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Capacitors are one of the most commonly used components on circuit boards. As the number of electronic devices (from mobile phones to cars) continues to increase, so does the demand for capacitors. The Covid 19 pandemic has disrupted the global component supply chain from semiconductors to passive components, and capacitors have been in short supply1.
Discussions on the topic of capacitors can easily be turned into a book or a dictionary. First, there are different types of capacitors, such as electrolytic capacitors, film capacitors, ceramic capacitors and so on. Then, in the same type, there are different dielectric materials. There are also different classes. As for the physical structure, there are two-terminal and three-terminal capacitor types. There is also an X2Y type capacitor, which is essentially a pair of Y capacitors encapsulated in one. What about supercapacitors? The fact is, if you sit down and start reading capacitor selection guides from major manufacturers, you can easily spend the day!
Since this article is about the basics, I will use a different method as usual. As mentioned earlier, capacitor selection guides can be easily found on supplier websites 3 and 4, and field engineers can usually answer most questions about capacitors. In this article, I will not repeat what you can find on the Internet, but will demonstrate how to choose and use capacitors through practical examples. Some lesser-known aspects of capacitor selection, such as capacitance degradation, will also be covered. After reading this article, you should have a good understanding of the use of capacitors.
Years ago, when I was working in a company that made electronic equipment, we had an interview question for a power electronics engineer. On the schematic diagram of the existing product, we will ask potential candidates “What is the function of the DC link electrolytic capacitor?” and “What is the function of the ceramic capacitor located next to the chip?” We hope that the correct answer is the DC bus capacitor Used for energy storage, ceramic capacitors are used for filtering.
The “correct” answer we seek actually shows that everyone on the design team looks at capacitors from a simple circuit perspective, not from a field theory perspective. The point of view of circuit theory is not wrong. At low frequencies (from a few kHz to a few MHz), circuit theory can usually explain the problem well. This is because at lower frequencies, the signal is mainly in differential mode. Using circuit theory, we can see the capacitor shown in Figure 1, where the equivalent series resistance (ESR) and equivalent series inductance (ESL) make the impedance of the capacitor change with frequency.
This model fully explains the circuit performance when the circuit is switched slowly. However, as the frequency increases, things become more and more complicated. At some point, the component starts to show non-linearity. When the frequency increases, the simple LCR model has its limitations.
Today, if I were asked the same interview question, I would wear my field theory observation glasses and say that both capacitor types are energy storage devices. The difference is that electrolytic capacitors can store more energy than ceramic capacitors. But in terms of energy transmission, ceramic capacitors can transmit energy faster. This explains why ceramic capacitors need to be placed next to the chip, because the chip has a higher switching frequency and switching speed compared to the main power circuit.
From this perspective, we can simply define two performance standards for capacitors. One is how much energy the capacitor can store, and the other is how fast this energy can be transferred. Both depend on the manufacturing method of the capacitor, the dielectric material, the connection with the capacitor, and so on.
When the switch in the circuit is closed (see Figure 2), it indicates that the load needs energy from the power source. The speed at which this switch closes determines the urgency of energy demand. Since energy travels at the speed of light (half the speed of light in FR4 materials), it takes time to transfer energy. In addition, there is an impedance mismatch between the source and the transmission line and the load. This means that energy will never be transferred in one trip, but in multiple round trips5, which is why when the switch is quickly switched, we will see delays and ringing in the switching waveform.
Figure 2: It takes time for energy to propagate in space; impedance mismatch causes multiple round trips of energy transfer.
The fact that energy delivery takes time and multiple round trips tells us that we need to move the energy as close as possible to the load, and we need to find a way to deliver it quickly. The first is usually achieved by reducing the physical distance between the load, switch and capacitor. The latter is achieved by gathering a group of capacitors with the smallest impedance.
Field theory also explains what causes common mode noise. In short, common mode noise is generated when the energy demand of the load is not met during switching. Therefore, the energy stored in the space between the load and nearby conductors will be provided to support the step demand. The space between the load and nearby conductors is what we call parasitic/mutual capacitance (see Figure 2).
We use the following examples to demonstrate how to use electrolytic capacitors, multilayer ceramic capacitors (MLCC), and film capacitors. Both circuit and field theory are used to explain the performance of selected capacitors.
Electrolytic capacitors are mainly used in the DC link as the main energy source. The choice of electrolytic capacitor often depends on:
For EMC performance, the most important characteristics of capacitors are impedance and frequency characteristics. Low-frequency conducted emissions always depend on the performance of the DC link capacitor.
The impedance of the DC link depends not only on the ESR and ESL of the capacitor, but also on the area of ​​the thermal loop, as shown in Figure 3. A larger thermal loop area means that energy transfer takes longer, so performance will be affected.
A step-down DC-DC converter was built to prove this. The pre-compliance EMC test setup shown in Figure 4 performs a conducted emission scan between 150kHz and 108MHz.
It is important to ensure that the capacitors used in this case study are all from the same manufacturer to avoid differences in impedance characteristics. When soldering the capacitor on the PCB, make sure that there are no long leads, as this will increase the ESL of the capacitor. Figure 5 shows the three configurations.
The conducted emission results of these three configurations are shown in Figure 6. It can be seen that, compared with a single 680 µF capacitor, the two 330 µF capacitors achieve a noise reduction performance of 6 dB over a wider frequency range.
From the circuit theory, it can be said that by connecting two capacitors in parallel, both ESL and ESR are halved. From the field theory point of view, there is not only one energy source, but two energy sources are supplied to the same load, effectively reducing the overall energy transmission time. However, at higher frequencies, the difference between two 330 µF capacitors and one 680 µF capacitor will shrink. This is because high frequency noise indicates insufficient step energy response. When moving a 330 µF capacitor closer to the switch, we reduce the energy transfer time, which effectively increases the step response of the capacitor.
The result tells us a very important lesson. Increasing the capacitance of a single capacitor will generally not support the step demand for more energy. If possible, use some smaller capacitive components. There are many good reasons for this. The first is cost. Generally speaking, for the same package size, the cost of a capacitor increases exponentially with the capacitance value. Using a single capacitor may be more expensive than using several smaller capacitors. The second reason is size. The limiting factor in product design is usually the height of the components. For large-capacity capacitors, the height is often too large, which is not suitable for product design. The third reason is the EMC performance we saw in the case study.
Another factor to consider when using an electrolytic capacitor is that when you connect two capacitors in series to share the voltage, you will need a balancing resistor 6.
As mentioned earlier, ceramic capacitors are miniature devices that can quickly provide energy. I am often asked the question “How much capacitor do I need?” The answer to this question is that for ceramic capacitors, the capacitance value should not be that important. The important consideration here is to determine at which frequency the energy transfer speed is sufficient for your application. If the conducted emission fails at 100 MHz, then the capacitor with the smallest impedance at 100 MHz will be a good choice.
This is another misunderstanding of MLCC. I have seen engineers spend a lot of energy choosing ceramic capacitors with the lowest ESR and ESL before connecting the capacitors to the RF reference point through long traces. It is worth mentioning that the ESL of MLCC is usually much lower than the connection inductance on the board. Connection inductance is still the most important parameter affecting the high frequency impedance of ceramic capacitors7.
Figure 7 shows a bad example. Long traces (0.5 inches long) introduce at least 10nH inductance. The simulation result shows that the impedance of the capacitor becomes much higher than expected at the frequency point (50 MHz).
One of the problems with MLCCs is that they tend to resonate with the inductive structure on the board. This can be seen in the example shown in Figure 8, where the use of a 10 µF MLCC introduces resonance at approximately 300 kHz.
You can reduce resonance by choosing a component with a larger ESR or simply putting a small value resistor (such as 1 ohm) in series with a capacitor. This type of method uses lossy components to suppress the system. Another method is to use another capacitance value to move the resonance to a lower or higher resonance point.
Film capacitors are used in many applications. They are the capacitors of choice for high-power DC-DC converters and are used as EMI suppression filters across power lines (AC and DC) and common-mode filtering configurations. We take an X capacitor as an example to illustrate some of the main points of using film capacitors.
If a surge event occurs, it helps limit the peak voltage stress on the line, so it is usually used with a transient voltage suppressor (TVS) or metal oxide varistor (MOV).
You may already know all of this, but did you know that the capacitance value of an X capacitor can be significantly reduced with years of use? This is especially true if the capacitor is used in a humid environment. I have seen the capacitance value of the X capacitor only drop to a few percent of its rated value within a year or two, so the system originally designed with the X capacitor actually lost all the protection that the front-end capacitor might have.
So, what happened? Moisture air may leak into the capacitor, up the wire and between the box and the epoxy potting compound. The aluminum metallization can then be oxidized. Alumina is a good electrical insulator, thereby reducing capacitance. This is a problem that all film capacitors will encounter. The issue I am talking about is film thickness. Reputable capacitor brands use thicker films, resulting in larger capacitors than other brands. The thinner film makes the capacitor less robust to overload (voltage, current, or temperature), and it is unlikely to heal itself.
If the X capacitor is not permanently connected to the power supply, then you don’t need to worry. For example, for a product that has a hard switch between the power supply and the capacitor, size may be more important than life, and then you can choose a thinner capacitor.
However, if the capacitor is permanently connected to the power source, it must be highly reliable. The oxidation of capacitors is not inevitable. If the capacitor epoxy material is of good quality and the capacitor is not often exposed to extreme temperatures, the drop in value should be minimal.
In this article, first introduced the field theory view of capacitors. Practical examples and simulation results show how to select and use the most common capacitor types. Hope this information can help you understand the role of capacitors in electronic and EMC design more comprehensively.
Dr. Min Zhang is the founder and chief EMC consultant of Mach One Design Ltd, a UK-based engineering company specializing in EMC consulting, troubleshooting and training. His in-depth knowledge in power electronics, digital electronics, motors and product design has benefited companies around the world.
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