Cooling at the heat source

23 October, 2009

Heat has always been an issue for system designers, but it is only recently that the problem has become so severe that thermal management solutions can no longer be introduced as an after-thought, but must be considered at the beginning of the design process. A new approach to thermal management involves using active thermal management at the source of heat.

The problem with thermal management is that it can be like pushing jelly; you can press it down in one direction but it will just flow in another. Heat flowing through a material will always create a temperature gradient across that material and as it flows from one material to another, will do so in a continuous manner.

Heat flow is equal to the work on the system plus the change in internal energy of the system. Hence, in thermodynamics, the change in heat is equivalent to the heat flowing into the system.

Active cooling and package size
As the size of electronics continues to shrink, the size of the thermal management system must shrink as well. Given that passive heat removal is only a linear function over distance of the temperature difference, work mus be put into the system to obtain at greater rate of cooling and hence a smaller device. One such example for optoelectronics of the continuing reduction in device size is shown in Figure 1. From right to left, the butterfly package, the TOSA style package and finally a TO-56 package. In some instances, customers choose to place the cooling device outside the package if it is too large to be placed inside.

Active thermal management devices such as thermoelectric coolers (TECs) have been employed to provide this additional heat pumping and temperature stabilisation capability.

Thermoelectric cooling uses the Peltier effect to create a heat flux between the junction of two different types of materials. A Peltier cooler, heater, or thermoelectric heat pump is a solid-state active heat pump which transfers heat from one side of the device to the other side against the temperature gradient (from cold to hot), with consumption of electrical energy. Such an instrument is also called a Peltier device, Peltier diode, Peltier heat pump, solid state refrigerator, or thermoelectric cooler.

One of the drawbacks to using TECs is that, in doing work to move the heat in an almost discontinuous manner, will add heat to the system. This heat will be added downstream of the device being cooled but will add to the overall system heat that needs to be rejected at the next level.

If TECs are applied for cooling in the same manner as passive thermal elements (in other words - everywhere), we will end up with a larger problem than the one being solved. A more cost-effective and efficient approach - and one that is only possible with TECs - is to cool only what is necessary. In other words, scale the thermal management system to the size of the heat problem.

Heat flow and regulation
Active thermal management must be employed close to the heat source. Since heat in the passive case flows linearly, any material between the TEC and the heat source will have a temperature drop across it. This will act to increase the temperature difference that the TEC must pull. The TEC is a heat pump working (moving heat) in a manner whose efficiency depends upon the temperature difference it is required to generate. Minimising this temperature difference will improve the efficiency of the TEC and reduce the additional heat added at the system level.

Today, TECs are manufactured using two methods. Bulk TECs are manufactured using pressed sintered powders; thin film TECs are manufactured using semiconductor manufacturing processes. Each has advantages, but the most obvious difference is that the thin film TECs are 10 to 20 times smaller in each linear dimension and can be placed in much smaller packages for integrated cooling and temperature control.

Dr. Paul A. Magill is Vice President of Marketing and Business Development, at Nextreme Thermal Solutions, Inc.