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When calculating a heat load for a thermoelectric cooler (TEC), factors to consider include total surface area to be cooled, amount of heat to be dissipated, ambient temperature, and temperature drop required between the hot and cold sides of the TEC.
The first step in selecting a TEC is determining the amount of heat it has to pump, the heat load Q. Heat loads can be active, passive, or a combination. An example of an active heat-load device is a transistor. The amount of power it dissipates is a function of its operating mode, however, using total input power is close enough for most calculations.
Steady-state active heat-load power is found from
Q sub a = V sup 2 /R = VI = I sup 2 R, where Q sub a = active heat load, W; V = applied voltage, V; R = device resistance, Omega; and I = device current, A.
For example, calculate the heat load for a lead selenide (PbSe) infrared detector. It has a resistance of 0.5 MuOmega and operates at a bias of 50 V. Thus, its active load is Q sub a = V sup 2 /R = (50) sup 2 /500,000 = 2,500/500,000 = 0.005 W = 5 mW.
Passive heat loads, on the other hand, are the result of radiation, conduction, and convection.
Radiation: Two objects at different temperatures brought near each other produce heat flow from the hot object to the cooler one. When the space between the objects is rarefied gas or a vacuum, the primary transfer mechanism is thermal radiation. Usually, radiation heat loads are not considered because conduction and convection heat flow in the same assembly typically are much greater. But radiation loading is significant in systems with small active loads and large temperature differences, especially in a vacuum.
Radiation loading is computed from
(Equation omitted)
where Q sub r = radiation heat load, W; F = heat sink shape factor (worst case = 1); e = emissivity (worst case = 1); s = Stefan-Boltzman constant, 5.667 X 10 sup -8 W/m sup 2 K sup 4 ; A = cooled area, m sup 2 ; T sub a...