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# CSD16570Q5B: Rds(on) Value in the SOA

Part Number: CSD16570Q5B

We try to calculate/get the Rds(on) value from the SOA of the D/S. However, we don't get the exact Rds(on) value from SOA curve and to match EC table.

For example, When the IDS is 400A and VDS is 0.4V, the Rds(on) should be 1mR. But in the EC table, the max. Rds(on) is 0.59 @10Vgs. is this correct?

1. How do you usually define the Rds(on) value in the EC table?

2. How do you draw the Rds(on) limitation on the SOA curve?

• Brian,
You are correct in looking at the grey line in the SOA as the resistance limitation line. In this region, the FET is fully enhanced assumed to behave like a constant resistance so the V/I ratio remains constant at the max resistance of the FET.

Your calculation is also correct. The only thing you aren't taking into account in your assumptions is this max resistance limitation line is drawn at the max junction temperature of the FET. So we would take the max resistance 0.59mOhm and multiply by the temperature coefficient at 150deg, provided in Figure 8. This value is ~1.6, so 0.59 * 1.6 = 0.944.

This is basically what you calculated from the SOA, the discrepancy being your estimate of 0.4V at 400A was probably a little aggressive.

So to answer your question, the datasheet Rdson values are characterized at 25degC, where as the SOA resistance limitation line is calculated from these characterized values at the max junction temperature (150deg C in this part's case).

I did some blogs and videos about datasheet SOAs and current ratings that you may find helpful. I have posted links to these below:

training.ti.com/understanding-mosfet-datasheets
e2e.ti.com/.../understanding-mosfet-data-sheets-part-2-safe-operating-area-soa-graph
e2e.ti.com/.../understanding-mosfet-data-sheets-part-3
e2e.ti.com/.../understanding-mosfet-data-sheets-part-4-mosfet-switching-times

Best Regards!

Brett Barr

Product Marketing Engineer