The Ripple Rating of a Capacitor

Engineers do not prefer having ripples in their circuits and do their best to minimize its effects. For example, an AC source delivers power to an AC-DC converter that subsequently converts it to a steady DC output. It can be very inconvenient if the output were to have any source AC power appearing on top of the DC output in the form of small, frequency dependent variations. However, ripple may not be considered evil in all cases, as some digital signals could be useful to engineers as a necessary design function. Among these are signals that use changes in voltage levels to switch the state of a device and those generating clock timings.

As capacitors can store charge, they are useful for smoothening ripples in circuits. However, the designer must take care that the peak voltage does not exceed the voltage rating of the capacitor. It must also be noted that since there can be DC bias present in the circuit, the peak voltage will be the sum of the maximum ripple voltage and the DC bias. However, that is not enough for electrolytic capacitors.

Electrolytic capacitors are usually made with aluminum, tantalum and niobium oxide technologies and they have polarity. If the negative voltage of the ripple is allowed to drop below zero, this will cause a connected capacitor to operate under reverse bias conditions. Class II ceramic capacitors used in low frequency applications also suffer from this restriction.

A capacitor functions as a charge reservoir, charging with the rise of the incoming voltage and discharging into the load as it decreases – smoothening out the ripples in the process. Therefore, capacitors will see varying voltage. Additionally, depending on the power applied, the current through the capacitor will also vary, as will the intermittently pulsed and continuous power. This causes resultant changes in the electric field of the capacitor regardless of the incoming form and creates oscillating dipoles within the dielectric material, thereby self-heating the capacitor. Any parasitic inductance or ESL and resistance or ESR contributes to the energy dissipation.

That means a capacitor with low ESR, ESL and DF (dissipating factor), will heat up less than one with a dielectric characterized by high ESR and DF. However, as these parameters also depend on frequency, different dielectric materials offer optimum performance (lower heat generation) over different frequency ranges.

The dielectric in a capacitor is usually very thin constituting only a small amount of the overall mass of the capacitor. Other materials used in the construction also contribute to the heating when considering ripple – capacitor plates being one of the major contributors. Additionally, the conductive contacts also heat up to some degree when the capacitor carries an AC signal or current.

For example, at a certain frequency, if the capacitor with a 100mOhms ESR carries a 1A rms current, the power dissipated internally will be 100mW. If this power is supplied continuously, it will heat the capacitor internally until thermal balance is reached. Since this depends on ESR, the power dissipation is a function of frequency. However, the total thermal management will also depend on the capacitor’s environmental conditions, governing the heating up of the capacitor in an application.