Capacitance is the accumulation of stored charge in a potential difference when conductors have a dielectric between them, impeding charge movement. distributor of electronic components serve important functions in electronics and power, such as filtering and energy storage.
Capacitance, also known as “capacitive reactance,” refers to the amount of free charge stored in a given potential difference, denoted as C, with the international unit being a Farad (F). In general, charges move in an electric field, but when there is a dielectric material between conductors, it impedes the movement of charges and causes them to accumulate on the conductor, resulting in stored charge known as capacitance.
Capacitance refers to the ability to hold a charge. Any static electric field consists of many capacitors, meaning that where there is an electric field, there is capacitance, which is described by the static electric field. It is commonly believed that an isolated conductor and infinity form a capacitor, and connecting the conductor to the ground is equivalent to connecting it to infinity and forming a whole with the earth.
Capacitance (or capacitance value) is a physical quantity that expresses the ability of a capacitor to hold a charge. From a physical point of view, capacitance is a type of static charge storage medium, where the charge may exist permanently, which is its characteristic, and it has a wide range of applications. Capacitors are essential electronic components in the fields of electronics and power, mainly used in power filtering, signal filtering, signal coupling, resonance, compensation, charging and discharging, energy storage, DC blocking circuits, and more.
The capacitance of a capacitor is defined as the ratio of the charge Q stored in the capacitor to the voltage U across its two terminals. In circuit theory, the ability of a capacitor to store charge for a given potential difference is known as capacitance, denoted as C. The international unit of capacitance is the farad (F).
Bypass Capacitors
Bypass capacitors serve as energy storage devices that provide power to local components, smoothing out the output of voltage regulators and reducing load demands. Similar to small rechargeable batteries, bypass capacitors can be charged and discharged to supply energy to devices. To minimize impedance effectively, bypass capacitors should be placed as close as possible to the power supply and ground pins of the load device. This placement is crucial for preventing excessive input values that could cause a rise in ground potential and noise. Ground potential refers to the voltage drop at the ground connection when there is a surge of large current spikes.
Decoupling Capacitors
Decoupling, also known as decoupling, involves distinguishing between the driving source and the driven load in a circuit. When the load capacitance is significant, the driving circuit needs to charge and discharge the capacitor to facilitate signal transitions. During steep rising edges, the current demand is high, which causes the driving current to draw substantial power supply current. Due to inductance and resistance within the circuit (especially inductance at chip pins), this can lead to ringing—a form of noise relative to normal conditions, disrupting the operation of preceding stages, known as “coupling.”
Decoupling capacitors act like a “battery,” meeting the changing current demands of the driving circuit and preventing mutual coupling interference. They further reduce the high-frequency impedance between the power supply and the reference ground in the circuit.
Understanding bypass and decoupling capacitors together makes it easier. Bypass capacitors also serve a decoupling function, but they are generally referred to as high-frequency bypasses, providing a low-impedance path for high-frequency switching noise. High-frequency bypass capacitors are typically smaller, with values like 0.1μF or 0.01μF chosen based on resonant frequencies. In contrast, decoupling capacitors are larger, possibly 10μF or more, determined by distributed parameters in the circuit and the size of the driving current changes. Bypassing targets interference in the input signal for filtration, whereas decoupling targets interference in the output signal to prevent noise from returning to the power source. This is the essential difference between them.
Filtering
Theoretically (assuming pure capacitance), the larger the capacitance, the lower the impedance and the higher the frequency that can pass through. However, in reality, capacitors larger than 1μF are mostly electrolytic and have significant inductive components, so their impedance actually increases at higher frequencies. Sometimes, a large electrolytic capacitor is seen in parallel with a smaller one; the larger capacitor filters low frequencies, and the smaller one filters high frequencies. The role of a capacitor is to allow AC to pass while blocking DC, passing high frequencies and blocking low ones. The larger the capacitor, the easier it is for high frequencies to pass through. In filtering applications, large capacitors (1000μF) filter low frequencies, while small capacitors (20pF) filter high frequencies. An analogy likens filtering capacitors to “water ponds” because the voltage across a capacitor does not change abruptly. Therefore, the higher the signal frequency, the greater the attenuation. Capacitors, like ponds, do not fluctuate significantly in volume with the addition or evaporation of a few drops of water. They convert voltage variations into current changes, with higher frequencies leading to larger peak currents, thereby buffering the voltage. Filtering is essentially the process of charging and discharging.
Energy Storage
Energy storage capacitors collect charges through rectifiers and transmit the stored energy to the power supply’s output via converter leads. Aluminum electrolytic capacitors with voltage ratings from 40 to 450VDC and capacitance values ranging from 220 to 150,000μF are commonly used. Depending on different power supply requirements, devices may sometimes use series, parallel, or a combination thereof. For power sources exceeding 10KW, large can-type screw terminal capacitors are often utilized.