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Tantalum capacitors are products with small volume, high capacitance and excellent performance. They were first developed by Bell Labs in the United States in 1956. They come in a variety of shapes and are made into small and chip components suitable for surface mounting, which are not only used in military communications, aerospace and other fields but also used in industrial control, film and television equipment, communication instruments and other products.
The full name of Tantalum Capacitors is tantalum electrolytic capacitors, which are also a type of electrolytic capacitor. Metal tantalum is used as the dielectric. Unlike ordinary electrolytic capacitors, which use electrolytes, tantalum capacitors do not need to use aluminum-coated capacitor paper for firing. There is almost no inductance in a tantalum capacitor, which also limits its capacity. In addition, because there is no electrolyte in it, it is suitable for working at high temperatures.
Figure 1. Tantalum Capacitors in Different Styles
Tantalum capacitors are characterized by long life, high-temperature resistance, high accuracy, and excellent high-frequency filtering and wave-changing performance. In the working process, they can automatically repair or isolate the defects in the oxide film, so that the oxide film medium can be strengthened and restored to its proper insulation capacity at any time without being subject to continuous cumulative damage. This unique self-healing performance guarantees its advantages of long life and reliability. Also, they have a very high working electric field strength larger than some types of capacitors, thereby ensuring their miniaturization.
Tantalum capacitors have excellent performance. They are small in volume, large in capacitance, and very convenient to use, which have few competitors in power filtering, AC bypass, and other applications.
Besides, they have the ability to store electricity, charge, and discharge, and are mainly used for filtering, energy storage and conversion, bypass marking, coupling and decoupling, and time constant components. In the application, pay attention to the performance characteristics of the tantalum capacitor such as the working environment and heating temperature, and take measures such as derating. Proper use will help to give full play to its function. While improper use will affect the product's working life.
Figure 2. RC Time Constant Calculator
If a voltage is applied to a capacitor of Value C through a resistance of value R, the voltage across the capacitor rises slowly. The time constant is defined as the time it will take to charge to 63.21% of the final voltage value.
Solid tantalum capacitors have excellent electrical properties, a wide operating temperature range, various forms, and excellent volume efficiency.
Tantalum capacitors also have unique characteristics. The working medium of the tantalum capacitors is a very thin tantalum pentoxide film formed on the surface of tantalum metal. This layer of oxide film dielectric cannot exist independently, it should be integrated with one end of the capacitor. Therefore, its capacitance in a unit volume is particularly large, indicating a very high specific capacity, which is particularly suitable for miniaturization.
The marked (one horizontal line) end of the capacitor body is the positive pole, and the other end is the negative electrode. The long lead of the lead tantalum capacitor is the positive end and the short lead is the negative end. On a chip tantalum capacitor, the positive pole is identified by a dark strip or beveled edge. Of course, you may not understand with plain text descriptions, so the following pictures are collected for you to distinguish the positive and negative electrodes of tantalum capacitors.
Figure 3. Polarity of Tantalum Capacitors
According to their polarity, capacitors can be divided into two types: non-polarity capacitors and polarized capacitors. Non-polarity capacitors are generally used to store charge, and are mainly used in circuits such as coupling and frequency selection. Polarized capacitors are usually used to store and release electric charges, and need to be selected according to the actual situation.
During the installation of polarized tantalum capacitors, we need to pay attention to distinguish their positive and negative poles. Misconnection will cause the instantaneous failure of tantalum capacitors. In pulse circuits, the positive or negative electrodes of two tantalum capacitors are connected to each other
Figure 4. Non-Polarity Capacitors
Solid tantalum capacitors have polarity. If the two poles are reversed, it will cause permanent failure. And if a reverse voltage is mistakenly applied to a high-impedance circuit, the capacitor may cause damage even if it is not short-circuited. To protect the circuit from overvoltages and reverse voltages, be careful that the end rod of the tester can not touch the capacitors.
Figure 5. A Solid Tantalum Capacitor
When the reverse voltage is unavoidably used in the circuit, it must be 10% of the rated voltage or 1V at 85° , and 5% of the rated voltage or 0.5 V at 85° . A smaller value is recommended. If the reverse voltage is applied for more than 240 hours, a resistor of minimum resistance of 33R or more should be added to the circuit.
The reverse connection of the positive and negative electrodes of the tantalum capacitor will not only cause failure, but lead to unnecessary expenses and losses for customers or enterprises with large demand. Therefore, it is critical to accurately identify the positive and negative electrodes.
Because tantalum capacitors have the danger of explosion, we must pay special attention when using them.
1. Tantalum capacitors are electrolytic capacitors with polarity (the terminal with sign "+" is positive). Do not connect the polarity reversely, or it will increase electric leakage or may cause a short circuit, smoke, or even explosion.
2. The circuits that it cannot be applied to are as follows: high-impedance voltage holding circuits; coupling circuits; time constant circuits; circuits that have leakage current effects; circuits that increase the withstand voltage in series.
Figure 6. Circuit to Illustrate RL Time Constant
3. Do not use it above the rated voltage, or it may cause a short circuit.
4. Limit rapid charging or discharging. It is recommended to add a current limiting resistor in the charging and discharging circuit to make the impulse current less than 20A.
5. During the designing process, allow a certain margin for the capacity, withstand voltage, and impedance of the capacitor to make the procedure more secure and reliable.
6. Make sure that the temperature range used is within the operating temperature range of the capacitor. The power supply current does not exceed the allowable ripple current, or the heat inside the capacitor will increase and reduce the service life.
7. The voltage applied by the capacitor is recommended to be 90% of the rated voltage. If the rated voltage is greater than 10V, 80% of the rated voltage is applied; if the DC voltage plus the alternating voltage, the peak voltage cannot exceed the rated voltage; if DC voltage plus negative peak alternating voltage, the negative voltage is not allowed to appear.
Manufacturers offer a wide range of tantalum capacitor products, which are optimized for specific characteristics and targeted at different applications and market segments. These different product families offer optimizations including lower ESR(equivalent series resistance), smaller size, higher reliability (for military, automotive, and medical applications), smaller DC leakage currents, lower ESL(equivalent series inductance), and higher operating temperatures. The following focuses on two of these areas: lower ESR and smaller size.
Reducing ESR has been one of the important research areas for the tantalum capacitor design. The choice of tantalum powder and the process used to coat the cathode material during production have a significant impact on ESR. However, for the given rated value (capacitance, voltage, size), these factors are mainly design constraints and are basically resolved on the most advanced devices available today. The two most important factors that reduce ESR are the replacement of MnO2 with a conductive polymer for the cathode material and the change of the lead frame material from iron-nickel alloy to copper (Cu).
Figure 7. A Simple Model of ESR Measurement
The ESR of traditional tantalum capacitors is mainly derived from the cathode material MnO2. As shown in Figure 8, the conductivity of MnO2 is about 0.1S/cm. In contrast, the conductivity of conductive polymers, such as poly3,4-ethylene dioxythiophene, is in the range of 100S/cm. This increase in conductivity directly causes a significant reduction in ESR.
Figure 8. Electric Conductivity of Different Materials
In Figure 9, the ESR-frequency curves at different rated values show the advantages of using a polymer cathode system for tantalum capacitors. By directly comparing the ESR-frequency curves of the A casing in MnO2 and polymer designs at the 6.3 V / 47 μF rated value, it can be seen that polymer designs reduce ESR by up to an order of magnitude at 100 kHz.
Figure 9. ESR-Frequency Curves at Different Rated Values
When we use Lead frame materials of more conductive materials, ESR can be improved. As shown by the capacitor cross-section in Figure 10, the lead frame provides the internal capacitor element and the electrical connection outside of the package.
Figure 10. Capacitor Cross Section
Iron-nickel alloys such as Alloy 42 have been the traditional choice for leadframe materials. The advantages of these alloys include the low coefficient of thermal expansion (CTE), low cost, and ease of use in manufacturing. Improvements in the processing of copper leadframe materials have enabled them to be used in tantalum capacitor designs. Since the conductivity is 100 times that of Alloy 42, the use of copper has a significant impact on ESR. For example, Vishay's 100μF / 6.3V T55 polymer tantalum capacitor with a case (EIA 3216) and traditional lead frame provides a maximum ESR of 70mΩ at 100kHz, 25° C. But the maximum ESR can be reduced to 40mΩ by changing the traditional leadframe to a copper lead frame.
The two main factors that improve the volume efficiency (capacitance density) of the tantalum capacitor design are the development of tantalum powder and the improvement of packaging.
The quality factor of the tantalum powder used in the capacitor design is: (capacitance voltage) / mass, which is abbreviated as CV / g. The evolution of tantalum powder used in massive production is shown in Figure 11. These increases in CV / g are associated with smaller particle sizes and improved powder purity. The use of these materials in capacitor design is a complex task, which requires a large investment in research and development.
Figure 11. Development of Tantalum powder Used in Mass Production
Another important factor that reduces the size of tantalum capacitors is the development of ultra-efficient packaging technology. The most common packaging technology used in the industry is leadframe design. This structure has a very high manufacturing efficiency, which can reduce costs and increase productivity. For applications not constrained by space, these devices are still viable solutions.
However, in many electronic systems where the main design criterion is increasing density, the ability to reduce element size is an important advantage. In this regard, manufacturers have made several advances in packaging technology. As shown in Figure 12, leadless frame designs can improve volumetric efficiency compared to standard lead frame structures. After we reduce the size of the mechanical structure required for external connections, these devices can take advantage of this additional available space to increase the size of the capacity cells, thereby increasing capacitance or voltage.
Figure 12. Volume Efficiency of Different Packaging Technologies
In the latest generation of packaging technology, Vishay's patented multi-array package (MAP) structure further improves volume efficiency by using metallization layers at the end of the package to provide external connections. This structure maximizes the size of the capacitive elements within the available volume by completely eliminating the internal anode connection. Figure 13 further illustrates the improvement in volumetric efficiency. It can be clearly seen that the volume of the capacitive elements has increased by more than 60%, which enables them to be used to optimize the device to increase capacitance and voltage, reduce DCL, and improve reliability.
Figure 13. Vishay's Patented Multi-array Package Structure
Another benefit of the Vishay MAP architecture is reducing ESL. The MAP structure can significantly reduce the size of the existing current loop by eliminating the mechanical lead frame of the loop package. By minimizing the current loop, ESL can be significantly reduced. As shown in Figure 14, this reduction can be as much as 30% compared to a standard lead frame structure. The decrease in ESL corresponds to an increase in the self-resonant frequency, which can expand the operating frequency range of the capacitor.
Figure 14. Performance of Vishay's MAP Structure Compared to Standard Lead Frame Structure
Advances in tantalum capacitor technology have resulted in lower ESR, lower ESL, and smaller size. The maturity of processes and materials used in conductive polymer cathode systems has brought us stable and reproducible performance. Improvements in packaging technology have led to higher capacitance density and reduced ESL. All this makes tantalum capacitors no longer limited to traditional uses but used in more designs.
All these improvements enable design engineers to significantly improved electrical performance with low parasitic effects and higher packaging densities.
Many customers often discuss the issue of tantalum capacitor explosion, and burning or explosion of the tantalum capacitor is the hardest problem for R&D technicians especially in switching power supply, LED power supply, and other industries. Because of the danger of the failure mode of tantalum capacitors, many R&D technicians dare not use them anymore.
In fact, if we can fully understand the characteristics of tantalum capacitors and find out the cause of failure (in the form of burnout or explosion), tantalum capacitors are not so horrible. After all, the benefits of tantalum capacitors are obvious.
The reasons for the failure of tantalum capacitors can be divided into two categories: the quality of tantalum capacitors and circuit design issues. This time we will analyze the circuit design problem.
Circuit design and product selection require the performance and parameters of tantalum capacitors to meet the characteristics of circuit signals. However, we cannot often guarantee that both of the above tasks are done well. Therefore, the failure problem will inevitably occur in the using process, which is briefly summarized as follows:
There are only two types of circuits using tantalum capacitors: circuits protected by resistors and low impedance circuits without resistor protection.
For circuits with resistors, since resistors will reduce the voltage and inhibit large currents, the working voltage can reach 60% of the rated voltage of the tantalum capacitor.
There are two types of circuits without resistors for protection:
(1) The charging and discharging circuit where the front level input has been rectified and filtered and the output is stable. In this type of circuit, the capacitor is used as a discharge power source. Since the input parameters are stable and there is no surge, even though it is a low impedance circuit, the voltage can still reach 50% of the rated voltage, which can ensure considerable reliability.
Figure 15. Charging and Discharging Circuit Schematic Diagram
(2) The power supply of the electronic machine. Capacitors are used in parallel in such circuits. In addition that the input signal should be filtered, discharging is also required to be at a certain frequency and power. Because it is a power circuit, the loop impedance of such circuits is very low to ensure that the output power density of the power supply is sufficient.
Figure 16. Two Capacitors in Parallel in a Power Supply Circuit
In this type of switching power supply circuit(also called DC-DC circuit), a high-intensity spike pulse with a duration of less than 1 microsecond will be generated in the circuit at each instant of power on and power off. The pulse voltage value can reach at least three times the stable input value, and the current can reach more than ten times the steady-state value. Due to the extremely short time of duration, the energy density per unit time is very high. If the operating voltage of the capacitor is too high, the pulse voltage actually applied to the product at this time will far exceed the product's rated value and the capacitor will be broken down.
Therefore, the permissible operating voltage of tantalum electrolytic capacitors used in this type of circuit cannot exceed 1/3 of the rated value. If we don’t consider the types of circuit impedance, and derate the voltage by 50%, as soon as the power is turned on, a short-circuit or explosion may occur in the DC-DC circuit with the lowest circuit impedance. To find out how much should the capacitors used in such circuits be derated, the size of the circuit impedance and the size of the input and output power, and the AC ripple in the circuit must be considered Because the circuit impedance can determine the magnitude of the switching instantaneous surge. The lower the internal resistance is, the more derating value the circuit should have. The magnitude of the derating must not be generalized but be determined by accurate reliability calculations.
The maximum DC current shock I that a tantalum capacitor can safely withstand during operation has the following mathematical relationship with the product's equivalent series resistance ESR and the rated voltage UR:
I = UR / 1 + ESR.
If a low-capacity tantalum capacitor is used in a circuit with a large peak output current, this product may be burned due to current overload.
Figure 17. The Steady-state, Inrush and Peak current When a Device is Turned on
When a tantalum capacitor with an excessively high ESR is used in a filter circuit with excessively high AC ripple, even if the voltage used is far lower than the derating range, sometimes a sudden breakdown will still occur at the moment of power on. The main reason for this kind of problem is that the ESR of the capacitor and the AC ripple in the circuit are seriously unmatched. The capacitor is a polar component that will heat up when the AC ripple passes through, and products of different case sizes can maintain different allowable heat generation of thermal balance. Because the ESR values of products with different capacities differ greatly, the AC ripple values that tantalum capacitors of different specifications can safely withstand also vary greatly. Therefore, if the AC ripple in a circuit exceeds the AC ripple value that the capacitors can safely withstand, it will cause a thermal breakdown. Similarly, if the AC ripple in the circuit is constant, and the actual ESR value of the selected tantalum capacitor is too high, the same phenomenon will also occur.
Generally speaking, in filtering and high-power charging and discharging circuits, tantalum capacitors with the lowest possible ESR value must be used. For circuit failure caused by high AC ripple in the circuit, many circuit designers ignore its harmfulness or don’t have enough understanding of it, and many of them simply determine that there is a problem with the quality of the capacitor.
This problem generally occurs because the actual withstand voltage of the tantalum capacitor is not enough. When a certain field strength is applied to the capacitor for a long time, if the insulation resistance of the dielectric layer is low, the actual leakage current of the product will be large at this time. For products with a large current, the actual withstand voltage will decrease.
Figure 18. The Flow of the Leakage Current in a Circuit
Another reason for this problem is that the standards for leakage current of tantalum capacitors are too loose, which has led some companies that do not have the production capacity of tantalum electrolytic capacitors to produce inferior tantalum capacitors. If the leakage current of the product at room temperature is too large, its leakage current will increase exponentially at a higher temperature, so the actual withstand voltage at high temperature will be greatly reduced. When the temperature is high, the breakdown will occur very easily.
The small change in leakage current at high temperature is one of the most important goals of all capacitor manufacturers. Therefore, this indicator has a decisive impact on reliability.
If the leakage current of the tantalum capacitor you choose to use is too large, it is actually a waste product, and a problem therefore inevitably occurs.
Many users often only pay attention to the selection and design of the performance of tantalum capacitors, and ignore the problems that tend to occur when installing and using chip tantalum capacitors, for example:
(1) Using automatic installation rather than manual soldering. Not preheating the product, and using an electric soldering iron with a temperature higher than 300 degrees to heat the capacitor for a long time, which causes the performance of the capacitor to be affected by excessive temperature shocks and to break down.
(2) The product is repeatedly heated with a soldering iron when cold welding and virtual welding occur if manual welding is not heated by the preheating table.
Figure 19. A Preheating Machine
(3) The temperature of the soldering tip reaches 500 degrees. This can weld quickly, but it is very easy to cause the failure of the chip components.
The reliability of the chip tantalum capacitors in actual use can actually be obtained through calculation, and many of our users have insufficient design margins during use, and the robustness is very poor. Though these tantalum capacitors passed a small batch of experiments, consistency and quality problems occur during batch production. At this time, the cause of the problem is often attributed to the capacitor manufacturer, and design reliability is ignored.
For many users, MTBF(mean time between failures) is still a strange concept. They do not have an in-depth understanding of reliability engineering and pay too much attention to experiments and ignore mathematical calculations. As a result, the reliability of the sub-circuit design is lower than the reliability of the whole machine. Therefore, problems continue to arise in mass production. In fact, there are many causes and phenomena of failure that are easy to cause failure when tantalum capacitors are used, which cannot be discussed one by one. If there are new problems during use, you can communicate with us in time.
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