Output Voltage Ripple⚠ unverified
Electrical / Power Electronics · Compute the approximate output voltage ripple from capacitance and ESR
Parameters
| Input | Symbol | Unit | Default | Description |
|---|---|---|---|---|
| Iout | Iout | A | 1.0 | Output (or ripple) current amplitude |
| C | C | F | 1.0 | Output capacitance |
| f | f | Hz | 1.0 | Switching frequency |
| ESR | ESR | — | 0.0 | Equivalent series resistance of the capacitor, in ohms (Ohm). Default is 0 |
| Output | Symbol | Unit | Description |
|---|---|---|---|
| result | ΔV | V | Approximate peak-to-peak output voltage ripple, in volts (V) |
The science & history
Understanding the Parameters
-
$I_{out}$ — larger load current means more charge pulled from the capacitor each cycle and a larger ESR voltage step.
-
$C$ — larger bulk $C$ reduces the $\Delta Q/C$ portion of ripple.
- $f$ — higher frequency shortens the interval between refreshes, cutting the capacitive term.
-
$ESR$ — often dominates at high $f$ or with electrolytic caps. The live calculator labels $ESR$ as dimensionless; it is ohm. Ceramic banks can have very low ESR so the $I/(8Cf)$ term dominates; polymer/electrolytic often ESR-limited.
-
$\Delta V$ — total estimated peak-to-peak ripple (capacitive + ESR). ESL (inductance) spikes are not included.
Derivation (Approaching a Proof)
Capacitive term (classic CCM buck). In continuous conduction the inductor current is a triangle of peak-to-peak $\Delta I_L$ sitting on $I_{out}$. The capacitor absorbs the AC part of that triangle. For the standard analysis with $\Delta I_L$ spanning the full cycle symmetrically, the peak-to-peak capacitor voltage from charge alone is
$$\Delta V_C = \frac{\Delta I_L}{8\,C\,f}.$$
A common design approximation sets $\Delta I_L \sim I_{out}$ (or uses a fixed fraction); the registry form collapses to $\Delta V_C = I_{out}/(8 C f)$, i.e. the case $\Delta I_L = I_{out}$. (If you know $\Delta I_L$ from Buck Inductor Ripple, prefer $\Delta I_L/(8Cf)$ for accuracy.)
ESR term. The capacitor ESR sees the ripple current; a simple bound uses load current magnitude, $\Delta V_{ESR} \approx I_{out}\,ESR$ (conservative order-of-magnitude / full-load step style). More precise designs use $\Delta I_L\cdot ESR$ for the ESR contribution.
Sum (first order): $\Delta V \approx \Delta V_C + \Delta V_{ESR}$ when the peaks roughly align — the registry formula adds the two contributions directly.
History
Output-cap ripple equations are textbook buck-converter design (Middlebrook, Erickson/Maksimović, and app notes from every power IC vendor). As ceramic MLCC banks replaced bulk electrolytics, the balance shifted from ESR-dominated to $C$- and ESL-dominated ripple.
Related Concepts: Buck Inductor Ripple, Buck Converter Duty Cycle, Capacitor Energy, Conduction Loss
Notes: Registry calculator output-voltage-ripple (unverified). Approximate CCM buck model;
registry unit bug: $ESR$ labelled dimensionless — should be ohm. Does not model ESL or
control-loop interaction.