170

)T T(A

P

4

0

4

S,th

− ⋅ ⋅σ⋅ε=

P

th,S

: Radiant power of abody [W]

A: Radiating surface [m²]

σ

: Stefan-BoltzmannConstant

4 2

8

Km

W 10

67040 .5

⋅

⋅

−

ε

:

Emission factor

T: Temperature of the radiating surface [K]

T

0

: Ambient temperature [K]

It is important to note that every radiating body not only emits thermal radiation but can

also absorb it (as described above). In the case of power electronics, the thermal

radiator is the heatsink. As long as this is warmer than its environment, it will emit

thermal output. But this does not apply to the area between the fins of a heatsink, where

the thermal radiation emitted is reflected and equilibrium is established between the

absorbed, emitted and reflected thermal radiation, as Tequals T

0

.

Thermal radiationas illustratedby a heatsink

The emittance, described by the emission factor

ε

, provides a quantitative description of

the amount of radiation that a body emits in comparison to an ideal black body. A black

body represents a theoretical body that completely absorbs radiation of any frequency

and can emit the maximum possible thermal radiation. Compared with such a black

body, all other real bodies have a unitless emission factor ina range from 0 to 1.

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