Output Voltage Ripple

Output voltage ripple

The output of a switching converter is never a perfectly flat DC voltage — riding on the average value there is always a small residual oscillation called ripple, at the switching frequency. It arises because the output capacitor, which filters the pulses, charges and discharges a little every cycle, feeding current to the load in between. A simple estimate of the amplitude is ΔV = I ÷ (f × C), where I is the output current, f the switching frequency and C the output capacitance. The formula points to the two routes for reducing ripple: raise the frequency (pulses come closer together, so the capacitor discharges less between them) or raise the capacitance (more charge in reserve). Keeping ripple low matters, since sensitive circuits (microprocessors, A/D converters, RF, audio) may misbehave or generate noise on a dirty supply — the typical specification is ripple below 1% of the output voltage. One practical detail: in real capacitors, part of the ripple stems from the equivalent series resistance (ESR) of the capacitor (ΔV_ESR = I × ESR), which sometimes dominates over the capacitive term — which is why quality supplies use low-ESR capacitors (ceramic, polymer). Ripple also carries a high-frequency component (switching spikes) that demands good physical layout to keep under control. Enter the current, the switching frequency and the capacitance.