Inductors work by storing energy in a magnetic field. As long as the field is changing, the resistance of the inductor will remain high (inductive reactance). My question is: If a strong magnet is placed on the inductor in a switching regulator, why doesnt the external field of the magnet interfere with it's operation? Surely when the inductors field collapses due to current being switched off, the permanent magnet's field will remain, preventing the inductor from generating a back-emf? This is not the case, I tried it and a magnet has no effect.
External magnet close to the inductor will only have effect when it is moving OR inductor core is close to saturation. With the presence of external magnetic field inductor core could saturate sooner (or later - depending on polarity and strength). If the inductor has no core (i.e. air core) it will be indeed extremely challenging to see the effect.
This principle is used in early magnetometers (fluxgate magnetometers).