Analog Circuits Custom Design

Tag: Capacitor

  • Useful Stuff: Capacitor (part 2)

    SRF:

    A capacitor doesn’t always behave like a capacitor, especially at high frequencies. As shown below, the equivalent circuit of a capacitor includes a series inductance, known as the Equivalent Series Inductance (ESL). The ESL varies depending on the capacitor type, packaging, and physical dimensions.

    Capacitor Model

    At low frequencies, the impedance of the ESL (ZL​) is negligible compared to the impedance of the capacitor (ZC​):

    ZL=2πfLZ_L=2\pi fL
    ZC=12πfCZ_C=\frac{1}{2\pi fC}

    As the frequency increases, ZL increases while ZC​ decreases. At high frequencies, the inductive impedance dominates, causing the capacitor to act more like an inductor. But what do we mean by “low frequency” and “high frequency”? How can they be distinguished?

    At a specific frequency, the two impedances ZL​ and ZC​ become equal. This is known as the self-resonant frequency (SRF) of the capacitor:

    ZL=ZCZ_L = Z_C
    2πfL=12πfC2\pi f L = \frac{1}{2\pi f C}
    SRF=f=12πLCSRF = f = \frac{1}{2\pi \sqrt{LC}}

    Larger capacitors tend to have a lower SRF. This is why, in some cases, you may see a nano-Farad capacitor placed in parallel with a multi-micro-Farad capacitor. The smaller capacitor is used to maintain capacitive behavior at higher frequencies where the larger capacitor cannot.

  • Useful Stuff: Capacitor

    Capacitor is a basic and one of the most important components in electronics.

    Capacitors come in various types, each suited for specific applications. Here are some common types of capacitors:

    Ceramic Capacitors: Widely used in electronic circuits for their small size and stability. They come in various temperature coefficients and voltage ratings.

    Electrolytic Capacitors: Typically used for power supply filtering and energy storage due to their large capacitance values. They are polarized and have higher capacitance per unit volume compared to ceramic capacitors.

    Tantalum Capacitors: Similar to electrolytic capacitors but offer better performance in terms of stability and reliability. They are also polarized and used in applications requiring high capacitance and low leakage current.

    Film Capacitors: Known for their high precision, stability, and low loss. They are used in applications where a stable and reliable capacitor is required, such as in audio equipment and power electronics.

    Supercapacitors (Ultracapacitors): Offer very high capacitance values and can store large amounts of energy. They are used in applications requiring rapid charge and discharge cycles, such as in energy storage systems and backup power supplies.

    Mica Capacitors: Provide high precision and stability, often used in RF and microwave applications due to their low loss and high-frequency performance.

    Polymer Capacitors: Similar to electrolytic capacitors but use a solid polymer electrolyte. They offer better performance in terms of ESR (Equivalent Series Resistance) and stability.

    Variable Capacitors: Allow adjustment of capacitance value, commonly used in tuning circuits for radios and other communication devices.

    Each type of capacitor has its unique characteristics and is chosen based on the specific requirements of the application.

    ESR:

    ESR (Equivalent Series Resistance) is an important characteristic of capacitors. In some applications like switching-mode power supplies (SMPS) this parameter plays a critical role to select the capacitor.

    The capacitor model is shown below.

    Capacitor model

    Some manufacturers produce capacitors specifically for low-ESR. The specifications of Low-ESR Series can be found in data sheets published by manufacturers. The table below shows some popular Low-ESR series of capacitors.

    ManufacturerSeriesStyleTechnologyLow ESR down to [mΩ @ 20ºC / 100kHz
    AVXTCJChipPolymer aluminium10
    AVXTCQChipPolymer aluminium25
    AVXTCMChipPolymer tantalum multi-anode6
    AVXTPSChipTantalum25
    AVXTPMChipTantalum12
    KEMETA700ChipPolymer aluminium4.5
    KEMETA759RadialPolymer aluminium12
    KEMETA768SMDPolymer aluminium15
    KEMETT528ChipTantalum multi-anode4
    KEMETT520/T530ChipTantalum multi-anode4
    MurataECASChipPolymer aluminium6
    NichiconGYBChipAluminium electrolytic20
    NichiconGYCChipAluminium electrolytic20
    NichiconPCHChipAluminium electrolytic13
    NichiconPCRChipAluminium electrolytic13
    NichiconUCMChipAluminium electrolytic50
    NichiconUCZChipAluminium electrolytic32
    PanasonicFNSMDAluminium electrolytic80
    PanasonicFTSMDAluminium electrolytic60
    PanasonicFSRadialAluminium electrolytic12
    PanasonicOS-CON™ChipPolymer aluminium14
    PanasonicSP-CapChipPolymer aluminium6
    PanasonicZASMDPolymer hybrid20
    PanasonicZCSMDPolymer hybrid20
    RubyconZLHRadialAluminium electrolytic12
    RubyconPC-CONChipPolymer aluminium4.5
    Vishay190 RTLRadialAluminium electrolytic17
    Vishay170 RVZRadialAluminium electrolytic17
    popular Low-ESR series of capacitors in the market. (Source: avnet.com)