# MOSFET Difference between Normalised On Resistance and Output characteristics

In the datasheet for STP36NE06, it has two graphs which seem to contradict each other:

In the graph on the left, for Vgs=10v and Id=18A, the slope of the curve indicates that Rds is approximately 0.05 ohms. (Vds would be about 1v). However the graph on the right suggests it would be between 0.6 and 1.8 ohms.

What am I not understanding here?

Thanks "Normalized" is what you are missing. The Y axis on the RH graph is dimensionless- it's a ratio.

If the MOSFET has an $R_{DS(ON)}$ of 43m$\Omega$ at ~27°C, then it will be something like 1.45 times that at 100°C, and 0.6 times that ~=25.8m$\Omega$ at -40°C.

A rough rule of thumb I use is that the datasheet $R_{DS(ON)}$ will be about 50% higher at the maximum junction temperature that I'd like to operate at. That's because it's usually quoted at a junction temperature of 25°C. They test that by using short pulses of current so the junction doesn't get a chance to heat up. So, like many datasheet numbers, the $R_{DS(ON)}$ number in the big print can mislead the unwary.  What's missing is the meaning of "normalised" ... 1.0 on the RH graph means not 1.0 ohm but 1.0* the normal value of the resistance. Therefore the graph indicates how the resistance scales with, in this case, junction temperature.

The value of a normalised graph is that it is easier to apply to different related situations - perhaps Rds(on) at different Vgs or different Id.

• +1 echo in here? – Spehro Pefhany Feb 1 '14 at 15:57