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The datasheet for the IPG20N06S4L has an SOA curve that shows the minimum on resistance is 50mOhm, whereas its output characteristics curve shows that you can achieve a 21.27mOhm on resistance. How do I reconcile these two graphs?

I must misunderstand what at least one of these graphs is telling us. I’ll list what I think are my main points of understanding so it is easier to identify why I am confused:

  • The Rdson limit is a physical limit that is shown on the SOA curve – despite “S” in SOA meaning “Safe”, this is less about safely operating the device and more about what is actually physically possible – operating outside of this region would imply you are getting a lower on resistance from the device than is physically possible;
  • The output characteristics curve shows us the electrical behaviour of the FET for various gate-source voltages, hence looking at the highest Vgs curve to determine minimum on resistance is sensible here;
  • On resistance in the ohmic region of the FET usually goes down with higher VGS for N channel enhancement mode FETs;
  • Both graphs are made with the same measurement or calculation assumptions;

enter image description here

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    \$\begingroup\$ The "5. Typical output characteristics" graph shows typical values, the "3 Safe operating area" graph shows it for worst case parameters of a device - as you rightly discovered, VGS parameter is not noted for that graph - I assume 4.5V, because the rDSonMax is 46 mOhm at VGS 4.5V and at ID = 10A on page 2 of the datasheet. (Even if the "Safe operating area graph" would be shown for VGS 10V, they should use rDSonMax of 27 mOhm and not rDSonTyp of 21 mOhm - values also from page 2). \$\endgroup\$
    – BitLauncher
    Commented Sep 26 at 6:23
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    \$\begingroup\$ Another difference: Graph 5 is for Tj (junction) and graph 3. is for Tc (case), so Tj is higher --> therefore rdsOn is also higher according to Fig. 8! Therefore the figure 3 is perhaps even for VGS 10V. \$\endgroup\$
    – BitLauncher
    Commented Sep 26 at 6:29

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The safe operating area has to give numbers that are "reliable" rather than typical. I mean, if the data sheet gave typical values for the safe regions, 50% of the devices wouldn't be safe to operate at those limits.

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  • \$\begingroup\$ I've not done myself any favours asking a question in the title that differs from the question in my body. The fact that they're showing worst case (and thus derated) values makes sense for the SOA. I guess what I wanted to know is how to make the connection between what they show in the SOA and the other parameters and graphs in the datasheet. \$\endgroup\$
    – daviegravee
    Commented Sep 27 at 4:33
  • \$\begingroup\$ I'm one step closer - i've found this in they show that the Rdson limit is calcuated using the RDSon for a Vgs of 10V and working at the max junction temperature: community.infineon.com/t5/Knowledge-Base-Articles/…. Graph 8 in the datasheet I linked shows that at max junction temperature and Vgs=10V, you would expect about 38mOhm Rds, which is closer to the worst case 50mOhm assumed by the SOA limit line. \$\endgroup\$
    – daviegravee
    Commented Sep 27 at 4:36
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    \$\begingroup\$ I'm not sure I follow where you are going with this. \$\endgroup\$
    – Andy aka
    Commented Sep 27 at 6:46

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