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What are Capacitors?
Capacitors are passive two-terminal energy storage elements composed of two conductors separated by an insulating (dielectric) material. The most basic parallel-plate capacitor is a device consisting of two parallel conducting plates of area A separated by a distance d.
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The tolerance value of a capacitor describes the extent to which the actual capacitance can vary from its specified nominal capacitance value. Tolerance is often expressed as a percentage, and may also be expressed as an actual value (i.e., 20pF+-1pF). In general, capacitors should be selected with an understanding of the given application’s sensitivity.
Tolerance values of +-5% and +-10% are ±5%/10%/20% are commonly used for coupling and decoupling applications. More sensitive applications may require the use of capacitors with tolerances of +-1% and +-2% or ±1% and ±2% or better. For example, ceramic capacitors used for coupling and decoupling applications are typically rated at +-5% and +-10%±5% and ±10%, or ±20%. Electrolytic capacitors often have a tolerance of -20% and +80%.
Voltage Rating (Maximum Voltage / Working Voltage)
The voltage rating of a capacitor is the maximum voltage that can be safely applied across it. Exceeding the maximum voltage can result in permanent damage to the capacitor. This specification is typically quoted for DC voltages. For AC applications, an RMS value is typically quoted. In general, capacitors should be selected with a sufficiently large safety margin from the maximum voltage rating.
Equivalent Series Resistance (ESR)
The equivalent series resistance of a capacitor is the impedance of the capacitor to alternating current and is particularly important to consider at high frequencies. This specification is the accumulation of the resistance of the dielectric material, the DC resistance of the terminal leads, the DC resistance of the connections to the dielectric, and the capacitor plate resistance. These resistances are all measured at a particular frequency.
Ripple Current
The ripple current of a capacitor refers to the maximum amount of current that can flow into it such that it does not cause a significant rise in temperature (due to ESR heat dissipation). High levels of ripple current can lead to noticeable levels of heat dissipation, which can diminish a capacitor’s lifespan.
This specification is important for circuits with high current flow (i.e., smoothing capacitors for power supply circuits).
Leakage Current
The leakage current of a capacitor refers to the amount of current that flows through its insulating barrier. Ideally, zero current should flow through the dielectric between the two plates of a capacitor. This phenomenon occurs because capacitors are not perfect insulators. Leakage current may also be quoted with voltage and temperature. It must be noted that leakage current increases with increasing temperature.
A previously charged capacitor will slowly lose its stored energy due to leakage. When a fully charged capacitor is continuously supplied, current will also leak and flow through it. Leakage current is typically quoted for large capacitors and when leakage is a significant factor.
Supercapacitors and aluminum electrolytic capacitors normally have values of leakage current quoted.
Leakage Resistance / Insulation Resistance
The leakage/insulation resistance is related to leakage current by Ohm’s law. Insulation resistance is typically quoted when very high values of resistance are encountered.
Ceramic capacitors or plastic film capacitors (where leakage current is insignificant), the values of resistance are typically quoted.
Leakage current and resistance can impact circuits in a variety of ways. High voltage circuits can experience large quantities of heat dissipation with even the smallest levels of leakage current. In general, leakage current can greatly impact high impedance circuits.
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Types of Capacitors
Electrolytic Capacitor
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An electrolytic capacitor is a type of polarized capacitor which uses an electrolyte as one of its conducting plates (electrodes) to achieve relatively large capacitances or high charge storage (an electrolyte is a liquid or gel that contains a high concentration of ions).
Electrolytic capacitors can be subcategorized into three families based on the material used to construct the dielectric:
Aluminum electrolytic capacitors
Tantalum electrolytic capacitors
Niobium electrolytic capacitor
Structure: Electrolytic capacitors are polarized as their internal structures are asymmetrical in nature. Typically, the anode (positive plate) is made of a metal which forms an insulating oxide layer that acts as the dielectric. The (true) cathode (negative plate) is made of a solid, liquid, or gel electrolyte which covers the surface of the dielectric oxide layer. The enlarged anode surface and thin dielectric oxide layer of electrolytic capacitors give them their large capacitances.
Important Considerations: Since electrolytic capacitors are polarized, the voltage at the anode (positive) must be higher than at the cathode (negative) at all times. Applying a reverse polarity voltage, or exceeding the maximum rated working voltage by even a small magnitude can destroy the dielectric, rendering the capacitor useless. It must also be noted that electrolytic capacitors typically have large tolerances. Capacitance drift (divergence from nominal capacitance) is prevalent among electrolytic capacitors.
Applications: Electrolytic capacitors are widely used for decoupling or noise filtering in power supplies and DC link circuits for variable-frequency drives, for coupling signals between amplifier stages, and for storing energy. In general, electrolytic capacitors are used for passing or bypassing low-frequency signals, and storing large amounts of energy.
Advantages:
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DC Bias
One thing you might have noticed using a multimeter to measure capacitance with a circuit on is that the capacitance of the capacitor you’re measuring looks drastically off. This could be normal, for this exact reason. DC Bias is effectively the voltage “coefficient” of your capacitor, as it states how capacitance changes with a DC voltage applied over the terminals. This occurs because applying a DC Voltage across the capacitor affects the electric field and the di-electric, modifying the capacitance(if you’re an ECE student, think like ECE 106). For example, X7R capacitors can have their capacitance decrease to less than 30% (!!) of their nominal rated capacitance at a higher applied voltage, due to the DC bias effect. For NP0 capacitors, the DC bias effect is a lot less significant since the dielectric material used for those capacitors is more resistant to DC bias.
When selecting capacitors especially for a specific application requirement, this is one factor that is often overlooked and should be taken into account. In the datasheet you should find the expected voltage you’d be applying over the capacitor and check how much the capacitance is affected at that voltage. rom there, you need to consider if it’s necessary to choose a different capacitor or add multiple capacitors in parallel in order to add capacitance.
Voltage Rating (Maximum Voltage / Working Voltage)
The voltage rating of a capacitor is the maximum voltage that can be safely applied across it. Exceeding the maximum voltage can result in permanent damage to the capacitor. This specification is typically quoted for DC voltages. For AC applications, an RMS value is typically quoted. In general, capacitors should be selected with a sufficiently large safety margin from the maximum voltage rating. Usually the rule of thumb is select something 1.5x - 2x your maximum rated voltage to account for transient spikes. For our specific standards, see Electrical Standards
Equivalent Series Resistance (ESR)
ESR is defined as the resistance that appears in series with the ideal capacitance of a capacitor. It accounts for the energy losses that occur when current flows through the capacitor, primarily due to the resistance of the electrode materials and the dielectric medium. This resistance leads to power dissipation in the form of heat and higher voltage ripple.
Equivalent Series Inductance (ESL)
The physical parameters of any wire/component will mean that unwanted parasitic inductance of some kind will always exist. This paracitic inductance will define the high frequency impedance (and thus performance) of the capactior. The ESL of a capacitor is dictated by it’s physical construction. Generally, the larger the physical size of a capacitor, the larger the ESL.
Ripple Current
The ripple current of a capacitor refers to the maximum amount of current that can flow into it such that it does not cause a significant rise in temperature (due to ESR heat dissipation). High levels of ripple current can lead to noticeable levels of heat dissipation, which can diminish a capacitor’s lifespan.
This specification is important for circuits with high current flow (i.e., smoothing capacitors for power supply circuits).
Leakage Current
The leakage current of a capacitor refers to the amount of current that flows through its insulating barrier. Ideally, zero current should flow through the dielectric between the two plates of a capacitor. This phenomenon occurs because capacitors are not perfect insulators. Leakage current may also be quoted with voltage and temperature. It must be noted that leakage current increases with increasing temperature.
A previously charged capacitor will slowly lose its stored energy due to leakage. When a fully charged capacitor is continuously supplied, current will also leak and flow through it. Leakage current is typically quoted for large capacitors and when leakage is a significant factor.
Supercapacitors and aluminum electrolytic capacitors normally have values of leakage current quoted.
Leakage Resistance / Insulation Resistance
The leakage/insulation resistance is related to leakage current by Ohm’s law. Insulation resistance is typically quoted when very high values of resistance are encountered.
Ceramic capacitors or plastic film capacitors (where leakage current is insignificant), the values of resistance are typically quoted.
Leakage current and resistance can impact circuits in a variety of ways. High voltage circuits can experience large quantities of heat dissipation with even the smallest levels of leakage current. In general, leakage current can greatly impact high impedance circuits.
...
Types of Capacitors
Electrolytic Capacitor
...
An electrolytic capacitor is a type of polarized capacitor which uses an electrolyte as one of its conducting plates (electrodes) to achieve relatively large capacitances or high charge storage (an electrolyte is a liquid or gel that contains a high concentration of ions).
Electrolytic capacitors can be subcategorized into three families based on the material used to construct the dielectric:
Aluminum electrolytic capacitors
Tantalum electrolytic capacitors
Niobium electrolytic capacitor
Structure: Electrolytic capacitors are polarized as their internal structures are asymmetrical in nature. Typically, the anode (positive plate) is made of a metal which forms an insulating oxide layer that acts as the dielectric. The (true) cathode (negative plate) is made of a solid, liquid, or gel electrolyte which covers the surface of the dielectric oxide layer. The enlarged anode surface and thin dielectric oxide layer of electrolytic capacitors give them their large capacitances.
Important Considerations: Since electrolytic capacitors are polarized, the voltage at the anode (positive) must be higher than at the cathode (negative) at all times. Applying a reverse polarity voltage, or exceeding the maximum rated working voltage by even a small magnitude can destroy the dielectric, rendering the capacitor useless. It must also be noted that electrolytic capacitors typically have large tolerances. Capacitance drift (divergence from nominal capacitance) is prevalent among electrolytic capacitors.
Applications: Electrolytic capacitors are widely used for decoupling or noise filtering in power supplies and DC link circuits for variable-frequency drives, for coupling signals between amplifier stages, and for storing energy. In general, electrolytic capacitors are used for passing or bypassing low-frequency signals, and storing large amounts of energy.
Advantages:
Large capacitance or high charge storage
Low cost
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Passive Device: NTC thermistor (Negative Temperature Coefficient): Works as an electric resistor whose resistance is very high at low-temperature values. The NTC thermistor connects to the power supply input line in series. It exhibits high value of resistance at ambient temperatures. When the device is turned on, the high resistance limits the inrush current to flow into the system. As the current flow continuously increases, the temperature of resistance at ambient temperatures. When the device is turned on, the high resistance limits the inrush current to flow into the system. As the current flow continuously increases, the temperature of the thermistor rises which reduces the resistance significantly. Hence, the thermistor stabilizes the inrush current and allows the steady current to flow int the circuit. NTC thermistors are widely used for the current limiting purpose due to its simple design and low cost. This implementation has some drawbacks, which includes not being able to rely on the thermistor in extreme weather conditions.
Active Device: Costlier option and also increases the size of the system or circuit. This implementation consists of sensitive components which switch high incoming current. Some of the active devices are soft starts, voltage regulators, and DC/DC converters.
These protections are used to protect the electrical and mechanical system by limiting the instantaneous inrush current.
Applications of Capacitors
Decoupling (Bypass) Capacitors
Decoupling capacitors suppress high-frequency noise in power supply signals. They aid in removing voltage ripples (caused by electrical noise) that harm delicate ICs.
In a sense, decoupling capacitors act as very small, local power supplies for ICs. If the power supply temporarily drops in voltage, a decoupling capacitor can briefly supply power to the correct voltage (hence the term bypass capacitor, as they can temporarily act as a power source, bypassing the power supply).
Only high-frequency signals can run through the capacitor to ground. DC signals proceed along to their respective ICs as desired. The high frequencies bypass the IC by running through the capacitor to ground.
Decoupling capacitors connect between the power source and ground. Different valued and different types of capacitors may be used to bypass the power supply in order to filter out select frequencies of noise.
De-rating: It is important to ensure that selected capacitors in a circuit design have a much higher tolerance than the potentially highest voltage spike possible in the system.the thermistor rises which reduces the resistance significantly. Hence, the thermistor stabilizes the inrush current and allows the steady current to flow int the circuit. NTC thermistors are widely used for the current limiting purpose due to its simple design and low cost. This implementation has some drawbacks, which includes not being able to rely on the thermistor in extreme weather conditions.
Active Device: Costlier option and also increases the size of the system or circuit. This implementation consists of sensitive components which switch high incoming current. Some of the active devices are soft starts, voltage regulators, and DC/DC converters.
These protections are used to protect the electrical and mechanical system by limiting the instantaneous inrush current.
Applications of Capacitors
Decoupling (Bypass) Capacitors
Decoupling capacitors connect between the power source and ground to suppress high-frequency noise in power supply signals. They aid in removing voltage ripples (caused by electrical noise) that harm delicate ICs.
In a sense, decoupling capacitors act as very small, local power supplies for ICs. If the power supply temporarily drops in voltage, a decoupling capacitor can briefly supply power to the correct voltage (hence the term bypass capacitor, as they can temporarily act as a power source, bypassing the power supply).
Only high-frequency signals can run through the capacitor to ground. DC signals proceed along to their respective ICs as desired. The high frequencies bypass the IC by running through the capacitor to ground.
De-rating: It is important to ensure that selected capacitors in a circuit design have a much higher tolerance than the potentially highest voltage spike possible in the system.
Paralelling Capacitors: Multiple decoupling capacitors, sometimes of different values, may be paralleled. Smaller capacitance values allow for smaller package sizes, which result in lower impedance at higher frequencies, while larger capactiors have lower impedance at lower frequencies. Combining smaller capacitors with larger ones allows decoupling over a greater frequency span. Note: The best decoupling is achieved by using the largest capacitance for a given package size. Capacitor impedance at high frequencies is dominated by package ESL. Placing a smaller value capacitor without reducing the package size often yields no tangible benefit. The plots below compare a 1uF and 10nF capactior of the same package size, with both showing similar HF characteristics but the larger capacitor having substantially better LF performance.
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Placement of Decoupling Capacitors: When placing decoupling capacitors, a good rule of thumb is to minimize the distance between the component’s voltage pin and the capacitor. For IC pins, this implies placing decoupling capacitors as close as possible to it. Note that this rule does not necessarily result in good decouplingIf multiple decoupling capacitors are present, they should be placed in the order of lowest ESL → closest to pins. When designing a multilayer PCB, the capacitor can be placed below the component’s pad. On single-layer designs, these capacitors are placed near the pin and routed with a short trace.
Decoupling capacitors can be placed before ground plane connections. When using vias to connect to a power plane, connect the capacitor to the component pin, and then to the via to ensure that current flows through the plane.
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https://forum.digikey.com/t/what-are-esd-capacitors/1019
https://www.tempoautomation.com/blog/the-most-effective-decoupling-capacitor-placement-guidelines/
Old Content
As with the other pages here, I’ll just link to pages that already explain this instead of writing everything out again - as the pages I have linked explain everything way better than I could.
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