Performance Evaluation of Isolated Bi-directional DC/DC Converters with Buck, Boost operations MD.Munawaruddin Quadri *1, Dr.A.Srujana *2 #1 PG student, Power Electronics Department, SVEC, Suryapet, Nalgonda, Telangana, India. #2 Professor and head of the department, Power Electronics Department, SVEC, Suryapet, Nalgonda, Telangana,India Abstract In this paper a new bidirectional dc to dc converter with two bridges is proposed. Operational analysis of converter is done with different voltage levels, both buck and boost converters are designed. The two bridges are designed for constant gain and electrical isolation. The batteries are taken as input and output voltage sources. A control strategy of PI controller is proposed for the generation of switching pulses to the converter switching devices, MOSFETs are used as switching devices in this paper. The MOSFETs are operated at Zero Voltage Switching condition. The comparative analysis is done with conventional converter. The performance of proposed bidirectional dc to dc converter is verified in MATLAB/simulink software. The converter is having high switching frequency and it is one the type of series resonant converter. Index terms - DC/DC converter, bidirectional operation, battery source, high current operation, high efficiency, and buck, boost operation conditions. I. INTRODUCTION T he bi-directional converters are having application of transfer of energy in between two voltages sources at different levels [1]. The dc to dc converter with current ripple reduction is proposed in this paper. Numerous different topologies for isolated, bi-directional DC/DC converters have been previously described but no comprehensive comparative evaluation of the systems has been presented in the literature so far. The resonant converter with two bridges takes in charge of electrical isolation and constant gain, and the bidirectional control is accomplished using only the second stage with a single bridge. The converters are referred to as topology DAB1 and DAB2 for the dual active bridge (DAB) converter and SRC1 and SRC2 for the series resonant converter (SRC). The schematic circuit of bidirectional dc/dc converter is shown in fig.1. Fig.1. schematic circuit of bi-directional dc to dc converter All the topologies discussed in this paper comprise a similar isolation stage implemented with a full bridge converter on the primary side (low voltage side) and on the secondary side (high voltage side) of the transformer. The dual active bridge utilizes the leakage inductance L as a buck or boost inductor to cover the desired output voltage range. Adding a capacitor in series to the leakage inductance leads to the series resonant converter which can also be operated with bidirectional power flow within a wide range of input and output voltages. Another category of bi-directional converters are topologies with a current-fed full bridge arrangement on the low voltage side. These converters have reduced rms switch and transformer current values at the expense of higher voltage ratings of the switches [2]. A control strategy is proposed in this paper with PI controller interleaved with PWM controller, for the operation of the converter at constant high frequency, a constant frequency pulse generator is designed. For the generation of reference voltage and current s for the PI controller forward voltage and current controller, reverse voltage and current controller is proposed in this paper. IJCSIET-ISSUE4-VOLUME2-SERIES4 Page 1
In this paper, Section II describes designing converter topologies and operating principle. The control strategy of bi-directional converter is studied in section III. Section IV describes the simulation results of converter. And finally section V concludes with future scope. II. DESIGN AND OPERATION OF CONVERTER In this section designing of bi-directional converter and operational analysis is done. In this paper the bidirectional converter is designed as to operate in both in buck (step-down) and buck (step-up operation). The operation of the proposed converter is explained in this section. The designed topology of bidirectional dc/dc converter is dual active bridge converter, it can be operated in both buck and boost condition [3]. Step-up and step-down operations depends on the transformer turns ratio. The input voltage is multiplied with transformer turns ratio of linkage inductor. The converter is operated in the triangular current mode of operation. The duty cycle generated from the controller for primary and secondary is smaller than 50%, it allows operating in low switching loss condition [4]. Transformer turns ratio N is For achieving of specified input and output voltage values are depends on the transformer ratio n, and inductor L, for minimum rms transformer current. At the time instant t 0, the voltage applied to the transformer primary side that generates current. And at time instant t 1, with voltage V 2 the secondary side bridge is turned on and transformer current starts to decrease and becomes zero at t 2. The next cycle is stars at time t 3, the power transferred in the circuit gets reversed, when time intervals t 1 and t 2 are transposed. This limitation can be circumvented with trapezoidal current mode modulation by allowing a certain switch current IL2 = il(t2) at time instant t2. The waveforms for transformer current il and voltages vt1 and vt2 in again correspond to a power transfer from the low voltage side to the high voltage side. Expect transformer current i L is greater than zero at time instant t 2, time intervals T 1 and T 2 are identical to those of the triangular current mode operation [5]. The amount of transferred power is depends on switching frequency f s, primary and secondary dc voltages V 1 and V 2, turns ratio n of transformer, converter inductor L, and primary side transistor current I L2. N = Converter design The designing method of proposed converter is based on the achieving of low switching losses. The converter designing is depends on the inductor value, further inductor value s depends on the transformer ratio (N). Here n =N. The requirement for the converter to be operated within a wide input and output voltage range results in operation regions of inefficient switch and transformer utilization. Therefore, a two stage solution is considered which consists of two separate conversion stages: an isolation stage and a non-isolated voltage converter. Initially four different possibilities were investigated: Primary side voltage converter with V 1 < V i Primary side voltage converter with V 1 > V i Secondary side voltage converter with V i <V 2 Secondary side voltage converter with V i >V 2 IJCSIET-ISSUE4-VOLUME2-SERIES4 Page 2
The first two methods are having low conversion efficiency for primary side voltage converters, below 85% for the boost converter when operated with an output power rating 2kW [6]. The fourth proposal is implemented in this paper. To get higher efficiency for the converters primary voltage should be near to reflected secondary voltage V 2 /n. To get balanced application of two converter stages, the isolation stage is operated with amplification of V 1 >V 1,0. III. PROPOSED CONTROL STRATEGY The bidirectional dc/dc converter is used for the transferring of energy between two batteries at different voltage levels. For the efficient operation of the bidirectional converter in under different circumstances controlling system must be designed with maximum energy conversion ratio. Conversion ratio of the converter in depends on the duty cycle given to switching devices. In the proposed paper, for the generation of switching pulses to MOSFETs, the PI controlling technique is proposed. The PI (proportional, integral) controller is adopted for the closed loop operation for the efficient transfer of energy between two batteries. In this paper the bidirectional dc/dc converter with PI controller, PWM technique is proposed [7]. The block diagram for the generation of switching pulses for MOSFETs is shown in fig.2. Here V 1, 0 is the voltage transformation ratio of the isolation stage needed for maximum input and output voltage, V 1 the amplification of the isolation stage And v2 the amplification of the non-isolated voltage converter With this, the transistors are utilized up to the maximum operating voltage in order to reduce the secondary side currents. Fig.2. control block diagram The designing of control circuit for the bidirectional DC/DC converter in the MATLAB/simulink is having the combinational circuit of PWM pulse generator and constant frequency pulse generator. The generation of the switching pulses for the MOSFETs M1, M3, M2, and M4 are from constant frequency pulse generator circuit, and for the MOSFETs M5, M6 are by using PI controller. Simulation model circuit for the bidirectional DC/DC converter is shown in fig.3. IJCSIET-ISSUE4-VOLUME2-SERIES4 Page 3
IV. SIMULATION RESULTS The proposed bidirectional dc/dc converter performance is verified by designing in MATLAB/simulink software. In the designing the energy transferred between two batteries. The bidirectional converter is designed in buck (stepdown), step-up (boost) operating conditions. Bi-directional dc/dc converter The proposed converter has been designed in step-down operation with high voltage battery nominal Fig.4. simulation model of control circuit voltage (VH) = 200v, rated capacity = 2000Ah and low voltage battery nominal voltage (VL) = 24v, rated capacity = 300Ah. The linear linkage transformer between two bridges is connected with nominal power 2kW and nominal frequency 5 MHZ. with primary inductance 12* H and magnetizing inductance Lm = 165* H. simulation model for bi-directional dc/dc buck converter is shown in fig.5. Fig.5. simulation model of bi-directional DC/DC converter. IJCSIET-ISSUE4-VOLUME2-SERIES4 Page 4
The simulation results for proposed bidirectional dc/dc converter are described as below. The voltage value of low voltage battery connected at secondary (V L ) is 26.13v and primary battery at lower voltage side (V H ) is 218.3. The simulation wave forms of higher and lower voltage are shown in fig.6. Simulation wave forms of voltage levels across the MOSFETs connected in the first leg of the h- bridge are shown in fig.8. And current in the primary winding of the linear transformer connected in H-bridge is shown in fig.9. Fig.6. simulation wave forms of higher (V H ) and lower voltage (V L ) The current values of bi-directional dc/dc buck converter at primary battery (I VH ) and secondary battery connected (I VL ), when simulation model is designed are shown in fig.7. Fig.8. simulation waveforms of MOSFETs voltages at primary leg. Fig.7. simulation wave forms of currents at higher voltage battery side (I VH ) and lower side (I VL ) Fig.9. simulation waveform of current at primary winding of linear transformer. IJCSIET-ISSUE4-VOLUME2-SERIES4 Page 5
V.CONCLUSION The bidirectional isolated dc/dc converter for the transfer energy conversion between two energy sources like batteries at two different levels is proposed in this paper. The relation between input sources to output sources is derived for the efficient energy conversion. A simple control technique is proposed for bidirectional dc/dc converter. A control system with constant frequency pulse generator and PWM technique is evaluated. The controller is designed as that it can operate in both step-up and step-down operations. The converter is having high efficiency and it will operate with high switching frequency of 5MHZ. The proposed bidirectional isolated dc/dc converter efficiency is achieved up to 91.7% in MATLAB/simulink software. REFERENCES [1] M. J. Vasallo, J.M. Andujar, C. Garcia, and J. J. Brey, A methodology for sizing backup fuelcell/battery hybrid power systems, IEEE Trans. Ind. Electron., vol. 57, no. 6, pp. 1964 1975, Jun. 2010. [2] C. S. Leu and M. H. Li, A novel current-fed boost converter with ripple reduction for highvoltage conversion applications, IEEE Trans. Ind. Electron., vol. 57, no. 6, pp. 2018 2023, Jun. 2010. [3] J. Wang, L. Zhu, D. Qu, H. Odendaal, J. Lai, and F. C. Lee, Design, implementation, and experimental results of bi-directional fullbridge DC/DC converter with unified softswitching scheme and soft-start capability, in Proc. Power Electron. Spec. Conf., 2000, pp. 1058 1063. [4] N. Schibli, Symmetrical multilevel converters with two quadrant DCDC feeding, EPFL, Thèse Nr. 2220, pp. 99-171, 2000. [5] M. B. Gerber, On packaging techniques for a high power density DC/DC converter, M.Sc. thesis Johannesburg, South Africa: Rand Afrikaans Univ., Dec. 2001. [6] A. Levy, Comparing the push-pull, forward converter and full bridge topologies in low voltage, high power DC-DC converter, High Frequency Power Converter Conference (HFPC) proceedings, pp. 470-476, September 2001. [7] J. Miniböck, J. W. Kolar, Experimental analysis of the application of latest SiC diode and CoolMOS power transistor technology in a 10kW three-phase PWM (VIENNA) rectifier., Power Electronics Conference (PCIM) proceedings, pp. 121-125, June 2001. MD.Munawaruddin Quadri: He received the B.Tech degree in electrical engineering from JNTU Hyderabad University in 2012 and currently he pursuing M.Tech in power electronics from Sri Venkateswara Engineering College, Suryapet,Nalgonda. His interests include power electronics, distribution level control and renewable energy systems. Dr.A.Srujana has obtained her B. Tech degree from KITS, Warangal, in 1998 and M.Tech degree From J.N.T.U, Hyderabad, India, in 2002.She has obtained her Ph.D from J.N.T.U, Hyderabad in 2011. She has 14 years of teaching experience. Presently she is working as Professor & H.O.D E.E.E at Sri Venkateswara Engineering College, Suryapet. IJCSIET-ISSUE4-VOLUME2-SERIES4 Page 6