# Rds(on) calculation in MOSFET datasheet

This question pertains to the N-channel MOSFET Fairchild FDBL0065N40.

While extracting the Rds(on) values from figures in the datasheet, I came across a discrepancy I can't explain away. Here's the $I_D$ vs $V_{gs}$ figure.
From eye-balling it, you can see the $I_D$ at $V_{gs}=5V$ is about 300A (at $V_{DD}=5V$). That results in a Rds(on) of: $$Rds(on)=\frac{V_{DD}}{I_D}=\frac{5V}{300A}=16.67m\Omega$$

Next, this figure of Rds(on) vs $V_{gs}$:
Which shows an Rds(on) of about $2.75m\Omega$ at $V_{gs}=5V$.

To summarize, in the first chart, I calculate Rds(on) to be $16.67m\Omega$ and the second chart, I get $2.75m\Omega$ at the same $V_{gs}$. Can someone help me understand this apparent discrepancy?

Look at figure 9 - I've copied it and extended it just to show that both on-resistance numbers are consistent: -

Normally fig 9 goes up to a drain voltage of 2 volts but if you project the lines you will see that at 5V drain voltage the current is about 300 amps and therefore on-resistance is about 17 mohms.

At 80 amps the on resistance is about 3 mohms. Figure 10 is like figure 9 but at 175 degC.

The MOSFET 'resistance' goes up considerably once Vds approaches and exceeds a certain point (a couple of volts in these case).

At 'only' 80A Vds is 220mV typically which is much less than 5V.

In fact, at higher Vds the current becomes more-or-less constant so the dynamic resistance (slope of Vds over Id) is very high.

Note the '5V' curve in figure 10- the slope is much steeper (and linear) near the origin.

• That makes sense. What is the value of Figure 10 then? Ultimately, I'm trying to find the Rds values from Vgs(th) and up. Figure 11 is only useful starting at 4.2V or so. Vgs(th) on this FET is 3V, so I'm missing Rds data between 3V and 4.2V. Sep 29 '15 at 0:20
• Above the threshold voltage it's not behaving like a resistance. You shouldn't really depend on th e values in that region anyway- they're very poorly controlled as a result of Vgs(th) being so variable unit-to-unit. Some MOSFET datasheets show curves or you could just use a SPICE model. Sep 29 '15 at 15:55