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Thermal Impedance of MOSFET

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Generally all datasheets of mosfets have thermal impedance curve of thermal impedance(thermal resistance and thermal capacitance) vs time. Please refer fig below:
11.PNG

What does this signify?
Should the settling time be higher or lower?
Complete datasheet is listed below:
 

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dknguyen

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Super short, it signifies how quickly heat distributes and equalizes in the component. It is important for very short, very high power pulses.

You can't use thermal resistance which assumes that the heat is generated slowly enough that it can equalize and spread throughout the component. Only using thermal resistance implies that if you halve the pulse duration, you can double the power dissipation during that time, ad infinitum since the average heat dissipation will remain the same. Obviously this isn't true in reality because at some point all the heat that is generated cannot distribute and equalize throughout the component before the next pulse hits, so heat continues to build up in the same small localized area and the component has spot failures due to imperfections in the material.
 
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