Study of Star-Delta Soft Switch to Extend AC Motor High Speed Operation Region

In traction application,speed range of motor is an important index for motor drives.In AC motor control,the maximum speed of the motor is limited by the output voltage capability of the traditional three-phase inverter with star connenction of windings.Switching the star connection to delta connection in high-speed range can extend the speed range.In order to extend the speed operation region,star-delta switch proposed by many literatures relies on mechanical relay,which needs a dead zone of tens of milliseconds and seriously affects torque output.Besides,the traditional method will cause current overshoot during the switch transient process,decreasing the device security and reliability.Aiming at the defects existing in the star-delta hard switching,this paper proposes a star-delta soft switching method.Without adding extra power electronics devices,DC-bus capacitor is used to provide the path of zero axis current in the transient process,which helps to achieve the smooth torque output and zero current switch in the transient.Experiments have been done to validate the performance of the proposed method.The switching transient from star to delta connection in the motor drive can be much more stable than hard switching method.

Closed Loop Control of Bi-Directional Soft Switched Quasi Z-Source DC-DC Converter

Quasi Z-source converter is a single stage soft switched power converter derived from Z-source converter topology, employing an impedance network coupling the source with the converter. The quasi Z-source source converter can buck or boost the voltage and current flow is bidirectional. The duty cycle of the switch can be adjusted to maintain constant voltage during load change. To obtain constant output voltage, proper controller design is a must. This paper presents closed loop control of quasi Z-source converter using PI controller where controller parameters are estimated using the small signal model of the entire system. The transfer function of the system with AC sweep is used to obtain appropriate proportional and integral gain constants to reduce transient dynamics and to reduce steady state error.

Design and Experimentation of FPGA-Based Soft-Switched Interleaved Boost Converter for Telecommunication System

This paper proposes the design and experimentation of digital control of soft-switched interleaved boost converter using FPGA for Telecommunication System. The switching devices in the proposed converter are turned on and off with Zero Voltage Switching (ZVS) and Zero Current Switching (ZCS) respectively. The circuit is operated in Continuous Conduction Mode (CCM) with various load ranges having duty cycle of more than 50%. The proposed converter is studied by developing the simulation module in MATLAB/SIMULINK. A PI controller is designed and implemented in FPGA to obtain a regulated DC output for line and load variations. Simulation and experimentation results are verified with a prototype development of the proposed converter. The results indicate that the converter performance is enhanced with closed loop control.

Critical Transient Processes of Enhancement-mode GaN HEMTs in High-efficiency and High-reliability Applications

Wide-bandgap devices,such as silicon-carbide metal-oxide-semiconductor field-effect transistors(MOSFETs)and gallium-nitride high electron mobility transistors(HEMTs),exhibit an excellent figure of merits compared to conventional silicon devices.Challenges of applying such fast switches include accurate extraction and optimization of parasitics especially when 6high-efficiency operation,all of which require the comprehensive understanding of such switch especially its interaction with peripheral circuits.Particularly for the enhancement-mode GaN HEMTs without the intrinsic body diode,when reverse conducting,its high voltage drop causes a high dead-time loss,which has rarely a concern in silicon devices.This paper focuses on 650V/30~60A enhancement-mode GaN HEMTs provided by GaN Systems,analytically models its switching behaviors,summarizes the impact of parasitics and dead time,and applies it in two DC/DC converters.Systematic design rules are generated not only for soft switching but also for hard switching applications.

A Novel Polymorphic Topology with Hybrid Control Strategy Based LLC Resonant Converter for Ultra-Wide Input Voltage Range Applications

To realize effective utilization of renewable energy sources,a novel polymorphic topology with hybrid control strategy based LLC resonant converter was analyzed and designed in this paper.By combining the merits of a full bridge LLC resonant converter,three-level half bridge LLC resonant converter,and variable frequency control mode,the converter realizes an intelligent estimation of input voltage by automatically changing its internal cir-cuit topology.Under this control strategy,different input voltages determine different operation modes.This is achieved in full bridge LLC mode when the input voltage is low.If the input voltage rises to a certain level,it operates in three-level half bridge LLC mode.These switches are digital and entirely carried out by the DSP(Digi-tal Signal Processor),which means that an auxiliary circuit is unnecessary,where a simple strategy of software modification can be utilized.Experimental results of a 500W prototype with 100V~600V input voltage and full load efficiency of up to 92%are developed to verify feasibility and practicability.This type of converter is suitable for applications with an ultra-wide input voltage range,such as wind turbines,photovoltaic generators,bioenergy,and other renewable energy sources.

A Photovoltaic Generation System Based on Wide Voltage-Gain DC-DC Converter and Differential Power Processors for DC Microgrids

In this paper,a photovoltaic(PV)generation system based on front-stage differential power processors(DPP)and BACK-stage centralized wide voltage-gain converter is proposed.The resonant switched capacitor(ReSC)converter,which features high power density and high efficiency,is employed as the differential power processor and aims at processing the differential power among series-connected PV modules,avoiding the great power loss caused by power mismatch of PV modules.The ReSC converter in PV system can operate in equalization mode or MPPT mode and both modes are researched.The BACK-stage DC-DC converter is implemented as the dual-phase-shift controlled full-bridge converter(DPS-FBC).It has high conversion efficiency and wide voltage gain,suitable for PV generation applications.The current source control for grid-connected mode and the voltage source control for islanding mode are developed for DPS-FBC.Therefore,for the whole system with front and BACK stage converters,the dual-mode control strategy is proposed.When the system is connected to the grid,DPS-FBC operates as the current source for global MPPT for PV modules while ReSC converters operate in maximum power point tracking(MPPT)mode for local MPPT.When the system operates in an island,DPS-FBC works as the voltage source for constant output voltage control and ReSC converters work in equalization mode.The mode switch is based on the voltage of output DC bus.Simulations and experiments verify the feasibility of the proposed PV generation system and the effectiveness of the proposed control algorithm.