High-frequency Power Converters Research Articles

High-Accuracy Calibration-Free Calorimeter for the Measurement of Low Power Losses

Wide-band-gap technologies enable ultra-high efficiencies in the high-frequency power conversion. However, inadequate accuracies in electrical measurements could lead to spurious efficiency measurements, even above 100%. Furthermore, bandwidth limitations, particularly of current probing, delay mismatches between current and voltage probes, loading effects, and electromagnetic interferences, make electrical techniques inappropriate for measuring extremely-low losses in switches with high dv/dt and di/dt values and magnetic components under high-frequency excitations. Calorimetric methods overcome this issue by a direct loss measurement through the generated heat. Nevertheless, limited ranges and accuracies of the existing systems hinder their application in sensitive measurements. Moreover, time-consuming calibrations with extensive data processing impede rapid design assessments. In this work, we propose a closed-type double-chamber calorimeter based on a real-time calibration between two identical chambers to eliminate extra compensation for ambient variations or changes within the system itself. By streaming the same coolant in both chambers, our system removes the need for flow measurements, enabling extremely low flow rates, which improves sensitivity and reduces complexity and system cost. The system provides an adjustable cooling and can measure losses down to 500 mW, enabling the evaluation of high-performance power converters and their components with no dependence on the geometry of the evaluated devices or systems.

Effect of CaCu3Ti4O12 dopant on the magnetic and dielectric properties of high-frequency MnZn power ferrites

To obtain miniaturized, lightweight, and integrated power conversion devices, the core material must be suitable for operation at higher frequencies (in the megahertz range) and must exhibit excellent performance, specifically, low core losses. In this study, MnZn power ferrites, a common core material, were doped with calcium copper titanate (CCTO) using a solid-state reaction method. The effects of CCTO doping (0.0–0.5 wt%) on the structural, magnetic, and dielectric properties of the MnZn ferrites were studied. The addition of 0.2 wt% CCTO to the MnZn ferrites improved the grain uniformity and decreased the porosity; thus, the material exhibited a fine microstructure. In addition, the saturation induction, initial permeability, and resistivity of the MnZn ferrites were increased, and the high-frequency core losses were suppressed. The core losses of the 0.2 wt% CCTO sample were only 495 kW/m3 at 3 MHz, 30 mT, and 25 °C. The lower core losses resulted mainly from reductions in the hysteresis loss and eddy current loss. The real permittivity of two representative MnZn ferrite samples was also investigated. The application of state-of-the-art MnZn ferrites with superior electromagnetic properties in high-frequency power conversion devices is anticipated. This study demonstrates that adding a dielectric material improves the electromagnetic properties of MnZn power ferrites.

Accurate Measurement of Dynamic on-State Resistances of GaN Devices Under Reverse and Forward Conduction in High Frequency Power Converter

Because of trapped charges in GaN transistor structure, device dynamic on -state resistance $R_\mathrm{DSon}$ is increased when it is operated in high frequency switched power converters, in which device is possibly operated by zero voltage switching (ZVS) to reduce its turn- on switching losses. When GaN transistor finishes ZVS during one switching period, device has been operated under both reverse and forward conduction. Therefore its dynamic $R_\mathrm{DSon}$ under both conduction modes needs to be carefully measured to understand device power losses. For this reason, a measurement circuit with simple structure and fast dynamic response is proposed to characterize device reverse and forward $R_\mathrm{DSon}$ . In order to improve measurement sensitivity when device switches at high frequency, a trapezoidal current mode is proposed to measure device $R_\mathrm{DSon}$ under almost constant current, which resolves measurement sensitivity issues caused by unavoidable measurement circuit parasitic inductance and measurement probes deskew in conventional device characterization method by triangle current mode. Proposed measurement circuit and measurement method is then validated by first characterizing a SiC- mosfet with constant $R_\mathrm{DSon}$ . Then, the comparison on GaN-HEMT dynamic $R_\mathrm{DSon}$ measurement results demonstrates the improved accuracy of proposed trapezoidal current mode over conventional triangle current mode when device switches at 1 MHz.

The X-Ray Emission Effectiveness of Plasma Mirrors: Reexamining Power-Law Scaling for Relativistic High-Order Harmonic Generation

Ultrashort pulsed lasers provide uniquely detailed access to the ultrafast dynamics of physical, chemical, and biological systems, but only a handful of wavelengths are directly produced by solid-state lasers, necessitating efficient high-power frequency conversion. Relativistic plasma mirrors generate broadband power-law spectra, that may span the gap between petawatt-class infrared laser facilities and x-ray free-electron lasers; despite substantial theoretical work the ultimate efficiency of this relativistic high-order-harmonic generation remains unclear. We show that the coherent radiation emitted by plasma mirrors follows a power-law distribution of energy over frequency with an exponent that, even in the ultrarelativistic limit, strongly depends on the ratio of laser intensity to plasma density and exceeds the frequently quoted value of −8/3 over a wide range of parameters. The coherent synchrotron emission model, when adequately corrected for the finite width of emitting electron bunches, is not just valid for p-polarized light and thin foil targets, but generally describes relativistic harmonic generation, including at normal incidence and with finite-gradient plasmas. Our numerical results support the ω−4/3 scaling of the synchrotron emission model as a limiting efficiency of the process under most conditions. The highest frequencies that can be generated with this scaling are usually restricted by the width of the emitting electron bunch rather than the Lorentz factor of the fastest electrons. The theoretical scaling relations developed here suggest, for example, that with a 20-PW 800-nm driving laser, 1 TW/harmonic can be produced for 1-keV photons.

Mechanism Model and Prediction Method of Common Mode Radiation for a Nonisolated Very-High-Frequency DC–DC Converter With Cables

Research on electromagnetic interference of high-frequency power converters remains a challenging and important topic, particularly in light of rapidly emerging wide band-gap power devices operating with ns level turn- on /turn- off times. Building on the substantial contributions that have been made on the formation mechanisms of common mode (CM) radiation, a new mechanism for an equivalent CM driving voltage source caused by imbalanced conductors or components is proposed. Applying this mechanism, the CM driving voltage source model of a nonisolated boost dc/dc power converter with input/output cables is introduced. The CM far-field electric strength algorithms of an asymmetric cable antenna are theoretically analyzed and derived in detail. Subsequently, a prototype for a low-power 30-MHz very-high-frequency boost power converter based on gallium nitride is built and its 10-m CM far-field radiation strength is predicted according to the proposed model. The radiation pattern characteristics are analyzed and several radiation suppression methods are also designed. The comparison with the measured radiation results shows that the proposed CM radiation model and the prediction method achieve good effect below 300 MHz.

Analysis of Intrinsic Switching Losses in Superjunction MOSFETs Under Zero Voltage Switching

Switching losses of power transistors usually are the most relevant energy losses in high-frequency power converters. Soft-switching techniques allow a reduction of these losses, but even under soft-switching conditions, these losses can be significant, especially at light load and very high switching frequency. In this paper, hysteresis and energy losses are shown during the charge and discharge of the output capacitance (COSS) of commercial high voltage Superjunction MOSFETs. Moreover, a simple methodology to include information about these two phenomena in datasheets using a commercial system is suggested to manufacturers. Simulation models including COSS hysteresis and a figure of merit considering these intrinsic energy losses are also proposed. Simulation and experimental measurements using an LLC resonant converter have been performed to validate the proposed mechanism and the usefulness of the proposed simulation models.

Novel Fe-based amorphous and nanocrystalline powder cores for high-frequency power conversion

The present work demonstrates the high-frequency core loss performance of Fe-based amorphous and nanocrystalline powder cores, initially produced by gas atomised powder, consolidated using sieved particles ≤20 µm, and isolated by a precise insulating layer of polymer to limit the inter- and intra-particle eddy currents to attain enhanced performance. The large glass forming ability (GFA) of the gas atomised powder, reflected by different glass forming instruments, such as the supercooled region (ΔTX = 54 °C) and the reduced glass transition temperature (Trg = 0.56), is consistent with the substantial amorphisation capability of the alloy. To the best of our knowledge, this is the first-ever report to reveal a large ΔTX in the Finemet-type alloy powders, an essential parameter to gas-atomise the amorphous powders with significantly lower cooling rates compared to the melt-spun ribbons. Further, subsequent annealing of the amorphous powders, between the exothermic events guided by differential scanning calorimetry (DSC), lead to the growth of a fine nanocrystalline structure of grains ≤15 nm, thanks to the positive enthalpy of mixing of Cu with the constituents to act as a nucleation agent, to retain the excellent soft magnetic properties. The DC soft magnetic properties of the powders were significantly improved on thermal annealing, confirmed by hysteretic loops, quantified by reduced coercivity HC <1 Oe of annealed powders at <575 °C, and attributed to the reduced magnetoelastic contribution due to zero/near-zero magnetostriction anisotropy, attained due to the homogenous nanocrystalline structure. The amorphous and nanocrystalline powder cores, consolidated by compression moulding, show ultra-high loss performance, due to the ultra-low coercivity attained on nanocrystallisation, and negligible eddy currents loss, owning to efficient insulation of small particles, for high-frequency power conversion applications, such as voltage regulator (VR), and resonant converters, in automotive industry and data storage centres.

Estimation of Power Losses Caused by Supraharmonics

Nowadays increases the number of power electronic devices in distribution electrical networks rapidly. Modern generation and consumption units use high switching frequency power converters for the network connection and therefore cause the voltage and current distortion in the frequency range over 2 kHz (so-called “supraharmonics”) in addition to conventional harmonics. Supraharmonics cause additional power losses in electrical equipment. The goal of the offered paper is the estimation of power losses caused by supraharmonics. The estimation is based on the measurement results obtained in a real MV/LV network in Germany.

A Review of High Frequency Power Converters and Related Technologies

Development of power electronic converters tend to achieve high efficiency and at the same time high power density in many industrial applications. In recent years, with emerging third-generation semiconductor materials i.e. Silicon Carbide (SiC) and Gallium Nitride (GaN), the switching frequency of several MHz has become a widely studied frequency band, therefore traditional technology can no longer meet the demand, and many new challenges appear. This paper presents a comprehensive review of high frequency (HF) converters, the essential challenges are analyzed such as topology selection, soft-switching technologies, resonant gate drivers, magnetic components design and optimization.

New Insights on Output Capacitance Losses in Wide-Band-Gap Transistors

The low on-resistance of wide-band-gap (WBG) transistors is a key feature for efficient power converters; however, the anomalous loss in their output capacitance (COSS) severely limits their performance at high switching frequencies. Characterizing COSS losses based on the large-signal measurement methods requires an extensive effort, as separate measurements are needed at different operation points, including voltage swing, frequency, and dv/dt. Furthermore, there is a practical tradeoff in the maximum voltage and frequency applied to the device. Here, we introduce a new circuit model, including an effective COSS and a frequency-dependent series resistance, along with a simple small-signal method to fully characterize COSS losses in WBG transistors. The method accurately predicts COSS losses at any voltage swing or frequency. Contrary to other methods, this technique directly leads to a general identification of COSS losses at different operation points, revealing new insights on COSS losses in WBG transistors, especially the dependence of EDISS on voltage and frequency. Based on the proposed approach, the issue of COSS losses in enhancement-mode GaN and SiC transistors was assigned to the limited quality factor of COSS. The precise characterization of COSS losses proposed in this letter is essential for designing efficient high-frequency power converters.

Cascode GaN/SiC: A Wide-Bandgap Heterogenous Power Device for High-Frequency Applications

Wireless power transfer systems and plasma generators are among the increasing number of applications that use high-frequency power converters. Increasing switching frequency can reduce the energy storage requirements of the passive elements that can lead to higher power densities or even the elimination of magnetic cores. However, operating at higher frequencies requires faster switching devices in packages with low parasitics. Wide-bandgap (WBG) power devices, such as gallium nitride (GaN) and silicon carbide (SiC) devices, have high-critical fields and high-thermal conductivity that make them good candidates for efficient high-voltage and high-frequency operations. Commercially available GaN and SiC devices have ratings targeting different applications. Lateral GaN devices dominate in lower voltage ( $C_{\text{oss}},C_{\text{iss}}$ ), which make them easy to drive at high frequencies. On the other hand, vertical SiC devices are often used in high-voltage and low-frequency applications since they have higher blocking voltages and larger gate charge than their GaN counterparts. As a result, SiC devices usually require high power and complicated gate drive circuitry. Recent work shows that in both GaN and SiC devices, losses in device $C_{\text{oss}}$ can exceed the conduction losses at high-switching frequencies and relatively high voltages under zero-voltage-switching conditions. Moreover, the $C_{\text{oss}}$ energy loss ( $E_{\text{oss}}$ ) per switching cycle increases with frequency in GaN devices but remains roughly independent of frequency in SiC devices. This means that at high frequencies, SiC devices can be preferable due to their smaller $C_{\text{oss}}$ energy loss even when taking into consideration the complexity of the gate drive circuit. In this article, we present a WBG high-voltage cascode GaN/SiC power device, combining the advantages of both a GaN and an SiC device—namely, simple gate drive requirements, $E_{\text{oss}}$ loss per cycle roughly independent of frequency, and relatively high-voltage blocking capability. Comparing this cascode GaN/SiC device with an SiC mosfet and a SiC junction gate field-effect transistor of similar voltage ratings and $R_{ds,{\text{ON}}}$ , we find that the inverter using the cascode GaN/SiC device has the highest efficiency and simplest auxiliary gate drive circuitry. Finally, integrating the cascode GaN/SiC device has the potential benefits of achieving lower $C_{\text{oss}}$ losses, higher device ratings, and better heat removal capability.

Evolution of Hybrid Inductive and Capacitive AC Links for Wireless EV Charging—A Comparative Overview

The inductive link in a wireless power transfer (WPT) system, fed by various high-frequency power converters with appropriate control strategies is widely accepted for medium power ( 95%) in static wireless charging scenarios. However, considering the dynamic wireless charging, the inductive link implementation for millions of kilometers is quite expensive due to the need for sophisticated resources. Therefore, relatively low-cost capacitive link, which initially focused on low power ( 90%) and significant misalignment tolerance. This paper aims to find the significant roles played by inductive and capacitive ac link designs in static and dynamic expansion of wireless electric vehicle (EV) charging. Through a relative analysis, this paper emphasizes the need for mutual promotion of these ac links as a hybrid system in order to boost wireless EV charging technology.

On the Techniques to Utilize SiC Power Devices in High- and Very High-Frequency Power Converters

In this paper, we explore the challenges of implementing resonant converters using silicon carbide (SiC) power devices at high frequency: namely, the issue of high parasitic inductance packages and the ability to drive and enhance the mosfet at these frequencies. Although power circuit designers have many alternative device technologies to choose from, such as silicon and gallium nitride materials, SiC devices have several advantageous attributes especially in high power applications. As a solution, we study the device performance and parasitics of SiC mosfet s in different packaging schemes. We further offer a solution to the challenges of driving SiC devices by demonstrating a multiresonant gate driver and use this scheme to drive an SiC mosfet at 30 MHz and a SiC JFET at 13.56 MHz in a class-E inverter, achieving 85.7 $\%$ drain efficiency for the mosfet and 93.8 $\%$ for the JFET.

Diode Evaluation and Series Diode Balancing for High-Voltage High-Frequency Power Converters

Miniaturization of high-voltage power converters is severely limited by the availability of fast switching, low-loss high-voltage diodes. This article explores techniques for using discrete low-voltage diodes in series as one high-voltage diode in high-frequency applications (e.g., hundreds of kHz and above). We identify that when series connecting diodes, the parasitic capacitance from the physical diode interconnections to common can result in voltage and temperature imbalance among the diodes, along with increased loss. We quantify the imbalance and propose two related compensation techniques. To validate the approaches, a full-bridge rectifier is tested with each branch consisting of four 3.3 kV SiC diodes in series. Experimental results showcase the imbalance and demonstrate the effectiveness of the compensation techniques. Additionally, we characterize the performance of a range of diodes for use in high-frequency, high-voltage converters. The proposed technique and evaluation results will be valuable for the design of lightweight and miniaturized high-voltage power converters.

A Non-Invasive DC-10-MHz Wideband Current Sensor for Ultra-Fast Current Sensing in High-Frequency Power Electronic Converters

To enhance the performance of the converters along with improving their efficiency and miniaturization in many power electronics applications, a need for higher switching frequency is highly demanded. In high switching frequency converters, due to high common mode voltage issue, wideband, isolated, and low loss current sensor is required for the robust control design and ultra-fast protection. In this paper, we propose a non-invasive hybrid, isolated, and loss-less wideband current sensing method applicable for high-frequency power converters (dc-10 MHz). The proposed hybrid current sensing scheme combines two different isolated sensing technologies—a magnetoresistor sensor and a differential Rogowski coil sensor, which have complementary characteristics to achieve a wide sensing bandwidth. The proposed hybrid current sensor also utilizes planar magnetic field concentrators (MCON) using conductive materials to normalize the non-uniform magnetic field distributions generated by the current over a wide frequency range. We demonstrate through hardware prototypes and experiments that the proposed hybrid sensor yields to a very high sensing bandwidth of dc to 10 MHz. The performance of the proposed hybrid sensor is also evaluated in a 1-MHz dc–dc converter as well as a high-frequency H-bridge inverter module.

Real-Time Deep Learning at the Edge for Scalable Reliability Modeling of Si-MOSFET Power Electronics Converters

With the significant growth of advanced high-frequency power converters, online monitoring and active reliability assessment of power electronic devices are extremely crucial. This paper presents a transformative approach, named deep learning reliability awareness of converters at the edge (Deep RACE), for real-time reliability modeling and prediction of high-frequency MOSFET power electronic converters. Deep RACE offers a holistic solution which comprises algorithm advances, and full system integration (from the cloud down to the edge node) to create a near real-time reliability awareness. On the algorithm side, this paper proposes a deep learning algorithmic solution based on stacked long short-term memory for collective reliability training and inference across collective MOSFET converters based on device resistance changes. Deep RACE also proposes an integrative edge-to-cloud solution to offer a scalable decentralized devices-specific reliability monitoring, awareness, and modeling. The MOSFET convertors are Internet-of-Things (IoT) devices which have been empowered with edge real-time deep learning processing capabilities. The proposed Deep RACE solution has been prototyped and implemented through learning from MOSFET data set provided by NASA. Our experimental results show an average miss prediction of 8.9% over five different devices which is a much higher accuracy compared to well-known classical approaches (Kalman filter and particle filter). Deep RACE only requires 26-ms processing time and 1.87-W computing power on edge IoT device.

A Multiphysics Design and Optimization Method for Air-Core Planar Transformers in High-Frequency LLC Resonant Converters

Air-core planar transformers are attractive alternatives to ferrite-core transformers for high-frequency power converters, such as LLC resonant converters. Although many design methods have been proposed for LLC converters, most of them are based on traditional ferrite-core transformers. There is a lack of efficient methods to design and optimize air-core planar transformers for high-frequency LLC converters. This paper presents a multiphysics optimal design framework that combines multiple physical models, the progressive bisection design method, and the Bayesian optimization technique to address air-core planar transformer design issues. It has the following contributions. 1) Multiple physical models, including circuit model, magnetoquasistatic model, full-wave electromagnetic model, are built for transformer design and efficiency evaluation. 2) The progressive bisection method and Bayesian optimization algorithm are applied to accelerate the optimal design process. 3) The proposed framework can accurately and efficiently design optimal air-core planar transformers and optimize efficiency in LLC converters. 4) A high-frequency flexible and thin LLC converter is constructed using the optimal design method.

Modulation Induced Current Imbalance and Its Sensorless Control of a GaN-Based Four-Phase DC–DC Power Amplifier

The development of wide band-gap semiconductor devices is rapidly promoting the development of power electronics technology, especially in high frequency (HF) power converter region. A category of HF power converter is switch-mode power amplifier (PA). This paper focuses on the implementation of a multiphase dc–dc PA with high bandwidth and high power rate. When naturally sampled phase-shifted carrier modulation is implemented in the practical digital control system to control this HF power converter, the pulsewidth error problem caused by limited domain frequency of control chip becomes much severer. As a result, dc voltage component of each phase becomes different under several conditions, leading to current imbalance among phase inductors. This paper analyzes this problem and proposes a sensorless control method to solve it, based on the injection of random sequence in reference wave. A four-phase prototype employing GaN devices is fabricated, switching at 1 MHz. Current imbalance and effectiveness of the proposed control method are verified by experimental results. The prototype is capable of uniformly amplifying sine signal within 200 kHz and a 350 kHz bandwidth random signal, with peak output power up to 810 W.

Three-Dimensional Frequency-Dependent Thermal Model for Planar Transformers in LLC Resonant Converters

In this paper, a thorough thermal analysis is carried out in order to find an analytical three-dimensional (3-D) frequency-dependent thermal model of planar transformers (PTs) used in high-frequency power converters. Because of the tendency of having higher power density as well as miniaturization of power converters, the components have to work near their thermal limit margins. This issue is more critical for planar magnetics in LLC converters due to their variable high-frequency operation, making thermal modeling and heat transfer analysis tools necessary. The proposed 3-D frequency thermal model is obtained based on the lumped parameters network (LPN) according to different thermal resistances of the PT including convection, conduction, and radiation heat exchanges. It should be noted that not only the 3-D geometry effect of the core is applied to the proposed model, but thermal surface resistances are also considered to model the high-frequency operation effect. Moreover, the lumped capacitance thermal model is used to analyze the transient thermal variation of the transformer. The LLC resonant converter with two PTs, including EE58/11/38 and ER51/10/38 planar cores, is considered to show the compatibility of the proposed 3-D-frequency LPN model. In addition, the temperature distribution of different parts of the transformers is evaluated using finite element method modeling and compared with the proposed 3-D frequency-dependent LPN model. Experimental results confirm the improved accuracy of the proposed LPN model and show the proposed model predicts the temperature distribution in PTs with an error of less than 3%. Considering the improved accuracy and low computational time of the method, the proposed 3-D frequency-dependent model is a powerful and fast design tool to evaluate the temperature distribution for different designs, and so can be used to effectively optimize the transformer from thermal point of view.

Investigation on filter requirements and stability effects of SiC MOSFET‐based high‐frequency grid‐connected converters

The new era of power semiconductor devices is exploiting the high potential of wide bandgap device-based high-frequency power converters. The advanced capabilities of Silicon carbide (SiC) semiconductors allow the power converters to switch at higher switching frequencies with lower switching and conduction losses. Operation under higher switching frequencies can drastically reduce the output filter requirements in grid-connected converters. At higher frequencies, an L-type filter can sufficiently eliminate the switching frequency harmonics. This will reduce the size and cost of the filter, which eventually results in the optimised system cost. In this paper, the filter requirements of SiC based high-frequency converters are investigated. Usage of LCL filters to remove high-frequency harmonic will shift the resonance frequency to high-frequency region, where these harmonics can create electromagnetic compatibility issues. In industries, passive damping is utilised for mitigating these oscillations. This will result in additional losses and compromise the advantages of SiC devices. Even though active damping can be utilised to eliminate resonance oscillations, the high bandwidth processors and measurement devices may increase the cost further. Simulation study is conducted on PSCAD/MATLAB and the experimental verification using Cree SiC devices verify the filter requirements, stability issues, and possible mitigating solutions for high-frequency grid converters.