An LC filter is used to smooth the output voltage of a buck converter since a rectangular voltage is produced at the terminals of the switching elements. This applet examines the relationship between the duty cycle, switching frequency, L and C values, and the “smoothness” of the output voltage. The “smoothness” can be directly viewed from the time plot of the output voltage and from the switching frequency harmonic plot of the output voltage. As always there is a compromise between the size of the passive filter components and the amount of attenuation in the switching frequency ripple.

• Click on the transistor/diode symbol to visually change the representation to a two position switch. Both representations are equivalent and produce the rectangular voltage at the output terminal.
• Adjust the switching frequency and view the change to the magnitude-frequency plot and on the level of switching frequency harmonics appearing in the output voltage.
• Adjust the duty cycle. The switching frequency harmonics magnitudes change with duty cycle and see how well the LC filter does at attenuating them.
• Adjust the values of the inductor and capacitor in turn. This changes the cutoff frequency of the filter and the level of attenuation of the switching frequency in the output.