How do I calculate the safe operating temperature of a MOSFET or BJT?
To determine the temperature rise you need to calculate the junction
to ambient thermal resistance (Tja
||thermal resistance between the IC junction and IC case. This is
from the IC datasheet.
||thermal resistance between the IC case and your heatsink. This
should also be in the IC datasheet. Usually you place a thermal
compound between the IC case and the heat sink to decrease the
thermal resistance. The decrease occurs because the thermal
compound fills the air gaps between the two surfaces and conducts
heat better than the air.
||thermal resistance between the heatsink and ambient air. This is
in the heatsink datasheet. Most heatsink datasheets specify Tsa
with and without airflow. You can determine if you need a fan by
reviewing this value.
For a single IC heat-source the junction temperature would be
where P is the power dissipated in the IC and Tamb
is the ambient
|For a MOSFET
|For a BJT
Make sure to use the Rds(on)
values at the
device operating temperature not at 25°C.
The maximum juction temperature is listed in the datasheet (usually in
the "Absolute Maximum" section). A conservative design limit would be
a maximum junction temperature less than 80% of the absolute maximum
Unless you have very short on-times pulse-width modulation (PWM) is
not going to significantly reduce the power dissipation. To see how
much look at the transient thermal response graphs in the datasheet.