A very important built-in parameter of a FET transistor (such as popular MOSFET) called RDS(ON) or On Resistance, can be found in most datasheets. It describes the transistor’s internal resistance, but remember that only under special circumstances. The conditions for which On Resistance can be valued: fully conducting state, which is also indicated by the word ON. It means that the VGS – gate to source voltage must be equal to 0. You should not confuse On Resistance with VGS, because, as it was explained before, despite the fact that those two values are connected and one depends on the other, they must be differentiated as they are two separate values. On Resistance, just like every resistance, is expressed in ohms, and usually ranges between 10 and 1000 ohms.
Low- RDS MOSFET
Sometimes when reading a specification, you may see the above expression. It indicates the value of On Resistance in this particular MOSFET is low. Practically it means that less power will be lost across your MOSFET and thus it will be more power efficient than other FET transistors with On Resistance of a higher value.
Some factors may have an impact on described here parameter. On Resistance can be improved (its value can be decreased) by increasing the gate voltage. On the other hand, it increases with junction temperature.
must be clear that saturation of mosfet makes the resistance between D (DRAIN) AND S (SOURCE) VERY SMALL . in CRT TV it can be 1Ω (low switching=15khz) .
but nowa days we are working with high switching >200khz . RDS(ON) are in milliΩ (1/1000 of ohm) . very important when replacing mosfet to choose RDS(ON) equal or less value of the original mosfet or it will be hot and burned .
beside that VDS and IDS must be equal or greater .