An in-depth analysis of capacitors

What is a capacitor? What technical knowledge does it have? Capacitance is mainly controlled by two parameters. One is ESR and the other is ESL. What do they do and what role do they play?

Theoretically, a perfect capacitor will not cause any energy loss on its own, but in fact, because the material used to make the capacitor has resistance and the insulating medium of the capacitor has losses, the capacitor becomes imperfect for various reasons. This loss is external and behaves like a resistor and capacitor connected in series, so it is called "equivalent series resistance".

For example, we believe that the voltage on the capacitor cannot change suddenly. When a current is suddenly applied to the capacitor, the voltage of the capacitor will rise from 0 due to its own charging. But with ESR, the resistor itself will produce a voltage drop, which will cause a sudden change in the voltage across the capacitor. Undoubtedly, this will reduce the filtering effect of the capacitor, so many high-quality power supplies use low-ESR capacitors.

Similarly, in situations such as oscillator circuits, ESR can also cause functional changes in the circuit, leading to serious consequences such as circuit failure or even damage. Therefore, in most cases, low ESR capacitors tend to perform better than high ESR capacitors. However, there are exceptions. Sometimes, this ESR is also used to do some useful things.

For example, in a voltage stabilizing circuit, a capacitor with a certain ESR will immediately generate fluctuations when a transient occurs in the load, causing the feedback circuit to act. This rapid response, at the expense of a certain transient performance, will gain subsequent results. Fast adjustment capability, especially when the response speed of the power tube is relatively slow and the volume/capacity of the capacitor is strictly limited. This situation is seen in some three-terminal voltage regulators or similar circuits that use mos tubes as adjustment tubes. At this time, too low ESR will reduce overall performance.

In fact, there are more occasions where lower ESR is required, and large-capacity capacitors with low ESR are relatively expensive. Therefore, many switching power supplies adopt a parallel connection strategy, using multiple aluminum electrolytics with relatively high ESR in parallel to form a large capacitor with low ESR. capacity capacitor. Sacrificing a certain amount of PCB space in exchange for reducing device costs is often cost-effective.

Another concept similar to ESR is ESL, which is equivalent series inductance. Early rolled capacitors often had very high ESL, and capacitors with larger capacities generally had larger ESLs. ESL often becomes part of ESR, and ESL can also cause some circuit faults, such as series resonance. However, relative to the capacity, the proportion of ESL is too small, and the probability of problems is very small. Coupled with the advancement of capacitor manufacturing technology, ESL has been gradually ignored, and ESR is used as the main reference factor besides capacity.

By the way, capacitors also have a quality coefficient Q similar to that of inductors. This coefficient is inversely proportional to ESR and related to frequency, so it is rarely used.

Circuit faults caused by ESR are often difficult to detect, and the impact of ESR can easily be overlooked during the design process. The simple approach is that during simulation, if you cannot select the specific parameters of the capacitor, you can try to artificially connect a small resistor in series with the capacitor to simulate the impact of ESR. Generally, the ESR of tantalum capacitors is usually below 100 milliohms, and Aluminum electrolytic capacitors are higher than this value, and the ESR of some types of capacitors can even be as high as several ohms.

The relationship between ESR value and ripple voltage can be expressed by the formula V=R(ESR)×I. V in this formula represents the ripple voltage, R represents the ESR of the capacitor, and I represents the current. It can be seen that when the current increases, the ripple voltage will increase exponentially even if the ESR remains unchanged. The above is some technical knowledge about capacitors, I hope it can help you.