Research On Control Technology Of Three-Port Isolated Bidirectional DC-DC Converter

The application of solar and super capacitor hybrid power supply devices in the field of electric vehicles and distributed renewable energy power generation systems has attracted widespread attention in recent years.In order to effectively combine the hybrid power supply device,the main power supply and the load,currently multiple single-input DC-DC converters are mainly used as the interface circuit.However,using a multi-port DC-DC converter to replace the original single-input DC-DC converter can not only improve the reliability and power utilization of renewable energy generation systems,but also simplify the system's topology,save costs,and achieve Centralized control.Aiming at the energy management of multi-port converters in electric vehicles,this paper chooses a three-half bridge DC-DC converter as the interface circuit of the hybrid solar and supercapacitor power supply device.Its two ports are energy input ports connected to solar cells and super capacitors respectively,and the other port is an energy output port for connecting electric vehicle loads.The three ports are connected through a three-winding transformer to meet the low-level,low-current ripple,and electrical isolation requirements of the input ports.The main research is as follows:(1)Analyze the Y-type and ?-type equivalent circuit models of the three-port converter.Based on the ? equivalent circuit,analyze the steady-state characteristics of the three-port converter with dead time.Analyze the soft switching range,calculate the leakage current of the transformer in the 18 switching states of the converter,and analyze the relationship between port voltage,phase shift angle and power transmission.(2)Based on the simplified circuit of the converter's eighteen working modes,the inductor current,capacitor voltage and equivalent inductor current of the delta-type equivalent circuit are selected as state variables.By introducing a switching function,a reduced-order state-space average mathematical model for a three-half bridge converter with dead time is established.Based on the state space average model,a linear small signal model and its equivalent circuit are derived.Subsequently,the open-loop transfer function of the controlled quantity and the controlled quantity is derived.(3)Since the three-port isolated half-bridge DC-DC converter is a current-type input structure,it is easier to control the input inductor current.Therefore,the inner loop current control and the outer loop voltage control are adopted.There is a coupling relationship between the two input terminals of the current inner loop system.The change of each input quantity will affect the output quantity,and the system needs to be decoupled.By designing a series compensator,the transfer function matrix is diagonalized,and the transfer function matrix of the decoupling network is derived.(4)Through the open-loop logarithmic frequency characteristic curve method in automatic control theory,this paper analyzes the amplitude-frequency characteristics,phase-frequency characteristics,and open-loop root locus distribution of the decoupled system.And the compensation network of converter inner current loop and outer voltage loop is designed.In Matlab/Simulink software,the soft switching problems of the traditional three-half bridge structure and the new three-half bridge structure with dead time are compared and simulated.At the moment when the switch is turned on and off,the converter with dead time can realize soft switching.Then compare and simulate the double closed-loop system before and after decoupling.In the current inner loop decoupling simulation,when a step change in the reference current occurs,a maximum fluctuation of about 0.4A occurs.Compared with the system before decoupling,it follows more quickly and the current fluctuation is smaller.Under the effect of the decoupling network,it realizes the autonomous control of the two input inductor currents;In the double closed-loop simulation,when the input power supply voltage disturbance and load disturbance occur,the maximum fluctuation of about 0.4V occurs,and it quickly stabilizes at 50 V.Compared with the system before decoupling,the voltage fluctuation is smaller,and the speed to restore stability is faster.This shows that the decoupled voltage outer loop control system has better noise immunity,dynamic performance and output accuracy.Finally,the three working modes of the auxiliary power supply system composed of solar cells and super capacitors are simulated,and it is verified that under different working modes,they can provide stable power for the DC load.