Three-level Dc Research Articles

Capacitor-Clamped, Three-level GaN-Based DC–DC Converter With Dual Voltage Outputs for Battery Charger Applications

Gallium nitride (GaN) heterojunction field-effect transistors are an enabling technology for high-density converter design. This paper proposes a three-level dc–dc converter with dual outputs based on enhancement-mode GaN devices, intended for use as a battery charger in aircraft applications. The charger can output either 28 or 270 V, selected with a jumper, to satisfy the two most common dc bus voltage requirements in airplanes. It operates as an LLC converter in the 28 V mode and as a buck converter in the 270 V mode. In both operation modes, the devices can realize zero voltage switching (ZVS). With the chosen modulation method, the converter can realize automatic voltage balancing of the flying capacitor and the frequency doubling function to act as an interleaved converter. For the LLC mode, the resonant frequency is twice the switching frequency of primary-side switches, and for the buck mode, the frequency of the output inductor current is also twice the switching frequency. This helps to reduce the size of magnetics while maintaining a low switching loss. Also, the converter utilizes a matrix transformer, with resonant parameters designed to reduce conduction loss and avoid ZVS failure. The operating principle of the converter is analyzed and then experimentally verified on a 1.5-kW prototype with 1 MHz resonant frequency.

Improved ZVS Three-Level DC–DC Converter With Reduced Circulating Loss

An improved three-level (TL) dc–dc converter is proposed in this paper. The converter contains two transformers. Like the conventional TL dc–dc converter, there are no additional switches on the primary side of the transformer. The rectifier stage is composed of four diodes in the center-tapped rectification. On the primary side of the transformer, the two transformers are connected in series. The middle node of the two transformers is connected to the neutral point of the split flying capacitors. Because it cooperates with the four-diode rectifier stage, the circulating current on the primary side of the transformer decays to zero during the freewheeling period. The zero-voltage switching (ZVS) of the leading switches is determined by energy stored in the output filter inductor, which is similar to the conventional TL converter. The ZVS of the lagging switches is determined by the energy stored in the magnetizing inductor of a transformer, rather than the energy stored in the leakage inductor. The proposed converter can reduce the output filter inductance. Because of the advantages given above, the efficiency of the proposed converter is far better than that of traditional methods. Finally, a 1-kW prototype was built to verify the performance of the proposed converter.

Effective Voltage Balance Control for Bipolar-DC-Bus-Fed EV Charging Station With Three-Level DC–DC Fast Charger

The development of high-power charging stations with fast chargers is a promising solution to shorten the charging time for electric vehicles (EVs). The neutral-point-clamped (NPC) converter-based bipolar-dc-bus-fed charging station brings many merits, but it has inherent voltage balance limits. To solve this issue, a voltage balance control (VBC) method based on a new modulation together with three-level (TL) dc–dc converter-based fast charger is proposed. Additionally, an effective VBC coordination between the TL dc–dc converter and the NPC converter is formulated. Through the proposed VBC coordination, the controllable balancing region is extended so that additional balancing circuits are eliminated. Meanwhile, the quality of the grid-side currents is improved as the NPC converter has more freedom to control currents. The low-frequency voltage fluctuations in dc buses are removed because the TL dc–dc converter performs most of the balancing tasks. Faster VBC perturbation performance is achieved due to higher available balancing current at TL dc–dc converter side. In addition, the voltage balance limits of both the TL dc–dc converter and the NPC converter are explored. The voltage balancing performances are compared when VBC is located at different sides. Simulation and experimental results are provided to verify the proposed VBC and the VBC coordination.

Wide Load Range ZVS Three-Level DC–DC Converter: Four Primary Switches, Capacitor Clamped, Two Secondary Switches, and Smaller Output Filter Volume

A novel soft-switching PWM three-level (TL) dc–dc converter with two added secondary switches is proposed. The proposed converter has following good features: four primary switches sustain half of the input voltage; the primary circuit is simpler and more compact due to only one flying capacitor with smaller current stress is required to clamp off state voltage of the primary switches; the secondary rectified voltage before the output filter is a TL waveform, which significantly reduces the requirement of the output and input filters; all primary switches can obtain ZVS in wide load range. Furthermore, less conduction loss is added. The added secondary switches can obtain zero-current switching independent of the load current. The proposed converter can be extended to higher voltage level easily due to simpler and more compact primary structure. In addition, the presented converter is also well suitable for high-input dc interfaces with independent controllable multioutput ports. The operation principle, soft switching characteristics, and some important analyzes are presented in this paper. The proposed converter is verified by the experimental results from a 10-kW prototype.

A ZVS Bidirectional Three-Level DC–DC Converter With Direct Current Slew Rate Control of Leakage Inductance Current

A high-frequency isolated bidirectional three-level dc–dc converter is proposed for high voltage applications. Direct current slew rate (DCSR) control of leakage inductance is proposed to minimize conduction loss and current stress in facing the load variation, the mismatch of turns ratio and circuit parasitic parameters. The mode analysis and the disadvantages of conventional PWM plus phase-shift (PPS) control are addressed while these disadvantages can be dealt with by the proposed control. Comprehensive comparison between conventional PPS control and the proposed DCSR control are made within the designed low voltage side (LVS) voltage range. Besides, the implementation of the proposed DCSR control is also given. With the proposed DCSR control, lower conduction loss, lower peak current, lower voltage spike over switches can be obtained in spite of the turns-ratio mismatch, load variation, and system parasitic parameters. Zero voltage switching (ZVS) can be achieved for all power switches in spite of the power flow direction. The effectiveness of the proposed DCSR control on the proposed topology is verified by simulation and experimental results.

Function‐integrated network for flying‐capacitor three‐level DC–DC conversion system

Flying-capacitor (FC) three-level (TL) DC–DC topology is an attractive candidate for the switching-mode power supply due to its low-voltage stresses on semiconductors, small size of filter and common-ground feature. However, to avoid the semiconductors to be damaged from an initial high voltage, the FC must be precharged to half of the input voltage by a bulky precharger. An idea to realise the precharging function by adding only one switch and cooperating it with the existent circuits is proposed. In this way, the necessary soft-starting circuit and Resistor-Capacitor-Diode (RCD) snubbers for reliability purpose can be fully utilised to precharge the FC. Merits of the proposed idea include the lower cost, the more compact structure and the higher overall efficiency. The principles of the proposed function-integrated network are analysed, and a 1.2 kW FC-TL conversion system is also built to verify the analysis.

Power conversion system for high altitude wind power generation with medium voltage AC transmission

High Altitude Wind Power (HAWP) generating system provides clean energy at low cost and high capacity factor due to reduced size of the turbine and high speed streamlined wind at high altitude. An air-borne wind turbine (AWT) at high altitude extracts kinetic energy from wind using buoyancy provided by the blimp/aerostat. The generated electrical power is then transmitted to the ground based station (without any power conditioning) using the transmission lines (tether). The power conversion system (PCS) for harnessing HAWP is proposed in this paper. The proposed PCS consists of a three-level neutral point clamped (NPC) rectifier, a three-level NPC DC–DC converter followed by a two-level inverter. Modelling, design and control of the PCS are presented and discussed. The PCS provides generation side maximum power-point tracking (MPPT) using sensorless optimal torque control technique. The DC–DC converter provides electrical isolation as well as voltage step-down functions. A modified proportional resonant (PR) control which can selectively eliminate lower order current harmonics of the grid-connected inverter is also presented. The proposed control scheme of the PCS is evaluated through simulation studies using software programs like PSIM and MATLAB. A scaled-down 1 kW laboratory prototype of the complete PCS is designed, built and tested. The experimental test results obtained validate the proposed control scheme for efficient power generation from high altitude wind and interface to the grid/load.

An Integrated Inductor for Eliminating Circulating Current of Parallel Three-Level DC–DC Converter-Based EV Fast Charger

The parallel three-level dc–dc converter has some merits over the conventional two-level buck converter when employed for fast chargers in a bipolar-dc-bus charging station. It can be directly connected to the bipolar dc bus, and can operate with the total neutral-point current eliminated under the 180°-interleaved mode, which guarantees balanced power consumption and helps decrease dc-bus voltage fluctuation. However, this incurs circulating currents, leading to increased losses and additional current stress to switching devices. To solve this issue, an integrated inductor is proposed in this paper. The inductor has an integrated magnetic core with both output and circulating windings placed on it, producing inductances to filter output currents and to attenuate circulating currents respectively. The circulating inductance can be flexibly adjusted to be much higher than the output inductance, so that the circulating currents can be well attenuated. The output and circulating inductances, along with the ratio between them, are deduced in detail. The current ripples are analyzed and compared under two operation modes with separate and integrated inductors, respectively. The weight, losses, and volume have also been compared between separate and integrated inductors. Both simulation and experimental results are presented to verify the effectiveness of the proposed integrated inductor.

Comprehensive DC Power Balance Management in High-Power Three-Level DC–DC Converter for Electric Vehicle Fast Charging

With the increasing popularity of electric vehicles, there is an urgent demand to shorten the charging time, so the development of high-power charging stations with fast chargers is necessary to alleviate range anxiety for drivers. The charging station based on the neutral-point-clamped (NPC) converter can bring many merits, but it has unbalanced power problems in the bipolar dc bus. To solve this issue, comprehensive dc power balance management (PBM) in conjunction with high-power three-level dc–dc converter based fast charger is proposed in this paper. The active dc power balance management (APBM) is proposed to assist the central NPC converter in balancing power so that the additional balancing circuit is eliminated; while the passive dc power balance management (PPBM) is proposed to eliminate the fluctuating neutral-point currents and to ensure the balanced operation of fast chargers. The principles of APBM and PPBM are researched, the efficient integration between them is studied, and the overall control scheme for the fast charger is proposed. The power balance limits of APBM are explored, while the circulating currents of PPBM are analyzed. Simulation and experimental results are presented to verify the effectiveness of the proposed fast charger with PBM functions.

Coupled Inductors in Interleaved Multiphase Three-Level DC–DC Converter for High-Power Applications

This paper investigates and evaluates coupled inductors (CIs) in the interleaved multiphase three-level dc-dc converter. If non-CIs are used in the multiphase three-level dc-dc converter, interleaving operation of the converter will increase inductor current ripple, although the overall output current ripple and common-mode (CM) voltage will become smaller. To reduce inductor current ripple, inverse-CIs are employed. The current ripple in the CI is analyzed in detail. The benefits of the three-level dc-dc converter with CIs under interleaving operation are evaluated. By adding CIs and working under interleaving operation, smaller inductor current ripple, smaller overall output current ripple, and smaller CM voltage can be achieved simultaneously compared with the noninterleaving case. The analysis results are verified by simulations and 10 kW scale-down experiments.

Soft Switching PWM Cascaded Three-Level Combined DC–DC Converters With Reduced Filter Size and Wide ZVS Load Range

Two soft switching cascaded three-level (TL) combined dc–dc converters are proposed in this paper. The first converter is composed of a half-bridge cascaded TL dc–dc converter (CTLC) and a full-bridge (FB) CTLC, and the other one is composed of two FB CTLCs. The presented topologies have following common advantages: off-state voltage on all switches is only half of the input voltage; the structure of the primary circuit is simple and compact; all primary switches are directly clamped by the input capacitors and no added clamping devices are required; and the voltage across the output LC filter is a TL waveform, which results lower output filter volume and faster system dynamic response. In addition, the input filter can also be minimized. All power switches in the proposed converters can obtain zero-voltage switching in wide load range; furthermore, less conduction loss is added. The operation and technical analysis of the proposed converters are given. Experiments are carried out to validate the proposed converters, and efficiency curves are tested and given.

Hybrid Three-Level and Half-Bridge DC–DC Converter With Reduced Circulating Loss and Output Filter Inductance

A hybrid three-level (TL) and half-bridge (HB) dc–dc converter is proposed in this paper. The TL dc–dc converter and HB converter have their own transformers, respectively. Compared with conventional TL dc–dc converters, the proposed one has no additional switch at the primary side of the transformer, where the TL converter shares the lagging switches with the HB converter. In order to reduce the circulating current in the primary side, a blocking capacitor is used to reset the primary winding current of the TL converter. Moreover, the rectifier stage is composed of four diodes in the center-tap rectification, forcing the circulating current at the primary side to stay zero during the freewheeling period. The magnetizing inductor of the HB transformer can extend the zero voltage switching operation range of the lagging switches even at light loads. Furthermore, the proposed converter can reduce the output filter inductance. Due to the advantages mentioned above, the efficiency of the converter is improved dramatically. The features and design guidelines of the proposed converter are given in the paper. Finally, the performance of the converter is verified by a 1-kW experimental prototype.

Three-Level DC Converter for Balancing DC 800-V Voltage

This paper studies a three-level dc converter for balancing the dc input voltage, which has a similar topology with the conventional three-level dc converter. It can easily build a neutral line for changing a dc voltage source into two dc voltage-level sources. A control strategy of conducting time extension of partial switch devices is also presented for achieving three-level states of each power switch. The operating principle of the converter is analyzed in detail based on the presented control strategy. The small-signal model and the effects of the proposed control strategy are also driven. Finally, a prototype is fabricated and tested in the lab and the experimental results verify the converter to achieve three-level characteristics and to have a good ability of balancing the input voltage.

Comparative Analysis of Bidirectional Three-Level DC–DC Converter for Automotive Applications

In battery/ultracapacitor electric vehicles, a bidirectional dc/dc converter is employed to process the power according to the power references obtained from the energy management controller. The selection of this converter is of critical importance for the overall system efficiency and size. This study proposes using a three-level dc/dc converter and provides a comprehensive comparison with the conventional two-level and interleaved bidirectional buck/boost converters in terms of magnetic component size/weight and overall efficiency. Unlike the comparative studies presented in the literature, where the efficiency comparison of converters is conducted based on given fixed input and output parameters, power references obtained from a wavelet-transform-based energy management strategy with varying energy source voltages and traction power are considered in this paper. The results of the analyses show that a three-level converter exhibits higher overall efficiency and has smaller size inductor. A 1-kW bidirectional three-level dc/dc converter is designed as a proof of concept, which exhibits 93.2% peak efficiency at 200-kHz switching frequency.

A Solar Power Generation System With a Seven-Level Inverter

This paper proposes a new solar power generation system, which is composed of a dc/dc power converter and a new seven-level inverter. The dc/dc power converter integrates a dc-dc boost converter and a transformer to convert the output voltage of the solar cell array into two independent voltage sources with multiple relationships. This new seven-level inverter is configured using a capacitor selection circuit and a full-bridge power converter, connected in cascade. The capacitor selection circuit converts the two output voltage sources of dc-dc power converter into a three-level dc voltage, and the full-bridge power converter further converts this three-level dc voltage into a seven-level ac voltage. In this way, the proposed solar power generation system generates a sinusoidal output current that is in phase with the utility voltage and is fed into the utility. The salient features of the proposed seven-level inverter are that only six power electronic switches are used, and only one power electronic switch is switched at high frequency at any time. A prototype is developed and tested to verify the performance of this proposed solar power generation system.

Three-Level Converters Back-to-Back DC Bus Control for Torque Ripple Reduction of Induction Motor

This paper proposes a regulation method of back-to- back connected three-level converters in order to reduce the torque ripple in induction motor. First part is dedicated to the presentation of the feedback control of three-level PWM rectifier. In the second part, three-level NPC voltage source inverter balancing DC bus algorithm is presented. A theoretical analysis with a complete simulation of the system is presented to prove the excellent performance of the proposed technique.

New Three-Level PWM DC/DC Converter - Analysis, Design and Experiments

This paper studies a new three-level pulse-width modulation (PWM) resonant converter for high input voltage and high load current applications. In order to use high frequency power MOSFETs for high input voltage applications, a three-level DC converter with two clamped diodes and a flying capacitor is adopted in the proposed circuit. For high load current applications, the secondary sides of the proposed converter are connected in parallel to reduce the size of the magnetic core and copper windings and to decrease the current rating of the rectifier diodes. In order to share the load current and reduce the switch counts, three resonant converters with the same active switches are adopted in the proposed circuit. Two transformers with a series connection in the primary side and a parallel connection in the secondary side are adopted in each converter to balance the secondary side currents. To overcome the drawback of a wide range of switching frequencies in conventional series resonant converters, the duty cycle control is adopted in the proposed circuit to achieve zero current switching (ZCS) turn-off for the rectifier diodes and zero voltage switching (ZVS) turn-on for the active switches. Finally, experimental results are provided to verify the effectiveness of the proposed converter.

Analysis and implementation of a zero‐voltage switching pulse‐width modulation resonant converter

This study presents an interleaved pulse-width modulation (PWM) converter to achieve the functions of zero-voltage switching (ZVS) for all power switches and zero-current switching (ZCS) for rectifier diodes at the secondary side. A three-level hybrid DC converter is adopted to reduce the voltage stress of power switched at one-half of the DC bus voltage and automatically balance the two input capacitor voltages. Therefore MOSFETs with 600 V voltage stress can be used at the second stage DC/DC converter after the three-phase power factor correction circuit. The fixed frequency PWM is used in three-level PWM resonant converter to regulate output voltage. Since the selected switching frequency is less than the series resonant frequency, MOSFETs can be turned on at ZVS and rectifier diodes can be turned off at ZCS. Thus, the switching losses of power switches are reduced and the reverse recovery losses of rectifier diodes are eliminated. Interleaved PWM scheme is adopted to share load current and further cancel current ripple at output capacitor side. Thus, output capacitance can be reduced. Finally, experiments are provided to verify the effectiveness of the proposed converter.

Analysis of an interleaved zero‐voltage switching/zero current switching resonant converter with duty cycle control

An interleaved series resonant converter with fixed frequency pulse-width modulation (PWM) is presented to achieve load current sharing, ripple current cancelation, zero-voltage switching (ZVS) for power switches and zero-current switching (ZCS) for rectifier diodes. Two three-level DC converters with clamped diodes and flying capacitor are adopted to share load current. The voltage stress of power switches is clamped at one-half of DC bus voltage. The interleaved PWM scheme is used to control two converters in order to reduce the output current ripple and size of output capacitor. The series resonant tank in three-level PWM converter is adopted to realise ZVS turn-on for all power switches andZCS for rectifier diodes. Thus, the switching losses of active switches are reduced and the reverse recovery losses of rectifier diodes are eliminated. The fixed frequency PWM operation is adopted to regulate output voltage, so that the drawback of a wide range of switching frequency in the conventional series resonant converters is overcome. Finally, experiments based on a scale-down prototype are provided to verify the effectiveness of the proposed converter.

Voltage Balancing Control Strategy in Converter System for Three-Level Inverters

Outcome of DC-link capacitor voltage variation on inverter switching states is accessible and designed for three-level inverter. In this paper for back-to-back system by including five-level diode clamped topologies we are proposing a novel DC link balancing method. The algorithm which we proposed here is the improvement of variable switching frequency control policy which was previously introduced by means of three-level back-to-back system which depends on calculations of adjacent capacitor voltages which focuses on three-level DC link network to identify the information about potential variation in consecutive nodes. As per the above proposal, all four capacitors in DC link network are effectively balancing the voltage. Due to optimization of switching losses the proposed method has advantages over the variable switching frequency. DOI: http://dx.doi.org/10.11591/ijece.v3i1.1471