High-speed PCB design in the role of capacitors

The first part: the classification of capacitors

Capacitors in the design of the circuit from the application of classification, the capacitance can be divided into four categories:

The first category: AC coupling capacitor. Mainly used for AC coupling Ghz signal.

The second category: decoupling capacitor. Mainly used to keep high-speed circuit to filter out the power or ground noise.

The third category: active or passive RC filter or frequency-selective network capacitors used.

The fourth category: Analog integrator and sample and hold circuit used in the capacitor.

In this article we will mainly discuss the second category decoupling capacitors.

Capacitors are categorized from the materials and processes that are manufactured, mainly in the following different forms of capacitance:

1, NPO ceramic capacitors

2, polystyrene pottery capacitors

3, polypropylene capacitors

4, PTFE capacitors

5, MOS capacitor

6, polycarbonate capacitors

7, polyester capacitors

8, monolithic ceramic capacitors

9, mica capacitors

10, electrolytic capacitors

11, tantalum electrolytic capacitors

In the actual design, the price, procurement and other reasons often used capacitors: ceramic capacitors, electrolytic capacitors, tantalum capacitors.

The second part: the specific model of capacitance and distribution parameters

To correctly and reasonably apply the capacitor, it is naturally necessary to know the concrete model of the capacitor and the specific meaning and function of each distribution parameter in the model. Like other components, the actual capacitance is different from the “ideal” capacitor, which has an additional characteristic of inductance and resistance due to its packaging, material, etc., and must be replaced with an additional “parasitic “Elements or” non-ideal “properties to characterize them in the form of resistive and inductive components, non-linear and dielectric memory properties. The “actual” capacitor model is shown below. The characteristics of these capacitors as a result of these parasitic elements are generally specified in the capacitor manufacturer’s product description. Knowing these parasites in every application will help you choose the right type of capacitor.

From the figure above we can see that the capacitor should actually be composed of six parts. In addition to its own capacitance C, there are the following components:

Equivalent Series Resistance ESR RESR: The equivalent series resistance of a capacitor consists of the capacitor’s pin resistance in series with the equivalent resistance of the two plates of the capacitor. When there is a large AC current through the capacitor, RESR causes the capacitor to dissipate energy (thereby creating a loss). This has serious consequences for RF circuits and power supply decoupling capacitors that carry high ripple current. But the precision of high impedance, small signal analog circuit will not have a great impact. The lowest RESR capacitors are mica capacitors and film capacitors.

2, Equivalent series inductance ESL, LESL: Equivalent series inductance of the capacitor is composed of the capacitor’s lead inductance and the equivalent inductance of the capacitor’s two plates in series. Like RESR, LESL can also have serious problems in RF or high frequency operation, though the precision circuitry itself operates normally at DC or low frequency. The reason for this is that transistors used in precision analog circuits still have gain when the transition frequencies are extended to several hundred megahertz or several gigahertz, amplifying a low inductance resonant signal. This is the main reason for decoupling the power supply terminals of such circuits at high frequencies.

- Equivalent Shunt Resistor EPR RL: This is what we commonly refer to as capacitor leakage resistance, which is used for AC-coupled applications, storage applications such as analog integrators and sample-and-hold and when capacitors are used for high impedance circuits Important parameters, the ideal capacitor of the charge should only change with the external current. However, the RL in the actual capacitor slowly leaks charges at a rate determined by the RC time constant.

4, or two parameters RDA, CDA is the distribution of capacitance parameters, but the actual impact should be relatively small, not introduced here. Therefore, there are three important parameters of capacitance distribution: ESR, ESL, EPR. The most important of these are ESR and ESL. In fact, only the RLC simplified model is actually used in the analysis of capacitance models. That is, the capacitance of C, ESR and ESL is analyzed. This time, we will focus on analyzing the simplified model of capacitance in the next week.

5, below we introduce the detailed model on the basis of the design, we often talk about two kinds of capacitors:

- Electrolytic capacitors (such as tantalum capacitors and aluminum electrolytic capacitors) have a large capacity. Due to their low isolation resistance, the equivalent parallel resistance EPR is very small, so the leakage current is very large (typically 5-20nA / μF). Therefore, It is not suitable for storage and coupling. Electrolytic capacitors are more suitable for the power bypass capacitor, used to stabilize the power supply. The most suitable capacitors for AC coupling and charge storage are polytetrafluoroethylene capacitors and other polyester (polypropylene, polystyrene, etc.) capacitors.

7, monolithic ceramic capacitors, more suitable for high-frequency decoupling capacitors, because they have a very low equivalent series inductance, ESL is equivalent series inductance is small, with a wide decoupling frequency band. This has a great relationship with his structure. Monolithic ceramic capacitors are made of multilayered metal and ceramic films, and these multilayered films are arranged in a parallel manner rather than in a serial manner of.

8, this week we talked about the capacitance of the equivalent model in detail, I believe we should now have a deeper understanding of the capacitor, and next week we will continue to talk about, we actually analyze the application of capacitors to be used frequently simplified equivalent Model, and the origin and meaning of his impedance curve.