Overview

Capacitors have electrical properties specified in their datasheets, such as capacitance.

This time, we will explain the main electrical characteristics specified by taking Wurth Elektronik 's electrolytic capacitor WCAP-ASLU series 865090449009 as an example.

Figure 1: 865090449009 electrical characteristics

capacitance

The most important characteristic of a capacitor is its capacitance. The symbol is C. Capacitance describes the property of a capacitor that can store electrical energy when a certain voltage is applied, and is measured in farads. Farad is named after Michael Faraday, a prominent British experimental physicist.

The following relationship applies to capacitance, voltage, current and time.

If the 100 μF /25V capacitor in this example is charged at 1A, it will reach 25V in 2.5ms.

Rated voltage

Rated voltage is defined in DC voltage and is the maximum voltage that can be applied to a capacitor continuously. The voltage ratings given in the datasheet apply from the specified lower to upper temperature limit of the capacitor. The rated voltage is the peak value when AC components such as ripple voltage are added to the DC voltage, and must be used under conditions that do not exceed this value.

surge voltage

Different from the rated voltage, it is a regulation of overvoltage in the event of an abnormality. In this example, it is specified as 1.15 times the rated voltage, and it should be noted that the temperature and cycle conditions are specified.

The conditions are described in the General Information of the product data sheet 865090449009, 1000 cycles @ 20 °C, the cycle is 30 seconds for charging and​ ​5 minutes and 30 seconds for discharging.

leakage current

It refers to the current that flows through a capacitor when a voltage is applied. Since it changes depending on the capacitance of the capacitor in Fig. 2, voltage, temperature, and the time after voltage application, the value for 2 minutes after voltage application @ 20 °C is specified in this example. Figure 2 is a reference example of the time change of leakage current.

Figure 2: Time variation of leakage current
From Wurth Elektronik SN019​

loss tangent

There are cases defined as DF (Dissipation Factor) and tan δ, but they have the same meaning. A capacitor has a phase shift, ideally the current is exactly 90° out of phase with the voltage. A real capacitor has ESL and ESR components, which causes a phase shift.

The difference δ from the ideal phase angle of 90 ° is called the loss angle. The tangent of the loss angle δ is expressed as the induction tangent tan δ, and the smaller this is, the smaller the loss of the capacitor becomes. Figure 3 shows this relationship.

Figure 3: Tangent of loss angle

In this example, it is written as Dissipation Factor (DF), but the relationship is tan δ = DF.

tan δ, ESR and capacitive reactance Xc are calculated as follows.

ripple current

Specified as rated ripple current. In this example the values are specified at 120Hz @ 85 °C. Some low-impedance products are specified at 100 kHz.

Since heat is generated by ripple current and ESR, the rated ripple current is determined, and the relationship between the power consumption of the capacitor and each parameter can be calculated as follows.

P: Condenser power consumption

Ir: ripple current

V: Applied voltage

IL: leakage current

Also, since the rated ripple current varies with frequency, it must be corrected when used at frequencies other than those specified in the datasheet.

In this example, the following correction table is shown.

Figure 4: Frequency compensation for ripple current

It can be seen that 1.5 times the ripple current is allowed when using this capacitor under the condition of 10kHz.

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