SMPS Input Filter Inductor

Formula

Description

The input filter inductor in a switching power supply limits the high-frequency ripple current drawn from the input source. The required inductance depends on the input voltage, the allowable ripple current, and the switching frequency. Higher switching frequencies allow smaller inductors for the same ripple current. The typical design target is a ripple current of 20-40% of the maximum DC input current. This inductor also forms part of the EMI filter, preventing conducted emissions from reaching the input power bus.

Variables

• L — Required inductance (H)
• V_in — Input voltage across the inductor (V)
• ΔI — Allowable peak-to-peak ripple current (A)
• f — Switching frequency (Hz)

Practical Notes

This simplified formula assumes a rectangular voltage waveform. For buck converters, the actual voltage across the inductor alternates between (Vin − Vout) during on-time and Vout during off-time. A more precise formula accounts for the duty cycle. Select an inductor with saturation current rating above Idc + ΔI/2 and an RMS current rating above the total RMS current.