Ⅰ. Introduction and basic structure of tantalum capacitors
Solid tantalum capacitors are made by pressing tantalum powder into an anode body and sintering it in a high-temperature furnace. The dielectric is energized by forming a porous amorphous Ta2O5 dielectric film by immersing the anode body in acid. MnO2 is formed by high-temperature breakdown and is employed as a lead-out connection through the graphite layer.
Tantalum capacitors have high performance, can reach great capacities while maintaining a small volume, are easy to manufacture into small and chip components, are suited for current electronic device assembly automation, miniaturization development, and have been widely employed. It has a long service life, good temperature resistance, and excellent accuracy, but it has limited withstand voltage and current capabilities, and it is generally used in large-capacity filtering parts of circuits.
Basic structure
Ⅱ.Production process flow chart
Forming → sintering → volume test → frame assembly → energizing → PTFE coating → film → graphite silver paste → film dispensing and curing → spot welding → molding curing → rib cutting → sand blasting → electroplating → marking → edge trimming → Leakage prediction→aging→testing→inspection→taping→warehousing
When the operating temperature of tantalum capacitors rises, the leakage current rises with it. The leakage current temperature curve is the name for this curve. Due to differences in production procedures and the precision of raw materials and equipment employed, goods of the same specification produced by different manufacturers frequently change the leakage current at high temperatures. There is a significant distinction. Due to the constant accumulation of heat generated by themselves at high temperatures, products with considerable changes in high temperature leakage current would eventually fail. Products with modest fluctuations in high temperature leakage current work for a long time at high temperatures, and the product's stability is improved. As a result, the tantalum capacitor's reliability can be determined by the product's leakage current change rate at high temperatures. The high temperature performance of chip tantalum capacitors has a significant impact on reliability.
IⅤ. Precautions for tantalum capacitor design
Since the failure of tantalum capacitors is prone to fire and explosion, the following points should be paid attention to during the design, storage and welding process:
4.1 Voltage
1. The voltage is applied to the tantalum capacitor mostly in the form of an electric field, causing the capacitor to fail locally. The voltage is derated to at least 1/3 of the rated voltage when used in a switching power supply (the US military standard prohibits the use of solids in the power filter). Other uses (such as tantalum capacitors) should reduce the voltage by 2/3. It is recommended that the voltage be reduced by at least 50% or perhaps 1/3 for our application.
2. The rated voltage value cannot be exceeded by the sum of the DC bias voltage and the peak value of the AC partial voltage.
3. The AC negative peak value and DC bias voltage must not exceed the capacitor's reverse voltage limit.
Tantalum capacitors are not recommended for DC voltage filtering above 4.15V, especially at the input port where power up and down is fast; for low-voltage but fast power up and down situations, a slow-start circuit should be used instead, or solid tantalum capacitors should be avoided as much as possible.
4. Regardless of polarity, avoid using a multimeter to measure resistance in the application.
5. When several tantalum capacitors are used in parallel, it is recommended to place small-capacity capacitors in the previous stage, and large-capacity capacitors in the latter stage.
4.2 Ripple current
1. The ripple current generates active power consumption via the tantalum capacitor's ESR, which causes the capacitor's temperature to rise, leading to thermal breakdown failure. As a result, the power loss produced by the ripple current passing through the capacitor must be minimized (tantalum capacitors should not be used for a long time. in high AC components or in pure AC circuits).
2. The impact of instantaneous high current on tantalum capacitors should be regulated in the circuit (the national military standard and suppliers normally recommend connecting a resistor with R>3/V in series to alleviate this impact and limit the current to less than 300mA; when a series resistor is used When it falls below 3/V, more derating design should be considered; otherwise, the product's reliability would suffer as a result. The failure rate will increase by around 10 times if the circuit resistance is decreased from 3/V to 0.1/V). The derating factor in filter circuits should be at least 0.5, and the low impedance circuit should be 1/3. Solid tantalum capacitors are not often robust to significant current shocks, and it is simple to build heat in the weak area of the oxide film, causing the oxide film to crystallize sooner and lowering the withstand voltage capabilities. As a result, capacitors should avoid repeated charging and discharge to extend their service life.
4.3 Thermal Design & Power Consumption Considerations
1. Tantalum capacitors should be stored as far away from heat sources as possible, such as transformer disks, batteries, and so on.
2. To reduce the risk of heat failure, it's important to understand the power loss and derating factor of tantalum capacitors of varied shell sizes.
Ⅴ. Reasons for Failure, Explosion, Burnout and Damage of Tantalum Capacitors
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.
Charging and Discharging Circuit Schematic Diagram
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.
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.
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.
(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.
FAQs
Q1: What are the advantages of tantalum capacitors?
Answer: Solid tantalum capacitors have excellent electrical properties, wide operating temperature range, robust performance, and excellent high-frequency characteristics. Tantalum capacitors offer several advantages that make them a preferred choice for many electronic applications.
Q2: What is tantalum capacitor used for?
Answer: Tantalum capacitors are chosen for applications requiring stable capacitance, low leakage current, and good performance over a wide range of temperatures. It is widely used in telecommunications, aerospace and military industries, medical devices, industrial control systems, automation equipment, civil appliances, radar systems, satellite communications, avionics, and many other aspects.
Q3: What can replace tantalum capacitor?
Answer: a Ceramic capacitors(MLCCs) are probably the most frequently used substitute for SMD tantalum capacitors, A low-ESR ceramic output capacitor with a discrete series resistor can be used to replace a tantalum output capacitor. especially in applications requiring high-frequency performance and low ESR. They are available in small sizes and can handle a wide range of temperatures.
b Aluminum-electrolytic capacitors are also viable options in some cases, they are generally larger and have higher Equivalent Series Resistance (ESR) than tantalum capacitors, but can be cost-effective for less space-constrained designs.
c Film Capacitors are common alternatives, they are used in applications requiring high precision and stability over time and temperature, it’s excellent for applications needing very low ESR and inductance
Q4: What is the difference between yellow and black tantalum capacitors?
Answer: The production process is different. Yellow tantalum capacitors are made of tantalum powder wrapped with polyoxy resin, while black tantalum capacitors are directly molded from tantalum capacitor molds.
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