Dielectric absorption

The dielectric absorption ( Latin absorbere "suck, suck") describes the losses or deviations from the simple ideal behavior of a dielectric in the electric field . In capacitors , the effect manifests itself on the one hand as a dielectric loss in AC operation and on the other hand as a time-dependent voltage across the capacitor despite constant charge, e.g. B. in the form of a voltage in the capacitor that recurs after discharging, which u. U. can also be dangerous (reloading effect). This can lead to errors in time-determining circuit parts or holding circuits. In high-frequency lines, dielectric absorption can contribute to attenuation. In electrolytic capacitors, the dielectric absorption is part of the residual current when switched on.

Explanation

The undirected permanent molecular dipoles in a dielectric (top) align under the influence of an electric field (bottom)

Exponential relaxation of a quantity from the initial value to the equilibrium value in the case . ${\displaystyle f(t)}$ ${\displaystyle f_{0}}$ ${\displaystyle f_{\infty }}$ ${\displaystyle f_{\infty }>f_{0}}$

Circuit model to explain a time-delayed voltage build-up through parallel-connected RC timing elements

In a real dielectric, the polarization cannot immediately follow a changing electric field. It takes a certain amount of time for permanent, electrical dipoles in the dielectric to adapt their mean alignment to a changed field through polarization . The dielectric absorption leads to a conversion of energy from the alternating field into heat, both unintentionally in the non-ideal capacitor or specifically in the microwave oven .

The alternating electric field created between the electrodes aligns the disordered permanent electric dipoles in the dielectric with the respective field direction by means of polarization . The polarization of the dipoles results in losses due to dielectric heating and is accompanied by heating of the capacitor. In figurative terms, dielectric absorption absorbs the energy required for polarization. Dielectric absorption and dielectric loss are synonymous per se. With many capacitors, dielectric absorption is the main source of electrical losses and determines - neglecting the feed line losses - the loss factor , ESR or quality factor of the capacitor.

These properties are determined in the frequency range from approx. 0.01 Hz to 1 GHz, the range in which commercially available capacitors are usually operated, by a material- dependent relaxation time constant . Compared to the space charge process of the capacitor, this leads to a time-delayed alignment of the polarized permanent molecular dipoles in the dielectric. The time constant explains the frequency dependence of the relative permittivity of many dielectrics for capacitors. Since the dipoles in the mentioned frequency range are predominantly polarized by an orientation polarization in which the dielectric relaxation largely corresponds to the dielectric absorption, the dielectric absorption is also often called dielectric relaxation . ${\displaystyle \varepsilon _{r}}$

The material-dependent relaxation time constant also has the effect that after a capacitor has completely discharged, a material-dependent number of molecular dipoles are polarized in the field direction without a voltage being measurable at the connections. However, the remaining polarization in the dielectric relaxes over time, so that a voltage in the polarity of the previously applied voltage arises again at the electrodes of the capacitor, which is "recharged" so to speak. In older publications, dielectric absorption is therefore also described as a recharging effect .

The voltage from the reloading effect builds up slowly, similar to an exponential function. Depending on the material, it can take days to weeks until all dipoles are discharged. The “recharged” voltage can last for months with the high insulation resistance of today's capacitor dielectrics - even with electrolytic capacitors . Unloading with subsequent reloading can be repeated several times.

Measurement

The measurement method for the recharging effect of the dielectric absorption is specified in EN 60384-1: The capacitor is charged for 60 minutes with nominal voltage, then discharged via a resistance of 5 Ω for 10 seconds. After removing the discharge resistor, the resulting voltage is measured after a 15-minute recovery time. The magnitude of the voltage generated by the dielectric absorption is given in percent in relation to the originally applied voltage and depends on the dielectric used. It is specified in the data sheets of many manufacturers.

Capacitor type	Dielectric absorption
Air and vacuum capacitors	not measurable
Class 1 ceramic capacitors , NP0	0.6%
Class 2 Ceramic Capacitors, X7R	2.5%
Polypropylene film capacitors (PP)	0.05 to 0.1%
Polyester film capacitors (PET)	0.2 to 0.5%
Polyphenylene sulfide film capacitors (PPS)	0.05 to 0.1%
Polyethylene Naphthalate Film Capacitors (PEN)	1.0 to 1.2%
Tantalum electrolytic capacitors with solid electrolytes	1 to 5%, 10%
Aluminum electrolytic capacitors with liquid electrolyte	about 10%
Double layer capacitors	?

For double-layer capacitors there are no manufacturer-guaranteed values ​​for the size of the dielectric absorption, which is why no numerical value can be given in the table above.

Effects in circuits

This effect has two effects on the capacitors used today. The voltage at the connections caused by the dielectric absorption can under certain circumstances lead to problems in the functioning of a circuit. For sensitive analog circuits such as sample and hold circuits , integrators or measuring amplifiers , class 1 ceramic or polypropylene capacitors are used instead of class 2 Kerkos, polyester film capacitors or electrolytic capacitors. In the vast majority of circuits, especially when the capacitors are used to filter undesired frequencies, this often minimal electrical recharge voltage has no effect on the electrical function.

In the case of aluminum electrolytic capacitors with liquid electrolytes, however, the voltage created by the recharging effect for components that have not yet been installed can pose a risk to the environment due to the formation of sparks during installation. This voltage, which can be 50 V with 400 V electrolytic capacitors, can cause damage to semiconductors or other components when installed in the circuit. Larger aluminum electrolytic capacitors , but also high-voltage and power capacitors , must therefore be transported or delivered short-circuited.

The second effect of the dielectric absorption effect has only recently been known and is due to the significantly improved properties of modern capacitors. When looking more closely at the temporal course of the residual current of tantalum electrolytic capacitors with polymer electrolytes after applying a voltage, it was found that the current increases to a value that is greater than the actual residual current. The difference is explained by the energy requirement that is required to adapt the time-delayed spontaneous alignment of the molecular dipoles in the dielectric to the field direction. This flow is therefore part of the residual flow, but can be separated from it by separate considerations.

See also

• Dielectric Spectroscopy

Web links

• Paul Falstad: Circuit Simulator Applet. (Java applet) falstad.com (circuit simulator with a simulation of an example for the recharging effect of a capacitor).
• Introduction to capacitors

Footnotes and individual notes

1. ↑ K. Kundert: Modeling Dielectric Absorption in Capacitors. (PDF).
2. ↑ ^{a b c} Analysis of solid tantalum capacitor leakage current (PDF).
3. ↑ Rod Elliott: 2.1 - Dielectric Absorption. (No longer available online.) In: Capacitor Characteristics. September 24, 2005, archived from the original on December 2, 2013 ; Retrieved November 24, 2013 . Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.  
4. ↑ WIMA, Characteristics of Metallized Film Capacitors in Comparison with Other Dielectrics ( Memento of the original dated November 5, 2012 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.  
5. ↑ Film Capacitors, TDK Epcos, General technical information (PDF; 1.4 MB)
6. ↑ AVX, Dielectric Comparison Chart (PDF; 161 kB)
7. ↑ Holystone, Capacitor Dielectric Comparison, Technical Note 3 (PDF; 64 kB)
8. ↑ Hardware Design Techniques - Analog Devices, HARDWARE DESIGN TECHNIQUES, 9.1 PASSIVE COMPONENTS, Dielectric Absorption, Page 9.4 PDF
9. ↑ CDE, Aluminum Electrolytic Capacitor Application Guide cde.com (PDF)
10. ↑ Danger from dielectric absorption (English).