Nmos datasheet setup IRF7201n International Rectifier website ... part 1

Four variations of a basic Nmos setup. (many more variations are possible once you know how). All four mosfets have identical settings (which you can check for yourself). Just one mosfet therefore could operate any of these setups. The Nmos (ie all four) is set up as per International Rectifier datasheet IRF 7201n Look at the voltage across the drain to source terminals ds volts ie Vds (the s terminal is at earth zero). Look at the current passing through the drain to source ie Id or Ids amps V = IR ie R = V/I letter I ( not the number 'one' ). R is the Resistance between the d and s terminals ie: Rds So... Rds = Vds/I Rds = 0.219 / 7.3 Rds = 0.03 Ohm for all four circuits (check it out) The bottom right circuit shows Vds = 0.0015 volts with the shown current of Id = 0.05 amps Rds = 0.0015/0.05 = 0.03 Ohm just the same as the other three circuits here, with the same mosfet. We are only interested in the above whilst the Nmos is switched ON ie: when the gate volts ( ie Vgs), is above the threshold volts of the settings (ie: VTO) We therefore place a reminder there with that Rds and write it as Rds(on) rather than just Rds If we now alter the Vgs of our circuit from its present shown value of 10v down to say 8v, we can then see how the Id and the Vds and the Rds(on) alter. Note therefore that altering Vgs will alter the Rds(on) as you can see in any of these circuits. Likewise, if we alter the VTO threshold setting, then Rds(on) will change to a different value. (Vgs - VTO) is called Vov or the 'overdrive' ... more about that later, but for now remember: Altering Vov will alter the Rds(on) ... check it yourself as above here. Note that the kiani library for that Nmos (7 along from top left there) uses an incorrect value for VTO of 1.5v there (not as per datasheet seen 1v) ... That means the settings have been fiddled (ie not now matching the datasheet Beta value) in order to obtain the Rds(on) value of 0.03 Ohm. Thus an invalid EC model for the datasheet. The correct values for the datasheet mosfet are: Vgs = 10v VTO = 1v Rds(on) = 0.03 Ohm at current 7.3 amp with these Vgs and VTO values.... note Vov = 9 being (10-1) Once those values have been used in an EC setup, then the settings obtained create the correct Beta. The Beta, Vov and Rds(on) can then be used mathematically to suit the requirements of our EC setup... eg if we wish to use a different Vgs in our circuit, ... the beta settings will remain unchanged as per the datasheet. Note however, that the Rds(on) will change, but that altered value is then understood and can be accounted for in our circuit. See now part 2 here ... http://everycircuit.com/circuit/4865219233579008

published 6 years ago