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The correct selection of Capacitor in Modern Power supply Technology(I)

Views:0     Author:Site Editor     Publish Time: 2018-03-22      Origin:Site

Filter capacitor 

After rectifying AC (power frequency or high frequency), it is necessary to use capacitor filter to smooth the output voltage, which requires a large capacitor capacity, and generally uses aluminum electrolytic capacitor. The main problem in the application of aluminum electrolytic capacitors is the relationship between temperature and life. For example, the cheap ambient temperature is more than 85 ℃, which can guarantee the performance parameters, especially the capacitance, within 1000 h, and the performance indexes will not be guaranteed. Although in many cases it still works. If the service temperature is reduced, the service life can be prolonged, and the rule of 50 ℃ can be followed. Therefore, in many cases requiring high temperature and high reliability, electrolytic capacitors with long life (such as 5000h or even 105 ℃ / 5000h) should be selected. Generally, electrolytic capacitors with small size have a relatively short life.

For the input filter capacitor of DC/DC switching and stabilizing power supply, because the switching converter draws the electric energy from the power supply in the form of pulse, so the high frequency current in the filter capacitor is larger, when the equivalent series resistance of the electrolytic capacitor is larger, Will produce the big loss, causes the electrolytic capacitor to heat. The low ESR electrolytic capacitor can obviously reduce the heat generated by the ripple (especially the high frequency ripple) current.

Because aluminum electrolytic capacitors can not function well in high frequency band, they should be supplemented with ceramic or non-inductive thin film capacitors with good high frequency characteristics. The main advantages of these capacitors are: good high frequency characteristics and low ESRs, such as 1 μ F MMK5 capacitors with a resonant frequency above 2MHz. The equivalent impedance is less than 0. 02 Ω, which is far lower than the electrolytic capacitor, and the smaller the capacity, the higher the resonant frequency (up to 50MHz, see figure 3), which will get a good output frequency response or dynamic response of the power supply.


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