Électronique et tranmiion de l information PERFORMANCE EVALUATION OF LLC RESONANT FULL BRIDGE DC-DC CONVERTER FOR AUXILIARY SYSTEMS IN TRACTION VEERA VENKATA SUBRAHMANYA KUMAR BHAJANA 1, PAVEL DRABEK 2, MARTIN JARA 3 Key word: Zero voltage witching (ZVS), Zero current witching (ZCS), Auxiliary power upply, Soft witching. Thi paper preent uperior performance of 8 kw LLC reonant full bridge dc-dc converter which ue a high frequency tranformer for auxiliary power upply in traction application. The comparion of hard witching and zero voltage witching (ZVS) of a Full bridge dc-dc converter with witching frequency of 100 khz i provided. Zero voltage witching wa achieved by employing the appropriate LLC reonant network. Operating principle are verified by imulation analyi which i upported by experiment on 8 kw laboratory prototype. The converter efficiency curve under both witching condition are preented a well. 1. INTRODUCTION Typical equipment of a traction vehicle involve variou ytem like cooling ytem, air preure, fan, etc. Thee ytem are uually upplied from a tandard 3 400V AC on-board power grid. The grid i generated by a tructure of dedicated converter which uually alo provide battery charging. The deign of uch auxiliary power upply ha to repect the fact that the catenary voltage of nominal voltage 600V DC or 750V DC may vary in the range of 400V DC 950V DC. Alo the output (either the power grid or battery charger) mut be iolated in order to provide a high level of afety. The converter efficiency play a ignificant role in the deign a well, ince it directly affect the ize of thermal management mean and thu the ytem cot. Employing one of the oft witching technique may therefore lead to a deired efficiency improvement. 1 Univerity of Wet Bohemia, Regional Innovation Centre for Electrical Engineering, Pilen, Czech Republic, kumarbvv@rice.zcu.cz 2 Univerity of Wet Bohemia, Department of Electromechanic and Power Electronic, Pilen, Czech Republic, drabek@ieee.org 3 Univerity of Wet Bohemia, Regional Innovation Centre for Electrical Engineering, Pilen, Czech Republic, jara@rice.zcu.cz Rev. Roum. Sci. Techn. Électrotechn. et Énerg., 60, 1, p. 79 88, Bucaret, 2015
80 Veera Venkata Subrahmanya Kumar Bhajana, Pavel Drabek, Martin Jara 2 Fig. 1 Auxiliary power upply block diagram. Figure 1 how concept of an auxiliary power upply (without a battery charger branch) which i ued in practice. It conit of the input voltage tabilizer (IVS) upplied from the catenary, tabilizing the output voltage to the catenary lower range level. Due to additional requirement from vehicle manufacture, uch a: mall ize, low weight and high frequency operation capability etc. the election of converter topology become crucial. Thi paper focue on the iolated dc-dc (Fig. 2) converter which convert tabilized 380V DC 400V DC to the level uitable for the power grid generator. Well etablihed LLC reonant full bridge topology [1] wa choen for the tudy ince it provide oft witching condition for all witche and promie high peed operation [2]. Thi topology wa propoed in a number of traction related application panning from MW (megawatt) locomotive drive [3] to fuel cell management in hybrid vehicle [4]. The diadvantage of the LLC converter i it capability to keep ufficient voltage gain only within a certain input voltage range [5]. The attempt to overcome thi drawback uually reult in more complex power circuitry [6] or low frequency operation [7]. Thi paper decribe the deign of an LLC converter operating in the ZVS region at witching frequency of 100 khz. The experimental data from the 8 kw laboratory prototype and efficiency comparion with hard-witching converter are preented.
3 Performance evaluation of LLC reonant full bridge dc-dc converter 81 Fig. 2 Full bridge reonant dc-dc converter. Section 2 preent the important operating mode of the converter, Deign criteria for LLC reonant converter are decribed in Section 3 and ection 4 preent imulation reult. The experimental reult and efficiency comparion are preented in Section 5 conequently. 2. SYSTEM DESCRIPTION AND OPERATION PRINCIPLES 2.1. SYSTEM DESCRIPTION Figure 2 how the chematic of a typical full bridge reonant dc-dc converter. The primary ide contain four active inulated gate bipolar tranitor (IGBT) S 1 S 4, the reonant capacitor (C), the reonant inductor (L) and the magnetizing inductance (L m ).The turn ratio of the tranformer i 1:1.5. The econdary ide of the tranformer include rectifying diode D 1 D 4, the filter capacitor (C o ) and the load reitance (R L ). The duty cycle of the primary-ide active witch i about 50%. When the S 1 and S 4 or S 2 and S 3 conduct, the input power i tranferred to the output load. Zero voltage witching i achieved by the reonance of L and C. 2.2. OPERATION PRINCIPLES When the reonant frequency of the full bridge dc-dc converter i et below the witching frequency zero voltage turning on of the converter power witche i poible. Thee condition enure that the reonant current (i Lr ) freewheel through the body diode of main witche prior to turn on, thu preventing turn on loe. Figure 3 how the key waveform of reonant full bridge converter, at t = t 0 the IGBT S 2 and S 3 are turned off what force the reonant tank current (i Lr ) to freewheel through the body diode of S 1, reonant tank and body diode of S 4. The voltage on S 1 and S 4, baically the diode drop, become cloe to zero when the gate turn on pule for S 1 and S 4 are applied at t = t 1. During the time t 1 to t 2, S 1 and S 4 are turned on, the output rectifier diode D 1, D 4 conduct and energy i tranferred
82 Veera Venkata Subrahmanya Kumar Bhajana, Pavel Drabek, Martin Jara 4 to the econdary through the tranformer. During the interval t 2 t 3, the power witche are turned off, the body diode of S 2 and S 3 create the path for freewheeling reonant tank current (i Lr ) which make ZVS condition for the following S 2 and S 3 turn on. Fig. 3 Key waveform ZVS operating region, where: V g1, V g2 are gate voltage of S 1, S 2 ; V P primary voltage; i Lr reonant current; i S1, i S2 current of S 1 and S 2. 3. DESIGN CRITERIA FOR RESONANT PARAMETERS The circuit component parameter were derived by firt harmonic analyi approach (FHA). Thi method i generally ued for reonant mode converter decription thank to it implicity while providing ufficient accuracy in the proximity of erie reonant pole. It i baed on the replacement of the rectangular input voltage waveform V in of the reonant network by it firt harmonic compound V in(fha) (1). The circuit can be then analyed by oberving the output harmonic voltage V out(fha). The rectifier and the load are modelled by a ingle component R EQU (Equivalent Reitance) derived by the equivalence of real DC and modelled AC diipated power (2). In (2) V out i output DC voltage; P out i output DC power and alo R DC i the load of the converter. With thee aumption the converter power circuitry can be implified according to Fig. 4, where L and C are the reonant component and L m i a magnetizing inductance of the tranformer. The leakage inductance i conidered to be a part of L in the model.
5 Performance evaluation of LLC reonant full bridge dc-dc converter 83 Fig. 4 Equivalent circuit of LLC reonant network. 4 V V in FHA π f t 2 8V out 8 REQU = = 2 2 π P π R ( ) = in ( 2π ) out Voltage gain (M) of the circuit i a function of a witching frequency, load and component parameter. It can be expreed a: Vout( FHA) 1 M = n = (3) V 2 2 in( FHA) k 2 1 1+ k Q f 2 + n fn fn f r k L DC. (1) (2) = (4) L m 1 = (5) 2 π LC f f n = (6) fr Z 0 L = (7) C Z Q = (8) nr 0. 2 EQU where f i witching frequency, f r reonant frequency, f n normalized frequency, Z o characteritic impedance, n tranformer ratio and Q i the quality factor repreenting load. Deign parameter k affect the hape of the gain-normalized frequency characteritic and it proper choice i a crucial deign tep of a LLC converter. Lower value allow broader regulation range conidering load and V in change at the expene of higher circulating current which increae conductive loe. For the decribed application the good regulation performance i not
84 Veera Venkata Subrahmanya Kumar Bhajana, Pavel Drabek, Martin Jara 6 required and alo the circulating current hould be minimized to keep the converter efficiency high. Therefore high value k = 15 wa elected. Together with tranformer magnetizing inductance L m = 217 μh, deigned for hard witched operation a well, it give L 15 μh uing (4). The converter wa intended to operate at witching frequency of f = 100 khz and 10% above the erie reonance. Applying (5) with the reonant frequency f r = 91 khz reult in C value of 200 nf. The gain veru normalized frequency chart preented in Fig. 5, gain (V o /V in ) i the ratio of output voltage (V o ) to input voltage (V in ) of the converter calculated for the load in the range from 3 kw to 9 kw uing the value derived above. The erie reonant frequency wa adjuted to 91 khz while the witching frequency of laboratory prototype wa 100 khz. Fig. 5 Gain-normalized frequency characteritic of the ZVS reonant converter. 4. SIMULATION RESULTS The LLC reonant full bridge dc-dc converter wa imulated in MATLAB environment uing PLECS block et. The imulation parameter were a follow: Input voltage = 400 V Output voltage = 600 V Switching frequency = 100 khz Reonant inductor (L ) = 15 µh Reonant capacitor (C ) = 200 nf Magnetizing inductance (L m ) = 217 µh Output power = 8 kw The reulting waveform howing ZVS turn-on of S 1 and S 2 witche are depicted in Fig. 6. Tranformer primary voltage and current are preented in Fig. 7.
7 Performance evaluation of LLC reonant full bridge dc-dc converter 85 Fig. 6 S 1 and S 2 voltage (a) and (b); S 1 and S 2 current (c) and (d). Fig. 7 Tranformer primary: a) voltage; b) current. 5. EXPERIMENTAL VERIFICATIONS The laboratory tet were conducted in teady tate for both ZVS operating converter uing LLC network and the hard witched one. The deign pecification for thi converter are V in = 400 V, V out = 600 V, P out = 3 kw to 8 kw and the witching frequency of 100 khz. The witche comprie of two IKW40N120H3
86 Veera Venkata Subrahmanya Kumar Bhajana, Pavel Drabek, Martin Jara 8 (Infineon) IGBT in parallel. Output rectifier i baed on SiC power module APTDC20H1201G (Microemi). The LLC network involve the reonant inductor L = 15 µh, reonant capacitor C = 200 nf and magnetizing inductance of the tranformer L m = 217 µh with ratio n = 1:1.5. Figure 8a how the primary voltage and current waveform of the hard witched converter and the witch voltage and current; Fig. 8b depict imilarly the waveform for ZVS operation; Fig. 8c how the primary voltage and current waveform, and Fig. 8d how the collector to emitter voltage and current. Figure 9 preent the witch commutation in detail. Fig. 8 a) Tranformer primary voltage and current Ch1:10A/Div Ch2: 100V/Div; b) collector to emitter voltage and current Ch1:10A/Div Ch2:100V/Div; c) tranformer primary voltage and current for ZVS operating region Ch1: 20A/Div Ch2: 250V/Div; d) collector to emitter voltage and current of the converter operate in ZVS operating region Ch1:10A/Div Ch2:100V/Div. The experimental reult obtained in Fig. 8d repreent the zero voltage turn on operation for the main witche, by etting the value of reonant inductor L = 15 µh and capacitor C = 200 nf, the reonant frequency remain below the witching frequency. Figure 9 how the collector to emitter voltage and current, when the main witche are turned on and turned off again.
9 Performance evaluation of LLC reonant full bridge dc-dc converter 87 Fig. 9 a) Collector to emitter ZVS turn on voltage and current Ch1: 10A/Div Ch2: 100V/Div; b) collector to emitter turn off voltage and current Ch1: 10A/Div Ch2: 100V/Div. Fig. 10 Efficiency comparion between hard witched and ZVS reonant full bridge dc-dc converter. 5.1. EFFICIENCY COMPARISON BETWEEN HARD SWITCHED AND ZVS FULL BRIDGE CONVERTERS In thi part, the efficiency of the dc-dc converter for auxiliary drive (fullbridge converter with high frequency tranformer and SiC diode rectifier) wa preented, comparing two topologie (uing reonant and hard witching). Fig. 10 how the efficiency comparion between hard witched and ZVS operating converter with 50% duty cycle. The efficiency of hard witching full bridge topology at maximum output power 8 kw wa 92.9% while ZVS reonant converter achieved 93.3%. 6. CONCLUSION Thi paper primarily focue on the performance of LLC reonant full bridge dc-dc converter for auxiliary ytem in traction with the maximum efficiency of 93.3% at 8 kw output power level. The experimental reult meaured for the
88 Veera Venkata Subrahmanya Kumar Bhajana, Pavel Drabek, Martin Jara 10 output power level from 3 kw to 8 kw with the 100 khz witching frequency are provided. The performance of reonant full bridge topology wa compared to the conventional hard witched variant of the converter. The experimental reult how an improvement by 0.4% at maximum power output. Thee reult confirm the fact that the ZVS full bridge dc-dc converter i uitable for auxiliary power upplie in traction application, where high efficiency i eential. ACKNOWLEDGEMENTS Thi work wa upported by the European Regional Development Fund and Minitry of Education, Youth and Sport of the Czech Republic under project No. CZ.1.05/2.1.00/03.0094: Regional Innovation Centre for Electrical Engineering (RICE) and alo thi work i realized by the project NEXLIZ CZ.1.07/ 2.3.00/30.0038, which i co-financed by the European Social Fund and the tate budget of the Czech Republic. Received on May 9, 2014 REFERENCES 1. X. Fang, H. Hu, Z. J. Shen, I. Batareh, Operation mode analyi and peak gain approximation of the LLC reonant converter, IEEE tranaction on power electronic, 27, 4, pp. 1985 1995, 2012. 2. Y. Liang, W. Liu, B. Lu, J. D. van Wyk, Deign of integrated paive component for 1MHZ 1kW half bridge LLC reonant converter, International conference on Indutry Application, Kowloon, Hong Kong, 2 6 October 2005, pp. 2223 2228. 3. Chuanhong Zhao, Wei M., Meter A., Lewdeni-Schmid S., Dujic D., Steinke J. K., Chaudhuri T., Power electronic tranformer converter deign of a 1.2MW demontrator for traction application, International Sympoium on Power Electronic, Electric Drive, Automation and Motion, Sorrento, Italy, 20 22 June 2012, pp. 529 551. 4. H. Kato, H. Matuo, T. Ito, S. Kawazu, D. Kawahara, S. Motomura, R. Hamaguchie, Comparative analyi of full bridge and half bridge current reonant dc-dc converter, Proceeding of Telecommunication Energy conference, Amterdam, Netherland, 9 th 13 th October 2011, pp. 1 6. 5. A. Florecu, S. Oprea, High efficiency LLC reonant converter with digital control, Rev. Roum. Sci. Techn. Électrotechn. et Énerg., 58, 2, pp. 183 192, 2013. 6. Z. Zhe, O. Thomen, M. Anderen, A dc-dc converter with wide input voltage range for fuel cell and upercapacitor application, IEEE International Conference on Power Electronic and Drive Sytem, pp. 706 711, Taipei,Taiwan, 2 5 th November 2009. 7. A. Coccia, F. Canale, P. Barboa, S. Ponnaluri, Wide input voltage range compenation in DC/DC reonant architecture for on-board traction power upplie, European Conference on Power Electronic and Application, Aalborg, Denmark, 2 5 September 2007, pp. 1 10.