Surge and ripple currents
It is important to know the maximum application surge current (single peak) allowed through the capacitor in order to avoid overloading the capacitor on power-up or start-up. Overload current can be calculated using the internal voltage of the power source and the internal resistances of all devices in series with the capacitor, including its ESR. The maximum surge current should be lower than the capacitor's maximum allowed surge current Ipmax
)/ (0.45+ESR). If the application current is too high, additional derating can be applied and a higher Vr
capacitor must be chosen.
The capacitor’s maximum ripple current is dependent on the maximum AC current flowing through the capacitor. Ripple has two main parameters, effective value (rms, ACIrms
) and frequency (f). Ripple current is limited by maximum power dissipation (Pd
), which is generated by the ESR of the capacitor. The bigger the case size, the higher the allowed power dissipation; each case size has a constant value. A lower ESR results in less power to dissipate, allowing a higher ripple current, according to:
= ESR x (Ir
Thus, for an application that calls for a high ripple current and low ESR, where the case size is not an issue, a multi-anode device is the best choice
The operating frequency mainly affects two parameters: capacitance and ESR (see figures below).
The upper figure (above) displays decreased capacitance at higher frequencies, while the lower one shows increased ESR at lower frequencies. Both dependencies should be considered to assure sufficient capacitance and an ESR rate low enough for the required ripple current.
The combination of application guidelines listed above will result in the correct capacitor selection. Alternatively, if the case size has to be prioritized for miniature or low profile applications, the selection process must be adjusted accordingly.
Sometimes, one capacitor alone is not sufficient, so two or more devices may be necessary. In these instances, only the same capacitor types are recommended to be used in combination. Parallel connection increases capacitance (multiplies) and decreases ESR (divides); serial connection increases total allowed DC voltage (rated voltage multiplied), but decreases capacitance (divided) and increases ESR (multiplied). For serial connection, capacitors should be connected in parallel with a resistor divider, where resistance of the divider resistors is calculated using a figure of 10 times the DC leakage current of the capacitor’s catalog value.
is a field application engineer at AVX
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