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Capacitor selection rules in power supply design

by:Shenmao     2021-04-29
The power supply is often the most easily overlooked link in the circuit design process. As an excellent design, power supply design should be very important, it greatly affects the performance and cost of the entire system. The use of capacitors in power supply design is often the most easily overlooked place in power supply design. 1. The working principle of capacitors in power supply design In power supply design applications, capacitors are mainly used for filtering and decoupling/bypass. Filtering is the operation of filtering out specific band frequencies in the signal, and is an important measure to suppress and prevent interference. According to the results of observing a certain random process, the probability theory and method of estimating another related random process. The term filtering originated from communication theory, which is a technique for extracting useful signals from interference-containing received signals. 'Received signal' is equivalent to the observed random process, and 'useful signal' is equivalent to the estimated random process. Filtering mainly refers to filtering out the incoming noise, and decoupling/bypass (a kind of decoupling effect in the form of bypass, later replaced by 'decoupling') is to reduce the external noise interference of local circuits. Many people tend to confuse the two. Let's look at a circuit structure below: the power supply in the picture is A and B power supply. After the current passes through C1, it passes through a section of PCB trace and is divided into two channels and supplied to A and B respectively. When A needs a large current at a certain moment, if there are no C2 and C3, then the voltage of terminal A will be lower due to the line inductance, and the voltage of terminal B will also be affected by the voltage of terminal A and reduce, so the local circuit The current change of A causes the power supply voltage of the local circuit B, which affects the signal of the B circuit. Similarly, the current change of B will also interfere with A. This is 'common path coupling interference'. After adding C2, when the local circuit needs a large instantaneous current, the capacitor C2 can temporarily provide current for A. Even if the common circuit part inductance exists, the voltage at terminal A will not drop too much. The impact on B will also be much reduced. So the decoupling function is played by the current bypass. Generally, large-capacity capacitors are mainly used for filtering, and the speed requirements are not very fast, but the requirements for the capacitance value are relatively large. If the local circuit A in the figure refers to a chip, and the capacitor is as close as possible to the power pin of the chip. If “local circuit A” refers to a functional module, ceramic capacitors can be used. If the capacity is not enough, tantalum capacitors or aluminum electrolytic capacitors can also be used (provided that each chip in the functional module has decoupling capacitors-ceramic capacitors ). The capacity of the filter capacitor can often be calculated from the data sheet of the power chip. If the filter circuit uses electrolytic capacitors, tantalum capacitors and ceramic capacitors at the same time, place the electrolytic capacitors closest to the switching power supply to protect the tantalum capacitors. The ceramic capacitor is placed behind the tantalum capacitor. In this way, the best filtering effect can be obtained. Decoupling capacitors need to meet two requirements, one is capacity requirements, and the other is ESR requirements. That is to say, the decoupling effect of a 0.1uF capacitor may not be as good as two 0.01uF capacitors. Moreover, 0.01uF capacitors have lower impedance in higher frequency bands. If a 0.01uF capacitor can meet the capacity requirements in these frequency bands, then it will have a better decoupling effect than 0.1uF capacitors. Many high-speed chip design guides with more pins will give the power supply design requirements for decoupling capacitors. For example, a BGA package with more than 500 pins requires a 3.3V power supply with at least 30 ceramic capacitors and several large capacitors. Capacitance, the total capacity should be more than 200uF... 2. The correct selection of capacitors in various power supplies. Capacitors play an important role in electronic circuits as basic components. In traditional applications, capacitors are mainly used for bypass coupling, power supply filtering, and direct current blocking. And the oscillation and delay in the small signal. With the development of electronic circuits, especially power electronic circuits, different special requirements are put forward for capacitors in different applications. Speaking of the structure of the capacitor. The simplest capacitor is composed of plates at both ends and an insulating dielectric (including air) in the middle [1]. After being energized, the plates are charged to form a voltage (potential difference), but due to the insulating material in the middle, the entire capacitor is non-conductive. However, this situation is under the premise that the critical voltage (breakdown voltage) of the capacitor is not exceeded. We know that any substance is relatively insulating. When the voltage across the substance increases to a certain level, the substance can conduct electricity. We call this voltage breakdown voltage. The capacitor is no exception. After the capacitor is broken down, it is not an insulator. However, in the middle school stage, such a voltage is not visible in the circuit, so all work below the breakdown voltage and can be seen as an insulator. However, in an AC circuit, because the direction of the current changes with time as a certain function. The process of charging and discharging a capacitor takes time. At this time, a changing electric field is formed between the plates, and this electric field is also a function of time.
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