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Grasping capacitors, ripple and self-heating

Posted: 19 Mar 2015 ?? ?Print Version ?Bookmark and Share

Keywords:Ripple? converter? capacitors? parasitic resistance? inductance?

Ripple is often assessed in terms of its two components: ripple voltage and ripple current. In most applications, it's a circuit condition that you want to minimise. For example, in an ACCDC converter, which takes power from an AC source and converts it to a steady DC output, you want to avoid any of the source AC power appearing as a small, frequency dependent variation on top of the DC supply. However, in other cases, ripple can be a necessary design function, such as a clocking signal or a digital signal capable of using changes in voltage level to switch the state of the device.

In the latter case, ripple considerations can be quite straightforward: don't let the peak voltage exceed the voltage rating of the capacitor. Although, it's important to bear in mind that the peak voltage will be the sum of the maximum ripple voltage plus any DC bias in the circuit. Additionally, there is a second caveat for electrolytic capacitors C such as tantalum, aluminium, and niobium oxide technologies C due to their polarity: don't let the minimum voltage of the ripple drop below zero, as this will cause the capacitor to operate under reverse bias conditions. This caveat also applies to Class II ceramic capacitors in low-frequency applications, but more on that below.

Since capacitors act as charge reservoirs, they charge as the incoming voltage increases and discharge into the load as it decreases, essentially smoothing out the signal. Capacitors will see varying voltage and, depending on the power applied, varying current, as well as both continuous and intermittently pulsed power. Regardless of the incoming form, the resultant changes in the capacitor's electric field cause the dipoles in the dielectric material to oscillate, which creates heat. This reaction, known as self-heating, is one of the primary reasons that dielectric properties are important, as any parasitic resistance (ESR) or inductance (ESL) will add to the energy dissipation.

A dielectric with low losses (i.e., low ESR / DF and low ESL) will heat less than a dielectric with high ESR and DF; however, these parameters also vary with frequency, as different dielectric materials provide optimum performance (i.e., generate the least heat) over different frequency ranges.

Capacitor dielectrics are thin, and may only constitute a small amount of the capacitor's overall mass, so the other materials used in their construction need to be considered when evaluating ripple as well. For example, the capacitor plates in a non-polar device (e.g., ceramic or film capacitors) are metallic, while polar devices, such as tantalum or aluminium, have a metallic anode (or, in the case of niobium oxide technology, a conductive oxide anode) and a semiconducting cathode, such as manganese dioxide or conductive polymer. There are also a variety of conductive contacts, including metals (e.g., copper, nickel, silver palladium, solder, etc.) and conductive epoxies, on the external terminations or leads, and all of these materials heat to some degree when passing an AC signal or current.

To see how these factors come into play, let's use a solid tantalum capacitor employed to smooth residual AC ripple current in the output stage of a DC power supply as an example. Firstly, since this is a polar technology, it will need a positive voltage bias to prevent an AC component from causing reverse bias to occur. This voltage will typically be the nominal output voltage of the supply.

Figure 1: Ripple Voltage superimposed on Bias Voltage.


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