SMPS Input Filter Inductor

L = Vin / (ΔI × f)

Quick answer

This calculator solves L = V_in / (ΔI × f_sw) for Inductance. With Input Voltage 12 V, Ripple Current 2 A, Switching Frequency 100 kHz, the result is 60 µH. It solves in 4 directions, so you can enter the values you have and get the one you need.

Calculator

Inductance (µH)
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Formula

L = V_in / (ΔI × f_sw)

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.

Symbols and units

Symbol Meaning Default unit Accepted units
L Inductance µH nH, µH, mH, H
Vin Input Voltage V pV, nV, µV, mV, V, kV
dI Ripple Current A pA, nA, µA, mA, A, kA
f Switching Frequency kHz Hz, kHz, MHz, GHz

Step by step

  1. 1. What you know: Input Voltage 12 V, Ripple Current 2 A, Switching Frequency 100 kHz
  2. 2. The relation: L = V_in / (ΔI × f_sw)
  3. 3. Substitute: Vin = 12 V, dI = 2 A, f = 100 kHz
  4. 4. Result: Inductance = 60 µH

Related quantities from the same inputs

  • P = V × I = 24 W

Nearest standard component values

Series Nearest value Error
E12 56 µH 6.7%
E24 62 µH 3.3%

Worked examples

Minimum range

Given: Input Voltage 0.33 V, Ripple Current 2 mA, Switching Frequency 100 Hz

Result: Inductance = 1.65 H

Low range

Given: Input Voltage 3.3 V, Ripple Current 20 mA, Switching Frequency 1 kHz

Result: Inductance = 0.165 H

Typical

Given: Input Voltage 12 V, Ripple Current 2 A, Switching Frequency 100 kHz

Result: Inductance = 60 µH

High range

Given: Input Voltage 0.23 kV, Ripple Current 100 A, Switching Frequency 0.5 MHz

Result: Inductance = 4.6 µH

Maximum range

Given: Input Voltage 2.3 kV, Ripple Current 1 kA, Switching Frequency 5 MHz

Result: Inductance = 0.46 µH

Solving for each variable

Solve for Inductance

Input Voltage 0.33 V, Ripple Current 2 mA, Switching Frequency 100 Hz → 1.65 H

Solve for Input Voltage

Inductance 100 nH, Ripple Current 2 mA, Switching Frequency 100 Hz → 20 nV

Solve for Ripple Current

Inductance 100 nH, Input Voltage 0.33 V, Switching Frequency 100 Hz → 33 kA

Solve for Switching Frequency

Inductance 100 nH, Input Voltage 0.33 V, Ripple Current 2 mA → 1.65 GHz

How Inductance changes with Input Voltage

Input Voltage (input) Inductance (result)
1.2 V 6 µH
3 V 15 µH
6 V 30 µH
9 V 45 µH
12 V 60 µH
18 V 90 µH
24 V 120 µH
48 V 0.24 mH
84 V 0.42 mH
120 V 0.6 mH

The same result in other units

Unit Value
nH 60000
µH 60
mH 0.06

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Frequently asked questions

How do you calculate SMPS Input Filter Inductor?

Use L = V_in / (ΔI × f_sw). With Input Voltage 12 V, Ripple Current 2 A, Switching Frequency 100 kHz, Inductance works out to 60 µH.

What is the formula for SMPS Input Filter Inductor?

The formula is L = V_in / (ΔI × f_sw). The input filter inductor in a switching power supply limits the high-frequency ripple current drawn from the input source.

How do you find Input Voltage when you know the other values?

Rearrange for Input Voltage: with Inductance 100 nH, Ripple Current 2 mA, Switching Frequency 100 Hz, Input Voltage is 20 nV.

How do you find Ripple Current when you know the other values?

Rearrange for Ripple Current: with Inductance 100 nH, Input Voltage 0.33 V, Switching Frequency 100 Hz, Ripple Current is 33 kA.

What units does SMPS Input Filter Inductor use?

Inductance is shown in nH, µH, mH. Values are held in SI base units internally, so you can mix prefixes without converting by hand.

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