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COMPARISON OF N-STAGE CASCADECOCKCROFT-WALTON VOLTAGE MULTIPLIER

APPLIED TO TRANSFORMER-LESS DC-DC BOOSTCONVERTER

S Priya1*, M.Mohamed Muzammil1, K Jeberdson Abraham1 and P Selavanathan1

*Corresponding Author: S Priya,priyas@jeppiaarinstitute.org

This paper proposes a high step-up DC-DC converter based on Cockcroft-Walton (CW) voltagemultiplier without using step up transformer. The low input DC voltage is boost up by boostinductor (Ls) in DC-DC converter. The n-stage CW-voltage multiplier is applying low input ACvoltage to high output DC voltage. It provides continuous input current with low ripple, highvoltage gain, reduced switching losses, low voltage stress on the switches, diodes and capacitorsand also improving efficiency of the converter. Step-up dc-dc converters have been proposed toobtain high voltage ratios without extremely high duty cycle by using isolated transformers orcoupled inductors. In order to obtain low input current ripple and high voltage ratio current-fedconverters are used. Current-fed converters are more superior to the voltage-fed inverters.Finally the proposed converter is validated by MATLAB simulation for n-number of stages.

Keywords: Duty ratio, Performance parameters, Boost converter

INTRODUCTIONNon isolated dc-dc boost converter has foundapplication in a variety of fields, owing to itsability to step-up low voltage dc input to highvoltage dc-output. In recent years, with thedevelopment of power electronic devices andincrease in its utility in renewable energyapplications, boost converters have found itsuse due to high voltage gain, reduced voltageand current stress in the switching devices,

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Int. J. Elec&Electr.Eng&Telecoms. 2015

1 Department of EEE, Jeppiaar Institute of Technology, Sri Perumbudur, India.

improved efficiency and better dynamicresponse. Boost converters and derivative ofits topologies have been implemented for PVsystems, battery charging and in fuel cellapplications [2-5]. The analysis of the behaviorof the steady state and dynamic characteristicsof a boost converter has also been underinvestigation [6].

Theoretically, the conventional boost dc-dcconverter can provide a very high voltage gain

Research Paper

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by using an extremely high duty cycle. However,practically, parasitic elements associated withthe inductor, capacitor, switch, and diodecannot be ignored, and their effects reduce thetheoretical voltage gain. Up to now, many step-up dc-dc converters have been proposed toobtain high voltage ratios without extremelyhigh duty cycle by using isolated transformersor coupled inductors. Among these high step-up dc-dc converters, voltage-fed type sustainshigh input current ripple. Thus, providing lowinput current ripple and high voltage ratio,current-fed converters are generally superiorto their counterparts. A traditional current-fedpush–pull converter was presented to providethe aforementioned merit. However, in orderto achieve high voltage gain, the leakageinductance of the transformer is relativelyincreased due to the high number of windingturns. Consequently, the switch is burdenedwith high voltage spikes across the switch atthe turn-off instant. Thus, higher voltage-ratingswitches are required.

BOOST CONVERTERThe conventional boost dc-dc converter canprovide a very high voltage gain by using anextremely high duty cycle. However, practically,parasitic elements associated with theinductor, capacitor, switch, and diode cannotbe ignored, and their effects reduce thetheoretical voltage gain. Up to now, many step-up dc-dc converters have been proposed toobtain high voltage ratios without extremelyhigh duty cycle by using isolated transformersor coupled inductors. Among these high step-up dc-dc converters, voltage-fed type sustainshigh input current ripple.

The system is operated with the high step-up converter based on the CW voltage

multiplier with 5 or more stages. Replacing thestep-up transformer with the boost-typestructure, the proposed converter provideshigher voltage ratio than that of theconventional CW voltage multiplier. Thus, theproposed converter is suitable for powerconversion applications where high voltagegains are desired. Moreover, the proposedconverter operates in Continuous ConductionMode (CCM), so the switch stresses, theswitching losses, and EMI noise can bereduced as well.

In recent years, extensive use of electricalequipment has imposed severe demands forelectrical energy, and this trend is constantlygrowing. Consequently, researchers andgovernments worldwide have made efforts onrenewable energy applications for mitigatingnatural energy consumption and environmentalconcerns. Among various renewable energysources, the photovoltaic (PV) cell and fuel cellhave been considered attractive choices.However, without extra arrangements, theoutput voltages generated from both of themare with rather low level.

Thus, a high STEPUP dc-dc converter isdesired in the power conversion systemscorresponding to these two energy sources.In addition to the mentioned applications, ahigh step-up dc-dc converter is also requiredby many industrial applications, such as high-intensity discharge lamp ballasts forautomobile headlamps and battery backupsystems for uninterruptible power supplies.

The proposed converter consists of oneboost inductor Ls, four switches (Sm1, Sm2,Sc1, and Sc2), and one n-stage CW voltagemultiplier. Sm1(Sc1) and Sm2(Sc2) operatein complementary mode, and the operating

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frequencies of Sm1 and Sc1 are defined asfsm and fsc, respectively. For convenience,fsm is denoted as modulation frequency, andfsc is denoted as alternating frequency.Theoretically, these two frequencies should beas high as possible so that smaller inductorand capacitors can be used in this circuit. Inthis paper, fsm is set much higher than fsc,and the output voltage is regulated bycontrolling the duty cycle of Sm1 and Sm2,while the output voltage ripple can be adjustedby fsc. As shown in Figure 1, the well-knownCW voltage multiplier is constructed by acascade of stages with each stage containingtwo capacitors and two diodes. In an n-stageCW voltage multiplier, there are N (=2n)capacitors and N diodes. For convenience,both capacitors and diodes are divided intoodd group and even group according to theirsuffixes, as described in Figure 1.

STEADY STATE ANALYSISOF BOOST CONVERTERFigure 1 shows the proposed converter, whichis supplied by a low-level dc source, such asbattery, PV module, or fuel cell sources. Theproposed converter consists of one boostconverter.

In order to simplify the analysis of circuit

operation, the proposed converter with a three-stage CW voltage multiplier, as shown inFigure 7, is used. Before analyzing, someassumptions are made as follows.

1. All of the circuit elements are ideal, andthere is no power loss in the system.

2. When a high-frequency periodic alternatingcurrent is fed into the CW circuit and all ofthe capacitors in the CW voltage multiplierare sufficiently large, the voltage drop andripple of each capacitor voltage can beignored under a reasonable load condition.Thus, the voltages across all capacitors areequal, except the first capacitor whosevoltage is one half of the others.

3. The proposed converter is operating inCCM and in the steady-state condition.

Figure 1: 7-Stage Circuit Figure 2: Equivalent Circuit of theProposed Converter (a) Source-Side

Part, (b) Load-Side Part

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4. When the inductor transfers the storageenergy to the CW circuit, only one of thediodes in the CW circuit will be conducted.

5. Some safe commutation states areignored. According to the secondassumption, each capacitor voltage in theCW voltage multiplier can be defined as:

where vck is the voltage of the kth capacitorand Vc is the steady-state voltage of vc2 –vcN. For an n-stage CW voltage multiplier, theoutput voltage is equal to the total voltage ofall even capacitors, which can be expressedas:

Each capacitor voltage in an n-stage CWvoltage multiplier can also be expressed as:

where Vois the steady-state voltage of theoutput load side. Figure 8 shows the theoreticalwaveforms of the proposed converter, includingswitching signals, inductor current, v, i, anddiode currents. According to the polarity of i,the operation of the proposed converter can bedivided into two parts: positive conductinginterval [t0, t1] for i – 0 and negative conductinginterval [t1, t2] for i – 0.

Derivation of the Ideal Static GainIt can be seen that the terminal voltage of theCW circuit VAB = 0 in states I and III, while instates II and IV, VAB = Vo/2n. The inductorcurrent variation, during interval 0 < t < DTsm,can be represented as:

where Vin is the input voltage, Lsis the boostinductor, and D is the duty cycle of the switchSm1(Sm2) in the positive (negative)conducting interval over one modulationswitching period Tsm = 1/fsm. Then, duringinterval DTsm < t < (1 – D)Tsm, the inductorcurrent variation can be represented as:

Under the steady-state condition, by thevolt–second balance principle, the voltagegain of the proposed converter can be

where MV represents the static voltage gain ofthe proposed converter. Moreover, therelationship between i and iL can be obtainedby |i|/iL = 1 – D. The relationship betweenvoltage gain and duty cycle for the proposedconverter under n = 1 – 8 and the classic boostdc-dc converter. Obviously, the proposedconverter provides high voltage gain withoutextremely high duty cycle, while the classicboost dc-dc converter is operating atextremely high duty cycle.

SIMULATION RESULTS

Parameters Ratings

3-STAGE 7-STAGE

Input DC voltage, Vin 40-55 V

Output Voltage, Vo 430 KV 680KV

Modulation frequency, FSM 60 kHz

Alternating frequency, FSC 1 kHz

Resistive load, RL 1 K

Boost inductor, Ls 1.5 mH

Capacitors 470 F

Table 1: Comparison of PerformanceParameters of 3-Stage and 7-Stage

Boost Converter

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CONCLUSIONIn this paper, a high step-up DC-DC converterbased on CW voltage multiplier without a lineor high-frequency step-up transformer waspresented to obtain a high voltage gain. Sincethe voltage stress on the active switches,diodes, and capacitors is not affected by thenumber of cascaded stages, powercomponents with same voltage ratings canbe selected. The mathematical modeling,circuit operation, design considerations, andcontrol strategy were discussed. The controlstrategy of the proposed converter can beeasily implemented with a commercialaverage-current-control and continuouscurrent mode with adding a programmed.The proposed control strategy employs twoindependent frequencies, one of which

operates at high frequency to minimize thesize of the inductor, while the other oneoperates at relatively low frequency accordingto the desired output voltage ripple. Finally,the simulation and experimental resultsproved the validity of theoretical analysis andthe feasibility of the proposed converter. Thusthe design, simulation and analysis ofproposed DC-DC boost converter with threeand seven-stage Cockcroft Walton voltagemultiplier was done.

REFERENCES1. Jain M, Daniele M and Jain P K (2000),

“A Bidirectional DC-DC ConverterTopology for Low Power Applications”,IEEE Transactions on Power Electronics,Vol. 15, No. 4, pp. 595-606.

2. Mzoughi D, Allagui H and Mami A (2013),“Study of a Boost Converter for PEM FuelCell”, in 10 th International Multi-Conference on Systems, Signals andDevices (SSD), March, pp. 1-5, Hamlet,Tunisia.

3. Shreelakshmi M P, Das M and Agarwal V(2013), “High Gain, High Efficiency Bi-directional DC-DC Converter for BatteryCharging Applications in Stand-AlonePhoto-Voltaic Systems”, in IEEE 39 th

Photovoltaic Specialists Conference(PVSC), June, pp. 2857-2861, Tampa,Florida.

4. Vazquez N, Levya J, Cervantes I, Diaz Land Hernandez C (2013), “Analysis andStudy of High DC/DC Boost Converters”,39th Annual Conference of IEEE, IndustrialElectronics Society, IECON, November,pp. 435-440, Vienna.

Figure 3: 7-Stage Boost ConverterOutput Voltage Waveform

Figure 4: 7-Stage Boost ConverterOutput Current Waveform

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5. Wai R J, Wang W H and Lin C Y (2008),“High-Performance Stand-AlonePhotovoltaic Generation System”, IEEETransactions on Industrial Electronics,Vol. 55, No. 1, pp. 240-250.

6. Yu D and Yuvarajan S (2004), “Modelingand Performance Studies of a Fuel-CellPowered Boost Converter”, in 26th AnnualInternational Telecommunications EnergyConference, INTELEC, September,pp. 713-717.