How to Choose a Bypass Capacitor Size

Key Takeaways

• Understanding bypass capacitors

• The factors affecting the sizing and placement of bypass capacitors

• Relation of resistance and impedance in determining bypass capacitor size

Most engineers are aware of the issues associated with electric surges, which can generate high-frequency noise in a circuit. This can result in issues like ringing or impedance mismatch, resulting in interference or inadequate power transfer. This is the reason why almost every type of electrical equipment needs grounding. Why? Grounding is an effective way of preventing the excess voltage and associated noise from passing the electrical network, thus, stabilizing it. This can be warranted easily with the use of capacitors, especially bypass capacitors, in the design of electrical devices.

How and where these bypass capacitors are placed plays an important role in defining the function and safety of the electric circuit. Similar importance should be given to the bypass capacitor size. This article will discuss the basics of bypass capacitors as well as their sizing requirements in broad electric circuit layouts. 

What Is a Bypass Capacitor?

In a system circuit, it is necessary to maintain a clean signal. For bypass capacitors connected to a DC power supply, a clean DC signal can be achieved by shorting the high-frequency AC noise to the ground. One end of the bypass capacitor is connected to the power supply pin while the other end is connected to the ground. The pure DC signal directly passes to the circuit. However, as the capacitor detects the AC signal, it will filter this signal and bypass it to the ground. The advantage of this function is:

• Any type of resistance in the circuit is removed
• Voltage drop in the circuit can be eliminated by storing the charge

Oftentimes, we can see the terms decoupling capacitor and bypass capacitor being used interchangeably. These capacitors have a similarity of function, i.e., blocking AC noise to improve circuit performance. However, if we look closely, bypass capacitors are more concerned with deviating and shunting noise as opposed to the smoothing of the signal with a decoupling capacitor. 

Bypass Capacitor Size and Placement

When placing a bypass capacitor in any standard PCB, it should generally be located as close to the IC pin as possible. The larger the distance between the capacitor and power pin, the more the inductance increases, which severely decreases the signal quality. The signal quality is also heavily dependent on the bypass capacitor size. Bypass capacitor sizing is mostly done on the basis of the capacitance value. The commonly used values are 1μF and 0.1μF to handle lower and higher value frequencies, respectively.

Let us take the above circuit as an example. When the current passes through the circuit, there are two paths it can take:

1. Through the resistor
2. Through the capacitor

The current prefers to take the path of least resistance, which means if the bypass capacitor offers less resistance than the resistor, the goal of shunting AC signal to the ground can be achieved. Thus, the rule of thumb is that the value of a capacitor should be at least 10 times less than the value of R_E, emitter resistance.

In digital or analog devices, the general formula to identify a bypass capacitor value is:

X_c is the reactance and f is the operating frequency.

Impedance in Bypass Capacitors

The size of a bypass capacitor is also dependent on the impedance in the circuit. The capacitive impedance can be calculated using the following formula:

This is indicative of the inverse relationship between capacitance and impedance. When a capacitor is fully charged or discharged, the impedance is infinite, i.e.,  it acts like an open circuit and does not allow current to pass. This means large capacitors take a long time on charging and discharging while small capacitors can quickly do this to act like an open circuit, not allowing the current to pass (high impedance). Therefore, a large value capacitor can be used when low impedance is desired. 

Calculating Bypass Capacitor Size With PSpice

Understanding the signal behavior through the IC, including its frequency and impedance, provides an appropriate pathway to select an ideal bypass capacitor size. As discussed above, generally used capacitance values are 1μF and 0.1μF to handle low and high value frequencies. However, the verification of placement and the sizing of bypass capacitors are important parts of circuit design.

An ideal solution is to use PCB design and analysis software that can provide complete circuit design, simulation, verification, and optimization abilities. With the help of tools such as the Allegro PSpice Simulator, PCB design teams can easily predict the behavior of digital or analog circuits, evaluate their functionality, and optimize them to achieve a high-performance design and layout. 

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