Technical papers of interest at APEC 2017

Last month I reported on new passive components displayed at APEC 2017, the annual technology eventaimed at designers of power supplies, dc-dc converters, motor drives, uninterruptable power supplies, inverters and other power electronic circuits, equipment and systems.

But of course the exhibition floor represents only part of an APEC power show. Of perhaps greater importance in terms of impact on the power electronics business going forward is the concurrent technical program, which addresses the latest trends and concerns of power electronics researchers from all around the globe in industry, government, and academia. A rigorous peer review process is used to select APEC’s technical sessions, seeking to highlight the most innovative, highest quality solutions, with papers and in-depth discussions combining theory with practical application scheduled for oral presentation from Tuesday morning through Thursday afternoon at the show.

Again, passives played a visible role among the subjects discussed. Here’s just a small sample:

As the complexity of mobile devices including smartphones, wearables and tablet computers has increased in an attempt to squeeze in more functionality in limited space, for efficient power conversion Switched mode power supplies (SMPS) including inductor-based and switched-capacitor (SC) converters are generally used.  In their paper “An Inductor-Less Hybrid Step-Down DC-DC Converter Architecture for Future Smart Power Cable,” Gab-Su Seo and Hanh-Phuc Le of the Department of Electrical, Computer, and Energy Engineering at the University of Colorado presented a DC-DC step-down power conversion architecture that allows a power cable, such as a USB cable, to deliver both power transmission and power conversion functions.

The authors proposed an S-Hybrid converter topology to utilize the parasitic inductance readily available in the cable for the power conversion stage. As a result, the explicit inductor often required in a buck converter and its associated loss can be eliminated, promising better full integration and improved efficiency, especially in mid- to heavy-load conditions.

In addition, this S-Hybrid converter architecture is said to require less inductance and achieve smaller inductor current ripple that contributes to better performance. The architecture is verified in the paper with a proof-of-concept 15 W inductorless Li-ion battery charger prototype using 1m USB 3.0 cable. The converter, switched at 2 MHz from a 5 V input, is reported to experimentally achieve 89.7% maximum efficiency and 6.0% higher efficiency at full load than a buck converter counterpart. What is more, a relative 45.7% on-board loss reduction at full load promises excellent integration feasibility.

Inductive power transfer (IPT) technology, which can deliver energy from a power source to loads through magnetic coupling without physical contact, is attractive for use in charging  Electric Bicycles (EBs) because it is more convenient and safer than plug-in systems. However, the cost of IPT systems is generally considered too expensive to employ in large scale when, normally, a bicycle parking lot can host several tens of bicycles and even charge hundreds at the same time.

In their paper “Inductive Power Transfer for Electric Bicycles Charging Based on Variable Compensation Capacitor,” Yang Chen, et al of the School of Electrical Engineering Southwest Jiaotong University (Chengdu, China) presented a novel method with only one additional capacitor and one AC switch employed in the primary side achieving the required CC or CV charging. Sophisticated feedback control is not a must, they reported, so that the cost and the complexity of the overall system could be minimized. Once given the output of the inverter, the CC mode and CV mode can be achieved by turning on/off the AC switch. The proposed method was verified with an experimental prototype and the results demonstrated that the designed output CC and CV meet the charging profile of EBs.

Single-phase pulse width modulation H bridge active capacitors have second-order harmonic ripple voltages on the DC bus.

The second-order harmonic power is usually filtered by bulky passive components in order to maintain a constant DC-bus voltage. But this approach results in low power density. However, in applications like active capacitors, a larger DC bus voltage fluctuation is acceptable since the DC bus connects to no load. In their paper  “Real DC Capacitor-less Active Capacitors” Yunting Liu and Fang Zheng Peng of the  Department of Electrical and Computer Engineering, Michigan State University analytically plot the DC-bus voltage waveforms in terms of utilization levels of the DC-bus capacitor. Based on a ripple energy analysis, an infinite active capacitor with H-bridge structure is proposed to fully utilize the DC capacitor energy. The authors then propose three novel DC capacitor-less active capacitors realized by applying the duty-cycle based modulation on three single-phase DC/AC structures, which share a feature that no capacitor exists on DC bus. These new DC capacitor-less active capacitors can help to increase the power density. A simulation and was conducted for verification purposes. 

When analyzing the step load change response waveform of a digitally controlled SMPS, a low stability phenomenon caused by Equivalent Serial Resistance (ESR) is observed. Studying electrolytic capacitor degradation in a digitally-controlled SMPS, Hiroshi Nakao and colleagues at the Computer Systems Laboratory of Fujitsu Laboratories reported in their paper “Failure Prediction Using Low Stability Phenomenon of Digitally Controlled SMPS by Electrolytic Capacitor ESR Degradation,” that unlike an analog controlled SMPS, a time delay between an analog-digital (AD) sampling and a pulse width modulation (PWM) duty update created ringing on a voltage waveform at the step load change response. The amplitude and frequency of this ringing became larger and higher with a higher ESR and was easily evaluated with an MPU used for digital control. The authors proposed a new failure prediction method for a digitally controlled SMPS using this low stability phenomenon.