A Guide to Designing Copper-Foil Inductors

Jul 1, 2007 12:00 PM
By Patrick Scoggins Senior Design Applications Engineer, Datatronics, Romoland, Calif.

Bobbin Fabrication

Fig. 3 outlines a custom bobbin for the E71/33/32 core. There are many paper-tube companies that will fabricate a bobbin for prototypes. Two well-known tube-bobbin vendors are Dorco and Precision Paper Tube Co.

The size and thickness of the copper foil are chosen based on the data in the fabricated-bobbin drawing. A copper width of 1.50 in. is used in this example to match the bobbin in Fig. 3. An interactive process that begins with an arbitrary choice for the initial value determines the copper thickness. The thickness chosen for the initial value in this exercise is 0.008 in. The effective area in units of circular mils would then be calculated as follows:

A good rule of thumb is to operate the inductor at no less than 500 CM of conductor cross-sectional area per ampere of current. This ensures the copper will not overheat during operation. Given an anticipated current of 20 A, it is quickly seen that the selected dimensions for the copper foil will satisfy this requirement:

Despite the reduced danger to overheating, it is still important to examine copper losses. The following formula is used to calculate the total dc resistance of the copper foil, where the mean-length turn (MLT) for each of the bobbin's 12 turns is 6290 mils:

This will result in power dissipation from copper losses, P_COPPER, as follows:

A copper loss of 1.66 W is acceptable for this example. With the copper loss, core and number of turns known, the unit can be prototyped.

Prototype Materials Selection

There are different materials to insulate the copper foil. If the copper foil is not insulated, the turns would short together. One material is kraft paper. This material is available in a number of thicknesses and it also impregnates very well. The second material to insulate copper foil is tape (either mylar or Kapton). In this example, the copper foil will be cuffed with 3M, #1205 Kapton tape. Fig. 4 illustrates how the copper foil appears after it has been cuffed in this manner.

The start and finish lead wires will be attached to the copper foil with high-temperature solder, as detailed in Fig. 5. In this application, #10 heavy (MW80C) wire for the leads will suffice. Fig. 6 shows the appearance of the completed inductor after the foil has been wrapped around the bobbin. The bill of materials needed for the complete manufacture of this component is shown in Table 2.

Table 2. Bill of materials for design example copper-foil inductor. Note that for copper, 6.5 ft includes ~3% additional mrgin.

Item	Description
Ferroxcube core	E71/33/32-3F3, gapped to 0.045 inches (center leg)
Bobbin	Custom
Copper	1.50 in. × 0.008 in. ETP (6.5 ft)
Leads	#10 heavy (MW80C)
Adhesive	Manufacturer's choice
Dolph varnish	CC-1105
3M tape	1205 Kapton
Solder	Sn10 and Sn63

Designing a dc inductor with copper foil can be achieved when considering the proper electrical parameters. The turns, gap, flux density and power loss are all critical in designing magnetics. Understanding the different types of copper and their levels of hardness is essential. Even when these aspects have been considered, any inductor design will still involve several iterations to verify electrical parameters and ensure design adequacy. However, the advantages of inductors wound with copper foil over those wound with conventional magnet wire make this process worthwhile.

References

1. Electrotube, http://www.electrocube.com/products/pdf/FoilTransformersSpecificationsPI.pdf

2. MWS Wire Industries, http//www.mwswire.com/pdf_files/mws_tech_book/MWS_Tech_Book.pdf, p. 28.

3. Lowden, Eric, Practical Transformer Design Handbook, 2^nd Edition, Tab Books, pp. 9 and 290.

4. Ferroxcube Soft Ferrites and Accessories Data Book, 2005.

5. Ridsdale, R.E., Electric Circuits, 2^nd Edition, McGraw Hill Inc., 1984, p. 88.

6. Precision Paper Tube Co., Catalog No. P197, p. 4.

7. DORCO Electronics Inc., http://www.dorco.com/.