Due to the development of new energy, the battery energy storage system (BESS) has been an indispensible part of the power electronics system. In the BESS, the bidirectional DC-DC converter is the most important part. Its performance will affect the efficiency and robustness of the new energy power electronics system. Because of the existence of the battery’s parasite resistance and the variation of the battery’s intrinsic voltage in different state of charge (SOC), the DC-DC converter must be efficient and realiable when transferring enery in the wide voltage variation range condition.
Nowdays, most of the bidirectional DC-DC converters are transformed from the unidirectional DC-DC converter, and the performance of the backward operation is much worse than the forward operation due to their asymmetry in structure. However, the bidirectional full bridge converter has a very simple and symmetrical structure, and the switching of the power flow direction is very smooth. These characteristics makes it excellent in disposing the buck and boost bidirectional operation with high efficieny and low cost. Although there are many advantages, the bidirectional full bridge DC-DC converters will suffer big circulating current and loss of zero voltage switching (ZVS) which make a high current stress, and finally deteriorate the over performance when the voltage transfer ratio deviates from the transformer’s turn ratio. Besides, the problems like slow dynamic response, complex modulation algorithms and existence of DC bias current also makes this converter hard to be implemented in real application.
For the purpose of solving these problems, this thsis focuses on the study of the characteristics of the bidirectional full bridge DC-DC converter in the wide voltage variation range condition. By studying the converter’s efficiency model, small signal model, modulation algorithm and DC bias compensation method, the performance like overall efficiency and so on are improved. The main research contents and inovations of this thsis are concluded as:
(1) An overall efficiency model is proposed. Traditional efficiency calculation strongly relies on the simulation softwares. By using the P-spice model of the semiconductor device and magnetics provided by the suppliers, the efficiency of the bidirectional full bridge DC-DC converter can be estimated previously. However, there are always convergence problems during the simulation, and the simulation is time consuming. Besides, the backward effect of the temperature rise is too complicated to be implemented in the simulation. So when using the simulation method, the modulation method optimization and system design will be very inconvenient. The proposed model uses the concepts of equivalent parasote resistance to calculate the key waveforms of the bidirectional full bridge DC-DC converter. Based on these calculated waveforms, the core losses and switching losses are calculated out with consideration of the backward effect of the temperature rise. This model reduces the calculation time to 1/100 times of calculation time in P-spice simulation. By using this model, the selection of switch device, the selection and optimization of the modulation algorithm can be accomplished conveniently, and finally the development time can be reduced.
(2) A small signal model based on the behavior analogy is proposed. Conventional small signal model of the bidirectional full bridge DC-DC converter is derived by using the harmonic analysis method and the state space method; The accuracy relies on how many harmonics are used. This method is hard to get a trade off between the accuracy and the complexity, and the modeling process is complicated. The proposed model is an equivalent mathematic model that derived based on the behavaior analogy of the bidirectional full bridge DC-DC converter. This model can describe the steady state and dynamic behaviors perfectly. Based on this equivalent mathematic model, the small signal model in frequency domain is derived. When compared to the test result, the error of this small signal model is less than 3% below half of the switching frequency. The proposed model can be used in any modulation method, and it can guide the design of the control loop and the selection of modulation method.
(3) A modulation method is optimized based on the ZVS model. The fundamental duty modulation is proved to be the best one in the existing modulation methods. It shows a wide ZVS region, small circulating current and wide power transfer range. However, this modulation is derived based on the fundamental harmonic method. So the algorithm has many inverse trigonometric functions which will comsume a long time to calculate. The best way to reduce the calculation time is to use look-up table. But the look-up table will consume many memory resources. All in all, the cost is hard to be reduced. The proposed modulation method reduces the complexity of the algorithm while the performance is almost the same with the fundamental duty modulation. The calculation time is reduced from 347 clock cycles to 47 clock cycles.
(4) A DC bias compensation method based on the duty cycle mathematic model is proposed. The previous researchs only give the concept of eliminating the DC bias by using control loop. Through further analyzing and modelling the mechanism of the DC bias’ formation, this thsis derives the mathematic model of the interrelationship between the duty cycle and the DC current bias. Then, a DC bias compensation method based on this model is proposed. The test result shows that it can improve the overall efficiency for about 1%~2%.
(5) A magnetic integration method which can guide the selections of smallest magnetic core and Litz wire is proposed. The leakage inductance mathematic models of two typical transformer structures are given. Based on the combination of the leakage inductance model and the magnetizing inductance model, the size of the smallest magnetic core of the transformer can be derived. Besides, the eddy current loss model derived by Dowell is modified to be used in the Litz wire condition, and then the selection method of the Litz wire can be drived. These two contributions figure out the overall magnetic integration design method, and make the top down design of the magnetic integration in bidirectional full bridge DC-DC converter come ture.