Why should tantalum capacitors be used at a reduced rating?

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Why should tantalum capacitors be used at a reduced rating? [Copy link]

The suitable use condition of tantalum capacitors is -55-+125 degrees, and the rated voltage can be applied for testing within 85 degrees. In theory, such conditions have shown that the temperature characteristics of tantalum capacitors are very good, but there is one point that is often overlooked, that is, under this condition, the test is carried out with a 1000 ohm protection resistor. At the moment the product is powered on, the surge changes in the voltage and current in the circuit are very small. The excessive instantaneous current is suppressed by the series resistor, so the change causes an impact on the tantalum capacitor. In actual use, tantalum capacitors are often used in power circuits without any resistor protection for filtering or charging and discharging. If such circuits use an external power supply, at the moment of switching, very high surge voltage and current changes will be generated in the circuit. Therefore, when used in such circuits, tantalum capacitors will be subjected to voltage and large current impact exceeding the rated value at the moment of switching and will break down and fail. Therefore, after a large number of experiments, it has been proved that if tantalum capacitors used in such circuits want to ensure sufficient reliability, they must be greatly derated to ensure that the sum of the surge voltage and reverse voltage applied to the product instantly does not exceed the rated value. Tantalum capacitors used in such low-impedance circuits are therefore required to be derated to 1/3 of the rated value to ensure reliability. Such requirements have brought a lot of inconvenience to actual users; on the one hand, many users are not aware of the harsh conditions for the use of tantalum capacitors in such circuits, so the voltage used is too high, which often leads to breakdown. On the other hand, users cannot obtain chip products with higher voltage resistance under the conditions of strictly limited volume and capacitance. The reason why tantalum capacitors have such harsh conditions in specific use is actually the dangerous failure mode of tantalum capacitors; when the leakage current of tantalum capacitors is too large, the withstand voltage of the product will drop rapidly, and the breakdown that occurs soon can cause the rapid collapse of the dielectric layer and cause combustion or explosion. This defect makes tantalum capacitors very fragile in circuits with large surge currents; the surge resistance is the worst among all capacitors. Take Kemet's T491 series tantalum surface mount capacitors as an example to understand the relevant information about derating.

The graph above shows the percentage of the rated voltage applied to a capacitor versus temperature. The “Recommended Maximum Application Voltage” area of the graph shows the recommended steady-state operating voltage for continuous operation to ensure optimal reliability at the specified temperature. If the actual application temperature is below 85°C, it is recommended that tantalum capacitors only operate at 50% of the rated maximum operating voltage. Therefore, a capacitor rated for 50V is now considered to be 25V at 85°C (50V x 50% = 25V). Due to the different material structures of tantalum capacitors, their derating indicator tables are very different.