Insulation resistance and leakage current of ceramic capacitors

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Insulation resistance and leakage current of ceramic capacitors [Copy link]

Since the electrodes of a capacitor are insulated, the resistance value is theoretically infinite. However, actual capacitors have a finite resistance value because a small amount of current flows between the insulated electrodes.
This resistance value is called "insulation resistance" and is expressed in units of resistance [MΩ] or CR product [ΩF], [MΩμF].

Insulation resistance

When a DC voltage is applied directly to a capacitor, the flow of the inrush current (also called charging current) is shown in Figure 1. As the capacitor is gradually charged, the current decreases exponentially.

figure 1

The current I(t) increases with time and is divided into three categories (as shown in equation (1)), namely, the charging current Ic(t), the absorption current Ia(t) and the leakage current Ir.

I(t) = Ic(t) + Ia(t) + Ir　……方程（1）

The charging current indicates the current through an ideal capacitor. Compared with the charging current, the absorption current has a delayed process and is accompanied by dielectric losses in the low frequency range, causing the polarity of high dielectric constant capacitors (ferroelectric capacitors) to reverse and Schottky barriers to occur at the interface between ceramic and metal electrodes.

Leakage current is a constant current that appears at a certain stage after the influence of absorption current decreases.

Therefore, the current value described below varies with the amount of time voltage is applied to the capacitor. This means that only timed measurements under specified voltage applications can determine the insulation resistance value of a capacitor. The
insulation resistance of a monolithic ceramic capacitor represents the ratio between the applied voltage and the leakage current after a set time (e.g. 60 seconds) when a DC voltage (without ripple) is applied between the capacitor terminals.
In addition, the charging current, absorption current, and leakage current cannot be clearly distinguished.

Insulation resistance value regulations and units

As mentioned above, the unit of insulation resistance is expressed as resistance [MΩ] or CR product [MΩF]. CR product [MΩF] is the product of nominal capacitance and insulation resistance.
The unit varies depending on the product number, so please confirm it on the detailed specification sheet of each product number.

Calculation method of insulation resistance [MΩ] of CR products [MΩF]
For example: When the CR product is 500ΩF or higher and the capacitance is 1μF
-> 500ΩF/1μF = 500MΩ or more

The higher the capacitance value, the lower its insulation resistance value.

The reason is explained as follows: Considering that a monolithic ceramic capacitor can be regarded as a conductor, the insulation resistance can be calculated using Ohm's law based on the voltage and current applied to it. The
insulation resistance value R can be expressed by equation (2), where the length of the conductor is L, the cross-sectional area of the conductor is S, and the resistivity is ρ.

R = ρL/S ……Equation (2)

Similarly, the capacitance C can be expressed by equation (3), where the distance between the two electrodes of the monolithic ceramic capacitor (dielectric thickness) is represented by L, the area of the internal electrode is represented by S, and the dielectric constant is ε.

C ∝ εS/L ……Equation (3)

Equation (4) is derived from equation (2) and equation (3). It can be seen from equation (4) that R is inversely proportional to C.

R ∝ ρε/C ……Equation (4)

The greater the insulation resistance, the smaller the leakage current under DC voltage. Generally speaking, the greater the insulation resistance value, the higher the accuracy of the circuit.

Verifone

The explanation is quite detailed. The effect should be better under high pressure.