Low-voltage/high-current requirements for microprocessors are driving the demand for low-height, high-power inductors used in compact power supplies. To meet this need, power inductors must provide up to 100 µH at a few amperes. Critical to achieving these extremely low height requirements is selection of the shape and type of soft magnetic core material used in these inductors.
Washer-shaped toroidal cores provide the required low height. One of the core materials available in the washer shape is distributed air-gapped Molypermalloy powder (MPP) toroids with finished core heights down to 0.7 mm. The other low-height core material available for self-shielding power inductors is the gapped ferrite. We will compare the performance differences between these two core materials, with a goal of achieving finished component heights (including the coil) down to 1 mm.
Typically, these cores are used for the power inductor in a buck converter, as shown in Fig. 1. When switched, the inductor core must provide adequate inductance under high current. Inductance of a wound core is proportional to the core materials effective permeability:
L = 0.4 π µe N2 Ae 10-8/Le (1)
L = Inductance in henries
µe = Effective permeability
N = Number of turns
Ae = Effective cross sectional area in cm2
Le = Magnetic path length in cm
Most buck converters operate in the continuous mode where the current switches back on before it reaches zero. This average of the switched current entering an inductor is often viewed as a dc bias current so that we can estimate the decrease in inductance due to the current flow. H, the magnetizing force, is:
H = 0.4 π N I/Le (2)
H = DC magnetizing force in Oersteds
N = Number of turns
I = Current in amperes
Le = Magnetic path length in cm
By plotting the decrease in permeability against the increase in magnetizing force, we can observe the reduction of induction due to the increase in current.
The MPP and ferrite materials are available in low height toroids, which resemble washers. The toroid core shape offers several benefits besides the ability to meet the difficult height requirement. The toroid's advantages include low core cost, no two-piece core assembly needed, good heat dissipation, no bobbin needed, and good shielding.
MPP washer cores are currently available in three sizes and four permeabilities, as shown in the Tables, on page 20. MPP material costs are not a significant factor in cores this small. The cost of pressing the material into the toroid shape is the most significant factor. However, new MPP pressing technology for small cores allows a low core price — similar to that of gapped ferrite.
Introducing an air gap into the magnetic material allows the core to handle a significant amount of magnetizing force, shearing over the BH curve and extending the amount of current or magnetizing force a core can withstand (Fig. 2).
You can use either of two types of gaps to extend the H range. MPP cores use the distributed air gap. Ferrites use two discrete air gaps spaced 180° apart, as shown in Fig. 3. Increasing the air gap increases the range of the magnetizing force until encountering practical limitations, such as:
- Fitting enough turns of wire in a given inside diameter to drive the core to a higher H level.
- Maintaining a high enough permeability to keep the magnetic field within the core structure.
- Using too large an air gap can make core manufacturing uneconomical.
µe vs. Hdc: The material's effective permeability versus Hdc curve allows you to determine how much the inductance decreases as current increases. Fig. 4, on page 22, plots this for various MPP materials.
You can plot a similar curve for gapped ferrite cores, for an interesting comparison of materials. Fig. 5, on page 22, plots effective permeability versus Hdc for MPP and gapped ferrite when gapped to the same effective permeability. With its higher flux capacity, MPP offers better dc bias characteristics. At a typical 50% roll-off point, this can result in a reduction in core size and a more robust design that utilizes the soft saturation of MPP. The flux capacity difference is even more dramatic at high temperatures, since the flux capacity of ferrites decreases with temperature while MPP stays relatively constant. This results in an even greater dc bias advantage for MPP at high temperatures.
MPP's distributed air gap results in a soft inductance versus dc bias curve. This soft swinging inductance curve is often desirable because it can improve efficiency when operating over a wide current range. Even with a fixed current requirement, the soft inductance versus dc bias curve provides added protection against overloads.
The distributed air-gap structure of MPP minimizes potential EMI problems because almost all the magnetic flux is within the core structure. Discrete gapped ferrite cores have a magnetic field that fringes around the two air gaps and can couple into nearby circuit board traces, causing EMI problems.
Both MPP and gapped ferrite cores have a thin Parylene coating that provides high resistance to the environment. Parylene also minimizes the constriction of the inside diameter and minimizes height. The Parylene coating typically has a minimum breakdown of 300Vrms from wire to core. The maximum steady-state operating temperature for the Parylene coating is 130°C, but you can use it as high as 200°C for short periods, such as during infrared solder re-flow. High temperature operation of the MPP cores does not affect the magnetic properties. The Curie temperature of MPP is about 460°C.
MPP has no binder holding it together so there are no thermal aging concerns. It derives its core strength from the irregular-shaped MPP particles pressed under high pressure. The somewhat malleable MPP material slightly deforms, allowing the insulated particles to interlock while maintaining high particle insulation. Then, a stress relief anneal ensures optimal magnetic characteristics. Power ferrite materials typically have a Curie temperature of 250°C, but the gapped ferrite toroid limit is the epoxy holding two toroid halves together, which usually imposes a 150°C limitation.
Core, Copper Losses
The inductor's dc current ensures the core is not saturated, and the switched current, or ΔI, determines core loss. Generally, in a continuous mode buck converter, copper loss is a greater concern than core loss. However, core loss is higher for MPP material than ferrite. At a typical condition of 100 kHz and 1000 Gauss, a typical power ferrite material has a loss density of 100mW/cm3 while MPP would have 400mW/cm3. This apparent material loss advantage of ferrite is offset somewhat by the air gap. A relatively large air gap results in gap loss, a complex problem that is often overlooked when comparing material loss curves. Gap loss can drastically increase core losses due to fringing flux around the air gap. The fringing flux can intersect the copper windings, creating excessive eddy currents in the wire. Also, hot spots on the core can develop as flux crowds near the air gap.
Fig. 6 plots core loss taken on similar-sized 125µ MPP toroids and ferrite toroids gapped to an effective permeability of 125. Due to test equipment limitations associated with the extremely small size cores, we used larger core sizes to measure core loss.
Besides core selection, wire size selection is also an important design factor. Selecting a wire size based on a particular current density is a safe design approach but often does not yield an optimal design. How hot you can allow the wire to get is the practical limit. Sizing the wire for the worst-case current can result in larger-than-needed wire sizes. Often, you can only see the worst case current for a short period, such as during turn on at a cold temperature or charging a completely dead battery, so you can use a significantly undersized wire.
To facilitate good heat dissipation from the copper wire, choose a single layer winding design. Single-layer windings are less costly to wind, keeping the winding's distributed capacitance to a minimum. This maximizes the frequency response and provides consistent wound core dimensions, particularly a consistent height, which is critical to these designs.
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