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I'm trying to understand the upper-left corner of a typical MOSFET safe operating area (SOA) figure. Specifically why the region marked in red on the figure below, above the "Limit Rds(on)" line, is typically marked as outside of the SOA.

enter image description here

I understand that for any specific Vds voltage, the MOSFET can't pass more drain current in its linear mode than Id = Vds/Rds(on) due to Ohm's law. However I don't understand why the SOA always seems to use the Rds(on)max for the limit - the maximum specified Rds. I would think the minimum value would be more appropriate.

I took the SOA figure above from the datasheet for the Nexperia PMV37ENEA. This datasheet also has the following specification for Rds(on):

enter image description here

The "Limit Rds(on)" line on the SOA figure approximately matches an I(V) line for Rds(on)=100 mOhms. However the typical values for Rds(on) in the table are lower than that. This means that most devices will operate outside of the safe region.

A device with Rds(on)=50 mOhm (well within the Rds(on) spec) will fall somewhere on the blue line here:

enter image description here

For example, a device with Rds(on)=100 mOhm and Id=2 A will dissipate 0.4 W and is safe to operate according to this figure. However a device with Rds(on)=50 mOhm and the same current will dissipate less, only 0.2 W, but is not safe. Why is that? Strictly from the thermal perspective this does not make sense to me.

I've looked at datasheets for several different MOSFET devices and the SOA figures all look similar. Various applications notes seem to confirm that some maximum for the Rds(on) is typically used for the limit line.

Compare this to bipolar transistors, where the upper-left corner of the SOA figure is typically not cut off like in MOSFET datasheets (even though, technically, you're still limited by Vce(sat) and may not be able to push the transistor into that region).

enter image description here

Am I missing something here? Why are MOSFET datasheets cutting off the SOA at Rds(on)max? Or is the "Limit Rds(on)" line in SOA figures there just for guidance, and not really a limit for safe operation of the MOSFET? This application note seems to suggest so since it says that the line can "easily be recalculated using [...] datasheet parameters for Rds(on)". If so, why are maximum Rds(on) values always used and not typical or minimum?

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I think this simply means that (Id,Vds) points you painted red, where Vds < Id * RdsON are not part of the operating area because they cannot be reached: when Id is flowing, Vds will be at minimum Id * RdsON when the MOSFET is fully on.

These points are not part of the operating area, so they are not part of the safe operating area. That doesn't mean they're unsafe, rather that they just don't happen.

And... the interesting parts of the SOA that are relevant to using the device in a design are mostly the top right corner where you will look for second breakdown or Spirito instability. But the top left corner isn't really important: you won't use the SOA graph to calculate conduction losses due to RdsON when the MOSFET is fully on, instead you will use the RdsON spec, thermal resistance etc.

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However I don't understand why the SOA always seems to use the Rds(on)max for the limit

It's because maximum power dissipation at a certain drain current occurs when resistance is at a maximum value. Hence Rds(on)max tends to be used. Remember that this is a safe operating area curve and so it uses a certain degree of caution in the numbers. Look at it from the perspective of someone buying a MOSFET that has an RDS(on) value naturally much closer to the max value - those guys will want to feel secure that the SOA graph works for them.

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  • \$\begingroup\$ I understand that Rds(on)max is usually the worst case scenario as far as heat dissipation is concerned. My question is specifically why in some cases a lower Rds(on) falls outside of the safe operating area (the area I marked in red, or specifically the 100 mOhm vs. 50 mOhm example). \$\endgroup\$
    – avian
    Commented Jan 4, 2022 at 18:38
  • \$\begingroup\$ @avian It falls outside the area because numerically it has no option other than to do so. But, when reviewing a datasheet in order to procure this MOSFET or that MOSFET, you want conservative honesty when it comes to SOA curves. You said: why in some cases a lower Rds(on) falls outside of the safe operating area - it's not "in some cases" - it's all cases because of \$I^2R\$ power dissipation. \$\endgroup\$
    – Andy aka
    Commented Jan 4, 2022 at 20:00
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I understand that for any specific Vds voltage, the MOSFET can't pass more drain current in its linear mode than Id = Vds/Rds(on) due to Ohm's law.

True, but that's looking at it backwards. Usually if your circuit is limited by \$\mathrm{RDS_{on}}\$ it's because you're putting some known current through the FET, and you would like it to not burn up.

In that case, Ohm's law says that \$V_{ds} = \mathrm{RDS_{on}} \cdot I_d\$, and the power dissipated is the \$I^2R\$ losses through the FET as if it were a resistor.

That's what the graph is telling you.

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In all cases, where the current is externally controlled, such as these plots, the SOA curves are EACH constant power P=VI but vary in time duration. This determines the max junction temperature rise limits at 25'C.

The Red Zone is where heat transfer time has been reached and thus transient limits no longer apply.

Logically the more resistance an element has with forced current, the hotter it gets. This is why worst case RdsOn is used and why thermal runaway can be an issue with poor cooling, since FETS tend to have a positive Tempco.(PTC)

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