What does the absorption coefficient of film capacitors refer to? Is the smaller it is, the better?

Before introducing the absorption coefficient of film capacitors, let’s take a look at what is a dielectric, the polarization of a dielectric and the absorption phenomenon of a capacitor.

Dielectric

A dielectric is a non-conductive substance, i.e., an insulator, with no internal charge that can move.If a dielectric is placed in an electrostatic field, the electrons and nuclei of the dielectric atoms do “microscopic relative displacement” within the atomic range under the action of the electric field force, but not “macroscopic movement” away from the atom to which they belong, like the free electrons in a conductor. When the electrostatic equilibrium is reached, the field strength inside the dielectric is not zero. This is the main difference between the electrical properties of dielectrics and conductors.

Dielectric polarization

Under the action of the applied electric field, a macroscopic dipole moment appears inside the dielectric along the electric field direction, and a bound charge appears on the dielectric surface, which is the polarization of the dielectric.

The absorption phenomenon

 the time lag phenomenon in the charging and discharging process of capacitor caused by the slow polarization of dielectric under the action of applied electric field. The common understanding is that the capacitor is required to be fully charged immediately, but it is not filled immediately; the capacitor is required to release the charge completely, but it is not released, and the time lag phenomenon occurs.

Absorption coefficient of film capacitor

The value used to describe the dielectric absorption phenomenon of film capacitors is called absorption coefficient, and is referred to by Ka. The dielectric absorption effect of film capacitors determines the low frequency characteristics of capacitors, and the Ka value varies greatly for different dielectric capacitors. The measurement results vary for different test durations of the same capacitor; the Ka value also varies for capacitors of the same specification, different manufacturers, and different batches.

So there are two questions now-

Q1. Is the absorption coefficient of film capacitors as small as possible?

Q2. What are the adverse effects of a larger absorption coefficient?

A1:

Under the action of the applied electric field: the smaller Ka (the smaller absorption coefficient) → the weaker the polarization of dielectric (i.e. insulator) → the lower the binding force on the dielectric surface → the smaller the binding force of dielectric on charge traction → the weaker the absorption phenomenon of capacitor → the capacitor charges and discharges faster. Ideal state: Ka is 0, i.e. absorption coefficient is 0, the dielectric (i.e. insulator) has no polarization phenomenon under the action of applied electric field, the dielectric surface has no traction binding force on the charge, and the capacitor charge and discharge response has no hysteresis. Therefore, the absorption coefficient of film capacitor is the smaller the better.

A2:

The effect of a capacitor with too large a Ka value on different circuits manifests itself in different forms, as follows.

1) Differential circuits become coupled circuits

2)Sawtooth circuit generates increased return of the sawtooth wave, and thus the circuit cannot recover quickly

3) Limiters, clamps, narrow pulse output waveform distortion

4) The time constant of the ultra-low frequency smoothing filter becomes large

(5) DC amplifier zero point is disturbed, one-way drift

6）The accuracy of the sampling and holding circuit decreases

7) Drift of DC operating point of linear amplifier

8) Increased ripple in the power supply circuit

All the above performance of dielectric absorption effect is inseparable from the essence of the capacitor’s “inertia”, that is, in the specified time charging is not charged to the expected value, and vice versa discharge is also the case.

The insulation resistance (or leakage current) of a capacitor with a larger Ka value is different from that of an ideal capacitor(Ka=0) in that it increases with longer test time (leakage current decreases). The current test time specified in China is one minute.