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  1. 1. S.P.Narasimha Prasad, K.Vijaya Bhaskar / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 2, Issue 5, September- October 2012, pp.1272-1277A Closed Loop for Soft Switched PWM ZVS Full Bridge DC - DC Converter S.P.Narasimha Prasad. K.Vijaya Bhaskar. Dept of EEE, SVPCET. Dept of EEE, SVPCET. AP-517583, India. AP-517583, India.Abstract: - This paper propose soft switched PWM efficiency, and low EMI, so for medium to highZVS full bridge DC to DC converter. The power DC/DC applications it is a good choice.control of the proposed converter can beimplemented either with the phase-shift or pulse The phase-shifted PWM full bridge (FB)width modulated technique. This converter is converter incorporates the leakage inductance ofeffectively reduces the switching losses, stress the transformer to achieve zero-voltage switching,and elector magnetic interference. The input DC but only achieves it near the full load condition.voltage 48V is step down to 12V level. Thesimulation results and analytical results are Several new techniques for high frequencycompared. The PWM ZVS FB converter proto DC-DC conversion are there to reduce componenttype will operate at 20 KHz at a 48V DC. The stresses and switching losses while achieving highopen loop and closed loop of the circuit is power density and improved performance. Amongsimulated by using MAT LAB software. them, the full-bridge (FB) zero-voltage-switched (ZVS) converter is one of the most attractiveIndex Terms-DC-DC converter, Full Bridge (FB), techniques which are shown in Fig. 1. It is the mostZero voltage switching (ZVS) widely used soft-switched circuit in high-power applications, [1]–[3]. This constant-frequencyI. INTRODUCTION converter employs phase-shift (PS) control and The continuing success of square-wave features ZVS of the primary switches withPWM topology in switching converter can be relatively small circulating energy. However, fullattributed to its ease of operation. The harmonics ZVS operation can only be achieved with a limitedcan easily be eliminated by power filter and it has a load and input-voltage range, unless a relativelycapability in allowing continuous and linear control large inductance is provided in series with theof the frequency and fundamental component of the primary winding of the transformer either by anoutput voltage. But with the demands for higher increased leakage inductance of the transformerpower densities, the switching frequencies are and/or by an additional external inductor. Thisapproaching 1 MHz range. At these frequencies, increased inductance has a detrimental effect on thesquare wave converters’ switching losses become performance of the converter since it causes anvery high leading to excessive heat dissipation. increased loss of the duty cycle on the secondaryEven if the increased switching frequency does not side, as well as severe voltage ringing across thecause unacceptable switching losses, the secondary-side output rectifiers due to theoscillations caused by converter parasitic elements resonance between the inductance and the junctionmay cause high current and voltage stresses, which capacitance of the rectifier. The secondary-sideare almost unpredictable, depending on circuit ringing can be suppressed by an active snubberlayout. Suitable snubber circuits must therefore be described in [2]. For implementations with anadopted, which affect power density and converter external primary inductor, the ringing can also bereliability. The zero-voltage transition approach, as effectively controlled by employing primary-sidewell as the active-clamp snubber approach, leads to clamp diodes D and D1 shown in Fig. 1, aszero-voltage switching of the transistors and zero- proposed in [2]. While the snubber approaches incurrent switching of the diodes. These approaches [1] and [2] offer practical and efficient solutions tohave been successful in substantially improving the the secondary-side ringing problem, they do notefficiencies of transformer-isolated converters. offer any improvement of the secondary side duty- cycle loss. The Zero-voltage switching (ZVS) phase Several techniques have been proposed toshift modulated full bridge (PSM-FB) DC/DC extend the ZVS range of FB ZVS convertersconverter with MOSFET switches has been without the loss of duty cycle and secondary-sideproposed in [1],[2]. Low component count and zero ringing [4]–[7]. Generally, these circuits utilizefull load switching losses enable this topology to energy stored in the inductive components of anachieve low cost, high power density, high auxiliary circuit to achieve ZVS for all primary switches in an extended load and input voltage 1272 | P a g e
  2. 2. S.P.Narasimha Prasad, K.Vijaya Bhaskar / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 2, Issue 5, September- October 2012, pp.1272-1277range. Ideally, the auxiliary circuit needs to provide provide a path for the current through primaryvery little energy, if any, at full load because the inductor, Lp which is used to store ZVS energy.full-load current stores enough energy in the When the load voltage is regulated, as the loadconverter’s inductive components to achieve current and/or input voltage decreases, the dutycomplete ZVS for all switches. As the load current cycle of each PWM switch, i.e., switches Q3 and Q4decreases, the energy provided by the auxiliary decreases so that the volt-second product on thecircuit must increase to maintain ZVS, with the windings of power transformer TR also decreases.maximum energy required at no load. The energy At the same time, the volt-second product on thestored for ZVS is independent of load as described windings of auxiliary transformer TRA increases,in [4] and [5]. Adaptive energy storage in the which proportionally increases the energy stored inauxiliary circuit has been introduced in [6] and [7]. the primary inductor. Due to the adaptive nature ofHowever, these converters have to use large the energy available for ZVS stored in the primaryinductors so, high circulating energy that is inductor, which increases as the load current and/orneeded1 to achieve no-load ZVS and that is due to input voltage decreases, the proposed circuit cana relatively large inductor employed to assist ZVS. achieve ZVS in a very wide range of load current, including no load, with minimal circulating energy. Fig.1 PWM Full Bridge Converter In this paper, a FB ZVS converter with adaptive Fig.2 FB ZVS converter with auxiliary transformerenergy storage that offers ZVS of the primaryswitches over a wide load range with greatly In the modified circuit, since the ZVSreduced no-load circulating energy and with energy stored in the primary inductor is dependentsignificantly reduced secondary-side duty cycle on its inductance value and the volt-second productloss is introduced with PWM control. ZVS full of the secondary of auxiliary transformer TRA, thebridge DC to DC converter with ZVS over the size of the primary inductor can be minimized byentire range is given by[8].High power density properly selecting the turns ratio of auxiliarymulti-kilowatt DC to DC converter with galvanic transformer TRA. As a result, the size of theisolation is given by [9]. The literature [1] to [9] primary inductor is very much reduced compareddoes not deal with the modeling and simulation of to that of the conventional PS FB converter shownclosed loop controlled PWM ZVS full bridge in Fig.1. In addition, since the auxiliary transformerconverter. This works aims to develop circuit does not need to store energy, its size can be small.model for ZVS full bridge converter. Finally, because the energy used to create the ZVS condition at light loads is not stored in the leakageII. PWM ZVS FB CONVERTER WITH inductances of transformer TR, the transformer’s AUXILIARY TRANSFORMER. leakage inductances can also be minimized. As a Fig. 2 shows the FB ZVS converter circuit result of the reduced total primary inductance, i.e.,diagram that provides ZVS for the bridge switches the inductance of the primary inductor used forover a wide range of load current. It employs low- ZVS energy storage and the leakage inductance ofpower auxiliary transformer TRA to extend the the power transformer, the proposed converterZVS range. The primary of auxiliary transformer exhibits a relatively small duty-cycle loss, whichTRA is connected to the center tap of power minimizes both the conduction loss of the primarytransformer TR and the ground through blocking switches and the voltage stress on the componentscapacitor C1, where as its secondary is connected in on the secondary side of the transformer, whichseries with the primary winding of power improves the conversion efficiency. Moreover,transformer TR and inductor Lp .Auxiliary because of the reduced total primary inductance,transformer TRA is only used to adaptively store a the secondary- side parasitic ringing is also reducedrelatively small amount of energy into primary and is effectively controlled by primary side diodesinductor that is required for ZVS. Finally, two D and D1.diodes are connected from the node connecting theprimary of the power transformer and thesecondary of the auxiliary transformer to thepositive and negative (ground) rails of the bridge to 1273 | P a g e
  3. 3. S.P.Narasimha Prasad, K.Vijaya Bhaskar / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 2, Issue 5, September- October 2012, pp.1272-1277III. OPERATIONAL PRINCIPLE The circuit diagram of the modifiedconverter is shown in Fig.2.The primary sideconsists of four switches, two diodes, one inductor,and one capacitor. It employs low power auxiliarytransformer TRA to extend the ZVS range. At lightloads energy used to create ZVS is not stored in theleakage inductance of the transformer TR .So; thetransformer’s leakage inductance can beminimized. Energy stored in primary inductor Fig 3(c).The circuit diagram at (t2 to t3)depends on volt-second product of the secondary ofauxiliary transformer TRA and inductance value.So by selecting proper turn ratio of auxiliarytransformer TRA, the size of the primary inductorcan be minimized. Auxiliary transformer is notused to store energy. So, its size can be small.Several assumptions are made as follows. 1) Capacitance of capacitor C5 is large enough so that the capacitor can be modeled as a constant voltage source. Fig 3(d).The circuit diagram at (t3 to t4) 2) The inductance of output filter L1 is large enough so that during a switching cycle As shown in Fig.3(a), at t=t0, load current the output filter can be modeled as a flows through rectifier D3 and the lower secondary constant current source. of power transformer TR. when diagonal switches 3) The leakage inductance of auxiliary Q1 and Q2 are conducting. Since during this transformer TRA and the magnetizing topological stage diodes D and D1 are reverse inductances of both transformers are biased, the reflected primary current is flowing neglected. through closed switch Q1, primary inductor Lp 4) The resistance of each conducting switch winding N2 of auxiliary transformer TRA, primary is zero; where as the resistance of each winding Np of transformer TR, and closed switch non-conducting switch is infinite. Q4. Since the impendence of the primary inductor 5) Current through primary side of auxiliary Lp and winding N2 of auxiliary transformer TRA transformer TRA is zero. are very small compare to primary referred filter inductor Lo .Let Vo be the primary referred output DC voltage. Slope of the primary current is given by (VDC – Vo)/Lo. Centre tap of primary voltage is given by VP/2 =V/2 because impendence of primary inductor Lp and winding N2 of TRA are small. At t=t1 as shown in Fig. 3(b), switch Q4 is turned off, primary current starts charging output capacitance C4 of switch Q4 and discharges output capacitance C3 of switch Q3. The total required Fig 3(a).The circuit diagram at (t0 to t1) energy to charge C4 and discharge C3 is provided not only from the stored energy of Lp, but also from the stored energy of the output filter inductor. Since the stored energy in the output filter inductor is significantly larger than the required energy to charge C4 and discharge C3, these capacitors are assumed to be charged and discharged linearly. Voltage across switch Q4 increases towards V and voltage across switch Q3 decreases towards zero. Primary winding voltage of auxiliary transformer Fig 3(b).The circuit diagram at (t1 to t2) increases from zero to V/2 and secondary winding of auxiliary transformer increases from zero to V/2* ni where ni is the auxiliary transformer turn ratio. Diode D starts conducting because of increasing secondary voltage of auxiliary 1274 | P a g e
  4. 4. S.P.Narasimha Prasad, K.Vijaya Bhaskar / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 2, Issue 5, September- October 2012, pp.1272-1277transformer. After voltage across Q3 reaches zero  1 Io v(1  d )2   LP    CV 2diode across Q3 starts conducting at t=t2 as shown (4)in Fig. 4(c). 2 n 4niLPfs  Where, D is the duty cycle of the converter. When the voltage across switch Q3becomes zero, voltage across the power Then primary current continue to flowtransformer also becomes zero since the primary of through anti-parallel diode of switch Q2 so that Q2the transformer is shorted by the simultaneous can be turned on with ZVS. In this stage Voltageconduction of the body diode of Q3 and diode D. Vs1 across switch Q1, which is in opposition toAs a result, the secondary windings are also shorted voltage V2, is increasing and current Id startsso that rectifiers D2 and D3 can conduct the load decreasing. When Id becomes zero Diode D stopscurrent simultaneously. However, because of the conducting so that primary current startsleakage inductance of transformer TR, load current decreasing. Load current Io also begins toIo is still carried by the lower secondary through commutate from the lower secondary and D3 torectifier D3 since no voltage is available to upper secondary and D2. When the commutation ofcommutate the current from the lower secondary the load current from the lower to upper secondaryand D3 to the upper secondary and D2 if ideal is completed, the primary current commutationcomponents are assumed. With real components from the positive to negative direction is alsothis commutation voltage exists, but is too small to finished.commutate a significant amount of current from thelower to the upper secondary so that even with real The circuit stays with diagonal switchescomponents the majority of the current is still Q2 and Q3 turned on until the switch Q3 is turnedfound in the lower secondary and its corresponding off. Second half of the switching period is exactlyrectifier D3. So, during this stage when switches Q1 the same as the first half of the switching period.and Q3 are conducting, primary current stays nearlyunchanged. The circuit stays with diagonal switches Q2 and Q3 turned on until the switch Q3 is turned During this stage, diode D is conducting off. Second half of the switching period is exactlyand voltage V2 is applied directly across primary the same as the first half of the switching period.inductor Lp, Which increases current I2 until Q1 isturned off at t=t3 as shown in Fig 4(d).Current I2(t) IV. SIMULATION RESULTS.in the interval of t2 to t3 can be given as The ZVS DC to DC converter is simulated using Matlab Simulink are presented here.I2(t) = Ip+ Id(t)= Io/n + {(V/ 2)*ni *Lp(t-t2)}(1)Where Id(t) is the current across diode D. n=turn ratio of power transformer. During this stage, the voltage acrossswitch Q3 is kept zero due to D. So switch Q3 isturned on with ZVS. After Q1 is turned off, currentI2 begins charging output capacitance C1 of switchQ1 and discharging capacitance C2 of switchQ2.The total energy required to charge C1 and Fig.4 Simulink Model of ZVS DC to DC converterdischarge C2 is supplied from the stored energy inthe primary inductor Lp. To achieve ZVS energy Simulink model of DC to DC converter isstored in the primary inductor (ELP) must be shown in Fig 4. Driving pulses are shown inhigher than total energy required to charge C1 and Fig. 5.DC input voltage is shown in Fig 6.Outputdischarge C2. voltage across Q1 & Q2 is shown in Fig 7.Voltage across Q3 & Q4 are shown in Fig 8. .Secondary ELP≥CV2 (2) voltage is shown in Fig 9. DC output current andWhere C1=C2=C voltage are shown in Fig. 10. DC output voltage isUsing equation (1) 12V and the current is 1A. It can be seen that the DC output is free from ripple.  1 Io v(1  d )2 ELP   LP   (3) 2 n 2 Where, fs is the switching frequency.From equation (2) and (3) 1275 | P a g e
  5. 5. S.P.Narasimha Prasad, K.Vijaya Bhaskar / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 2, Issue 5, September- October 2012, pp.1272-1277 Fig.10 DC output current and voltage Fig.5 Driving Pulses For constant-frequency, variable dutycycle control of the proposed converter, switchesQ1 and Q2 always operate with approximately 50%duty cycle, whereas switches Q3 and Q4 have a dutycycle in the range from 0% to 50% as shown in Fig5. Fig.11 Open loop system Fig.6 DC Input Voltage Fig.12 Dc input voltage with disturbance Fig.7 Output Voltage across Q1 and Q2 Fig.13DC output voltage with disturbance Fig.8 Output voltage across Q3 and Q4 Fig.14 Closed loop system Fig. 9 Voltage across the secondary Fig.15 Dc input voltage with disturbance 1276 | P a g e
  6. 6. S.P.Narasimha Prasad, K.Vijaya Bhaskar / International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622 www.ijera.com Vol. 2, Issue 5, September- October 2012, pp.1272-1277 done. The simulation results are in line with the predictions. REFERENCES [1]. R. Redl, N. O. Sokal, and L. Balogh, “A novel soft- switching full bridge dc–dc Fig.16 Dc output voltage with disturbance converter: analysis, design considerations, at 1.5 kW, 100 kHz,” IEEE Trans. Power Electron., vol. 6, no. 4, pp. 408– 418,Jul. 1991 [2]. J. A. Sabaté, V. Vlatkovic´, R. B. Ridley, and F. C. Lee, “High-voltage,high- power, ZVS, full- bridge PWM converter employing an active snubber,” in Proc.Fig.17 Output current and voltage with disturbance IEEE APEC’91, 991, pp. 158– 163. [3]. W. Chen, F. C. Lee, M. M. Jovanovic´, and J. A. Sabaté, “A comparative study ofV. COMPARISONS OF OPEN LOOP a class of full bridge zero-voltage- SYSTEM WITH CLOSED. switched PWM converters,” in Proc. Simulink model of open loop system is IEEE APEC’95, 1995, pp. 893–899.shown in Fig.11 where input is given with [4]. P. K. Jain, W. Kang, H. Soin, and Y. Xi,disturbance. Fig.12 shows the DC input voltage “Analysis and design considerations of awith disturbance. Fig.13 shows DC output voltage load and line independent zero voltagewith disturbance. When input voltage changes due switching full bridge DC/DC converterto disturbance in Fig.12, output voltage also topology,” IEEE Trans. Power Electron.,changes. vol.17, no. 5, pp. 649–657, Sep. 2002. [5]. R. Ayyanar and N. Mohan, “Novel soft- Simulink model of closed loop system is switching DC-DC converter with fullshown in Fig.14. It consists of a feedback circuit. ZVS- range and reduced filter requirementThe R.M.S value of instantaneous voltage signal is – Part I: Regulated output applications,”taken from the output. To reduce the output, a gain IEEE Trans. Power Electron., vol. 16, no.of 0.95 is taken and given to the sub tractor. Other 2, pp.184–192, Mar. 2001.input to the sub tractor is the set voltage of 12V. [6]. A. J. Mason and P. K. Jain, “New phaseOutput of sub tractor is the error signal which is shift modulated ZVS fullbridge DC/DCgiven to the PI controller. The output of PI converter with minimized auxiliarycontroller is given to the two comparators whose current for medium power fuel celloutputs are PWM waves. They are fed to the gates application,” in Proc. IEEE Powerof MOSFETs 5&7as control signals. The Fig.15 Electron. Spec. Conf (PESC), 2005, pp.shows DC input voltage with disturbance and 244–249.Fig.16 shows DC output voltage with disturbance [7]. Y. Jang and M. M. Jovanovic´, “A newwhere output voltage changes with input. But the family of full- bridge ZVS converters,”output reduces to a value of 12V. Output current IEEE Trans. Power Electron., vol. 19, no.and voltage with disturbance are shown in 3, pp. 701–708, May 2004.Fig.17.Thus the closed loop system is able to [8]. Mangesh Borage,Sunil Tiwari,Shubhendumaintain constant voltage. Bharadwaj,and Swarna Kotaiah, “A Full- Bridge DC-DC Converter with Zero-VI. CONCLUSION. Voltage –Swiching over the Entire ZVS DC to DC converter is modeled Conversion Range,” IEEE Tras.Powerusing the blocks of Simulink. Soft switched ZVS Electron,vol.23,No.4,July 2008.PWM DC to DC Converter is analyzed and [9]. Martin Pavlovsky,Sjoerd Walter Hero desimulated and results are presented. Conversion Haan , and Jan Abraham Ferreira,”from 48V DC to 12V DC is done using soft Reaching High Power Density inswitched PWM converter. Switching losses and Multikilowatt DC-DC Converter Withstresses are reduced using zero voltage switching. Galvanic Isolation,” IEEE Trans. PowerThe simulation results are similar to the predicted Electron., vol. 24, no. 3, March 2009.results. This converter can be used for batterycharging and Electrolysis. The scope of this work isthe modeling and simulation of ZVS DC to DCconverter. Hardware implementation is yet to be 1277 | P a g e