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Introduce the application of high-speed PCB design capacitor
08Nov
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Introduce the application of high-speed PCB design capacitor

Introduce the applICation of high-speed PCB design capacitor
The Chinese name of PCB is printed circuit board because it is made of electronic printing, so it is calLED "printed circuit board" The development of PCB in China started in 1.956. From 1963 to 1978, it gradually expanded and formed the PCB industry Over the past 20 years of reform and opening up, thanks to the introduction of foreign advanced technology and equipment, single-sided, double-sided and multilayer boards have developed rapidly, and the domestic PCB industry has gradually grown from SMAll to large In 2002, it beCAMe the third largest PCB producer In 2003, the annual output value and import and export volume of PCB exceeded US $6 billion, making it the second largest global producer of PCB In recent years, China's PCB industry has maintained a rapid growth of about 20%. It is expected to surpass Japan in 2010 and become an active national PCB output value and scientific and technological development Capacity (or capacity) is a physical quantity that characteristics the ability belongs to a capacitor to hold charge The electric quantity needed to add 1V to the potential difference between the two plates of the capacitor is called the capacitance of the capacitor In physics, a capacitor is a static charge storage medium It is mainly used for power filter, signal filter, signal coupling, brightness, DC blocking and other circuits
Printed circuit board


pcb board


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.  It is mainly used for AC coupling of Ghz signal
2) Decoupling capacitors.  It is mainly used to prevent power or ground noise of high-speed circuit board
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 paper, we will mainly discuss the second type of decoupling capacitors Capacitors are 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 capacity
In order to apply capacitors correctly and reasonably, it is naturally necessary to understand the specific model of capacitors and the specific meaning and function of each distributed parameter in the model As with other components, actual capacitors are different from "ideal" capacitors Due to its packaging, "real" capacitors have additional inductance and resistance characteristics, information, etc, Additional "parasitic" must be used Characterized by "element" or "non ideal" characteristics in the form of resistance and inductance elements, nonlinear and dielectric memory characteristics From the above figure, we can see that the capacitor should actually be composed of six parts There are the following components:
2.1 Equivalent series resistance ESR RESR: the equivalent series resistance of the capacitor consists of the pin resistance of the capacitor and the equivalent resistance of the two plates of the series capacitor RESR causes the capacitor to dissipate energy (and thus losses) when there is a large AC current flowing through the capacitor.  This may have serious consequences for RF circuits carrying high ripple current and power decoupling capacitors However, it does not greatly affect the high impedance accuracy of small signal analog circuits The capacitors of RESR are mica capacitors and film capacitors
2.2 Equivalent series inductance ESL language, LESL: The equivalent series inductance of a capacitor consists of the pin inductance of the capacitor and the equivalent inductance of two plates of the series capacitor Like RESR, LESL may have serious problems in RF or HF operation, although the precision circuit itself works well under DC or low frequency The reason is that the gain of transistors used in precision analog circuits at transition frequencies can be extended to hundreds of MHz or gigahertz, and resonant signals can be amplified with very low inductance values
2.3 Equivalent parallel resistance EPR RL: This is what we usually call capacitor leakage resistance. RL is an important factor in AC coupling applications. It analogizes to memory applications such as integrators and sample and hold, when capacitors are used in high impedance circuits Parameter, the charge in the ideal capacitor should only change with the external current However, RL in a real capacitor causes the charge to leak slowly at a rate determined by the RC time constant
2.4 The two parameters RDA and CDA are also capacitance distribution parameters, but they have relatively little impact in practical applications, so they are not introduced here Therefore, capacitance has three important distribution parameters: ESR, ESL, and EPR The most important are ESR and ESL In fact, RLC is usually used to SIMplify the capacitance model
2.5 Now, on the basis of introducing the detailed model, we will discuss two kinds of capacitors often used in our design
2.6 Electrolytic capacitors (such as tantalum capacitors and aluminum electrolytic capacitors) have a large capacity.  So the leakage current is very large (typical value is 5~20nA/mF) due to its low isolation resistance and very small equivalent parallel resistance EPR Electrolytic capacitors are more suitable as bypass capacitors for power supply to stabilize power supply
2.7 Monolithic ceramic capacitors are more suitable for decoupling capacitors in high-frequency circuits because their equivalent series inductance is very low, that is, the ESL of equivalent series inductance is very small, and they have a wide decoupling band This has a lot to do with his structure The monolithic ceramic capacitor is composed of multilayer metal films and ceramic films, which are arranged in parallel with the bus instead of being wound in series of.
2. On August this week, we discussed the detailed equivalent model of capacitor I believe that now everyone should have a deep understanding of capacitors We will continue our discussion next week We will analyze the simplified equivalent model of capacitors often used in practical applications, The origin and significance of his impedance curve
3. Simplified model of capacitance and impedance curve
For the convention of analysis, RLC model consisting of series equivalent resistance ESR 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/long-term evolution technology In WCDMA system, RLC layer is located above MAC layer and is a part of L2. It provides segmentation and retransmission services for user 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), Acquired 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 service does not use the PDCP and BMC protocols, the RLC provides the radio bearer (RB) to the upper layer; Otherwise, RB services are provided by PDCP or BMC Our criteria for selecting capacitors are: 1. The lowest possible ESR capacitor 2. The resonant frequency value of capacitor is on PCB

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