Design And Research Of DC/DC Converter For Power Electronic Transformer

In this paper,to focus on the application of the output of DC/DC converter used in power electronic transformer in wide-range voltage conversion environment,researching and designing a two-stage LLC harmonic resonance DC/DC converter which the rated output power is 10.4kW,the input voltage is 500 V,and the output voltage is 350V-600 V.And in order to solve the problem of uneven current distribution in cascaded LLC converters,a new automatic current sharing scheme is proposed.(1)Designing a high frequency transformer which is applied to the LLC converter and the apparent power is 5.5 kW,including the selection of the core,the determination of the winding arrangement,and the arrangement of the winding.In the ANSYS environment,Simploer-Maxwell is used to simulate the designed high-frequency transformer,and the magnetic density,leakage magnetic density,the rises of temperature and iron loss distribution of the main magnetic circuit of the magnetic core under the worst conditions are analyzed.(2)Based on the characteristics of the LLC resonant topology,a single-stage LLC resonant converter with a rated output of 5.2 kW is designed.The simulation is performed in the PSIM environment in the frequency converting control.Analyzing the characteristics of the variable frequency in case of the system gain is equal to 1,the gain is greater than 1 and the gain is less than 1.(3)The controlling scheme using the frequency converting control in the boost mode and the phase shifting control in the buck mode which combining the advantages of frequency converting control and phase shifting control,and the controlling scheme is simulated and analyzed in the PSIM environment.(4)Cascading scheme is input series output parallel and in order to solve the problem of the uneven current of LLC converter during cascade operation so that the application of the power electronic transformer in high-voltage and high-power applications is realized,a new type of automatic current sharing topology is proposed on the basis of not increasing the control complexity of the system and not changing the working mode of the original main circuit.Using the principle of coupled inductor and capacitor charge balance,a coupling inductor is added between the resonators of the two-stage converter at two levels and acurrent sharing circuit with a current sharing capacitor as a core is added to the output end of the converter.When the current is not equal between two-stage circuit,the induced voltage of the coupled inductor drives the switch to charge the current sharing capacitor until the current difference of the two-stage LLC converter is reduced to an acceptable range.At this time,the current sharing switch is turned off and the current sharing capacitor discharges to a converter with a small current.By establishing the mathematical model of charging and discharging process,the relationship between system operating frequency,current sharing capacitance value and coupled inductance value is obtained which has a guiding significance for the design of current sharing circuit.In the PSIM environment,the difference of the primary resonance currents in the case of with the current sharing circuit and without the current sharing circuit are compared near the steady state and the control mode switching point under certain parameter differences.