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The ESR of the capacitor
2019-08-07
 
ESR, the abbreviation of the three words EquivalentSeriesResistance, translated is "equivalent series resistance".

In theory, a perfect capacitor does not generate any energy loss by itself, but in fact, because the material used to make the capacitor has resistance, the insulating medium of the capacitor is depleted, and the capacitor becomes not "perfect" for various reasons. This loss is external, and it appears like a resistor connected to a capacitor string, so it has a name called "equivalent series resistance."

The emergence of ESR led to the behavior of the capacitor deviating from the original definition.

For example, we believe that the voltage on the capacitor cannot be abrupt. When a current is suddenly applied to the capacitor, the capacitor will rise from 0 because it charges itself. But with ESR, the resistor itself produces a voltage drop, which causes 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, such as low-ESR capacitors.

Similarly, in the case of an oscillating circuit or the like, ESR also causes a change in the function of the circuit, causing serious consequences such as circuit failure or even damage. So in most cases, low ESR capacitors tend to perform better than high ESR.

But there are exceptions to this. Sometimes, this ESR is also used to do something useful.

For example, in a voltage regulator circuit, there is a certain ESR capacitor. When a load transient occurs, it will immediately generate fluctuations and trigger the feedback circuit action. This fast response is obtained at the expense of a certain transient performance. Quick adjustment capability, especially when the response speed of the power tube is slow, and the volume/capacity of the capacitor is strictly limited. This situation can be found in some three-terminal regulators or similar circuits that use mos tubes as adjustment tubes. At this time, too low ESR will reduce overall performance.

ESR is an equivalent "serial" resistor, meaning that connecting two capacitors in series will increase this value, while paralleling will reduce it. In fact, there are more occasions requiring lower ESR, and the low-ESR bulk capacitors are relatively expensive. Therefore, many switching power supplies adopt a parallel strategy in which multiple ESRs are connected in parallel to form a low ESR. Capacity capacitor. Sacrificing a certain amount of PCB space, in exchange for the reduction of device costs, is often cost-effective.

Another concept similar to ESR is ESL, which is the equivalent series inductance. Early coiled inductors often had high ESL, and the larger the capacity, the larger the ESL. ESL is often part of the ESR, and ESL can also cause some circuit failures, such as series resonance. However, the relative capacity is too small, the probability of problems is too small, coupled with the progress of the capacitor manufacturing process, ESL has now been gradually ignored, and ESR is the main reference factor in addition to capacity.

By the way, the capacitor also has a quality coefficient Q similar to that of the inductor. This coefficient is inversely proportional to the ESR and is frequency dependent and less used.

Circuit failures caused by ESR are often difficult to detect and the effects of ESR are easily overlooked during the design process. In a simple way, if you can't select the specific parameters of the capacitor during the simulation, you can try to artificially connect a small resistor in the capacitor to simulate the effect of ESR. Generally, the ESR of the tantalum capacitor is usually below 100 milliohms. Aluminum electrolytic capacitors are higher than this value, and the ESR of some types of capacitors can be as high as several ohms.

 
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