The two major thermal paths for a diode mounted to a PC board are through the diode
leads to the PC board and through the diode body and leads to the surrounding air or
other medium.
The thermal resistances of VMI diodes are given for several lead lengths in the diode
catalog section of this website. Multiplying the thermal resistance of a diode through
its leads by the power dissipated in the diode will give its junction temperature rise
over the temperature of its mounting location. The temperature or thermal impedance
of the diode mounting location will have a major effect on the junction temperature of
the diode since any temperature rise of the the mounting location will be added to the
temperature rise of the diode itself.
The medium surrounding the diode can reduce the junction temperature of the diode
by adding a parallel thermal path. If the surrounding medium is air, its cooling effect
will depend largely on its temperature and on its movement. Whereas forced air can
significantly reduce the junction temperature of a diode, the cooling effect of still air,
such as would be the case if the diode is mounted in an enclosed box, may have little
effect on the junction temperature of the diode.
If the surrounding medium is a potting material, its cooling effect will depend on the
temperature and thermal conductivity of the material, the power dissipation of any
components located near the diode, and on the thermal path through the material
to any outside surface or heat sink.
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