PCB Technical - PCB routing trace capacitance calculator

A capacitor,also known as "capacitance",refers to the amount of free charge stored under a given potential difference, and is one of the common and important passive devices in circuit design. Capacitors often play an important role in high-speed circuits.

PCB routing bypass capacitor refers to the addition of capacitors next to circuit routing in PCB design to reduce noise and improve signal integrity and stability. In high-frequency circuit design, bypass capacitance is a commonly used solution, which can effectively eliminate noise interference in the circuit and improve the quality of signal transmission.

A bypass capacitor is a type of capacitor typically used in parallel with a power supply or a specific signal line. Its primary function is to provide a rapid supply of current to the circuit to meet transient current demands and to filter out unwanted high-frequency signal interference. Bypass capacitors can be designed to help minimize power supply noise, protect circuit components, and improve the performance and stability of electronic equipment.

A capacitor is an abbreviation for a capacitor, consisting of two conductors that are close to each other, with a layer of non-conductive insulating medium sandwiched between them. When a voltage is applied between the two plates of a capacitor, the capacitor will store charge.

Capacitors connected in parallel with the load in the circuit can resist voltage changes caused by current changes, making the working voltage of the load more stable.

The use of capacitors, resistors, and inductors in the signal transmission circuit can form various filters to purify the signal.

How to place capacitors in PCB design

Capacitors play an important role in high-speed PCB design and are usually the most commonly used device on PCBs. In PCB, capacitors are usually divided into filtering capacitors, decoupling capacitors, energy storage capacitors, etc.

1.Power output capacitor,filtering capacitor

We usually refer to the capacitors in the input and output circuits of the power module as filtering capacitors. Simply understood, it is a capacitor that ensures stable input and output power supply. The principle of placing filter capacitors in the power module is "first large and then small".

When designing the power supply,attention should be paid to the wiring and copper sheet being wide enough, and the number of through holes being sufficient to ensure that the current capacity meets the requirements.Evaluate by combining the width and number of vias with the current magnitude.

2.decoupling capacitors

The power pins of high-speed ICs require sufficient decoupling capacitors, and it is best to ensure that each pin has one. In actual design, if there is no space to place decoupling capacitors, they can be removed as appropriate.

The capacitance value of the decoupling capacitor of the IC power supply pin is usually relatively small, such as 0.1 μ F. 0.01 μ F, etc; The corresponding packaging is also relatively small, such as 0402 packaging, 0603 packaging, etc. When placing decoupling capacitors, the following points should be noted.

1) Place as close to the power pins as possible, otherwise decoupling may not be effective. In theory, capacitors have a certain range of decoupling radius, so the principle of proximity should be strictly followed.

2) The lead from the decoupling capacitor to the power pin should be as short as possible, and the lead should be thickened, usually with a line width of 8-15 mils (1mil=0.0254mm). The purpose of bolding is to reduce the lead inductance and ensure power performance.

3) After the power and ground pins of the decoupling capacitor are led out from the solder pad, holes are drilled nearby and connected to the power and ground plane. The lead wire should also be thickened, and the through-hole should be as large as possible. If a hole with a diameter of 10mil can be used, an 8mil hole is not needed.

4) Ensure that the decoupling loop is as small as possible. The common BGA packaging typically has decoupling capacitors placed underneath the BGA, i.e. on the back. Due to the high pin density of BGA packaging, decoupling capacitors are generally not placed in large quantities, but they should be placed as many as possible.

3.Energy storage capacitor

The function of energy storage capacitors is to ensure that the IC can provide electrical energy in the shortest possible time when using electricity. The capacitance value of energy storage capacitors is generally relatively large, and the corresponding packaging is also relatively large. In a PCB, the energy storage capacitor can be located further away from the device, but it cannot be too far either.

The principles of capacitor fan holes and fan wires are as follows.

1) Try to make the lead as short and bold as possible to reduce parasitic inductance.

2) For energy storage capacitors or devices with high overcurrent, it is advisable to drill as many holes as possible.

3) Of course, the best electrical performance fan hole is the hole in the disc. Actual needs to be comprehensively considered.

In modern electronic design, bypass capacitors play a crucial role in PCB routing.They not only help to improve the stability and performance of the circuit, but also have a significant impact on signal integrity and power supply interference suppression.

1.Noise Filtering

One of the main uses of bypass capacitors is to effectively filter out noise on the power supply.These capacitors reduce the impact of power supply noise on a circuit by providing a low impedance path that directs high frequency noise signals directly to ground. Especially in high-speed digital circuits, bypass capacitors can significantly improve signal quality and ensure stable circuit operation.

2.Transient current support

In the circuit, some devices will instantly require a large amount of current when switching. Bypass capacitors can quickly provide these transient currents to ensure that integrated circuits (ICs) always receive a stable supply voltage during operation. By minimizing instantaneous changes in supply voltage, bypass capacitors help maintain system stability and reliability.

3.Input and Output Filtering

Bypass capacitors provide frequency selective filtering at the input and output, allowing DC signals to pass through while blocking AC signals. This feature is especially important in power supply design to minimize interference with sensitive components. They ensure the quality of signal transmission of the circuit at different frequencies and enhance the performance of the entire system.

4.Considerations in PCB layout design

In PCB layout design, bypass capacitors should be placed close to the power supply pins to ensure that they can respond quickly to current demand. At the same time, the designer needs to minimize the distributed inductance between the bypass capacitors and other components to improve their performance. This layout maximizes the filtering effect of bypass capacitors on high-frequency noise.

Bypass capacitors have multiple important uses in PCB routing,including noise filtering,transient current support and input/output filtering.Through proper layout and design, bypass capacitors can significantly improve the stability and performance of circuits,providing a more reliable operating environment for electronic devices.