> By giving it a burst of opposing magnetic field you are going some way to weakening it, but how much of a problem this is depends on loads of things, starting with the material used for the magnet itself.  No way of knowing beforehand how long any random magnet will still do it's job, I'm afraid!


I don't think so. 
One interesting trick for boost or buck switchers, is to use a magnet to bias the core, and roughly double the current the core can take without saturation. A few thousand hours at 100kHz is a lot of pulses.


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