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Old 15th April 2010, 08:56 AM   #21
AndrewT is offline AndrewT  Scotland
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no.
Maximum Pd quoted in the datasheet is for continuous DC power, when Tc=25degC.

The SOA plots, if shown, give the single shot transient I & V for instantaneous Pd
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Old 15th April 2010, 09:41 AM   #22
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Quote:
Originally Posted by guyst View Post
mnemneth, is it possible you are confusing case temp with junction temp?
please, some credit here...when i said the data is in the dsheet i didnt mean just take max Pd. there is a lot more dissipation data there: thermal resistances and max Tj is what i used, apparently naively so, as i use the steady state numbers derived from 50% eff
Hi guyst,
I'm more than happy to give you credit where credit's due.

As to your question!

No, I'm very specifically stating JUNCTION temperature.

Remember the power handling drops linearily from 25C to Tj(max) of 150C, hence the figures I gave in my last post.

At Tj of 118C, the power handling of the transistors is 25% of Max.

This entails your heat sinks needing to get rid of 37.5W per output device.

The board is running 5 pairs of devices, so you need to get rid of 162.5W of heat and that needs BIG heat sinks and probably fan cooling.

Sandy
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Old 15th April 2010, 01:17 PM   #23
guyst is offline guyst  Israel
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Quote:
Originally Posted by mnemneth View Post
No, I'm very specifically stating JUNCTION temperature.

Remember the power handling drops linearily from 25C to Tj(max) of 150C, hence the figures I gave in my last post.

At Tj of 118C, the power handling of the transistors is 25% of Max.
i think you might be wrong. ill explain:
max power dissipation is when Tj=Tj max.
power dissipation by heat conduction from junction to case is P=(Tj-Tc)/Rjc. right?
(only steady state speaking, no lumped heat transfer)
now, when Tj=150c, and Tc=25c, we get P=125/Rjc. needless to say its hypothetical etc.
when Tj=150c, and Tc=118c (lousy heatsink), we get P=32/Rjc, which is 25.6% of max Pd.
obviously we will take less then 150c if this is max Tj,
for headroom.
if i pointed out the obvious, this is a sign that i misunderstood you again, and i should seek professional help in english
oh and i might be wrong, im human.

Last edited by guyst; 15th April 2010 at 01:28 PM. Reason: just remmembered
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