FRA Capacitor Impedance Measurement

Ceramic capacitors are now commonly used to decouple power supplies to reduce the voltage ripple generated by the power supply. Common sources ripple are currents drawn at the switch mode frequency and its harmonics, and currents drawn in response to the load - such as oscillators, and FPGAs whose outputs may all switch together or at indeterminate frequencies.

If we know the ripple current, and the supply impedance, the ripple voltage is simply Ripple Current x Supply Impedance. Getting the supply impedance down reduces the ripple voltage. Standard switchmode power supplies have gains of >20 dB, and therefore can use feedback to present a low impedance, only up to a few kHz. Above that capacitors are used. So knowing the impedance/frequency of the capacitor is vital. Unfortunately not may manufacturers provide this in their data sheets. In addition, the Resr (effective series resistance of a Resistor) is not well specified, and determines the power dissipation of the capacitor, which is important in compact high current power supplies.

In the video we measure a 22uF ceramic capacitors' impedance characteristic, and check out how well it will perform in a power supply.

In another video we will look at the effects of paralleling capacitors.