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What is the connection between junction temp. and thermal resistance?

Latest Updated:08/01/2005

Question:

What is the relationship between the junction temperature and the thermal resistance?

Answer:

When using a power device, the relationship between the junction temperature and the thermal resistance, which indicates the resistance to the conductivity of heat, is important.
The junction temperature (Tj) can be expressed as follows:

Tj = Ta + θj-a x P
  Ta: Ambient temperature
  θj-a: Thermal resistance between a junction and ambient air 
    (= between junction and case + between case and ambient air) 
  P: Power consumption 


Based on this fact, the following can be stated 

In the case of the same power consumption, the smaller thermal resistance makes smaller rise in temperature at the junction.
Therefore, the device can be used in a higher ambient temperature.
Conversely, if using the device at the same ambient temperature, the smaller thermal resistance allowes greater power consumption.

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