How to correctly select the filter capacitor in the design of switching power supply?
The wave capacitor plays a very important role in the switching power supply. How to correctly select the filter capacitor, especially the selection of the output filter capacitor is a problem that every engineer and technician is very concerned about. We can see various capacitors on the power filter circuit, 100uF, 10uF, 100nF, 10nF with different capacitance values, so how are these parameters determined? Don't tell me that I copied someone else's schematic diagram, huh, huh.
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 main parameters of pros and cons. However, 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 the main indicator. The standard for measuring the quality of high-frequency aluminum electrolytic capacitors is "impedance- "Frequency" characteristics, it is required to have a lower equivalent impedance within the operating frequency of the switching power supply, and at the same time have a good filtering effect on the high-frequency spikes generated when the semiconductor device is working.
Ordinary low-frequency electrolytic capacitors begin to show inductivity at around 10kHz, which cannot meet the requirements of switching power supplies. The high-frequency aluminum electrolytic capacitor dedicated to the switching power supply has four terminals. The two ends of the positive aluminum sheet are respectively drawn out as the positive electrode of the capacitor, and the two ends of the negative aluminum sheet are also respectively drawn out as the negative electrode. 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.
Since the four-terminal capacitor has good high-frequency characteristics, it provides an extremely favorable 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, and multiple leads are connected in parallel to reduce the impedance component in the capacitive reactance. And the use of low-resistivity materials as lead-out terminals improves the ability of the capacitor to withstand large currents.
For digital circuits to operate stably and reliably, the power supply must be "clean", and energy replenishment must be timely, that is, filtering and decoupling must be good. What is filtering and decoupling, simply put, it is to store 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, at low frequencies they can handle it, but high speed digital systems are different.
