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Resonant Inductor for VHF Power Supply

IP.com Disclosure Number: IPCOM000059667D
Original Publication Date: 1986-Jan-01
Included in the Prior Art Database: 2005-Mar-08
Document File: 3 page(s) / 66K

Publishing Venue

IBM

Related People

Chung, CT: AUTHOR [+2]

Abstract

This article describes a design of a power inductor to operate at hundreds of kilohertz without overheating. A basic resonant converter off-line switching power supply topology is shown in Fig. 1. Present literature describes advancements in the frequency of operation into the hundreds of kilohertz (Very High Frequency (VHF), affording advantages in size and weight reduction of such power supplies. Special problems arise at VHF in the design of the primary inductor for output powers of 50W or more. The problems can be identified by reviewing a typical inductor designed for lower frequencies, or DC filter choke applications. Fig. 2 shows a typical low frequency inductor construction. The center portion of the core, bobbin, and winding is shown in cross-section.

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Resonant Inductor for VHF Power Supply

This article describes a design of a power inductor to operate at hundreds of kilohertz without overheating. A basic resonant converter off-line switching power supply topology is shown in Fig. 1. Present literature describes advancements in the frequency of operation into the hundreds of kilohertz (Very High Frequency (VHF), affording advantages in size and weight reduction of such power supplies. Special problems arise at VHF in the design of the primary inductor for output powers of 50W or more. The problems can be identified by reviewing a typical inductor designed for lower frequencies, or DC filter choke applications. Fig. 2 shows a typical low frequency inductor construction. The center portion of the core, bobbin, and winding is shown in cross-section. A standard "E" core 1 of magnetic material is inserted in a bobbin or coil-form 2 upon which a number of turns of magnet wire 3 are densely wound. An "I-bar" 4 of similar magnetic material is interfaced to the "E" core via a gapping spacer 5 of non-magnetic material. The spacer is usually required for inductors when high-permeability magnetic material is used. The assembly may be secured by cementing the pieces together, or by an encircling band or tape. At VHF, the typical construction of Fig. 2 experiences serious problems. "Proximity" and "skin" effects, two phenomena that occur in the presence of strong, oscillating magnetic fields, force the current in the winding to be distributed over a very small portion of the conductor cross-section, causing excessive power dissipation and overheating. Worse, conductors along the upper portion of the bobbin suffer intense heating from eddy currents induced by the fringing of the magnetic field at the gap. The result is an excessively dissipative inductor that overheats and may even burn-up. The construction of the VHF resonant inductor which eliminates these problems is shown in Fig. 3. The ferrite "E" core 1 may remain essentially the same as that of Fig. 2, on condition that the material is suited to VHF operation. The bobbin 2 can be basically the same as in Fig. 2, but the placement and type of conductors 3 must be modified. First, spacer 6 is introduced to keep the upper conductors at a distance from gap 9 roughly equal to, or greater than, the width of the gap. Spacer 7 keeps the innermost layer of conductors away from the core center-leg, and spacer 8 keeps the two layers of conductors apart. The spacers are of non...