Selection of Capacitor for Power Supply

Capacitors are one of the key passive components to enable a wide range of voltage and current combinations for power supplies. Although each type of capacitor can store electrical energy, the dielectric technology plays an important role in the choice of capacitor for a particular application.

The important applications of capacitors in power supply are energy storage, surge voltage protection, EMI suppression and control circuits. For different application areas, these dielectric technologies compete with each other or complement each other.

energy storage

The energy storage capacitor collects charge through the rectifier and transfers the stored energy to the output of the power supply through the converter leads. Aluminum electrolytic capacitors (such as EPCOS B43504 or B43505) with a voltage rating of 40 ~ 450VDC and a capacitance value of 220~150 000 M F are commonly used. According to different power requirements, devices are sometimes used in series, parallel, or a combination of them. For power levels exceeding 10KW, larger tank-shaped spiral terminal capacitors are usually used.

To select the appropriate capacitor value, check its rated DC voltage, allowable voltage ripple and charge/discharge cycle. However, in selecting an electrolytic capacitor for this application, the following parameters should be considered.

The capacitor ripple current in a typical power supply is a combination of ripple currents at various frequencies. The RMS (root mean square) value of the ripple current determines the temperature rise of the capacitor.

A common error is to calculate the RMS current load by adding the squared values of the ripple current at each frequency. In fact, it must be considered that as the ripple frequency increases, the ESR of the capacitor decreases.

The correct approach is to estimate the ripple current at high frequency (to 100HZ) according to the frequency diagram of the ripple factor. The estimated current squared value is used to determine the ripple current. This is the real current load.

Since the ambient temperature determines the life of the capacitor under load conditions, the relationship between ripple current load, ambient temperature and probabilistic life is precisely defined by reputable manufacturers. Under actual operating conditions, the ripple current load and the ambient temperature are used to determine the probabilistic lifetime, and the published probabilistic lifetime is used as the value.

Surge voltage protection

Modern power semiconductor devices with high switching frequencies are susceptible to potentially damaging voltage spikes. Surge voltage protection capacitors (such as EPCOS B32620-J or B32651..56) connected across the power semiconductor device protect the semiconductor device by limiting the peak voltage by absorbing voltage pulses, making the surge voltage protection capacitor an important member of the power component library.

The voltage and current rating of the semiconductor device and its switching frequency determine the choice of surge voltage protection capacitor. Since these capacitors are subject to steep DV/DT values, film capacitors are the right choice for this application.

Typical capacitor ratings are between 470PF and 47NF for voltage ratings up to 2000VDC. For high-power semiconductor devices, such as IGBTs, capacitance values can be as high as 2.2 MF and voltages in the range of 1200VDC.

The capacitor cannot be selected only based on the capacitance value/voltage value. When selecting surge voltage protection capacitors, the required DV/DT value should also be considered.

The dissipation factor determines the power dissipation inside the capacitor. Therefore, a capacitor with a lower loss factor should be selected instead.

EMI/RFI Suppression

These capacitors are connected to the input of the power supply to mitigate electromagnetic or radio interference generated by the semiconductors. Being directly connected to the main input line, these capacitors are subject to destructive overvoltages and transient voltages. As a result, different safety standards have been introduced in various regions of the world, including EN 132 400 in Europe, UL1414 and 1283 in the United States, and CSAC22.2 NO in Canada. 0,1 and 8.

X-class and Y-class capacitors using plastic film technology (such as EPCOS B3292X/B81122) provide one of the inexpensive suppression methods. The impedance of the suppression capacitor decreases with increasing frequency, allowing high-frequency current to pass through the capacitor. The X capacitor provides a "short circuit" for this current between the lines and the Y capacitor provides a "short circuit" for this current between the lines and the grounded device ".

There is a finer classification of X and Y capacitors according to the peak value of the surge voltage that can be tolerated. For example, an X2 capacitor with a capacitance value of up to 1MF has a rated peak surge voltage of 2.5KV, while an X1 capacitor with a similar capacitance value has a rated peak surge voltage of 4KV. The appropriate level of disturbance rejection capacitor should be selected based on the peak voltage during load power down.