Decoupling capacitor: Difference between revisions

Revision as of 16:05, 10 May 2024

Revision as of 08:56, 30 May 2024

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The decoupling capacitor works as the device’s local [[Electric field|energy storage]]. The capacitor is placed between the power line and the [[Ground (electricity)|ground]] to the circuit the current is to be provided. According to the [[Capacitor#Current–voltage relation|capacitor current–voltage relation]]

:<math>i(t) = C \frac{d\,v(t)}{dt},</math>

a voltage drop between a power line and the ground results in a current drawdrawn out from the capacitor to the circuit. When capacitance {{mvar|C}} is large enough, sufficient current is supplied to maintain an acceptable range of voltage drop. The capacitor stores a small amount of energy that can compensate for the voltage drop in the power supply conductors to the capacitor. To reduce undesired [[Parasitic element (electrical networks)|parasitic]] [[equivalent series inductance]], small and large capacitors are often placed in [[Parallel circuit|parallel]], adjacent to individual integrated circuits (see [[#Placement|§ Placement]]).

In digital circuits, decoupling capacitors also help prevent radiation of [[electromagnetic interference]] from relatively long circuit traces due to rapidly changing power supply currents.

Decoupling capacitors alone may not suffice in such cases as a high-power amplifier stage with a low-level pre-amplifier coupled to it. Care must be taken in the layout of circuit conductors so that heavy current at one stage does not produce power supply voltage drops that affect other stages. This may require re-routing printed circuit board traces to segregate circuits, or the use of a [[ground plane]] to improve the stability of power supply.

==Decoupling==

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A bypass capacitor is often used to decouple a subcircuit from AC signals or [[voltage spike]]s on a power supply or other line. A bypass capacitor can [[Shunt (electrical)|shunt]] energy from those signals, or transients, past the subcircuit to be decoupled, right to the return path. For a power supply line, a bypass capacitor from the supply voltage line to the power supply return (neutral) would be used.

High frequencies and transient currents can flow through a capacitor to circuit ground instead of to the harder path of the decoupled circuit, but DC cannot go through the capacitor and continues on to the decoupled circuit.

Another kind of decoupling is stopping a portion of a circuit from being affected by switching that occurs in another portion of the circuit. Switching in subcircuit A may cause fluctuations in the power supply or other electrical lines, but you do not want subcircuit B, which has nothing to do with that switching, to be affected. A decoupling capacitor can decouple subcircuits A and B so that B doesn't see any effects of the switching.

==Switching subcircuits==

In a subcircuit, switching will change the load current drawn from the source. Typical power supply lines show inherent [[inductance]], which results in a slower response to changechanges in current. The supply voltage will drop across these parasitic inductances for as long as the switching event occurs. This transient voltage drop would be seen by other loads as well if the inductance between two loads is much lower compared to the inductance between the loads and the output of the power supply.

To decouple other subcircuits from the effect of the sudden current demand, a decoupling capacitor can be placed in parallel with the subcircuit, across its supply voltage lines. When switching occurs in the subcircuit, the capacitor supplies the transient current. Ideally, by the time the capacitor runs out of charge, the switching event has finished, so that the load can draw full current at normal voltage from the power supply and the capacitor can recharge. The best way to reduce switching noise is to design a [[Printed circuit board|PCB]] as a giant capacitor by sandwiching the power and ground planes across a [[dielectric]] material.{{Citation needed|date=August 2018}}

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==Transient load decoupling==

[[Transient (oscillation)|Transient]] [[Electrical load|load]] decoupling as described above is needed when there is a large load that gets switched quickly. The parasitic inductance in every (decoupling) capacitor may limit the suitable capacity and influence the appropriate type if switching occurs very fast.

[[Logic]] circuits tend to do sudden switching (an ideal logic circuit would switch from low voltage to high voltage instantaneously, with no middle voltage ever observable). So logic circuit boards often have a decoupling capacitor close to each logic IC connected from each power supply connection to a nearby ground. These capacitors decouple every IC from every other IC in terms of supply voltage dips.

These capacitors are often placed at each power source as well as at each analog component in order to ensure that the supplies are as steady as possible. Otherwise, an analog component with a poor [[power supply rejection ratio|power supply rejection ratio (PSRR)]] will copy fluctuations in the power supply onto its output.

In these applications, the decoupling capacitors are often called ''bypass capacitors'' to indicate that they provide an alternate path for high-frequency signals that would otherwise cause the normally steady supply voltage to change. Those components that require quick injections of current can ''bypass'' the power supply by receiving the current from the nearby capacitor. Hence, the slower power supply connection is used to charge these capacitors, and the capacitors actually provide the large quantities of high-availability current.

==Placement==