How to select a particular value Y capacitor in a power supply design i.e. i want to know the parameters or specification what i should keep in my mind while choosing Y capacitor in a power supply design?
Y caps are from line voltage to chassis, so you choose them as large as you can, but the value needs to be low enought to fullfill the requirements on maximum earth current.
You need to see how much noise is conducted and radiated down the power line. The Y cap is part of a filter. With out knowing how noisy you power supply is you don't know how good of a filter you need.
I just attended a TI seminar on Power Supply Design that gave a good example of EMI filter design and selecting both X and Y capacitors. The documentation for the course should be available on the TI web site for the Power Supply Design Seminar 2012-2013.