Power supply is often the most neglected part of our circuit design process. In fact, as a good design, power supply design should be very important, which greatly affects the performance and cost of the entire system.
Most of the concept of capacitance remains in the ideal capacitance stage, generally believe that the capacitance is a C. But do not know that the capacitance has many important parameters, and do not know a 1uF ceramic capacitor and a 1uF aluminum electrolytic capacitor what is the difference. The actual capacitance can be equated to the following circuit form:
C: Capacitance value. This is generally measured at 1kHz, 1V equivalent AC voltage, and 0V DC bias, although there can be many capacitance measurements in different environments. However, it should be noted that the capacitance value C itself will change with the environment.
ESL: Equivalent series inductance of the capacitor. The inductance is present at the pin of the capacitor. In low frequency applications the inductance is small, so it can be disregarded. When the frequency is higher, it is necessary to consider this inductance. For example, a 0805 package of 0.1uF chip capacitors, each pin inductance 1.2nH, then the ESL is 2.4nH, you can calculate the resonant frequency of C and ESL for 10MHz or so, when the frequency is higher than 10MHz, the capacitance embodied in the inductive properties.
ESR: capacitor equivalent series resistance. No matter what kind of capacitor will have an equivalent series resistance, when the capacitor works in the resonance frequency, capacitance and inductive reactance of the capacitor is equal in size, so it is equivalent to a resistor, this resistance is the ESR, which is very different due to the different structure of the capacitor. Aluminum electrolytic capacitors ESR generally from a few hundred milliohms to a few ohms, porcelain chip capacitors are generally tens of milliohms, tantalum capacitors between aluminum electrolytic capacitors and porcelain chip capacitors.
Here we look at the frequency characteristics of some X7R material porcelain chip capacitors:
Of course, there are many more parameters related to capacitors, however, the most important ones in design are C and ESR.
Here is a brief introduction to the three types of capacitors we commonly use: aluminum electrolytic capacitors, porcelain chip capacitors and tantalum capacitors.
(1) Aluminum electrolytic capacitors are made from aluminum foil that has been oxidized by grooving, rolled with an insulating layer, and then dipped in an electrolyte solution. The principle of the electrolytic capacitor is a chemical one, where the capacitor charges and discharges by a chemical reaction, and the speed of the capacitor's response to a signal is limited by the speed of the charged ions moving through the electrolyte, and it is generally used in low frequency filtering applications (less than 1M). The ESR is the sum of the aluminum section resistance and the equivalent resistance of the electrolyte. The electrolyte in the aluminum capacitor will gradually evaporate and cause the capacitance to decrease or even fail, with the rate of evaporation increasing with temperature. The life of an electrolytic capacitor is halved for every 10 degrees of temperature rise. If the capacitor can be used for 10,000 hours at room temperature of 27 degrees, it can only be used for 1,250 hours at 57 degrees. So aluminum electrolytic capacitors try not to be too close to the heat source.
(2) Porcelain chip capacitors store electricity by physical reaction, and therefore has a high response speed, can be applied to the occasion of G. However, because of the dielectric, porcelain chip capacitors can be used for a wide range of applications. However, porcelain chip capacitors because of different media, also presents a big difference. The best performance is C0G material capacitance, temperature coefficient is small, but the material dielectric constant is small, so the capacitance value can not do too big. The worst performance is Z5U/Y5V material, this material dielectric constant is large, so the capacitance value can do tens of microfarads. But this material is affected by the temperature and DC bias (DC voltage will cause material polarization, so that the capacitance is reduced) is very serious. Here we look at the C0G, X5R, Y5V three material capacitors by the ambient temperature and DC operating voltage.
You can see that the C0G's capacity basically doesn't change with temperature, the X5R is slightly less stable, and the Y5V material becomes 50% of its nominal value at 60 degrees.
It can be seen that the 50V withstand voltage Y5V ceramic chip capacitor has only 30% of its nominal capacity when applied at 30V. Ceramic capacitors have a big disadvantage, that is, they are fragile. So you need to avoid bumping, try to stay away from the board is prone to deformation.
(3)Tantalum capacitors are like a battery in both principle and structure. The following is the internal structure of the tantalum capacitor schematic diagram:
Tantalum capacitors have the advantages of small size, high capacity, high speed, low ESR and higher price. What determines the capacity and withstand voltage of tantalum capacitors is the size of the raw material tantalum powder particles. The finer the particles, the larger the capacitance can be obtained, and if you want to get a larger withstand voltage requires thicker Ta2O5, which requires the use of larger particles of tantalum powder. So the same volume to get high withstanding voltage and high capacity tantalum capacitors is very difficult. Tantalum capacitors need to pay attention to another place is: tantalum capacitors are easier to break down and short circuit characteristics, poor surge resistance. It is likely that a large instantaneous current causes the capacitor to burn and form a short circuit. This should be taken into account when using very large capacity Tantalum capacitors (e.g. 1000uF Tantalum capacitors)
From the above, we can understand that different capacitors have different applications, and not the higher the price, the better.
Here is the role of capacitors in power supply design.
In power supply design applications, capacitors are mainly used for filtering (filter) and decoupling / bypass (decoupling / bypass). Filtering mainly refers to filter out foreign noise, and decoupling/bypass (a form of bypass to achieve the effect of decoupling, later replaced by "decoupling") is to reduce the local circuit external noise interference. Many people tend to confuse the two. Here we look at a circuit structure:
Figure switching power supply for A and B power supply. The current is supplied to A and B by the switching power supply through C1 and then through a section of PCB alignment (temporarily equivalent to an inductor, the actual electromagnetic wave theory analysis of such an equivalent is wrong, but for the sake of convenience of understanding, this equivalent way is still used), and is split into two circuits to supply A and B respectively. Separate two separate supply A and B. Switching power supply out of the ripple is relatively large, so we use C1 on the power supply filtering, to provide a stable voltage for A and B. C1 needs to be placed as close as possible to the power supply. C2 and C3 are bypass capacitors, play the role of decoupling. When A needs a large current at a certain moment, if there is no C2 and C3, then the voltage at A will be low because of the inductance of the line, and the voltage at B will also be lowered by the voltage at A. The current change of the local circuit A will cause the power supply voltage of the local circuit B, which will have an effect on the signal of the B circuit. Similarly, the current change of B will also form interference to A. This is called "common coupling". This is called "common circuit coupling interference".
With the addition of C2, when the local circuit needs a momentary high current, capacitor C2 can temporarily provide current for A. Even if the inductance of the common circuit part exists, the voltage at the end of A will not drop too much. The effect on B will also be much reduced. So the role of decoupling through the current bypass.
General filtering mainly use high-capacity capacitors, the speed requirements are not very fast, but the capacitance value requirements are large. Generally use aluminum electrolytic capacitors. In the case of smaller inrush currents, the use of tantalum capacitors instead of aluminum electrolytic capacitors will have a better effect. From the above example, we can see that as a decoupling capacitor, it is necessary to have a very fast response speed to achieve the effect. If the local circuit A in the diagram refers to a chip, then the decoupling capacitor should be a ceramic chip capacitor, and the capacitor should be as close as possible to the chip's power supply pins. If the "local circuit A" refers to a function module, you can use ceramic capacitors, if the capacity is not enough can also use tantalum capacitors or aluminum electrolytic capacitors (provided that the function module chips have decoupling capacitors - ceramic capacitors). The capacity of the filter capacitors can often be found in the datasheets of the switching power supply chips. If the filter circuit uses electrolytic capacitors, tantalum capacitors and ceramic chip capacitors at the same time, place the electrolytic capacitors closest to the switching power supply so as to protect the tantalum capacitors. Place the ceramic chip capacitor behind the tantalum capacitor. This will give the best filtering effect.
In the figure, the switching power supply for A and B power supply. Current through the C1 and then through a section of the PCB line (temporarily equivalent to an inductor, the actual electromagnetic wave theory to analyze this equivalent is wrong, but for ease of understanding, still use this equivalent way.) Separate two separate supply A and B. Switching power supply out of the ripple is relatively large, so we use C1 on the power supply filtering, to provide a stable voltage for A and B. C1 needs to be placed as close as possible to the power supply. C2 and C3 are bypass capacitors, play the role of decoupling. When A needs a large current at a certain moment, if there is no C2 and C3, then the voltage at A will be low because of the inductance of the line, and the voltage at B will also be lowered by the voltage at A. The current change of the local circuit A will cause the power supply voltage of the local circuit B, which will have an effect on the signal of the B circuit. Similarly, the current change of B will also form interference to A. This is called "common coupling". This is "common circuit coupling interference".
With the addition of C2, when the local circuit needs a momentary high current again, capacitor C2 can provide current for A temporarily, and the voltage at A won't drop too much even though the inductance of the common part is present. The effect on B will also be much reduced. So the role of decoupling through the current bypass.
General filtering mainly use high-capacity capacitors, the speed requirements are not very fast, but the capacitance value requirements are large. Generally use aluminum electrolytic capacitors. In the case of smaller inrush currents, the use of tantalum capacitors instead of aluminum electrolytic capacitors will have a better effect. From the above example, we can see that as a decoupling capacitor, it is necessary to have a very fast response speed to achieve the effect. If the local circuit A in the diagram refers to a chip, then the decoupling capacitor should be a ceramic chip capacitor, and the capacitor should be as close as possible to the chip's power supply pins. If the "local circuit A" refers to a function module, you can use ceramic capacitors, if the capacity is not enough can also use tantalum capacitors or aluminum electrolytic capacitors (provided that the function module chips have decoupling capacitors - ceramic capacitors). The capacity of the filter capacitors can often be found in the datasheets of the switching power supply chips. If the filter circuit uses electrolytic capacitors, tantalum capacitors and ceramic chip capacitors at the same time, place the electrolytic capacitors closest to the switching power supply so as to protect the tantalum capacitors. Place the ceramic chip capacitor behind the tantalum capacitor. This will give you the best filtering effect.
Decoupling capacitors need to meet two requirements, one is the capacity requirement, the other is the ESR requirement. That is to say a 0.1uF capacitor decoupling effect may not be as good as two 0.01uF capacitors. Moreover, 0.01uF capacitor has lower impedance in higher frequency bands, in these bands if a 0.01uF capacitor can meet the capacity requirements, then it will have a better decoupling effect than 0.1uF capacitor.
Many pins more high-speed chip design guide will give the power supply design decoupling capacitance requirements, such as a more than 500 pins of the BGA package requirements of the 3.3V power supply at least 30 ceramic capacitors, but also a few large capacitors, the total capacity to be more than 200uF!
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