An electronic system is comprised of active and passive devices. When the supply is provided to the active ones for example transistors, vacuum tubes, and so on, there is the indulgence of change in the value of impedances due to voltage drops. If the common path is shared among the multiple devices, the individual currents will be drawn from the devices that are vast enough for producing the large changes in voltages. To overcome this situation, a bypass path must be established for the flow of transient currents. For this situation, decoupling capacitors are preferred. It can decouple the AC signals from the DC signals. There is another kind of capacitors called Coupling Capacitors. These capacitors are known for making the AC component present in the signal to pass by blocking the DC component. This article discusses an overview of decoupling capacitor and its working.
What is a Decoupling Capacitor?
Any capacitor that has been placed in any circuit with the intention of decoupling of the AC signals which are present along with Dc signals can be defined as Decoupling Capacitors. These are proven to be very advantageous while driving the loads in the circuit. The purpose of the capacitor is to store the charges. It can be charging and discharging based on the requirements.
When these capacitors are utilized as decoupling ones it can react to the small changes in the values of the voltages. As this change occurs it tends to absorb the excess energy. Based on its nature these are also known as ‘Bypass Capacitors’. Mostly in the ‘Digital Systems’ the capacitors that are placed on the board almost all are used for decoupling.
Decoupling Capacitor Design
Based on the usage of decoupling capacitors as energy reservoirs. These are placed almost nearer to the IC or the layout of PCB. The purpose of this capacitor in the circuit is
- If there is any drop incurred at the applied input voltage to make the IC stabilize enough supply of power is provided by this decoupling capacitor.
- There are chances of increment in the voltage level at the input. So, the decoupling capacitors used in such cases are able to absorb the excess amounts and attain stability.
While working with these capacitors the points to be considered are:
- Placement – These capacitors are always connected in the circuit in between the source or power supply of about 5V or 3.3 V and the ground.
- Distance – The capacitor is placed always near the circuit to make it more effective.
In the circuits to remove noise and where the power supply is connected to drive the load these capacitors are used. Practically to obtain the hundred percent pure form of the Dc signal is impossible. To a certain extent, it is possible because of the removal of the glitches and it is important to protect the load. When the precision of the circuit is considered one can prefer these capacitors. The presence of the reactance in the capacitors makes it function as Decouplers. Hence these Decoupling Capacitors have many practical uses.
Selection and Placement
The selection of this type of capacitor is based on the essential and requirements. Let us consider a basic circuit of supply, load, and the capacitor that functions as a decoupling one. Based on the type of the preferred circuits the other types of capacitors can also be used along with these capacitors. The connection is done in such a way that it provides infinite or maximum value of resistance towards the Dc component of the signals.
Hence the supply signal when passes through the capacitor it can pass the Ac component of the signals through it. But it prevents the flow of the Dc signals through it. Once the Ac signals are passed through these capacitors it shunts them to the ground. This functionality or the process sometimes referred to as ‘Bypass’.
These capacitors are known for offering the ‘Low Impedance Path’ for the signals that possess high frequency. In this way, the noise tends to get eliminated from the circuitry. Hence these capacitors decouple the AC signals.
Decoupling Capacitor Value Calculation
The minimum required value of the capacitance can be found by using the formula
For example, if the total current passing through the circuit is 500 milli Amps with 0.5 volts at 6 nanoseconds duration of time. Then what will be the minimum required capacitance?
Sol: Given, the value of the current is 500 milli Amps
By substituting in the above formula
C= 500 (6/0.5)
On simplifying based on the units the value of the capacitance can be found by
C= 6 nano Farads
Hence, based on the given values of a circuit the value of the capacitance can be found.
Difference between Decoupling / Bypass Capacitor
Decoupling and the Bypass structure and the construction-wise both are similar or we call it as same. Coming to its functionality sometimes it may differ. Any generally designed capacitor can be utilized as the above capacitors. There is no special design criterion. Based on the requirements of gaining the integrity of the power value in the circuit these capacitors are preferred.
The integrity maintenance can be done by choosing the appropriate size of this capacitor.
The decoupling of the capacitors in the circuits is very useful to protect the devices. Further using these capacitors along with the other variants of the capacitor can enhance the functionality of the circuit. These capacitors can be used in two ways. Firstly, these are used for the transmission of the purest DC to the load. This decoupling is referred to as ‘Power Supply Decoupling’. Secondly, Decoupling can be provided to protect the loads from getting damaged. Hence this type of coupling referred to as the ‘Transient Load Decoupling’.
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In the circuits where analog signals are modulated but the microcontroller presence in the circuit requires special attention. Overall the conclusion can be derived that selection and the placement of these capacitors make the circuit efficient. For this process most frequently preferred capacitors are ‘Ceramic Capacitors’. We cannot prefer ‘Electrolytic Capacitors’ because it cannot tolerate or it is poor in terms of handling High Frequencies. Hence the placement of the Decoupling capacitor is worthwhile. Can you describe which circuit you have utilized and on what basis you have chosen the Decoupling Capacitor?