Power Tip 33: Beware of circulating currents in a SEPIC coupled-inductor – Part 2

Power Tip 33: Beware of circulating currents in a SEPIC coupled-inductor – Part 2

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By eeNews Europe

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

In this Power Tip, we continue our discussion from Power Tip 32 – Part 1 of establishing the leakage inductance requirements for a coupled inductor in a SEPIC topology. Previously, we discussed the fact that the coupling capacitor’s AC voltage is impressed across the leakage inductance of the coupled inductor. The voltage across the leakage inductance can induce large circulating currents in the power supply. In Part 2, we show measured results of a power supply built with a loosely coupled and tightly coupled inductor.

The circuit of Figure 1 was built and characterized. This circuit might find application in the automotive market. Here, there is a wide-ranging input of eight to 36 volts, which can be above or below the regulated 12-V output.

Figure 1: SEPIC converter can buck or boost with a single switch.
(Click on image to enlarge)

The automotive market prefers ceramic capacitors due to their wide temperature range, long life, high-ripple current rating and high reliability. Consequently, the coupling capacitor (C6) is ceramic. This means that it will have a high AC voltage across it compared to an electrolytic capacitor, and the circuit will be more sensitive to a low value of leakage inductance.

Two 47 μH Coilcraft inductors are characterized in this circuit: a MSD1260 with very low leakage inductance (0.5 μH); and a MSC1278 with a high leakage inductance (14 μH). Figure 2a and Figure 2b show the primary current waveforms for the two inductors.

Figure 2: (a-upper) Loosely coupled; (b-lower) Tightly coupled.
Low leakage (lower) causes severe circulating currents
with a coupled inductor

(Click on image to enlarge)

Looking at the upper waveform (Figure 2a), we see the input current (flowing into pin 1 of L1) with the MSC1278 inductor, and on the right is the MSD1260 input current waveform. The current in the upper figure is what is typically expected. The current is mostly DC with a triangular AC component to it.

The lower waveform (Figure 2b) is what you get with high AC voltage on the coupling capacitor and a low value of leakage inductance. The peak current is almost twice the DC input current and the RMS current is 50 percent more than that for the case of the high-leakage inductor.Obviously, electromagnetic interference (EMI) filtering of this power supply with the tightly coupled inductor is going to be more problematic. The ratio of AC input currents between the two designs is almost five-to-one, meaning another 14 dB of attenuation will be needed.

The second impact of this high circulating current is on the efficiency of the converter. With 50 percent more RMS current in the power supply, conduction losses will more than double. Figure 3 compares the efficiency for the two different inductors with nothing else changed in the circuit.

Figure 3: High leakage (MSC1278) yields better efficiency
due to reduced currents.

(Click on image to enlarge)

Both results are respectable at around 90 percent for 12 V-to-12 V conversion. However, the loosely coupled inductor yields 1-to-2 percent better efficiency over the load range, even though it has the same DC resistance as the tightly coupled inductor.

To summarize, a coupled inductor in a SEPIC converter can reduce the size and cost of the power supply. The inductor does not need to be tightly coupled. In fact, tight coupling will increase currents within the supply, complicating input filtering and degrading efficiency.

The simplest way to pick an acceptable amount of leakage is through simulation. However, you can also estimate the voltage on the coupling capacitor, set an allowable ripple current, and then calculate a minimum leakage inductance.

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

Betten, John; “SEPIC Converter Benefits from Leakage Inductance,”, May 2011.

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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