Decoupling Capacitor or Bypass Capacitor in Electronics, Complete Guide

(Last Updated On: July 7, 2020)

decoupling capacitor

Description:

Decoupling Capacitor or Bypass Capacitor in Electronics– The electronic components like Capacitors, Resistors, Transistors, Diodes, and Inductors are the major electronic components. These few electronic components are found in almost all the electronic circuits. If you want to learn electronics then you should master the use of these few electronic components.

I have very detailed articles on,

·         What is a Resistor, Different types of resistors & Applications?

·         What is a Capacitor? Capacitor Types, Capacitor Uses, and Capacitor Working

·         What is a Transistor? PNP Transistor and NPN Transistor, BJT

You can start with the above three electronic components for now. As I have already explained the maximum basic things about the Capacitor, so I will not go into the very details. In this article we will only focus on the Decoupling Capacitor most frequently used in electronic circuits. I will try to answer mostly commonly asked questions regarding the Decoupling Capacitors.

Without any further delay, let’s get started!!!


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Capacitor Energy Storage and Supply

It seems obvious that if a capacitor stores energy, one of its many applications would be supplying that energy to a circuit, just like a battery. The problem is capacitors have a much lower energy density than batteries; they just can’t pack as much energy as an equally sized chemical battery (but that gap is narrowing!).

The upside of capacitors is they usually lead longer lives than batteries, which makes them a better choice environmentally. They’re also capable of delivering energy much faster than a battery, which makes them good for applications which need a short, but high burst of power. A camera flash might get its power from a capacitor (which, in turn, was probably charged by a battery).

Decoupling Capacitor:

A decoupling capacitor is a capacitor used to decouple one part of an electrical network (circuit) from another. Noise caused by other circuit elements is shunted through the capacitor, reducing the effect it has on the rest of the circuit. An alternative name is bypass capacitor as it is used to bypass the power supply or other high impedance component of a circuit.

decoupling capacitor

Let me give you an example of the LM7805 5V linear voltage regulator with 2 decoupling capacitors C1 and C2. These two capacitors are connected at the input and output sides of the Voltage Regulator. These two capacitors are also called as the Bypass Capacitors.

I have been using this 5V regulated Power supply for years in different electronics projects. Even in my last project “ESP32 Firebase“, I have used the same 5V regulated power supply for powering up the ESP32 Wifi + Bluetooth Module.


Now let me explain why I have used these decoupling capacitors.

Active devices of an electronic system (transistors, ICs, vacuum tubes, for example) are connected to their power supplies through conductors with finite resistance and inductance. If the current drawn by an active device changes, voltage drops from power supply to device will also change due to these impedances. If several active devices share a common path to the power supply, changes in the current drawn by one element may produce voltage changes large enough to affect the operation of others – voltage spikes or ground bounce, for example – so the change of state of one device is coupled to others through the common impedance to the power supply. A decoupling capacitor provides a bypass path for transient currents, instead of flowing through the common impedance.

The decoupling capacitor works as the device’s local energy storage. The capacitor is placed between power line and ground to the circuit that current is to be provided. According to capacitor equation,

decoupling capacitor

Voltage drop between power line and ground results in current draw out from the capacitor to the circuit and when capacitance C is large enough, sufficient current is supplied to maintain an acceptable range of voltage drop. To reduce the effective series inductance, small and large capacitors are often placed in parallel; commonly positioned adjacent to individual integrated circuits. The capacitor stores a small amount of energy that can compensate for the voltage drop in the power supply conductors to the capacitor.

In digital circuits, decoupling capacitors also help prevent radiation of electromagnetic interference from relatively long circuit traces due to rapidly changing power supply currents. The decoupling capacitors improve the Power Supply stability.


Decoupling:

A bypass capacitor is often used to decouple a subcircuit from AC signals or voltage spikes on a power supply or other line. A bypass capacitor can 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.

You can practically see this effect, if you power up the ESP32 or Nodemcu ESP8266 Wifi Module using the Arduino board. These modules will keep resetting when they start to transmit any data, as during the transmission these modules need more current.

It’s pretty standard for beginner electronic designers to forget just how unstable input voltages can be, despite how sturdy that power supply might look. And when you’re working with microcontrollers or microprocessors in your digital circuit, the slightest fluctuation in your voltage can lead to undesired results. So what can you do to keep your ICs running with smooth, clean voltage? Use decoupling capacitors!

A decoupling capacitor or a bypass capacitor, acts as a kind of energy reservoir. You’ll find these guys commonly placed as close as possible to an integrated circuit (IC) on a PCB layout. Once fully charged, their job is to simply oppose any unexpected change in your input voltages from a power supply. When a decoupling capacitor is in place, it will do one of two things:

  1. If the input voltage drops, then a decoupling capacitor will be able to provide enough power to an IC to keep the voltage stable.
  2. If the voltage increases, then a decoupling capacitor will be able to absorb the excess energy trying to flow through to the IC, which again keeps the voltage stable.

All of this is needed because there’s a ton of electrical noise on a typical circuit board, and the steady 5V that we think we have flowing all over the place is actually jumping around as it moves from component to component.

Some components like integrated circuits rely on their input voltage being as steady as possible, so when you place a decoupling capacitor next to an IC, you’ll be able to protect those sensitive chips by filtering out any excess noise and creating a nice, steady source of power. What happens if you don’t use decoupling capacitors next to your IC? Well, you’ll likely wind up with a processor that starts skipping instructions and behaving abnormally.


Coupling Capacitor:

Coupling capacitors, on the other hand, provide DC isolation while creating an intentional path for audio, video, RF, and high-speed digital data. Coupling capacitors are often found on high speed interfaces to ensure that any DC potential difference on connected devices does not manifest as ground currents between the devices.

Decoupling Capacitor Placement:

A transient load decoupling capacitor is placed as close as possible to the device requiring the decoupled signal. This minimizes the amount of line inductance and series resistance between the decoupling capacitor and the device. The longer the conductor between the capacitor and the device, the more inductance is present.

Since capacitors differ in their high-frequency characteristics (and capacitors with good high-frequency properties are often types with small capacity, while large capacitors usually have worse high-frequency response), decoupling often involves the use of a combination of capacitors. For example in logic circuits, a common arrangement is ~100 nF ceramic per logic IC (multiple ones for complex ICs), combined with electrolytic or tantalum capacitor(s) up to a few hundred μF per board or board section.



Decoupling Capacitor Selection Guide:

While any decoupling capacitors are arguably better than none, there are several guidelines to consider when implementing a decoupling scheme. Because the capacitors will need to provide current very quickly, the first and most important aspect is to choose capacitors with low equivalent series resistance (ESR), which sums characteristic impedance with any impedance related to inductance. Ceramic capacitors are typically used for decoupling applications due to their wide temperature tolerance, ability to withstand wide voltage ranges, low ESR, stability, and reliability. However, the construction of the capacitor is as important as the size of the package, as inherent benefits from capacitor chemistry can be quickly offset by the added inductance of a larger package size.

The smallest available package that otherwise meets design parameters is often the best choice, although specialized bypass and decoupling capacitor packages that further reduce inductance may be available. Smaller packages also have the benefit of reducing loop size for the capacitor circuit, and this further minimizes the inductance of every decoupling capacitor.

Other ways to optimize the functionality of decoupling caps are to ensure that power and ground planes are continuous and adjacent, by ensuring that capacitors are mounted as closely as possible to the power and ground pins of ICs, by making circuit paths to ground and power planes as short as possible, and by ensuring vias are routed between or beside the pads of the capacitor.  Adjacent power and ground planes should be symmetrically placed in the design, and the number of layers between the planes and the decoupling capacitors should be minimized.  If possible, capacitors should also be distributed in the area they are decoupling. When this is not possible and a capacitor bank is used, it is best to alternate their orientation to spread their connection points and prevent effective splits in ground or power planed from multiple adjacent vias routed through the plane. The number of capacitors to use depends primarily on the number of power and ground pins present in a localized circuit area or IC, as well as the number of I/O signals present. Designs with analog and digital sections can require that decoupling and bypassing is handled for segments of a circuit or IC.

Today’s digital devices can have significant challenges in maintaining a stable and quiet power supply in the presence of switched loads and other sources of system noise. With the proper use of bulk power capacitors and bypass capacitors in an integrated decoupling scheme, designers can ensure that problems associated with intra-system power noise and other noise sources are mitigated properly and their products will work as designed.


Decoupling Capacitors Final Words!

When working with decoupling capacitors in your own design, keep these three things in mind:

Placement. You’ll always want to connect your decoupling capacitors between your power source, whether that’s 5V or 3.3V, and ground.

Distance. You’ll always want to place your decoupling capacitors as close as possible to your IC. The farther away they are, the less effective they’ll be.

Ratings. As a general guideline, we always recommend adding a single 100nF ceramic capacitor and a larger 0.1-10uF electrolytic capacitor for each integrated circuit on your board.

Decoupling Capacitors, Commonly asked questions!!!

Q: How do I know if I need a decoupling Capacitor?

Answer: Power supplies are slow, they take roughly 10 us to respond (i.e. bandwidth up to 100 kHz). So when your big, bad, multi-MHz microcontroller switches a bunch of outputs from high to low, it will draw from the power supply, causing the voltage to start drooping until it realizes (10 us later!) that it needs to do something to correct the drooping voltage.

To compensate for slow power supplies, we use decoupling capacitors. Decoupling capacitors add fast “charge storage” near the IC. So when your micro switches the outputs, instead of drawing charge from the power supply, it will first draw from the capacitors. This will buy the power supply some time to adjust to the changing demands.

The “speed” of capacitors varies. Basically, smaller capacitors are faster; inductance tends to be the limiting factor, which is why everyone recommends putting the caps as close as possible to VCC/GND with the shortest, widest leads that are practical. So pick the largest capacitance in the smallest package, and they will provide the most charge as fast as possible.


Q: How to select the value for a Decoupling or Bypass Capacitor?

Answer: The reactance of the capacitor added to the circuit should be 1/10th or less of the resistance in parallel. We all know that the current always take the low resistance path, if you want to shunt the AC signal to the ground the capacitor should have a lower resistance. The capacitance value of the bypass capacitor to be used can be calculated using the formula

C = 1/2πfXC

With the above bypass capacitor formulae, let’s consider you need to find the capacitance of the capacitor connected across the resistor of resistance 440Ω, we know the reactance is always 1/10th of the resistance, hence the reactance will be 44Ω and the standard frequency of the Pakistani and Indian electrical network is 50Hz, so the bypass capacitor value can be calculated as

C = 1/2(3.14)(50)(44)

The capacitance of the capacitor across the 440 Ω resistor should be 73µF. Using the same you can find out the value of capacitors that can be used in a circuit.


Q: What is the difference between Bypass and Decoupling Capacitor

Answer: When you look at the purpose they are used for, there is not much difference between the two types of capacitors. Surprisingly, most of the times the decoupling capacitors are also called as the Bypass capacitors.  This is because they are shunted to the ground sometimes.

Some of the few noticeable difference between the bypass capacitor and decoupling capacitors are , the bypass capacitor is designed to shunt the noise signals where as the decoupling capacitors are designed to smoothen the signal by stabilizing the distorted signal. For shunting the signal we can just use a single electrolytic capacitor but for soothing the signal we will need two different types of capacitor.

Read my articles on,

·         What is a Resistor, Different types of resistors & Applications?

·         What is a Capacitor? Capacitor Types, Capacitor Uses, and Capacitor Working

·         What is a Transistor? PNP Transistor and NPN Transistor, BJT

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About the Author: Engr Fahad

My name is Shahzada Fahad and I am an Electrical Engineer. I have been doing Job in UAE as a site engineer in an Electrical Construction Company. Currently, I am running my own YouTube channel "Electronic Clinic", and managing this Website. My Hobbies are * Watching Movies * Music * Martial Arts * Photography * Travelling * Make Sketches and so on...

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