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DC Bus Capacitor Calculator

Enter power, hold-up time and voltages to size your DC bus capacitor.

How it is calculated

The load must be fed for a time t from the energy the capacitor gives up as the bus sags from V1 to V2. That energy is ½·C·(V1² − V2²) and the load consumes P·t, so C = 2·P·t ÷ (V1² − V2²). Inputs: P in watts, hold-up time in milliseconds (converted to seconds inside), V1 the nominal bus voltage and V2 the lowest voltage the load still works at, both in volts. The result is in microfarads, with the energy delivered (P·t in joules) and the voltage drop V1 − V2. The tool needs V1 > V2 > 0 and a positive power and time. It assumes a constant-power load and ideal capacitors with no ESR, tolerance or ageing margin.

Worked example

A 2,000 W drive must ride through 20 ms with the bus at 600 V nominal and 450 V minimum. Step 1: time = 20 ms = 0.02 s, so energy needed = 2,000 × 0.02 = 40.00 J. Step 2: V1² − V2² = 360,000 − 202,500 = 157,500. Step 3: C = 2 × 40 ÷ 157,500 = 0.000508 F, or about 507.9 µF. Step 4: the permitted sag is 600 − 450 = 150.00 V. In practice you would choose the next larger standard value and allow margin for tolerance and ageing.

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Common mistakes

  • Entering the hold-up time in seconds. The field is in milliseconds, so 0.02 s must be typed as 20; typing 0.02 gives a capacitance a thousand times too small.
  • Using the capacitor's rated voltage as V1. Use the actual nominal bus voltage (for example the rectified level), and keep the rating comfortably above it.
  • Setting V2 to the undervoltage trip level. The load must still work at V2, so leave margin above the trip point.
  • Forgetting the efficiency. If the load power is output power, the bus must supply P ÷ efficiency, which raises the capacitance.
  • Taking the result as the final value. Tolerance, temperature, ageing and ripple current mean the fitted capacitance should be larger.

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.

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Comments (1)

  • Editorial team

    Read the energy line as what the load draws during hold-up, power times time, which is 40 J for the preset case. The capacitor bank itself holds more: at the nominal 400 V the preset 1,565.6 µF stores about 125 J, and only the part released as the bus sags to the minimum is the 40 J. What hold-up time and minimum bus voltage does your design target?

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