There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

The on-board low-voltage dc–dc converter (LDC) in electric vehicles (EVs) is used to connect the high-voltage battery with the low-voltage auxiliary system. With the advancement of auxiliary equipment in EVs, the output current of the LDC can be hundreds of amperes, which will cause high-conduction loss and severe thermal concern. In this article, a high-efficiency high-power-density on-board LDC is presented. To reduce current stress and improve efficiency, three-phase interleaved LLC dc–dc converters are paralleled to provide 270 A load current. Synchronous rectifier is used to reduce secondary conduction loss. zero-voltage-switching (ZVS) turn- on of primary switches and ZCS turn- off of secondary switches are achieved, thus switching loss can be reduced significantly. Moreover, phase-shedding technology is used to improve light load efficiency. Switch-controlled capacitor (SCC) technology is used to achieve accurate load current sharing among the three phases, which protects the devices against high-current stress, reduces the conduction loss, and improves the reliability of the system. As SCC switches achieve ZVS turn- on and turn- off by its nature, the loss of the SCC circuit is of less concern with regard to the rated output power. In addition, GaN HEMTs are used in the primary side to improve the power density and eventually help achieving light weight. A 3.8-kW (14 V/270 A) LDC prototype is developed and tested. Experimental results show good current balancing among the three phases. A peak efficiency of 96.7% at 140 A load and a full load efficiency of 95.8% are achieved with 3 kW/L power density and 1.5 kg weight.

A High-Efficiency High-Power-Density On-Board Low-Voltage DC–DC Converter for Electric Vehicles Application

The advantages of load commUtated high-frequency (HF) sinusoidal inverters as link stiiges in power conversion are well-known. An inverter configuration that employs parallel connection of the load to the resonating capacitor can be employed to realize the high- frequency link. A technique of dc to low-frequency ac power conversion using two, such high-fre4uency links is presented. The frequencies of the two links differ by twice the required output frequency. The difference between the two link voltages is then a high- frequency ac voltage whose envelope is a low-frequency ac wave at the required output frequency. The frequency of the envelope wave is decided by the difference between the two link frequencies while the amplitude depends on the amplitude of the individual voltages. Thus the output voltage can be regulated by varying the link frequencies, and so long as the frequency difference remains constant, the output (envelope) frequency is not affected. The output voltage can be obtained from the difference of the two link voltages by appropriate rectification/inversion. This process can be regarded as a form of cycloconversion in which the modulation process is incorporated in the input voltage instead of the switching.

A DC-ACPowerConversion Technique UsingTwin Resonant High-Frequency Links

This paper provides a comprehensive review of the control approaches and the power-decoupling topologies to mitigate 2ω-ripple problem in the single-phase inverters, its solutions, and discusses open challenges yet to be addressed. The cause and effects of 2ω-ripple problem and its solution based on the passive and active power-decoupling techniques are discussed. A subcategory of the active power-decoupling technique nominated as the control-oriented compensation technique is reviewed in detail, this technique can achieve the ripple-mitigation at the source through the control but not necessarily adds extra circuit or active filter to the system. The control-oriented compensation techniques can be applied in the two-stage DC-DC-AC converters and the single-stage inverters having a front-end control capability with the H-bridge such as in the quasi-switched-boost inverters. The merits and associated challenges of these techniques are listed and summarized in a tabular form. Finally, a conclusive discussion with open challenges is presented.

/pdf/control-strategies-and-power-decoupling-topologies-to-4oz6n87yxb.pdf

Control Strategies and Power Decoupling Topologies to Mitigate 2ω-Ripple in Single-Phase Inverters: A Review and Open Challenges

This paper presents a review of isolated matrix inverters. The study contributes to creating a point of reference for a comprehensive classification of existing solutions. Over 30 topologies were reviewed, and the main advantages and disadvantages discussed. Applications of isolated matrix inverters are summarized in a tabular form to demonstrate their flexibility for different power and voltage levels achieved due to the presence of a transformer. These inverters have been proposed for the uninterruptible power supplies, high and low-voltage/power photovoltaic systems, low-power fuel cell, different low- and high-voltage battery and/or electric vehicle chargers, audio amplifiers. The fully controlled switches on both terminals of these converters typically can provide the bidirectional power transfer capability, which is also addressed for most of the topologies, but requires some modification in their modulation strategy. Average efficiency of today’s isolated matrix inverters is comparable with the two-stage power converters; however, they can provide higher reliability and lower cost.

https://www.mdpi.com/1996-1073/13/9/2394/pdf

Review of Isolated Matrix Inverters: Topologies, Modulation Methods and Applications

One of the key technical challenges of the Google and IEEE Little Box competition, an international contest to build the world’s smallest 2-kW single-phase inverter in 2015, was to shrink the volume of the energy storage required to cope with the twice mains-frequency (120 Hz) pulsating power at the ac side and meet the stringent 2.5% input voltage ripple at the dc side. In this article, first, a full-power processing buck-type converter active buffer approach, selected by the first prize winner of the Little Box Challenge (LBC), is analyzed in detail. Being relieved from strict voltage ripple requirements, a larger voltage ripple is allowed across the buffer capacitor significantly reducing the capacitance requirement. Second, a partial-power active buffer approach, selected by the second prize winner of the LBC, where conventional passive capacitive buffering of the dc-link is combined with a series-connected auxiliary converter, used to compensate for the remaining 120-Hz voltage ripple across the dc-link capacitance, is studied in detail. In this article, both the selected concepts are comparatively evaluated in terms of achievable efficiency, power density, and ripple compensation performance under both stationary and transient conditions. Novel control schemes and optimally designed hardware prototypes for both considered buffer concepts are presented and accompanied by experimental measurements to support the claimed efficiency and power density and assess the performance of the implemented control systems. Finally, by means of comparison with conventional passive dc-link buffering using only electrolytic capacitors, it is determined at what voltage ripple requirement it actually becomes beneficial in terms of volume to employ the considered active buffer concepts.

Comparative Evaluation of a Full- and Partial-Power Processing Active Power Buffer for Ultracompact Single-Phase DC/AC Converter Systems

Given the demand for higher power density, higher efficiency and lower realization cost solutions in data centers, dc-dc converters using PCB winding magnetics and resonant operation have been intensively studied in literature. In this paper, a new approach for 300V-430V dc input and 12V dc output server power supplies is proposed, which is based on a series resonant converter topology using a synergy between boundary and discontinuous conduction mode operation, whereby low peak-to-average ratio transformer current waveforms, soft-switching throughout the full power range and tight regulation of the output voltage for a wide input voltage range are achieved. Furthermore, a PCB winding ”snake-core” matrix transformer is introduced, which provides only a single path for the magnetic flux and therefore ensures an equal flux linkage and/or induced voltage of the parallel-connected secondary windings despite possible geometric PCB layout asymmetries. This approach avoids the emergence of circulating currents between parallel-connected secondary windings and guarantees at the same time equal secondary-side voltages in power supplies with multiple isolated outputs. Thus, a higher degree of freedom in the design of the primary windings of PCB winding-integrated transformers is achieved, whereby the design of these transformers is considerably simplified.

New PCB Winding ”Snake-Core” Matrix Transformer for Ultra-Compact Wide DC Input Voltage Range Hybrid B+DCM Resonant Server Power Supply

Growing applications such as electric vehicles, data centers, and industrial robotics require power-dense and efficient converters systems to provide high output currents at low output voltages of generally 12 V Today’s converters achieve high efficiencies at a specific input voltage, but drastically lose efficiency once the input voltage deviates from its nominal value A large voltage swing, however, is a common situation in the aforementioned applications, as the respective input voltages, in worst-case, can easily drop by 50 %, whereby full functionality still needs to be maintained In this work, we demonstrate a wide-input-voltage-range 400 V to 12 V, 3 kW dc-dc converter with a novel control method and transformer design The employed boundary/discontinuous mode control scheme reduces circulating currents over conventional LLC techniques, and the "snake-core" matrix transformer achieves ideal parallel-connected secondary voltage balance The converter achieves 350 W/in3 power density while operating from 300 V to 430 V input voltage and from 10 % to full load An advanced design for a data center power supply application additionally utilizes the magnetizing inductance for boost operation in order to optimize the operating conditions of the converter system and to increase the overall efficiency A performance comparison is finally explored for comparing the advanced design with the current hardware demonstrator and with conventional LLC converters

Wide-Input-Voltage-Range 3 kW DC-DC Converter with Hybrid LLC & Boundary / Discontinuous Mode Control

Despite the increasing performance of power semi-conductors and passives components, limited timing resolution in off-the-shelf available digital control hardware often prevents the switching frequency in kW-scale dc/ac power conversion to be increased above several MHz for the sake of extreme power densities. In this paper an alternative approach to generate a sinusoidal output voltage, based on constant duty cycle frequency shift control of a high frequency resonant inverter stage and a subsequent synchronous cycloconverter, is analyzed. The design of the presented converter is facilitated by means of a derived mathematical model. A novel closed-loop control system is proposed which achieves tight regulation of the output voltage by means of controlling the switching frequencies of the involved bridge legs operated in resonant mode. Characteristic waveforms of the dc/ac converter during steady-state and load transients are presented. Two distinct implementations of the resonant inverter stage, constituting an intermediate voltage or intermediate current link, are analysed and compared.

Constant duty cycle sinusoidal output inverter with sine amplitude modulated high frequency link

The trend towards higher computation power per server rack has lead to an increased interest in high density server power supplies with high efficiency and low cost. This paper analyzes how the specific features of planar magnetics as transformers and inductors can be best utilized in combination with GaN power semiconductors to reach unprecedented system performance both for power supplies with 12V and with 48V output voltage. Based on the optimization results a hardware demonstrator with an ultra-high power density (350W/in3) of a 3kW DC/DC converter stage with a wide-input voltage range of 300V-430V and an output voltage of 12V is presented.

Gustavo C. Knabben

Papers

Comparative Evaluation of a Full- and Partial-Power Processing Active Power Buffer for Ultracompact Single-Phase DC/AC Converter Systems

New PCB Winding ”Snake-Core” Matrix Transformer for Ultra-Compact Wide DC Input Voltage Range Hybrid B+DCM Resonant Server Power Supply

Wide-Input-Voltage-Range 3 kW DC-DC Converter with Hybrid LLC & Boundary / Discontinuous Mode Control

Constant duty cycle sinusoidal output inverter with sine amplitude modulated high frequency link

Ultra-high Power Density Server Supplies Employing GaN Power Semiconductors and PCB-Integrated Magnetics