Pulsing is not too too important to the heat sink if thermal time
constant is long compared to pulse width. Note that thermal tc and
thermal resistance do not have to be at all well correlated BUT often
have some relationship as big fat bits of metal tend to be both heat
stores and have lower thermal resistance.

The Tjs is more susceptible to pulse effects as the junction thermal
mass is small. Most power devices have tables in the data sheet that
related Tj temperature rise w1ith pulse frequency and duty cycle. So
Tjs will probably be pulse affected but Tsa not so much so.

Again, once you get a =A0feel for what is happening it is truly trivial
in concept - BUT not necessarily trivial in practice due to various
practical effects. Wouters suggestion to use ohms law type
calculations and mindsets is entirely appropriate.

=A0Do note that device characteristics may be severely affected by Tj
but manufacturers stupidly usually quote them in the most favorable
and entirely unrealistic conditions. Marketing says to do this -
common sense says not to.

eg MOSFETS usually have Rdson quoted at eg 1% dutycycle at say 10
kHz.. Tj gets a chance to drop back between pulses. Rule of thumb -
MOSFET real world Rdsons are typically about 2:1 greater than data
sheet claims.




=A0 =A0 =A0 =A0 =A0 =A0 Russell McMahon

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