Electronic capacitors must be known for eliminating the voltage characteristics of the power line ripple design

Author: Murata Manufacturing Components Business Division Zakipedia

The phenomenon that the actual electrostatic capacitance value of a capacitor changes with direct current (DC) and alternating current (AC) voltage is called a voltage characteristic. The smaller the amplitude of the change, the better the voltage characteristics and the greater the amplitude, indicating that the voltage characteristics are worse. When a capacitor is used in an electronic device for the purpose of eliminating power line ripple, etc., it is necessary to design using a voltage condition.

The DC bias characteristic is a phenomenon in which the actual electrostatic capacitance changes (decreases) when a DC voltage is applied to the capacitor. This phenomenon is a phenomenon unique to a high dielectric constant sheet-like multilayer ceramic capacitor using barium titanate-based ferroelectrics, an aluminum electrolytic capacitor (polymer AI) of a conductive polymer, and a conductive polymer tantalum electrolytic capacitor. (polymer Ta), film capacitor (Film), titanium oxide, and sheet-like multilayer ceramic capacitor for temperature compensation using calcium zirconate-based paraelectric (MLCC) This phenomenon hardly occurs on the top (see Figure 1).

The following example shows how this actually happens. A 1.8V DC voltage is applied to a high dielectric constant sheet-like multilayer ceramic capacitor having a rated voltage of 6.3 V and a capacitance of 100 uF. At this time, in the case of a product having a temperature characteristic of X5R, the electrostatic capacity was reduced by about 10%, and the actual electrostatic capacity value was changed to 90 uF. In the Y5V product, the electrostatic capacity is reduced by about 40%, and the actual electrostatic capacity becomes 60uF.

Figure 1: Capacitance change rate of various capacitors - DC bias characteristics (example)

When a DC voltage is applied to a barium titanate-based ferroelectric, the electric field is small, and the electric displacement (D) is proportional to the electric field (E). However, as the electric field increases, the spontaneously polarized spontaneous polarization (Ps) starts along the electric field. The directions are neatly arranged, showing a very large dielectric constant, and the actual electrostatic capacitance value is increased. Further, as the electric field is further enhanced, the spontaneous polarization is aligned neatly, and after the polarization is saturated, the dielectric constant becomes small, and the actual electrostatic capacitance value becomes small (refer to FIG. 2).

Therefore, when selecting a multilayer ceramic capacitor, do not select it in full accordance with the electrostatic capacity described in the catalog. It is necessary to apply a DC voltage component to the applicable power supply (signal) line, measure the electrostatic capacitance, and grasp the actual electrostatic capacitance value. However, the lower the DC voltage component applied by such a DC bias characteristic, the smaller the capacitance reduction. Semiconductor chips such as FPGAs and ASICs that operate at a supply voltage (DC voltage) that exceeds 1V have recently appeared on the market. When a multilayer ceramic capacitor is used on the power line of such a chip, there is no obvious problem of DC bias characteristics.

Figure 2: State when a voltage is applied to a ferroelectric ceramic

The AC voltage characteristic is a phenomenon in which the actual electrostatic capacitance changes (increases or decreases) when an AC voltage is applied to the capacitor. This phenomenon is the same as the DC bias phenomenon, and is a phenomenon unique to a high dielectric constant sheet-like multilayer ceramic capacitor using barium titanate-based ferroelectrics. Conductive polymer aluminum electrolytic capacitor (polymer AI) and conductive Chip-shaped multilayer ceramic capacitor for temperature compensation of polymer electrolyte tantalum electrolytic capacitor (polymer Ta), film capacitor (Film), titanium oxide, and paraelectric body using calcium zirconate-based (MLCC) This phenomenon hardly occurs on the top (see Figure 3).

It is assumed that an alternating current voltage (frequency: 120 Hz) of 0.2 Vrms is applied to a high dielectric constant sheet-like multilayer ceramic capacitor having a rated voltage of 6.3 V and an electrostatic capacity of 22 uF. At this time, in the case where the temperature characteristic is the X5R product, the electrostatic capacity is reduced by about 10%, and the actual electrostatic capacitance value becomes 20 uF. The Y5V product is even worse, the electrostatic capacity is reduced by about 20%, and the actual electrostatic capacity becomes 18uF.

Figure 3: Capacitance change rate of various capacitors - AC voltage characteristics (example)

As described above, the grain of the ferroelectric ceramic has a domain, and the direction of each spontaneous polarization (Ps) is random, and the whole corresponds to a state of no polarization. When an electric field (E) is applied thereto, polarization is generated in the direction of the electric field to reach a saturation value. In this state, even if the electric field is removed, the polarization direction does not return to the original disordered state, and some will stay in the state of polarization, forming a residual polarization, which appears externally. In order to zero this residual polarization, an electric field in the opposite direction is required. When the inverse electric field is further enhanced, polarization inversion occurs and polarization is performed in the opposite direction. The polarization action of the ferroelectric caused by the external electric field like this is shown in the DE history curve (hysteresis curve) of FIG.

At high AC voltages, the current flowing through the capacitor creates large waveform distortion in the case of ferroelectrics and therefore cannot be directly applied to the definition of linear materials (*1). However, the relative dielectric constant (εr) obtained from the actual electrostatic capacitance value can also be said to be the average inclination of the hysteresis curve (dashed line in Fig. 4).

Figure 4: DE hysteresis curve of ferroelectrics

*1: The linear stress and stress-strain characteristics are linear, that is, the material properties in which the stress σ is proportional to the deformation ε.