Research On High Frequency Isolated Bidirectional Resonant DC-DC Converters

Future Intelligent Electricity Networks need to comprehensive the utilization of all kinds of energy. Diversified development of eight major energy sources which include oil, coal, wind power, hydropower, nuclear power, natural gas, solar, and biomass put forward new challenge for the grid structure of the traditional centralized power generation mode. As a result a new type of power grid management system is needed to deal with the rapid development of distributed generation model of the smart grid. UNIFLEX-PM(UNIversal and FLEXible Power Management in Future Electricity Networks) plan in Europe and FREEDOM(Future Renewable Electric Energy Delivery and Management) in the United States are working on an interactive interface project of parallel operation intelligent network, power electronics conversion device, to meet the needs of the future smart grid. Our country also put increasingly attention to new energy generation and the construction of the smart grid is also on the agenda. But we don't have mature application in the research of smart grid interface technology of distributed generation, so it becomes more urgent in the domestic research on this field.The National Natural Science Foundation Project replied by this topic, “no power frequency transformer cascade multilevel converter”, is a tertiary structure of converter. This article focuses on the middle level of bidirectional DC-DC converter. For research of this paper, the switching losses of the power switch tubes in the traditional bidirectional DC-DC converter are proportional increased with the increase of switching frequency, which reduces the efficiency of the converter, limit the increase of the converter operation frequency. And improving the switching frequency makes it possible for the replacement of the traditional power frequency transformer to the high frequency transformer. That reduces the whole volume of the equipment and improves the power density of the system. At the same time, as the resonance of the resonant network makes the working current of the converter change with sine rule, that smooth curve of working current and the smaller voltage and current rate can inhibit higher harmonic current, so it has important practical significance for the research of resonant bidirectional DC-DC converter.Given the advantages of reducing the wastage of the power switch devices in the resonant DC-DC converter, this paper puts forward the basic macro model of high frequency isolated resonant bidirectional asymmetric DC-DC converter. In this model, one end of the high frequency isolated transformer series a resonant capacitance. Inverter and rectifier units can be full-bridge, half bridge or tri-level half bridge, etc., which constitute the bidirectional converter circuit of high frequency transformer asymmetry topology. Through different equivalent conversion for high frequency isolated transformer, there are three different types of two-way DC-DC converter circuit topology which includes LC series resonant topology, LLC resonant topology, LCC resonant topology. It needs special mention that for two-way DC-DC converter circuit topology, it has more research value and significance with bidirectional symmetrical structure. So for the equivalent circuit of series resonance, in order to construct the bidirectional symmetrical structure, the series resonant capacitance is divided into two series capacitors symmetrical about high frequency transformer. Combined with three-level half bridge as inverter unit and rectifier unit, these constitute the double tri-level half bridge bidirectional CLLC resonant DC-DC converter. The proposed CLLC resonant converter is equivalent to LC series resonance converter eventually. This is different from the CLLC resonance mentioned in exiting literatures which is analyzed and discussed as LLC resonance ultimately, now this circuit topology is called CLLLC resonance. To avoid confusion, it is informed.In this paper, there are three examples which are dual three-level half bridge CLLC resonant bidirectional DC-DC converter, three-level half bridge and full bridge LLC resonant bidirectional DC-DC converter and dual active bridge LCC resonant bidirectional DC-DC converter respectively corresponding to different equivalent of high frequency transformer based on the basic macro model of resonant converter. The key research focuses on the study and compare of the control strategy, power characteristics and soft switching of the CLLC resonant converter and LLC resonant converter. At the same time, the research, analysis and compare of the circuit characteristics and soft-switching range is also mentioned when the CLLC resonant converter works in Single-Phase-Shift(SPS) and Dual-Phase-Shift(DPS) control strategies under same working frequency. And this paper simply analyses the applicability of the LCC bidirectional DC-DC converter carried on this topic research. The comparison between LCC resonant with other resonant topology is not focus in the study of this article.For resonant converter, changing of the working frequency of the power switches causes the corresponding change of the input impedance of the resonant network. This verifies the input current of the resonant network. Through this way, it can realize the regulation of the output voltage or the output current under constant voltage, so as to realize the control of output power. At the same time, aim at the unbalanced volt-second problem caused by the switching speed, unequal conduction time and conduction voltage drop of different power switching devices, it can be solved without additional hardware circuit by joining the resonant network. When the load changes suddenly, the series resonant inductor in the network can prevent resonant current mutation. It also has the characteristics of short circuit from protection. These are the unique advantages of the resonant converter.When LC series resonant, LLC resonant and LCC resonant work under variable frequency control method, they have different working conditions and the situation of the soft switching is also different. Considered that the variable frequency control is more complex to implement, a new control strategy is needed to realize the simplified control. Therefore, introducing phase-shift control to CLLC resonant converter, the working principle is analyzed under fixed frequency phase-shift control strategy. It is known that power transmission can be dong by simple control in the symmetric structure of bidirectional DC-DC converter.In the power transmission analysis of the CLLC resonant converter, there are two cases which are resistance load and constant voltage load. In the past, all the research on the resonant converter is focused in the case of resistance load, and adjusting the working frequency of the converter realizes the change of output voltage. However, this topic research is for power transmission between two DC power supply. Therefore, there is another case of constant voltage load. Analysis results show that the working principle under these two cases of resistance load and constant voltage load with variable frequency control stays the same. Compared with variable frequency method and fixed frequency phase-shift method, variable frequency method can make the voltage gain achieve maximum 1 at the resonant frequency, while fixed frequency phase-shift method can boost the output voltage as well as buck the output voltage. The design of variable frequency method is more complex and it is difficult to design a filter which can adapt to a wide range of frequency change. Fixed frequency phase-shift control method can be simply realized and fixed frequency can suppress the high frequency harmonic. Therefore fixed frequency phase-shift control has more advantage than variable frequency control. This also is a key focus in this paper.In this paper, the working principle, circuit characteristics and soft-switching range is analyzed and compared in detail when the CLLC resonant convert works in SPS and DPS control methods. Then we can know that there exists a minimum circle power in DPS control, but it is very cumbersome in calculating. Although the SPS control has more circle power in same transfer power, but the peak resonant current is smaller than all working spots in DPS. Not only that the soft-switching range in SPS is larger then the soft- switching range in DPS. Beside it has simpler calculate and design, so the CLLC resonant converter in SPS is more suitable for this project.After determining the circuit topology and control strategy of the bidirectional DC-DC converter, this paper compares the power characteristics and soft switching conditions in the two cased of resistance load and constant voltage load between the CLLC series resonant converter under fixed frequency Single-Phase-Shift control method and the LLC resonant converter under the variable frequency control method.For CLLC series resonant converter in SPS control, there are approximate output power according to the sine law of change working under the conditions of resistance load and constant voltage load. Still there exists circle power in the working process at the same time. And circle power increase as the phase-shift angle grows larger. The transformation law is obtained between the circle power and output power with the growing of phase-shift angle. And the optimal working scope of phase-shift angle is obtained through analyzing the relation between the circle power and output power with phase-shift angle growing.By analyzing the soft switching range of CLLC resonant converter, it is known that no matter how phase-shift angel changes, the power switches in inverter unit and rectifier unit can achieve zero voltage switching working in the case of constant voltage load under fixed frequency single-phase-shift control strategy, as long as the working frequency of the converter is larger than the resonant frequency of series capacitance and inductance.For LLC resonant converter, when working under the condition of resistance load, and the switching frequency is less than the resonant frequency which the parallel inductance is not involved, the voltage gain of the LLC resonant converter has relations with the resonant frequency which the parallel inductance is not involved and the resonant frequency which the parallel inductance is involved. In this case, the power switches of the inverter unit can achieve soft switching. When working under the condition of constant voltage load, the parallel inductance does not take part in the resonant process in a cycle. And the power switches of inverter unit can not realize zero voltage conducting and zero current off. It does not belong to the soft switching. At the same time, there is also a circle power within a cycle.Although LLC resonant converter in one-way power transmission has the advantages of a wide range of input voltage and the output voltage regulating in all load range, this can not be reflected in two-way power transmission and the case of constant voltage load. By comparing the power characteristics and soft switching conditions of CLLC resonant converter and LLC resonant converter, the dual three level half bridge CLLC resonant bidirectional DC-DC converter is determined which is suitable for the research in this paper with fixed frequency phase-shift control method. In this paper, the small signal model of the dual three level half bridge bidirectional CLLC resonant DC-DC converter is established by Extended Describing Function Method.This paper gives the simulation results of the dual three level half bridge CLLC resonant bidirectional DC-DC converter working under the condition of resistance load and constant voltage load, the three level half bridge combined with full fridge LLC resonant bidirectional DC-DC converter under the condition of resistance load and constant voltage load, the dual active bridge LCC resonant bidirectional DC-DC converter under the condition of resistance load. The simulation results verify the validity of the front theoretical analysis.In the end, according to the circuit topology proposed in this paper, the experiment platform are set up which includes the dual three level half bridge CLLC resonant bidirectional DC-DC converter, the three level half bridge combined with full-bridge LLC resonant bidirectional DC-DC converter, and the dual active bridge LCC resonant bidirectional DC-DC converter. Hardware circuits include three level half bridge board and full bridge board in the platform. The control system and switch driving board are selected. The parameters of the resonant network are designed. The experimental results and analysis are listed respectively in the condition of resistance load, which validate the correctness of the front theoretical analysis. The simulation and experiment results laid a foundation for the next step of research.