Electronic Design - The role of pcb capacitors in PCB design

PCB capacitors are passive electronic components used in printed circuit boards (PCBs) that are primarily used to store an electrical charge and release it in a circuit. They usually consist of two conductive plates and a layer of insulating medium, a construction that allows the capacitor to store electrical energy and release it when needed.

A capacitor should actually consist of six parts.In addition to its own capacitance C,it consists of the following parts.

1.Equivalent series resistance ESR RESR:The equivalent series resistance of a capacitor consists of the capacitor's pin resistance and the equivalent resistance of the two pole plates of the capacitor in series. RESR causes the capacitor to dissipate energy (and thus cause losses) when a large AC current flows through it.This can have serious consequences for RF circuits with high ripple currents and power supply decoupling capacitors.The capacitors with the lowest RESR are mica capacitors and film capacitors,but they do not have much effect on precision high-impedance small-signal analog circuits.

2.Equivalent series inductance ESL LESL:The equivalent series inductance of a capacitor consists of the pin inductance of the capacitor and the equivalent inductance of the two pole plates of the capacitor in series. Like RESR,LESL can be a serious problem in RF or high-frequency operating environments,although the precision circuits themselves work fine in DC or low-frequency conditions.The reason for this is that the transistors used in precision analog circuits have gain even when the hopping frequency extends to hundreds of megahertz or gigahertz,allowing low inductance amplification of resonant signals.This is the main reason why the power supply side of the circuit should be properly decoupled at high frequencies.

3.Equivalent parallel resistance EPR RL is what we usually call capacitive leakage resistance.RL is an important parameter in AC coupling applications,storage applications (e.g. analog integrators and sample holders) and when using capacitors in high impedance circuits.The charge in an ideal capacitor should vary only with external current.However, RL in an actual capacitor causes the charge to leak slowly at a rate determined by the RC time constant.

4.Two parameters, RDA and CDA, are also capacitance distribution parameters,but they have little effect in practical applications and are not described here.So there are three important capacitance distribution parameters: ESR, ESL and EPR.The most important of these are ESR and ESL.In fact,only the RLC simplified model is used in analyzing the capacitance model, i.e., analyzing the capacitance in terms of C, ESR and ESL.

5.Based on the introduction of the detailed model,let's talk about the two types of capacitors that are often used in our designs.

6.Electrolytic capacitors (e.g.,tantalum and aluminum electrolytic capacitors) have large capacities.They are not suitable for storage and coupling due to their low isolation resistance, i.e., small equivalent parallel resistance EPR and high leakage current (typical value 520 na/μ f).Electrolytic capacitors are more suitable as bypass capacitors for power supplies to stabilize them. 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 high-frequency circuit decoupling capacitors,because its equivalent series inductance is very low,that is,the equivalent series inductance ESL is very small, and decoupling band is very wide.This has a lot to do with its structure.Monolithic ceramic capacitors consist of multiple layers of sandwiched metal and ceramic films that are arranged in parallel with the busbar rather than wound in series.

The role of capacitors in pcb

1. Coupling role

Capacitors in the PCB board is mainly responsible for the signal coupling role, especially in the low-frequency signal transmission. Through the coupling capacitance, low-frequency signals can be effectively transmitted between the circuits, to prevent the static work of the circuit before and after the mutual interference.

2.Filtering

Filtering is one of the very important functions of capacitors in the circuit.Capacitors can suppress unwanted high-frequency signals, as a filter on the power line or signal line,so that the power output is more stable.Large-capacity capacitors are usually responsible for low-frequency filtering,while small-capacity capacitors are used for high-frequency filtering to achieve the best signal quality.

3. Decoupling

The main function of decoupling capacitors is to provide a low impedance power path for integrated circuits (ICs) to eliminate noise in the power supply system, thus ensuring circuit stability. These capacitors usually need to be placed as close as possible to the power supply pins of the IC they serve to minimize inductance and impedance.

4. Energy Storage

The main purpose of an energy storage capacitor is to quickly provide the power needed when the circuit is powered. Energy storage capacitors usually have a large capacitance value, can buffer voltage fluctuations in the sudden increase in power demand, so as to ensure the stability of the power supply. In the design, the layout of the storage capacitor should also consider the relative position to the load to ensure its efficiency.

5. Noise filtering

Capacitors can be realized through the bypass circuit of the interference signal filtering. Bypass capacitors are often used to bypass high frequency noise, ensuring that when the signal is amplified, only low frequency signals make it to the next level of the circuit. This is critical to improving the signal integrity of the circuit.

6. Importance of Layout

The layout of capacitors on a PCB has a direct impact on circuit performance. A proper layout not only reduces signal interference, but also improves the integrity of the power supply. Capacitors should be placed in the current path as far as possible to ensure that it gives full play to the filtering and decoupling functions

How to choose the right capacitor when designing a PCB?

1. Determine the application requirements

Before choosing the right capacitor, you first need to define the specific application of the capacitor in the circuit. For example, capacitors can be used for different functions such as filtering, decoupling, coupling or energy storage. Different types of capacitors are suitable for different purposes, such as decoupling capacitors are mainly used to filter out high-frequency noise in the power supply, while filtering capacitors are used for power system stability and noise suppression.

2. Consider the type of capacitor

The selection of capacitors should be based on the actual need for the type to determine. Common types of capacitors include aluminum electrolytic capacitors, ceramic capacitors and tantalum capacitors. Aluminum electrolytic capacitors are suitable for high-capacity, low-frequency power bypass, while ceramic capacitors are suitable for high-frequency decoupling circuits due to their low equivalent series inductance.

3. Selection according to capacitance and voltage level

The correct choice of capacitance value is crucial,the capacitor capacity should be able to meet the circuit requirements,while its rated voltage needs to be higher than the highest operating voltage in the circuit.Ensure that there is sufficient voltage margin to avoid triggering capacitor failure under operating conditions.

4.Consider environmental factors

Factors such as temperature,humidity and vibration of the environment in which they are used can also affect the selection of capacitors.Certain types of capacitors perform poorly at high temperatures or in extreme environments,so capacitors suitable for the specific operating environment should be selected.

5.Frequency Response and ESR

In the design of high-frequency circuits, you need to pay attention to the frequency characteristics of capacitors and equivalent series resistance (ESR).Selection of capacitors with lower ESR helps to improve circuit performance, especially in the case of rapid signal changes,low ESR can reduce power loss and heat generation.

6.Package Type and Layout

The package type of the capacitor may affect the layout of the board and the density of electronic components.Therefore, it is necessary to choose a package form suitable for PCB design. A proper layout ensures that the capacitors can be connected to the circuit in an optimal way to minimize interference and improve performance.

PCB capacitors play a vital role in printed circuit boards (PCBs), assuming multiple functions such as coupling, filtering, decoupling, energy storage and noise filtering.Choosing the right capacitors not only improves circuit stability and signal integrity, but also optimizes circuit performance under different operating conditions.