Capacitor Derating Guidelines

Introduction

proper derating is essential for maximizing capacitor lifetime and ensuring reliable operation. This guide provides specific derating recommendations for voltage, temperature, and ripple current.

Voltage Derating

general recommendation: - Standard applications: 80% of rated voltage (20% derating)

  • High-reliability applications: 70% of rated voltage (30% derating)
  • Automotive applications: 80% of rated voltage minimum
  • Military/aerospace: 50-60% of rated voltage
why voltage derating matters: - Reduces stress on dielectric oxide layer
  • Decreases leakage current
  • Improves long-term stability
  • Provides margin for voltage transients
example: For a 100V capacitor in a 75V application:
  • Derating = 75V / 100V = 75% (25% derating)
  • This exceeds the 80% recommendation
  • Select next higher voltage rating: 160V or 200V

Temperature Derating

general recommendations: - Standard applications: Operate at 70% of rated temperature

  • High-reliability applications: Operate at 60% of rated temperature
  • Maximum case temperature: 85°C for 105°C rated capacitors
  • Maximum case temperature: 95°C for 125°C rated capacitors
temperature impact on lifetime: Following the Arrhenius relationship:
  • Every 10°C reduction doubles lifetime
  • Every 10°C increase halves lifetime
example calculation: Capacitor rated: 10,000 hours at 105°C Operating at: 85°C case temperature Lifetime multiplier: 2^((105-85)/10) = 2^2 = 4x Expected lifetime: 10,000 × 4 = 40,000 hours

Ripple Current Derating

general recommendations: - Standard applications: 80% of rated ripple current

  • High-frequency applications: 70% of rated ripple current
  • High-temperature applications: 60% of rated ripple current
ripple current and self-heating: Self-heating (ΔT) = I² × ESR × Rth Where:
  • I = ripple current (A RMS)
  • ESR = equivalent series resistance (Ω)
  • Rth = thermal resistance (°C/W)
maximum self-heating: - Standard: 5°C
  • High-reliability: 3°C

Combined Derating Example

application: Industrial power supply

  • Input voltage: 230V AC
  • DC bus voltage: 325V DC
  • Ambient temperature: 50°C
  • Required lifetime: 50,000 hours
capacitor selection: 1. Voltage rating: 325V / 0.8 = 406V minimum → Select 450V
  • Temperature rating: Need 105°C with derating to 85°C
  • Ripple current: Calculate actual and apply 80% derating

    • verification: - Voltage derating: 325V/450V = 72% (28% derating)

    • Temperature: Operating at 75°C (85°C max with 10°C margin)
    • Calculated lifetime: 10,000 × 2^3 × 2.9 = 232,000 hours

    Application-Specific Derating

    led drivers: - Voltage: 80% derating

    • Temperature: 70°C maximum
    • Lifetime target: 50,000 hours
    solar inverters: - Voltage: 70% derating (for 25-year life)
    • Temperature: 75°C maximum
    • Consider film capacitors for DC-link
    automotive: - Voltage: 80% derating
    • Temperature: 125°C rating for under-hood
    • Vibration: Verify mechanical ratings
    medical: - Voltage: 80% derating
    • Temperature: 70°C maximum
    • Full traceability required

    Derating Verification

    during design: 1. Calculate worst-case operating conditions

  • Apply derating factors
  • Select capacitors meeting derated requirements
  • Verify thermal design

    • during validation: 1. Measure actual operating voltage

  • Measure case temperature under full load
  • Measure ripple current with current probe
  • Verify all parameters within derated limits

    • in production: 1. Monitor field reliability data

  • Track capacitor failure rates
  • Adjust derating based on field experience