In circuit design, there is a common understanding that "device margin design in the absence of certainty, the margin as large as possible will be reliable", in fact, this view is wrong. For safety capacitors, the withstand voltage margin left too large, can also lead to a failure, called "low-voltage failure".
The mechanism of low voltage failure is the presence of dielectric leakage current. In the case of greater humidity, because the capacitor is not sealed, will lead to moisture infiltration in the capacitor poles voltage, infiltration of moisture on the surface will be due to the conductivity of the formation of leakage currents, excessive leakage currents will make the capacitor's energy storage characteristics greatly reduced, the results will be manifested as a loss of capacitance characteristics. This phenomenon is most likely to occur when the power is applied after the humidity storage test.
However, after a period of time (not less than 2h) of high temperature storage, and then power on, the performance of the capacitor can be restored.
Or the capacitor will be removed, to both ends of the higher voltage (not less than 0.7 times the rated voltage voltage value, such as 50V capacitor, plus not less than 35V, not higher than 50V voltage), after a short period of time, and then welded the capacitor on the circuit board, power on the phenomenon, the failure disappears.
The mechanism of the above phenomenon is shown in the figure. Filling medium, infiltration of moisture, will form a leakage current circuit as shown in the figure, which will produce leakage current, conduction on the resistance, so it will produce heat I2R, when the capacitor's rated withstand voltage value is large, and the actual applied voltage is very small (such as the application of 10% of the rated withstand voltage), the heat is very small, not enough to make the moisture evaporation, and therefore the performance of the capacitor fails.
However, when the applied voltage is large, the same resistance value can produce a large amount of heat, which will make the moisture evaporate quickly, and the capacitance characteristics are quickly restored.
Therefore, too large a derating of the capacitor's withstand voltage value can easily lead to low-voltage failure. Generally in accordance with the ** derating to 70% of the rated value is appropriate.
After the high temperature storage test, the moisture evaporates quickly at high temperature, and the capacitor characteristics can be restored.
