Showing papers by "Khai D. T. Ngo published in 2019"

Medium Voltage Dual Active Bridge Using 3.3 kV SiC MOSFETs for EV Charging Application

Abstract: Electric vehicle charging technology has come a long way and seen progressive outcomes toward a wide adoption of EVs. To further accelerate the EV evolution, though, the recharging time of electric vehicles will require much improvement. In order to address such a challenge, it is critical to develop a technology that would enable faster, more efficient, and more effective ways of charging an electric vehicle. In order to address such a challenge, it is critical to develop a technology that would enable faster, more efficient, and more effective ways of charging an electric vehicle. The current fast-charging solution involves a heavy and bulky MV-LV transformer, which adds installation complexity for EV charging stations. Therefore, this paper presents an alternative power-delivery solution utilizing a medium-voltage (MV) dual-active-bridge (DAB) converter. The proposed architecture is designed to directly interface the MV grid for high-power, fast-charging capability while eliminating the need for the installation of the MV-LV transformer. The MV DAB converter utilizes 3.3 kV SiC MOSFETs to realize the next 800 V EV charging system. This paper also provides an extended zero-voltage-switching (ZVS) scheme for the MV DAB converter by varying the input (DC-Link) voltage and the switching frequency of the converter. With the system specifications and requirements, a scaled-down 12 kW MV DAB prototype was developed, and its charging operation along with the extended ZVS scheme was simulated and verified by experiments.

Analytical Calculation of Proximity-Effect Resistance for Planar Coil With Litz Wire and Ferrite Plate in Inductive Power Transfer

Abstract: Planar coils are popularly used in inductive power transfer applications. The coils’ efficiency is improved with litz wire by eliminating the winding's skin-effect loss, but the proximity-effect loss still needs to be considered. To calculate the proximity-effect resistance, the magnetic fields across the windings are required. During the coils’ optimization process, numerical methods of field calculation are too time-consuming due to the parametric sweep of physical parameters. The analytical calculation of the magnetic field is preferred in this scenario, but it becomes complex with the existence of the ferrite plate, which is used to increase the coils’ coupling. In this paper, the method of image is applied to simplify the analytical calculation, in which the magnetic fields across the windings are derived from Ampere's law and Biot–Savart law. The proximity-effect resistance is then calculated with these fields. The study also proves that the size of the ferrite plate has negligible influence on the proximity-effect resistance as long as it is larger than the size of the winding. A planar coil was fabricated and measured to verify the analytical calculation of the proximity-effect resistance, using litz wire composed of 100 strands with 0.1 mm diameter. A square plate of 3C96 ferrite with the dimensions of 100 mm × 100 mm × 5 mm was placed below the winding. The difference between calculation and measurement is less than 15%.

Circuit Models and Fast Optimization of Litz Shield for Inductive-Power-Transfer Coils

Abstract: The metal shields with plates or rings are usually added to the inductive-power-transfer coils to attenuate the stray magnetic field. However, the coil-to-coil efficiency is reduced owning to the extra losses on the shields. This paper introduces the litz shield which attenuates the field with smaller shielding loss thanks to the uniform distribution of the shield current. Two types of litz shields, i.e., shorted litz shield and driven litz shield are discussed in detail. Their performances are analyzed with the circuit model composed of four coupled inductors. The efficiency and the field attenuation for the coils with litz shields are optimized with Pareto fronts. A fast method to derive the fronts using a lumped-loop model is described. Only tens of finite-element simulations are required in the entire optimization. The coils with different shields were optimized to transfer 100-W power across 40-mm gap with the same efficiency. The measurement results prove that the shorted litz shield is more effective than the other shields. It attenuated 23% more of the stray magnetic field compared to the coils with traditional metal shield.

Magnetic Integration Into a Silicon Carbide Power Module for Current Balancing

Abstract: Threshold-voltage mismatch among paralleled dies leads to unbalanced turn- on peak currents and switching energies, thus degrading reliability. A passive method employing inversely coupled inductors of tens of nH and drive-source resistors reduces current unbalance. An integrated design of the coupled inductors is required to facilitate their practical use in a power module. A layout to achieve inverse coupling, high coupling coefficient, and low voltage stress, magnetic materials suitable for operation at tens of MHz, and high current rating of tens of amperes with small magnetic core are challenging for its implementation. A module with integrated coupled inductors that achieve inverse coupling by utilizing the copper trace of the substrate and bond wires, size comparable to the silicon carbide die, coupling coefficient higher than 0.98, tens of nH operating at tens of MHz, and current rating of tens of amperes was designed, fabricated, and validated in this work. The coupled inductors with magnetic material of low-temperature cofired ceramics are compatible with existing packaging technology for module fabrication. Effectiveness on reducing transient-current mismatch at various input voltages, load currents, and gate resistances was verified by experiments. Compared with the baseline module resembling commercial modules, the module with integrated coupled inductors reduces current unbalance from 36% to 6.4% and turn- on energy difference from 28% to 2.6% while maintaining the same total switching energy and negligible change of voltage stress.

A Theory to Synthesize Nonisolated DC–DC Converters Using Flux Balance Principle

Abstract: This paper describes a theory to synthesize nonisolated dc–dc converters. It uses the fundamental flux balance equation across the inductors of a converter as a starting point in this synthesis process. The flux balance equations are the linear equations of the input voltage, capacitor voltages, and duty cycle (D). The coefficients of these linear equations can be selected from a finite set of choices. These choices define the converter topologies that are subsequently used to synthesize a converter. The synthesis procedure applies to a converter of multiple order. All the possible converters are identified for a first-order topology. In the case of second-order converters, all the choices of the flux balance equation are defined. Based on these choices, three new quadratic topologies are derived and verified to demonstrate the effectiveness of the theory. The procedure to synthesize a converter from a given voltage conversion ratio is also outlined.

Transient Core-Loss Simulation for Ferrites With Nonuniform Field in SPICE

Abstract: A methodology to develop a subcircuit model for core loss simulation in LTspice is presented. The subcircuit implements the dynamic core loss model used in the transient solver of finite-element analysis (FEA) in order to provide equivalent results in the time domain for ferrite materials. The wipe-out rule is applied in the simulation so that the core loss during the transient behavior can be predicted. A field factor is derived from an effective flux density for a nonuniform flux distribution to simplify the relationship between the current and the field for arbitrary magnetic core shapes. The interface of the subcircuit is capable of being integrated to any power stage simulations. Simulation results of core loss equivalent to the FEA on an exemplary plate-core inductor are obtained from the subcircuit that significantly reduces computational cost.

Design Methodology of a ZVS Class-E Inverter with Fixed Gain

Abstract: A ZVS Class-E inverter design is presented, aiming at Zero-Voltage Turn-On (ZVTO) and fixed voltage gain over a wide load range without adding components, changing duty cycle, or tuning switching frequency. Passive components could be designed and tuned one-by-one. The expectations were validated by a Class-E inverter switched at 6.78 MHz with 11-V input voltage, 16-V output voltage, and 25-W maximum output power. ZVTO is maintained with 9% output voltage increase over a 10:1 output power range.

Hetero-Magnetic Coupled Inductor (HMCI) for High Frequency Interleaved Multiphase DC/DC Converters

Abstract: A coupled inductor made of different magnetic materials was designed and fabricated for high frequency interleaved multiphase DC/DC converters. The coupled inductor, herein termed hetero-magnetic coupled inductor or HMCI, has a sandwich structure with two ferrite plates and metal windings in between. The gap region between the ferrite plates is filled with a low temperature curable magnetic paste to increase inductance density and reduce fringing effect. The winding structure combined with a custom-formulated magnetic paste provides flexible negative coupling between the different phases. Compared to the a commercial two-phase coupled inductor, FEA simulation of our HMCI shows reductions in footprint by 50% and AC resistance by 36% while having a similar transient inductance. Experimental results on the fabricated inductor were in agreement with the simulated results.

Design Methodology of a One-Turn Inductor With Significant DC and AC Fluxes—Demonstration in a Resonant Cross-Commutated Buck Converter

Abstract: One-turn inductors must cope with high dc flux in conventional hard-switched point-of-load converters. These inductors need to handle both high dc and ac fluxes in soft-switched converters with high current ripple, for example, resonant cross-commutated buck (rccBuck) converters. In order to understand the losses associated with those fluxes, the winding loss in megahertz range is modeled in this paper through calculating the fringing field in the window area. The dc-biased core loss for resonant waveforms is modeled by the equivalent elliptical loop (EEL) method. According to the loss models, winding and core dimensions are designed systemically to minimize the total loss within a volume smaller than that of a commercial product. Four 1-turn inductors were fabricated for a 12 to 1.2 V rccBuck converter switched at 2 and 2.5 MHz. The designed one-turn inductors demonstrated 2.1% higher efficiency at 20 A output and 50% lower total magnetic volume than commercial inductors with similar inductances, on the same rccBuck converter.

A (Permalloy + NiZn Ferrite) Moldable Magnetic Composite for Heterogeneous Integration of Power Electronics

Abstract: Soft magnetic moldable composites (SM2Cs) would be ideally suited for the integration of magnetic components in power electronic converters because they can be formed into magnetic cores by low-temperature and pressure-less processing. However, most SM2Cs have low relative magnetic permeability, typically less than 30, and high core-loss densities at switching frequencies over 1 MHz. To improve their magnetic properties, we combine powders of Permalloy and a NiZn ferrite with an acrylic polymer to formulate a paste of SM2C. The paste can be molded and then cured below 200 °C without pressure to form cores with a relative permeability over 35 and a core-loss density at 1 MHz, 30% lower than those of commercial cores. The ease of its processing and high-performance properties makes the SM2C a good candidate material for the integration of power magnetics.

A Comparison of DC and AC Output Inductors in Tunable Piezoelectric Transformer Based DC/DC Converters

Abstract: Tunable piezoelectric transformer (TPT) is a recently developed piezoelectric transformer with three ports: input, output and control ports. It can be treated as the integration of an LCC resonant tank and a transformer, where the series resonant capacitance can be tuned by the control port. There are two methods to design the output LC filter in TPT based converters. The traditional method is to place the output inductor after the rectifier. Such inductor is denoted by DC output inductor since it has a non-zero average current. The proposed method in this paper is to place output inductor before the rectifier. Such output inductor operates with zero average current and is denoted by AC output inductor. The regulation limitation caused by the DC output inductor is analyzed. This can be improved by the new resonance characteristic induced by the AC output inductor. The performance of TPT based converters with these two types of output inductors are compared. Experimental results show that the TPT based DC/DC converter with AC output inductor requires smaller output inductance and control capacitance than the one with DC output inductor. Also, the one with AC output inductor has better regulation and light load efficiency but worse full load efficiency.