PCB Blog - Introduce the application of high-speed PCB board design capacitors

The Chinese name of PCB is printed circuit boards because it is made by electronic printing, so it is called "printed" circuit board. my country's PCB board development work began in 1956, and from 1963 to 1978, it gradually expanded to form the PCB board industry. More than 20 years after the reform and opening up, due to the introduction of foreign advanced technology and equipment, single-sided, double-sided and multi-layer boards have achieved rapid development, and the domestic PCB board industry has gradually developed from small to large. In 2002, it became the third largest PCB board producer. In 2003, the output value and import and export volume of PCB board exceeded 6 billion US dollars, making it the second largest PCB board producer in the world. my country's PCB board industry has maintained a rapid growth of about 20% in recent years, and is expected to surpass Japan around 2010, becoming a country with active PCB board output value and technological development. Capacitance (or capacitance) is a physical quantity that characterizes the ability of a capacitor to hold charge. The amount of electricity required to increase the potential difference between the two plates of a capacitor by 1 volt is called the capacitance of the capacitor. In terms of physics, a capacitor is a static charge storage medium (like a bucket, you can charge the charge into it, in the absence of a discharge loop, it is more obvious to remove the dielectric leakage and self-discharge effect / electrolytic capacitors, there may be electric charge, which is its characteristic), it has a wide range of uses, and it is an indispensable electronic component in the field of electronics and electricity. Mainly used in power supply filtering, signal filtering, signal coupling, resonance, DC blocking and other circuits.

1. The classification of capacitors
Capacitors are classified by application in circuit design, and capacitors can be divided into four categories:
1) AC coupling capacitor. Mainly used for AC coupling of Ghz signals.
2) Decoupling capacitors. Mainly used to keep out noise from the power supply or ground of high speed circuit boards.
3) Capacitors used in active or passive RC filtering or frequency selection networks.
4) Capacitors used in analog integrators and sample-and-hold circuits.

In this article we will mainly discuss the second type of decoupling capacitors. Capacitors are classified from the materials and processes of manufacture, mainly in the following different forms:
1.1 NPO Ceramic Capacitors
1.2 Polystyrene Ceramic Capacitors
1.3 Polypropylene capacitors
1.4 PTFE capacitors
1.5 MOS capacitors
1.6 Polycarbonate Capacitors
1.7 Mylar capacitors
1.8 Monolithic Ceramic Capacitors
1.9 Mica capacitors
1.10 Aluminum Electrolytic Capacitors
1.11 Tantalum Electrolytic Capacitors

2. The specific model and distribution parameters of capacitance
In order 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 distribution parameter in the model. Like other components, the actual capacitor is different from the "ideal" capacitor. The "real" capacitor has the additional characteristics of inductance and resistance due to its packaging, materials, etc., and must use additional "parasitic". Characterized by "component or "non-ideal" properties in the form of resistive and inductive elements, nonlinear and dielectric memory properties. From the diagram above we can see that the capacitor should actually consist of six parts. In addition to its own capacitor C, there are the following components:
2.1 Equivalent series resistance ESR RESR: The equivalent series resistance of the capacitor is composed of the pin resistance of the capacitor and the equivalent resistance of the two plates of the capacitor in series. RESR causes the capacitor to dissipate energy (and thus losses) when there is a large AC current flowing through the capacitor. This can have serious consequences for RF circuits and power supply decoupling capacitors that carry high ripple currents. However, it will not have a great impact on precision high-impedance, small-signal analog circuits. RESR's capacitors are mica capacitors and film capacitors.
2.2 Equivalent series inductance ESL, LESL: The equivalent series inductance of the capacitor is composed of the capacitor's pin inductance and the equivalent inductance of the two plates of the capacitor in series. Like RESRs, LESLs can have serious problems with RF or high frequency operation, although the precision circuits themselves work fine at DC or low frequencies. The reason for this is that transistors used in precision analog circuits have gain at transition frequencies extending to hundreds of megahertz or gigahertz, and can amplify resonant signals with very low inductance values.
2.3 Equivalent Parallel Resistance EPR RL: This is what we usually call capacitor leakage resistance, and RL is an important factor in AC-coupled applications, storage applications such as analog integrators and sample-and-holds, and when capacitors are used in high-impedance circuits. parameter, the charge in an ideal capacitor should vary only with external current. However, RL in a real capacitor causes charge to leak slowly at a rate determined by the RC time constant.
2.4 The two parameters RDA and CDA are also the distribution parameters of the capacitance, but the influence is relatively small in the actual application, so it will not be introduced here. Therefore, there are three important distribution parameters of capacitance: ESR, ESL, and EPR. The most important ones are ESR and ESL. In fact, when analyzing the capacitance model, only RLC is generally used to simplify the model.
2.5 Now, on the basis of introducing the detailed model, we talk about two kinds of capacitors that are often used in our design.
2.6 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 (typical value is 5~20nA/μF), so it is Not suitable for storage and coupling. Electrolytic capacitors are more suitable for bypass capacitors of power supplies to stabilize power supply.
2.7 Monolithic ceramic capacitors are more suitable for decoupling capacitors in high-frequency circuits, because they have very low equivalent series inductance, that is, the ESL of the equivalent series inductance is small, and they have a wide decoupling frequency band. This has a lot to do with his structural composition. Monolithic ceramic capacitors are composed of multi-layer interlayer metal films and ceramic films, and these multilayer films are arranged in parallel with the bus bars instead of being wound in series. of.
2.8 This week, we talked about the detailed equivalent model of capacitors. I believe that everyone should have a deep understanding of capacitors now. We will continue to talk next week. We will actually analyze the simplified equivalent model of capacitors that are often used in applications., and the origin and significance of his impedance curve.

3. Simplified model of capacitance and impedance curve
For the convenience of analysis, the RLC model consisting of series equivalent resistance ESR, series equivalent inductance ESL and capacitance is often used in practical analysis. RLC (Radio Link Control) is a radio link control layer protocol in wireless communication systems such as GPRS/WCDMA/TD-SCDMA/LTE. In the WCDMA system, the RLC layer is located above the MAC layer and is a part of L2, providing segmentation and retransmission services for users and control data. Each RLC entity is configured by RRC, and there are three modes according to the service type: Transparent Mode (TM), Unacknowledged Mode (UM), Acknowledged Mode (AM). In the control plane, the service provided by the RLC to the upper layer is the radio signaling bearer (SRB); in the user plane, when the PDCP and BMC protocols are not used by the service, the RLC provides the radio bearer (RB) to the upper layer; otherwise, the RB service is provided by the PDCP or BMC bearer. Our criteria for selecting capacitors are: 1. The lowest possible ESR capacitor. 2. The resonant frequency value of the capacitor is as high as possible on PCB board.