DC Bus Capacitor Calculator

About this calculator

A DC bus capacitor bank provides hold-up energy so that a motor drive, UPS, or switching power supply keeps running through brief AC line dropouts or the ripple gaps between rectifier pulses. When line power drops, the bus voltage sags as the capacitor discharges into the load; this calculator finds the bulk capacitance needed to keep the bus above a minimum usable voltage for a required hold-up time.

The sizing formula comes from the energy stored in a capacitor, ½CV², compared before and after the sag: C = 2·P·t / (V_nom² − V_min²), where P is the load power, t is the hold-up time, V_nom is the nominal bus voltage, and V_min is the lowest voltage the downstream electronics can tolerate. Doubling the hold-up time or halving the allowed voltage droop both roughly double the required capacitance.

Power electronics engineers use this when designing variable-frequency drives, servo amplifiers, and off-line power supplies that must ride through a single missing AC half-cycle (around 8–10 ms at 50/60 Hz) or a longer utility sag without tripping an undervoltage fault.