Sepic Converter Inductors

Application Features

The SEPIC converter allows the output voltage to be greater than, less than, or equal to a required nominal voltage.

  • The output voltage polarity is the same as the input voltage thus allowing a common ground.
  • DC isolation is provided by a capacitor.
  • The two inductors can be wound on the same core.
  • Efficiency is significantly higher than most other DC-DC converters.

Requirements

The SEPIC converter works best in a continuous-conduction mode.  This is when the current through L1 never falls to zero.  This is similar to the familiar DC choke input filter where the inductive reactance minimum is 2/3 of the minimum load resistance.  One simple check is to make the nominal value equal to the load resistance to allow for a ± tolerance of the inductors and roll-off with DC.

  • The voltage across capacitor C1 can reverse every cycle, so a non-polarized capacitors should be used for C1.  They should also have a low Equivalent Series Resistance (ESR).
  • The diode’s switching time needs to be fast in order not to generate high voltage spikes across the inductors.
  • The inductor windings can be on the same core.  Therefore, standard DC chokes like MCE’s DL series with 2, 4, or 6 windings can be split.  The average current through L2 is the same as the average load current.