Capacitors are one of the most critical passive components for achieving a wide range of voltage and current combinations in power supplies. Although every capacitor can store electrical energy, dielectric technology plays an important role in the selection of capacitors for specific applications. The most important uses of capacitors in power supplies are in energy storage, surge voltage protection, EMI suppression, and control circuits.
Energy Storage
Energy storage capacitors collect charges through rectifiers and deliver the stored energy to the power supply output via converter leads. Aluminum electrolytic capacitors with voltage ratings of 40 to 450 VDC and capacitance values ranging from 220 to 150,000 μF (such as EPCOS B43504 or B43505) are commonly used. Depending on different power supply requirements, devices may be used in series, parallel, or a combination of both. For power supplies exceeding 10 kW, larger can-type capacitors with screw terminals are typically used.
To choose the right capacitance, the rated DC voltage, allowable voltage ripple, and charge/discharge cycle must be considered. However, when selecting electrolytic capacitors for this application, the following parameters should be taken into account. Capacitor ripple current in typical power supplies is a combination of ripple currents at various frequencies. The RMS value of the ripple current determines the capacitor's temperature rise.
A common mistake is calculating the RMS current load by simply adding the squares of the ripple currents at different frequencies. In fact, it is necessary to consider that the ESR of the capacitor decreases with increasing ripple frequency. The correct approach is to estimate the high-frequency ripple current (up to 100 Hz) based on the frequency chart of the ripple factor. The estimated current squared value is then used to determine the ripple current. This represents the true current load. Since ambient temperature determines the life of the capacitor under load, reputable manufacturers precisely define the relationship between ripple current load, ambient temperature, and probabilistic life. Under actual working conditions, the probabilistic life is determined using the ripple current load and ambient temperature, while the published probabilistic life is considered as an absolute value.
Surge Voltage Protection
Modern power semiconductor devices with high switching frequencies are susceptible to potentially damaging voltage spikes. Surge voltage protection capacitors bridged across power semiconductor devices (such as EPCOS B32620-J or B32651..56) absorb voltage pulses to limit peak voltage, thereby protecting semiconductor devices. This makes surge voltage protection capacitors an important part of the power component library.
The rated voltage, current, and switching frequency of semiconductor devices dictate the selection of surge voltage protection capacitors. Because these capacitors endure steep DV/DT values, film capacitors are an appropriate choice for this application.
Under rated voltage conditions up to 2000 VDC, typical capacitance values range from 470 pF to 47 nF. For high-power semiconductor devices like IGBTs, capacitance can reach up to 2.2 μF with voltages in the 1200 VDC range. Capacitors should not be selected based solely on capacitance/voltage values.When selecting a 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 a lower loss factor should be chosen as a replacement. EMI/RFI Suppression These capacitors are connected to the input of the power supply to mitigate electromagnetic or radio interference generated by semiconductors. Because they are directly connected to the main input line, these capacitors are susceptible to damaging overvoltages and transient voltages. As a result, different safety standards have been introduced in various regions 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- and Y-grade capacitors with plastic film technology, such as EPCOSB3292X/B81122, offer one of the most inexpensive methods of suppression. The impedance of the suppressor capacitor decreases with increasing frequency, allowing high-frequency current to pass through the capacitor. The X capacitor provides a "short circuit" to this current between the lines, and the Y capacitor provides a "short circuit" between the line and the grounded device. There is a more detailed classification of X and Y capacitors based on the peak of the surge voltage they can withstand. For example, an X2 capacitor with a capacitance value of up to 1ΜF 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 interference suppression capacitor should be selected based on the peak voltage during the load failure. Control and logic circuits A variety of capacitors are used in power control circuits and are general-purpose components with low voltage and low loss, except in harsh environmental conditions. Power supplies used in harsh environments usually use high-temperature components. For industrial or professional power supplies, low ESR components such as the EPCOSB45294 series are a good choice when high overall reliability is required. In order to take advantage of the automation of assembly, the compression of form factors, the reduction of assembly costs, and the resulting increase in productivity, most designers try to use the SMD capacitor technology used in control circuits. However, there are many engineers who choose hybrid technology to take advantage of the much lower cost of certain lead components.