Showing papers by "Khai D. T. Ngo published in 2018"

Systematic Design of Coils in Series–Series Inductive Power Transfer for Power Transferability and Efficiency

Abstract: The quality factor of coils should be high enough to deliver power efficiently during inductive power transfer. To achieve a higher quality factor, coils are designed either with a larger size and thicker wire or with a special structure, making the coils bulky and complicating the fabrication. This paper presents a systematic method to design the coils in series–series inductive power transfer with the smallest possible value of quality factor to realize the requirements of power transferability and efficiency. The electrical parameters of the coils are normalized, decoupled in equations, and designed sequentially. It is found that the smallest value of the quality factor for required efficiency is achieved using an optimal normalized impedance. The physical parameters of the coils are selected sequentially to realize the electrical parameters. This paper presents two operating conditions of series–series inductive power transfer, and example sets of coils with a 100-mm air gap are designed for these operating conditions using the same procedure. The coils were fabricated and tested in a 3.3-kW converter. The measured output voltage (200–400 V for 400 V input) and the coils' efficiency (98.9% at peak power) met the specifications. All switches turned on with zero voltage.

A Fast Method to Optimize Efficiency and Stray Magnetic Field for Inductive-Power-Transfer Coils Using Lumped-Loops Model

Abstract: Both the efficiency and stray magnetic field in inductive power transfer are influenced by the design of transmitter and receiver coils Their synergetic optimization is realized with Pareto front The conventional method to derive the front requires thousands of finite-element simulations to sweep the physical parameters of the coils, which is time consuming especially for three-dimensional simulations This paper demonstrates a fast method to optimize the efficiency and stray magnetic field The windings are replaced by several lumped loops As long as the number of turns for each loop is known, the efficiency and magnetic field are calculated using permeance matrices and current-to-field matrices Therefore, sweeping physical parameters in simulation is replaced by sweeping turns numbers of the lumped loops in calculation Only tens of simulations are required during the entire procedure, which are used to derive the matrices The Pareto fronts calculated using the lumped-loops model match well with those derived from simulation using parametric sweep An optimal design selected along the Pareto front was fabricated and measured to verify the calculation accuracy The verification shows the same efficiency and less than 125% difference of the stray magnetic field, for the results from calculation, simulation, and measurement

Design and Additive Manufacturing of Multipermeability Magnetic Cores

Abstract: Uneven distribution of magnetic flux in a conventional magnetic core limits the improvement of power density of the magnetic component. By making a magnetic core comprised of materials with different permeabilities, henceforth called multipermeability core, a more uniform flux distribution can be achieved without complex configuration and geometry of the core and winding. In this paper, we designed a three-ring toroid core to produce a flux uniformity factor, α , defined as B min/ B max, in each of the three rings by increasing permeability from the inner to outer ring. With the same inductance, volume of the three-permeability toroid core was smaller than that of a single-permeability toroid core. For ease of fabricating multipermeability magnetic cores, a commercial multiextruder paste-extrusion 3-D printer was explored to process different magnetic pastes into 3-D structures. The magnetic pastes were formulated in our laboratory. Two types of magnetic paste systems, a high-temperature (>900 °C) pressure-less sinterable ferrite and a low-temperature (<250 °C) pressure-less curable powdered-iron, were developed and tested in the 3-D printer. By varying the magnetic/organic composition in each of the two material systems, we produced pastes that are compatible with the 3-D printer and can be processed into core materials with relative permeability ranging from 20 to 70. With the same dimensions, a 3-D-printed three-permeability toroid core had higher inductance than a single-permeability core.

Colossal tunability in high frequency magnetoelectric voltage tunable inductors

Abstract: The electrical modulation of magnetization through the magnetoelectric effect provides a great opportunity for developing a new generation of tunable electrical components. Magnetoelectric voltage tunable inductors (VTIs) are designed to maximize the electric field control of permeability. In order to meet the need for power electronics, VTIs operating at high frequency with large tunability and low loss are required. Here we demonstrate magnetoelectric VTIs that exhibit remarkable high inductance tunability of over 750% up to 10 MHz, completely covering the frequency range of state-of-the-art power electronics. This breakthrough is achieved based on a concept of magnetocrystalline anisotropy (MCA) cancellation, predicted in a solid solution of nickel ferrite and cobalt ferrite through first-principles calculations. Phase field model simulations are employed to observe the domain-level strain-mediated coupling between magnetization and polarization. The model reveals small MCA facilitates the magnetic domain rotation, resulting in larger permeability sensitivity and inductance tunability. Voltage tunable inductors (VTIs) with high working frequency and tunability are desired for new design of energy efficient electronics. Here the authors demonstrate magnetoelectric VTIs operating up to 10 MHz with tunability over 750% via strain mediated magnetoelectric effect and magnetocrystalline anisotropy cancellation strategy.

Ferrite Paste Cured With Ultraviolet Light for Additive Manufacturing of Magnetic Components for Power Electronics

Abstract: Additive manufacturing (AM) has the potential to rapidly prototype innovative designs of magnetic components aimed at improving power density and efficiency of switch-mode power converters. However, only a few magnetic feedstocks are available for AM, and the fabricated parts have significantly poorer magnetic properties. Our previous studies showed the promise of an extrusion-based AM platform for making high-performance power magnetics. But, the magnetic paste feedstock developed in these studies suffered from the “slumping” problem, thus not suitable for building large or tall core structures. To improve the AM capability, a ferrite paste was developed that is cured upon ultraviolet (UV) illumination. Core structures with large aspect ratios were fabricated by retrofitting the printer with a UV-light-emitting diode module. By varying the magnetic composition of the paste and sintering the printed parts at and below 1000 °C, we made tall (>4 mm) ferrite cores with relative permeability ranging from 63 to 103 and resonance frequency higher than 30 MHz.

Impact of Accelerated Stress-Tests on SiC MOSFET Precursor Parameters

Abstract: Integrating SiC power MOSFETs is very attractive for advancing power electronic system performance, yet the system reliability with new devices remains in question. This work presents an overview of accelerated lifetime tests and the packaging and semiconductor failure mechanisms they excite. The experiments explained here includes High Temperature Gate Bias (HTGB), Switching Cycling, Power Cycling, and Thermal Cycling. These experiments stress different failure mechanisms, that show degradation in different device parameters including, but not limited to, threshold voltage and on-resistance. These four experiments help illustrate the spectrum between device and package degradation that can be used to design more reliable power electronic circuits.

Magnetic paste as feedstock for additive manufacturing of power magnetics

Abstract: Inductors and transformers are ubiquitous in switch-mode power converters. Additive manufacturing or 3D printing of these components has the potential to drastically accelerate their design and prototyping. However, there are very few reported activities claiming successful fabrication of power magnetics by 3D printing. One of the main reasons is the lack of suitable feedstock materials for printing platforms. In this effort, we developed two types of magnetic paste material as the feedstock for a commercial paste-extrusion 3D printer: (1) a low-temperature ( 900°) pressure-less sinterable NiZn ferrite paste. Of each type, the magnetic properties (relative permeability and core-loss density) after heat treatment were found to be comparable or better than its corresponding commercial products, which often require pressure for fabrication. The powder-iron material had a relative permeability of 35 and a core-loss density of 110 mW/cm3 at Bpeak of 10 mT and 1 MHz. The core-loss density was 33% lower than that of a commercial powder-iron core from Micrometals with the same relative permeability. The ferrite material had a relative permeability of 72 and a core-loss density of 200 mW/cm3 at Bpeak of 10 mT and 5 MHz. The loss density is almost 50% lower than that of a commercial 4F1 core with a relative permeability of 80. With these feedstock materials, one can start taking full advantage of the flexibility of the 3D printing platform to design and prototype high-performance, unique-shaped magnetic cores.

Steady-State Analysis of Resonant Cross-Commutated Buck Converter Under Continuous Voltage Mode

Abstract: Resonant current generated in one phase of two interleaved buck converters is injected into the switched node of the other phase to turn on the active switch at zero voltage over wide load range, and to turn off the synchronous switch at near-zero current at rated load This idea was introduced in a recent letter and applied to an interleaved two-phase buck converter with output voltage regulated by extended duty ratio This paper focuses on modeling its steady state when capacitor voltages are continuous Closed-form formulas of inductor current and capacitor voltage were derived during each switched stage The complex fourth-order circuit mode was solved by decomposing it to two second-order modes The current to discharge the parasitic capacitor of mosfet was calculated by using state-plane trajectory The method of state-space representation was employed to calculate inductor RMS current, mosfet turn- off current, and voltage stresses The expectations were validated by three resonant cross-commutated (rcc) converters switched at 2 MHz with 12-V input, 33 V at 20-A output, and peak efficiencies of 936%, 936%, and 933% The discrepancy between measured and modeled instantaneous state variables was less than 2%

Trends in SiC MOSFET Threshold Voltage and ON-Resistance Measurements from Thermal Cycling and Electrical Switching Stresses

Abstract: Integrating SiC power MOSFETs is very attractive for advancing power electronic systems, yet the system reliability with new devices remains in question. This work presents two accelerated test experiments to further investigate the packaging and semiconductor failures of a TO-247 SiC MOSFET. First, a variation on power cycling experiments - switching cycling - is introduced. Traditional power cycling experiments utilize conduction losses to self-heat the device where a large temperature swing causes degradation at the packaging level. However, by decreasing the on-time such that the device only turns on and immediately turns off, the temperature swing is decreased and the semiconductor itself is degraded. This work shows experimental device degradation caused by continuous switching events - switching cycling, at 90% of the device's breakdown voltage. Second, thermal cycling experiments were conducted between -40 °C and 175 °C to observe degradation in the mechanical packaging of the device. Experimentally-measured changes in threshold voltage and ON-resistance are recorded in both experiments and compared. These results also illustrate the spectrum between device and package degradation from accelerated test methods.

Design, Fabrication, and Characterization of a Low-Temperature Curable Magnetic Composite for Power Electronics Integration

Abstract: To simplify the integration process of embedding magnetic components in power electronics converters, we fabricated a magnetic-filled-benzocyclobutene composite that can be cured at temperatures below 250 °C without pressure. The magnetic fillers used in the formulation were a round-shaped particle of permalloy and a flake-shaped particle of Metglas 2705 M. To guide the formulation, we first constructed 3-D finite-element models of the composite consisting of periodic unit cells of magnetic particles and flakes in the polymer matrix and used Ansoft Maxwell to simulate magnetic properties of the composite. Then, flowable pastes of the composite with varying amounts of Metglas in the magnetic fillers up to 12.5 wt% were prepared, and toroid cores were poured and cured at 250 °C. Subsequently, magnetic properties of the cores, i.e., complex permeability and core loss density, were measured. We found that the real part of the composite’s relative permeability increased with Metglas addition, reaching a value of 26 at 12.5%. However, the core loss data at 1 and 5 MHz showed that the addition of Metglas flakes also increased the core loss density. The measured properties were consistent with the Maxwell simulation results. Microstructures of the cores were examined by scanning electron microscopy. We found that the magnetic particles were uniformly dispersed and isolated by the polymer, which explained the high electrical resistivities of the composite and relatively low core loss densities due to suppression of inter-particle eddy-current losses.

Design and Control of Tunable Piezoelectric Transformer Based DC/DC Converter

Abstract: Piezoelectric transformer is an alternative for magnetic transformers in power converters. Specifically, tunable piezoelectric transformer (TPT) is a recently developed device that has input, output, and control ports. When connected with different impedance at the control port, TPT has different voltage gain characteristics. In this paper, it is proposed to use this property for output voltage regulation while keeping constant switching frequency. To illustrate this concept, this paper introduces the working principle and characteristics of the TPT. A closed-loop control scheme is proposed, where the regulation is done by a duty cycle controlled switched capacitor at the TPT control port. Design of the converter implementing the proposed control scheme is presented. A converter with a 100 W TPT is constructed and tested, which shows the state-of-the-art performance and limitations of TPT based converters.

Reliability Assessment of 3D-Printed Pot-Core Constant-Flux Inductors

Abstract: Constant-flux inductors (CFIs) are designed to have more uniform flux distribution in the core for efficient use of magnetic material and higher inductance density. However, they require elaborate winding structures which pose challenges to fabrication by conventional fabrication techniques. Recently, we applied a paste-extrusion 3D printer to fabricate spiral windings with different turn-to-turn widths for a pot-core CFI. The printed windings were sintered at 850°C and then potted in a low-temperature curable magnetic composite to form inductors measured at $5 \text{mm x}\ 5 \text{mm x}\ 2\ \text{mm}$. In this work, reliability of the pot-core CFIs was evaluated by temperature cycling between −40°C and 125°C and storage testing at 125°C. After 1000 temperature cycles and 1000 hours at the storage temperature, the DC winding resistances of the pot-core CFIs had no significant change while the inductances dropped only 4.8% and 2%, respectively.

Static and Dynamic Characterization of a 2.5 kV SiC MOSFET

Abstract: In this work, a 2.5 kV discrete SiC MOSFET manufactured by GE with a 58 A current rating and a 40 m Ω on-resistance is characterized, both statically and dynamically. The high current capabilities within a discrete package at 2.5 kV MOSFET offers improved performance differentiation when compared to both traditional 1.7 kV discrete devices and 3.3 kV power modules. This device is characterized statically, dynamically and under increased temperature conditions up to 200°C. A clamped, inductive test circuit is used to conduct the double pulse tests necessary to determine the switching energy losses. These measurements are compared to a commercially available SiC MOSFET that is similarly rated.

Power Switch Drivers with Equalizers for Paralleled Switches

Abstract: Capacitors connected between gate terminals of a plurality of parallel-connected power transistors are charged and discharged in each switching cycle to provide a plurality of power transistor control waveforms from a single gate driver waveform that equalize power losses/temperatures or steady-state currents among the plurality of power transistors. The capacitors are charged to different voltages by diverting current from one transistor driver by disabling another power transistor driver at different respective times in response to measured transient or steady state current or temperature or other operational parameter.

Hetero-Magnetic Swinging Inductor (HMSI) and Its Application for Power Factor Correction Converters

Abstract: A variable inductor concept based on a magnetic structure consisting of a composite of magnetic materials is introduced to meet designed inductance function of current. A design of the inductor, termed hetero-magnetic swinging inductor (HMSI), was fabricated using a toroid core with a gap that was filled by a custom-developed low-permeability magnetic powder + polymer material. Having the gap filled with the magnetic material allowed for greater flexibility in L-I design with added benefit of low fringe field. We applied this swinging inductor to a Critical Conduction Mode (CRM) Boost power factor correction (PFC) converter to reduce its switching frequency variation range. First, the required L-I curve of the HMSI needed for narrow range of switching frequency was calculated. Then, the HMSI core structure consisting of two magnetic materials was designed. Circuit simulations showed that for a 110-Vrms input, the converter with the HMSI had a switching frequency variation factor, defined as $\Delta\text{f}/\text{fmin}$ , six times less than that of the converter using a linear inductor. A prototype of the HMSI was fabricated and tested. The effectiveness of using the HMSI to narrow the range of switching frequency variation was verified in a 150 W CRM PFC converter.

A Voltage-Controlled Capacitor with Wide Capacitance Range

Abstract: A Voltage-Controlled Capacitor (VCC) consisting of a voltage dependent capacitor and a series capacitor aiming at wide capacitance range within its voltage rating is presented. The operational principle, structure, and component selection are illustrated based on the requirements of working frequency, required maximum and minimum capacitance, and rated voltages. One VCC was fabricated and tested at 250 kHz and its capacitance is varied from 34 nF to 163 nF under the lowest dc capacitor voltage and from 32 nF to 150 nF under the highest dc capacitor voltage.

Cooler with emi-limiting inductor

Abstract: A power device package includes a dielectric substrate having an upper conductor layer and a lower conductor layer, a semiconductor die coupled to the upper conductor layer of the dielectric substrate via conductive adhesive, a cooler including a protruding hillock having a top surface and outer sides, the lower conductor layer of the dielectric substrate being coupled to the surface of the protruding hillock via an adhesive, and a magnetic material attached mateably around the protruding hillock. The magnetic material includes inner sides abutting the outer sides of the protruding hillock.

Magnetic Integration into SiC Power Module for Current Balancing

Abstract: High-current power module contains dice in parallel. Threshold-voltage mismatch among paralleled dice 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. Layout to achieve inverse coupling, high coupling coefficient, and low voltage stress, as well as magnetic materials suitable for operation at tens of MHz, and high current rating of tens of Amperes with small magnetic core are the challenges 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 (SiC) 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 co-fired ceramics (LTCC) 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 following commercial practice, 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 increase of voltage stress.

Effect of Surface Roughening of Temperature-Cycled Ceramic-Metal-Bonded Substrates on Thermomechanical Reliability of Sintered-Silver Joints

Abstract: We studied the reliability of power chip die-attach by sintered-silver on two types of ceramic-metal-bonded substrates: 1) aluminum-nitride direct-bond-aluminum (DBA) and 2) active-metal-brazed (AMB) silicon nitride-copper. The samples underwent a temperature-cycling test from −55 °C to 250 °C. Consistent with what we reported earlier, the surface roughness on both substrates grew with number of temperature cycles. The roughening rate on the DBA substrate was two times faster than that on the AMB substrate. To evaluate the effect of surface roughening on the sintered-silver joint reliability, we measured the transient thermal impedance of the bonded power chips. Despite the higher roughening rate, the sintered-silver bond on the DBA substrate had a longer lifetime—defined by a 20% increase in the transient thermal impedance of the bonded device—than that on the AMB substrate. Cross-sectional scanning electron microscopy showed the formation of vertical cracks in the sintered bond-line on the DBA substrate, as opposed to horizontal cracks on the AMB substrate. Formation of the vertical cracks in the bond-line relieved stresses without significantly impacting the thermal performance.

Resonant Cross-Commutated Point-of-Load Converter

Abstract: Resonant current generated in one phase of two interleaved buck converters is injected into the switched node of the other phase to turn on the active switch at zero voltage within a wide load range, and to turn off the synchronous switch at near-zero current at rated load. This idea is applied in a point-of-load converter with 12 V input and 1.2 V at 20 A output, and compared to a two-phase synchronous buck converter with the same specifications. They employ same GaN switches and filter capacitors, and are designed to have same magnetic volume, so that the overall volumes are comparable. The resonant cross-commutated buck (rccBuck) converter achieved 1.1% higher efficiency at rated load by switching at 2 MHz, and 0.8% higher efficiency at medium load by switching at 2.5 MHz. All switches have low noise and clean waveforms even with zero gate resistances.

Novel Actively Tuned Resonant Filter Based Buck Converter Using Tunable Capacitor

Abstract: In power electronic converters, resonant filters can be significantly smaller compared to the conventional low-pass filters. Such improvement is achieved due to the high attenuation offered by the resonant filter at the Pulse Width Modulation (PWM) frequency or switching frequency of the power converter. However in traditional resonant filters, due to the variation of filter component values under dynamic operating conditions, the high attenuation at the switching frequency cannot be maintained. In prior research, Tuned Resonant Filter (TRF) based on tunable capacitor has been introduced to address this problem. Tunable capacitor is intended to compensate the variations of the filter components. In this work, a novel control scheme for resonant filters with tunable capacitor is proposed that can automatically tune the capacitor value to achieve the desired filter performances under varying operating conditions. The proposed control scheme is realized through combination of tank voltage sensing, feedback, PI controller and driver circuits that can regulate capacitor bias voltage such that the effective impedance of the resonant filter is tuned to achieve the intended filtering operation. Analysis, design, and implementation of the proposed control scheme in a synchronous buck converter are presented to demonstrate an efficient circuit implementation. Experimental results are presented to validate the hypothesis that the proposed controller can successfully provide automatic tuning of the resonant filter under changing operating conditions and maintain the desired filter performance.