Capacitors 101: How They Work, Their Functions, and Types

Capacitors are indispensable components in PCB (Printed Circuit Board) design, providing functionality from energy storage and noise filtering to voltage regulation. Whether you're designing a simple circuit or a complex multi-layer board, understanding the different types of capacitors and their applications is crucial. The basic structure of a capacitor consists of two metal plates separated by a layer of dielectric. Capacitors can be of fixed or variable type.

Capacitance is a fundamental concept in electrical engineering, representing the ability of a system to store electrical charge. The ability of the capacitor to hold electric charge is called capacitance and is measured in Farads. Like resistors, capacitors can be arranged in series or parallel combinations, and thus effective capacitance can be varied. There are several types of capacitors that have been developed for use in electronic circuits. In this comprehensive article, we will explore the working principles of capacitors, their functions, calculation methods, and various types used in electronic circuits. To know more about electronics and PCB designs, see our latest guide on PCB manufacturing process.

What Are Capacitors and their Working Principle?

Capacitors are passive electrical components that store and release electrical energy in the form of an electric field. They are made of two conductive plates separated by an insulating material called a dielectric. When a voltage is applied, the plates store electric charge. The charges stored on the plates are complementary to each other. Capacitors play many roles in electronic circuits. The capacitor terminals are taken out from the metal plates for external connections.

The fundamental working principle of a capacitor involves the separation and storage of electric charges. When connected to a power source:

• Electrons flow from the negative terminal to one plate (negative plate). An equal number of electrons are repelled from the other plate (positive plate) to the positive terminal.

• This charge separation creates an electric field in the dielectric material. The capacitor continues to charge until the voltage across its plates equals the applied voltage.

• Once fully charged, the capacitor can act as a temporary battery, storing the electrical energy until it's needed in the circuit.

Calculating Capacitance:

The capacitance of the structure can be given by the following equation:

C = εA / D

Where:

ε is the permittivity (Dielectric constant) measured in

C is the capacitance measured in farads

D is the distance between the plates

A is the overlapped area between two plates

The capacitance is the ratio of charge (Q) to voltage (V), and its mathematical expression is as follows:

C = Q / V

Where:

C is the capacitance measured in farads

Q is the charge accumulated on the plates

V is the voltage applied to capacitor

Factors Influencing Capacitance:

The capacitance of a capacitor can be varied by changing the following parameters:

• Area of the plate: The capacitance increases by increasing the area of the plate.
• Overlap of the plates(A): As the parallel plate overlap area increases, the capacitance value increases.
• Distance between the plates(D): The closer the proximity of the parallel plates, the greater the capacitance value.
• The nature of the dielectric(ε): Choosing a dielectric material of high permittivity increases the capacitance value.

Classification of Capacitors Based on Polarity:

Polarized capacitors: They are the capacitor that has positive and negative two different terminals. Common examples include aluminum electrolytic and tantalum electrolytic capacitors. These capacitors have distinct positive and negative terminals, and it’s crucial to connect the positive terminal to the positive side of the power source, and the negative terminal to the negative side.

Non-polarized capacitors: These capacitors can be connected in either direction without any concern for polarity. They are often used in applications where the voltage direction changes frequently.

Types of Capacitor in Electronics:

Use of Capacitors in a Circuit:

• Decoupling and Bypassing: They prevent noise from propagating in the circuit by acting as small reservoirs of charge.
• Filtering: Capacitors smooth out fluctuations in voltage, often used in power supply circuits to filter out AC ripple.
• Timing Circuits: Combined with resistors, capacitors are used to create timing intervals.
• Energy Storage: They temporarily store electrical energy and release it when needed, particularly in backup applications.
• Tuning: Variable capacitors are used in radio tuning circuits to select specific frequencies.

High-Frequency Considerations for a Capacitor on PCB:

While a capacitor's behavior is predictable at low frequencies, it becomes less ideal at higher frequencies. Use these tips to maintain stable operation and avoid issues such as Equivalent Series Inductance (ESL) and self-resonance. Here are some key factors to consider while working on high frequency circuit

• Instead of electrolytes, use ceramic caps or ESR films.
• Tiny case sizes have a lower ESL, with chip caps being better than Lead.
• Reduce trace lengths and return paths to the ground plane.
• Avoid large loop areas to prevent parasitic inductance.
• Simulate the circuit using S-parameter models.
• Test for ringing and other frequency-dependent issues during prototyping.
• The distance between capacitor pads on the PCB affects resonant frequency.
• If the capacitor operates near the self-resonant frequency, characterize its impedance.

What is Parasitic Capacitance, How to Reduce it:

The parasitic capacitance on a PCB is due to the high-frequency signals and different adjacent traces passing through the circuit board. This capacitance then introduces EMI, which propagates down to adjacent traces. Parasitic capacitance can exist between adjacent PCB traces, component leads and semiconductor junctions. To keep this capacitance minimum:

• A guard ring reduces the capacitive effect between two traces.
• Increase the space between adjacent traces.
• Use low permittivity dielectric materials since they produce less stray capacitance.
• Avoid routing the traces in parallel, as this leaves a maximum area between the two traces.

Understanding and minimizing parasitic capacitance is crucial in high-frequency circuit design and layout.

Conclusion:

Capacitors are fundamental components in PCB design, providing various functionalities essential for circuit stability, noise reduction, and energy management. Understanding the different types of capacitors, their properties, and best practices for placement can make a significant difference in the performance and reliability of your circuits. In summary:

• PCB capacitors store energy between conducting plates separated by a dielectric.
• Different capacitor types suit various applications and conditions.
• Proper rating of voltage, capacitance, ESR, and other factors prevents failures.
• Correct selection, placement, and handling ensure stable PCB performance.
• Bypass and decoupling capacitors need careful design for clean power delivery.

Whether you're working on high-speed digital designs, audio circuits, or power electronics, selecting the right capacitor type and placement is key to achieving your design goals. By mastering the use of capacitors, you’ll be better equipped to handle complex PCB projects and avoid common pitfalls.