How should the filter capacitor be chosen properly when creating a switching power supply?
The switching power supply depends heavily on the filter capacitor. Every engineer and technician is extremely concerned with the issue of how to choose the filter capacitor appropriately, especially the selection of the output filter capacitor. We can observe different capacitors on the power filter circuit, with capacitance values of 100uF, 10uF, 100nF, and 10nF, respectively. How are these parameters determined? Please refrain from accusing me of stealing another person's schematic diagram.
The pulsating voltage frequency for typical electrolytic capacitors used in 50Hz power frequency circuits is only 100Hz, and the charging and discharging period is on the order of milliseconds. The necessary capacitance can reach hundreds of thousands of F in order to get a lower pulsation coefficient. In order to improve capacitance, standard low-frequency aluminum electrolytic capacitors are designed. the primary pros and disadvantages criteria. However, the switching power supply's output filter electrolytic capacitor has a sawtooth wave voltage frequency that can reach tens of kHz or even MHz.Capacitance isn't the primary indicator right now. The criteria for judging the quality of high-frequency aluminum electrolytic capacitors are their "impedance-" "Frequency" characteristics. These capacitors must have a lower equivalent impedance within the operating frequency of the switching power supply and, at the same time, exhibit good filtering of the high-frequency spikes produced when the semiconductor device is operating.
Switching power supply cannot be used because standard low-frequency electrolytic capacitors cannot operate above about 10 kHz before they start to exhibit inductivity. The switching power supply's high-frequency aluminum electrolytic capacitor has four connections. The capacitor's positive electrode is made up of the two ends of the positive aluminum sheet, while its negative electrode is made up of the two ends of the negative aluminum sheet.The current flows in from one positive terminal of the four-terminal capacitor, passes through the inside of 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.
The four-terminal capacitor offers a very advantageous method for minimizing the pulsing component of the voltage and suppressing the switching spike noise since it has strong high-frequency properties. The aluminum foil is cut into several smaller portions, and several leads are linked in parallel to lower the impedance component in the capacitive reactance, which is another form of high-frequency aluminum electrolytic capacitor. Additionally, the capacitor's capacity to handle heavy currents is increased by using low-resistivity materials as lead-out terminals.
The power supply must be "clean" and energy replenishment must be timely for digital circuits to run steadily and dependably, which means that filtering and decoupling must be effective. Simply expressed, filtering and decoupling are methods of energy storage so that energy can be quickly replenished when the chip requires current. Don't you dare tell me that DCDC and LDO are not in charge of this? Yes, they can manage it at low frequencies, but high-speed digital systems work differently.
First, let's look at the capacitor. The capacitor's sole purpose is to serve as a charge storage device. We are all aware that the power supply needs capacitor filtering, and that each chip's power pin needs to have a 0.1uF capacitor installed for decoupling. Why are some board chips' capacitors close to the power pin 0.1uF or 0.01uF? What's the point, indeed? We must comprehend the actual features of capacitors in order to comprehend this truth. A perfect capacitor is nothing more than a C-based charge storage. The real made capacitor is not as straightforward, though.
