Integrated interleaved active balancing converter for battery management applications

03/02/2015
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Integrated interleaved active balancing converter for battery  management applications

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application/pdf Integrated interleaved active balancing converter for battery management applications Kremena Vladimirova, Thanh Hai Phung, Fabien Mestrallet, Alexandre Collet, Jean-Christophe Crebier, Thierry Creuzet, Boris Franitch
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Integrated interleaved active balancing converter for battery  management applications

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Integrated interleaved active balancing converter for battery management applications Kremena VLADIMIROVA (1), Thanh Hai PHUNG (1), Fabien MESTRALLET (1), Alexandre COLLET (1), Jean- Christophe CREBIER (2), Thierry CREUZET (1), Boris FRANITCH (1) 1 : Freemens, 1 place Notre Dame, 38000 Grenoble, kremena.vladimirova@freemens.fr 2 : G2Elab, UGA, 38000 Grenoble, jean-christophe.crebier@g2elab.grenoble-inp.fr 2 : G2ELab, CNRS, 38000 Grenoble Abstract This paper presents an efficient and innovative concept for the optimal management of electrochemical storage systems. The basic idea is to introduce a compact and highly reliable silicon integrated real time active balancing converter into the battery, thus allowing efficient energy transfer between the series connected battery cells. The presented concept allows to guarantee permanent and real time SOC equalization among the cells while maximizing the energy potential of the battery pack as well as its lifetime extension. The design of a silicon integrated CMOS multiple legs power converter is presented. First experimental results are provided and discussed, highlighting the advantages of the proposed concept. Introduction The constantly growing needs of the portable and embedded power applications have led to the expansion of the lithium batteries market. Furthermore, lithium batteries are known as being extremely sensitive to overcharges and over- discharges and therefore require efficient battery management systems to prevent early state of health reduction and failure occurrence. Battery Management Systems are intended to observe and act on battery cells to keep them balanced and correctly used. Real time active balancing rise the opportunity to permanently keep the battery cells with identical State of Charge (SOC), simplifying significantly the battery pack management. Various balancing methods can be considered [1, 2, 3] to guarantee the voltage balance among the cells of the pack. Nevertheless, only a few topologies can meet the objectives of being simple to integrate and implement while being able to carry large currents and to perform real time active balancing. Recently, an innovative active balancing topology was proposed [4] allowing to transfer high energy quantities form any overcharged cell(s) to any undercharged cell(s), under any operating conditions applied to the stack, charge or discharge, high or low rates. In order to greatly reduce the volume of the inverter, an interleaved converter approach was also presented. Nevertheless, in that case many inverter arms are needed, thus leading to complex implementation and reliability problems due to the increased amount of active devices, drivers, supplies and interconnections. A possible solution to this issue is the monolithic integration of the active devices, their drivers and associated functions [5, 6]. This paper presents deeper investigation of this original approach for lithium battery real time active balancing. The paper focuses on the monolithic integration of an interleaved converter allowing a significant simplification of the implementation of the battery itself as well as its management system (BMS). The paper will first briefly recall the cell balancing operating principle and the topology of the active balancing converter. Then, the design of the proposed integrated converter will be presented and the resulting advantages will be discussed. The second part of the paper is dedicated to the perspective of a high level integration of the balancing converter. The design of a silicon integrated CMOS multiple power converters integrated circuit is also presented. First experimental results are provided and discussed, highlighting the advantages of the proposed concept. The last section of the paper is dedicated to the evolutions of the battery management system thanks to the introduction of real time active balancing architectures within the battery stack. Active balancing Real time active balancing is considered to guarantee voltage and/or SOC balance among series connected cells while offering access to all available energy in the stack even if the elementary cells are non- identical in terms of storage capability. There are many families of equalizing topologies for active balancing that can operate under natural or forced balancing control schemes. The natural balancing operation allows natural transfer of energy to where it is needed, without any control and measuring. The forced balancing mode controls the currents that flow inside the equalizer and the energy is delivered to the most undercharged cells. This paper focuses on a particular topology that can operate both in natural and forced cell balancing principles. The cell balancing circuit (Fig. 1) is composed of N-1 parallel converter legs, where N is also the number of cells in the pack. Each phase is connected at the potential available between two consecutive battery cells through an inductor and is able to maintain and regulate its potential as a fraction of the total voltage available across the battery stack. The cell balancing operating principle of this structure is presented with more details in [4]. Cell 1 Cell 2 Cell 3 T1 C T2 T4 T6 Cell N Tn T3 T5 Tn-1 L1 L2 L3 Ln-1 Vpack Battery stack Fig. 1: Initial active balancing topology The structure main advantages concern the bidirectional energy transfer from any cell(s) to any cell(s), the choice of natural or forced operating mode, as well as the ease of implementation and the integration feasibility due to the generic arrangement of the power devices. As for the disadvantages, the structure has very large amount of components and it presents non-optimal volume and size due to the design constraints of the passive components. Based on this consideration, it is clear that the design of the inductors must be a subject of an important optimization effort. An interesting approach that can be considered is the interleaving of the cell balancing device with coupled inductances. Considering that the duty cycles of each leg in the initial topology will operate at or near a fraction k/N, k being an integer from 1 to N-1, the interleaved coupled inductors can be greatly optimized [4]. In order to demonstrate the operation of the proposed concept, Fig.2.a) shows the practical realization with discrete devices. a) 0 10 20 30 40 50 60 2.95 3 3.05 3.1 3.15 3.2 3.25 3.3 3.35 3.4 Time (min) Cell voltages (V) Cell1 Cell2 Cell3 Cell4 Cell5 Cell6 Cell7 Cell8 b) Fig. 2: a) Photograph of an interleaved leg (40Vmax, 10Amax) of the balancing topology and b) practical results of voltage (V) as function of time (min) during natural equalization of an 8 cell battery pack. With the proposed interleaved converter the total volume of the passive components is 5 times smaller compared to ones needed for the basic converter topology (under comparable design ratings and switching frequency). As it is for the operation of the converter Fig.2.b) shows the experimental results for 60min of running the converter under natural balancing mode for a 24V - 10A.h Li-ion battery stack of 8cells with one cell fully discharged compared to the others. The results correspond to what is expected of the interleaved active balancing converter. 60 minutes after the beginning of the experimental measurement, all cells have a voltage difference of less than 100mV. Integration motivation Multiphase converters [7] offer the possibility to replace the standard magnetic cores with coupled inductors in order to reduce the inductors size and the total volume of the converter. The efficient current sharing and the better thermal management due to the sharing of the current and the resulting conduction and switching losses between the different active devices are also key advantages of these power electronic architectures. Nevertheless, as shown in Fig. 3, in that case the numbers of the active devices and the interconnections are significantly increased leading to very complex implementation and reliability problems. The solution to this issue is the monolithic integration of the active devices, their drivers and associated functions. This leads to a significant reduction of the number of power dies, drivers and PCB interconnections. Fig.3 shows a schematic view of a battery pack composed of four battery cells needed to achieve the required voltage level for the portable applications. Four inverter legs are interconnected between two neighbour cells. The required power converter is therefore composed of twelve inverter legs. Fig. 3: An interleaved cell balancing topology Design and realization of the integrated active balancing converter The integrated converter was designed to contain 12 CMOS inverter legs based on the 0.35µm CMOS high- voltage technology (20V up to 50V) from Austria Microsystems (ams). The CMOS transistors were designed with nominal current through each inverter leg of about 0,5A allowing a current flow among the cells in the range of 2A. The maximum battery stack voltage level must be kept below 20V. Fig.4 shows a photograph of the integrated active balancing converter assembled in QFN package. Fig. 4: Photograph of the 12 inverter legs all integrated in a single die, including drivers and level shifters (the die is 5*2.5mm) Practical results A prototype for four cells balancing was realized for the practical validation of the operation of the integrated active balancing converter. The prototype is shown on Fig.5. It is made out of three integrated converters allowing to perform active balancing currents up to 6A per inverter leg. The PCB integrates on one side the passive components and on the other side, the integrated converters, the microcontroller, voltage and current sensors and the required supplies. Fig. 5: Photograph of the active balancing converter Fig.6 shows the results of the realized practical tests. The characterization is focused on the top balancing structure, implemented between Cell1 and Cell2. The battery stack voltage is 12V. The structure is operating at 500kHz switching frequency with a duty cycle of 0.75. Fig. 6: Practical results (efficiency: red curve, switching losses: green curve, control part consumption: blue curve) The balancing current is 2.5A, the power transferred by the converter is 22W and as shown in Fig.6 the efficiency reached at this point is 91%. From a global point of view, the active balancing circuit power density is 3.5kW/L. Benefits of real time active balancing for battery management Real time active balancing enables to maximize the available energy from the battery stack. Moreover, it perfectly compensates cells’ State of Health (SOH) disparities by maintaining permanently their respective SOC equal. This feature is a key factor for the secure operation and optimal use of the battery. But the most unexpected benefit relies on the resulting simplification of the battery management needs. Since all battery cells are kept equally charged, no matter their operating conditions, the energy management of the battery itself can be greatly simplified. For instance, the charging unit only needs to monitor the battery stack voltage to determine correctly the level of charge of all cells. In such a way, there is no risk to overcharge the weakest element, or to enter in deep discharge when the battery is completely emptied. Real time active balancing definitively maximizes the safe operation of the battery as long as it performs effective SOC equalization among the cells of the battery stack. If this work has outlined that active balancing hardware can be a reality, its qualification still needs to be carried out. This will be further presented at the conference. Conclusions This paper presented the interest of the introduction of an integrated active balancing converter for battery management application. The active balancing topology allows to transfer high energy quantities form any overcharged cell(s) to any undercharged cell(s), under any operating conditions, thus preventing early state of health reduction and failure occurrence of the electrochemical storage equipment. In order to greatly reduce the size of the passive components an interleaved converter topology is proposed. Nevertheless, the increase of the number of the active devices in this case leads to reliability problems. Thus, a monolithic integrated version of the active balancing converter is proposed. The concept is based on the integration of the active devices and their associated drivers in the same silicon die. The practical results demonstrated that proposed active balancing concept is a key enabling factor for optimal battery management, maximizing the safe operation of the battery stack. References 1 Jian Cao et al, “Battery Balancing Methods: A Comprehensive Review”, IEEE Vehicle Power and Propulsion Conference (VPPC), September 3-5, 2008, Harbin, China 2 Stephen W Moore et al, “A Review of Cell Equalization Methods for Lithium Ion and Lithium Polymer Battery Systems”, 2001 Society of Automotive Engineers 3 T. H. Phung et al, "Optimized Structure for Next-to- Next Balancing of Series-Connected Lithium-ion Cells", IEEE APEC 2011, 2011, USA 4 F. Mestrallet et al, "Multiphase Interleaved Converter for Lithium Battery Active Balancing", IEEE APEC 2012 5 K. Vladimirova et al, “Single Die Multiple 600V Power Diodes with Deep Trench Terminations and Isolation”, IEEE Trans. on Power Electronics 2011, vol 26, issue 11, pp:3423-3429 6 Van Nguyen et al,” A new compact, isolated and integrated gate driver using high frequency transformer for interleaved Boost converter”, IEEE ECCE2011,pp:1889-1896 7 T. Meynard et al, “Multicell converters: basic concepts and industry applications”, IEEE Trans. on Industrial Electronics 2002, vol 49, no. 5, pp:955-964