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RE: Which 0.1uF polypropylene bypass caps?

Posted by 13th Duke of Wymbourne (A) on July 7, 2017 at 18:11:07

In Reply to: RE: Which 0.1uF polypropylene bypass caps? posted by petercapo on July 7, 2017 at 16:12:14:

Just to recap, the impedance across a capacitor falls as frequency rises until you reach the self-resonant frequency (SRF). Above this frequency the parasitic inductance within the capacitor makes the impedance increase with frequency. The idea of bypassing is to place another capacitor in parallel that has a higher SRF to push out the frequency at which the impedance of the parallel pair starts to rise. Getting a higher SRF means using a smaller bypass capacitance. A smaller capacitor will have a higher impedance than the original capacitor at lower frequencies so you have to choose the right combination of bypass capacitance and bypass SRF for it to be effective. The ratio of the bypass SRF to electrolytic SRF must be greater than the SQRT(ratio of the electrolytic capacitance to the bypass capacitance) for the bypass to be effective. Or, looking at it the other way around, the ratio of electrolytic capacitance to bypass capacitance must not be greater than SQUARE(ratio of bypass SRF to electrolytic SRF).

For example, a 100uF electrolytic with SRF=1MHz and a 0.1uF bypass with a SRF= 31MHz have the same high frequency impedance, so there is little bypassing benefit. If you pick a 0.1uF bypass it must have an SRF > 31MHz to be beneficial.

Hope that helps
13DoW

ps. I was about to answer that the ratio of SRFs should be greater than the ratio of capacitances but I ran a few circuit simulations and realized that SRF varies with frequency squared hence the square/square root relationship.

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