Reducing input voltage spikes on buck converter
As with any o-scope measurements on switching regulator circuits you can get false indictaions of spikes if you do not use best practise when probing: -



The "bad measurement" technique is susceptible to induced voltages from the switching currents - the loop formed by the probe's earth connection in effect acts like a receiver for magnetic fields. The bigger the loop area, the more interference is picked up.

I'm not saying this is always to blame but it can be a very common mistake when probing high current/frequency circuits.

You can prove this by connecting the probe tip to the probe ground wire and, without making a physical connection to your regulator, move the loop around to see what you pick-up.