What is the concept of decoupling capacitor
When it comes to PCB design, you may have heard of the term decoupling capacitor, which in fact can always be found surrounding main components such as regulators or load supply elements and integrated circuits. In fact, a decoupling capacitor, is an important isolation element for high-frequency noise elements (sometimes know as AC elements also known an AC signal) of a power supply or line signal in order to provide a stable output to the receiving element as it can be. Therefore, decoupling capacitors are provided to improve the feedback and IC supply elements of a specific circuit it is connected to.
What is the purpose advantage of a decoupling capacitor
The objective of using decoupling capacitors is to stabilize as much as possible the supply for sensitive components, providing removal of artifact noise on the circuit to specific set of components. Nonetheless, it is important to consider the different types of power supplies within the rails on the design, since there will be different kind on noises and capacitance considerations to implement a resilient network with higher life span. Consider also that capacitors produce impedance, and in such sense, decoupling capacitors are also suitable for signal filtering to cut off specific frequencies according to the relative RLC network, which produce good applications for audio design.
What type of capacitor is use for decoupling
There are different types of capacitors and special properties according to their core material. In the case of decoupling capacitors, it is important to consider precision of the capacitance value, equivalent resistance value (ESR), maximum DC voltage rating as well as cost.
While tantalum based decoupling capacitors are very precise on their capacitance value (suitable for high-sensitivity components and power networks), the bigger the size, the higher the price. On the other hand, ceramic capacitors prize is lower than that of tantalum, but might not be as suitable in precision, however they are still an option for middle-sensitivity networks such as RF circuits.
Finally, there are the aluminum based electrolytic capacitors, which will come bigger in size and DC maximum rating, but the life span could be also a factor to worry about when using them on special designs.
Besides, The film capacitor, as a decoupling capacitor, is equivalent to a battery, which satisfies the change of current in the circuit and avoids coupling interference.
Film capacitors are electronic components made of metal foil, polypropylene, polystyrene and other materials to contain electric charges and commonly used on the high power side of power supplies.
What are decoupling capacitors used
As stated above, decoupling capacitors provide a discharge whenever there is no voltage on the network, hence their use for stabilizing high-frequency signals to a single value. Therefore, they should be used to parts of the circuit network in which high-sensitivity and special feedback conditions must be implemented against random voltage changes.
How to place decoupling capacitor
The placement of a decoupling capacitor will have to be surrounding and as close as possible of the decoupling element, more on this below. However, the placement of nearby vias for ground decoupling among planes must also follow standard alignment, which can be better placed closer to the decoupling capacitor footprint on a symmetric design.
In simple terms, place the decoupling capacitor as close to the supply pin as possible and use the shortest possible trace. Therefore, the arrangement of the decoupling capacitor is generally placed as close as possible to the PIN pin of the electric chip VCC and GND.
What is difference between decoupling and bypass capacitor
As we said above, capacitors can be divided into many kinds of capacitors according to the application and characteristics, and decoupling capacitors and bypass capacitors are often compared and used. Here we will introduce the difference between decoupling capacitors and bypass capacitors.
The way a decoupling capacitor and a bypass capacitor are used have to do with the impedance and voltage regulation (sensitivity). While a decoupling capacitor is utilized after power up in order to maintain a stabilized supply of voltage to a component or network, the bypass capacitor must ensure low impedance for high-frequency signals providing a flow path of the signal through a nearby resistor. In this way, a low-pass signal is guaranteed to provide electromagnetic compliance and proper component functionality.
In addition, they differ in that coupling capacitors are typically used to couple two linearly independent sources and shield the signal from power supply noise on one side, while the function of bypass capacitors is to avoid that of nonlinear components, such as field-effect tube (FET).
How do you calculate decoupling capacitors
Typical decoupling capacitor values are around one to forty-seven nanofarads. The calculation of the overall decoupling capacitance is aligned with the resonant frequency. To avoid inductor behavior of the capacitor, it should not fall above the target frequency. It should be adapted to conventional commercial value of capacitors and divided among as many numbers of capacitors of similar values that can be placed on a “symmetrical” manner. Say, a total decoupling capacitance of 100nF may be split among ten 10nF capacitors in parallel that placing one 47nF and five 10nF capacitors, in order to ensure similar distribution of undesired signals.
Finally, consider that the size of the utilized decoupling capacitors has to do with the loop inductance, hence the balancing of proper component packages shall adequate the appropriate loop inductance within the specific network.
Where do you place a decoupling capacitor
The positioning of decoupling capacitor is according to the component it will be “attached” to. Sometimes appropriate guidelines or rules of thumb are provided by the integrated circuit manufacturer, but also attending to common considerations. Among the main considerations, decoupling capacitors should be placed as near as possible from the relative component pin in series, while they should be placed in parallel on power networks and resistors for EMI reduction. Furthermore, it is a good design practice to put the decoupling capacitors on the component plane and via to ground plane. Finally, it is a good practice to place more than one capacitor on the network where the capacitance have high values, in order to distribute the noise and prevent for a single capacitor failure.
What happens if a decoupling capacitor fails
The most common failure of a decoupling capacitor will result into shorts within the circuit which can burn the board in the case it shorts the power network. On the other hand, and with less probability, they can get physically cracked and provide issues within the board layers due to the cracking effect. In any case, distributing the overall capacitance over two or more capacitors is a good practice to prevent these kinds of failures, which might now always result on catastrophic events for the board, but can prevent undesired noise emissions. Finally, consider the life span of capacitors in order to prevent wear-off effect of the overall circuit through periodic maintenance.