Selection criteria for a MOSFET during designing of circuit

[shaswat]

I want to know that how to select a MOSFET during the designing of a circuit.On internet I find a datasheet for the MOSFET (20V P-channel). In that datasheet it shows differnet parameter


PRODUCT SUMMARY

VDS (V) RDS(on)(mΩ) ID (A)
-20 130 @ VGS = -4.5V -2.8
-20 190 @ VGS = -2.5V -2.0

How these parameters would be useful during the designing of a circuit.
I am newbie in this field so any idea would be apprecible.

 

[Ow@i$]

before jumping directly on to the datasheet you must first define what is that you are designing .. and depending upon the need of the project then you search for the relevant Mosfet..!!!

 

[shaswat]

Actually iam working on a device having low internal MOSFET voltage(500V). In my application the power supply used to sustain and work perfectly at 440 V test . So at 440 V test after rectifying and converting the AC into DC I got approx 640 V. So at this voltage the device doesn't work and fail at 440 V test. So in order to used this device I want to add an external MOSFET in series with the circuit so that it become safe at 440 V test. For safety purposr I have to add 200 V MOSFET at the external of the circuit. Here I got the problem.The problem is that I don't have any idea about how to select the MOSFET. What parameter should I have to kept in mind during the selection of the device. What is Rds on value and how these values affects the circuit...