Coupled Inductors for SEPIC Converter Applications

This post will explain the functionality, typical applications, typical layout, and benefits of a SEPIC converter.

What Is a SEPIC?

Before going into detail about coupled inductors for SEPIC converters, we must define what a SEPIC converter is. SEPIC stands for single-ended primary inductance converter. This circuit offers the possibility to use the function of a buck and boost regulator in one circuit.

A buck converter regulates a high voltage into a smaller voltage, so input voltage is greater than output voltage. For example, for a car charger for a cell phone, 12 V is converted down to 5 V.

On the other hand, a boost converter regulates a small voltage into a higher voltage, so input voltage is smaller than output voltage. For example, an LED driver with a 3 V input converts to a 9 V output.

A SEPIC converter combines those functions and allows a circuit to have flexible input voltages with a stable output voltage. This type of converter can also be described as a buck/boost converter.

Typical SEPIC Applications

Typical SEPIC applications include the following:

  • Battery-operated equipments and handheld devices
  • NiMH chargers
  • LED lighting applications
  • DC power supplies with a wide range of input voltages

For more information, please see our product training modules on Digikey.com.

Typical Layout of a SEPIC Converter

In this image, you can see a typical layout of a SEPIC converter from Linear Technology, which uses two 22 uH inductors, or L1 and L2, in the circuit. The capacitor, C6, basically splits the windings into a primary and a secondary.

This design can now be done as shown in this example by using two individual inductors with 22 uH, but there are clear advantages when using one of Wurth Electronics Midcom’s coupled inductors in this design.

A coupled inductor basically consists of two identical windings wound onto one core. The important criteria is that the windings are exactly identical to generate the coupling effect in a SEPIC converter.

You might be asking yourself why you would use a coupled inductor instead of two inductors. Here are the main advantages:

  • Less space on PCB
  • Less cost
  • Input ripple current cancellation (resulting in smaller input capacitor and simpler EMI input filter)
  • Increased efficiency (due to smaller inductor value [half of uncoupled SEPIC] and therefore less DCR and less winding losses)

Stay tuned to our blog to learn more about which coupled inductor families are available from Wurth Electronics Midcom as well as how to calculate an inductor for a SEPIC converter. To watch the entire training video, visit our company page on DigiKey.com

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