Sihun Yang et al., Vol., No.4, nideration of Operating Characteritic for Bidirectional LLC Reonant nverter Sihun Yang*, Seiya Abe**, Tohiyuki Zaitu***, Junichi Yamamoto***, Maahito Shoyama*, Tamotu Ninomiya**** *Dep. of Electrical and Electronic Engineering, Faculty of Information Science and Electrical Engineering, Kyuhu Univerity **The International Centre for the Study of Eat Aian Development ***Texa Intrument Japan Ltd ****Faculty of Engineering, Nagaaki Univerity yang@ee.kyuhuu.ac.jp, abe@icead.or.jp, tzaitu@ti.com, j_yamamoto@ti.com, hoyama@ee.kyuhuu.ac.jp, ninomiya@nagaakiu.ac.jp rreponding Author; Sihun Yang, Dep. Of Electrical and Electronic Engineering, Faculy of Information Science and Electrical Engineering, Kyuhu Univerity, Fukuoka, 89395, Japan, 8 9 8 374, yang@ee.kyuhuu.ac.jp Received:.. Accepted: 3.. Abtract In the field of uing renewable energy uch a the PV ytem, the batterie are connected to the bue with bidirectional DCDC converter for enuring a table energy upply. Therefore, the bidirectional power conditioner have a key role in renewable power ytem. In thi paper, bidirectional LLeonant converter a the power conditioner i propoed. The ac analyi and operating characteritic of bidirectional LLeonant converter are invetigated. Finally, experimental reult are hown to confirm the validity of the propoed converter. Keyword bidirectional, LLeonant converter, AC equivalent model, operating characteritic.. Introduction With the depletion of foil fuel reerve and the increae in pollution, renewable energy ytem uch a the PV ytem have been carried out. In thee application, the batterie are connected to the bue with bidirectional DC DC converter for enuring a table energy upply. Therefore, a bidirectional DCDC converter i one of the mot important topic for power electronic [, ]. The bidirectional DCDC converter tend to require high efficiency, low noie and mall ize etc. The LLeonant converter i one of the profitable converter to achieve above demand, becaue of oftwitching operation and imple circuit contruction [36]. Fig. how the circuit diagram of bidirectional LLeonant converter. In the analyi of the LLeonant converter, the Fundamental Harmonic Simplification technique (FES) i uually ued. So far, the detail analyi of the LLeonant converter for forward operating mode ha been dicued. However, the revere direction mode of the LLeonant converter i le well argued o far, and it operating characteritic have not been clarified yet. Thi paper preent the ac analyi of bi direction mode by uing FES method.. Analyi of Reonant Topology The operating characteritic of reonant network can be evaluated by AC analyi baed on the FES method. And it i analyzed by uing the fmatrix. The ac equivalent model of the bidirectional LLeonant converter i hown in Fig. [7].
Sihun Yang et al., Vol., No.4,.. AC analyi of Forward Direction Mode When forward direction mode, the fmatrixe are multiplied from left ide of ac equivalent model. Forward direction mode V H Q 4 V L Revere direction mode Q 3 Fig.. Bidirectional LLeonant converter. Forward direction mode Revere direction mode I n: I V V ' ' Fig.. AC equivalent model of LLeonant converter. n V L r V C r I I L m n The load reitance which connected between port of AC equivalent model of the converter i converted AC equivalent reitance a follow ; 8 Rac R L From eq. (), the operating characteritic of forward direction can be derived a following equation of (3)(5). L m Z o _ F n 3 Lm Lm n Rac Lm n Rac Zin _ F L C n m r Rac Lm nrac MF 3 L m n R ac n R ac Output impedance: Z o_f, Input impedance: Z in_f and Voltage converion ratio: M F Fig. 3 how the impedance characteritic and the ltage converion ratio of forward direction mode uing eq. (3)(5). A hown in Fig. 3 (a), the two reonance peak of output impedance appear at fr (erie reonant peak) and fpr (parallel reonant peak). On the other hand, only one reonance peak appear between fr and fpr in the input impedance. One of the advantage of LLeonant converter i oft witching operation (ZVS turnon of primary ide witche and ZCS turnoff of econdary witche). In order () () (3) (4) (5) to achieve the oft witching operation, the witching frequency range hould be et optimal. For the ZCS operation of econdary ide witche, the range of witching frequency f i f<fr. Moreover, the range of witching frequency f i f>fzin (fzin : peak frequency of input impedance) for ZVS operation of primary ide witch network. Hence, the range of witching frequency for oftwitching operation (ZVS & ZCS) i fzin<f<fr. Voltage converion ratio MH Impedance : Zo, Zin ZVS & ZCS Region Zin Zo Peak of Zin ZCS Region (a) Impedance characteritic Heavy load M= (b) Voltage converion ratio Fig. 3. Operating characteritic of the forward direction A hown in Fig. 3 (b), the peak of converion ratio of forward direction mode i dumped at heavy load, and the peak frequency i ifted to high frequency ide. The movable range of the peak frequency i fpr<f<fr. From analyi reult, in the forward mode, the operating characteritic depend on load reitance and reonant parameter largely... AC Analyi of Revere Direction Mode When revere direction mode, the fmatrixe are multiplied from right ide of ac equivalent model. V V n C r I I n L m In revere direction mode, the rectifier operation become ltage doubler. Therefore, the AC equivalent reitance i derived follow ; fpr fzin fr Frequency (Hz) fr Frequency (Hz) ZVS Region (6) 79
Sihun Yang et al., Vol., No.4, Rac R L (7) From eq. (6), the circuit characteritic of revere direction mode can be derived a following equation of (8) (). Output impedance: Zo_R, Input impedance: Zin_R and Voltage converion ratio: MR Zo_ R (8) 3 L m Lm Rac Lm Zin _ R n R ac n C R (9) r ac M R () Rac Fig. 4 how the impedance characteritic and ltage converion ratio of revere direction mode uing eq. (8)(). A hown in Fig. 4 (a), only one reonance peak appear at fpr in the output impedance. On the other hand, there are two reonance peak appear at fr (erie reonant peak) and fpr (parallel reonant peak) in the input impedance. The frequency of all reonance peak of both impedance do not depend on load reitance. In revere direction mode, the frequency range of f>fr i good for oftwitching operation (ZVS turnon of econdary witche). Fig. 4 (b) how the ltage converion ratio of revere direction A hown in Fig. 4 (b), the peak frequency doe not change from fr. Alo, the peak ltage converion ratio doe not change, and doe not depend on load reitance a well. Furthermore, the ltage converion ratio doe not depend on magnetizing inductance due to eq. (). Hence, operation near the erie reonant frequency fr i good range for revere Voltage converion ratio MH (b) Voltage converion ratio Fig. 4. Operating characteritic of revere direction 3.. The circuit operation of forward direction mode The key waveform and operating tate of the forward direction mode are hown in Fig. 5 and 6, repectively. The operating witching frequency i between the erie reonant peak fr and the peak frequency of input impedance fzin in thi cae. In a half witching cycle, the operation of the LLC reonant converter for forward direction mode can be divided into ix tate [8]. State Thi tate begin when MOSFET turn off. During thi period, the current id flowing through MOSFET i negative, and it dicharge paraitic capacitor to enure ZVS operation. Thi tate will be end with the ltage of MOSFET V d being zero and the ltage of MOSFET V d being the power upply ltage Vin. v g v g v d v d i d /n fr Frequency (Hz) Heavy load 3. Analyi of Circuit Operation In thi ection, the circuit operation of the forward direction mode and the revere direction mode are invetigated in detail, repectively. i v v AK State State State3 State4 State5 State6 Fig. 5. Operation waveform of the converter (forward (a) Impedance characteritic 79
Sihun Yang et al., Vol., No.4, i d v g v d v g i Lm v cr i v d D vak D (a) Circuit diagram of the converter D (b) State D (c) State D (d) State 3 (g) State 6 Fig. 6. Operating mode of LLeonant converter (forward State The current id flow through the body diode of Q in thi tate. Thi tate will be end when the gate ignal of Q i inputted. State 3 Thi tate tart when turn on under the zero ltage. The drain current of flow from revere ide becaue of negative reonant current. The energy tored in the inductance and Lm and the reonant capacitor i fed back to the input terminal, dicharging. Thi tate will be end to the reonant current reache to magnetizing current. State 4 When the reonant current reache to magnetizing current, the diode D turn on. In thi tate, power i fed back to the input ide and power i inputted to the output ide. Thi tate will be end with the current i d of Q being zero. State 5 Thi tate tart when the reonant current i direction i revere to poitive. The current i d flowing through become poitive. The energy i upplied from the input terminal, charging. State 6 Thi tate tart when the reonant current equal to magnetizing current. The current i d flowing through D become zero. The energy flow from the input terminal, charging and the inductance and. For the next half witching cycle, the operation i ame a analyzed above. (e) State 4 D 3.. The Circuit Operation of Revere Direction Mode The key waveform and operating tate of the revere direction mode are hown in Fig. 7 and 8, repectively. The operating witching frequency i higher than the erie reonant peak fr in thi cae. In a half witching cycle, the operation of the LLeonant converter for revere direction mode can be divided into five tate. (f) State 5 D State Thi tate begin when MOSFET turn off. During thi period, the current id flowing through MOSFET i negative, and it dicharge paraitic capacitor of to enure ZVS operation. Thi tate will be end with the ltage of MOSFET V d being zero. State The current id flow through the body diode of in thi tate. The gate ignal of i inputted to turn on under the zero ltage. State 3 Thi tate tart when turn on under the zero ltage. The drain current of flow from revere ide 793
Sihun Yang et al., Vol., No.4, becaue of negative reonant current. Thi tate will be end to the reonant current reache to magnetizing current. State 4 When the reonant current reache to magnetizing current, the diode D turn on. The drain current of flow from revere, continuouly. Thi tate will be end to the reonant current reache to zero. D v g v g v d v d (c) State Q i d i D Q v (d) State 3 v AK D State State State3 State4 State5 Fig. 7. Operation waveform of the converter (revere v g (e) State 4 Q D vak i i Lm v d v d D (a) Circuit diagram of the converter D v cr i d v g (f) State 5 Fig. 8. Operating mode of LLeonant converter (revere State 5 Thi tate tart when the reonant current i become poitive. The current i d flowing through become poitive. The energy i upplied from the input terminal, charging the capacitor and i outputted to the output terminal through D. For the next half witching cycle, the operation i ame a analyzed above. Q Q (b) State 4. Experimental Reult In order to clarify validity of operating characteritic of forward and revere direction mode, the prototype circuit i 794
Output Voltage (V) INTERNATIONAL JOURNAL of RENEWABLE ENERGY RESEARCH Sihun Yang et al., Vol., No.4, implemented. The circuit parameter and pecification are hown in Table. In thi cae, the reonance peak fpr and fr are around 5kHz and 7kHz, repectively. 4.. Forward Direction Mode The output ltage characteritic are meaured a hown in Fig. 9. A hown in Fig. 9, the gain peak frequency of ltage characteritic i changed by load reitance. Table. The circuit parameter and pecification. Value Sign Parameter Revere Forward mode mode Input ltage 48V V n Turn ratio of Tranformer 3 Magnetizing inductance 37H Leakage inductance 6H Reonant capacitance nf Output Voltage (V) 5 45 4ohm 4 3ohm 35 ohm 3 ohm 5 5 5 5 5 3 Switchign Frequency (Hz) Fig. 9. Voltage gain of forward direction The peak frequency hift to high frequency range at heavy load, and the peak value of gain curve decreae at heavy load. Thee characteritic are the ame a analytical characteritic. Figure and how the experimental key waveform of forward direction The operating condition i W load. In thi cae, the witching frequency i khz and 3 khz, repectively. From experimental verification, the operating frequency range for ZVS operation i fzin<f<fr. 4.. Revere Direction Mode The output ltage characteritic are meaured a hown in Fig.. A hown in Fig., the gain peak frequency of ltage characteritic i not changed by load reitance, and the lop of gain curve i decreaed at light load. In thi cae, the peak frequency i around 7kHz. Thee characteritic are the ame a analytical characteritic. The peak value of ltage gain i lightly changed becaue the ltage drop i changed by load current. V g V d I d I V g : V/div V d : 5V/div I d : A/div : 5V/div I : A/div : V/div : A/div Time : u/div Fig.. Experimental waveform of the converter operating at khz (f < fr)and W load for forward direction V g V d I d I V g : V/div V d : 5V/div I d : A/div : V/div I :.5A/div : V/div : A/div Time : u/div Fig.. Experimental waveform of the converter operating at 3 khz (f = fr)and W load for forward direction 7 6 5 4 3 ohm 5ohm ohm 5ohm 5 5 3 35 Switching Frequency (Hz) Fig.. Voltage gain of revere direction 795
Sihun Yang et al., Vol., No.4, Figure 3 and 4 how the experimental waveform of the converter operating for revere direction From Fig. 3, the converter operate under ZVS operation when the converter operate at reonant frequency fr. Alo, turnoff urge ltage of witche i cloe on zero. From Fig. 4, the ZVS operation i confirmed when the converter operate at upper reonant frequency. However, when the converter operate at other frequency except in reonant frequency, turnoff urge ltage of witche i occurred, becaue the current phae of rectifier hift depending on witching frequency. 5. ncluion In thi paper, operating characteritic of bidirectional LLeonant converter i preented. Thi paper preent operation and ac analyi of the converter by uing FES method. Impedance characteritic and ltage converion ratio of bidirectional LLeonant converter i confirmed. Moreover, the circuit operation of bidirection mode are invetigated in detail. From experimental reult, it i confirmed that the ltage converion characteritic are imilar to analytical characteritic. From experimental waveform, the oftwitching operation i confirmed. V g V d I d I Fig. 3. Experimental waveform of the converter operating at 3 khz (f = fr)and W load for revere direction V g V d I d I V g : V/div V d : V/div I d : A/div : V/div I : A/div : V/div : A/div Time : u/div V g : V/div V d : V/div I d : A/div : V/div I : A/div : V/div : A/div Time : u/div Fig. 4. Experimental waveform of the converter operating at 35 khz (f > fr)and W load for revere direction mod. Reference [] R. Li, et al, Analyi and deign of improved fullbridge bidirectional DCDC converter, IEEE PESC Record, pp. 556, June 4. [] S. B. Kjaer, J. K. Pederen, and F. Blaabjerg, A review of inglephae gridconnected inverter for photoltaic module, IEEE Tran. Ind. Appl., l. 4, No. 5, pp. 936, Sep./Oct. 5. [3] M. Jain, M. Daniele, and P. K. Jaine, "A bidirectional DCDC converter topology for low power application", IEEE Tran. Power electron., l. 5, no. 4, pp. 59566, July. [4] B. Yang, F. C. Lee, A. J. Zhang, G. Huang, LLC Reonant nverter for front end DC/DC converion, in IEEE APEC, pp. 8, March. [5] Z. Pavlovic, J. A. Oliver, P. Alou, O. Garcia, J. A. bo, "Bidirectional Dual Active Bridge Serie Reonant nverter with Pule Modulation," IEEE APEC, pp. 53 58, February. [6] J. H. Jung, H. S. Kim, J. H. Kim, M. H. Ryu, J. W. Baek, "High Efficiency Bidirectional LLC Reonant nverter for 38V DC Power Ditribution Sytem Uing Digital ntrol Scheme," IEEE APEC, pp. 53538, February. [7] S. Abe, T. Zaitu, J. Yamamoto, T. Ninomiya, Principle of Overall AC Equivalent Model for Bidirectional Serie Reonant DCDC nverter, IEEE COMPEL, PS3, June. [8] K. Morita, Novel Ultra Lownoie oftwitchmode Power Supply, IEEE INTELEC, pp. 5, October 998. [9] Y. Zhang, D. Xu, K. Mino, K. Saagawa, MHzkW LLC Reonant nverter with Integrated Magnetic, IEEE PESC, pp. 95596, June 7. [] T. Liu, Z. Zhou, A. Xiong, J. Zeng, J, Ying, "A Novel Precie Deign Method for LLC Serie Reonant nverter," IEEE INTELEC, pp. 6, September 6. [] B. Lu, W. Liu, Y. Liang, F. C. Lee, J. D. van Wyk, "Optimal Deign Methodology for LLC Reonant nverter," IEEE APEC, pp. 533538, March 6. 796