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The Role of Capacitors in Power Supply
Capacitor is one of the key passive components to realize the combination of voltage and current in a wide range 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.
1: Energy storage
Energy storage capacitor collects charge through rectifier and transfers stored energy to the output of power supply through converter lead. According to different power supply requirements, devices sometimes adopt the form of series, parallel or combination. For power supply whose power level is more than 10 KW, the larger tank spiral terminal capacitor is usually used.
To select the appropriate capacitance value, the rated DC voltage, the allowable voltage ripple and the charging/discharging cycle should be checked. However, the following parameters should be taken into account when selecting electrolytic capacitors for this application. 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 mistake is to calculate RMS current load by adding the square values of ripple currents at each frequency. In fact, it must be considered that the ESR of capacitors decreases with the increase of ripple frequency. The correct method is to estimate the ripple current at high frequency (up to 100HZ) according to the frequency diagram of the ripple factor. The square value of the estimated current is used to determine the ripple current. This is the real current load. Because ambient temperature determines the life of capacitors under load conditions, the reputable manufacturers have precisely defined the relationship between ripple current load, ambient temperature and probabilistic life. Under actual working conditions, the probabilistic life is determined by the ripple current load and ambient temperature, and the published probabilistic life is taken as the absolute value.
2: Surge Voltage Protection
Modern power semiconductor devices with high switching frequency are vulnerable to potentially damaging voltage spikes. Surge voltage protection capacitors (e.g. EPCOSB32620-J or B32651.56) that span the two ends of power semiconductor devices limit the peak voltage by absorbing voltage pulses, thus playing a protective role for semiconductor devices, making surge voltage protection capacitors an important part of the power component library.
The rated voltage and current values of semiconductor devices and their switching frequencies determine the selection of surge voltage protection capacitors. Because these capacitors bear very steep DV/DT values, thin film capacitors are the appropriate choice for such applications. Typical capacitance ratings range from 470PF to 47NF at rated voltage up to 2000VDC. For high-power semiconductor devices, such as IGBT, the capacitance can be as high as 2.2 F, and the voltage is in the range of 1200 VDC. Capacitors cannot be selected only according to capacitance/voltage values. When choosing surge voltage protection capacitor, the required DV/DT value should also be considered. The dissipation factor determines the power dissipation inside the capacitor. Therefore, a capacitor with low loss factor should be selected as a replacement.
3: EMI/RFI suppression
These capacitors are connected to the input end of the power supply to mitigate electromagnetic or radio interference caused by semiconductors. These capacitors are vulnerable to destructive overvoltage and transient voltage due to their direct connection to the main input line. As a result, different safety standards have been introduced in various parts of the world, including EN132400 in Europe, UL1414 and 1283 in the United States, and CSAC 22.2NO.0, 1 and 8 in Canada.
X-level and Y-level capacitors using plastic film technology (such as EPCOSB3292X/B81122) provide one of the cheapest suppression methods. The impedance of the restraining capacitor decreases with the increase of frequency, allowing high frequency current to pass through the capacitor. X capacitor provides "short circuit" for this current between lines, while Y capacitor provides "short circuit" for this current between lines and grounding equipment.
According to the peak value of surge voltage, X and Y capacitors are classified in more detail. For example, the rated peak surge voltage of an X2 capacitor with capacitance value up to 1_F is 2.5KV, while that of an X1 capacitor with similar capacitance value is 4KV. The appropriate level of interference rejection capacitor should be selected according to the peak voltage during load interruption. All kinds of capacitors in control and logic circuits are used in power supply control circuits. These capacitors are universal components with low voltage and low loss unless they are in harsh environment. High temperature components are usually used for power supply in harsh environments. For industrial or professional power supply, low ESR components such as EPCOSB45294 series can be selected, which is a good choice when requiring higher overall reliability.
In order to make use of the automation of assembly, the reduction of size, the decrease of assembly cost and the increase of productivity, most designers try to follow the SMD capacitor technology used in the control circuit. However, there are not a few engineers who choose hybrid technology to take full advantage of the much lower cost of some lead elements.