The absence of nickel in the formulation helps make XFlux much more economical than the MPP or High Flux materials. The XFlux material exhibits slightly better DC bias performance than High Flux, and much better than MPP or Kool Mμ. XFlux ® cores are distributed air gap cores made from a silicon-iron alloy powder. In addition to toroids, Kool Mμ is available in E-core shapes, so that winding costs may be minimized as well. It is designed to be a practical alternative when iron powder is too lossy, typically because the frequency is moderate or high, but MPP is too expensive. The main tradeoff is that Kool Mμ has higher AC losses than MPP. The absence of nickel in the formulation helps make Kool Mμ much more economical than the MPP. The Kool Mμ material is similar in DC bias performance with MPP. Kool Mμ ® (or, “sendust”) cores are distributed air gap cores made from an iron, aluminum, silicon alloy powder. Like MPP cores, High Flux is not widely available in shapes other than toroids. In other words, higher Bsat translates into best inductance stability (least shift) under high DC bias or high AC peak current. High Flux exhibits higher core loss than MPP and Kool Mμ, but due to its higher Bsat, High Flux exhibits the best performance in permeability vs. Containing 50% nickel, and with processing costs comparable with MPP, High Flux pricing is typically 5%-25% less than MPP. High Flux cores are distributed air gap toroidal cores made from a nickel-iron alloy powder. MPP toroids are available from 3.5 mm to 125 mm in outside diameter. MPP exhibits the lowest core loss of the powder core materials, but it has the highest core cost due to processing costs and its 80% nickel content. MPP (Molypermalloy Powder) cores are distributed air gap toroidal cores made from a nickel, iron, and molybdenum alloy powder. Then the designer has a variety of options in choosing among the powder cores. In the many cases powder cores have the clear advantage. The inductor designer must meet the energy storage (inductance) requirement, as well as requirements for total loss, space, cost, EMI, fault-tolerance, temperature performance, and reliability. Discrete gaps are also used in amorphous and nanocrystalline tape wound cut cores, which have improved AC loss performance compared with powder cores, but often at a cost disadvantage. Discrete gaps also result in inductors that are vulnerable to eddy current losses in the coil due to fringing, and to generating EMI. The discrete gap structure results in an inductor that reaches a sharp saturation point, requiring lots of headroom in the design. Ferrites are at the low end of the available range for Bsat, and they shift down in Bsat significantly with increasing temperature. The main performance advantage of ferrite is low AC core loss at high frequency, due to high material resistivity in the ceramic material, compared with metal alloys. (This is not at the magnetic domain level domains are vastly smaller than powder core grains.) Distributing the gap throughout the powder core structure serves two main purposes: (1) eliminating the disadvantages of a discrete gap structure, which are sharp saturation, fringing loss,and EMI, and (2) controlling eddy current losses so that higher Bsat alloys may be used at relatively high frequencies, despite comparatively low bulk resistivity in the alloy.ĭiscrete gaps are most commonly used in ferrite cores.
At a microscopic level, magnetic alloy powder grains are separated from one another by binder insulation or by high temperature insulation coating each grain. Distributed gap materials are powder cores. The power inductor gap may be realized in one of two fashions, discrete or distributed.
The physics of soft magnetic materials result in the case that commericially useful materials range from about 0.3T to 1.8T in Bsat. One envelope constraint is that Bsat is not widely variable. Since μ = B/H, the lower the value of μ, the greater the value of H (or current) that is supported at a level of B that is less than the maximum value of flux density (Bsat) inherent in the magnetic material. Another way to express the function of the air gap is to say that it reduces and controls the effective permeability of the magnetic structure. The purpose of the gap is to store the energy, and to prevent the core from saturating under load. Power inductors require the presence of an air gap within the core structure. By resisting change in current, the filter inductor essentially accumulates stored energy as an AC current crests each cycle, and releases that energy as it minimizes. An inductor is a current filtering device.