How to choose capacitors in switching power supply - News - Global IC Trade Starts Here.

SMD aluminum electrolytic capacitor

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The most important applications of capacitors in power supplies are in terms of stored energy, surge protection, EMI suppression, and control circuitry. We can see through Figure 1 that these dielectric technologies compete with each other or complement each other for different application areas.
The energy storage energy storage capacitor collects charge through the rectifier and transfers 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 to 450 VDC and a capacitance between 220 and 150 000 ÎœF are more commonly used. Depending on the power requirements, the devices are sometimes used in series, in parallel, or a combination thereof. For power supplies with power levels above 10 kW, bulky screw-type terminal capacitors are typically used.
To select the appropriate capacitor value, look at its rated DC voltage, allowable voltage ripple, and charge/discharge cycle. However, the following parameters should be considered when selecting an electrolytic capacitor 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 the RMS current load by adding the squared values ​​of the ripple currents at each frequency. In fact, it must be considered that as the ripple frequency increases, the ESR of the capacitor drops.
The correct approach is to estimate the ripple current at high frequencies (to 100 Hz) based on the frequency plot of the ripple factor. The estimated current squared value is used to determine the ripple current. This is the real current load.
Since ambient temperature determines the life of the capacitor under load conditions, those reputable manufacturers precisely define the relationship between ripple current load, ambient temperature and probabilistic lifetime. Under actual operating conditions, the ripple current load and ambient temperature are used to determine the probabilistic lifetime, and the published probabilistic lifetime is taken as the absolute value.
Modern power semiconductor devices with high surge voltage protection switching frequencies are susceptible to potentially damaging voltage spikes. Surge voltage protection capacitors (such as EPCOS B32620-J or B32651..56) across the power semiconductor device limit the peak voltage by absorbing voltage pulses, thereby protecting the semiconductor device and making the surge voltage protection capacitor An important member of the power component library.
The rated voltage and current values ​​of the semiconductor device and its switching frequency are selected around the surge voltage protection capacitor. Because these capacitors are subject to very steep DV/DT values, film capacitors are the right choice for this application.
Typical capacitor ratings are between 470 PF and 47 NF at rated voltages up to 2000 VDC. For high-power semiconductor devices, such as IGBTs, the capacitance can be as high as 2.2 Μ F and the voltage is in the range of 1200 VDC.
It is not possible to select a capacitor based solely on the capacitance value/voltage value. 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 an alternative.
EMI/RFI inhibits these capacitors from being connected to the input of the power supply to mitigate electromagnetic or radio interference generated by the semiconductor. These capacitors are susceptible to damaging overvoltages and transient voltages due to their direct connection to the main input line. As a result, different safety standards have been introduced in various regions of the world, including EN132 400 in Europe, UL1414 and 1283 in the US, and CSA C22.2 NO.0, 1 and 8 in Canada.
X-stage and Y-stage capacitors using plastic film technology (such as EPCOS B3292X/B81122) offer one of the cheapest suppression methods. The impedance of the suppression capacitor decreases as the frequency increases, 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" to this current between the line and the grounded device.
There is a finer classification of X and Y capacitors depending on the peak value of the surge voltage that can be withstood. For example, an X2 capacitor with a capacitance value of up to 1 ÎœF has a peak surge voltage of 2.5 kV, while an X1 capacitor with a similar capacitance value has a peak surge voltage of 4 kV. The level of the appropriate interference suppression capacitor should be chosen based on the peak voltage during the power outage of the load.
Control and Logic Circuits All types of capacitors are used in power control circuits, and unless they are subjected to harsh environmental conditions, these capacitors are general purpose components with low voltage and low loss.
High-temperature components are usually used for power supplies used in harsh environments. Industrial or professional power supplies with low ESR components, such as the EPCOS B45294 series, are a good choice when demanding high overall reliability.
In order to take advantage of the automation of the assembly, the compression of the external dimensions, the reduction in assembly costs, and the resulting increase in productivity, most designers attempt to follow the SMD capacitor technology used in control circuits. However, there are also a handful of engineers who choose hybrid technology to take advantage of the much lower cost of certain lead components.