1. Field of the Invention
The subject invention generally pertains to magnetic circuit elements, and more specifically to power magnetic circuit elements used in high frequency switched mode power electronic converter circuits.
2. Description of Related Art
Planar transformers and inductors have been used in switched mode power converter circuits for some time. Planar magnetics offer packaging flexibility in designs where component height is limited. Planar magnetics also offer assembly advantages over machine wound magnetic components because the planar magnetics typically have their windings machine etched on a printed wiring board or similar insulating substrate and no hand soldering is required. The printed wiring board winding method results in lower labor costs and simplified assembly. The printed wiring board winding method also results in greater uniformity. Typically one layer of the printed wiring board will contain one or more turns of a winding. Using a multi-layer printed wiring board the winding segments on each layer can stand alone as a complete winding or combine with other layers in a series or parallel combination to yield the desired number of turns and the desired winding resistance for a given winding. When more than one turn is placed on a single layer the winding is wound in a spiral pattern to accomplish the desired number of turns.
In a planar magnetic circuit element the electrical conducting material is copper foil which is bonded to an insulating substrate such as a glass fiber filled epoxy. Current near the outer edge of the spiral winding creates flux perpendicular to the plane of the foil in the foil segments nearer to the center of the winding. This flux creates an eddy current that flows in a loop such that the net current towards the outside of the winding is decreased or reversed and the current towards the center of the winding is significantly increased. The AC current in the foil is forced to the edges of the foil. This problem is magnified as the number of turns increases and as the center of the winding is approached. For a copper trace on the outside perimeter of the spiral winding current is forced to the inner edge of the winding by the eddy current effects, so that the total AC current is confined to the inner edge of the trace and the AC current in the remainder of the trace is zero. For the next trace in from the outermost trace a current equal to the total AC current is forced to the outer edge, but reversed in direction. At the inner edge of this second trace in from the outer perimeter the current is in the direction expected but the magnitude is doubled. All of the AC current is confined to the inner and outer edges of the trace due to the eddy currents. For the third trace in from the outer perimeter the current at the outer edge of the trace is equal to twice the total net AC current in the trace and the current at the inner edge is equal to three times the total trace current. As the center of the spiral is approached the magnitude of the flux causing eddy currents increases along with the conduction losses. This problem is well known and is called proximity effect. The proximity effect forces AC current towards the edges of the copper foil segments and out of the interior of the copper foil segment. The proximity effect causes a large increase in AC winding resistance and an increase in conduction losses. A planar magnetic that is constructed to avoid these proximity effects has a significant performance advantage in reduced AC conduction losses and extended frequency range.
Another problem with spiral wound planar magnetics is that the area and volume of the circuit that is dedicated to providing a return path for winding currents outside of the core window is large and results in a low space utilization factor, higher winding resistance, and associated conduction losses, both DC and AC. A planar magnetic that provides a short, low volume, return path for winding currents offers a significant advantage.