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The ratio of actual ripple current to rated ripple current (IA/IR) 

more about capacitors in this good guide: https://www.vishay.com/docs/28356/alucapsintrobcc.pdf
  • IA stands for actual ripple current - the RMS current actually flowing through the capacitor due to applied voltage ripple/noise.
  • IR stands for rated ripple current - the maximum RMS current rating specified by the manufacturer for the capacitor.
  • The ratio IA/IR is an important parameter that should not exceed 1. Operating a capacitor where IA/IR > 1 risks overheating and reducing its lifespan.
  • IA/IR provides an indication of how heavily stressed the capacitor is under the actual operating conditions compared to its rated specifications.
  • A lower IA/IR ratio means longer expected lifetime as there is headroom under the rated ripple current limit.
  • Power supply designers aim to keep IA/IR < 0.5-0.7 for electrolytic filter caps to ensure long lifespan.