This is the first of two articles on the topic of planar magnetics. In this first part, a general overview of this technology is presented to acquaint the reader with these construction concepts.

Not long ago, an eye-catching advertisement appeared in Electronic Products magazine. What made the photo in this advertisement so notable was the presence in it of a large towering magnetic assembly sitting in the rear section of a densely populated computer
board, surrounded by numerous surface-mounted IC chips and other miniature omponents. Figure 1 is a reproduction of that photo.

While the amusement purpose of the advertisement was to encourage readers to visit the magazine's web site, it was visual evidence for many of us working in the magnetics industries today of the way users and system engineers often view power magnetics. The physical size of these components is not the only reason that power magnetics are
often viewed as "necessary-but-evil" parts in many designs. Other reasons include high costs to manufacture and assemble. In addition, they can and do add significant weight to the power supply of an system.

Thinking Thin….
To reduce the size and cost problems mentioned earlier, designers are now turning to new assembly design techniques for power magnetic components. These techniques revolve around the use of lowheight magnetic cores and windings that do not require mounting bobbins. Figure 2 is an illustration of these new planar construction methods for circular ("potcore- like") and rectangular ("box-core-like") magnetic assembles.

In the circular case in this figure, the windings are realized using printed-circuit-board (PCB) assembly methods while, in the case of the rectangular E-E or E-I core assembly shown, the winding(s) could be made from flat coils of formed wire. In either case, there is no wiring "bobbin" in either assembly approach. Figure 3 is a photo of some PCB windings used in a typical planar E-E core magnetic assembly.

Looking at the circular core-and-winding arrangement depicted in Figure 2, it is easy to see why planar magnetic construction approaches can be inexpensive, requiring no significant labor talent for assembly nor special assembly equipment. Because the windings are
"built" using PCB design techniques, turns positioning are precise and consistent, yielding magnetic designs with highly controllable "parasitics" (e.g., winding resistances, leakage inductances and capacitances).

As far as component height is concerned, a magnetic in a medium power application today is considered a planar design if its height dimension is on the order of 0.5 inches or less. For power magnetics in modern telecommunications rack-mounted power supplies, the
height requirement can be even lower, on the order of 0.25 inches!

On the other hand, low-profile power magnetics often have larger surface areas than their wired, high-profilecounterparts, requiring more packaging space as a result. However, larger surface areas gives the user more effective unit areas for cooling the part, which
can be an important advantage in those cases where the magnetics in question are used in high-power applications.

Most planar magnetics assemblies today utilize power core materials of the manganese-zinc
(Mn-Zn) or nickel-zinc (Ni-Zn) ceramic varieties, with the Mn-Zn material being the most
popular today in applications where power-processing frequencies are less than 2 MHz. Above this frequency, the Ni-Zn material becomes more efficient, as its resistivity is several orders or magnitude larger than Mn-Zn. Because of the ease of manufacture, ferrite cores can be made in many shapes and sizes and, for this reason, is the primary
core material for planar magnetics designs. The only disadvantage of ferrite material is the relatively low value of saturation flux density when compared to other core material types available today.