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Hi,

That is a very general question. There are three main regions of
MOSFET operation:

1) Fully off
2) resistive (fully on)
3) Partly on (also called "linear region")

Assuming you are using the MOSFETs as switches, then the power
dissipated is approximately:

P=3DI^2*R+f * (Tr+Tf) * 0.5 * V * I

The first part is simple on resistive losses (R is on resistance -
beware that this is temperature dependent).  The second part is the
switching losses, or the power dissipated by being in state #3 while
turning on and turning off. f is the switching frequency, Tr and Tf
are rise and fall times of the gate drive (taking gate charge into
account).

Once you have P, you have to compute the temperature difference
between ambient and the MOSFET junction. If you will not have great
variations in P, then you can simply add up the thermal resistance
from junction to ambient (usually junction to case + case to heatsink
+ heatsink to ambient) and multiply by P in Watts. If P can make
sudden jumps, then you need to make a dynamic thermal model. The
internal dynamic thermal model of the MOSFET may be included in the
datasheet. One way is when they give a "Transient Thermal Impedance"
In this case, you look up the length of the pulse in P and it gives
you an adjusted thermal resistance from junction to case. For some
duration and longer, it will be the same value as continuous. It will
be less for shorter durations.

Sean


2008/5/7 gardenyu <gardenyu2004@hotmail.com>:
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