Electronic – Watt-squared-seconds: unit of safe operating area

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In the datasheet of Linear's LTC4364, page 15, the safe operating area (SOA) of a MOSFET is described as:

For short duration transients of less than 100ms, MOSFET survival is increasingly a matter of SOA, an intrinsic property of the MOSFET. SOA quantifies the time required at any given condition of VDS and ID to raise the junction temperature of the MOSFET to its rated maximum. MOSFET SOA is expressed in units of watt-squared-seconds (P2t), which is an integral of P(t)2dt over the duration of the transient. This figure is essentially constant for intervals of less than 100ms for any given device type, and rises to infinity under DC operating conditions. Destruction mechanisms other than bulk die temperature distort the lines of an accurately drawn SOA graph so that P2t is not the same for all combinations of ID and VDS. In particular P2t tends to degrade as VDS approaches the maximum rating, rendering some devices useless for absorbing energy above a certain voltage.

I've never encountered W^2s (watt-squared-seconds) before, and no datasheet that I've seen expresses SOA in W^2s — rather in graphs of I_D vs. V_DS. Does anyone have any insight into how it's calculated based on datasheet graphs? Or how it's derived?

Best Answer

When the pulse length is sufficiently short, the die temperature change is approximately constant for \$P^2t\$. So if you know the initial die temperature and the maximum die temperature you can estimate the maximum pulse the die can handle. See the below curve.

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If you calculate \$P^2t \$ for the three points shown, it's around \$300W^2 s\$ for all three.

This will be derated to 0 for elevated die temperatures. See below from the LTC4233 datasheet:

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You can see the cause of this effect in the transient thermal response curve of MOSFETs:

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