# Why is the inductor necessary in an LC filter?

I am using an LC filter for the output of a buck converter. Dont both the inductor and capacitor just attenuate high frequency signals? Does having the inductor affect the output voltage or does it just affect the noise of it?

• Have you got a schematic for reference? Add it into your question. – Transistor Aug 22 '18 at 18:32
• The inductor in a buck converter isn't just filtering a signal. It's fundamental to how the buck converter works. – The Photon Aug 22 '18 at 18:41
• Here is some reference material on inductors. – rdtsc Aug 22 '18 at 18:47
• BTW with a handle like that, you should definitely add a user image, like pbs.twimg.com/profile_images/1190856641/… – The Photon Aug 22 '18 at 18:48
• An L is necessary in an LC filter, otherwise it would just be a C filter!. But seriously, it increases the order of the filter. It does for current and voltage what the capacitor does for voltage and current. A ladder of series Ls and shunt Cs gets one filter order per component. – Neil_UK Aug 22 '18 at 18:49

Why is the inductor necessary in an LC filter?

This is a bit of a nomenclature problem. LC means "Inductive"(L) "Capacitive"(C). An inductor is not necessary in all types of filter, for instance an RC filter limits the charge rate of the capacitor with a resistor, but is less efficient by nature, since an inductor would use impedance rather than resistance to achieve the voltage drop.

I am using an LC filter for the output of a buck converter. Dont both the inductor and capacitor just attenuate high frequency signals?

Specifically, what an inductor or capacitor do are:

An Inductor generates a magnetic field proportional to the current flowing in it. This magnetic field stores energy and when applied voltage is removed and current decreases, the inductor will deposit the energy back into the circuit to maintain current flow through itself. The effect is that the inductor resists changes in the current flowing through it, and it does so proportionally to it's inductance. Inductors will cause current to lag behind voltage. A Capacitor stores energy on the surface of two parallel plates separated by an insulator. When a voltage is applied to a capacitor, the capacitor sinks or sources current proportional to the difference between the applied voltage and it's current voltage. The closer the voltage on the capacitor is to the applied voltage, the less current flows. Whereas an inductor resists changes in the current flowing through it, a capacitor resists changes in the voltage on it's terminals. A capacitor will cause current to lead voltage. A good mnemonic for remembering which component that leads and lags the current is the word "CIVIL".

Different types of filters will attenuate different frequencies.

Does having the inductor affect the output voltage or does it just affect the noise of it?

Especially if you are trying to build an efficient power supply, resistance is to be avoided, but the side effect of low resistance is that if the output is connected at the same time as the input, input voltage will be applied directly to the output while the input is on, so if the device powered is sensitive to instantaneous voltage, it could be destroyed. Using an inductor, you can average out the output voltage on a switched mode supply without limiting capacitor charging speed with a large resistance. This is far from the only application of an inductor, but suffice to say you've got to do some math to figure out their effect on a complex signal type.

Draw a switched mode circuit with a capacitor and load across the output, but no series inductor and resistor, and you will see that when the switch is on, you have applied full source voltage to both a capacitor and the load.

I am using an LC filter for the output of a buck converter. Dont both the inductor and capacitor just attenuate high frequency signals?

Yes, they attenuate high frequency signals and leave the output with less switching converter ripple noise. That is what is desired in a buck converter; it's a voltage regulator so, the removal of high frequency signals/noise on the output is usually very desirable.

Does having the inductor affect the output voltage or does it just affect the noise of it?

It smooths the DC content of the output and removes switching noise. It basically restores a switched waveform having an average value of $V$ to a DC output of $V$ with far fewer switching artefacts.

For describing the efficiency of lowpass filtering we use the pole distribution in the complex s-plane - expressed by the so-called quality factor "pole-Q, Qp".

• Passive RC filters do not allow values Qp>0.5 (bad transition from pass band to stop band)

• For complex poles with Qp>0.5 (better filtering; classical approximations to brickwall filtering: Bessel, Butterworth, Chebyshev) we need LC combinations, which allow resonant effects (passive LC structures).

• As an attractive alternative, the funcions of the inductor (enabling resonance effects) can be replaced by amplifiers. This leads to very popular active RC-filter structures with frequency dependent feedback networks.