Selection and Calculation of Capacitance in Power Supply(III)

Selection of capacitors.

When the electrolytic capacitor is in use, if there is no large ripple, the withstand voltage can only be 20% larger than the actual value. That is, the output of 7805 is enough for 6V or 16V, and the output of 7805 is enough for 10V, and there will be no big problem with 10V. The output of the three-terminal regulator does not need to be connected with a very large capacitance. As far as the actual load is concerned, the general 100mA can be connected with 47-100uF to connect 470-1000uF, and preferably a small porcelain chip or monolithic capacitor of 0.01-0.1uF.

Primary filter capacitance.

In general, the role of electrolytic capacitance is to filter out the low frequency signal in the current, but even the low frequency signal, its frequency is also divided into several orders of magnitude. Therefore, in order to be suitable for use at different frequencies, electrolytic capacitors are also divided into high-frequency capacitors and low-frequency capacitors (in this case, high-frequency is relatively speaking).

The low-frequency filter capacitor is mainly used for the filter after the municipal power filter or transformer rectifier, and its working frequency is 50 Hz, while the high-frequency filter capacitor mainly works in the filter after rectifying in switching power supply, and its working frequency is from several thousand Hz to tens of thousands Hz. When we use the low-frequency filter capacitor in the high-frequency circuit, because of the low frequency filter capacitor's high frequency characteristic, the internal resistance is larger and the equivalent inductance is higher when it is charged and discharged at high frequency. Therefore, in the use of the electrolyte due to the frequent polarization and produce a larger amount of heat. The higher temperature will make the electrolyte gasification inside the capacitor, the pressure inside the capacitor will increase, and eventually lead to the bulge and burst of the capacitor.

Selection of filter Capacitance.

After the rectifier bridge is pulsating DC, the fluctuation range is very large. At the back, two capacitors are usually used to stabilize the output. It is known that the voltage at the two ends of the capacitor can not change, so it can make the output smooth; the small capacitance is used to filter out the high frequency interference and make the output voltage pure. The smaller the capacitance is, the higher the resonant frequency is, and the higher the interference frequency is.