Inductors

Core material

Recommended core material depends on switching frequency

  • < 100 kHz : Iron powder, ferrite, Superflux
  • between 100 kHz and 1 MHz : Ferrite, Superflux
  • > 1 MHz : Ferrite

Inductance value

  • Higher inductance value = smaller ripple current
  • Lower inductance = higher ripple current

Rule of thumb formula :

Step down converter

L={(U_inmax-U_out)*(U_out)}/{(U_inmax)*0.3*I_out*f}

Step up converter

L={(U_out-U_inmin)*{U_{i n}}^2}/{2*0.2*I_out*(U_out)^2}

Inductance current rating

Maximum current can be replaced by saturation current.

Step down converter

Nominal current : I_n=I_out
Maximum current : I_max=1.5*I_n

Step up converter

Nominal current : I_n={U_out/U_{i n}}*I_out
Maximum current : I_max=2*I_n

Inductor DC resistance

For a same inductor package size :

  • Higher inductance means higher DC resistance
  • Lower inductance means lower DC resistance
  • Shielded inductors have a lower DC resistance at same size

EMC

To reduce EMC, use magnetically shielded power inductors.
Do not route any conductor tracks under the inductor.
Inductors packages can be changed from shielded to unshielded while maintaining solder pad compatibility.

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