Wireless Low-Power Transfer for Galvanically Isolated High-Voltage Applications

For various applications, such as gate drivers for transistors, wireless chargers for mobile devices and cars, and isolated measurement equipment, an isolated DC power supply for electronic components is required. In this work, a new concept for an isolated power supply with insulation strength of 50 kV and power transmission of up to 60 W to supply measurement equipment with 12 or 24 V is presented. Furthermore, high overall efficiency of 82.5% at 55 W is achieved. Feasibility is demonstrated in a real application powering data acquisition electronics at high reference potential. Our new concept uses a coreless printed circuit board (PCB) transformer (15 cm × 10 cm × 4 cm and a weight of 480 g) designed for maximum efficiency via a coil layout and close proximity of adjacent coils on one PCB while reaching high isolation strength via the PCB material and potted coils. To increase efficiency, we investigated different coil geometries at different frequencies. A low-cost design consisting of two Qi charging coils mounted on one PCB is compared with our integrated PCB transformers manufactured from a four-layer PCB with ferrites applied on the outside. With this new design, high isolation voltages are possible while reaching high transformer efficiency of up to 90%. © 2022 by the authors. Licensee MDPI, Basel, Switzerland

Generic closed loop controller for power regulation in dual active bridge DC-DC converter with current stress minimization

This paper presents a comprehensive and generalized analysis of the bidirectional dual active bridge (DAB) DC/DC converter using triple phase shift (TPS) control to enable closed loop power regulation while minimizing current stress. The key new achievements are: a generic analysis in terms of possible conversion ratios/converter voltage gains (i.e. Buck/Boost/Unity), per unit based equations regardless of DAB ratings, and a new simple closed loop controller implementable in real time to meet desired power transfer regulation at minimum current stress. Per unit based analytical expressions are derived for converter AC RMS current as well as power transferred. An offline particle swarm optimization (PSO) method is used to obtain an extensive set of TPS ratios for minimizing the RMS current in the entire bidirectional power range of - 1 to 1 per unit. The extensive set of results achieved from PSO presents a generic data pool which is carefully analyzed to derive simple useful relations. Such relations enabled a generic closed loop controller design that can be implemented in real time avoiding the extensive computational capacity that iterative optimization techniques require. A detailed Simulink DAB switching model is used to validate precision of the proposed closed loop controller under various operating conditions. An experimental prototype also substantiates the results achieved

A new active power controller in dual active bridge DC-DC converter with a minimum-current-point-tracking technique

This article proposes a new controller for power regulation in dual active bridge (DAB) dc-dc converter based on a new scheme that tracks minimum RMS current to ensure minimum losses. The proposed controller is based on an implementation of perturb and observe (PO) tracking method that enables minimum current point tracking (MCPT) at any desired level of active power transfer and dc voltage ratio. The PO is embedded in a closed-loop control scheme which simultaneously regulates active power in DAB converter. The nonlinear I - V characteristic of DAB presents the basis for this proposed controller and the rationale of using PO algorithm. The proposed controller does not require complex nonlinear converter modeling and is not circuit parameter dependent. Design procedure of the proposed controller is presented, and extensive simulation is carried out using MATLAB/Simulink to validate the effectiveness of the proposed MCPT closed-loop controller. An experimental prototype also substantiates the results achieved

Gallium Nitride Superjunctions for Power Electronic Applications

Gallium Nitride (GaN) is a wide-band gap semiconductor that has found market acceptance in applications such as lighting, power electronics and radio-frequency electronics. The high breakdown electric field and electron saturation velocity of GaN are favorable relative to Silicon, but manufacturing costs and material defects have limited the market penetration of GaN. Commercial GaN devices are based on a two-dimensional electron gas (2DEG) that arises from spontaneous and piezoelectric polarization at the hetero-interface between GaN and Aluminum Gallium Nitride (AlGaN). These 2DEG-based devices, termed High Electron Mobility Transistors (HEMTs), have under-performed to date when comparing semiconductors with the Baliga Figure of Merit (BFOM) for power electronics. However, Silicon and Silicon Carbide (SiC) devices have exceeded their respective material limits due to lower defect density materials and device engineering. Currently, no native substrate for GaN is economically viable or commercially available so Group III-Nitrides are deposited on Si, Sapphire or SiC. This hetero-epitaxy leads to high defect densities in the epitaxial films which hinder device performance and reliability. Therefore, the reduction of defect densities and manufacturing costs is critical to meeting goals set-forth by government agencies to improve power conversion efficiencies while being cost competitive. The work presented herein provides a potential solution to manufacturing low defect density GaN devices that have both a low on-state resistance, RvON and high blocking voltage capability, VvBR, on Silicon substrates. This is accomplished by utilizing selective area epitaxy (SAE), which reduces the defect densities of the epitaxial films, and adopting the superjunction (SJ) device architecture from Silicon that reduces the engineering trade-off between RvON and VvBR. A SJ device requires n-type and p-type GaN materials (n-GaN and p-GaN, respectively), of which p-GaN has proven to be a material that is difficult to achieve in low resistivities. However, the doping ranges required to achieve high voltage operation have been frequently demonstrated. The SAE process requires extra lithography and deposition steps relative to a commercial GaN-on-Si HEMT technology, therefore the high growth rates of the SAE are leveraged to reduce deposition time and thinner buffers layers could enable larger wafer diameters. Parametric device simulations are presented to determine which design parameters are most sensitive in the superjunction process. The SJ layer charge and metal contact resistance to the pGaN were found to be the two most critical design parameters. Next, an epitaxial GaN process was developed on sapphire substrates using Metal Organic Chemical Vapor Deposition (MOCVD). The GaN-on-Sapphire films were characterized using Hall measurements and diodes to understand the background doping concentration of the unintentionally doped GaN (UID-GaN). These GaN films and devices serve as a baseline for comparing GaN films grown on Silicon substrates. Integration of GaN with Silicon control electronics is an attractive technology to reduce interconnect losses and minimize footprint. However, commercial GaN devices are grown on Silicon (111) substrates while CMOS devices are fabricated using Silicon (100). A GaN-on-Silicon (100) process was developed using Atomic Layer Deposition (ALD) Aluminum Oxide (Al2O3) as a nucleation layer, with the goal of understanding how GaN could be grown on Silicon (100) substrates to avoid costly bonding and substrate removal processes. X-ray Diffraction (XRD) and electrical characterization was utilized to compare the GaN quality between Silicon substrates and the GaN-on-Sapphire. A SAE process for GaN on Sapphire substrates was developed using a Plasma Enhanced CVD (PECVD) Silicon Nitride (Si3N4) dielectric as a mask. Scanning Electron Microscopy (SEM) was utilized to monitor lateral epitaxy and provide information on how to control the film morphology. Significant defect density reduction of the SAE GaN films was accomplished by using Aspect Ratio Trapping (ART), which is a form of SAE where the aspect ratio of the dielectric window is greater than 1. Electron beam lithography (EBL) and Reactive Ion etching (RIE) processes were developed to obtain an aspect ratio of 1.2. Transmission Electron Microscopy (TEM) was then performed on the ART GaN-on-Silicon (111) which verified defect trapping inside the windowed region. Lastly, p-GaN ohmic contacts were developed using Nickel and Gold as a baseline process. Improvements to the baseline ohmic contact process was made by using Magnesium as an interlayer between the p-GaN and the Nickel

Efficient, High Power Density, Modular Wide Band-gap Based Converters for Medium Voltage Application

Recent advances in semiconductor technology have accelerated developments in medium-voltage direct-current (MVDC) power system transmission and distribution. A DC-DC converter is widely considered to be the most important technology for future DC networks. Wide band-gap (WBG) power devices (i.e. Silicon Carbide (SiC) and Gallium Nitride (GaN) devices) have paved the way for improving the efficiency and power density of power converters by means of higher switching frequencies with lower conduction and switching losses compared to their Silicon (Si) counterparts. However, due to rapid variation of the voltage and current, di/dt and dv/dt, to fully utilize the advantages of the Wide-bandgap semiconductors, more focus is needed to design the printed circuit boards (PCB) in terms of minimizing the parasitic components, which impacts efficiency. The aim of this dissertation is to study the technical challenges associated with the implementation of WBG devices and propose different power converter topologies for MVDC applications. Ship power system with MVDC distribution is attracting widespread interest due to higher reliability and reduced fuel consumption. Also, since the charging time is a barrier for adopting the electric vehicles, increasing the voltage level of the dc bus to achieve the fast charging is considered to be the most important solution to address this concern. Moreover, raising the voltage level reduces the size and cost of cables in the car. Employing MVDC system in the power grid offers secure, flexible and efficient power flow. It is shown that to reach optimal performance in terms of low package inductance and high slew rate of switches, designing a PCB with low common source inductance, power loop inductance, and gate-driver loop are essential. Compared with traditional power converters, the proposed circuits can reduce the voltage stress on switches and diodes, as well as the input current ripple. A lower voltage stress allows the designer to employ the switches and diodes with lower on-resistance RDS(ON) and forward voltage drop, respectively. Consequently, more efficient power conversion system can be achieved. Moreover, the proposed converters offer a high voltage gain that helps the power switches with smaller duty-cycle, which leads to lower current and voltage stress across them. To verify the proposed concept and prove the correctness of the theoretical analysis, the laboratory prototype of the converters using WBG devices were implemented. The proposed converters can provide energy conversion with an efficiency of 97% feeding the nominal load, which is 2% more than the efficiency of the-state-of-the-art converters. Besides the efficiency, shrinking the current ripple leads to 50% size reduction of the input filter inductors

Design and analysis of current stress minimalisation controllers in multi-active bridge DC-DC converters.

Multi active bridge (MAB) DC-DC converters have attracted significant research attention in power conversion applications within DC microgrids, medium voltage DC and high voltage DC transmission systems. This is encouraged by MAB's several functionalities such as DC voltage stepping/matching, bidirectional power flow regulation and DC fault isolation. In that sense this family of DC-DC converters is similar to AC transformers in AC grids and are hence called DC transformers. However, DC transformers are generally less efficient compared to AC transformers, due to the introduction of power electronics. Moreover, the control scheme design is challenging in DC transformers, due to its nonlinear characteristics and multi degrees of freedom introduced by the phase shift control technique of the converter bridges. The main purpose of this research is to devise control techniques that enhance the conversion efficiency of DC transformers via the minimisation of current stresses. This is achieved by designing two generalised controllers that minimise current stresses in MAB DC transformers. The first controller is for a dual active bridge (DAB). This is the simplest form of MAB, where particle swarm optimisation (PSO) is implemented offline to obtain optimal triple phase shift (TPS) parameters, for minimising the RMS current. This is achieved by applying PSO on DAB steady-state model, with generic per unit expressions of converter AC RMS current and transferred power under all possible switching modes. Analysing the generic data pool generated by the offline PSO algorithm enabled the design of a generic real-time closed-loop PI-based controller. The proposed control scheme achieves bidirectional active power regulation in DAB over the 1 to -1 pu power range with minimum-RMS-current for buck/boost/unity modes, without the need for online optimisation or memory-consuming look-up tables. Extending the same controller design procedure for MAB was deemed not feasible, as it would involve a highly complex PSO exercise that is difficult to generalise for N number of bridges; it would therefore generate a massive data pool that would be quite cumbersome to analyse and generalise. For this reason, a second controller is developed for MAB converter without using a converter-based model, where current stress is minimised and active power is regulated. This is achieved through a new real-time minimum-current point-tracking (MCPT) algorithm, which realises iterative-based optimisation search using adaptive-step perturb and observe (P&O) method. Active power is regulated in each converter bridge using a new power decoupler algorithm. The proposed controller is generalised to MAB regardless of the number of ports, power level and values of DC voltage ratios between the different ports. Therefore, it does not require an extensive look-up table for implementation, the need for complex non-linear converter modelling and it is not circuit parameter-dependent. The main disadvantages of this proposed controller are the slightly slow transient response and the number of sensors it requires