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The main technical indexes have capacitor voltage electric capacity, anti-scar, value. Besides the three main index, the index of more important is equivalent to a series resistor (ESR). Some capacitor on a golden ribbon line, picturing a big hollow letters ", "it said the capacitance, LOW loss belongs to LOW ESR capacitance. Some capacitance (the emblem also ESR equivalent series resistance), ESR, the smaller the loss, low output current is bigger, the higher the quality of capacitor.

Better is the Equivalent Resistance ESR abbreviations, namely "Equivalent Series Resistance". The ideal capacitance itself would not have any energy losses, but actually, because the material is made capacitance of the medium of electrical insulation resistance, capacitance has loss. This loss in the external, like a resistor in series with the capacitor together, so called "equivalent series resistance". And another concept ESR similar ESL, namely is equivalent series inductance. Early roller-rolling inductance often have high capacity of capacitance, ESL, ESL generally. ESL often become part of the ESR and ESL causes series, etc. But the relative electric capacity, the proportion of ESL, small chance of small problems, afterwards because of capacitance, now has gradually, and ignore ESL ESR as pressure, temperature value capacity, the selection of main capacitor value beyond the reference elements.

The equivalent resistance series of high-grade unit is the European ESR Ω (m). Usually the ESR usually tantalum capacitors in Europe, and below 100 nanobots are higher than the aluminum electrolytic capacitor, some kinds of numerical ESR even up several ohms. The height, and ESR capacity of capacitor voltage and frequency, and temperature are fixed, when the rated voltage, capacity greater ESR less. Likewise, when the capacity of fixed by the rated voltage of the varieties can also reduce ESR, So choose high voltage capacitor have many benefits, Low frequency when high, ESR ESR low, The temperature will cause the ESR.

Now electronic technology is in low voltage high current circuit design, supply the development direction of the components of the lower voltage appears to power, but the requirement but does not decrease. According to the P = UI, the formula for calculating the power to get the same voltage reduced, it must increase current. INTEL, for the latest AMD cpus, voltage, and less than 3, 2V before 4V much lower than the voltage. But, on the other hand, these chips since transistors and frequency of the power, but increased demand for electricity, request higher. Example 2 power is the CPU, the former 70W voltage is the voltage is 3.3 V, 1.8 V. So, the former current I = P/U = = 21.2 70W/A 3.3 V, While the latter's current I = P/U = 70W/f = 1.8 V, nearly twice the current is the former. In the current through the capacitance under the condition of higher value, if the capacitance ESR cannot keep in a small range, it will produce higher ripple voltage (ideal output should be a dc voltage ripple voltage, and horizontal level is the peaks and troughs), so to engineer in the design, want to use the smallest ESR capacitor.

And the ripple voltage ESR relations can use formula V = R (ESR) x I said. This formula is the V, and said the ripple voltage capacitance ESR, r. says I said current. As you can see, when the current increases, even in the ESR remain unchanged, under the situation of ripple voltage will be doubled, so using lower capacitance is imperative ESR value.

In addition, even the same ripple voltage, low voltage circuit of the influence than in high voltage more cases. For example, the CPU 3.3 V 0.2 V ripple voltage, small in proportion to the great influence, but to 1.8 V CPU, also is 0.2 V ripple voltage, which is sufficient to cause the proportion of digital circuit of judgement.

Such as the "2007 (17 edition of the NCP1200 by the composition of switch power supply, 1A 12V, switch rectifier transformer subprime diode after type filter capacitance coupling LC in C6 and C7 requirement is" to choose the equivalent series resistance small quality capacitors, equivalent resistance will not only affect the conversion can also affect the output ripple voltage."

The ESR is equivalent to "series" resistance, capacitance series, two will make the ESR value, while the parallel will make decreases. Therefore in the need to lower the occasion, and low ESR ESR large capacity and relatively expensive price of capacitance of the cases, with multiple ESR relatively high aluminum electrolytic capacitor, forming a parallel low ESR large capacity is a kind of common capacitance. Many of the capacitor switching power supply adopt the strategy to sacrifice, parallel to the PCB space for certain devices cost reduction.

But the existence of equivalent series resistance must have good aspect. Such as voltage circuit, have certain ESR capacitance, the transient load occurred in time, will produce immediate fluctuations caused by feedback circuit, the rapid response to the transient performance of sacrifice must obtain the expense of follow-up, quick adjustment ability, especially the response speed of power tube is slow, but in the volume of capacitor, capacity is restricted. This kind of situation in some use MOS tube of three components, adjusting voltage or similar circuits, using low ESR capacitor may reduce overall performance.