How to Choose Filter Capacitors Correctly in Switching Power Supply Design?
Filter capacitors play a very important role in switching power supplies, and how to correctly select filter capacitors, especially the selection of output filter capacitors, is a matter of great concern for every engineering and technical personnel. We can see various capacitors on the power filter circuit, with different capacitance values of 100uF, 10uF, 100nF, and 10nF. How are these parameters determined?
The ordinary electrolytic capacitor used in a 50Hz power frequency circuit has a pulsating voltage frequency of only 100Hz and a charging and discharging time of milliseconds. To achieve a smaller pulsation coefficient, the required capacitance can reach hundreds of thousands μ F. Therefore, the goal of ordinary low-frequency aluminum electrolytic capacitors is to increase their capacitance, and the capacitance, loss tangent, and leakage current of the capacitor are the main parameters for distinguishing its advantages and disadvantages. The output filtering electrolytic capacitor in a switching power supply has a sawtooth voltage frequency of up to tens of kHz, or even tens of MHz. At this time, capacitance is not its main indicator. The standard for measuring the quality of high-frequency aluminum electrolytic capacitors is the "impedance frequency" characteristic, which requires a lower equivalent impedance within the operating frequency of the switching power supply, and has a good filtering effect on the high-frequency peak signals generated during the operation of semiconductor devices.
Ordinary low-frequency electrolytic capacitors begin to exhibit inductance around 10kHz, which cannot meet the requirements of switching power supply usage. The high-frequency aluminum electrolytic capacitor dedicated to switching power supply has four terminals, with the two ends of the positive aluminum sheet being led out as the positive electrode of the capacitor, and the two ends of the negative aluminum sheet also being led out as the negative electrode. 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 returned from the load also flows from one negative end of the capacitor and then from the other negative end to the negative end of the power supply.
Due to the excellent high-frequency characteristics of the four terminal capacitor, it provides an extremely advantageous means to reduce voltage ripple components and suppress switch spike noise. High frequency aluminum electrolytic capacitors also come in the form of multiple cores, which divide the aluminum foil into shorter sections and connect them in parallel with multiple leads to reduce the impedance component in the capacitance. And the use of low resistivity materials as lead out terminals improves the capacitor's ability to withstand large currents.
Digital circuits must 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 decoupling? Simply put, it means storing energy when the chip does not need current, and I can replenish energy in a timely manner when you need current.
