Here are the slides from my presentation at the APEC magnetics industry session.
Industry session sullivan 2018.pdf
They provide an overview of the topics at the PSMA/IEEE PELS saturday magnetics workshop, focusing on dimensional effects in core loss and fringing effects.
I skipped over discussion of core loss with non-sinusoidal waveforms, because that was discussed in last year’s talk and a previous presentation providing an overview of different methods.
Our second presentation at the PSMA/IEEE PELS workshop on magnetics was on fringing, including:
- The effect of fringing on gap reluctance,
- The effect of fringing on core loss in laminated and tape-wound cores
- The effect of fringing on winding loss
Here are the Fringing Effects Presentation Slides including references at the end.
Of the three main component types needed in power converters—switches, capacitors and inductors—the most difficult to integrate on a semiconductor chip or in a planar package is the inductors. This difficulty arises partly from process compatibility challenges with magnetic materials, and is exacerbated by the fact that, because most types of electronics don’t need inductors, there has been relatively little development effort. But a more fundamental challenge is the way magnetics performance scales with size.
Capacitors and semiconductor devices can be made from thousands of small cells connected in parallel, but that approach would severely undercut the performance of magnetic components.
In this work, we examine the scaling relationships for magnetics to demonstrate the inherent difficulty of small size and low profile magnetics. Cases considered include those with winding designs limited by skin and proximity effect and those constrained by efficiency and thermal dissipation. Small-scale magnetic components are typically limited by efficiency rather than heat dissipation. With efficiency constrained, and considering high frequency winding loss effects, it is shown that power density typically scales as the linear dimension scaling factor to the fifth power.
For the full analysis, see the attached paper, Sullivan, C.R., Reese, B.A., Stein, A.L. and Kyaw, P.A.,. “On size and magnetics: Why small efficient power inductors are rare.” IEEE International Symposium on 3D Power Electronics Integration and Manufacturing (3D-PEIM), 2016.
There’s a lot we know about magnetic core loss, and at lot we don’t know. The situation is summarized in presentation slides from my presentation at the PSMA/IEEE Magnetics Workshop. The slides have the references added at the end with reference numbers sprinkled through.
People were intrigued by the three simple flux crowding simulations shown here. These aren’t intended to be highly accurate–they are based on constant permeability, and as pointed out by Bruce Carsten, the real behavior is nonlinear. But the results are still interesting and somewhat surprising. Based on loss proportional the flux density raised to the 2.5 power, the second picture–the circular hoop–doesn’t reduce loss as you might expect. Rather, it raises loss by about 3%, compared to the simple square corners at the top. But the bottom design does reduce loss, by about 8%, compared to the simple square corners.
Our new paper accepted for publication in IEEE Transactions on Power Electronics (pdf) explores the potential for new types of passive components and shows that there are exciting opportunities.
- Analysis of volumetric energy densities of various storage mechanisms shows that mechanical storage may offer order-of-magnitude improvement over conventional electromagnetic passive components.
- Considering only the limitations imposed by material properties and not by available fabrication methods, both piezoelectric and LC resonators have fundamental performance limits that are much higher than the capabilities of commercial passive components in use today.
- A prototype 1 cm3 integrated LC resonator optimized for low loss is capable of handling 7.42 kW with only 0.06% loss attributable to the resonator when used in a resonant switched-capacitor circuit.
The high-Q achievable by self-resonant structures increases the range and efficiency of wireless power transfer (WPT). However, to date implementations of this structure have been thick, which limits their practical implementations. In the attached paper, we explore the design of thin self-resonant structures.
1) a computationally efficient 2-D optimization algorithm is proposed to design thin
resonant structures and illustrate the trade-offs in the design, and
2)a new magnetic core shape is proposed which shapes the magnetic field lines to be parallel to the conductive layers and reduces current crowding.
These advances results in a prototype 3.5 mm thick self-resonant structure, which has a measured quality factor of 560 despite having a diameter of only 6.6 cm; this provides a 3.03× improvement over the state-of-the-art WPT coils in the literature.
See a full description of the thin structure in the attached paper
Matching networks have useful applications in transforming voltages and impedances in resonant inverters and dc-dc converters. Stacking multiple stages of matching networks can, in some cases, increase the efficiency because each stage is responsible for smaller transformation, but it also reduces the available inductor volume for each stage which can increase the loss.
At COMPEL 2017, we presented a paper (link for pdf) on optimization of matching networks with volume constraints to determine the optimum number of stages and other design choices for various transformation ratios, volumes and impedances. Adding the volume constraint to the typical matching network design process helps provide a better perspective on the number of stages that should be used. Simple design rules for designing matching networks, with a constraint on the available volume, are presented for voltage transformation ratios lower than 20.
High-frequency power conversion is useful for miniaturization of power electronics, but requires low loss passive components to achieve high power densities without thermal issues.
At COMPEL 2017, we presented a paper (link for pdf) investigating the lowest achievable ESR and the highest achievable power capability of a resonant tank using an air-core inductor with a single-layer foil winding and commercially available capacitors. A loss model is presented and online catalogs of multilayer ceramic capacitors are searched for components that can provide a low ESR when combined with an optimally designed inductor for various resonance frequencies.
The resulting resonator has a measured sub-mΩ ESR and high efficiency with 250 V dc rating in a 1 cm3 volume. The resonant tank, when used in a resonant switched-capacitor converter, has kilowatt-range power capability. A power converter, using this resonant tank, will be limited by the power density of switches and interconnects rather than by passive components.
Here are the slides from my talk today at G2ELab in Grenoble.
Discussion of modeling winding and core loss, designs for kHz frequencies, designs and challenges for MHz frequencies, and new approaches to passive components for MHz frequencies. It includes a list of several dozen references at the end, and many of the slides include references numbers, so this can be a good entry point for finding more information on those topics.
At APEC 2017 we presented a resonant structure that improves the range and efficiency of wireless power transfer. High quality factor in resonant coils is essential for both goals, so developed a new technology that achieves Q values that weren’t previously possible. The new structure integrates inductive and capacitive effects to behave as an LC resonator. It’s made by stacking alternating layers of thin foil and dielectric material in a magnetic core. The high-Q is achieved through these effects:
- Thin foil layers mitigate proximity effect.
- Inductive coupling of sections and integration of capacitance eliminates terminations in high-current paths.
- Capacitive ballasting forces equal current sharing between all layers.
We experimentally validated this structure and measured a Q of 1173 at 7 MHz despite a coil diameter of only 6.6 cm. Next, we integrated 2 of the structures into a wireless power transfer system. We were able to improve the range over which we could maintain efficiency above 94% by a factor of two when compared to the current state-of-the art. For more details see our presentation slides linked here, or our paper linked here.