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Shown above is the academic definition of MOSFET SOA. Rdson limit is not considered as a real limit in most academic texts - that's ok- understandable.

But why is the max drain current limit different for DC and rest of the pulsed cases?

Divya K.S
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    Bondwire or package ratings, most likely. – Tim Williams Jun 15 '23 at 13:33
  • The real question isn't, why the DC case is *lower* (see eg @TimWilliams' comment), but why is the current limit **independent of the pulse length**. – tobalt Jun 15 '23 at 15:55
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    @tobalt Presumably such limits simply aren't controlling for shorter time scales; or, perhaps the shorter-duration curves are there, they're just bunched up tightly (i.e. the curves go 1, 9, 9.9, 9.99... A), enough that you don't see them as separate at this scale (note log-log axes). – Tim Williams Jun 15 '23 at 16:18

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It's the same reason why a 10 amp fuse can carry 50 amps for a few milliseconds without blowing. If you keep heating the fuse (aka MOSFET bond-wire for example) it's going to reach unsustainable temperatures and melt.

It's not limited to figure 22-18 in your question either; all MOSFET SOA curves indicate this.

Andy aka
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    But the heating or power limit is by the diagonal part of the curve. – Divya K.S Jun 15 '23 at 15:50
  • All points are, in effect, powers @DivyaK.S – Andy aka Jun 15 '23 at 15:54
  • This is presumably because of bond wire current carrying capacity (limit) and subsequent heating. I was wondering why it was not clubbed with power limit – Divya K.S Jun 15 '23 at 16:18
  • https://www.infineon.com/dgdl/Infineon-ApplicationNote_Linear_Mode_Operation_Safe_Operation_Diagram_MOSFETs-AN-v01_00-EN.pdf?fileId=db3a30433e30e4bf013e3646e9381200 – Divya K.S Jun 15 '23 at 16:18
  • Power of *what elements?* @DivyaK.S quoting from the link, "When following the RDS(on) limit-line towards higher currents and voltages the current-limit-line is found. This limit-line is typically defined by the maximum current the package can handle. Therefore the amount of this current will be strongly package dependent." – Tim Williams Jun 15 '23 at 16:18
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    @ Tim Williams- Please refer my above comment. Things are clear except the statement "The package limit-line will not change in case application conditions such as ambient temperature vary". Not sure why it doesn't depend on ambient temperature - as with any damage due to thermal rise – Divya K.S Jun 15 '23 at 16:24
  • https://www.vishay.com/docs/74670/74670.pdf This doc mentions how current limit is estimated. – Divya K.S Jun 15 '23 at 16:24
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Could you please check whether the explanation given in the below link helps.

Why pulsed drain current is higher than continuous drain current in MOSFETs?

Hari
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  • The unusual case of "should not be an answer", but not to be [just] a comment, but a reason to close! I agree, this seems a duplicate. – Tim Williams Jun 16 '23 at 04:49
  • (I think the comment to that question to be spot on. I don't find the answer significantly helpful. Note the secondary breakdown ...) – greybeard Jun 16 '23 at 05:05