How to design this filter?
As others have said, this is a common mode choke, or filter.  It's job is to attenuate noise coming back out of the SMPS and prevent it from getting onto your input power lines, 48V in this case.

You need two key pieces of information to design such a filter.  First, you need to know your conducted emissions requirement.  This comes from your customer or maybe from a government or industry document, and will specify the amount of "noise" allowed as a function of frequency.  This is the easy part.

Second, you need to know the amount of noise created by your SMPS and put onto the input lines.  It usually takes a detailed analysis of your power system to do that.  This is the hard part.

Once you have those two pieces of information, you can determine how much attenuation your filter has to provide as a function of frequency.

A third component to this is the amount of current your supply draws from the 48V source.  A 1 amp filter is a lot easier to realize (smaller components, particularly magnetics) than is a 30 A filter.

You've got a pretty sporty filter there, with a 30 KHz switching frequency and 1600 W (33.3 A) of power draw.

To clarify the comments on common mode filters, this is a type of schematic symbol, or some variant thereof, that we use for a common mode choke.