Selection of Capacitors for Power supply(I)

Capacitor is one of the most important passive components to realize the combination of wide range voltage and current of power supply. Although each capacitor can store electrical energy, dielectric technology plays an important role in the selection of capacitors for specific applications.

The most important applications of capacitors in power supply are energy storage, surge voltage protection, EMI suppression and control circuits.

Energy storage

The energy storage capacitor collects the charge through the rectifier and transmits the stored energy through the converter lead to the output of the power supply. Aluminum electrolytic capacitors (such as EPCOS B43504 or B43505) with a voltage rating of 40 / 450VDC and a capacitance of between 220 and 150,000 μ F are commonly used. According to different power supply requirements, the device sometimes adopts the form of series, parallel or its combination. For power supply with power level exceeding 10KW, the large volume spiral terminal capacitor is usually used.

To select the appropriate capacitance value, check its rated DC voltage, the allowable voltage ripple, and the charge / discharge cycle. However, the following parameters should be considered when selecting electrolytic capacitors for this application. The corrugated current of capacitor in typical power supply is the combination of corrugated current on each frequency. The RMS value of corrugated current determines the temperature rise of the capacitor.

A common mistake is to calculate the RMS current load by adding the square value of the ripple current at each frequency. In fact, it must be taken into account that the ESR of the capacitor decreases as the ripple frequency increases. The correct method is to estimate the ripple current at high frequencies (up to 100 HZ) based on the frequency diagram of the ripple factor. The corrugated current is determined using the estimated current squared value. This is the real current load. Because ambient temperature determines the life of capacitors under load conditions, the reputable manufacturers define the relationship between corrugated current load, ambient temperature and probabilistic life accurately. Under the actual working conditions, the probabilistic life is determined by corrugated current load and ambient temperature, and the published probabilistic life is taken as the absolute value.