Ripple Current Calculations for Chemi-Con Capacitors

Ripple current is the AC current flowing through a capacitor, causing self-heating and affecting lifetime. Proper calculation and management are essential for reliable designs.

Understanding Ripple Current

Ripple current generates heat in the capacitor through I²×ESR losses. This self-heating raises the capacitor temperature, reducing lifetime according to the Arrhenius relationship.

Key Factors:

  • RMS ripple current magnitude
  • ESR at operating frequency
  • Thermal resistance of capacitor
  • Ambient temperature
  • Airflow and cooling

Calculating Ripple Current

For Buck Converters:

Iripple = Vout × (Vin - Vout) / (Vin × f × L)

Where:

  • Vout = Output voltage
  • Vin = Input voltage
  • f = Switching frequency
  • L = Inductance

For Boost Converters:

Iripple = Vin × (Vout - Vin) / (Vout × f × L)

RMS Calculation:

For triangular ripple: Irms = Ipeak-to-peak / (2 × √3)

ESR Frequency Dependence

ESR varies significantly with frequency:

  • At 100-120Hz: Maximum ESR (datasheet value)
  • At 1kHz: 70-80% of 120Hz value
  • At 10kHz: 50-60% of 120Hz value
  • At 100kHz: 40-50% of 120Hz value

Always use ESR at your actual operating frequency for accurate calculations.

Self-Heating Calculation

Temperature Rise = I² × ESR × Rth

Where:

  • I = RMS ripple current
  • ESR = Equivalent series resistance at frequency
  • Rth = Thermal resistance (°C/W)

Typical thermal resistances:

  • Small radial (10-16mm): 35-45°C/W
  • Medium radial (18-25mm): 25-35°C/W
  • Large radial (30-35mm): 20-30°C/W
  • Snap-in (35-40mm): 15-25°C/W

Temperature Derating

Ripple current ratings are specified at 105°C. At lower temperatures, higher ripple current is allowed:

Correction Factor = √((105 - Ta) / (105 - Tmax))

Where:

  • Ta = Actual ambient temperature
  • Tmax = Maximum rated temperature

Example: At 65°C ambient, ripple current can be increased by approximately 25%.

Design Best Practices

  • Measure Actual Ripple: Use current probe to verify calculations
  • Include Margin: Design for 80% of rated ripple current
  • Consider Frequency: Use frequency-corrected ESR
  • Thermal Management: Ensure adequate airflow and heat sinking
  • Verify Temperature: Measure case temperature under full load

    • Parallel Capacitors

    For high ripple current applications, parallel multiple capacitors:

    • Ripple current shares (ideally equally)
    • Total ESR reduced
    • Better thermal distribution
    • Use identical capacitors for best current sharing