with moving heat from the heatsink into the air.
  4. A Peltier element has a power consumption of its own, so the heatsink now has to dissipate the CPU heat + the Peltier element's waste heat. Peltier elements are not very efficient, so the heat load might increase by 20%, so that 20% more heatsink is required to maintain the same CPU temperature.
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