PCB Blog - Talking about the design of decoupling capacitors in PCB board

On the PCB board, bypass and decoupling refer to preventing useful energy from passing from one circuit to another and changing the transmission path of noise energy, thereby improving the quality of the power distribution network. It has three basic concepts: power, ground planes, and power connections for components and inner layers. Decoupling is the release of RF energy from the power supply side of the high-frequency device to the power distribution network when the device switches at high speed. Decoupling capacitors also provide a localized DC source for devices and components, which is useful in reducing current-borne surge spikes across the board.

In digital circuits and IC controller circuits, power supply decoupling is necessary. A momentary spike occurs in the power distribution network without decoupling capacitors when the DC energy is dissipated by the element switching. This is because there is a certain inductance in the power supply network, and the decoupling capacitor can provide a local power supply with no inductance or a small inductance. By keeping the voltage at a constant reference point through decoupling capacitors, false logic transitions are prevented, and noise generation is reduced because it provides a loop area for high-speed switching currents instead of components and remote power supplies a large recirculation area in between. Decoupling capacitors in the PCB can greatly reduce the current loop area. Another function of the decoupling capacitor is to provide a local energy storage source, which can reduce the radiation path of the power supply. The generation of RF energy in a circuit is proportional to I·A·f, where I am the return current; A is the area of the loop, and f is the frequency of the current. Since the current and frequency are determined at the time of device selection, it is important to reduce the current loop area in order to reduce radiation. In circuits with decoupling capacitors, current flows in small RF current loops, reducing RF energy. Small loop areas can be obtained by placing decoupling capacitors.

ΔU is the noise generated by L·di/dt on the ground wire, which flows in the decoupling capacitor. This ΔU drives the ground structure on the board and the common-mode voltage in the distribution system to flow across the board. Therefore, reducing ΔU is related to ground impedance, as well as the usage and location of decoupling capacitors.

Decoupling is also a way to overcome physical and timing constraints by providing a low impedance power supply between signal and power lines and planes. Before the frequency increases to the self-resonance point, the impedance of the decoupling capacitor will become lower and lower as the frequency increases, so that the high-frequency noise will be effectively discharged from the signal line, and the remaining low-frequency radiation energy will be There will be no impact. According to the principle of decoupling capacitors, if the difficulty of absorbing energy from the power line is increased, most of the energy will be obtained from the decoupling capacitor, giving full play to the role of the decoupling capacitor, and at the same time, the power line will also produce a smaller di/ dt noise. According to such a method, the impedance on the power supply line can be artificially increased.

It is a common method to connect ferrite beads in series on the power supply line of the IC. Since the ferrite beads present a large impedance to high-frequency current, the effect of the power supply decoupling capacitor is enhanced. Bypass is the discharge of unnecessary common-mode RF energy away from components or cables. Its essence is to create an AC branch to discharge the undesired energy from the susceptible area. In addition, it also provides filtering functions. Its filtering capability is obviously also limited by its own bandwidth. Bypasses are sometimes referred to collectively as filtering designs. Bypassing or filtering is usually applied between power supply and ground, between signal and ground, or between different grounds. It is different from decoupling. But the same is true for how capacitors are used, so what is generally described capacitors applies to both decoupling and bypassing. Energy storage is used to maintain a constant DC voltage and current supplied to the device when the signal pins used are switched simultaneously under a capacitive load. It also prevents power dips due to di/dt current surges in the device. If decoupling is the category of high frequency, then energy storage can be understood as the category of low frequency on PCB board.