How to Select the Right Inductor for Your SMPS Design

How to Select the Right Inductor for Your SMPS Design

Switch Mode Power Supplies (SMPS) rely heavily on inductors for energy storage, filtering, and maintaining output stability. Choosing the wrong inductor can lead to excessive ripple, overheating, EMI problems, or even power supply failure.

Why the Inductor matters in SMPS design

  • Stores energy when the switch is ON
  • Releases energy when the switch is OFF
  • Smooths output current
  • Controls ripple and efficiency
Warning: A poorly selected inductor can cause high ripple, overheating, and reduced efficiency.

Step 1: Identify Your SMPS Topology

Common converter types include Buck, Boost, Buck-Boost, and Flyback. Each topology requires different inductance calculations.

Step 2: Define Key Electrical Parameters

ParameterExample
Input Voltage (Vin)24V
Output Voltage (Vout)12V
Output Current (Iout)2A
Switching Frequency200 kHz
Ripple Current Target30% of Iout

Step 3: Calculate Required Inductance (Buck)

L = [ Vout × (Vin − Vout) ] / [ Vin × fsw × ΔIL ]

Design Tip: Higher inductance reduces ripple but increases size.

Step 4: Check Saturation Current (Isat)

Peak current:

Ipeak = Iout + (ΔIL / 2)
  • Choose Isat ≥ 1.25 to 1.5 × Ipeak
  • Prevents core saturation and failure

Step 5: Verify RMS Current Rating

  • Irms should be ≥ Iout
  • Low rating causes overheating

Step 6: Choose Core Type

Ferrite Core Inductors

Ferrite Core Inductor
  • Best for high-frequency SMPS
  • Low core losses
  • Sharp saturation behavior

Powdered Iron Inductors

Powdered Iron Inductor
  • Soft saturation
  • Good for high current
  • Higher losses at high frequency

Step 7: Check DCR

Ploss = Irms² × DCR

Lower DCR improves efficiency and reduces heating.

Step 8: Verify Size and Temperature Rise

  • Ensure PCB footprint fits
  • Keep temperature rise below 40°C
  • Check manufacturer thermal curves

Step 9: Check Self-Resonant Frequency

  • SRF ≥ 5–10 × switching frequency
  • Avoid capacitive behavior at high frequency

Quick Selection Checklist

    ✅ Correct inductance value
    ✅ Isat margin available
    ✅ Irms adequate
    ✅ Low DCR
    ✅ Proper core material
    ✅ Shielding as required
    ✅ Size fits PCB
    ✅ SRF sufficiently high

Common Design Mistakes

    ❌ Selecting by inductance only
    ❌ Ignoring saturation current
    ❌ Ignoring DCR losses
    ❌ Skipping thermal testing

Final Thoughts

Selecting the right inductor is a balance of electrical performance, thermal behavior, EMI, size, and cost. Proper calculation and lab validation ensure a reliable SMPS design.