PCB News - The role of capacitors in circuit board design

Introduction: Capacitor is a kind of passive component of storage container. All aspects of our life are inseparable from electronic products related to capacitors. Capacitors can be seen everywhere. Everything is due to the existence of capacitors. Let's learn together Let's look at the role of capacitance in the circuit.

The role of capacitors-filtering

Filtering is a very important part of the role of capacitors. It is used in almost all power circuit board capacitors. Theoretically (that is, assuming that the capacitor is a pure capacitor), the larger the capacitor, the smaller the impedance, and the higher the passing frequency. But in fact, most of the capacitors over 1uF are electrolytic capacitors, which have a large inductance component, so the impedance will increase when the frequency is high. Sometimes you will see a large electrolytic capacitor with a small capacitor connected in parallel. At this time, the large capacitor is connected to the low frequency and the small capacitor is connected to the high frequency. The function of the capacitor is to pass the high frequency and block the low frequency. The larger the capacitance, the easier it is to pass low frequencies, and the smaller the capacitance, the easier it is to pass high frequencies. Specifically used in filtering, large-capacity capacitors filter low frequencies, and small-capacity capacitors filter high frequencies.

Some netizens compare the filter capacitor to a "pond." Since the voltage at both ends of the capacitor does not change suddenly, it is vividly said that the capacitor is like a pond, because the water in the pond will not change due to the addition or evaporation of a few drops of water. It converts the change in voltage into a change in current, thereby buffering the output voltage. Filtering is the process of charging and discharging. Play a role in stabilizing the output voltage.

The role of circuit board capacitors bypass

The main function of the bypass capacitor is to generate an AC shunt, that is, when the signal mixed with high frequency and low frequency is amplified by the amplifier, it is required that only low frequency signals are allowed to be input to the next stage when passing through a certain stage, without high frequency When the signal enters, add a proper size grounding capacitor at the input end of the stage, so that the higher frequency signal can be easily bypassed through this capacitor (this is because the capacitor has a small impedance to high frequency), and the low frequency signal is due to the capacitor. The impedance to it is large and it is sent to the next stage of amplification.

The role of capacitors-decoupling

"Decoupling" is also called "decoupling". The decoupling capacitor is connected to the signal output terminal. Both of these capacitors play an anti-interference role. Let us first talk about the role of the decoupling capacitor in the circuit.

The decoupling capacitor functions as a battery to meet the change of the drive circuit current and avoid mutual coupling interference. Combining bypass capacitors and decoupling capacitors will make it easier to understand. The bypass capacitor is actually decoupling, but the bypass capacitor generally refers to high-frequency bypass, that is, to provide a low-impedance discharge path for high-frequency. High-frequency bypass capacitors are generally relatively small, while decoupling capacitors are generally relatively large, which are determined according to the distribution parameters in the circuit and the magnitude of the drive current change.

Bypass is to take the interference in the input signal as the filtering object, and decoupling is to take the interference of the output signal as the filtering object to prevent the interference signal from returning to the power supply. This should be their essential difference.

The role of capacitors-energy storage

When it comes to energy storage, the first thing that comes to mind is the battery, but what the capacitor collects is that the charge is a physical reaction, and the battery is a decomposition chemical reaction. The common capacitor energy storage has a magnetizer, a capacitor welding machine and other occasions that pass high voltage and high current. When using capacitors to store energy, a large capacitor or a capacitor group composed of a number of small capacitors in parallel is generally used. The specific capacity and withstand voltage should be selected according to demand.

The role of capacitance-coupling

Capacitive coupling is also called "electric field coupling". Coupling refers to the process of signal transmission from the first stage to the second stage. Generally, it usually refers to AC coupling when it is not specified.

From the circuit point of view, it can always be divided into the driving power supply and the driven load. If the load capacitance is relatively large, the drive circuit must charge and discharge the capacitance to complete the signal jump. When the rising edge is relatively steep, the current is relatively large, so that the drive current will absorb a large power supply current. The inductance and resistance (especially the inductance on the chip pins will bounce). Compared with the normal situation, this current is actually a kind of noise, which will affect the normal operation of the previous stage. This is coupling.

The role of capacitance-resonance

The change between voltage and current generated by capacitors and other passive components actually uses the characteristics of capacitor charging and discharging. Generally, there are parallel resonance and series resonance of capacitors, and can also be combined into filters for engineering applications such as notch filters through the series-parallel combination of resonant capacitors.

The role of capacitors-time constant

The time constant refers to a constant that represents the time course of the transition reaction. Refers to the time required for the physical quantity to decay from the maximum value to 1/e of the maximum value. The time constant in the capacitor is common in the RC circuit. When the input signal voltage is applied to the input terminal, the voltage on the capacitor (C) gradually rises. The charging current decreases as the voltage rises. The characteristics of current through resistance (R) and capacitance (C) are described by the formula: i = (V/R)e-(t/CR)

The above is an introduction to the role of capacitors in PCB board design. Ipcb is also provided to PCB manufacturers and PCB manufacturing technology