As to the ripple wave of the switching power supply, it certainly exists both theoretically and practically. Generally speaking, here are some approaches restraining or reducing it.
Reducing EMI - diode back connection inductance
The metal shell can be adopted as the shield which is used to reduce the radiated interference of the external electromagnetic field. In order to reduce the electromagnetic interference of the power line input, the power input terminal is equipped with the EMI power line filter, just as the below diagram shows (EMI filter is also named as the power line filter). This is also a method usually used to restrain the high-frequency noise.