As can be seen from the equation, the corner frequency can be reduced by increasing either the inductance or capacitance or both. Typically, c is set to be 1/10th of the switching frequency of the converter to obtain a good attenuation.
Although it is easy to choose a filter corner frequency to reduce ripple effectively at the converter switching frequency, it is less easy to predict the attenuation of noise spikes which comprise a whole spectrum of harmonic frequencies. This is because at a certain frequency when the value of ZL and ZC become equal, the LC network may start to ‘resonate’ and noise can be amplified rather than attenuated. Above resonance, although there is still some noise attenuation, other parasitic effects begin to occur.
For example, the self-capacitance of the inductor produces another resonance peak, at a much higher frequency. This capacitance also tends to allow noise to ‘bypass’ the inductor. At higher frequencies, core losses in the inductor increase, and the AC resistance of the inductor wire increases because of the ‘skin effect’; moreover, the capacitor begins to act as a resistor as its impedance becomes small compared with its equivalent series resistance (ESR).