Design and Development of Bidirectional DC-DC Converter using coupled inductor with a battery SOC indication

Design and Development of Bidirectional DC-DC Converter using coupled inductor with a battery SOC indication Sangamesh Herurmath #1 and Dr. Dhanalakshmi *2 # BE,MTech, EEE, Dayananda Sagar institute of Technology, Bangalore, Karnataka India. * Professor,EEE,Dayananda sagar institute of Technology, Bangalore, and Karnataka India Abstract: - Renewable energy is becoming more important now a day. High gain DC/DC converters are the key part of renewable energy systems, Thus to reduce the effect of leakage-inductance bidirectional DC-DC converters are developed where the battery can balance the energy between the power source and the load. But the voltage difference between the battery and the DC bus is large, thus, a bidirectional DC-DC converter with high step-up/down voltage conversion ratio is required. Earlier proposed bidirectional DC-DC converters use many active components, which not only increase the cost and conduction losses of the converter, but also complicate the control circuit. The converter acts as two-stage boost converters, controlling one power switch to achieve high voltage step-up conversion in discharging mode, and the converter acts as two cascaded buck converters that control two power switches simultaneously to achieve high voltage step-down conversion in charging mode. The operating principles and analysis of the steady-state characteristics are discussed in great in detail. Finally, a prototype of 12/200V circuit is implemented to verify the feasibility of the proposed converter. One of its important functions is to execute algorithms that continuously estimate battery state-of-charge (SOC) and available power. Index Terms Bidirectional converter, high conversion ratio, coupled-inductor,soc indication I. INTRODUCTION Since the usage of the non renewable fuel results in environmental pollution, the clean energies become very important in the world. In recent years, the renewable energy systems, including photo-voltaic systems, fuel-cell systems, wind-power generating systems, are developed rapidly. Because the renewable systems cannot provide a stable power for user, the renewable energy systems and battery can be utilized for the hybrid power systems. When the renewable energy systems cannot supply enough power for the load, the battery must supply that insufficient power. The whole power of the renewable energy systems cannot be used completely by the load, the surplus energy can be used to charge the battery. The bidirectional DC-DC converter is widely used in renewable energy applications because it plays an important 1012 Role in system back-up or in reserving energy for the battery. These converter are able to transfer or balance energy between two different DC sources. The renewable energy systems like solar and fuel cells are the source with low voltage. The low voltage can be boosted to high voltage by using a step-up dc-dc converter and also to combine ac utility voltage [1], [2]. A step-up dc-dc converter is expected to give high efficiency. Many topologies have been proposed to improve the efficiency and achieve high step-up voltage gain.using a Voltage-lift technique, a switched capacitor circuit, boost-fly back step-up converter; coupled inductors, switched coupled inductor cell and voltage lift technique. [5]- [6] These earlier proposed topologies of high step-up converters help to increase the efficiency, but the influence of the leakage inductor is always neglected. The topology titled A NOVEL HIGH STEP-UP DC-DC CONVERTER WITH COUPLED-INDUCTOR [7] utilizes coupled-inductor and a voltage doubler circuit to achieve high step-up voltage gain. The voltage stress of power switch is reduced by a passive clamp circuit, but the leakage inductance is not completely absorbed by using this topology. The bidirectional converter use coupled-inductor technology [8] to achieve a high voltage conversion ratio. However, the energy stored in the leakage inductor of the coupled inductor causes a high voltage spike on the power switches. The bidirectional dc-dc converter is often used to transfer the solar energy to the capacitive energy source during the sunny time, while to deliver energy to the load when the dc bus voltage is low. The bidirectional dc-dc converter is regulated by the solar array photovoltaic level, thus to maintain a stable load bus voltage and make fully usage of the solar array and the storage battery. The battery can balance the energy between the power source and the load. The voltage difference between the battery and the DC

bus is large, thus, a bidirectional DC-DC converter with high step-up/down voltage conversion ratio is required. The conventional boost/buck bidirectional converter can be used for such applications, but it is not practically suitable because the conversion ratio will be significantly reduced by parasitic elements. The proposed bidirectional converter includes dual boost/buck converter to achieve a high voltage conversion ratio by employing a coupled-inductor technique, the Mode1: proposed topology has the following features: 1)Boost/Buck structure achieves high voltage conversion ratio at step-up or step-down stage. 2) An effectively simplified control circuit. 3) The leakage-inductance energy of the coupled inductor is recycled, thus reducing the voltage stress on power switches. 4) A low R DS -ON switch can be selected to improve system efficiency. Fig 1 circuit operation in mode 1 In mode 1, switch S1 and diode Ds3 are switched on. Fig 1 shows the equivalent circuit. The energy stored in the leakage inductor LK2 is released to capacitor C2, and ILk2, (IS3) are reduced gradually. The battery voltage VL releases energy into the leakage inductor LK1. Thus, the leakage inductor current ILk1 rapidly increases. Moreover, the magnetizing inductance current ILm is equal to ILk1 + n*ilk2, where n = NS/NP. Mode 1 ends when the current IS3 is decreased to zero and the diode DS3 is switched off. Mode2: Fig 2 circuit operation in mode 2 Fig. 1 proposed bidirectional converter configuration II.PROPOSED CONVERTER & MODE OF OPERATIONS The proposed converter is used for the bidirectional transfer of energy between the low voltage side VL, which is connected to a 24 V battery, and the high voltage side VH, which is connected to a 180 V DC bus. Fig. 1 shows the proposed converter circuit with leakage inductances and major current paths. The following conditions were assumed in analyzing the steady-state characteristics of the proposed converter. 1. All the circuit components are ideal. 2. The capacitors CL, C2, and CH are large enough, and the voltages can be treated as constant. 3. The magnetizing inductance Lm of the coupled inductor is large enough, and the converter is operated. Helpful Hints MODES OF OPERATIONS Discharging mode: In mode 2, S1 and D4 are switched on. Fig 2 shows that the equivalent circuit. VL charges the magnetizing inductor Lm and the leakage inductor LK1. The magnetizing-inductor current ILm and the leakage-inductor current ILk1 are increased linearly. In addition, VL transfers its energy into C2 via the secondary winding NS and D4. Thus, the voltage across C2 is charged to nvl. This mode ends when S1 is switched off. Mode 3: Fig 3 circuit operation in mode 3 1013

In mode 3, S1 and DS3 are switched off and DS2 is turned on. Fig 3 shows the equivalent circuit. The energy of the leakage inductors LK1 and LK2 are released into C2 through DS2 and D4, respectively. This mode ends when the current ILk2 and ID4, is equal to zero and D4 is switched off. Mode 4: In mode 6, S1 and DS2 are switched off and DS3 is turned on. Fig 6 shows the equivalent circuit. The energy of Lm is released to CH and RH via the secondary side of the coupled-inductor and DS3. The energy stored in C2 is also transferred to CH and RH. This mode ends when S1 is switched on. Waveform of Discharging Fig 4 circuit operation in mode 4 In mode 4, S1 is switched off and DS2 and DS3 are turned on. Fig 4 shows the equivalent circuit. The energies of VL, Lm, and LK1 are released into C2 through DS2. Moreover, part of the Lm energy is transferred to CH and load RH via the secondary side of the coupled inductor. This mode ends when the voltage across C2 is equal to nvin. Mode 5: Charging Mode: Fig 7 waveform of discharging Power switches S2 and S3 are controlled simultaneously and S1 is off. Mode 1: Fig 5 circuit operation in mode 5 In mode 5, S1 is switched off and DS2 and DS3 are turned on. Fig 5 shows the equivalent circuit. The energy of Lm is released into CH via the coupled-inductor and DS3. The ILm decreases linearly, and the energy stored in C2 is transferred to CH and RH. This mode ends when ILk1 is equal to zero. Mode 6: Fig 8 circuit operation in mode 1 In mode 1, S1 is switched on. Fig 8 shows the equivalent circuit. Lm releases its energy to capacitor CL and load RL. The magnetizing current ILm decreases linearly. The energy stored in the leakage inductor Lk2 is recycled to C2. This mode ends when the current ID4 is reduced to zero. Fig 6 circuit operation in mode 6 1014

Mode 2: and LK2 are released into CL and C2 via DS1 and D4, respectively. This mode ends when the energy stored in LK2 is released to zero. Mode 5: Fig 9 circuit operation in mode 2 In mode 2, S2 and S3 are switched on and D4 is switched off. Fig 9 shows the equivalent circuit. The voltage source VH charges Lm. The voltage across the primary winding is equal to VP. The ILm increases linearly. The DC bus voltage VH releases its energy to C2, CL, and RL. This mode ends when the energy stored in C2 is released to RL. Mode 3: Fig 12 circuit operation in mode 5 In mode 5, S2 and S3 are switched off and DS1 is switched on. Fig 12 shows the equivalent circuit. Lm not only releases its energy into CL and RL but also transfers energy to C2 via NS and D4. The ILm decreases linearly. Waveform of Charging Fig 10 circuit operation in mode 3 In mode 3, S2 and S3 are switched on and DS1 and D4 are switched off. Fig 10 shows the equivalent circuit. VH and C2 release their energy into Lm, CL, and RL. The ILm increases linearly. This mode ends when S2 and S3 are turned off. Mode 4: Fig 13 Waveform of charging Fig 11d circuit operation in mode 4 In mode 4, S2 and S3 are switched off and DS1 is switched on. Fig 11 shows the equivalent circuit. The energies of LK1 III. PARAMETERS & DESIGN: VIN= 24V For the circuit discussed in previous section, D was taken more than 0.5 and the output waveforms are obtained by simulation. Here also we consider the same. (1) 1015

The coupling-coefficient of the coupled-inductor turns ratio n are assumed (3) Moreover, the magnetizing inductance current ilm is equal to ilk1 + nilk2, where n = NS/NP. (4) (5) (7) The energy of the leakage inductors LK1 and LK2 are released into C2 through DS2 and D4, respectively. This mode ends when the current ilk2 (2) (6) C0= (16) Input power &Output power of circuit calculated by using the basic equations = * (17) = * (18) Efficiency: = *100 (19) PARAMETERS Input Voltage Switching Frequency Lm, L k VALUES 24v 50khz Np:Ns 1:3 37uH,0.33uH (8) The energies of VL, Lm, and LK1 are released into C2 through DS2. Moreover, part of the Lm energy is transferred to CH and load RH via the secondary side of the coupled inductor. This mode ends when the voltage across C2 is equal to n Vin. C L C 2 C H C 0 R L 220uF 300uF 300uF 10uF 500Ω IV. SIMULATION RESULTS INPUT VOLTAGE AND INPUT CURRENT: (9) (10) The inductor ripple current is 20% to 40% of the output current IL=(0.2to0.4)*I0(max)* (12) L= (13) According to basic principles of boost converter taking the equation V0=ESR* (14) Basic equation of boost converter ripple factor taking in this circuit the equation derived as C= (15) fig 14:Waveform of input voltage and input current The above figure 14 shows the simulation results of output voltage and output current of Bidirectional dual bo/bu DC-DC converter using coupled inductor for the input voltage of 24 V and input current 15A 1016

Fig 17: Waveform of leakage inductance current OUTPUT VOLTAGE AND OUTPUT CURRENT: Fig 15:Waveform of output voltage and output current SIMULATION RESULTS CURRENT ACROSS MUTUAL INDUCTANCE ( The above figure 17 shows the simulation results of inductor for (leakage inductor current) current is 15 A. For the time period of t 1 the inductor current linearly increases because the inductor Lm starts charging until it reaches its peak value, And at time period t 2 inductor Lm starts discharging until it reaches zero and the current through it decreases linearly. Simulation across switches is taken in these results. The above figure a shows the simulation results of inductor converter current switch 2 and 3. In mode 2, S2 and S3 are switched on and D4 is switched off. The voltage source VH charges Lm. The voltage across the primary winding is equal to VP. The ILm increases linearly. The DC bus voltage VH releases its energy to C2, CL, and RL. This mode ends when the energy stored in C2 is released to RL. The above figure b shows the simulation results of Bidirectional dual bo/bu DC-DC converter using coupled inductor converter current switch 1. In mode 1, S1 is switched on. Lm releases its energy to capacitor CL and load RL. The magnetizing current ILm decreases linearly. The energy stored in the leakage inductor Lk2 is recycled to C2. Fig 16: Waveform of inductor current of Fig 18 Waveform of current across switch 2 The above figure 16 shows the simulation results of inductor converter (mutual inductor current) current is1.8a. For the time period of t 1 the inductor current linearly increases because the inductor Lm starts charging until it reaches its peak value, And at time period t 2 inductor Lm starts discharging until it reaches zero and the current through it decreases linearly. SIMULATION RESULTS CURRENT ACROSS LEAKAGE INDUCTANCE : Fig 19 Waveform of ) Simulation across capacitors is shown in these results. The above figure a shows the waveform of inductor converter capacitor C2 voltage, which is 23 V DC for the input voltage of 24 V DC. The voltage is built up when the switch S1 is turned on and maintained constant at 23 V throughout the process. The above figure 31 shows the 1017

waveform of of Bidirectional dual bo/bu DC-DC converter using coupled inductor converter capacitor C2 voltage, which is 120 V DC for the input voltage of 180 V DC. The voltage is built up when the switch S1 is turned on and maintained constant at 120 V throughout the process. V. CONCLUSION This paper has presented a novel bidirectional DC-DC converter for renewable energy systems. The proposed converter can achieve high conversion ratio using the coupled-inductor technique. The experimental waveforms agree with the theoretical analysis. The efficiency in discharging and charging mode is over 90% in full load condition. The highest efficiency levels discharging mode and charging mode are 95% and 92%, respectively. The operating principles and analysis of the steady-state characteristics are discussed in great in detail. Finally, a prototype of 12/200V circuit is implemented to verify the feasibility of the proposed converter. One of its important functions is to execute algorithms that continuously estimate battery state-of-charge (SOC) and available power. Fig 20 Waveform of voltage across capacitor Fig 21 Waveform of voltage across capacitor Simulation across diode is shown in these results. The above figure a shows the simulation results of inductor converter diode current. In mode 2, S1 and D4 are switched on. VL charges the magnetizing inductor Lm and the leakage inductor LK1. The magnetizing-inductor current ILm and the leakage-inductor current ILk1 are increased linearly. In addition, VL transfers its energy into C2 via the secondary winding NS and D4. Thus, the voltage across C2 is charged to nvl. REFERENCES [1 ] L. Palma, M. H. Todorovic, and P. Enjeti, "A high gain transfonnerlessdc-dc converter for fuel-cell applications," in Proc. IEEE PowerElectron. Spec. Conf., 2005, pp. 2514-2520. [2] V.Scarpa, S. Buso, and G. Spiazzi, "Low-complexityMPPT techniqueexploiting the PV module MPPT locus characterization," IEEE Trans.Ind. Electron., vol. 56, no. 5, pp. 1531-1538, May 2009. [3] S. K. Changchien, T. J. Liang, J. F. Chen, and L. S. Yang, "Novel highstep-up DC-DC converter for fuel cell energy conversion system,"ieee Trans. Ind. Electron., vol. 57, no. 6, pp. 2007-2017, 2010. [4] Step-Up DC-DC Converter with High Voltage Gain Using Switched-Inductor Technique ISSN: 2321-9939-Mayur N. Parmar, Prof.Vishal G. Jotangiya. [5] High Step up Switched Capacitor Inductor DC-DC Converter for UPS System with Renewable Energy Source Maheshkumar. K and S. Ravivarman K.S. Rangasamy College of Technology, Tiruchengode, Namakkal-637 215 [6] O. Abutbul, A. Gherlitz, Y. Berkovich, and A. Ioinovici, "Step-upswitching-mode converter with high voltage gain using aswitchedcapacitor circuit," IEEE Trans. Circuits Syst. I, vol. 50, no. 8, pp.1098-1102, Aug. 2003. Fig 22 Waveform of current across Diode D4 Efficiency curves of discharging and charging for using the experimental results all are calculated for the efficiency graph Be love shown in figures. 1018

[7] Novel Isolated High-step-up DC DC Converterwith Voltage LiftTsorng-Juu Liang, Member, IEEE, Jian- Hsieng Lee, Shih-Ming Chen, Jiann-Fuh ChenMember, IEEE,and Lung-Sheng Yang. [8] A Novel High Step-Up DC-DC Converter. withcoupled-inductor. 1019