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Parasitics & Capacitor Selection, Part 2D: Equivalent Series Resistance (ESR)

In ceramic capacitors, the dielectric effects outweigh the electrode and termination conductivity, since Class 1 dielectrics have lower ESR and experience the least change with frequency and temperature, and so their ESR only drops slightly with increasing temperature.

For film capacitors with polypropylene dielectric there is little ESR change due to temperature or frequency. However, for higher dielectric constant polyester, the ESR will be higher overall and will increase a little with temperature at low frequencies (~ 10 kHz) and markedly with temperature at higher frequencies (~ 1 MHz).

In the case of solid tantalum, the manganese dioxide counter-electrode permeates the inner pores of the capacitor element and also builds up a thick external coating that forms a base for the materials used for the external conductive bond. Consequently, at increasing frequencies, the counter-electrode acts as distributed resistance, which causes the internal regions to exhibit less conductive contribution. This results in reduced ESR as frequency increases, but with an associated loss of effective capacitance.

Figure 4: Cross Section of Tantalum Capacitor Element

This shows the interior of a tantalum capacitor element and how the solid electrolyte (manganese dioxide crystals) coats the convoluted contours of the tantalum pentoxide dielectric layer. This is not a good pathway for high-frequency signals.

This shows the interior of a tantalum capacitor element and how the solid electrolyte (manganese dioxide crystals) coats the convoluted contours of the tantalum pentoxide dielectric layer. This is not a good pathway for high-frequency signals.

Since the manganese dioxide material has the largest contribution to system resistance, its negative temperature coefficient means that there is a significant reduction in ESR as temperature increases. Alternatively, tantalum polymer capacitors, in which the semiconducting counter-electrode is replaced by conductive polymer, do not show a significant drop in ESR with increasing temperature, but do exhibit a lower overall ESR.

The most important consideration regarding ESR in power applications is that, when subjected to ripple current, ESR will act as an internal heating element for the component. So, even though the capacitor may be doing an effective job of smoothing the ripple in the application, the heat that is caused by it needs to be controlled. Although low dielectric constant technologies tend to have the lowest ESR, bulk capacitance at the application frequency is also needed for effective filtering. To this end, there are many software programs available that allow designers to select a rating and then plug in the application frequency, temperature, and voltage in order to calculate the maximum allowable ripple current and associated temperature rise for that capacitor.

The main takeaways for designers are:

  • In general, ESR is lower in larger parts with more conduction paths. For example, a 0805 0.1uF / 16V MLCC will have a lower ESR than the same rating in a 0402 size.
  • The thinner the dielectric for a given size part, the lower the ESR. For example, a tantalum C case 100uF / 6V will have lower ESR than the same size 10uF / 35V.
  • The lower the ESR, the better the bulk capacitance retention at higher frequency.
  • For all critical applications, be sure to check the online software with respect to potential self-heating and allowable ripple.

2 comments on “Parasitics & Capacitor Selection, Part 2D: Equivalent Series Resistance (ESR)

  1. RedDerek
    April 9, 2014

    Would you explain a bit more about the condition that the effective capacitance for ceramic caps drops as one uses the capacitor near the maximum voltage? From what I have heard the capacitance change can be as much as 20% between the 1/2 Vmax and 0.9 Vmax.

  2. Chris-R
    May 28, 2014

    Yes – this is known as the voltage coefficient of capacitance (VCC), and is a factor for class II ceramics (e.g. X7R & X5R) and not for Class I (e.g. NPO). The dielectric materials used for class II ceramics have the property that their relative permittivity (directly related to their dielectric constant) reduces as the electric field across them is applied. This has always been the case, and when using, say, a 0.1uF / 50v X7R on a 12v rail, the effect is measurable. However, as electric field strength is inversely proportional to the distance between the internal electrodes, for very densely stacked designs, such as a 100uF / 4v 1206, the effect becomes more significant. It's always advisable to check the VCC curves in the manufacturers' data sheets for the rating that you will be using, in terms of the voltage the part will be operating at. 

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