How new inductor cores meet demands for smaller, quieter, and more reliable power

by Patrik Kalbermatten, KEMET

Engineered composite cores have enabled inductor manufacturers to pack high inductance into a small volume. New FlakeComposite technology takes core performance to the next level, and adds extra mechanical resilience to enable new types of low-profile devices.

Power inductors are crucial devices for managing the flow of energy in switching converters, to ensure smooth power delivery and help coordinate commutation. The inductance value is selected to store sufficient energy to keep current flowing for long enough to operate the circuit correctly while the main switch is off.

While the inductance value is calculated differently depending on the type of converter, to support continuous- or discontinuous-current mode (CCM or CDM), or resonant operation, a high inductance value relative to size, for a given current rating, is generally desirable. Stable performance within the intended frequency range is also needed, while temperature stability and high maximum operating temperature are required for applications such as automotive or aerospace.

Engineering inductors to the limit

An inductor’s properties are limited by the laws of physics. Careful engineering of the core material helps to push these limitations to the limit to give engineers the best possible combination of parameters for their application. Commonly used core materials include manganese-zinc (Mn-Zn) and nickel-zinc (NiZn) ferrites, and metal-powder cores that comprise grains of specially formulated alloy separated by an insulating binder. Thin-film inductors can also be created by depositing cobalt-based alloys, and achieve high permeability with good saturation performance, although increasing the core volume to address power applications is difficult.

Ferrite cores have high permeability, up to about 300 for NiZn materials and even higher for MnZn, although there are some drawbacks. The materials tend to be brittle and therefore unsuited to embedding in PCBs or creating inductors in low-profile shapes such as planar lateral-flux devices. In addition, they can experience abrupt saturation leading to a sharp roll-off of inductance with increasing DC bias.

As far as powder cores are concerned, popular alloys include iron-silicon (FeSi) or iron-silicon-aluminium (FeSiAl), and other compositions including amorphous iron and permalloy. With their particle-based structure, these distributed air gap cores have a softer saturation characteristic than ferrite inductors that is less sensitive to small shifts in DC bias. On the other hand, permeability is typically about an order of magnitude less than ferrites and the organic binder cannot tolerate high operating temperatures.

A new metal-flake compacting technology now makes it possible to produce a distributed air gap core material with permeability equivalent to NiZn ferrite and a soft saturation characteristic comparable with that of conventional powder cores. Moreover, this new class of FlakeComposite cores also displays greater temperature stability, increased maximum operating temperature, and mechanical flexibility. With this increased flexibility comes not only the opportunity to create ultra low-profile inductors but also to embed robust inductors within the PCB to save real-estate and to explore the opportunities for new types of inductors such as lateral flux inductors to be integrated with active components in next-generation power-conversion designs.

Performance comparison

Figure 1 illustrates the key permeability and saturation properties of FlakeComposite core material in relation to ferrite, powder, and thin-film cores.

inductor cores — Figure 1. FlakeComposite has permeability comparable to ferrite with superior saturation performance.

Ferrite materials are known to lose permeability at high frequencies, high temperature, or at high values of DC bias current, causing rapid reduction in inductance value and hence impairing performance. To be certain that FlakeComposite core inductors can be sure of performing at least as well as ferrite inductors, frequency, temperature, and DC-bias performance are compared.

Figure 2 compares the frequency dispersion of complex permeability for FlakeComposite against NiZn ferrite. The graphs for both materials show that permeability reduces rapidly above about 6MHz, showing that FlakeComposite can perform equally well or better than NiZn in switching converters operating up to 1MHz.

Figure 2. FlakeComposite gives comparable performance to NiZn ferrite for power applications up to MHz frequencies.

Comparing magnetic saturation characteristics, the FlakeComposite benefits from softer onset of saturation compared to NiZn ferrite, as well as reduced temperature dependence (figure 3).

Figure 3. The magnetic saturation curve is softer and less temperature dependent compared to NiZn ferrite.

Figure 4 compares the DC bias performance of FlakeComposite and NiZn ferrite and with conventional metal composite (powder). FlakeComposite combines the strengths of both types, displaying comparable superior permeability of NiZn at low bias while retaining higher permeability at high bias with minimal temperature dependence.

Figure 4. DC bias characteristic displays higher permeability when high DC bias field is applied.

If the inductor operating temperature reaches the Curie temperature of the core material, at which the core loses its magnetic properties, the core permeability falls quickly resulting in rapid loss of inductance. As figure 5 shows, FlakeComposite also has a higher Curie temperature than typical NiZn or MnZn ferrites.

Figure 5. The higher Curie temperature of FlakeComposite ensures inductance value is retained at higher operating temperature.

Thinner inductors shrink footprint

In the ongoing quest to reduce the footprint of power-conversion modules, such as point of load (PoL) converters, new designs that integrate active and passive components have been proposed. These employ planar inductors that are specially designed to have a lateral flux pattern, unlike the conventional vertical flux pattern previously used to build low-profile inductors. As the inductor thickness is made lower, lateral flux inductors display increasingly superior inductance compared to conventional vertical-flux devices. The mechanical properties of FlakeComposite allow inductors from 50µm to 2mm thickness, making them well suited to creating ultra-thin lateral-flux inductors.

Extremely low profile yet robust inductors, built using FlakeComposite, are also inherently aligned to being embedded in PCBs to help save footprint, and allow up to 40% inductor height reduction compared to traditional ferrite cores.

Resilient high-permeability magnetic material

In addition to use in power inductors, FlakeComposite’s combination of magnetic and mechanical properties are ideal for electromagnetic shielding applications including EMI suppression and shielding wireless power-transfer coils to optimize charging performance and protect nearby electronic equipment. FlakeComposite technology is at the heart of KEMET’s Flex Suppressor products that have been shown to attenuate electromagnetic noise in a variety of applications.

Conclusion

FlakeComposite is a novel way to optimize inductor-core performance and further extend opportunities to miniaturize the power-conversion circuits of tomorrow, over and above the achievements of current ferrite core materials. By delivering similar permeability with superior saturation characteristic, DC-bias performance, and higher temperature capability, FlakeComposite enables lower-profile power inductors and delivers the mechanical properties needed to make PCB-embedded inductors a true space-saving reality.