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Current Sharing in Multiphase Interleaved Converters by Means of One Current Sensor Jens C. Schroeder, Marinus Petersen, Friedrich W. Fuchs Institute of Power Electronics and Electrical Drives Christian-Albrechts-University of Kiel Kaiserstr. 2, 24143 Kiel, Germany Email: [email protected], [email protected], [email protected] Abstract To improve the efficiency and to reduce the volume in dc-dc converters and their filters, interleaved converters are used. In these multiphase converters, appropriate active current sharing is necessary due to the inherent current inequality caused by the parameter deviations between the components and the dutycycles. The common method to realize current sharing is to use a single current sensor for each phase whereas the current in each phase is controlled independently. Due to the costs of the current sensors, a reduction is mandatory. In this contribution, a method is presented which allows the current sharing by means of only one current sensor without extra circuitry and computation effort. This method is analyzed and the success is verified in practice. 1 Introduction The main issues in electric vehicles are the maximization of range and the lifetime of the costintensive battery. In a common electric propulsion system, the battery is connected to the voltage source inverter (VSI) which feeds the machine. So the battery has to deliver or absorb all the power which is demanded or recuperated by the propulsion system in all operating points. Due to the internal battery resistance, the battery voltage varies significantly depending on the battery current. Operating under high currents, the lifetime decreases in an accelerated way and in general the recuperation efficiency is poor. To optimize battery lifetime, efficiency and voltage stabilization, a battery buffer system (BBS) can be implemented [1, 2]. The block diagram of an exemplary system can be seen in fig. 1. To improve the efficiency and to reduce the volume, a threephase interleaved converter is used here, which is shown in fig. 2. Interleaved converters show several benefits compared with conventional converters. Especially in automotive applications, the resulting volume reduction and the efficiency improvement by interleaved operation are fundamental [3, 4, 5]. Interleaved converters consist of N parallel arranged identical topologies whose pulse width modulation (PWM) is phase shifted 360◦ /N against one another. Interleaved operation results in the benefit of current ripple reduction and inductor volume reduction [4]. The components in an N-phase interleaved converter should be rated for 1/N part of the current. Due to possible component and dutycycle deviations between the different phases, the current might be not divided equally without an active current sharing. For a safe operation, an active current sharing is mandatory to load all phases with the same current. The common way to realize current sharing is to use N single current sensors, i.e. one for each phase, whereas the current in each phase is controlled independently. To minimize costs and CK + - Ibat Vlink Buffer System Ld Inverter Ud AC DC M 3~ Ud = Induction Motor IDCDC IL2 L2 DC DC EDLC IEDLC Ud IL1 L1 IEDLC VEDLC Id Power Management Figure 1: Lift truck propulsion system IDCDC IL3 L3 VEDLC EDLC Battery Vlink Figure 2: Three phase interleaved converter the complexity of the converter, the goal is the reduction to only one single current sensor. In this work, a threephase interleaved converter is optimized, which is used in an electric lift truck to connect a 24 V lead acid battery and electric double layer capacitors (EDLC) in a battery buffer system. The system power is 5 kW, the EDLC voltage level is in the range of 26V -38V. Several authors have designed methods of current sharing in the past. In [6], a method is presented in which a current sensor is switched between the different phases. Due to the additional switches, the component number increases considerably. In [7], a master slave method is developed. All slave phases have to be operated in discontinuous current mode there, thus it cannot be implemented in continuous current mode. The author in [8] measures the voltage at the output capacitor and can detect a phase unbalance via a fourier analysis, [9] adapts the method. This method demands high calculation capability and very fast sampling. In [10] a current sharing method for a two phase interleaved converter is presented. The current through the highside MOSFETs is measured by one sensor. By symmetrical sampling during a precalculated timestep, the phase currents can be detected and controlled independently for a limited range of dutycycles. This method is extended here by further investigations to realize a current sharing in a three phase interleaved converter with the given margins of the battery buffer system. In section 2, the demand for current sharing is shown. In section 3 the new current sharing method is introduced and analyzed, simulation results are presented. In section 4, measurement results verify the benefit of the new method and in section 5 a conclusion is given. 2 Verification of the demand for current sharing At first, the demand for a current sharing control is shown. Therefore, the current characteristics are presented for a system without current sharing. In this system, the target dutycycle in all three phases is the same. An open loop control or a closed loop control with a feedback of the resulting current can be used. To validate the consequences of such a control in practice, some deviations in the different phases are set. The phase resistances, for instance in the inductor, can deviate as well as the inductance and the dutycycle which is applied at the gates of the MOSFETs. In fig. 3 the current characteristics are shown in case of phase inequalities. In (a) a 1 % dutycycle increase in phase two is set, in (b) a 20 % increase of the phase resistance. It is obvious that especially an inequality in the dutycycles, which is rather a problem in analogue control circuits, results in high deviations between the phase currents. Phase resistance inequalities cause problems as well. Inductance deviations do not influence the average current, they only result in different current ripples. Inequal current distribution causes higher losses in the converter, and components which are not rated for currents higher than the maximum expected average could be destroyed in the worst case. Thus, a current sharing is needed which guarantees equal current distribution between the phases. 0.6 0.6 Phase 1 Phase 2 Phase 3 0.55 0.5 0.45 0.45 0.4 0.4 I [p.U.] I [p.U.] 0.5 0.35 0.35 0.3 0.3 0.25 0.25 0.2 0.2 0.15 0.15 0.1 160 162 164 t/T (a) 166 168 Phase 1 Phase 2 Phase 3 0.55 0.1 160 170 162 164 t/T (b) 166 168 170 Figure 3: Simulated current characteristics without current sharing: (a) 1 % dutycycle increase in phase 2 (b) 20 % resistance increase in phase 2 To get a reference on how the current can be shared under perfect circumstances, fig. 4 is shown. The currents of the inductors are measured by three sensors and controlled independently in each phase, thus the different phases carry the same current despite deviations in the dutycycle or the phase resistance. The goal is to achieve this behavior by means of only one current sensor. 0.6 0.6 Phase 1 Phase 2 Phase 3 0.55 0.5 0.45 0.45 0.4 0.4 I [p.U.] I [p.U.] 0.5 0.35 0.35 0.3 0.3 0.25 0.25 0.2 0.2 0.15 0.15 0.1 160 162 164 t/T (a) 166 168 Phase 1 Phase 2 Phase 3 0.55 170 0.1 160 162 164 t/T (b) 166 168 170 Figure 4: Simulated current characteristics with current sharing (3 sensors): (a) 1 % dutycycle increase in phase 2 (b) 20 % resistance increase in phase 2 3 Reducing the number of current sensors The challenge is to realize current sharing with only one current sensor. In [10] a method to realize current sharing in a twophase interleaved converter is presented in which the resulting current through the highside MOSFETs is measured. The topology is shown in fig. 5a. Due to the interleaving of the currents, each current can be measured independently during the time when only one current is passing the highside MOSFETs, which is during the freewheeling time interval in boost mode and during the switch-on time interval in buck mode. The limitations concerning the allowed operating dutycycles are presented as well: The minimum dutycycle in boost operation is dependent on the number of phases and the ratio between the minimum time Tmin,AD which is needed by the AD-converter to measure the current and the period time TP . dmin can be calculated in (1). The maximum dutycycle in boost operation can be calculated with (2) [10]. dmin,boost = N − 2 Tmin,AD + N TP W M dmax,boost = 1 − (1) Tmin,AD TP W M (2) In a threephase interleaved converter which is used in the lift truck buffer system, operated at a switching frequency of 16 kHz and an estimated Tmin,AD of 5 µs, it results in dmin,boost =0.413 and dmax,boost =0.92. The EDLC voltage level, which is within the range of 25V -38V, is always higher than the battery voltage, which is around 24V. The operated dutycycle can be calculated via (3). Vlink d=1− (3) VEDLC It results in 0.08 < d < 0.37. Thus, this method cannot be used to realize current sharing in this system. Now the idea is to change the location of the current sensor to the common branch below the lowside MOSFETs like it is shown in fig. 5b. A IEDLC IEDLC IDCDC VEDLC IL3 L3 EDLC IL2 L2 IL2 L2 IDCDC VEDLC IL3 L3 EDLC IL1 L1 IL1 L1 Vlink Vlink A (a) ILS (b) Figure 5: Topology of converter with current measurement (a) above highside MOSFETs (b) below lowside MOSFETs In fig. 6, the current characteristics of the converter are shown for one period both for boost and for buck mode. ILS exceeds zero only during conducting of the lowside switches, which is during the switch-on time in boost mode and during switch-off time in buck mode. If the sampling of the current is done in the middle of the respective current slope, the average currents of the three single phases can be detected by only one current sensor. The minimum and maximum dutycycles can be calculated by (4) and (5). VG, M2 VG, M1 VG, M4 VG, M3 VG, M6 VG, M5 IL1 -IL1 IL2 -IL2 IL3 -IL3 ILS ILS topt,1 1 3 T topt,2 (a) 2 3T topt,3 Tt topt,2 1 3 T topt,3 (b) 2 3T topt,1 T t Figure 6: Current characteristics in a three phase interleaved converter and optimum sample time (a) boost mode (b) buck mode dmin,boost Tmin,AD = TP W M (4) dmax,boost = 1 − N − 2 Tmin,AD + N TP W M (5) For the margins of the three phase converter in the lift truck propulsion system, it results in 0.08 < d < 0.587, so it can be applied here. The optimal sampling times topt,boost and topt,buck for continuous current operation can be calculated by (6) and (7), whereas i is the phase number: topt,boost = T · d i−1 + 2 N (6) topt,buck = T · d 1 i−1 + + 2 2 N (7) In fig. 7, the simulation results are presented, again for an inequality in the dutycycle and the phase resistance. It is shown that the current sharing control works well and that the characteristics are nearly the same as in the three sensor current sharing. This nearly perfect behavior is caused by the fact that each current can be controlled seperately. The only drawback of this kind of current sharing method is the location of the current sensor. It has to be implemented between the lowside MOSFETs and the ground plane, which is within the commutation path of the semiconductors. Thus, on-circuit sensors have to be used to keep the stray inductance as low as possible. 0.6 0.6 Phase 1 Phase 2 Phase 3 0.55 0.5 0.45 0.45 0.4 0.4 I [p.U.] I [p.U.] 0.5 0.35 0.35 0.3 0.3 0.25 0.25 0.2 0.2 0.15 0.15 0.1 160 162 164 t/T (a) 166 168 Phase 1 Phase 2 Phase 3 0.55 170 0.1 160 162 164 t/T (b) 166 168 170 Figure 7: Simulated current characteristics with current sharing by means of one sensor below the lowside MOSFETs: (a) 1 % duty cycle increase in phase 2 (b) 20 % resistance increase in phase 2 4 Laboratory setup and measurement results The developed current sharing method is implemented in the laboratory test bench in an FPGA. The FPGA generates the threephase interleaved PWM and realizes the symmetric sampling of the three currents. IRFS3006-7PPbF MOSFETs (60V /240A ) are chosen. The printed circuit board (PCB) of the converter with the Allegro ACS758xCB current sensor is shown in fig. 8. The symmetrical sampling in boost mode is shown in fig. 9. The AD-converter sampling is started at the falling edge of the trigger signal of CH3. It is shown that there are three symmetrical samplings in every period and that the output signal of the current sensor displays the rising current slopes of the different phases. Figure 8: PCB of the three phase interleaved converter; red circle marks the current sensor between lowside MOSFETs and power ground In the measurement results in fig. 10 it can be seen that the currents are not equally divided for both unbalanced cases without current sharing. By means of the single sensor current sharing operation, the currents can be controlled independently and so they can be divided equally. Figure 9: Symmetrical current sampling in boost mode for d=0.4; CH1: Phase current (5 A/div); CH2: Current sensor signal (200 mV/div); CH3: AD-converter SPI chipselect signal (500 mV/div); time: 10 µs/div Thus, the success of the method is shown. (a) (b) (c) (d) Figure 10: Measured current characteristics (20 A/div, 4 ms/div) (a) unbalanced equivalent serial resistances without current sharing (b) unbalanced equivalent serial resistances with current sharing (c) unbalanced equivalent serial resistances without current sharing (d) unbalanced equivalent serial resistances with current sharing To analyze the influence of the current sensor in the commutation path to the MOSFETs overvoltage during switch-off, further measurements are performed. First, a maximum voltage of 66.8V occurs during maximum EDLC voltage operation of 36V. The voltage can be reduced by an additional 10 nF capacitor, which is applied between the drain source connectors of the MOSFETs. The maximum voltage can be reduced to 54.4V, which is acceptable. 5 Conclusion In this contribution, the demand for current sharing in a threephase interleaved converters has been shown and a method has been investigated to realize current sharing in a threephase interleaved converter by means of only one current sensor and without further effort. Simulation results show the ability of this method to replace the three sensor current sharing without further complexity or drawbacks. A three phase interleaved converter with an on-circuit current sensor between the lowside MOSFETs and power ground has been built up in the laboratory and its functionality has been successfully approved. The occuring overvoltage during switching due to the current sensor in the commutation path has been analyzed and the operation could be satisfactorily improved by an additional capacitor. Thus, the single sensor current sharing has been succesfully designed and tested and can be implemented in the lift truck’s battery buffer system. References [1] M. Ortuzar, J. Moreno, and J. Dixon. Ultracapacitor-based auxiliary energy system for an electric vehicle: Implementation and evaluation. Industrial Electronics, IEEE Transactions on, 54(4):2147 –2156, aug. 2007. [2] M.B. Camara, H. Gualous, F. Gustin, and A. Berthon. Design and new control of dc/dc converters to share energy between supercapacitors and batteries in hybrid vehicles. Vehicular Technology, IEEE Transactions on, 57(5):2721 –2735, sep. 2008. [3] M. Hirakawa, M. Nagano, Y. Watanabe, K. Ando, S. Nakatomi, S. Hashino, and T. Shimizu. High power density interleaved dc/dc converter using a 3-phase integrated close-coupled inductor set aimed for electric vehicles. In Energy Conversion Congress and Exposition (ECCE), 2010 IEEE, pages 2451 –2457, sept. 2010. [4] J.C. Schroeder, B. Wittig, and F.W. Fuchs. High efficient battery backup system for lift trucks using interleaved-converter and increased edlc voltage range. In IECON 2010 - 36th Annual Conference on IEEE Industrial Electronics Society, pages 2334 –2339, nov. 2010. [5] J.C. Schroeder and F.W. Fuchs. Design of a powermanagement for a battery buffer system in an electric lift truck by means of fuzzy control and genetic algorithm. In Power Electronics and Applications (EPE 2011), Proceedings of the 2011-14th European Conference on, pages 1 –10, 30 2011-sept. 1 2011. [6] R.P. Singh and A.M. Khambadkone. Current sharing and sensing in n-paralleled converters using single current sensor. Industry Applications, IEEE Transactions on, 46(3):1212 –1219, may-june 2010. [7] L. Huber, B.T. Irving, C. Adragna, and M.M. Jovanovic. Implementation of open-loop control for interleaved dcm/ccm boundary boost pfc converters. In Applied Power Electronics Conference and Exposition, 2008. APEC 2008. Twenty-Third Annual IEEE, pages 1010 –1016, feb. 2008. [8] G. Eirea and S.R. Sanders. Phase current unbalance estimation in multiphase buck converters. Power Electronics, IEEE Transactions on, 23(1):137 –143, jan. 2008. [9] S. Mariethoz, A.G. Beccuti, and M. Morari. Model predictive control of multiphase interleaved dc-dc converters with sensorless current limitation and power balance. In Power Electronics Specialists Conference, 2008. PESC 2008. IEEE, pages 1069 –1074, june 2008. [10] Hongrae Kim, M. Falahi, T.M. Jahns, and M.W. Degner. Inductor current measurement and regulation using a single dc link current sensor for interleaved dcdc converters. Power Electronics, IEEE Transactions on, 26(5):1503 –1510, may 2011.