Thankyou very much for spending your time in reply, I appreciate that you are just coming to this in passing, and not having had time to read it fully as have I myself. Your points are of course, very worthy and interesting.
Adding copper that oscillates with rise times of 10ns or so will not improve your EMI
Sorry but this was not what was suggested. You are absolutely right to suggest that it would be a mistake to go putting large thermal copper pours of switching node copper....I would agree with that...but the inductor has one terminal pad connected to the cathode of the "bottom" led in the load.....this does not have high dv/dt and should be used under the inductor to give extra cooling. The inductor carries 9 amps here. Adding copper in such a way under the inductor is free of charge and does not increase the board size, wheras picking a new inductor, a bigger one, may make it higher in profile etc.
I did indeed forget about the hot loop as you describe.....the current that is high di/dt in this loop that you describe, is the reverse recovery current of the power diode...if its a schottky diode , then as you know, its not reverse recovery, but the capacitive current associated with the capacitance of the schottky.
In truth, if the power switch loop and the rectifier loop are kept narrow area, then the hot loop is going to be narrow area too, since its comprised of bits of each.
The inductor current is indeed continuous, but it suffers a high di/dt at the peaks and troughs.....not as much as say the power switch loop in a flyback which is trapezoidal current though, I would confess. However, it is high di/dt, as in a very short interval of time, the current (at the peak) very suddenly ceases to rise, and then goes down....and that in very very short time.
Also, I am absolutely sure you would agree that current always flows in loops, and the loop responsible for radiating electromagnetic waves is the loop in which the current is flowing in at the time that the radiation is emitted. Electromagnetic radiation is only emitted from the "hot loop" when current flows round that entire "hot loop" (eg the reverse recovery current that I described)
So many thanks for pointing out the "hot loop", however, I would add here that the jist of this thread is that more thermal cooling copper should be used. It is free of charge to use it , because the board area under the inductor is there anyway. You are quite right to warn about adding thermal copper of the switching node...I agree that is a bad idea if drain node copper is splashed all over the place due to its high dv/dt.....however, the discussion here is that there are other copper nodes that could be used as cooling copper which aren't high dv/dt.
I'd always like to just pick a bigger component in order to get extra cooling..but typically, there are always size restrictions, and as you know, one thus needs to maximise what one has got...eg the board area under the inductor.