The use of capacitors in power supply design: aluminum electrolytic capacitors / ceramic capacitors / tantalum capacitors

The power supply is often the most easily overlooked link in the circuit design process. In fact, as an excellent design, the power supply design should be very important, it greatly affects the performance and cost of the entire system.

The power supply is often the most easily overlooked link in the circuit design process. In fact, as an excellent design, the power supply design should be very important, it greatly affects the performance and cost of the entire system.

Here, only introduce the use of capacitors in the circuit board power supply design. This is often the most overlooked place in power supply design. Many people are engaged in ARM, DSP, and FPGA. At first glance, they seem to be very advanced, but they may not be able to provide a cheap and reliable power supply solution for their systems. This is also one of the main reasons why our domestic Electronic products are rich in functions and poor in performance. The root cause is the research and development atmosphere. Most of the R&D engineers are boring and unreliable; and the company wants short-term benefits and only needs rich functions. Regardless of whether there will be eggs tomorrow, “there are starving bones on the road” is not a pity.

Closer to home, let me introduce capacitors first.

Most of everyone’s concept of capacitance still stays at the ideal capacitance stage. Generally speaking, capacitance is a C. But I don’t know that there are many important parameters of the capacitor, nor do I know the difference between a 1uF ceramic capacitor and a 1uF aluminum electrolytic capacitor. The actual capacitance can be equivalent to the following circuit form:

The use of capacitors in power supply design: aluminum electrolytic capacitors / ceramic capacitors / tantalum capacitors

C: Capacitance value. Generally it refers to the measurement under the condition of 1kHz, 1V equivalent AC voltage, and DC bias voltage of 0V. However, there can be many different capacitance measurement environments. But one thing to note is that the capacitance value C itself will change with the environment.

ESL: Capacitor equivalent series inductance. The pin of the capacitor is inductance. In low frequency applications, the inductive reactance is small, so it can be ignored. When the frequency is higher, this inductance must be considered. For example, a 0.1uF chip capacitor in a 0805 package, with an inductance of 1.2nH per pin, then the ESL is 2.4nH. You can calculate that the resonant frequency of C and ESL is about 10MHz. When the frequency is higher than 10MHz, the capacitance is reflected as Inductance characteristics.

The use of capacitors in power supply design: aluminum electrolytic capacitors / ceramic capacitors / tantalum capacitors

ESR: Capacitor equivalent series resistance. No matter what kind of capacitor has an equivalent series resistance, when the capacitor works at the resonance frequency, the capacitance and inductance of the capacitor are equal, so it is equivalent to a resistance, and this resistance is ESR. There are great differences due to different capacitor structures. The ESR of aluminum electrolytic capacitors is generally from a few hundred milliohms to several ohms, and ceramic capacitors are generally tens of milliohms. Tantalum capacitors are between aluminum electrolytic capacitors and ceramic capacitors.

Let’s look at the frequency characteristics of some X7R ceramic capacitors:

Of course, there are many more capacitance-related parameters, but the most important thing in the design is C and ESR.

Here is a brief introduction to the three commonly used capacitors: aluminum electrolytic capacitors, ceramic capacitors and tantalum capacitors.

1) Aluminum capacitors are made by oxidizing aluminum foil and then sandwiching an insulating layer, and then immersing in electrolyte solution. The principle is chemical principle. The charging and discharging of capacitors rely on chemical reactions, and the response speed of capacitors to signals is affected by the electrolyte. The movement speed limit of the charged ions is generally applied to the filtering occasions with low frequency (below 1M). The ESR is mainly the sum of the aluminum` resistance and the equivalent resistance of the electrolyte, and the value is relatively large. The electrolyte of aluminum capacitors will gradually volatilize, causing the capacitor to decrease or even fail, and the volatilization speed increases as the temperature rises. Every 10 degrees of temperature increase, the life of electrolytic capacitors will be halved. If the capacitor can be used for 10,000 hours at a room temperature of 27 degrees, it can only be used for 1250 hours at a temperature of 57 degrees. Therefore, the aluminum electrolytic capacitor should not be too close to the heat source.

2) Ceramic capacitors rely on physical reactions to store electricity, so they have a very high response speed and can be applied to G applications. However, ceramic capacitors also show great differences due to different dielectrics. The best performance is the capacitor made of C0G, which has a small temperature coefficient, but the dielectric constant of the material is small, so the capacitance value cannot be too large. The worst performance is the Z5U/Y5V material. This material has a large dielectric constant, so the capacitance can be tens of microfarads. However, this material is seriously affected by temperature and DC bias (DC voltage will polarize the material and reduce the capacitance). Let’s take a look at the influence of the ambient temperature and DC working voltage on the capacitors of C0G, X5R, and Y5V materials.

The use of capacitors in power supply design: aluminum electrolytic capacitors / ceramic capacitors / tantalum capacitors

The use of capacitors in power supply design: aluminum electrolytic capacitors / ceramic capacitors / tantalum capacitors

It can be seen that the capacitance value of C0G basically does not change with temperature, the stability of X5R is slightly worse, and when the Y5V material is at 60 degrees, the capacity becomes 50% of the nominal value.

The use of capacitors in power supply design: aluminum electrolytic capacitors / ceramic capacitors / tantalum capacitors

It can be seen that when the 50V withstand voltage Y5V ceramic capacitor is applied at 30V, the capacity is only 30% of the nominal value. Ceramic capacitors have a big disadvantage, that is, they are fragile. Therefore, it is necessary to avoid bumps and try to stay away from the place where the circuit board is prone to deformation.

3) Tantalum capacitors are like a battery in principle and structure. The following is a schematic diagram of the internal structure of a tantalum capacitor:

The use of capacitors in power supply design: aluminum electrolytic capacitors / ceramic capacitors / tantalum capacitors
 

Tantalum capacitors have the advantages of small size, large capacity, fast speed, low ESR, etc., and the price is relatively high. 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, and if you want to get a larger withstand voltage, you need thicker Ta2O5, which requires the use of larger particles of tantalum powder. Therefore, it is very difficult to obtain tantalum capacitors with high withstand voltage and large capacity for the same volume. Another point that tantalum capacitors need to pay attention to is that tantalum capacitors are relatively easy to break down and exhibit short-circuit characteristics, and their anti-surge capability is poor. It is possible that a large instantaneous current causes the capacitor to burn out and form a short circuit. This needs to be considered when using ultra-large-capacity tantalum capacitors (such as 1000uF tantalum capacitors).

It can be understood from the above that different capacitors have different applications, not that the higher the price, the better.

Let’s talk about the role of capacitors in power supply design.

In power supply design applications, capacitors are mainly used for filtering and decoupling/bypass. Filtering mainly refers to filtering out 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. Below we look at a circuit structure:

The use of capacitors in power supply design: aluminum electrolytic capacitors / ceramic capacitors / tantalum capacitors

The switching power supply in the picture supplies power for A and B. The current passes through C1 and then passes through a section of PCB trace (for the time being, it is equivalent to an inductance. Actually, it is wrong to analyze this equivalent with electromagnetic wave theory, but for the convenience of understanding, this equivalent method is still used.) Separate the two paths. Supply A and B. The ripple from the switching power supply is relatively large, so we use C1 to filter the power supply to provide a stable voltage for A and B. C1 needs to be placed as close to the power supply as possible. C2 and C3 are bypass capacitors, which play a decoupling role. When A needs a large current at a certain moment, if there is 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 reduced by the voltage of terminal A, 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. Generally, aluminum electrolytic capacitors are used. When the surge current is small, it is better to use tantalum capacitors instead of aluminum electrolytic capacitors. From the above example, we can know that as a decoupling capacitor, it must have a fast response speed to achieve the effect. If the local circuit A in the figure refers to a chip, ceramic capacitors should be used for the decoupling capacitors, and the capacitors should be as close as possible to the power pins 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 found in the data sheet of the switching power supply 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.

The decoupling capacitor needs to meet two requirements, one is the capacity requirement, and the other is the ESR requirement. In other words, 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 instruction manuals 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 ones. Capacitors, the total capacity should be more than 200uF.

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