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In a hot-swap application the FET spends some time in the linear region/Saturation region before it moves into the ohmic region.

Here is a snippet of the turn on waveform seen for a FET: Yellow is battery voltage (VD). Green is output (VS). Blue is inrush current on turn on.

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As can be seen the VDS varies in the linear region as the output reaches the battery voltage.

Here is the SOA for the FET used in this application:

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How do i determine the SOA point or margin? The time spent in linear region is ~2.5ms But during this time VDS varies.

The total energy dissipated is in this area in red right (for linear region)?: Or do we consider the worst case VDS (51.4V), max current (27.3A) and time in linear ~2.5ms and use this to set a point on the SOA curve?

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  • \$\begingroup\$ You can plot your VDS and Id from your scope photo on the SOA curve, then see for what time period you exceed any of the single-pulse lines. Keep in mind that the SOA curves are typical, not guaranteed, and for 25C. So apply a reasonable derating factor. Also keep in mind that planar FETs tend to do much better than trench FETs in hot-swap applications. \$\endgroup\$
    – John D
    Sep 14, 2022 at 18:51
  • \$\begingroup\$ Ah okay that makes sense. Planar FETs tend to have pretty bad RDSon and this is a high continuous current application. I couldn't find a Trench FET that was comfortably in the SOA region. By the sounds of it i should planar FETs which are comfortably in the SOA region and then parallel them up to distribute current? \$\endgroup\$
    – Hasman404
    Sep 15, 2022 at 8:17
  • \$\begingroup\$ Trench FETs suffer from the Spirito effect (you can Google it) so for best reliability paralleling planar FETs (if done correctly, e.g. individual gate resistors, careful layout) is a good choice. \$\endgroup\$
    – John D
    Sep 15, 2022 at 14:18

1 Answer 1

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Whilst reading more on hotswap design i came across the answer I was lookign for and so posting it here if it's ever useful to anybody else.

TI covers this in the following app note: https://www.ti.com/lit/an/slva673a/slva673a.pdf?ts=1663233798502&ref_url=https%253A%252F%252Fwww.google.com%252F

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