On Size and Magnetics: Why Small Efficient Power Inductors are Rare

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.

APEC 2018 – Thin Self-Resonant Structures for Wireless Power Transfer

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.

We describe:
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

APEC 2017 High-Q Self-Resonant Structure for Wireless Power Transfer

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.

A Step-by-Step Guide to Extracting Winding Resistance from an Impedance Measurment

At APEC 2017, Benedict Foo presented a method for extracting winding resistance from an impedance measurement.  For details on this method see the paper.

Impedance analyzer measurements can be helpful in assessing inductor a  transformer winding resistance and predicting winding loss, but the measured ESR does not directly correspond to winding resistance. Neglecting the effects of core loss and winding capacitance can yield significant errors in the prediction. A step-by-step method to account for such effects and extract winding resistance from an impedance measurement is described. The proposed methodology is applicable to both inductors and multi-winding transformers. Several measurements are needed in this method; one is to determine the effects of core loss and the others yield the impedance from which winding resistance is extracted to form a resistance matrix. The winding resistance of a transformer was determined experimentally and the interactions between the winding resistance, effects of core loss, winding capacitance and inductance and their contributions to the measured impedance are demonstrated.