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?
Best Answer
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.