Magnetics Business & Technology - Summer 2014 - (Page 12)

FEATURE ARTICLE A Fresh Look at Design of Buck and Boost Inductors for SMPS Converters By Weyman Lundquist and Carl Castro | West Coast Magnetics Inductors are a critical component in buck and boost circuits. New developments in winding and core technologies are making it advantageous to specify buck and boost inductors at higher power levels, at higher frequencies and at higher values of ripple current. This article examines four different alternatives for boost inductor design including a gapped ferrite e-core with a new proprietary foil winding (WCM's Shaped Foil technology), an iron powder toroid wound with solid wire, an iron/nickel blend powdered toroid with solid wire and an iron aluminum silicon powdered e-core with a helical winding. While these are clearly not the only alternatives for this type of inductor, they are the most favorable when the design objective is to develop the smallest and lowest cost alternative for medium to high power (1 kW to 100 kW) buck and boost inductors operating at high ripple values at frequencies above 10 kHz. The choice of core requires an examination of a number of variables including loss density, saturation flux density and cost. The core material with the lowest loss density (measured in mW/cm3) is the manganese zinc ferrite, followed by the nickel iron and the Fe Al Si powdered core with the pure iron powder core having the greatest losses. The cost of a ferrite core (measured in $/cm3 of material) is slightly higher than powdered cores with high iron content, and less than powdered cores with iron as a base metal and nickel and or silicon and aluminum added for greater performance. The main disadvantage of the ferrite core is a much lower saturation flux density requiring a longer magnetic path length. Since cost was an important factor in the present comparison not all core and winding options were considered. Amorphous, nanocrystalline and MPP cores are notably more expensive than the ferrite and powdered cores. Similarly, litz wire is significantly more expensive than solid wire and copper foil. So these materials were excluded from the present comparison. As a reasonable approximation of an inductor that would be chosen for a medium power application, we decided to fix the inductor value at 10 uH and 55 amps of rms current. The inductor was further required to handle 65 amps of peak current without saturating. The four design approaches are described in more detail as follows. The first approach was with a manganese zinc ferrite E core, with a gap. The material was a low loss manganese zinc material with an initial permeability of 2,000. The winding was six turns of copper foil with a cross section of 31,600 cir mils. The winding utilized in the ferrite E core design was a new generation of WCM's Shaped Foil technology. Developed in collaboration with the Thayer School of Engineering at Dartmouth, the technology takes advantage of the strong field in a gapped core structure to equalize the distribution of AC current with the foil winding cross section. The result is a winding that has lower DC resistance than a solid wire winding, AC resistance similar to litz wire and a low cost. It also has significantly lower losses than helical windings at high ripple values as will be shown. Improvements made to the Shaped Foil technology over the last year have been significant and a new patent application is in process. The second approach to the design is a conventional high iron content low permeability toroid wound with solid wire. This core was wound bifilar with 10 turns of 10 awg wire. The third design used a toroid with iron and nickel wound with 13 turns of 7 awg solid wire. For these two designs the cores were sized to sustain 10 microhenries minimum at 65 amps of peak current. WCM does not wind helical coils, so for this technology we had to choose from available standard products. We were limited in this choice, so as a result the core chosen was slightly oversized from the perspective of energy storage capacity and current handling. Nevertheless, WCM decided to look at the cost and losses of this slightly oversized inductor. While we knew the cost would be on the high side, we were very curious what the loss curve would look like. We chose two helical windings for this core. The first had 22 turns with a copper cross section of 22,600 circular mils and the second had 12 turns with a cross section of 38,200 circular mils. After building prototypes, WCM load tested each of the inductors to insure that they did maintain 10 uH minimum inductance Figure 1. Figure 2. 12 Magnetics Business & Technology * Summer 2014 Inductors (from left to right): Shaped Foil, Iron Nickel Toroid, High Iron Toroid, Helical Winding www.MagneticsMagazine.com http://www.MagneticsMagazine.com

Table of Contents for the Digital Edition of Magnetics Business & Technology - Summer 2014

Editor's Choice
Moving Beyond Helium in Magnetics Research
Advances in Ferrite-Integrated On-Chip Inductors Using Aerosol Deposition
A Fresh Look at Design of Buck and Boost Inductors for SMPS Converters
Research & Development
Handheld Three-Axis Magnetometers
Magnetics, Materials & Assemblies
Software & Design
Test & Measurement
Industry News
Marketplace / Advertising Index
Spontaneous Thoughts: A Different Paradigm

Magnetics Business & Technology - Summer 2014