Minimize Winding Losses in High-Frequency Inductors

Jul 1, 2008 12:00 PM
By Weyman Lundquist, President and Engineering Manager, West Coast Magnetics, Stockton, Calif.

Shaped-Foil Technology

Foil windings are a compelling alternative because of the comparatively high window utilization and very low resulting dc resistance. West Coast Magnetics has worked with the Thayer School of Engineering at Dartmouth to develop a shaped-foil technology^[1] that has both low dc and low ac copper losses. The new technology shapes the foil in the vicinity of the gap to use the gap-fringing flux to equalize the current distribution throughout the foil and minimize the skin and proximity effects.

An inductor employing this technology combines the very low dc resistance of a copper-foil winding with the low ac resistance of a Litz-wire winding. In particular, for high-current, high-ripple inductors, the shaped-foil winding technology can be the lowest-loss solution.

An experiment was conducted to compare the new foil technology to conventional windings including solid-wire, Litz-wire and full unmodified-foil windings.^[2] For this experiment, a series of 90-µH, 40-A inductors were built and tested using best practices. Each of the prototype inductors was constructed using the same E70/33/32 ferrite core of material EPCOS N67, which had a 2.63-mm gap in the center leg (designated B66371-G-X167). Fifteen turns were wound on all the inductors, producing an inductance of 90 µH to 95 µH.

The windings are shown in Fig. 1. In each case, best winding practices were employed, which included single-layer windings for the Litz and solid-wire samples, and in every case, the conductor area was maximized in the winding window.

The solid-wire sample consisted of four layers of 10-gauge wire, 15 turns per layer, with the windings connected in parallel. Two Litz-wire samples were constructed, one using 500 strands of 40 AWG Litz wire and the other 80 strands of 32 AWG Litz wire. As with the solid-wire prototype, these inductors were wound in four parallel layers of 15 turns.

A standard foil inductor was constructed of 15 turns of 0.020-in. copper, measuring 1.55 in. wide and separated by 0.003 in. × 1.69 in. of Nomex insulator. Two samples were built using the new shaped-foil technology. One sample had a foil shape optimized for a current of 40 Adc with a 15% (6-A peak-to-peak) ripple at 50 kHz, and the other sample was optimized for a ripple of 22.5% at 50 kHz.

The total core and copper losses for each of these inductors were determined experimentally and the results are plotted in Fig. 2. The hatched area illustrates the loss reduction that is observed with the new foil-winding technology versus conventionally wound inductors. Core losses were the same for all the inductors, so the loss reduction is observed in the windings exclusively.

Experimental data was collected for these sample inductors at ripple-current values from 1% to 30% and frequencies from 10 kHz up to 500 kHz. This data demonstrates that the shaped-foil technology was the lowest-loss solution for ripple-current values exceeding 10% and at frequencies from 25 kHz up to 500 kHz.

Litz-Wire Windings

Other work done by the Thayer School of Engineering and West Coast Magnetics has led to advances in the use of Litz wire for winding gapped-power inductors. The field around the gap in a power inductor can be quite strong and create localized losses in windings close to the gap.

For a given core and bobbin geometry, it has been shown that there is an optimal solution for Litz-wire stranding and placement inside the bobbin. By choosing the Litz stranding and placement of the winding inside of the bobbin window, it is possible to minimize winding losses.

A freeware simulation program called ShapeOpt allows the user to optimize the stranding and positioning of the winding inside of the available winding window. This program is available for use by designers at www.thayer.dartmouth.edu/inductor/shapeopt. The program is simple to use and the inputs include:

• Core-window width and height

• Bobbin-window width and height

• Ripple-current magnitude and frequency

• Gap length

• Bobbin-fill factor

• Litz-wire strand diameter

• Turn length

• Number of turns.

With this information as input, the program will generate an output detailing the field strength in the bobbin window as well as the ideal placement of the winding in the bobbin window. The program also will determine the total winding losses and choose the number of strands necessary to fill the available window area.

By way of example, consider a 10.6-µH inductor operating at 250 kHz with a 4-A rms ripple. This inductor uses an E19/8/5 core with a 0.65-mm gap and 13 turns. With a Litz-wire strand diameter of 0.05 mm (44 AWG) selected, ShapeOpt yields an optimal result of 314 total strands and a total winding loss of 0.28 W. Fig. 3 details the placement of the winding inside the bobbin window with the green area being acceptable for winding and the white area being void space.

In practice, this bobbin could quickly be prepped with a tape to mask out a rectangular cross section, which closely approximates the result shown in Fig. 3. This type of tape prep is very similar to the margin-tape application widely used to maintain creepage and clearance values to meet isolation requirements in transformers.

From this simulation, it can be seen that it is not optimal to completely fill the bobbin with wire. In fact, that approach would result in much higher losses. This problem becomes even more acute as the frequency increases.

Fig. 4 is an example of an optimal solution for a single inductor as a function of frequency. As the frequency of the power inductor increases, the placement of the winding inside the core window becomes critical to obtaining the minimum-loss solution.

Another topic investigated by Thayer researchers using the ShapeOpt program was the optimal ratio of core-winding window height to core-winding window width. Obtaining this optimal ratio involves solving for the total winding losses, while holding two factors constant (winding-window perimeter and Litz-wire stranding) and varying the window height to window breadth. When this is done, some interesting conclusions can be drawn.

At low frequencies, in the 1-kHz to 10-kHz range, the optimal ratio of window width to height is about 1. By the time the frequency increases to 500 kHz, the optimal ratio of width to height increases to about 2.

Comparing this result to the ratio of height to width for several typical E-core geometries (Fig. 5), it quickly becomes apparent that most core and bobbins available on the market today do not have optimal shapes for power inductor designs using gapped cores.

References

1. The shaped-foil technology is patented by Dartmouth College, and West Coast Magnetics is a licensee.

2. This experiment was designed by West Coast Magnetics and electrical engineer Jennifer Pollock, Ph.D., of Dartmouth's Thayer School of Engineering. Professor Charles Sullivan of the Thayer School of Engineering and Ryan Goldhahn, a Duke University Ph.D. candidate, also were involved with the experiment.

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