Power Tip 32: Beware of circulating currents in a SEPIC coupled-inductor – Part 1

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(Editor’s note: you can see a linked list of all entries in this series here.)

 In this Power Tip, we establish the leakage inductance requirements for the coupled-inductor in a SEPIC topology. The SEPIC is a useful topology when electrical isolation between the primary and secondary circuit is not required and when the input voltage is higher or lower than the output voltage. It can also be used in place of a boost converter when short circuit protection is required.

A SEPIC converter features single-switch operation and continuous input current, resulting in low electromagnetic interference (EMI). The topology (Figure 1) can use two separate inductors or, since the voltage waveforms across inductors are similar, can use a coupled-inductor, as shown.

Figure 1: A SEPIC converter uses a single switch to step-up or -down the output voltage.

The coupled-inductor is attractive because its volume and cost are less than two individual inductors. The downside is standard inductors are not always optimized for the entire range of possible applications.

The current and voltage waveforms in this circuit are similar to a continuous current mode (CCM) flyback design. When Q1 is turned on, it applies the input voltage across the primary of the coupled inductor to build energy in the circuit. When Q1 is turned off, the voltage on the inductor reverses and is clamped to the output voltage.

Capacitor C_AC is what differentiates the SEPIC from the flyback; when Q1 is on, the secondary inductor current flows through it to ground. When Q1 is off, primary inductor current flows through C_AC, adding to the output current that is flowing through D1. The big advantage to this topology over a flyback is that both the FET and diode voltages are clamped by C_AC, and there is little to no ringing in the circuit. This results in the ability to select lower-voltage, and hence more-efficient, power devices.

Since this topology is similar to a flyback, many people think that a tightly coupled set of windings is required. However, this is not the case. Figure 2 shows the two states of operation for the continuous SEPIC where the transformer has been modeled with leakage inductance (LL), magnetizing inductance (LM), and an ideal transformer (T).


(DC component cancels)



(DC component cancels)

Figure 2a and 2b: Both states of operation of the SEPIC converter. The AC voltage on the leakage inductance equals the coupling capacitor voltage.

Upon inspection, the voltage across the leakage inductance is equal to the voltage across C_AC. So a large AC voltage from a small value of C_AC, or a small leakage inductance, creates a large circulating current. A large circulating current will degrade the efficiency and EMI performance of a converter, which is undesirable.

One method of reducing this large circulating current is to increase the coupling capacitance (C_AC). However, this carries a cost, size, and reliability penalty. A more prudent approach is to increase the leakage inductance, which is easily done when specifying a custom magnetic component.

Interestingly, few vendors have recognized this fact and many have produced inductors with low-leakage inductance for SEPIC applications. On the other hand, Coilcraft has a 47 μH MSD1260 with a leakage inductance of approximately 0.5 μH, and has recently developed alternate versions of this design with leakage inductances over 10 μH, which we will characterize in March’s Power Tip. Please rejoin us then.

For more information about this and other power solutions, visit:


  1. Betten, John; “SEPIC Converter Benefits from Leakage Inductance”,,
  2. Coilcraft Catalog, MSD1260 Data sheet, also contact for parts with higher leakage inductances,

About the author

Robert Kollman is a Senior Applications Manager and Distinguished Member of Technical Staff at Texas Instruments. He has more than 30 years of experience in the power electronics business and has designed magnetics for power electronics ranging from sub-watt to sub-megawatt with operating frequencies into the megahertz range. Robert earned a BSEE from Texas A&M University, and a MSEE from Southern Methodist University.


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