In the actual design because of the price, procurement, capacitors and other reasons often used are: ceramic capacitors, aluminum electrolytic capacitor,** tantalum capacitors** .

Application of capacitance to correct, the natural need to know the specific model and distributed parameter model of each capacitor in the specific meaning and function. Like other components of different capacitors in practice and the "ideal" capacitor, " actual" capacitors because of its packaging, materials and other aspects, it has an additional characteristic inductance and resistance, must use additional parasitic elements or "non ideal" to characterize the performance. In the form of resistance and inductance element element, nonlinear and dielectric storage performance. The "real" capacitor model as shown below. Because of these characteristics of capacitor parasitic elements determined, usually in capacitor manufacturers products that are detailed said ming. Understand the parasitism in each application, will help you choose the right type of capacitor.

From the above chart we can see that the capacitor should actually consists of six parts.In addition to their capacitance C, and the following parts:

1, the equivalent series resistance of ESRRESR: the equivalent series resistance of the capacitor is the equivalent resistance of the capacitor lead resistance and capacitor plate two series. When there is a large AC current through the capacitor , RESR capacitor energy consumption (resulting in loss). This will cause serious consequences for the RF circuit and with high ripple current power supply decoupling capacitor. But the precision of high impedance, small signal analog circuit will not affect very large . The capacitor RESR is the lowest in mica capacitors and film capacitors.

2, the equivalent series inductance ESL, LESL: the capacitor equivalent series inductance is the equivalent inductance capacitor pin inductor and capacitor plates which are connected in series two. Like RESR LESL, there will be serious problems in high frequency RF or conditions, although the precise circuit itself in DC or low frequency under the condition of normal work. The reason is used in precision analog circuits in transistor at the transition frequency (transitionfrequencies) extended to several hundred MHz or GHz case, still has a gain, can enlarge the resonant inductance value is very low. The main reason is that in case of high frequency power supply for the circuit is appropriate to decoupling.

3, the equivalent parallel resistance capacitor leakage resistance is EPRRL: we usually say, in AC coupled applications, storage applications (such as analog integrator and sampling holder) and when the capacitor is used for high impedance circuit , RL is an important parameter, the ideal charge in the capacitor should only change with the external current. However, the actual rate of capacitor RL to charge decided to RC time constant of the slow leak.

The distribution parameters of 4, or two RDA, CDA is the capacitance parameter, but in the actual effect should be relatively small, not presented here. So the capacity distribution parameters are three: ESR, ESL, EPR. The most important of which is ESR and ESL, the actual capacity model when analyzing the general simplified model is only RLC, C, ESR, capacity of ESL, which we will do in the next week focuses on the analysis of electric volume simplified model.

5, we introduced in detail based on the model, we talk about the design often used two kinds of capacitors:

6, electrolytic capacitors (for example:** tantalum capacitors** And aluminum electrolytic capacitor) because of its large capacity, low isolation resistance is EPR, equivalent parallel resistance is very small, so the leakage current is very large (typically 5 20nA/? F), therefore it is not suitable for storage and coupling. Electrical capacitance solution is suitable for bypass capacitor power supply, power supply for stable power supply. The most suitable for the AC coupling capacitor and charge storage capacitor is PTFE and other polyester (PP and PS) electric container.

7, monolithic ceramic capacitors, more suitable for high frequency circuit decoupling capacitor, because they have very low series inductance, equivalent series inductance ESL is very small, with a very wide band back coupling. This node and his structure have great monolithic ceramic capacitor is composed of multilayer laminated film and metal ceramic films, and these films are arranged in parallel bus way, and not in accordance with the serial mode of winding.

8, this week we talked with the capacitor equivalent model, we believe that now on capacitor should have deeper understanding, next week we will continue to talk, we analysis of the actual application of the simplified equivalent model capacitor is often used, and his origin and significance of impedance curve.

**The simplified model and the impedance curve of capacitance**

For the convenience of analysis, in the actual analysis should be often used in RLC model by the equivalent series resistance ESR series equivalent inductance ESL, capacitance. Because of the high frequency characteristics of the capacitance effect is the largest ESR and ESL, we usually adopt the actual model of simplified analysis:

The RLC model consists of the impedance above if the mathematical formula can be expressed as follows:

Then the expression of the model are as follows:

The expression of type is the capacitance of the capacitance change with frequency, if 2 pi fLs = 1 / 2 pi fC, then Z min = Rs, the:

Draw the capacitor capacitance curve diagram as follows:

From the above, we see very clear: the capacitance in the whole frequency band, not all is the performance for the capacitance characteristics, but in the case of low frequency (resonant frequency below), performance for the capacitive device, when the frequency increases (more than resonant frequency) when it became the inductive device. It is said that the impedance increases with frequency decreases first and then increases, the equivalent impedance of the minimum value in the series resonant frequency, at this time, capacitor capacitance and inductance is exactly offset, impedance is exactly equal to the parasitic series resistance ESR.

Understand the above curve, should not difficult to understand in practice should be, we are choosing standard capacitor:

1, as far as possible low capacitance ESR.

2, as far as possible the high resonant frequency of capacitance value.

February 18, 2017