Derating of tantalum capacitors
The tantalum capacitors are popular due to their superior frequency characteristics, long-term stability, and capacitance value independent of the DC bias. However, they have a potentially dangerous failure mode that needs to watch out for. It is because even a tiny overvoltage will lead to field crystallization. That means the dielectric will convert from an amorphous state to a crystalline form. While the amorphous tantalum pentoxide has a very high resistivity to direct electrical current, the conductivity of the crystalline form is high. Once the tantalum capacitor has failed, it will present as a short in the circuit. To prevent this failure, the working voltage should never exceed the rated voltage, including the DC voltage, AC ripple, transients, and spikes. For safety reasons, the applicable voltage is lower than the capacitor's rated voltage. The voltage rail usually must be between 0.5 to 0.6 of the rated value.
The table below gives a reference to the minimum rated voltage according to the voltage of the rail.
High-temperature applications
Above 85°C, the dielectric will be prone more to crystallize due to its higher energy level. It causes that even the rated and the recommended voltage must be derated. The diagram and the table below give an overview of the voltage derating vs. Temperature.
Ripple current
The ripple current is related to equivalent series resistance (ESR), thus causing joule heating. So it has to be decreased at a higher temperature. The diagram and the table below show the temperature compensation multipliers for the maximum ripple current at higher temperatures.
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The equations below can be used to determine power dissipation.
,where the P is the maximum power dissipation in watts, I is the RMS value of the ripple current in amperes, V is the RMS value of the ripple voltage in volts, R is the ESR at the specified frequency in ohms, Z is the impedance of the capacitor at the specified frequency in ohms.
The table below shows examples of the maximum power dissipation of the typical packages.
The table below shows the derating of the power dissipation due to the temperature.
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