Dual Ferrite-Bead Inductor Saves Space and Cost in Multi-Phase Converters

An integrated dual inductor developed by Pulse saves space and improves efficiency in multiphase dc-dc converters such as those used to power CPU cores and DDR memory.

The PA0766NLT Power Bead inductor combines two independent inductors into a single ferrite core structure with no magnetic coupling between the two inductors. The component has a footprint measuring 0.55 in. × 0.53 in. × 0.28 in. In the multiphase converter, this dual ferrite-bead inductor replaces two surface-mount powdered iron inductors. In so doing, the PA0766NLT promises a 20% savings in board space, a 47% reduction in dc resistance, an 80% lower core loss and reduced ac proximity loss in the winding. At the same time, the integration is projected to lower cost by 25%.

The PA0766NLT is available in four different inductance values and current ratings ranging from 296 nH at 38 A peak current per phase to 568 nH at 18.5 A peak current per phase. The two independent inductors can be connected in series or parallel to form a single inductor with an inductance and current range from 150 nH at 76 A peak current to 770 nH at 25 A peak current. In addition, the direct current resistance is 0.9 mΩ maximum per phase and the part is rated for operation up to 130°C. The PA0766NLT is also RoHS compliant.

Until now, multiturn, powered iron inductors with an E core have typically been the output inductor of choice in multiphase dc-dc converters because they provide sufficient inductance at the desired switching frequencies. These devices saturate gradually with increasing current, so that they maintain the needed inductance even in the high-current CPU power applications.

In contrast, ferrite bead inductors have typically been ill suited to these low-frequency switching applications. Though multi-turn ferrite beads could potentially be used in multiphase converters, they are generally avoided because they are either too large or too costly for the applications. On the other hand, single-turn ferrite beads have lacked sufficient inductance and sufficiently high saturation currents to be usable at the desired switching frequencies.

According to John Gallagher, field applications engineer at Pulse, the inductance of these single-turn ferrite beads is limited by the physical constraints of their construction. In a single-turn ferrite inductor, the core consists of an I-shape section sitting a top a U-shape section with two air gaps separating the two sections. The material used to fill these gaps establishes a minimum spacing requirement, which limits the maximum inductance achievable.

However, in the integrated power bead inductor, smaller air gaps are feasible. In this design, an I–shaped section of core material rests atop an E-shaped core material with air gaps formed between the outer legs of the “E” and the “I.” A single-turn winding is formed on each side of the I (see the diagram). In this case, a high-temperature epoxy is used to fix the spacing in the air gaps and the spacing achieved here is smaller than that achieved in the single inductor ferrite bead. Moreover, in the integrated design, each inductor has just one air gap, which further helps achieve a higher value on each inductor.

Though conceptually simple, the production of such a component in high volume poses process challenges. Specifically, the spacing of each gap must be kept equal. If the gaps become unequal, a teeter-totter affect comes into play, which can introduce coupling between the two inductors. The selection of the proper epoxy, which must withstand the rigors of leadfree, 260°C reflow, is a critical factor in maintaining equal spacing of the air gaps.

Though the initial version of the integrated inductor is a 2-phase design, the company is now working on four and five-phase versions. Pricing for the PA0766NLT averages $0.46 per unit in quantities of 100,000.

For a datasheet see www.pulseeng.com/pdf/P624.pdf.