# SOA and varying VDS

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.

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:

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?

• 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. Commented Sep 14, 2022 at 18:51
• 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? Commented Sep 15, 2022 at 8:17
• 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. Commented Sep 15, 2022 at 14:18

## 1 Answer

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