Research On Isolated SWISS-Type Three-Phase AC/DC Converters

With the promotion of new energy technologies,AC and DC hybrid microgrid technologies that can better include various energy sources and past technologies have developed rapidly.The AC/DC converter is an important part of the hybrid microgrid.High efficiency,low cost,small size,and stable output are always the goals pursued in converter research.The topology and control strategy of three-phase AC/DC converters still need to be developed.At the same time,with the development of material technology,power semiconductor devices have also entered the era of wide bandgap,and various high-performance third-generation devices have begun to challenge the dominance of traditional silicon-based devices.Combined with application requirements and the development of new devices,this paper studies the technology of high-performance modular three-phase bidirectional AC/DC converters.The application technology of Si C MOSFET is first studied in this paper.The crosstalk problem of Si C MOSFET bridge arms under high-frequency switching seriously affects the reliability of the device.Firstly,the mechanism of crosstalk is analyzed,and a simplified equivalent circuit model of the crosstalk problem is established.It is pointed out that crosstalk is not only caused by the di/dt of the body diode’s reverse recovery current but alos caused by the dv/dt generated by charging and discharging the parasitic drain-source capacitance.From the crosstalk analysis model,combined with the high parasitic gate resistance of the Si C MOSFET,it is pointed out that multi-level driving is a better alternative,and a novel multi-level driving circuit is designed.The gate drive circuit uses magnetic isolation technology to generate two negative gate drive levels without the need of additional isolation power supply.It can provide reasonable driving levels for the positive and negative pulses.At the same time,the drive circuit can also provide a short high level to accelerate the turn-on period of the Si C MOSFET,and switch to another high level required for steady-state conduction after the turn-on.This four-level drive circuit can effectively ensure that the gate-source voltage of the Si C MOSFET is in a safe range when applied in the bridge arm structure.Secondly,the topological structure of three-phase AC/DC converters is studied.It is pointed out that the current source AC/DC converters have the natural anti-surge capability and can save additional starting pre-charging circuits.These reasons make the current source AC/DC converter quite competitive in AC/DC grid applications.Combined with the electrical isolation and power density requirements,there is an urgent need for a three-phase AC/DC converter topology with the capabilities of current source output,single-stage isolation and high-frequency soft switching.The existing current source single-stage high frequency isolation AC/DC converter is derived from the non-isolated SWISS-type rectifier topology.It has the advantages of simple structure and high efficiency when using Si and Si C devices in combination.However,there are many problems with this structure such as the high switching loss of power devices and the low utilization of isolation transformer.In order to effectively utilize the advantages of SWISS-type converters,this paper establishes the modulation principle model of the SWISS-type converter,which can derive a series of single-stage AC/DC converters based on the existing DC/DC topologies.Thus,the switching function model of SIWSS-type converter is deduced,which can be used for the controller design.A noval single-stage isolated SWISS-type AC/DC converter based on dual full-bridge structures is proposed.The power coupling problem of the two full-bridge structures is solved through the research of modulation strategy,so that the main power devices can realize soft switching.Besides,the relationship between the（6N±1）^th harmonic of the AC current and the loss of duty cycle caused by leakage inductance is proved by a mathematical model.And a corresponding adaptive compensation strategy is proposed to improve the AC current power quality.At last,the high-quality operation of the proposed converter is realized.After the in-depth research on the dual full-bridge structures based SWISS-type AC/DC converter,a single-stage isolated SWISS-type AC/DC converter based on single full-bridge with midpoint-clamped is proposed in order to improve the power density.The transformer design of the proposed dual full-bridge structure requires two equivalent transformers with the rated output power.Besides,there are too many switches in the power flow path which is not conducive to the optimization of efficiency.The new single full-bridge structure can completely inherit the original topology modulation strategy and control method.Through the adjustment of the switches,the voltage clamp branches of the transformer terminal are constructed without adding any other device.Therefore,the utilization of high-frequency isolation transformer is improved and the optimization of conduction loss of high-frequency power devices is realized,which can further improve the power density of converter.The above-mentioned research systematically show the high-performance three-phase bidirectional AC/DC converter technology mixed using Si C devices and Si devices.It can provide an innovative solution for the interface converter of the distributed AC/DC microgrid.