PCB News - The role of capacitors in high-speed PCB design

Part 1: Classification of capacitors

Capacitors are classified by application in circuit design. Capacitors can be divided into four categories:

The first category: AC coupling capacitors. Mainly used for AC coupling of Ghz signals.

The second category: Decoupling capacitors. It is mainly used to keep out the noise of the power supply or ground of the high-speed circuit board.

The third category: capacitors used in active or passive RC filtering or frequency selection networks.

The fourth category: Capacitors used in analog integrators and sample-and-hold circuits.

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

Capacitors are classified according to the materials and processes of manufacture, and there are mainly the following different forms of capacitors:

1. NPO ceramic capacitors

2. Polystyrene ceramic capacitors

3. Polypropylene capacitor

4. PTFE capacitor

5. MOS capacitor

6. Polycarbonate capacitors

7. Polyester capacitor

8. Monolithic ceramic capacitors

9. Mica capacitor

10. Aluminum electrolytic capacitors

11. Tantalum electrolytic capacitors

In the actual design, due to various reasons such as price and purchase, the capacitors that are often used are: ceramic capacitors, aluminum electrolytic capacitors, and tantalum capacitors.

Part 2: The specific model and distribution parameters of the capacitor

To apply capacitors correctly and reasonably, it is naturally necessary to understand the specific model of the capacitor and the specific meaning and function of each distributed parameter in the model. Like other components, the actual capacitance is different from the "ideal" capacitor. The "real" capacitor has an additional characteristic of inductance and resistance due to the influence of its packaging, material, etc., and additional "parasitic" must be used. "Components or "non-ideal" performance to characterize, its manifestations are resistance components and inductive components, non-linear and dielectric storage performance. The "real" capacitor model is shown below. Due to the characteristics of the capacitor determined by these parasitic elements, it is usually described in detail in the product description of the capacitor manufacturer. Understanding these parasitic effects in each application will help you choose the right type of capacitor.

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

1. Equivalent series resistance ESR RESR: The equivalent series resistance of a capacitor is composed of the pin resistance of the capacitor and the equivalent resistance of the two plates of the capacitor in series. When a large AC current flows through the capacitor, RESR causes the capacitor to dissipate energy (and thus generate loss). This has serious consequences for radio frequency circuits and power supply decoupling capacitors carrying high ripple currents. But it will not have a big impact on precision high-impedance, small-signal analog circuits. The capacitors with the lowest RESR are mica capacitors and film capacitors.

2. Equivalent series inductance ESL, LESL: The equivalent series inductance of a capacitor is composed of the pin inductance of the capacitor and the equivalent inductance of the two plates of the capacitor in series. Like RESR, LESL also has serious problems in radio frequency or high frequency working environments, although the precision circuit itself works normally under DC or low frequency conditions. The reason is that the transistors used in precision analog circuits still have gain when the transition frequencies are extended to hundreds of megahertz or gigahertz, and can amplify resonant signals with very low inductance values. This is the main reason for proper decoupling of the power terminal of this circuit under high frequency conditions.

3. Equivalent parallel resistance EPR RL: It is what we usually call the capacitor leakage resistance. In AC coupling applications, storage applications (such as analog integrators and sample-and-hold devices), and when capacitors are used in high-impedance circuits, RL is an item An important parameter, the charge in an ideal capacitor should only change with the external current. However, the RL in the actual capacitor causes the charge to leak slowly at a rate determined by the RC time constant.

4. The two parameters RDA and CDA are also the distribution parameters of the capacitor, but the actual effect is relatively small, so they will not be introduced here. So there are three important distribution parameters of capacitance: ESR, ESL, EPR. The most important ones are ESR and ESL. Actually, only RLC is used to simplify the model when analyzing the capacitor model, that is, to analyze the C, ESR, and ESL of the capacitor. We will focus on the simplified model of the capacitor next week.

5. On the basis of introducing the detailed model below, we will talk about two kinds of capacitors that are often used in our design:

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

7. Monolithic ceramic capacitors are more suitable for decoupling capacitors in high-frequency circuits, because they have a very low equivalent series inductance, that is, the equivalent series inductance ESL is very small and has a wide decoupling frequency band. This has a great relationship with his structure. Monolithic ceramic capacitors are composed of multi-layer interlayer metal films and ceramic films, and these multi-layer films are arranged in parallel to the bus bars instead of being wound in series. of.

8. This week we talked about the detailed equivalent model of capacitors. I believe everyone should have a deeper understanding of capacitors. Next week we will continue to talk about the simplified equivalent of capacitors that we often use in practical analysis applications. Model, and the origin and meaning of his impedance curve.

The above is an introduction to the role of capacitors in high-speed PCB design. Ipcb is also provided to PCB manufacturers and PCB manufacturing technology.