Pi filter with resonance in power supplies
It sounds great, but I never seen it used in any device, why?


It sounds great until you start to realize that the inductance needed to give you a blocking resonance (on regular AC supplies with bridge rectifiers) is probably physically larger in size than a modern wall wart and then some again. I remember being taught this technique at college in the 1970s but nobody used them back then either. Semiconductor regulators have marched so far forwards that any ripple that might be suppressed by a chunky inductor resonated with a capacitor is easily dealt with by them. Then along came switch mode power supplies and changed the game again.

I'm not saying that they aren't used in places but I am saying the need for them is the tiniest of a fraction of a percent compared to what might be called state of the art in the 1950s.