B. Lehman

Northeastern University, Boston, MA, United States

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Publications (109)138.85 Total impact

  • Chung-Ti Hsu, Brad Lehman, Ting Qian
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    ABSTRACT: This paper proposes a new approach to efficiently track the desired maximum power point for photovoltaic systems under partial shading conditions. The power stage architecture achieves fast input current change rate by combining a current-adjustable converter with a few converters operating at a constant input current. By taking advantage of fast current change, the control scheme effectively combines perturb-and-observe (P&O) tracking and periodic global maximum power point (MPP) searching, and fulfills fast tracking as well as maintaining ease of implementation.
    2014 IEEE Applied Power Electronics Conference and Exposition - APEC 2014; 03/2014
  • Su Sheng, Brad Lehman
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    ABSTRACT: This paper proposes a method to parallel m digital controlled cascaded H-bridge (CHB) multilevel inverters directly with high bandwidth current sharing capabilities. The proposed method utilizes switch signal synchronization with dead time control to eliminate circulating current and adopts “piecewise” master-slave strategy to achieve equal current distribution in less than one cycle. A prototype is designed, built, and tested to verify the proposed method.
    2014 IEEE Applied Power Electronics Conference and Exposition - APEC 2014; 03/2014
  • Su Sheng, Peng Li, B. Lehman
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    ABSTRACT: This paper proposes a method to parallel N digital controlled low cost modified sine wave inverters with current sharing capabilities. Existing current sharing methods used to parallel pure sine wave inverters and DC-DC converters cannot be applied directly to modified sine wave inverters because of possible circulating currents. The proposed method utilizes switch signal synchronization with dead time control to eliminate circulating current, implements digital compensator with on/off control to improve the dynamic response of the first stage DC-DC converter, and adopts a “piecewise” master-slave current sharing strategies to achieve equal current distribution. A prototype is designed, built, and tested.
    Energy Conversion Congress and Exposition (ECCE), 2013 IEEE; 01/2013
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    ABSTRACT: Inductive magnetic harvesting technologies have been developing rapidly and have great potential in applications that rotate or vibrate. However, magnetic energy harvesters are often limited by their low voltage outputs, and normally, their placement are in applications that work at low frequency (typically around 100Hz). The traditional rectification with a diode bridge becomes inefficient with the resulting low harvester output voltage due to the diode forward voltage drop. This paper presents a new energy harvesting circuit, which can operate with magnetic harvesters limited by low output voltage. By adding one resonant capacitor and one bi-directional switch, the rectifier input voltage can be increased dramatically with the high voltage generated by resonance between the resonant capacitor and harvester leakage inductance. The simulation and experimental results verify the theoretical predictions.
    Energy Conversion Congress and Exposition (ECCE), 2013 IEEE; 01/2013
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    ABSTRACT: This paper presents a new system architecture for a low cost photovoltaic (PV) battery charging station that can balance: 1) the charging time of each individual battery and 2) the total charging time of all batteries in the system. The control strategy for the new system first charges each individual battery to either the same voltage or same state of charge (SOC) level and then charges multiple batteries in parallel simultaneously. As the paper demonstrates, this method can increase solar energy exploitation so that the total charging time of all batteries is decreased. Experimental results of a prototype of the system validate the effectiveness of the proposed control strategy.
    Control and Modeling for Power Electronics (COMPEL), 2013 IEEE 14th Workshop on; 01/2013
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    ABSTRACT: For most linear loads, finding the maximum power point by using the traditional perturb-and-observe algorithm to track the maximum output current of the DC-DC converter in a PV system is a proven technique. However, when the load is nonlinear and exhibits negative impedance, traditional methods might no longer be applicable. This paper presents a method to overcome this challenge. The proposed maximum power point tracking algorithm still only requires typically measured signals, yet is suitable for both linear and periodic nonlinear loads.
    Energy Conversion Congress and Exposition (ECCE), 2013 IEEE; 01/2013
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    ABSTRACT: Methods to estimate the DC wire loss in photovoltaic (PV) systems are proposed. Root-mean-square (RMS) of the irradiance data is first related to wire losses. Then a statistical model is used to characterize the irradiance data for the PV systems. The β-distribution gives reasonable match of the RMS value for the irradiance data. The result is consistent for different geographic locations.
    Telecommunications Energy Conference (INTELEC), 2012 IEEE 34th International; 01/2012
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    ABSTRACT: This paper describes an algorithm for managing a system consisting of “dissimilar” battery packs in parallel without multiple DC/DC converters interfacing each battery pack to the load. The power loss from the circulating current can be avoided so that the operating time of the parallel battery system is prolonged. The proposed algorithm is able to recognize the load demand and determine the number and sequence of the battery packs needed to be connected to the DC bus.
    Telecommunications Energy Conference (INTELEC), 2012 IEEE 34th International; 01/2012
  • F. Boico, B. Lehman
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    ABSTRACT: A method to track the maximum power of multiple-input, portable, photovoltaic systems is proposed. The method shares a single current sensor by interleaving Perturb and Observe operations. The system has reduced size and cost, making it attractive for compact portable solar panels and solar battery chargers, such as for cell phones, laptops, and other portable electronics with rechargeable batteries. A flexible total system architecture design is proposed and tested.
    Solar Energy - SOLAR ENERG. 01/2012;
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    Song Chen, Peng Li, David Brady, Brad Lehman
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    ABSTRACT: This paper proposes a general method of sizing the inverter for a PV system. The method evaluates effects of PV incentive policies, inverter efficiency curves, and inverter protection schemes on optimum inverter sizing through system-level cost analysis. Specifically, different scenarios of PV incentives are discussed and compared to show that the optimal inverter size varies notably by location and context.
    01/2011;
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    T. Qian, B. Lehman
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    ABSTRACT: This study presents a concept of connecting two-stage DC-DC converters in an input-series connection. An application example is discussed in detail where the first stage utilises two series connected buck converters that have reduced voltage stress. A single second stage is a half-bridge converter and is able to regulate the charge balance of the first stage. The benefits of the topology include: reduced primary switch voltage stress, simple self-driven synchronous rectification for wide input voltage range, self-voltage balancing on intermediate bus capacitors and simple housekeeping power supply. Further, the topology exhibits an unusual ripple match concept that can be utilised to suppress the current ripple of the second stage. Based on the detailed analysis, prototypes with 500-700-V input and 5-V/30-A output are built. Experimental results verify the principle and performance of the new topology.
    IET Power Electronics 12/2010; · 1.52 Impact Factor
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    Song Chen, Peng Li, D. Brady, B. Lehman
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    ABSTRACT: This paper investigates the time behavior of over-irradiance events in which the photovoltaic (PV) array outputs more power than the rated power of the inverter. A new dynamic interpretation of such events is proposed and is compared to the conventional static viewpoint. Facts revealed under such dynamic view may lead to new guidelines for system integrators and inverter designers in both sizing and designing inverters. A way to extend overload operation time of inverters is also proposed.
    Control and Modeling for Power Electronics (COMPEL), 2010 IEEE 12th Workshop on; 07/2010
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    R. Ciprian, B. Lehman
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    ABSTRACT: Variations in the behavior of power supplies caused by environmental conditions require accurate characterization of the electrical behavior dependence with environmental conditions. This paper introduces models to help predict RH and other environmental factors influence on sensitive circuitry in power electronic systems. An application example of a high power density, high voltage DC-DC converter is used to verify the results.
    Energy Conversion Congress and Exposition, 2009. ECCE 2009. IEEE; 10/2009
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    D.D. Nguyen, B. Lehman, S. Kamarthi
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    ABSTRACT: This paper proposes a neural network based approach to estimating the maximum possible output power of a solar photovoltaic array under the non-uniform shadow conditions at a given geographic location. Taking the solar irradiation levels, the ambient temperature, and the sun's position angles as inputs, a multilayer feed-forward neural network estimates the output power of the solar photovoltaic array. Training data for the neural network is generated by conducting a series of experiments on a shaded solar panel at different hours of a day for several days. After training the neural network, its accuracy and generalization properties are verified on test data. It is found that the neural network, which is an approximation of the actual shading function, is able to estimate the maximum possible output power of the solar PV arrays accurately. Further, the network is able to estimate the maximum output power for field data and gives rise to the possibility that the proposed approach can be used for making decision regarding the installation of solar PV arrays in the field.
    Energy Conversion Congress and Exposition, 2009. ECCE 2009. IEEE; 10/2009
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    Dzung Nguyen, B. Lehman
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    ABSTRACT: This paper proposes an adaptive reconfiguration scheme to reduce the effect of shadows on solar panels. A switching matrix connects a solar adaptive bank to a fixed part of a solar photovoltaic (PV) array, according to a model-based control algorithm that increases the power output of the solar PV array. Control algorithms are implemented in real time. An experimental reconfiguration PV system with a resistive load is presented and is shown to verify the proposed reconfigurations.
    IEEE Transactions on Industrial Electronics 08/2008; · 6.50 Impact Factor
  • Dzung Nguyen, B. Lehman
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    ABSTRACT: This paper proposes an adaptive reconfiguration scheme to reduce the effect of shadows on solar panels. A switching matrix connects a solar adaptive bank to a fixed part of a solar PV array, according to a model based control algorithm that increases the power output of the solar PV array. Control algorithms are implemented in real-time. An experimental reconfiguration PV system with the battery load is presented and is shown to verify the proposed reconfigurations.
    Applied Power Electronics Conference and Exposition, 2008. APEC 2008. Twenty-Third Annual IEEE; 03/2008
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    ABSTRACT: One way to create white light is with a RGB color mixing system, in which light from multiple monochromatic LEDs (red, green and blue) is mixed. Unfortunately, there are several variations/uncertainties between individual LEDs that can create unwanted problems, such as variations in: forward voltage drop, peak wavelength, spectral radiant intensity, and the width of the spectral power distributions (SPD) of the LEDs. This paper will explain, through experiments, behavioral models and probability theory, how the variations affect the design of LED based lighting system. We especially describe the effects such as the influence of SPD variation and Forward Voltage drop variation.
    Applied Power Electronics Conference and Exposition, 2008. APEC 2008. Twenty-Third Annual IEEE; 03/2008
  • Ting Qian, B. Lehman
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    ABSTRACT: This paper proposes an integrated magnetic dc-dc converter suitable for high input voltage application. The converter is based on a coupled input-series and output-parallel dual interleaved Flyback converter concept. All the center and outer legs are gapped, and the transformers are integrated into one magnetic core with not so tight coupling. The gap is beneficial for suppressing current spike caused by the voltage mismatch between the windings. The two transformers are inversely coupled, and current ripple reduction can be achieved with suitable coupling design. A prototype with 350-450-V input and 24-V/4-A output is built. Experimental results verify the performance of the new topology.
    IEEE Transactions on Power Electronics 02/2008; · 5.73 Impact Factor
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    Ting Qian, B. Lehman
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    ABSTRACT: This paper proposes a new dual interleaved active-clamp forward topology suitable for high input voltage and high-density application. Two interleaved forward circuits are connected in series on the primary side and in parallel on the secondary side. Therefore, only two high-voltage MOSFETs are needed for the primary switches. Also, the two interleaved parts utilize auxiliary windings to share one capacitor to fulfill active-clamp. Charge balance of the two channels is, thus, regulated by the shared clamping capacitor without demand for extra control schemes. Through the resonance between the magnetizing inductors and the equivalent capacitors of the switches during the transition time, switching loss of the primary switches can be reduced. Interleaved operation reduces the size of the output filter inductor. Based on detailed analysis, a prototype with 220-400-V input and 5-V/30-A output is built. Experimental results verify the performance of the new topology.
    IEEE Transactions on Power Electronics 02/2008; · 5.73 Impact Factor
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    Ting Qian, Wei Song, B. Lehman
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    ABSTRACT: This paper proposes a new self-driven synchronous rectification scheme for dc-dc converters with symmetrically driven transformers and wide input voltage range. The driving signals are obtained from the output inductor. Two auxiliary switches are utilized to turn off the main switches properly. Two extra auxiliary switches, which are controlled by a delayed signal from the inductor, are added to avoid the undesired gate discharge. This approach can achieve high efficiency and is suitable for practical applications. Experimental results based on a 36-75 V input, 2.5 V/30 A output prototype are shown to verify the proposed scheme.
    IEEE Transactions on Power Electronics 02/2008; · 5.73 Impact Factor

Publication Stats

827 Citations
138.85 Total Impact Points

Institutions

  • 1996–2012
    • Northeastern University
      • Department of Electrical and Computer Engineering
      Boston, MA, United States
  • 2009
    • Diversified Technologies, Inc
      Bedford, Massachusetts, United States
  • 1996–2007
    • Boston University
      • Department of Electrical and Computer Engineering
      Boston, MA, United States
  • 2000
    • Rockwell Collins
      Cedar Rapids, Iowa, United States
  • 1997
    • Clark Atlanta University
      • Department of Engineering
      Atlanta, GA, United States
  • 1994–1996
    • Mississippi State University
      • Department of Electrical and Computer Engineering
      Starkville, MS, United States
  • 1992–1994
    • Georgia Institute of Technology
      • School of Electrical & Computer Engineering
      Atlanta, GA, United States
  • 1989
    • University of Illinois, Urbana-Champaign
      Urbana, Illinois, United States