Why is the ESR of the capacitor not marked?

It seems that all the power engineers talk about the quality of the electrolytic capacitors. In the end, there is always one less choice. Choose a capacitor with a lower ESR parameter. However, when the company purchaser purchases the capacitor according to this requirement, he can only choose a good brand. Because the buyer knows that the capacitor ESR parameters of the good brand are low, because the ESR value of the electrolytic capacitor is never indicated.

Someone asked why the ESR of the capacitor is not indicated and the temperature characteristics are not indicated. In fact, I don't know. Here are three sentences to analyze the parameters of the capacitor.

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An output rectifier filter capacitor switching power supply, the capacity is usually the first choice, the capacitance of an aluminum electrolytic capacitor is fully meet the requirements, and the relatively high ESR. ESR can be reduced by multiple parallel methods. You can also choose a larger capacity to reduce ESR.

ESR is the most important performance parameter in high-frequency electrolytic capacitors. Many capacitor suppliers emphasize the performance characteristic of “LOW ESR”, which means that the ESR value is small. So, how do we correctly understand the practical significance of LOW ESR? Due to the development of electronic technology, the voltage supplied to the hardware is showing a trend of lower and lower. For example, the power supply voltage of FPGA, DSP and RAM series is very low. The circuit voltage is less than 2V, which is much lower than the previous 3, 4V voltage. However, on the other hand, due to the explosion of transistors and frequencies, the power consumption of these chips is increasing. Therefore, according to the formula of P=UI, the current requirements of these devices are getting higher and higher.

For example, on a computer motherboard, for example, two CPUs with the same power consumption of 70W, the former voltage is 3.3V, and the latter voltage is 1.8V. Then, the current of the former is I=P/U=70W/3.3V, which is about 21.2A. The current of the latter is I=P/U=70W/1.8V=38.9A, which is nearly double that of the former. In the case where the current through the capacitor is getting higher and higher, if the ESR value of the capacitor cannot be kept in a small range, then a higher chopping voltage than before (the ideal output DC voltage should be a horizontal line, The chopping voltage is the peak and trough on the horizontal line).

In addition, even the same chopping voltage has a greater impact on the low voltage circuit than in the high voltage case. For example, for a 3.3V MCU, the 0.2V chopping voltage is a small percentage, which is not enough to form a fatal effect, but for an FPGA and DSP used for 1.8V power supply, it is also a 0.2V chopping voltage. The proportion of it is enough to cause the judgment of the digital circuit to be wrong.

So what is the relationship between the ESR value and the chopping voltage? We can use the following formula:

V=R(ESR)×I

In this formula, V represents the chopping voltage, and R represents the ESR of the capacitor, and I represents the current. It can be seen that when the current is increased, the chopping voltage is multiplied even when the ESR remains unchanged, and it is imperative to use a capacitor with a lower ESR value. This is why the capacitors used in hardware devices such as today's boards are increasingly emphasizing the low ESR.

Considering the production process of electrolytic capacitors, the resistance of the electrolyte is the main part of the equivalent series resistance (ESR) of the aluminum electrolytic capacitor. The main reason why most aluminum electrolytic capacitor manufacturers do not give ESR data is that the ESR of aluminum electrolytic capacitors is too large compared to capacitors of other media. For example, a typical aluminum electrolytic capacitor of 1μF/16V has an ESR of about 20Ω; a 100μF aluminum electrolytic capacitor has an ESR of 1.5 to 2Ω.

Imagine that such data written in the data sheet will definitely affect the applicator's confidence in the application of aluminum electrolytic capacitors. Therefore, in some cases, the application of aluminum electrolytic capacitors is a helpless choice. Will affect the application of aluminum electrolytic capacitors.

For aluminum electrolytic capacitors for general applications, most aluminum electrolytic capacitor manufacturers do not give ESR data. This data is given for low-ESR aluminum electrolytic capacitors for switching power supplies or pin-type aluminum electrolytic capacitors with relatively large capacitance.

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In fact, as a hardware engineer, there is always such a feeling, the parameters of the capacitor, we divide it into "dominant parameters" and "implicit parameters" . The so-called "dominant parameter" is some basic parameters printed on the surface of the capacitor. These parameters are often directly known after we see a capacitor. For example, the capacity of the capacitor (such as "470μF", etc.), the range of capacity deviation, the temperature range, and the voltage value (such as "16V").

The so-called "recessive parameter" is the parameter we need to query according to the model of the capacitor. For example, the ESR value we often say has become an important parameter to distinguish the performance of the capacitor, and we can't see this parameter in the capacitor. We have to go to the relevant website to query by the model of the capacitor.

There are quite a few similar parameters for the same fate as ESR, including the following:

1. The rated ripple current value ; the rated ripple current will cause loss in the ESR of the aluminum electrolytic capacitor, and the limit of the heating current is the rated ripple current value. It is defined as the maximum ripple current value that ensures the rated life of the aluminum electrolytic capacitor at the highest operating temperature. For aluminum electrolytic capacitors for general applications, most aluminum electrolytic capacitor manufacturers do not give rated ripple current data. For low-ESR aluminum electrolytic capacitors for switching power supplies or pin-type aluminum electrolytic capacitors with large capacitance, this is given. data.

2. The value of the chopping current that can be tolerated; as explained above.

3. Tangent (TAN) of the loss angle; equivalent to the ratio of reactive power to active power. This value is related to the quality of the capacitor and the amount of heat generated. The smaller the value, the better the capacitor performance.

4. Leakage current value; no matter how large the insulator is, there will always be a slight current leakage through the capacitor. This value represents the specific leakage. And this parameter manufacturer is not willing to be detailed.

5. ESL equivalent inductance; the equivalent inductance ESL often becomes part of the ESR, and ESL also causes some circuit failures, such as series resonance. However, the relative capacity, ESL ratio is too small, the probability of problems is small, coupled with the progress of the capacitor manufacturing process, has now gradually ignored the ESL, and the ESR as a major reference factor in addition to capacity.

In fact, concerned about these capacitor parameters, is concerned about the service life of the capacitor, specifically not to mention, other materials or online have a lot of introduction.

The main reason why most aluminum electrolytic capacitor manufacturers do not give ESR data is that the ESR of aluminum electrolytic capacitors is too large compared to capacitors of other media (except for individual companies in Japan). Among the many aluminum electrolytic capacitor manufacturers in the world, the equivalent series resistance (ESR) of the aluminum electrolytic capacitors that can be produced is small. Some do not have this data (such as many domestic manufacturers of aluminum electrolytic capacitors), while others are in need of technical secrecy. Of course, there are also feelings that the ESR value of the aluminum electrolytic capacitors produced by them is too large, too shy, and it is reported that it is hurtful. .