SMPS Inrush Current Calculation for Circuit Breaker Selection

Calculating Inrush Current of SMPS for Selecting Circuit Breakers

When connecting a switched-mode power supply (SMPS) to the grid, a peak inrush current occurs due to capacitor charging, reaching 100+ times the rated current. This can trip circuit breakers even with a power margin. The pulse duration is 50–2000 µs, with maximum amplitude when switching on at the peak grid voltage (325 V for 230 V). SMPS topologies determine the magnitude: classic with a capacitor on the primary side yields tens of amps, while PSR yields single-digit amps.

Main SMPS Topologies and Their Inrush Characteristics

SMPS are built on two topologies.

Topology 1 (Classic): A large electrolytic capacitor (units to hundreds of µF) is placed after the diode bridge on the primary side. It charges to the grid peak voltage. Inrush current is mainly from charging this capacitor (up to 98%), plus small capacitances from the EMI filter. Duration is hundreds of µs, peak is tens of A. Used for powers of 50+ W, including SMPS with power factor correction (PFC).

Topology 2 (PSR): A large capacitor is on the secondary side, with nanofarad capacitances on the primary. Inrush current comes from oscillations of reactive grid and filter elements. Peak is single-digit amps, duration is tens of µs. Common in power supplies up to 60 W.

Measuring Inrush Current Parameters

Inrush current is characterized by a duo: I_max (amplitude) and T_i (duration at I_max/2 level). It depends on the switching phase: maximum at the sine wave peak, minimum near zero (difference of 8–10 times).

For topology 1, the pulse is RC-CR: the leading edge is defined by L_sum/R_sum (τ_n = L_sum / R_sum), the trailing edge by C1 R_sum (τ_c = C1 R_sum).

Real pulses include ringing (T_ring) from reactive elements. Duration T_u = T_o.i + T_ring if ringing energy is negligible (I_ring < 0.1 I_max T_o.i) or T_o.i > 200 µs.

For topology 2, it's damped oscillations. Convert to RC-CR equivalent:

• RMS of each half-cycle: I_n = 0.7 * |a_n|
• I_max = 0.7 * a1
• T_i based on envelope at I_max/2 * 0.7 level

Recommendation: Use an oscilloscope digital filter (5–20 kHz) for noisy signals.

Inrush Currents of Parallel SMPS

For N parallel SMPS, I_max_sum ≠ N * I_max (ideal grid). Consider grid parameters: Rc, Lc. Standard IEC63129 suggests Lc = 100 µH, short-circuit current 400–460 A (Rc ≈ 0.5–0.6 Ω for 230 V).

Simulation shows:

• In a real grid, peaks are distributed across phases, total I_max is less than N-fold.
• Grid inductance smooths edges, reducing overall peak.

Calculation steps for a group of SMPS:

• Measure I_max, T_i for each.
• Estimate Lc, Rc of the grid (from design or standard).
• Simulate total pulse in Excel: account for switching phase shifts.
• Check against circuit breaker trip curve (integral i²t).

Selecting a Circuit Breaker Considering Inrush Current

Circuit breakers trip via thermal (long-term) or electromagnetic (instantaneous) releases. For SMPS, the magnetic release is critical: proportional to i²t.

• Use characteristic curves (B, C, D): D-class for high inrush (motor starters, SMPS).
• Margin: breaker rating 1.5–2x total operating current.
• For groups: sum impulse energies, not peaks.

Example: 10 LED fixtures (SMPS topology 2). Each I_max=5 A, T_i=50 µs. Ideally 50 A, in grid with Lc=100 µH—real peak ~20–30 A. A 10–16 A type D breaker won't trip.

Key selection factors:

• SMPS topology.
• Switching phase.
• Grid parameters (Lc, Rc).
• Release characteristic.
• Number of parallel loads.

Key Takeaways

• SMPS inrush current is capacitor charging + oscillations, peak up to 100x rated at 325 V switching.
• Topology 1: tens of A, hundreds of µs; topology 2: single-digit A, tens of µs.
• For parallel SMPS, total peak < N * I_max due to grid effects.
• Choose D-class breakers, simulate i²t in Excel.
• Measure with 5–20 kHz filter, T_i at I_max/2.

— Editorial Team