How can I calculate the channel-to-ambient thermal resistance, Rth(ch-a), of a small-signal MOSFET?

You can calculate the channel-ambient thermal resistance Rth(ch-a) from the drain loss.

Rth(ch-a) can be obtained from the following formula using the datasheet as a reference.

Formula: Rth(ch-a) = (Tch Max-Ta)/PD Max

  • Rth(ch-a): Channel to Ambient Thermal Resistance
  • Tch Max : Max. Channel Temp.
  • Ta: Ambient temperature (measured temperature condition of PD Max)
  • PD Max : Max. Power Loss
Fig. 1: Cross-sectional view of an example of TO-220
Fig. 1: Cross-sectional view of an example of TO-220

See also the documentation below.

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