In the switching power supply, how to choose the filter capacitor correctly and how to determine the parameters?

The filter capacitor plays a very important role in the switching power supply. How to select the filter capacitor correctly, especially the choice of the output filter capacitor, is a problem that every engineering and technical personnel is very concerned about. We can see a variety of capacitors in the power filter circuit, 100uF, 10uF, 100nF, 10nF different capacitance values, then how to determine these parameters? Common electrolytic capacitors used in 50Hz power frequency circuits have a pulsating voltage frequency of only 100Hz, and the charging and discharging time is on the order of milliseconds. In order to obtain a smaller pulsation coefficient, the required capacitance is as high as hundreds of thousands of μF. Therefore, the goal of ordinary low-frequency aluminum electrolytic capacitors is to increase the capacitance. The capacitance, the loss tangent value and the leakage current of the capacitor are to identify them. The main parameters of pros and cons. The output filter electrolytic capacitor in the switching power supply has a sawtooth voltage frequency of up to tens of kHz or even tens of MHz. At this time, the capacitance is not its main indicator. The standard to measure the quality of high-frequency aluminum electrolytic capacitors is 'impedance- 'Frequency' characteristics require a lower equivalent impedance within the operating frequency of the switching power supply, and at the same time, it has a good filtering effect on the high-frequency spikes generated when the semiconductor device is working. Ordinary low-frequency electrolytic capacitors begin to appear inductive at around 10kHz, which cannot meet the requirements of switching power supplies. The special high-frequency aluminum electrolytic capacitor for switching power supply has four terminals. The two ends of the positive aluminum sheet are respectively led out as the positive electrode of the capacitor, and the two ends of the negative aluminum sheet are also led out as the negative electrode. The current flows in from one positive terminal of the four-terminal capacitor, passes through the capacitor, and then flows from the other positive terminal to the load; the current returning from the load also flows in from one negative terminal of the capacitor, and then flows from the other negative terminal to the negative terminal of the power supply. Because the four-terminal capacitor has good high-frequency characteristics, it provides an extremely advantageous means for reducing the pulsating component of the voltage and suppressing the switching spike noise. High-frequency aluminum electrolytic capacitors also have a multi-core form, that is, the aluminum foil is divided into several shorter sections, which are connected in parallel with multiple lead pieces to reduce the impedance component in the capacitive reactance. And the use of low-resistivity materials as the lead-out terminals improves the capacity of the capacitor to withstand large currents. The digital circuit must run stably and reliably, the power supply must be 'cleanWhat is filter decoupling? Simply put, it stores energy when the chip does not need current, and I can replenish energy in time when you need current. Don't tell me that this responsibility is not for DCDC and LDO? Yes, they can be handled at low frequencies, but high-speed digital systems are different. Let's take a look at the capacitor first. The function of the capacitor is simply to store charge. We all know that capacitor filtering should be added to the power supply, and a 0.1uF capacitor is placed on the power supply pin of each chip for decoupling. Wait, why do I see that the capacitors next to the power pins of some board chips are 0.1uF or 0.01uF, what's the point? To understand this, it is necessary to understand the actual characteristics of the capacitor. The ideal capacitor is just a storage of charge, namely C. The actual manufactured capacitor is not so simple. When analyzing power integrity, the commonly used capacitor model is shown in the following figure: ESR is the series equivalent resistance of the capacitor, ESL is the series equivalent inductance of the capacitor, and C is A truly ideal capacitor. ESR and ESL are determined by the manufacturing process and materials of the capacitor and cannot be eliminated. What effect do these two things have on the circuit? ESR affects the ripple of the power supply, and ESL affects the filter frequency characteristics of the capacitor. We know that the capacitive reactance of the capacitor Zcu003d1/ωC, the inductive reactance of the inductor Zlu003dωL,(ωu003d2πf), the complex impedance of the actual capacitor is Zu003dESR+jωL-1/jωCu003d ESR+j2πf L-1/j2πfC . It can be seen that the capacitance plays a role when the frequency is very low, and the role of the inductance cannot be ignored when the frequency is high to a certain level, and the inductance plays a leading role when the frequency is high. The capacitor loses its filtering function. So remember that capacitors are not simply capacitors at high frequencies. The filter curve of the actual capacitor is shown in the following figure: The equivalent series inductance of the capacitor is determined by the manufacturing process and material of the capacitor. The ESL of the actual SMD ceramic capacitor ranges from a few tenths of nH to a few nH. The smaller the ESL of the package The smaller. From the filtering curve of the capacitor above, we can also see that it is not flat. It is like a'V', which means that it has frequency selection characteristics. At that time, we hope that it is as flat as possible (previous board-level filtering). Sometimes it is hoped that the sharper and sharper the better (filtering or notching). What affects this characteristic is the quality factor Q of the capacitor, Qu003d1/ωCESR. The larger the ESR, the smaller the Q and the flatter the curve. On the contrary, the smaller the ESR, the larger the Q and the sharper the curve. Usually tantalum capacitors and aluminum electrolysis have relatively small ESL, but large ESR, so tantalum capacitors and aluminum electrolysis have a wide effective frequency range, which is very suitable for the previous board level filtering. That is, in the input stage of DCDC or LDO, a larger capacity tantalum capacitor is often used for filtering. And put some 10uF and 0.1uF capacitors close to the chip for decoupling, ceramic capacitors have very low ESR. Having said so much, whether we put 0.1uF or 0.01uF close to the pin of the chip, the following is listed for your reference. So, don't see 0.1uF capacitors in everything in the future. In some high-speed systems, these 0.1uF capacitors won't work at all.
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