Non-Inverting Buck Boost Converter for Charging Lithium-Ion Battery using Solar Array A. SRILATHA 1, M. KONDALU 2, S. ANANTHASAI 3

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1 ISSN Vol.03,Issue.11 June-2014, Pages: Non-Inverting Buck Boost Converter for Charging Lithium-Ion Battery using Solar Array A. SRILATHA 1, M. KONDALU 2, S. ANANTHASAI 3 1 Assoc Prof, Joginapally BR Engineering College, Hyderabad, Andhrapradesh, India, a.srilatha11@gmail.com. 2 Prof & HOD, Joginapally BR Engineering College, Hyderabad, Andhrapradesh, India, kondalu_m@yahoo.com. 3 Asst Prof, Joginapally BR Engineering College, Hyderabad, Andhrapradesh, India, ananthasai.somasi@gmail.com. Abstract: As the demand for rechargeable batteries increases, so does the demand for battery chargers. There are different kinds of design solutions available for implementing battery chargers. Generally the Buck converter topology [5] is used as a DC- DC converter to provide the controlled output power supply to the batteries. But in this case a problem may arise, for example, if you want to charge 5.2V Li-ion batteries from a 6V supply due to the presence of the protection diode and other small drops across other components. This drop is generally about 1V which makes it very difficult to provide 5.2V to the Liion batteries [3] using the buck converter topology. This application note describes a simple technique for implementing a noninverting buck-boost converter [2] which requires only one inductor. This converter is basically the result of cascading a Buck converter with a Boost converter. This converter can be controlled by two PWM signals from the PWM controller [10] and can be used as a Buck converter or Boost converter whenever required. Keywords: Non-Inverting Buck-Boost Converter, Lithium-Ion Battery, Constant Current And Voltage Charging. I. INTRODUCTION Non-inverting Buck-Boost converter can be used to charge a wide range of the batteries using the same hardware, which uses rechargeable batteries. The Batteries are charged in a constant current mode till they reach full charge voltage [3]. Once the Battery reaches the full voltage, charging current has to be reduced in order to maintain the voltage constant. Also for Li-ion battery [3], charging has to be stopped once the battery is fully charged. Solar array is a constant current source; this is simple and reliable but involves a lot of hardware. An Electronic charger can reduce the hardware in addition to providing superior charge control characteristics. This work describes the Design and Implementation of Noninverting Buck Boost Converter [2] which performs the task of Efficient Battery charge regulator [1], during sun lit conditions. This configuration can be considered as an optimized model as it saves space, weight, cost, and which are significantly high, in the existing method of battery charging string switching method. II.BATTERY CHARGING METHODS There are several battery charging methods namely [3] Constant current chargers vary the voltage to maintain a constant current flow, switching off when the voltage reaches the level of a full charge [3]. A Constant voltage charger sources current into the battery in an attempt to force the battery voltage up to a pre-set value (usually referred to as the Set-point voltage or set voltage).once this voltage is reached, the charger will Source enough current to hold the voltage of the battery at this constant Voltage [3].Constant Current-Constant Voltage charging. Used for charging Lithium ion batteries [3] which are vulnerable to damage if the upper voltage limit is exceeded, Special precautions are needed to ensure the Battery is fully charged while at the same time avoiding overcharging. For this reason it is recommended that the charging method switches to constant voltage before the cell voltage reaches its upper limit. In PWM Type the generated output voltage is less than the input voltage. Used for most of the battery charging applications due to single inductor topology and low complexity. [4].Buck converter cannot be used since the input current is pulsating so we require high input filter. In Buck Boost converter the generated output voltage is less than or greater than the input voltage.[5]buck Boost converter cannot be used due to the following limitations: The output voltage generated is opposite in polarity to the input which is not suitable for charging the battery as battery needs positive voltage for charging, Polarity inverting circuitry must be employed to get positive output, which increases the cost of the circuit. III.PROPOSED SCHEME Fig1. Non-Inverting Buck-Boost converter SEMAR GROUPS TECHNICAL SOCIETY. All rights reserved.

2 Non-inverting Buck-Boost converter as charger [1] is shown in Fig 1: IV. PRINCIPLE OF OPERATION Phase SW1 ( PWM1 ) SW2 ( PWM2 ) A. SRILATHA, M. KONDALU, S. ANANTHASAI Operating Modes 1 ON OFF BUCK 2 ON ON BUCK- BOOST 3 OFF ON N/A SW1 and SW2 driven by the PWM1 and PWM2[2] signals output by the SG1524B PWM IC[10]. V. LIMITATIONS To minimize the input current ripple at the input power supply, an input capacitor of high value is needed at the DC- DC converter input/output current ripple is also high due to output current pulsating, so a high value output capacitor is needed. It requires additional MOSFET switch & diode, so power loss is high. VI. NON-INVERTING BUCK-BOOST BASED BATTERY CHARGER In Phase 1, switch SW1 (PWM1) ON and Switch SW2 (PWM2) is OFF and operating mode is Buck. In Phase 2, switch SW1 (PWM1) is ON and Switch SW2 (PWM2) is ON and operating mode is Buck- Boost. In Phase 3, switch SW1 (PWM1) is 0FF and switch SW2 (PWM2) is ON this condition will never occur either in a Buck converter or Boost converter. The following guidelines are used to manage the PWM signals shown in Fig 1. Keep the Frequency of both PWM signals same, to control the synchronization of the two PWM signals. The Duty cycle D1 of control signal PWM1, must be greater than the Duty cycle D2 of control signal PWM2. PWM1 signal should be enabled before the PWM2 signal. PWM1 signal should be disabled after the PWM2 signal. Fig3. Battery is connected to the circuit. VII. LITHIUM-ION BATTERY CHARGING TERMINOLOGY The rate of charge or discharge is expressed in relation to battery capacity known as the C-Rate, this rate of charge equates to a charge or discharge current and is defined as [3] I = M x Cn (1) I charge or discharge current, M is multiple or fraction of C, C is the numerical value of rated capacity AH, n Time in hours at which C is declared. Fig2. Timing diagram for two PWM signals. From fig1: Output voltage is less than or greater than the input voltage, Input and output voltages are of same polarity, Single inductor Topology. Low complexity and fewer components, there is an additional diode which prevents the output voltage going Negative, Non-Inverting Buck-Boost converter can be used as a Buck converter or as a Boost converter by selecting different combinations of switches Fig 4:Li-ion Battery Charging. From fig4 batteries are charged in constant current mode till they reach full charge voltage once the battery reaches

3 Io(amps) Io(amps) Non-Inverting Buck Boost Converter for Charging Lithium-Ion Battery using Solar Array around 100mv to 200mv. From Fig (5)&(6) battery is charged in constant current mode till it reaches a full charge voltage charging current has to be reduced in order to maintain voltage constant. full charge voltage, charging current has to be reduced in order to maintain the voltage constant, Also for lithium-ion the charging has to be stopped once the battery is fully charged. They are generally much lighter than other types of Rechargeable Batteries of same size. The Electrodes of lithium ion Battery are made of light weight lithium and carbon. Lithium is a highly reactive element, meaning a lot of energy can be stored in atomic bonds. This translates in to a very high energy density for lithium-ion Batteries. A lithiumion Battery pack looses only about 5% of its charge per month. They have no memory effect, which means that you do not have to completely discharge them before recharging. Lithium-ion batteries can handle hundreds of charge/discharge cycles. VIII.CURRENT AND VOLTAGE MODE CONTROL When it comes to control of the duty cycle of the switch of a DC-DC converter, a question arises which mode of control is to be used. There are two modes [7] Voltage mode, Current mode. IX.DESIGN Input voltage: 31 V to 32.0V, Output: To charge lithiumion Battery 50 AH.[3],Charge Current: 2A,Battery Voltage: 28V to 31.8V, Battery lowest voltage is 20V,the PWM controller is made use to design control circuit. It consists of all circuits needed for generating PWM signal. [10], This converter uses a switching frequency of 100KHZ [10], & the switching element used is mosfet, the driver transformer is constant current constant current Fig6. VIN=32.5V, EOC=31V, I=2A. X. RESULTS&DISCUSSIONS Fig5. VIN=32.0V, EOC=29V, I=2A. selected such that number of primary turns equal to number of secondary turns, Duty cycle of the series switch should always be greater than the Shunt Switch [5], this is to ensure Buck-Boost operation. The value of the output capacitor controls the ripples in output voltage as well as settling time. Higher the value of capacitor, lower the value of ripple but higher settling time and vice versa Cmin=61.42 µf. Capacitance of the input filter capacitor=61.42 µf. The value of inductor chosen is 100 µ H. Current sensing resistor (Rs)[6] is selected such that the maximum drop across it to be Fig7. PWM IC Wave forms (simulation result).

4 A. SRILATHA, M. KONDALU, S. ANANTHASAI Fig(7) shows the simulated waveforms of PWM IC Fig (8)shows the charger circuit gate wave forms for both the switches & inductor current. Fig(9)&(10)shows the hardware results at totem pole,gate of mosfet & pin no 7 of PWM IC. Fig(11)shows the hard ware results of PWM IC. Fig (12)shows the hard ware results of the charger circuit for 1A from min to max voltage &from the wave forms we can observe that the shunt switch is off initially & on after 32V.Fig(13)shows the series switch gate wave forms for 1A.Fig (14)shows the ripple voltage waveforms for 1A. Fig8. Gate wave forms of both switches current(simulation result). & inductor Fig9. Collector of Q1, Base of totem pole Gate of Mosfet(hardware result). Fig10. Gate of mosfet, Anode of diode,pin 7of PWM IC(hardware result).

5 Non-Inverting Buck Boost Converter for Charging Lithium-Ion Battery using Solar Array Fig11.Charger circuit wave forms for 1A (hardware result). Fig12. Series switch gate wave forms for 1A (hardware Fig13. Ripple voltage wave forms for 1A (hardware result). result).

6 From the results & waveforms the power loss will occur in the control circuit, switches, diodes, input capacitor, output capacitor, sense resistor& inductor. So the efficiency of the system will be reduced. In order to improve the efficiency of the system Operate at the optimum switching frequency to minimize losses. Use of improved semiconductor switches. Charge at lower current rate. Disconnect charger when charging is not required. Under light-load conditions,the converter can operate in buck-mode with a lower switching frequency,there by reducing switching losses and consequently improving system efficiency.use of Resonant switching concept can also improve the efficiency.use of synchronous rectification. XI. CONCLUSION Non-inverting Buck-Boost type Battery charger has been proposed for charging Batteries using solar array. The circuit structure is simpler and much cheaper compared to other control mechanisms where much hard ware is required. It operates in constant current and constant voltage (taper charge) mode to efficiently and fully charge Batteries, with protection features in built. The main advantage is it reduces the hardware the disadvantage is that losses will be more in the Buck-Boost operation Efficiency needs to be improved to make it more optimal. XII.SCOPE FOR FUTURE WORK The present charger is designed for a charge current of 3A. The charge current with a higher rating can be designed. Microcontroller based charger can also be designed with improved closed loop control characteristics. Power MOSFET & diodes with improved characteristics can be used to boost the efficiency. XIII. REFERENCES [1]. AN2389 Application note,an MCU-based low cost noninverting buck-boost for battery chargers. [2].Anon-invertin buck-boost converterwith reduced components using a microcontroller(ieee)byrobert S.Weissbach, Member IEEE KEVIN M.TORRES,Member IEEE. [3].Design of high energy lithium-ion battery charger M.F.M. Elias*, A.K. Arof**, K.M. Nor* *Department of Electrical Engineering, Faculty of Engineering University of Malaya. [4].Digital Combination of Buck and Boost Converters to Control a Positive Buck-Boost Converter,Arindam Chakraborty,Alireza Khaligh,and Ali Emadi Illinois Institute of Technology 3301 South Dearborn Street Chicago, IL 60616, USA emadi@iit.edu.arthur Pfaelzer Intronics, Inc Providence Highway Norwood, MA 02062, USA. [5].Combination of Buck and Boost Modes to Minimize Transients in the Output of a Positive Buck-Boost Converter Arindam Chakraborty, Alireza Khaligh, and Ali Emadi Illinois Institute of Technology Electrical and Computer A. SRILATHA, M. KONDALU, S. ANANTHASAI Engineering Department 3301 South Dearborn Street Chicago, IL 60616, USA. [6].Current sensing techniques for DC-DC converters,hassanpooyaforghanizadeh,studentmember,ieee,and GabrielA.Rincon Mora,seniormember,IEEE Georgia Tech Analog consortium. [7].10MHz Current Mode 4 Switch Buck BoostConverter (4SBBC) for Polar Modulation Jinseok Park, Jiwei Fan, KevinG.Gard,AlexQ.HuangSemiconductorPowerElectronics Center(SPEC)Department of Electrical and Computer EngineeringNorth Carolina State UniversityRaleigh, North Carolina USA jpark3@ncsu.edu. [8].Design And Application GuideFor High Speed MOSFET Gate Drive Circuits By Laszlo Balogh. [9].Design Calculations for Buck-Boost Converters Application ReportSLVA535A August 2012 Revised September [10].Regulating pulse width modulator SG1524B/ SG2524B/ SG3524B.