Direct power control of dual‐active‐bridge dc–dc converters based on unified phase shift control
2018; Institution of Engineering and Technology; Volume: 2019; Issue: 16 Linguagem: Inglês
10.1049/joe.2018.8598
ISSN2051-3305
AutoresFeng An, Wensheng Song, Kexin Yang,
Tópico(s)Silicon Carbide Semiconductor Technologies
ResumoThe Journal of EngineeringVolume 2019, Issue 16 p. 2180-2184 Session – Report Session AOpen Access Direct power control of dual-active-bridge dc–dc converters based on unified phase shift control Feng An, Feng An School of Electrical Engineering, Southwest Jiaotong University, Chengdu, Sichuan Province, People's Republic of ChinaSearch for more papers by this authorWensheng Song, Corresponding Author Wensheng Song songwengsheng@163.com School of Electrical Engineering, Southwest Jiaotong University, Chengdu, Sichuan Province, People's Republic of ChinaSearch for more papers by this authorKexin Yang, Kexin Yang School of Electrical Engineering, Southwest Jiaotong University, Chengdu, Sichuan Province, People's Republic of ChinaSearch for more papers by this author Feng An, Feng An School of Electrical Engineering, Southwest Jiaotong University, Chengdu, Sichuan Province, People's Republic of ChinaSearch for more papers by this authorWensheng Song, Corresponding Author Wensheng Song songwengsheng@163.com School of Electrical Engineering, Southwest Jiaotong University, Chengdu, Sichuan Province, People's Republic of ChinaSearch for more papers by this authorKexin Yang, Kexin Yang School of Electrical Engineering, Southwest Jiaotong University, Chengdu, Sichuan Province, People's Republic of ChinaSearch for more papers by this author First published: 06 December 2018 https://doi.org/10.1049/joe.2018.8598Citations: 10AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Abstract Dual-active-bridge (DAB) dc–dc converters are the important part of the transformerless electric traction drive system in railway application. Also the dynamic characteristic of DAB converters in the electric traction drive system is very significant under the input voltage fluctuation. In order to improve the efficiency and enhance the input voltage step response, a direct power control with current-stress-optimised (DPC-CSO) method based on unified phase-shift control for DAB converters is proposed in this study. Compared with traditional current-stress-optimised scheme, DPC-CSO scheme can reduce current stress of switching devices and improve efficiency further. Meanwhile, it can keep the output voltage constant when it faces with the input voltage step-change. In addition, only two voltage sensors are used in DPC-CSO scheme to reduce system cost. Finally, a scale-down DAB converter experimental platform is developed, and the experimental results have verified the excellent performance of the proposed DPC-CSO scheme and correctness of theoretical analysis. 1 Introduction Dual-active-bridge (DAB) dc–dc converters are proposed at the beginning of 1990s [1], and they have been widely used in hybrid electric vehicle [2], photovoltaic systems [3-5] and electric locomotive traction [6, 7] etc., due to its salient advantages such as high power density, bidirectional energy flow, electrical isolation and ease of realising soft-switching control [8, 9]. Single phase-shift (SPS) control is the simplest and extensively used modulation method [10]. However, transferring power of the adopted DAB converter is primarily related to the auxiliary inductor, which will lead to high current stress when the input voltage and output voltage does not match [11]. The excessive current stress may result in the increase of device cost, low efficiency and even the damage of insulated gate bipolar transistor (IGBT) devices [12]. In order to reduce the current stress of DAB converters, a current-stress optimised (CSO) control with dual phase-shift control is proposed in [13]. Although the scheme can reduce the current stress significantly, but the calculated process of optimal phase-shift and on-line logic judgement are very complex. Moreover, the same optimisation method is applied to extended phase-shift control to reduce power-circulating flow and conduction losses [14]. Furthermore, a simple current stress optimisation scheme with unified phase-shift (UPS) modulation is proposed in [15]. The scheme can boost the efficiency of the DAB converters and reduce the computation complexity and alleviate the burden of controller effectively. However, the dynamic response of converters under this method is a little bit poor because only the proportional-integral (PI) controller is used to realise the power control of converters. Rapid dynamic response of the adopted DAB converters is very significant, especially for the input voltage fluctuation condition, such as in railway locomotive traction application [16]. In [17, 18], the small signal model is developed to study the dynamic characteristic of DAB converters. On the basis of this, a model-based feedback optimised strategy is discussed to enhance the dynamic response. In addition, a boundary control scheme using the natural switching surface is proposed in [19] to realise the fast transient response on-line. However, the switching frequency of the proposed scheme is variable, which may cause transformer saturation. In addition, a virtual direct power control for DAB converters is reported in [20]. Although the control scheme can improve the dynamic response effectively, the control method is based on SPS control, and effect of the scheme combined with other phase-shift modulation method needs to be further discussed. Throughout the existing literature, the optimised scheme, which can improve the efficiency and dynamic performance for DAB converters simultaneously, is rarely reported. In order to realise the improvement of efficiency and dynamic performance, a direct power control with current-stress-optimised (DPC-CSO) scheme with UPS modulation is proposed. Firstly, UPS modulation is introduced to boost the efficiency of converters, and then the dynamic response of output voltage can be further improved by combining the DPC and CSO schemes under the input voltage fluctuation condition. Finally, DPC-CSO scheme can be modified to enhance the robustness and reduce the dependence on the circuit parameters. Meanwhile, compared with traditional CSO scheme, only two voltage sensors are used in DPC-CSO scheme to reduce system cost. The paper is further organised as follows: In Section 2, the UPS control is introduced and the power characteristic is analysed in detail. The CSC scheme is developed under UPS control in Section 3. Subsequently, the integrated description of DPC-CSO scheme is shown in Section 4. In addition, the theoretic analysis and the proposed scheme are verified by the experimental comparison of traditional CSO and DPC-CSO schemes. Finally, the work is concluded in Section 6. 2 Analysis of UPS modulation The circuit topology of DAB dc–dc converters is shown in Fig. 1, which consists of two H-bridges located on the two sides of a medium-frequency isolated transformer. Where U in and U o are the input voltage and output voltage, respectively, C 1 and C 2 represent the buffer capacitor of power supply and support capacitor of load side, respectively, Uab and Ucd are the output ac voltage of two H-bridges, and R represents the equivalent load resistor, L is the storage inductor, which is the sum of auxiliary inductor and transformer leakage inductor, i o is the output current, and n is the turn ratio of isolated transformer. Fig. 1Open in figure viewerPowerPoint Topology of DAB dc–dc converters The switching sequences, voltage and current waveforms of DAB converters in UPS control are shown in Fig. 2, where T s is the switching cycle, D 1 T s /2 represents the phase-shift ratio between S 1 and S 3, D 2 T s /2 represents the phase-shift ratio between S 1 and S 5, D 3 T s /2 represents the phase-shift ratio between S 1 and S 7. In UPS control, all the phase-shift ratios are referred to the same modulation signal S 1, which is very convenient to implement in digital controllers [15]. Fig. 2Open in figure viewerPowerPoint Switching sequences, main voltage and current waveforms of DAB converters in UPS modulation In addition, operational state of converters in UPS control can be divided into three conditions according to the relationship of three phase-shift ratios D 1, D 2 and D 3 [15]: 0 ≤ D 1 ≤ D 2 ≤ D 3 ≤ 1, 0 ≤ D 2 ≤ D 1 ≤ D 3 ≤ 1 and 0 ≤ D 2 ≤ D 3 ≤ D 1 ≤ 1. Assuming the voltage conversion ratio k = U in /U o, and k ≥ 1 (the other condition k < 1 can be analysed similarly); thus the averaging transmission power of DAB converters during a switching cycle can be deduced as [15] (1) where f is the switching frequency and f = 1/T s, and the current stress of converters under UPS modulation can be obtained as (2) In order to simplify the analysis, the unified transferring power and inductor current stress of DAB converters under UPS control can be further expressed as (3) where PN and IN are the maximum transmission power and the maximum averaging input current of DAB dc–dc converters in UPS control, respectively, which can be expressed as (4) In addition, the unified transferring power and current stress of converters under SPS control can be deduced as (5) 3 Current-stress-optimised strategy in UPS control From the mathematical perspective, searching the optimal phase-shift ratios combination at the desired transferring power to minimise the current stress belongs to the extreme optimisation problem with equivalent constraint, which can be solved with Lagrange multiplier method. In order to establish the relationship between the transferring power and current stress of converters, the Lagrange multiplier is introduced and the Lagrange function can be defined as [15] (6) where E is the Lagrange function; λ is the Lagrange multiplier; and p i * is the desired transferring averaging power. Furthermore, the relationship between phase-shift ratios D 1, D 2 and D 3 to minimise current stress in UPS modulation can be obtained taking a derivative with respect to phase-shift ratios Di 1, Di 2 and Di 3 as (7) where the operational state 0 ≤ D 2 ≤ D 1 ≤ D 3 ≤ 1 is corresponding to the power range 0 ≤ p u ≤ 2(k − 1)/k 2; the operational state 0 ≤ D 1 ≤ D 2 ≤ D 3 ≤ 1 is corresponding to the power range 2(k − 1)/k 2 < p u ≤ 1, and the operational state 0 ≤ D 2 ≤ D 3 ≤ D 1 ≤ 1 is unsolvable. Specifically, the phase-shift ratios D 1, D 2 and D 3 can be further obtained based on the voltage conversion ratio and desired transferring power with (3) and (7) as (8) Meanwhile, the minimum current stress in SPS and UPS control schemes can be expressed with the voltage conversion ratio and the desired transferring power by combining (3), (4) and (8) as follows: (9) where i pu and i ps are the unified minimum current stress of DAB dc–dc converters with UPS and SPS control schemes, respectively. In order to reflect the difference of unified minimum current stress in SPS and UPS control schemes, the current stress ratio M from i pu to i ps can be defined as (10) According to (10), the 3D curves of the current stress ratio M versus to the voltage conversion ratio and desired transferring power can be obtained as shown in Fig. 3. Clearly, when the desired transferring power is constant, the current stress ratio increases with the increase of the voltage conversion ratio. Also the current stress ratio decreases with the increase of desired transferring power for given voltage conversion ratio. Fig. 3Open in figure viewerPowerPoint 3-D curves of the current-stress ratio M versus to p* and k In conclusion, compared with SPS control, the current stress of DAB converters in UPS modulation can be reduced effectively, especially for large voltage conversion ratio and light load operational condition. 4 Proposed direct power control with current-stress-optimised scheme In the traditional CSO scheme, one of phase-shift ratios is controlled with the PI controller to improve the power of converters to the desired level, and the other phase-shift ratios are calculated with the transferring power and the voltage conversion ratio to realise the optimised operation. It undoubtedly leads to the poor dynamic response of output voltage to a certain extent. In order to improve the efficiency and enhance the dynamic response of DAB dc–dc converter, a DPC-CSO scheme with UPS modulation is proposed. In view of the idea of direct power control, the unified desired transmission power can be expressed as (11) where P * can be obtained from the PI controller with output voltage and desired voltage as input value. Combining with (4) and (11), the unified desired transmission power can be further derived as (12) Meanwhile, the storage inductance L, the switching frequency f and the turn ratio of transformer n can be considered as constant values which can be omitted due to the use of integrator. Thus, the unified desired transferring power of converters can be further simplified as (13) Based on the above analysis, the proposed DPC-CSO scheme can be realised through the following steps: firstly, the input and output voltage can be sampled by using voltage sensors, and then the unified desired transmission power and voltage conversion ratio can be calculated based on (13). Finally, the optimal phase-shift ratio is estimated from (7) and the driving pulse signal can be obtained through UPS pulse modulator. Moreover, the block diagram of the proposed DPC-CSO scheme is shown in Fig. 4. Fig. 4Open in figure viewerPowerPoint Block diagram of the proposed DPC-CSO scheme The proposed DPC-CSO scheme can boost the efficiency of converters and enhance the dynamic response under the input voltage fluctuation at the same time compared with the traditional CSO scheme. Meanwhile, the DPC-CSO scheme has strong robustness and it has no dependence on the circuit parameters. Only two voltage sensors are adopted to reduce the system cost. 5 Experiments In order to verify the correctness of proposed DPC-CSO scheme, a scale-down laboratory prototype of DAB dc–dc converter is built using TMS320F28335 as core controller. The photo of experimental prototype is shown in Fig. 5. Fig. 5Open in figure viewerPowerPoint Photo of the adopted DAB converter experimental prototype Also electrical parameters are shown in Table 1. Table 1. Electrical parameters of the adopted DAB converter experimental prototype Parameters Value turn ratio of transformer n = 1 Ω switching frequency f = 10 kHz storage inductance L = 184.5 μH input-side capacitor C 1 = 2.2 mF output-side capacitor C 2 = 1.12 mF load resistor R = 30/20 Ω With the desired voltage U o * = 50 V, load resistor R = 15 Ω, Fig. 6 shows the experimental results when the input voltage steps change from 90 to 70 V. It can be seen that the transient response of output voltage in the traditional CSO scheme is very slow, over 160 ms. Also the proposed DPC-CSO scheme can reduce the settling time and voltage fluctuation significantly. Clearly, the output voltage of converters under DPC-CSO scheme is almost constant during the input voltage step-change process. Fig. 6Open in figure viewerPowerPoint Experimental results when the input voltage steps down from 90 to 70 V (a) Traditional CSO scheme, (b) DPC-CSO scheme With the desired voltage U o * = 50 V, load resistor R = 25 Ω, Fig. 7 shows the inductor current stress and efficiency of converters versus to the input voltage in the traditional CSO and DPC-CSO control schemes, respectively. Fig. 7Open in figure viewerPowerPoint Experimental curves of inductor current stress and efficiency of converters versus to the input voltage under the traditional CSO and DPC-CSO schemes (a) Current stress, (b) Efficiency Clearly, DPC-CSO scheme can further reduce the current stress and improve the efficiency of converters compared with the traditional CSO scheme. Also the proposed DPC-CSO scheme can realise the global current stress optimisation to boost the efficiency. 6 Conclusion Aimed at the DAB dc–dc converter, a DPC-CSO scheme is proposed to improve the efficiency and dynamic performance under the input voltage fluctuation simultaneously. By combining the DPC and CSO schemes, the dynamic response of output voltage can be improved effectively on the basis of achieving minimum current stress optimisation under the input voltage steps-change condition. Meanwhile, only two voltage sensors are adopted in DPC-CSO scheme to reduce the system cost. 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