Efficiency analysis of a bidirectional DC/DC converter in a hybrid energy storage system for plug-in hybrid electric vehicles
Introduction
Nowadays, the development of electric vehicles (EVs) and plug-in electric vehicles (PHEVs) has gained more attention as they can reduce greenhouse gas emissions and improve air quality. EVs, especially PHEVs, require energy storage systems (ESSs) with high energy and power density [1], [2], [3], [4], [5]. A single battery system cannot meet such requirement. A hybrid ESS which consists of a bidirectional (Bi) DC/DC converter, batteries and ultracapacitors is a solution, where the batteries mainly store energy and the ultracapacitors mainly release and absorb power. This can be realized by adjusting the voltage level of the ultracapacitors through Bi DC/DC converter in the ESS [6], [7], [8], [9]. Therefore, it is very important to understand the efficiency of Bi DC/DC converters under different working conditions, making efficient hybrid ESSs [10].
The influences of topology, frequency, switching method, temperature and duty cycle on the efficiency of Bi DC/DC converters have been investigated. In [11], [12], the influences of two topologies on the efficiency of the converter are studied. The simulation and experimental results show that the proposed topologies have the merit of high efficiency. The effect of the frequency on the efficiency of the converter is investigated by applying the adaptive frequency modulation [13]. The converter with low ripple achieves the maximum efficiency up to 87%. In [14], a new digital PWM technique is used to explore the relationship between frequency and efficiency. The results show that high switching frequency leads to low efficiency and vice versus. In order to increase efficiency of the converter, zero voltage switching based on resonant technique is proposed to reduce switching loss [15], [16], [17].
Some researchers study the influence of temperature on the efficiency of the converter [18], [19], [20]. They select SiC as the switching devices to evaluate the performance of the converter at high ambient temperatures. The high efficiency can be achieved at high temperature environment in the converter with the SiC switching devices. Different duty cycles are adopted in the converter to study their effects on the efficiency based on the proposed efficiency model [21]. The experimentally measured and theoretically calculated efficiencies are compared at two different duty cycles: D = 0.33 and D = 0.5 to show their effects.
There are two drawbacks in the above mentioned studies. First, they only explore the influence of one factor on the efficiency. Second, they do not consider the effect of the switching devices used in the converter: MOSFET SiC or IGBT Si, on the efficiency. One purpose of this study is to analyze the influences of multi-factors, such as temperature, switching frequency, duty cycle and material of switching device, on the efficiency of a Bi DC/DC converter. Another purpose is to develop the efficiency model of the converter. The experimental results verify the accuracy of the proposed efficiency model.
The rest of the paper is organized as follows. In Section 2, a Bi DC/DC buck-boost converter is analyzed. The efficiency model of the converter is established in Section 3. The experimental platform is introduced and the efficiencies from the experiments under different operating conditions are analyzed in Section 4. The efficiencies from the model and those from the experiments are compared in Section 5. Finally, the conclusions are presented in Section 6.
Section snippets
Selection of Bi DC/DC converter
Many topologies of Bi DC/DC converter are used in hybrid energy storage systems (HESSs) [2]. Fig. 1 shows the fundamental topology of the HESS which has been chosen in this study, where a battery pack is connected to a Bi DC/DC converter and the converter is then connected to an ultracapacitor pack.
Three reasons are considered in the selection of this Bi converter. First, the energy in HESSs should be allowed to transfer in two directions, where the current can either flow from the battery pack
Efficiency model of Bi buck-boost converter
To develop the efficiency model, the detailed analysis of the losses for each of components in a Bi buck-boost converter is conducted when the converter is operating in steady state. Generally, the losses include switching loss and conduction loss. In this paper, the efficiency model for the buck mode will be only developed and analyzed in detail since the procedure to develop the efficiency model for the boost mode is similar to that for the buck mode.
Experimental design scheme
Different temperatures, materials of the switching devices, duty cycles and switching frequencies as presented in Table 2 as well as two working modes for the Bi DC/DC converter will be considered in the experiments. In the buck mode, the input voltage is set to 300 V, the output voltage will be 100 V at KD1 = 33.33%, 150 V at KD1 = 50% and 200 V at KD1 = 66.67%, respectively. In the boost mode, the input voltage is set to 50 V, the output voltage is 75 V at KD2 = 33.33%, 100 V at KD2 = 50% and 200 V at KD2 = 75%,
Comparison of model and experimental efficiencies
The model and experimental efficiencies are compared under different working modes, duty cycles and the materials of the switching devices. Fig. 14, Fig. 15 show their comparison results and absolute errors (AEs), respectively.
It can be seen from Fig. 15(a)–(c) that the maximum AE is around 10% and the minimum AE is nearly 0.7% for the buck mode. It can also be seen from Fig. 15(d)–(f) that the maximum AE is 15% and the minimum AE is nearly 0.5% for the boost mode. The reasons to cause these
Conclusions
This paper develops the efficiency model for a Bi DC/DC converter. The model and experimental efficiencies are compared to show that the proposed efficiency model has high accuracy which can be applied to predict the efficiency of the Bi DC/DC converter under different working conditions. Furthermore, the influences of the multi-factors, such as temperatures, materials of the switching devices, duty cycles and switching frequencies, on the efficiency of the converter has been experimentally
Acknowledgements
This work was supported by the National Natural Science Foundation of China (Grant No. 51507012, 51675042) and the Joint Funds of the National Natural Science Foundation of China (Grant No. U1564206). Any opinions expressed in this paper are solely those of the authors and do not represent those of the sponsor.
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