Capacitor

Capacitor #

Stores electrical energy in an electric field. Essential for filtering, coupling, decoupling, and timing circuits. Breadpad models temperature effects and tolerances for accurate analysis.

Basic Properties #

  • Terminals: 2 (Positive and Negative for polarized types)
  • SPICE Designation: C
  • Default Value: 100 nF (0.0000001 F)

Key Parameters #

Primary Parameter #

  • Capacitance: Energy storage capacity in Farads (F)
    • Common ranges: picofarads (pF) to millifarads (mF)
    • Automatically formatted with SI prefixes (e.g., 100nF, 10µF)

Temperature Coefficients #

  • TC1: Linear temperature coefficient (ppm/°C)

    • Varies widely by dielectric type
    • Default: 0 (temperature independent)

  • TC2: Quadratic temperature coefficient (ppm/°C²)

    • Second-order temperature effects
    • Default: 0

Temperature-dependent capacitance:

C(T) = C₀ × (1 + TC1×(T-T₀) + TC2×(T-T₀)²)

Tolerance (Premium Feature) #

  • Tolerance: Manufacturing variation as percentage
    • Common values: 5%, 10%, 20%
    • Precision types: 1%, 2%
    • Used in Monte Carlo analysis

SPICE Netlist Format #

Basic capacitor:

C1 node1 node2 100n

With temperature coefficients:

C1 node1 node2 100n TC1=200e-6 TC2=0

With tolerance (Monte Carlo):

C1 node1 node2 {100n*(1+gauss(0,0.10/3,6))}

Capacitor Types and Characteristics #

Ceramic (MLCC) #

  • Capacitance: 1pF - 100µF
  • Voltage: 6.3V - 1kV
  • TC1:
    • C0G/NP0: ±30 ppm/°C
    • X7R: ±15%
    • Y5V: +22% to -82%
  • Applications: Decoupling, filtering, timing

Electrolytic (Aluminum) #

  • Capacitance: 0.1µF - 1F
  • Voltage: 6.3V - 450V
  • TC1: -40% to +10% over temperature
  • Polarized: Yes
  • Applications: Power supply filtering, energy storage

Tantalum #

  • Capacitance: 0.1µF - 1000µF
  • Voltage: 2V - 50V
  • TC1: ±200 ppm/°C typical
  • Polarized: Yes
  • Applications: Low ESR filtering, portable devices

Film (Polyester, Polypropylene) #

  • Capacitance: 1nF - 10µF
  • Voltage: 50V - 1kV
  • TC1: -200 to +200 ppm/°C
  • Applications: Audio, precision timing, motor run

Common Applications #

Power Supply Decoupling #

  • 0.1µF ceramic near each IC
  • 10-100µF electrolytic for bulk storage
  • Place close to power pins

AC Coupling #

  • Blocks DC while passing AC signals
  • Value: 1/(2π × f_cutoff × R)
  • Typical: 0.1µF - 10µF

RC Filters #

  • Low-pass: f_c = 1/(2πRC)
  • High-pass: Same formula, different configuration
  • Active filters with op-amps

Timing Circuits #

  • 555 timer: T = 1.1RC
  • RC oscillators
  • Delay circuits

Energy Storage #

  • Flash circuits: 100-1000µF
  • Backup power: Supercapacitors (1F+)
  • Peak current delivery

Important Considerations #

ESR (Equivalent Series Resistance) #

Not explicitly modeled in basic SPICE, but important for:

  • Power supply design
  • High-frequency performance
  • Ripple current handling

Voltage Derating #

  • Electrolytic: Use at 80% of rated voltage
  • Ceramic: Capacitance drops with DC bias
  • Tantalum: Use at 50% for reliability

Frequency Response #

  • Self-resonant frequency: f_r = 1/(2π√(LC))
  • Above f_r, capacitor acts as inductor
  • Important for decoupling effectiveness

Simulation Tips #

  1. Initial Conditions: Use IC=voltage for startup analysis
  2. Parasitic Inductance: Add series L (1-10nH) for accurate HF modeling
  3. Leakage Current: Add parallel R (1-100MΩ) for electrolytics
  4. Temperature Sweeps: Use TC1/TC2 for thermal analysis
  5. Monte Carlo: Include tolerance for worst-case analysis

See Also #