HIGH CURRENT, LOW VOLTAGE MODULAR POWER CONVERTER FOR LEAD ACID BATTERY CHARGING

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1 ICSET 2008 HIGH CURRENT, LOW VOLTAGE MODULAR POWER CONVERTER FOR LEAD ACID BATTERY CHARGING lknur Çolak (1) Nejat Tuncay (2) stanbul Technical University Mekatro R&D, TUBITAK MRC Electrical Engineering Department, Ayazaga Campus Technology Free Zone Section Maslak, Istanbul, Turkey Gebze, Kocaeli, Turkey Key words: Modular converters, battery chargers, phase shifting ABSTRACT The submarine lead acid batteries require high current (~8kA) and low voltage (~10V) battery chargers because of their high capacities. The need to install the power converters in small place is the driving force for reduced volume and high efficiency. Moreover, the high power lead acid batteries require very low voltage and current ripples and a very high level of performance from the power converters, particularly in terms of DC stability and dynamic response. To meet these requirements full bridge phase shifted, hard switching PWM technique is used in this application. By allowing zero voltage transition of phase shifted switching power modules, the power density of the system is considerably increased. This paper describes this quasi resonant (8A, 10V) power converter intended as a stable high current source for series connected 4 lead acid battery cells. The converter is made with a modular concept with 8 current sources (1kA, 10V) in parallel. Also 45 phase shift is added between these 8 modules to reduce the output voltage and current ripple. I. INTRODUCTION A high current, low voltage DC power supply is required for the submarine lead acid battery charging. At present, there are several types of power supplies, which are used to charge lead acid batteries. Conventional rectifiers are the most common and simple topologies, in the battery power supplies. But most of these rectifiers are not suitable for the multi-stepped charge operation. Especially, if the voltage level of the battery to be charged is low and the current level is high, formation of the power topology and the control of recharging levels become exceedingly hard. Therefore, with their clumsy structures, slow system responses, and high ripple levels, the conventional rectifiers make not so convenient solutions for modeling high current-low voltage battery power supplies. In this study, suitable topologies have been investigated for modeling a high current- low voltage converter having low current and voltage ripples, which is the most critical point of the system design. As a result of the studies, various converter models composed of modular structures, having different current and voltage values, and different operation modes have been simulated. Simulated circuits have been examined from different points, and their advantages and disadvantages over others have been emphasized. As a result of the findings, proposed circuit models and operation modes have been evaluated to be inadequate for the system in question, and system s operation modes and /08/$25.00 c 2008 IEEE 1042

2 modular structure have been altered in accordance. Application of Full Bridge, Phase Shift PWM Converter model has enabled high power soft switching. Phase shifting method used in the circuit has increased the power density of the system by allowing zero voltage transition of power switches. Experimental results obtained show that the phase shifting PWM method is a good choice for low voltage, high frequency, and high current applications. They also show that zero voltage transition method reduces the switching losses, and 45 phase shifted modules decrease the output voltage and the output current ripples. II. POWER UNIT (8kA, 10V) Output Filter Primery Current Sensor High Frequency Transformers Figure 1. 8kA, 10V Power Unit Output Rectifier To meet the system requirements; - The chosen topology was separated into 8 paralleled modules [1kA, 10V] - All of the modules have an input three phase rectifier, a single phase inverter which has 20kHz switching frequency, high frequency transformers, output rectifiers and filters - Full Bridge, Phase Shifting PWM method is used to enable high power and soft switching - 45 constant phase shift is used between the 8 modules to reduce output ripples and the size of output filters - Phase shifting method used in the circuit has increased the power density of the system by allowing zero voltage transition of power switches. III. TRANSFORMER 4 high frequency center tapped transformers are used at the outputs of every converter blocks and both their inputs and outputs are paralleled. The primary sections of the transformers consist of a single coil, and the secondary sections consist of 3 high current coil. Transformers, which are wound by using Litz wire, are made up of U93/76/30 type Ferrite cores. IV. OUTPUT RECTIFIER In case of the only one battery cell s charging operation the output voltage must be very low (2V rated). Because of the very low voltage level, the secondary coils of each center tapped high frequency transformers are rectified by water-cooled schottky diodes and the DC outputs are paralleled after the filters. After the filtering of the DC voltage, an MDK950 type dual diode module is implemented to prevent the backflow of the battery current over the rectifier. The implementation of the given diode module also prevents converter modules to affect each other when there is a potential difference between the converter modules. V. PASSIVE FILTER Each of the converter modules can operate by itself according to the desired current level. For this reason, a passive filter is implemented at the outputs of each module. A 5.5uH DC inductance with a rated current of 1000A and a capacitor group are used at the outputs of the modules in order to reduce the ripples of output current and voltage below 1%. The inductances are made up of U93/76/30 type ferrite cores as in the case of transformers. The coils of the inductance consist of copper foils. 1043

3 VI. COOLING SYSTEM All the semi-conductor coolers in the system are of water-cooled structure. Water inflow of each cooler is parallel. On the other hand, high level heating components, such as high frequency transformer, output coil, and output series diode have thermostats and fan coolers that start to operate when these components reach above certain temperatures. In addition, there are auxiliary fans placed on the ceiling of the cabins which start operating in situations when temperature rises due to operating in closed environment or in an environment with a relatively high ambient temperature. VII. CONVERTER OPERATION The converter is operated in isolated buck derived full bridge mode. In this mode main semiconductors are driven at zero voltage turn on time. This is made possible by introducing a phase shift between the two legs of the inverters. The main advantages of this topology include constant frequency operation which allows optimum design of the magnetic filter components, pulse width modulation (PWM) control, minimum voltage and current stresses on the semiconductors and good control range and controllability. There is also 45 constant phase shift between the 8 parallel inverters. With 45 phase shifting, the ripple frequency of the outside is increased from 40kHz to 320kHz. By using this methodology the output filters values and sizes are decreased. On-time of semiconductors is increased as the load current increases. Only one module is on when load current is below 330A. Whenever the load current rises above this level, two modules are on, until the load current reaches 660A. Above 660A, a third module turns on. Whole of the modules turn on and turn off in 330A intervals. Supervision of these on-off controls are based on hysteresis principle. On the other hand, it is very critical point to prolong the battery life, especially for the submarines. To ensure long battery life and high performance during the battery s life, the applied charging method is as important as the proper battery usage and proper maintenance. Suitable charge management includes a multistage charging method and also temperature compensation. Control strategy of the DC/DC converter determined due to the charge states of the battery. Figure 2. Block Diagram 1044

4 VIII. EXPERIMENTAL RESULTS t on time period of the IGBTs Dead time between two IGBTs on one (a) Phase shift = %20 (b) Phase shift = %0 Figure 3: Gate Signals Voltage spike on the IGBT while it is turned off Figure 4: One IGBT s V CE Voltage IX. CONCLUSION This paper represents a low voltage high current capability quasi resonant DC/DC converter, which has minimum ripple at the outside. With this purpose 8, 45 phase shifted inverter modules are used in the main part. To Figure 5: 8kA Power Converter 1045

5 reduce the output semiconductors current stress a dual wired, center tapped, high frequency transformers are used. With phase shift method the size of the converter is extremely decreased and on the other hand the efficiency of the system is increased. X. REFERENCES [1] Andreycak, B., 1993a. Designing a Phase Shifted Zero Voltage Transition (ZVT) Power Converter, Unitrode Power Design Seminar Note, 1993 [2] Andreycak, B., Phase Shifted, Zero Voltage Transition Design Considerations and the UC3875 PWM Controller, Unitrode Application Note, May 1997 [3] Balogh, L., 1999a. The Current-Doubler Rectifier: An Alternative Rectification Technique For Push-Pull And Bridge Converters, Unitrode Corporation 7 Continental BLVD. Merrimack, NH [8] Jauregi, E., Zabaleta, J. R., Tellería, M., de la Fuente, J.M., Del Río, J. M., Figueres, E., Montabonnet, V. and Bordry, F., High Current Switch Mode Power Converter Prototype for LHC Project 6kA, 8V, JEMA GJ Lasarte-Oria, Spain & Eeropean Organization for Nuclear Research CERN, CH Geneve 23, Switzerland [9] Leslie, L. G., Design and Analysis of a Grid Connected Photovoltaic Generation System With Active Filtering Function, MSc Thesis, Virginia State University, Polytechnic Institute, Virginia [10] Mammano, B. and Putsch, I., 1991a. Phase-Shifted PWM Control A New Integrated Controller Eases the Design of Efficient High-Frequency Bridge Power Switching, Applied Power Electronics Conference Proceedings, March 1991, [4] Chen, W., Lee, F. C., Jovanovic M. M. and Sabate, J. A., A Comparative Study of a Class of Full Bridge Zero-Voltage- Switched PWM Converters, IEEE Applied Power Electronics Conference, March 1995, Vol.2, [5] Dahono, P., Firmansyah, M., Pramasti, D., A high Current, Low Voltage DC Power Supply, IEEE, 2001 [6] Dehmlow, M., Heumann K. and Sommer R., Comparison of Resonant Converter Topologies, Institut für Allgemeine Elektrotechnik Technische Universitat, Berlin Einsteinufer 19 D-1000 Berlin 10 FRG [7] Hamo, D. J., A 50W, 500kHz, Full- Bridge, Phase-Shift, ZVS Isolated DC to DC Converter Using the HIP4081A, Intersil Application Note,