Server Power Architecture & Energy Storage Sizing
DC-Link Hold-Up Capacitor Line-Dropout Sizing Formula
Calculate Server PSU Dropout Energy Demand (Joules), Usable Capacitor Discharge Fraction 1 - (V_min / V_nom)^2, Required Bulk Hold-Up Capacitance C_uF = (2000 * P_out * t_ms) / (eta * (V_nom^2 - V_min^2)), and Nominal Stored Joules.
Size bulk PFC hold-up capacitors from V_nom^2 - V_min^2—never 0.5*C*V_nom^2 alone!
Data-center server PSUs (ITIC / CBEMA curve) and semiconductor fab tools (SEMI F47) must ride through a full 20-millisecond AC grid cycle dropout without dropping regulated DC rails! A classic beginner error is setting total stored capacitor energy 0.5 * C * V_nom^2 equal to load energy—forgetting that the downstream DC-DC converter shuts down at its under-voltage lockout threshold V_min > 0, leaving (V_min / V_nom)^2 of stored energy TRAPPED inside the capacitor! Equating extractable electrostatic energy 0.5 * C * (V_nom^2 - V_min^2) to DC-bus demand (P_out_W / eta_dc) * (t_hold_ms / 1000) yields Required Bulk Capacitance in microfarads: C_uF = (2,000 * P_out_W * t_hold_ms) / (eta_dc * (V_nom^2 - V_min^2))!
Verify on a 630 W Server PSU—Rated_DC_Load P_out = 630 W (B2), Downstream_DC_DC_Efficiency eta = 0.90 (B3 -> Bus Power = 700 W), Required_Holdup_Time t_hold = 20 ms (B4 -> Required Dropout Energy = 700 * 0.020 = 14.00 J in E2), Nominal_PFC_Bus V_nom = 400 V (B5), and Brownout_UVLO_Threshold V_min = 300 V (B6): Usable Energy Fraction is 1 - (300 / 400)^2 = 0.4375 (43.75% in E3); Required Hold-Up Capacitance is (2,000 * 630 * 20) / (0.90 * (160,000 - 90,000)) = 28,000,000 / 70,000 = 400.00 uF (E4); and Total Nominal Stored Energy at 400 V is 0.5 * 0.0004 * 160,000 = 32.00 J (E5, of which 18.00 J stays trapped at 300 V)!
| ITIC 20-ms Hold-Up Metric (630W Load, 90% Eff, 20ms, Vnom=400V -> Vmin=300V) | Formula | Verified Output |
|---|---|---|
| Dropout Energy Demand (Joules) | (P_out_W / eta_dc) * (t_hold_ms / 1000) | 14.00 J |
| Usable Capacitor Energy Fraction | 1 - (V_min / V_nom) ^ 2 | 0.4375 (43.75% Extractable) |
| Required Bulk Capacitance (uF) | (2000*P_out*t_ms) / (eta*(Vnom^2 - Vmin^2)) | 400.00 uF (1,600 uF @ 200->150V!) |
| Total Stored Energy at V_nom (J) | 0.5 * (C_uF / 10^6) * V_nom^2 | 32.00 J (18.00 J Residual @ 300V) |
Bulk PFC Hold-Up Capacitance (uF) Sizing Formula
Validate P_out > 0, 0 < eta <= 1, t_hold > 0, and strictly ordered bus voltages 0 < V_min < V_nom!
=LET(p_ot, B2, et_d, B3, t_ms, B4, v_nm, B5, v_mn, B6, IF(OR(p_ot <= 0, et_d <= 0, et_d > 1, t_ms <= 0, v_mn <= 0, v_nm <= v_mn), "Invalid Holdup Bus Spec", ROUND((2000 * p_ot * t_ms) / (et_d * ((v_nm ^ 2) - (v_mn ^ 2))), 2)))Voltage-Squared Scaling Check (Halve Bus to 200 V -> 150 V)
Halve V_nom to 200 V and V_min to 150 V: Usable Fraction E3 stays 0.4375, yet Required Capacitance E4 QUADRUPLES from 400.00 uF to 1,600.00 uF—explaining why front-end Boost PFC stages step up to 400 V!
=IF(ISTEXT(E4), E4, ROUND(0.5 * (E4 / 1000000) * (B5 ^ 2), 2))Try the example
Use the original values or change them to test your understanding.