Filter capacitors play a very important role in switching power supplies. How to correctly select filter capacitors, especially the selection of output filter capacitors, is a problem that every engineer and technician is very concerned about. We can see a variety of capacitors in the power filter circuit, 100uF, 10uF, 100nF, 10nF different capacitance values, so how are these parameters determined? Don't tell me I copied someone else's schematic, huh.
The common electrolytic capacitor used in the 50Hz power frequency circuit has 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, so the goal of ordinary low-frequency aluminum electrolytic capacitors is to increase the capacitance. The main parameters of pros and cons. The output filter electrolytic capacitor in the switching power supply has a sawtooth wave voltage frequency as high as tens of kHz or even tens of MHz. At this time, the capacitance is not its main indicator. The standard for measuring the quality of high-frequency aluminum electrolytic capacitors is "impedance- Frequency" characteristic, requires a lower equivalent impedance within the operating frequency of the switching power supply, and at the same time has a good filtering effect on the high-frequency spike signal generated when the semiconductor device is working.
Ordinary low-frequency electrolytic capacitors begin to show inductance around 10kHz, which cannot meet the requirements of switching power supplies. The high-frequency aluminum electrolytic capacitors dedicated to switching power supplies have four terminals. The current flows from one positive end of the four-terminal capacitor, passes through the inside of the capacitor, and then flows from the other positive end to the load; the current returning from the load also flows from one negative end of the capacitor, and then flows from the other negative end to the negative end of the power supply.
Since 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 peak noise. High-frequency aluminum electrolytic capacitors also have a multi-core form, that is, the aluminum foil is divided into several shorter sections, and multiple lead-out sheets are connected in parallel to reduce the impedance component in the capacitive reactance. And the material with low resistivity is used as the lead-out terminal, which improves the ability of the capacitor to withstand large currents.
The digital circuit must run stably and reliably, the power supply must be "clean", and the energy supply must be timely, that is, the filtering and decoupling must be good. What 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 the responsibility of DCDC and LDO? Yes, they can handle it 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 the charge. We all know that capacitor filtering should be added to the power supply, and a 0.1uF capacitor should be placed on the power supply pin of each chip for decoupling, etc. Why do I see that the capacitor next to the power supply pin of some board chips is 0.1uF or 0.01uF Yes, does it matter? To understand this, it is necessary to understand the actual characteristics of capacitors. An ideal capacitor is just a storage of charge, C. However, the actual manufactured capacitor is not so simple. When analyzing the power integrity, we commonly use the capacitor model.
How to correctly select filter capacitors in switching power supply design?
ESR is the series equivalent resistance of the capacitor, ESL is the series equivalent inductance of the capacitor, and C is the real ideal capacitor. ESR and ESL are determined by the manufacturing process and material 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 Zc=1/ωC, the inductive reactance of the inductor Zl=ωL, (ω=2πf), the complex impedance of the actual capacitor is Z=ESR+jωL-1/jωC=ESR+j2πf L-1/j2πf C. It can be seen that when the frequency is very low, the capacitance plays a role, and when the frequency reaches a certain level, the role of the inductance cannot be ignored, and when the frequency is high, the inductance plays a leading role. Capacitors lose their filtering effect. So remember, capacitors are not just capacitors at high frequencies.
As mentioned above, the equivalent series inductance of the capacitor is determined by the manufacturing process and material of the capacitor. The ESL of the actual chip ceramic capacitor ranges from a few tenths of nH to several nH. The smaller the package, the smaller the ESL.
On the filter curve of the capacitor, we can also see that it is not flat, it is like a 'V', which means that it has frequency selection characteristics. Sometimes you want it to be as sharp as possible (filtered or notch). What affects this characteristic is the quality factor Q of the capacitor, Q=1/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. Generally, tantalum capacitors and aluminum electrolysis have relatively small ESL and large ESR, so tantalum capacitors and aluminum electrolysis have a wide effective frequency range, which is very suitable for pre-stage board-level filtering. That is, the input stage of the DCDC or LDO is often filtered with a larger-capacity tantalum capacitor. And put some 10uF and 0.1uF capacitors close to the chip for decoupling, ceramic capacitors have very low ESR.
