ABSTRACT
Hybrid operation in a system where the thermal analysis was performed with a wrong assumption, that the power is dissipated by the entire case surface, may be seriously jeopardized. For the analyzed example the actual case-to-ambient temperature rise will be almost double that of anticipated value. If we postulate that the case-to-ambient thermal resistance calculated with the wrong assumption is
and the power dissipation is PD = 75 watts, we find that
The actual ambient-to-case temperature rise would be
The differential of
will be added to the actual junction temperature of the semiconductor device. This unexpected temperature deviation may seriously affect hybrid electrical performance and reliability. In some cases it may cause the transistor or diode overstress and failure.
