Types of Capacitor - Electrical Characteristics - Ohmic Losses, ESR, Dissipation Factor, and Quality Factor

Ohmic Losses, ESR, Dissipation Factor, and Quality Factor

The summarized losses in discrete capacitors are ohmic AC losses. DC losses are specified as "leakage current" or "insulating resistance" and are negligible for an AC specification. AC losses are non-linear, possibly depending on frequency, temperature, age or humidity. The losses result from two physical conditions:

  • line losses including internal supply line resistances, the contact resistance of the electrode contact, line resistance of the electrodes, and in "wet" aluminum electrolytic capacitors and especially supercapacitors, the limited conductivity of liquid electrolytes and
  • dielectric losses from dielectric polarization.

The largest share of these losses in larger capacitors is usually the frequency dependent ohmic dielectric losses. For smaller components, especially for wet electrolytic capacitors, conductivity of liquid electrolytes may exceed dielectric losses. To measure these losses, the measurement frequency must be set. Since commercially available components offer capacitance values cover 15 orders of magnitude, ranging from pF (10−12 F) to some 1000 F in supercapacitors, it is not possible to capture the entire range with only one frequency. IEC 60384-1 states that ohmic losses should be measured at the same frequency used to measure capacitance. These are:

  • 100 kHz, 1 MHz (preferred) or 10 MHz for non-electrolytic capacitors with CR ≤ 1 nF:
  • 1 kHz or 10 kHz for non-electrolytic capacitors with 1 nF < CR ≤ 10 μF
  • 100/120 Hz for electrolytic capacitors
  • 50/60 Hz or 100/120 Hz for non-electrolytic capacitors with CR > 10 μF

A capacitor's summarized resistive losses may be specified either as ESR, as a dissipation factor(DF, tan δ), or as quality factor (Q), depending on application requirements.

Capacitors with higher ripple current loads, such as electrolytic capacitors, are specified with equivalent series resistance ESR. ESR can be shown as an ohmic part in the above vector diagram. ESR values are specified in datasheets per individual type.

The losses of film capacitors and some class 2 ceramic capacitors are mostly specified with the dissipation factor tan δ. These capacitors have smaller losses than electrolytic capacitors and mostly are used at higher frequencies up to some hundred MHz. However the numeric value of the dissipation factor, measured at the same frequency, is independent on the capacitance value and can be specified for a capacitor series with a range of capacitance. The dissipation factor is determined as the tangent of the reactance and the ESR, and can be shown as the angle δ between imaginary and the impedance axis.

If the inductance is small, the dissipation factor can be approximated as:

Capacitors with very low losses, such as ceramic Class 1 and Class 2 capacitors, specify resistive losses with a quality factor (Q). Ceramic Class 1 capacitors are especially suitable for LC resonant circuits with frequencies up to the GHz range, and precise high and low pass filters. For an electrically resonant system, Q represents the effect of electrical resistance and characterizes a resonator's bandwidth relative to its center or resonant frequency . Q is defined as the reciprocal value of the dissipation factor.

A high Q value is for resonant circuits a mark of the quality of the resonance.

Comparization of ohmic losses for different capacitor types
for resonant circuits (Reference frequency 1 MHz)
Capacitor type Capacitance
(pF)
ESR
at 100 kHz
(mΩ)
ESR
at 1 MHz
(mΩ)
tan δ
at 1 MHz
(10−4)
Quality
factor
Silicon capacitor 560 400 2,5 4000
Mica capacitor 1000 650 65 4 2500
Class 1
ceramic capacitor (NP0)
1000 1600 160 10 1000

Read more about this topic:  Types Of Capacitor, Electrical Characteristics

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