Voltage drop problem in the use of tantalum capacitors

Here we take the TAJ series of AVX tantalum chip capacitors as an example to explain as follows:

1. Surge voltage of tantalum capacitors

The surge voltage of a tantalum capacitor refers to the maximum voltage that the capacitor can withstand in a short time through a circuit with a minimum series resistance of 33Ohms (1KΩ in CECC countries). The surge voltage can reach up to 10 times the rated voltage and last up to 30 seconds within one hour at room temperature. The surge voltage is only a reference parameter and cannot be used as a basis for circuit design. During normal operation, the capacitor should be charged and discharged regularly.

The value of surge voltage is different at different temperatures. At 85 degrees and below, the classification voltage VC is equal to the rated voltage VR, and the surge voltage VS is equal to 1.3 times the rated voltage VR; at 85 to 125 degrees, the classification voltage VC is equal to 0.66 times the rated voltage VR, and the surge voltage VS is equal to 1.3 times the classification voltage VC.

AVX tantalum capacitors are limited in the voltage and current surges they can withstand. This is due to the common property of all electrolytic capacitors that a high enough electrical stress will penetrate the dielectric and destroy it. For example, a 6 V tantalum capacitor will have an electric field of 167 kV/mm when operating at rated voltage. Therefore, it is important to ensure that the voltage across the capacitor terminals never exceeds the specified surge voltage rating. The semiconducting manganese dioxide used as the negative plate layer of tantalum capacitors has self-healing properties. However, this low resistance is limited. In the case of low impedance circuits, the capacitor may be broken down by the surge current. The capacitor is stepped down, which increases the reliability of the component . AVX recommends a derating table summarizing the rated voltage used on common voltage tracks. The recommended protection resistor for low impedance tantalum capacitors in circuits that are rapidly charging or discharging is 1Ω/V. If this requirement cannot be achieved, tantalum capacitors should be used with a derating factor of up to 70%. In this case, a higher voltage may be required than for a single capacitor. A series combination should be used to increase the working voltage of the equivalent capacitor: for example, two 22μF 25V parts in series are equivalent to one 11μF 50V part.

2. Reverse voltage of tantalum capacitor

The reverse voltage of AVX tantalum capacitors is strictly limited, as follows:

10% of rated DC working voltage at 1.0V 25°C maximum 3% of rated DC working voltage at 0.5V 85°C maximum 1% of rated DC working voltage at 0.1V 125°C maximum Reverse voltage values ​​are based on the highest voltage that the tantalum capacitor can experience at any time. These limits assume that the tantalum capacitor is polarized in the correct direction for most of its operating life. They are intended to cover short-term reversals such as those that occur during a switching transient during a small portion of an impressive waveform. Continuous application of reverse voltage will result in polarization that will cause increased leakage current.

In situations where continuous reverse applied voltage may occur two similar capacitors should be connected together in a back-back configuration with the negative termination connected. In most cases this combination will have a nominal capacitance that is half the capacitance of either capacitor. Under isolated pulse conditions or within the first few cycles the capacitance may approach the full nominal value. The reverse voltage ratings are designed to cover small level excursions resulting from conditions of mistaken polarity. The values ​​quoted are not intended to cover continuous reverse operation.

3. Superimposed AC voltage of tantalum capacitor (Vr.ms) ------ also known as ripple voltage

This is the maximum rms AC voltage; superimposed on a DC voltage, that can be applied to a capacitor. The sum of the DC voltage and the peak superimposed AC voltage shall not exceed the category voltage.

4. Forming voltage of tantalum capacitors

This is the voltage formed at the anode oxide. The thickness of this oxide layer is proportional to the voltage formed on a capacitor and is a factor in setting the rated voltage.