Hold-Up Time Capacitor

Formula

Description

Hold-up time is the duration a power supply can maintain its output after the input power is interrupted, using energy stored in the bulk electrolytic capacitors. The energy available is E = ½C(Vmax² − Vmin²), where Vmax is the fully charged DC bus voltage and Vmin is the minimum voltage at which the downstream converter can still regulate. For server and telecom power supplies, the typical requirement is 20 ms (one full cycle of 50 Hz mains) of hold-up time to ride through brief power interruptions without disrupting the load.

Variables

• t — Hold-up time (s)
• C — Bulk capacitance on the DC bus (F)
• V_max — DC bus voltage when fully charged (V)
• V_min — Minimum DC bus voltage for regulation (V)
• P — Load power (W)

Practical Notes

Example: 400 V DC bus, 300 V minimum, 200 W load, 20 ms required: C = 2 × 200 × 0.02 / (400² − 300²) = 8/70000 ≈ 114 µF. Use 120 µF or 150 µF standard value. The hold-up capacitor is typically the largest and most expensive component in the power supply. For higher power or longer hold-up times, consider adding a separate energy storage stage with supercapacitors.