Output Cap Ripple Voltage
Estimate output-capacitor ripple in a switching converter from capacitance, ESR, and inductor ripple current.
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The engineering
Output ripple has two parts: the charge sloshing in and out of the capacitor (ΔV_C, set by C and frequency) and the voltage across the equivalent series resistance as ripple current flows through it (ΔV_ESR). In a buck the cap sees a triangular current, so the capacitive term collapses to ΔI/(8·f·C); in a boost or buck-boost the cap must supply the full load during the switch-on interval, giving a much larger, pulsed contribution.
The gotcha in real supplies: ESR usually wins. A 100 µF electrolytic at 30 mΩ carrying 1 A of ripple drops 30 mV across ESR before the capacitance term even matters. That is why designers reach for low-ESR ceramics or parallel banks — adding raw capacitance does nothing if ESR is the limiter. Summing the two terms is conservative; they don't peak at exactly the same instant.
Where this math comes from
Ripple estimation grew out of the linear-supply era, but switching converters made it a first-class design constraint once switchers went mainstream in the 1970s. The clean small-ripple derivation most engineers carry in their heads — ΔI/(8fC) for the buck — is the one drilled by Robert Erickson at the University of Colorado.
Erickson's Fundamentals of Power Electronics (first edition 1997, with Dragan Maksimović on later editions) formalized the volt-second and charge-balance bookkeeping that turns these into one-line results. The ESR term became dominant in practice as switching frequencies climbed and capacitor technology — not the math — set the achievable ripple floor.
- 1976R. D. Middlebrook & S. ĆukState-space averaging gives a rigorous basis for converter ripple analysis.
- 1997Robert EricksonFundamentals of Power Electronics codifies the small-ripple ΔI/(8fC) result.
- 2001Erickson & MaksimovićSecond edition extends charge-balance ripple methods across topologies.
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