Reverse Polarity in rush Protection 2 p fet parallel
Modified to better suit what the OP probably requires


  i want simple calculation of capacitance of capacitor where it will
  affect in rush current


Firstly and importantly, the capacitor on each MOSFET needs to be in parallel with the zener in order to achieve inrush current limiting but, the confusion here surrounds both MOSFETs being in parallel. They are actually positioned to be reverse polarity protection devices and cannot be used as inrush limiters (contrary to what I originally understood and wrote so, sorry about this).

So, instead, think about the reverse polarity circuit (without a gate capacitor) then, after that MOSFET circuit, think about using another P channel MOSFET with its source after the reverse polarity MOSFET's source: -



So, assuming you do this, the rest of my answer is relevant....

You need to think about the CR time of the capacitor and resistor to ground. Quite literally C x R is the time constant and for this type of circuit, the MOSFET might be regarded as being fully conducting at about 50 % of CR. This assumes that the MOSFET fully conducts at about 12 volts between gate and source.

Given that your supply is 48 volts, at the end of the first CR time period you’ll get about 24 volts across the capacitor, it isn’t too far wrong to estimate that at 50% of CR, the voltage will be 12 volts.

However, a lot of MOSFETs might be pretty well conducting at 3 volts so, there’s a fair degree of slop in this gearbox. That’s why I recommend simulating and messing around with values etc..

As for paralleling MOSFETs, it’s not clear cut by any means when operating them linearly through the dangerous zone so play safe and choose MOSFETs that are intended for linear operation. The main clue is page 1 of the data sheet. If it says typical applications are switching circuits or converters then make absolutely sure you give enough headroom. Alternatively go to a company called IXYS and pick the appropriate range designed for linear applications.