| This paper is mainly about Bi-directional DC/DC converters used in hybrid energy storage system of new energy vehicles, and a kind of 6-channel interleaved magnetic integration Bi-directional DC/DC converter is designed considering both the output current ripple and the dynamic response speed.Vehicle exhaust is one of the important factors that deteriorated the environment, with the increasing number of urban cars, many cities which have no heavy industry occur serious hazy weather. Therefore, research works on new energy vehicles which have the advantage of low emission or even zero emission have been global hotspot in recent years. Hybrid electric vehicle is an important branch of new energy vehicles, which usually uses a kind of hybrid energy storage device choosing Bi-directional DC/DC converter for charging and discharging management of battery and super capacitor. Batteries are used as main power source connected to the load directly, while super capacitors as the auxiliary power source connected to the load through DC/DC converter, in order to realize the energy recovery in regenerative braking period. When DC/DC Converter is working in Boost mode, the supercapacitor provides peak power to the load through the converter; conversely, when DC/DC converter works in the Buck mode, the supercapacitor receives the energy feedback from the load. The designer of hybrid energy storage system usually expects the output current ripple of the Bi-directional DC/DC converter as small as possible, and a common solution is multiple processing methods, that is, using multiple basic Buck-Boost parallel circuit.The advantage of multi-channel Bi-directional converter is that it can increase the output power, reduce the current stress of each channel and improve the efficiency of the converter simultaneously. However, the increasing of the channel number makes the analysis and design of the converters more complicated. Thus, relevant domestic and foreign research work is relatively less. This paper is based on research project Research on theory and control method of Interleaved Magnetic Integration Bi-directional DC/DC converter from National Science Foundation, and we designed a 6-channel interleaved magnetic integration converter on the basis of former research results on 3-channel and 4-channel.How to reduce the output current ripple and how to improve the dynamic response speed is a pair of conflicting factors in designing the DC/DC convertors. By using interleaved magnetic integration technology, a converter with 6-channel coupled inductors designed in this paper can improve the dynamic performance when reducing the current ripple at the same time. Under the same dynamic response speed, the converter with 6-channel coupled inductors can significantly reduce the phase current ripple, the total output current ripple and the output voltage ripple compared with the converter with discrete inductors.The topology structure of the 6-channel interleaved converter is given and the Boost working mode of it is analyzed. The equivalent circuit of the converter under different mode is also drawn, and the rule table of the main switch in the 12 working modes during a switching period is obtained on the basis of the 6 different value range of the duty cycle. Due to the increased number of channels, the design of integral core coupled inductor becomes more difficult than that of the array type core structure. This kind of coupled inductors uses matrix topology, which can satisfy the demand of channel number change and further expansion. Three kinds of main inductor magnetic integration scheme of 6-channel Bi-directional DC/DC converter are given in this paper. The first kind of coupling mode is realized between the first channel and the fourth channel, the second channel and the fifth channel, the third channel and the sixth channel respectively. The second mode is among 1-3-5 and 2-4-6 channels, and the third one is sequential coupling mode in which the first channel couples with the second one until the sixth channel couples with the first channel.According to the above three coupling schemes, the array topology matrix of the corresponding scheme is designed respectively. The relationship between the leading diagonal core inductance, the non leading diagonal core inductance, the turn ratio, the magneto resistance ratio and the degree of coupling is also given for the three schemes. According to the circuit topology structure of three kinds of schemes, we get the voltage equation, the working mode waveform of the main switch during a switching cycle and the waveform of the inductance current and Channel current in the boost model. Taking the first channel as an example, we calculate the twelve equivalent inductances of the six kinds of duty cycle of the three methods. The calculation results of the equivalent inductance establish the mathematics foundation for solving the steady-state equivalent inductance and equivalent transient inductance. Equivalent inductance solution needs a lot of Mathematical calculations. It is particularly important to look for regular and effective calculation method for multi-channel converter. The calculation method used in this paper can be extended to the equivalent inductance calculation of N channel Bi-directional converter.Under the circumstance of using discrete inductors, when the inductance increases the current ripple can be reduced but the dynamic response is degraded. Apparently, the reduction of the current ripple and the dynamic response speed can not be optimized simultaneously, that is, they are a pair of conflicting factors. The advantage of magnetic integration technology is that it can obtain the optimal coupling degree through correct analysis and calculation when considering the output current ripple and the response speed at the same time. When the load increases suddenly, the duty ratio of the converter will increase, thus, the steady state and the transient current ripple can be solved mathematically through dynamic analysis. The specific method is as follows: first, draw the waveform of the channel current under coupling and uncoupling condition, then calculate the steady state current ripple and the transient current increment in the same condition, therefore, the steady state equivalent inductance and transient equivalent inductance of the converter can be determined through contrast between the mathematical relationship of coupling and uncoupling condition. For the three designed coupling schemes, the corresponding dynamic response curves of the phase current for the uncoupling mode and each coupling mode can be drawn, and the expressions of equivalent steady state inductance and equivalent transient inductance can be deduced according to different duty ratio range. Furthermore, the expressions of phase current ripple and total output current ripple under coupling and uncoupling condition and the expression of dynamic response speed can be derived. Finally, the mathematical relationship between the coupling degree, the duty ratio, the steady state phase current ripple and the transient current increment can be derived, which provides mathematical basis for the design of the optimal coupling degree of the three coupling modes.Coupling degree is the most important factor in the application of magnetic integration, and the design thought of this paper is as follows: in order to consider both the current ripple and the dynamic response speed, assume that the transient inductance of coupling mode is equal to the discrete inductance of the uncoupling mode according to the mathematical expression of the dynamic response speed. That is, ensuring that the respond speed of the coupling mode and the uncoupling mode is the same, choose an appropriate coupling degree to make the current ripple of the coupling mode less than that of the uncoupling mode. Under the assumption that the response speed of coupling and uncoupling condition is equal, we can readily draw a conclusion according to the expressions of current ripple. When the difference of equivalent steady state inductance and equivalent transient inductance is the largest, the current ripple under coupling cases is the smallest compared with the uncoupling case. This is the criterion of designing the 6-channel coupling inductors to obtain optimal coupling degree.The concrete implementation approach of designing coupling degree is based on the expressions of the coupling degree, the duty ratio, the equivalent steady state inductance and the equivalent transient inductance. Using normalized inductance method and Math CAD software, the relationship curves between the normalized equivalent steady state inductance and the coupling degree, the normalized equivalent transient inductance and the coupling degree, and the normalized inductance difference and the coupling degree can be plotted for the three coupling schemes. Then we can determine the area where the difference between normalized equivalent steady state inductance and normalized equivalent transient inductance is the largest as the optimal coupling degree area in the curves. The results of calculation and analysis show that the 6-channel converter can work steadily in both 2-2coupling mode and 3-3 coupling mode, and the optimal coupling degree area can also be determined for each mode. But as for the sequential coupling mode, when the duty ratio is larger than one third, the equivalent steady state inductance may increase to infinity in some cases. Thus, the converter can not work steadily, this means that the sequential coupling mode is not feasible. The subsequent simulation and experimental research in this paper is only aiming at the 2-2 and 3-3 coupling mode.In order to ensure the continuity of the current, the minimum value of the equivalent steady state inductance under uncoupling conditions is calculated, and what's more, the minimum value of self-inductance of each phase under the coupling condition is calculated. This paper also solves the leakage inductance and mutual inductance value of each unit under the coupling conditions, based on the principle of array type matrix structure. Using iron silicon aluminum magnetic ring KS068125 A, winding inductance coils of 2-2coupling and 3-3 coupling of the six phase coupling inductance are made manually. The simulation model of the system is built by Saber software, and the simulation analysis of the three coupling cases are carried on. Simulation results including the phase current of 2-2 coupling, 3-3 coupling and uncoupling mode, the total output current and the load mutation have been given. The simulation results validate the correctness of the theoretical analysis and deduction. The experimental prototype of the converter system is set up, in order to test the circuit performance under 2-2 coupling, 3-3 coupling and uncoupling cases respectively. Through testing of single-phase current ripple, the total output current ripple and the total output voltage ripple, the experimental results proved that using coupling inductors can largely reduce the phase current ripple, the total output current ripple and the output voltage ripple under the same conditions.According to the results of theoretical analysis, mathematical deduction, simulation analysis and experimental test, we can draw a conclusion that using a coupled inductor can obviously reduce the single-phase current ripple,the total output current ripple and the total output voltage ripple, and improve the response speed of the system for 6-channel interleaved bi-directional DC/DC converter. In addition, the performance of 2-2 coupling scheme is superior to that of the uncoupled one, while 3-3 coupling scheme is better than 2-2 coupling scheme. |
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