Views:1 Author:Site Editor Publish Time: 2018-03-23 Origin:Site
Fully consider ripple voltage
Generally electrolytic capacitors have positive and negative electrodes. When used in pulsating circuits with both DC and AC voltage components, special attention should be paid to its working characteristics. When the electrolytic capacitor is in use, it must meet the voltage polarity requirements at both ends of the electrolytic capacitor. When used in interstage coupling or pulse circuits, the DC voltage applied on the capacitor also superimposes the amplitude of the AC voltage component, which in some cases may cause the negative peak voltage of the AC component to exceed the positive DC voltage value, As a result, the polarity capacitor is in the reverse working state, which will cause the leakage current of the capacitor to increase sharply, and then destroy the positive working characteristic and cause the failure. Therefore, when there is a pulsating AC component at both ends of the capacitor, the sum of the AC peak voltage and the added DC voltage should not exceed the rated operating voltage of the capacitor. The reason is that the temperature rise and heat of the capacitor caused by the AC component is more serious than that of the DC component, so the size of the ripple should be strictly controlled and generally should not exceed 20% of the rated working voltage of the capacitor. Even for tantalum electrolytic capacitors, should also be controlled within 10%. Because the ripple voltage can polarization the electrolyte and has a great influence on the loss resistance RS, the peak voltage of ripple applied to the capacitor should be controlled effectively. The allowable AC component specified in the general technical conditions refers to the allowable value at 50 Hz power frequency. If the frequency used exceeds the above conditions, it may be calculated according to the following formula:
Ssh is the surface area of the capacitor shell (unit cm2t is the allowable temperature rise at a certain ambient temperature) (the sinusoidal frequency of the pseudo-ripple voltage at ℃ / Hz-C is the capacitance (μ FU tg δ is the tangent of the loss angle at the actual operating frequency).
To ensure the reliable operation of the capacitor, the ripple voltage added to the capacitor should be less than the peak ripple voltage calculated from the above formula. Because there are generally polarity capacitors can not withstand reverse voltage, so. Polarity capacitors are not allowed in polarity conversion or pure AC circuits, but non-polar tantalum decompression capacitors (such as CA74 fully sealed solid tantalum capacitors) should be used. Electrodeless tantalum capacitors are actually connected back to back with two polar tantalum capacitors, even though there is always a tantalum capacitor in the AC circuit in the positive polarity state.