Calculating Safe Operating Area of Mosfet: P^2 t Stress Explained

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SUMMARY

The discussion focuses on the P2t stress calculation for MOSFETs, specifically how it relates to the Safe Operating Area (SOA) curve in MOSFET datasheets. The P2t value, expressed in watts squared seconds (W2·s), represents the accumulated heat effect over time, combining instantaneous power (P) and thermal effects (t). This calculation is crucial for understanding the thermal limits of MOSFETs, particularly in short-circuit scenarios, where the device must maintain thermal equilibrium to prevent damage. The P2t value remains constant for intervals under 100 milliseconds, indicating that heat generation does not exceed the heat sink's capacity during this period.

PREREQUISITES
  • Understanding of MOSFET operation and characteristics
  • Familiarity with thermal management principles in electronics
  • Knowledge of power dissipation and heat transfer concepts
  • Basic engineering mathematics for interpreting datasheet specifications
NEXT STEPS
  • Research the Safe Operating Area (SOA) curves for various MOSFETs
  • Study the thermal resistance and heat sink design for power electronics
  • Learn about short-circuit protection mechanisms in MOSFETs
  • Explore the implications of P2t stress in different MOSFET applications
USEFUL FOR

Electrical engineers, power electronics designers, and anyone involved in the thermal management of MOSFETs will benefit from this discussion, particularly those focused on optimizing device performance and reliability under stress conditions.

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Saw a calculation of stress in a datasheet (page14) here -
http://tinyurl.com/43465sv
(The device drives a Mosfet.)
How do you relate the P^2 t stress calculation in the datasheet to the Safe operating area curve in a Mosfet datasheet.
If the Stress is 20W^2 sec, then how do you interpret it? The Mosfet should be able to withstand 20watts for 1sec.
The watt square is confusing me. Why watt square*t? why not joules.
 
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The first section on the same page says what it is but you have to apply engineering knowledge to infer where it comes from.

The units come from two separate effects:

  • P t which results in units of energy delivered as heat that modulates to heat transfer effectiveness - this is the "accumulated heat over time" or integrated heat effect
  • P which is the instantaneous power delivered to the load and dissipated as heat

These combined as function: Damage(Pshort-circuit, Eheat) which are modeled multiplicatively, hence P2t

This value is "essentially constant for intervals of less than 100ms for any given device type" so basically you have thermal equilibrium because you aren't generating heat fast enough to exceed the heat sink capacity of the package. Above that you start to have a positive feedback loop on heat that accelerates the effect of the short circuit power (P).
 
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