Get to know "tantalum capacitors"

2023-11-21

Overview

The full name of tantalum capacitor is tantalum electrolytic capacitor, which is also a type of electrolytic capacitor. It uses metal tantalum as the dielectric, hence its name. Tantalum capacitors were first developed by Bell Laboratories in the United States in 1956. They have excellent performance and are passive components that are small in size and can achieve large capacitance among capacitors. Tantalum capacitors come in a variety of shapes and are manufactured into small and chip components suitable for surface mounting. Tantalum capacitors are not only used in military communications, aerospace and other fields, but also are widely used in industrial control, film and television equipment, communication instruments and other products.

Basic structure (taking chip tantalum capacitor as an example)

The solid tantalum capacitor is formed by pressing tantalum powder and sintering it into an anode body in a high-temperature furnace. Its dielectric is to energize the anode body in acid to form a porous amorphous Ta2O5 dielectric film. Its working electrolyte is manganese nitrate solution. High temperature decomposition forms MnO2, which is used as a lead connection through the graphite layer.

 Manufacturing process

The general process flow is as follows (blue bold fonts are key processes):

Raw material inspection-molding process-sintering process-wet inspection QC-welding process-empowering process-coating process-graphite silver paste process-immersion silver QC-assembly process-molding process-sandblasting process-printing process-trimming process- Pre-testing process - Experienced process - Testing process - Appearance process - Taping process - Inspection process - Finished product QC - Warehousing and storage - Packaging - Shipping QC

Main characteristic parameters

In the actual manufacturing process of tantalum capacitors, due to the differences in the performance of the raw materials used, the different process levels, and the different performance of the equipment, although the performance of the mass-produced products all conforms to the standards, in fact there are obvious differences in the performance of the products produced by different manufacturers. quality difference. Even in the same production batch, there are actually quality differences between different products. The underlying reason for this phenomenon is that the complex production process of tantalum capacitors makes it impossible to maintain absolutely consistent product parameters. Therefore, the pursuit of quality consistency and high performance has become an important goal for all manufacturers. For users, there are two main reasons for failure during use: 1. The product performance parameters do not match the circuit usage conditions. 2. The products provided by users have quality problems.

The impact of various electrical performance parameters of tantalum capacitors on reliability during use. The actual main parameters include:

①Rated capacity, that is, capacitance [uF];

②DC, DC leakage current [uA];

③Loss [%], dissipation factor (DF value);

④ESR, equivalent series resistance, [Ω]

1.Capacity value

The impact of capacity accuracy on use and reliability is briefly explained; in actual working conditions, the capacity can determine the response speed and amplitude of the filtered signal. In the pulse charge and discharge circuit, it can determine whether the output current waveform meets the requirements. However, unless the signal that needs to be filtered is weak in a circuit with a very high operating frequency, high capacity accuracy must be required. For filtering and pulse charging and discharging in general DC-DC circuits, small deviations in capacity will not affect the use effect at all. It will not affect reliability. Therefore, in actual use, the magnitude of the deviation in capacity will not have any impact on reliability.

General test conditions for capacitance: ambient temperature: 25 degrees room temperature, frequency: 120HZ, voltage: maximum AC effective value 1V or maximum DC 2.5V. Generally speaking, the capacitance decreases as the frequency increases, and the capacity increases slightly as the temperature increases. Because solid tantalum capacitors use solid MnO2 electrolytes, the capacity changes are relatively stable. In filtering applications, the capacity changes are basically negligible. Referring to the figure below, it can be seen that the measured capacitance values are different under different test frequencies.

1) Rated working voltage & surge voltage

The operating voltage indicated on the general specifications is the maximum DC voltage within a certain temperature range (below 85 degrees). When the temperature is higher than 85 degrees, the rated voltage will decrease, usually at 125 degrees, and the rated voltage will drop to 2 times the original rated voltage. /3.

Working voltage VS temperature change curve:

surge voltage

Generally, the surge voltage that tantalum capacitors can withstand is about 1.3 times the rated voltage or category voltage. Exceeding the surge voltage can easily lead to the breakdown of the Ta2O5 dielectric.

Reverse voltage

It is generally not allowed to apply reverse voltage to tantalum capacitors, and it cannot be used in a pure AC environment. A small amount of reverse voltage is allowed under certain circumstances. In 25℃ environment: less than or equal to 10% Ur or 1V (whichever is smaller). In 85℃ environment: less than or equal to 5% Ur or 0.5V (whichever is smaller). In 125℃ environment: less than or equal to 1% Ur or 0.1V (whichever is smaller), IEC60384-3 reverse voltage test conditions are: 125 ℃ environment, 3Vdc or 10% UR (whichever is smaller) test for 125 hours.

2. DC leakage current

1) Ripple current & surge current

The damage mechanism of surge current and ripple current is mainly caused by overheating of the device, causing the device to burn out. The relationship between power loss (Phas) and ripple current (Irms) is expressed by the following formula: Phas = V-·I drain + Irms2·R≈Irms2·Rs,

Among them: V-: DC bias voltage (V);

I drain: leakage current (A);

Rs: equivalent series resistance (Ω);

Irms: ripple current.

It can be seen from the above formula: when Rs increases or when Irms increases, the power loss increases. Therefore, in high-frequency lines, the ripple current through the tantalum electrolytic capacitor is required to be small and the tantalum electrolytic capacitor with a small equivalent series resistance is required. capacitor. In all applications, it should be noted that the ripple current and surge current in the circuit should not be too large. Generally speaking, the surge current and ripple current are not written in the tantalum capacitor specification book. You need to consult the supplier, or apply I =U/R is a rough estimate, where U is the rated voltage and surge voltage. R is the equivalent series impedance. National military standards and suppliers generally recommend connecting a resistor in series with a low-impedance circuit to reduce the impact of surge current (it is generally recommended to connect 1V/ohm or 3V/ohm in series)

2) Leakage current

The DCL value is the most important decisive parameter for the performance of tantalum capacitors. In particular, the attenuation speed of the product's leakage current and the rate of change of the leakage current at high temperatures will have a decisive impact on the reliability of the product. Therefore, the reliability of a product during use mainly depends on the leakage current of the product and the change rate of the leakage current of the product at high temperatures. Especially when used in low-impedance switching power supply circuits without resistance protection [also called DC-DC circuits or low-impedance circuits] and high-power pulse charge and discharge circuits, the above characteristics of this product will have a great impact on circuit reliability. Almost decisive. Because there are frequent surge voltages and surge currents in such circuits, products with insufficient withstand voltage and large leakage current changes at high temperatures cannot withstand the surge impact at all, and may be broken down and fail or explode in an instant.

The relationship between tantalum capacitor leakage current and charging time:

Products with the same specifications produced by different manufacturers have completely different leakage current attenuation speeds, although they are all qualified products. The leakage current of tantalum capacitors will gradually decrease as the charging time increases, reaching a stable state within 3 minutes. However, the leakage current attenuation speed of products of different qualities during charging varies due to different production conditions. Products with fast decay speed generate less heat due to the small current passing through in a very short period of time. Therefore, there is almost no excessive heat concentration that can cause instant failure of the product, so the product is not prone to thermal failure. The leakage current is small, indicating that the dielectric layer of the product is of good quality and can safely withstand higher voltage and current impacts. Products with slow leakage current attenuation are not only prone to crashes due to the large passing current when surges occur. Wear it, and it is extremely easy to explode and burn, causing devastating effects to the user. Therefore, users can test the tantalum capacitor's leakage current attenuation speed to identify the tantalum capacitor's ability to withstand voltage shocks and current shocks.

The relationship between volt-ampere characteristics of tantalum capacitors:

Judging from the leakage current of products with the same specifications tested at different voltages, the leakage current of tantalum capacitors will increase with the increase of the test or use voltage, until it breaks down [this phenomenon is called the volt-ampere characteristics of tantalum capacitors]. However, the degree of increase in leakage current of products produced by different manufacturers will be very different. In fact, tantalum capacitors allow the application of 1.3 times the rated voltage without any quality problems. However, if the leakage current of a product changes too much with the change of voltage, such a product will not be able to withstand excessively high operating voltage. Likewise, it cannot withstand even high surge voltages.

The leakage current of higher quality products will change less within the specified test or use voltage range. On the contrary, the quality of the product will not meet the basic requirements of users. Because such products have poor surge resistance, they are very prone to breakdown when used in circuits with large pulse currents.

The relationship between leakage current and operating temperature of tantalum capacitors:

The leakage current of tantalum capacitors will increase with the increase of the operating temperature. This curve is called the leakage current temperature curve. However, products of the same specifications produced by different manufacturers often have high-temperature leakage current changes due to different production processes and the accuracy of the raw materials and equipment used. There is a very big difference. Products with large changes in high-temperature leakage current will eventually experience breakdown due to the continuous accumulation of heat generated by themselves at high temperatures. Products with small changes in high-temperature leakage current will work at high temperatures for a long time, and the stability of the product will be affected. And the reliability will be higher. Therefore, the change rate of product leakage current at high temperature can determine the reliability of tantalum capacitors. For chip tantalum capacitors, the high temperature performance has a decisive impact on reliability.

The leakage current test is generally performed by applying the rated voltage at 20°C. In the measurement circuit, a 1000OHM protection resistor is connected in series with the capacitor and charged for one to five minutes (two minutes for KEMET, VISHAY, and AVX, and five minutes for SANYO). Then measure the leakage current.

3. Dissipation factor (DF value)

The dissipation factor is a physical quantity that determines the internal power dissipation of the capacitor. The smaller the better, generally the DF value increases as the frequency increases.

Explanation of the impact of loss on product use and reliability: Loss (DF value) is a parameter that characterizes the proportion of ineffective power loss caused by the resistance of the tantalum capacitor itself. Products with smaller losses will also have smaller ESRs. However, the slight difference in loss size will not have a significant impact on the use. The impact on the reliability of the product in working condition is greater than the impact of the capacity deviation, but it is smaller than the impact of the product leakage current and ESR size on the reliability during use. The ratio is still small (the influence of leakage current and ESR size > the influence of loss size > the influence of capacity deviation). If the loss of the product is large during filtering, the filtering effect will be worse. At the same time, products with larger losses also have poorer surge resistance.

4. Impedance, equivalent series resistance (ESR) & inductive reactance

ESR is an important indicator that determines the filtering performance of a capacitor. The ESR of a tantalum capacitor is mainly caused by the impedance of the pin and internal electrode. It is a very important parameter for the performance of the capacitor at high frequencies. Generally speaking, the same capacitance and the same voltage value The ESR of tantalum capacitors is lower than electrolytic capacitors, but higher than multilayer ceramic capacitors. ESR decreases with the increase of frequency and temperature, ESR=DF/WC. Below the resonant frequency, the impedance of the capacitor is the vector sum of the capacitive reactance and ESR of the capacitor. After the capacitor resonates, the impedance of the capacitor is the vector sum of the inductive reactance and ESR of the capacitor.

In pulse charging and discharging circuits, tantalum capacitors will continue to withstand surge current impacts with peak power that may reach tens of amps, and sometimes the frequency of charging and discharging may reach hundreds or even thousands of Hz; in this type of voltage, the voltage is basically stable and the surge In circuits with constant current flow, the reliability of tantalum capacitors not only depends on the product's withstand voltage, volt-ampere characteristics and high and low temperature performance, but also depends on the equivalent series resistance ESR of the product, because products with larger ESR values are more susceptible to high surges. In an instant, more heat will accumulate, which can easily lead to breakdown of the product. Therefore, the ESR value of tantalum capacitors can directly determine the product's ability to withstand DC surges.

In addition, the heat generated by products with different ESR values in a circuit with AC ripple within a certain period of time is also proportional to the ESR value. Products with higher ESR generate higher heat within a certain period of time. Therefore, different Products with different specifications have different ripple current resistance capabilities due to different impedance ESR values. Products with low ESR not only have less capacity attenuation when used at high frequencies, but also have better filtering effects and can be used in higher frequency circuits. At the same time, because they have greater surge resistance, they also meet the requirements of continuous high reliability requirements. Basic requirements for pulse charge and discharge circuits that pass instantaneous large currents.