How to choose the appropriate capacitance in the microelectronic circuit design?

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Joined: Fri Oct 13, 2017 6:55 am

How to choose the appropriate capacitance in the microelectronic circuit design?

Postby kingblert » Fri Oct 13, 2017 7:01 am

When you choose the capacitor and are in face of all kinds,how to choose it? I would like to talk about some of the common capacitor and its universal scope of application. Note: the type of capacitor is too much, so here will only be involved in which is common in the design of microelectronic circuit. Such as those what variable capacitors, super capacitors, etc. will not be covered for the time being.

Ceramic Capacitor
Pros: low ESR, good high frequency characteristics, high stability, low temperature coefficient.
Cons: capacitance is relatively low (a few pF to dozens of uF), fragile and vulnerable, the capacitance value is easier to be affected by the voltage.
Ceramic capacitors are currently the most widely used capacitor, commonly used for decoupling, signal coupling and analog filter design and so on. Now the process is getting better and better, large-capacity ceramic capacitors can be bought, but the price is relatively high. Now usually the density of PCB is very high with small size, especially consumer electronics, the use of leaded cap is rare, the commonly used package is now almost all patch. Due to the limitations of the manufacturing process, the capacitance of the low ESR capacitor is generally not too high.
Electrolytic capacitor
Pros: high capacity, relatively cheap, not easily damaged, the influence of capacitance value by the voltage is relatively small.
Cons: high ESR, high ESL, poor high frequency characteristics, low stability, high temperature coefficient
Electrolytic capacitors in the power circuit is very common, usually the capacitance value is relatively large,you can find that it is from a few uF to several thousand uF capacity. Because of its high capacitance, poor high frequency characteristics (usually less than 100kHz), it is often used for decouple and regulation of input power. It should be noted that the power supply here is not referring to the chip Vdd or Vcc, I refer to the power supply module such as your PCB power. For instance, a 5V DC input to a Buck convert into a 3.3V DC input to A variety of other chip of your PCB. Usually electrolytic capacitors are connected between the 5V power supply and ground.
On the map is a simple painting of a commonly used power module design. If 5V is the total power of the PCB, the power supply behind the decoupling capacitor will generally use electrolytic capacitors, because in this position we usually do not pay too much attention to high Frequency decoupling, so you can use a relatively low price to get a larger capacitor to decouple and regulate. Speaking of which, some people may ask, if I can use ceramic capacitors to replace the electrolytic capacitor? It can, it also can not. And before explaining this, we first talk about the relationship of capacity v.s. voltage of the two kind. See below

This is a cut of a capacitance change v.s. voltage from a specific ceramic capacitor product of Kynix . It is clear that the value of the capacitor is not a constant value for the voltage applied to it, that is, for the above capacitor, the capacitance value is 0.1uF when the voltage is 10V, and only 0.05uF when the voltage becomes 50V. The electrolytic capacitor is a bit more advantaged than the ceramic capacitor for having a smaller voltage dependency. In fact, the capacitance value will not only follow the voltage changes, but also according to the temperature changes, at this point the electrolytic capacitor is not as good as ceramic capacitors. From DFM (Design for manufacturability) and reliability point of view, electrolytic capacitors are generally larger, for the integration of ultra-high PCB is difficult to fit into. Electrolytic capacitors are much more resistant to physical damage than ceramic capacitors, especially on flex PCB, where ceramic capacitors are susceptible to damage. But in reliability speaking, ceramic capacitors are better than electrolytic capacitors.
Electrolytic capacitor is actually a major category, There are commonly aluminum electrolytic capacitor and tantalum electrolytic capacitors and so on. Such as tantalum capacitors, which improve some of the shortcomings of the aluminum electrolytic capacitor , the higher density density, the better high-frequency characteristics, more reliable. but! The tantalum capacitor is extremely tolerant of the reverse voltage, and if the voltage is reversed or the applied voltage is higher than the rated voltage, it is easy to "burst".
Decoupling Cap and PCB Layout
The frequency response characteristics of the capacitor is determined by ESR, ESL, and its own C. In everyday designs, the capacitance that is relatively critical to the layout is usually a decoupling capacitor. Common decoupling capacitors are used to be a large plus a small, because the capacitance of the smaller capacitance generally has a smaller ESR, ESL, which means that at high frequencies it looks like a capacitor rather than an inductor. So that you can make up for the high frequency that large capacitors can not cover . As shown in the figure below, putting a 100nF and a 1nF impedance frequency characteristic curve together can cover the overall capacitive up to nearly 200MHz, while the single 100nF can only reach about 20MHz.
So the most basic PCB layout rule, is putting the decoupling capacitor closer from your chip pin, to ensure that the parasitic resistance of the PCB trace and the inductance as small as possible, which is not only from the Power to the capacitor,but including capacitance to ground .Second, place the capacitor away from the high noise or high frequency digital signal lines.
For microelectronics design, it can be said that these two types of capacitance is almost covered the usual used ones by more than 90% . (In fact, so far I rarely contact other types of capacitors). Of course, if you are interested in understanding other capacitors, there are a lot of information on the Internet to see, nothing more than the performance difference. The key is to mention the advantages and disadvantages of different capacitors into account in the design process, thinking of what kind of impact will happen if these advantages and disadvantages are added to your circuit? What is the specific impact? Only to understand that you can make the right decision, so choose a most suitable capacitor.
Leave a few questions about the capacitor when I interview.
1, why the larger value of the ceramic capacitor when working with the voltage will make? Such as DC-DC converter's output capacitor.
2, as shown below, capacitor C is a common ceramic capacitor (non-ideal device), the capacitance value is C = 100nF, ESR = 30mΩ, ESL = 1nH, Rated Voltage is 5V. Current source I is the ideal current source. [Hints: Please consider everything that might happen]
When T = 0 switch off, please explain the voltage and capacitance value changes at both ends from T = 0. Draw the current through capacitor C and the voltage across the voltage versus time. If the ideal current source is replaced by the ideal voltage source V = 3V, how long does it take for the capacitor to charge to 99%?
Or use the ideal voltage source to charge the capacitor C to the voltage V1. According to the formula, the total energy stored on the capacitor is E = ½CV², and C = Q / V, so E = ½QV. But the total energy output from the ideal voltage source is P = QV. Why after charging the capacitance C gets only half of the energy?
And then further problem, what method can be charged to reduce the energy loss to close to zero?

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