Study on Performance Improvement of AC-DC Converter

Abstract

This project is about to develop the power converter with improved performance with low voltage and high current dc output.Supercapacitors require low voltage(12V) and high current (150A) to be charged.This paper describes the high frequency transformer and resonant inverter intended to obtain better efficiency by maintaining the constant low voltage. Keywords-Series Parallel Resonant inverter, Resonating components,High frequency transformer, Leakage inductance. I.Introduction Advent of devices with better switching capabilities has enriched the field of power electronics. This enables the development of converter circuits with improved and high performance[6]. The AC-DC converters are characterized so as to obtain low voltage and high current dc output. Conventional converters are most common but they have clumsy structures,slow system responses and high ripples level of voltage and current. At present there are several types of power supplies to charge supercapacitors but they do not make so convenient solutions for high current low voltage dc output with high performance and low losses. In this study several topologies have been investigated for modelling high current low voltage converter having low current and voltage ripples which is the most crucial point of system design. To improve the power factor of ac input line several topology has been proposed in the literature(i)(iv).High frequency resonant inverter provides novel solution to the problem offered by conventional inverter and also they can be operated in variable and high frequency mode. Owing to higher frequency of operation component sizes get reduced thereby component stress. On the other hand conventional converter was found to have several disadvantages such as bulky size, heavy weight, high di/dt value,high voltage stress,turn on and turn off losses, low efficiency,etc.The above demerits are overcome by high frequency resonant converter. It has numerous advantages such as light weight,higher frequency of operation,high efficiency,small size,fast response,low component stress,reduced electromagnetic interference(EMI),etc. This topological modification will have other advantages: (a) the minimum regulation of the DC voltage at the rectifier output becomes possible, (b) the power losses in the core are reduced, (c) the power efficiency is consequently increased, (d) the size, weight of the converter are reduced and (e) the electro-magnetic interference (EMI) disturbances are reduced. II.Operating Principle o f AC-DC Converter The above figure shows the block diagram of AC-DC converter for low voltage/high current dc output application with h igh performance.In the 1 st block,when ac supply is fed to the single phase uncontrolled diode rectifier,there is conversion of ac to dc.Filters has been added to avoid ripples and to have constant smooth dc output. In the 2 nd block,output from rect ifier is applied to s ingle phase full bridge high frequency inverter circuit.Mosfet switches are used in the inverter circuit as they have fast switching speed and gives better efficiency at high frequency.The switching frequency is 50Khz.The high frequency inverter has numerous advantages such as light weight,better system reliability and efficiency at higher frequency,small size,quick response,low component stress,reduced electromagnetic interference(EMI),etc. The 3 rd block is LC tank resonating circuit.The LC resonating components of high frequency inverter helps to minimize the leakage reactance of the transformer. There are three types of resonant converter: (a) Series resonant converter (b) Parallel resonant converter and (c) Series parallel resonant converter(v).Series Resonant Converter uses a series capacitor,which blocks the dc component avoiding high frequency(HF) transformer saturation. Its part load efficiency is high due to the decrease in device currents with decrease in load.Output dc filter is required to carry high ripple current.Parallel resonant converter works such that the entire current is limited by resonant inductor when the output terminal is short circuited(vi).The filter inductor limits the ripple current carried by the output capacitor.Series parallel resonant converter combines desirable features of series and parallel resonant converter(x).Thus the behaviour of series parallel resonant converter was found to be suitable for low voltage dc application and was observed in detail.At light loads it takes the property of parallel resonant converter.The load can be short circuited and light load regulation is possible.Current through high frequency switches does not decrease in proportion to decrease in load and hence produces high efficiency.The resonant tank circuit consists of capacitor and inductor which is placed across primary winding of high frequency transformer and thus forms LCC type series parallel resonant converter. In the 4 th block,resonant inverter output is applied to HF transformer. High frequency(HF) transformer specified fo r low International Journal of Engineering Research ISSN:2319-6890)(online),2347-5013(print) Volume No.3, Issue No.4, pp : 252-254 01 April 2014 IJER@ 2014 Page 253 voltage high current output applications have been useful for establishing more advanced power distribution with energy storage systems.The HF t ransformer non idealities such as leakage inductance and winding capacitance are utilized as a part of resonant tank circuit elements (viii).Th is configuration is well suited for low output voltage giving better performance in terms of efficiency and voltage regulation.The leakage inductance of HF transformer can be used as a part of resonating inductor.Hence the transformer leakage inductance need not be a troublesome parasitic.Thus power loss and voltage spike magnitude will not increase.The HF transformer has several advantages such as low eddy current losses at higher accurate frequency, skin effects are minimized, low leakage inductance, reduction in size, weight and cost etc.Skin effect is the tendency of an AC to become disturbed within a conductor such that the current density is largest near the surface of the conductor(vii-ix). It causes the effective resistance of the conductor to increase at higher frequency where the skin depth is smaller,thus reducing the effective cross section of conductor.Hence the transformer output will be low ac voltage and high ac current. In the 5 th block, output of HF transformer is applied to rectifier circuit to obtain constant low voltage and high current dc output.The diode bridge rectifier connected across the secondary of HF transformer is of schottky type.The feedback loop is used for maintaining the constant voltage output,to obtain min imum voltage regulation and also to meet output ripple specifications. The low voltage,high current dc output has several other applications such as heating of the bearings,short circuit and laboratory testing of circuit breakers(both ac and dc),microcontrollers,electric weld ing.arc furnace for s mall industries,etc.As a result of the studies, the ac-dc converter model have been simulated in the PSIM software. A lso to have a clear idea about the working of ac-dc converter simulated circuits have been examined from different points and their advantages over others have been observed. III.Simulat ion Parameters S.NO PARAMETERS SYM BOL VALUES (UNITS) 1. APPLIED AC VOLTAGE Vac 230 V 2. APPLIED AC VOLTAGE Iac 10 A 3. SWITCHING FREQUENCY freq 50 Khz 4. OUTPUT DC VOLTAGE Vdc 12 V 5. OUTPUT DC CURRENT Idc 150 A The above table shows the simulation parameters which were considered while simulating the circuit in PSIM software. Simulation Results 0.64 0.66 0.68 0.7 0.72 0.74 0.76 0.78 Time (s) 0