Multi-Phase DC-DC converters using a Boost Half Bridge Cell for High Voltage and High Power Applications

Transcription

1 Multi-Phase C-C converters using a Boost Half Bridge Cell for High Voltage and High Power Applications Changwoo Yoon, and ewan Choi, EEE enior Member eoul National University of Technology ept. of Control and nstrumentation Eng. 7 Kongneung-ong, Nowon-Ku, eoul , Korea schoi@snut.ac.kr Abstract n this paper multi-phase C-C converters are proposed for high voltage and high power applications. A generalized converter is configured such that the BHB cells and voltage doublers are connected in parallel or series to increase the output voltage and/or the output power. n addition to significantly reduced device voltage and current ratings the proposed converter has the following features : significantly reduced transformer turn ratio, ZV turn-on of switches and ZC turn-off of diodes, no additional clamping and start-up circuits required, high component availability and easy thermal distribution due to the use of multiple small components, flexibility in device selection resulting in optimized design. A design guideline of determining the optimum circuit configuration for given output voltage and power level is presented. Experimental results are also provided to validate the proposed concept.. NTROUCTON The isolated boost C-C converter has been increasingly needed in high power applications such as fuel cell systems, photovoltaic systems, hybrid electric vehicles, and UP where good voltage regulation with high step-up ratio and the use of high frequency transformers for galvanic isolation and safety purpose are required. The multi-phase C-C converter could be a choice of topology for high power applications. Generally, the multi-phase C-C converter has several advantages over the single-phase C-C converter ; easy MOFETs selection due to reduced current rating, reduction of the input and output filters volume due to increased effective switching frequency by a factor of phase number compared to the single-phase C-C converter, reduction in transformer size due to better transformer utilization. everal isolated boost three-phase C-C converters have recently been proposed for high power applications [-6]. Their circuit topologies are usually based on a three-phase bridge converter [-5] or three single-phase bridge converters [6]. The basic cell consisting of each phase of the converter is a switch leg [-5] or a single-phase bridge [6] having inductive filters on output side or input side according to whether it is a voltage-fed or current-fed type. n the meanwhile, the boost half bridge (BHB) converter has been presented [7-]. t demonstrates the following features: small input filter due to continuous input current, low EM due to ZV turn ON of all power switches, wide input voltage range application due to wide duty cycle range. The BHB converter with a voltage doubler rectifier at the secondary has no dc magnetizing current of the transformer, reduced voltage surge associated with diode reverse recovery, and no circulating current due to absence of output filter inductor [9]. n this paper multi-phase C-C converters using a BHB converter as a basic building block are proposed for high voltage and high power applications. A generalized form of the multi-phase C-C converter is configured in such a way that the BHB cell and the voltage doubler rectifier are connected in series, parallel, or combination of them at the primary and secondary, respectively, to increase the output voltage and/or the output power. Therefore, the device current rating of the proposed multi-phase converter is reduced by increasing the number of parallel connection, and the device voltage rating is reduced by increasing the number of series connection. n addition to the advantages of the conventional multi-phase converter which include reduced current rating and reduced volume of input and output filters resulting from the interleaved switching, the proposed multi-phase converter has the following features: a. Reduced turn ratio of the transformer and voltage rating of the diodes and capacitors, and therefore especially suitable to high voltage applications b. Natural ZV turn-on of main switches using energy stored in transformer leakage inductor, and ZC turn-off of rectifier diodes which results in greatly reduced voltage surge associated with the diode reverse recovery c. No primary clamping and start-up circuits required due to the proposed interleaved asymmetrical PWM switching d. High component availability and easy thermal distribution due to the use of multiple small components instead of single large component, and no dc magnetizing current of the transformer e. Flexibility in device selection by proper choice of topology resulting in optimized design under harsh design specification /09/$ EEE 780 PEMC009 Authorized licensed use limited to: eoul National University of Technology. ownloaded on August 7, 009 at 03:46 from EEE Xplore. Restrictions apply.

2 The interleaving effect of the multi phase configuration is analytically described. How to determine the optimum circuit configuration for given output voltage and power level is also presented.. PROPOE MULET-PHAE C-C CONVERTER A. Generalized Multi-phase C-C Converter Fig. shows the BHB cell that is used as a building block of the proposed multi-phase converter. Fig. shows the generalized circuit of the proposed multi-phase C-C converter for high voltage and high power applications. The generalized converter has N groups of converters, where each group of switch legs is connected in parallel at the low voltage high current side while each group of voltage doublers is connected in series at the high voltage low current side. That is, N is the number of voltage doublers connected in series to form the output voltage. Each of N groups also has P parallel connected legs, where P is the number of switch or diode legs connected in parallel to increase the output power. For example, the N-th group having P parallel-connected legs includes input inductors L N to L NP, upper switches U,N to U,NP (lower switches L,N to L,NP ), transformers T N to T NP, and upper diodes U,N to U,NP (lower diodes L,N to L,NP ) which are connected to the same output capacitors C OU,N (C OL,N ). n summary, N should be increased to get higher output voltage, and P should be increased to get higher output power. Fig. 3 shows an example circuit configuration (P=) which illustrates how to increase output voltage by increasing N. Fig. 4 shows an example circuit configuration (N=) which illustrates how to increase power level by increasing P. n both Fig. 3. An example circuit configuration of the proposed multi-phase converter which illustrates how to increase the output voltage (P=) Fig. 4. An example circuit configuration of the proposed multi-phase converter which illustrates how to increase the output power (N=) cases the interleaving technique can be applied to reduce the size of input filter inductors and input and output capacitors. Therefore, N and P could properly be chosen according to given output voltage and power level. This could give flexibility in device selection resulting in optimized design even under harsh design specifications. B. Operating Principles The key waveforms of the generalized multi-phase C-C converter are shown in Fig. 5. The interleaved asymmetrical PWM switching is applied to the multi-phase converter. That is, and - are the duty cycles of lower and upper switches of a leg, respectively, and each leg is interleaved with a phase difference of /(N P). The average value of the inductor current can be obtained by, Fig.. BHB cell as a building block for the proposed multi-phase converter Lav, V = VR O O N P () The average value decreases as N P increases. The magnitude of the inductor current ripple is obtained by, V Δ L = () L f The positive peak value of the transformer leakage inductor current can be obtained by, Fig.. Proposed multi-phase C-C converter (N is the number of series-connected voltage doublers, and P is the number of diode legs connected to the same output capacitors) V N V O lk, + pk = + ( ) RO NP P + ( ) Lk fs ( ) (3) 78 Authorized licensed use limited to: eoul National University of Technology. ownloaded on August 7, 009 at 03:46 from EEE Xplore. Restrictions apply.

3 C. ZV Characteristics of Main switch The ZV current of main switches is related with the difference of the inductor current and the transformer leakage inductor current, i L - i Lk, as shown in Fig. 5. The negative peak of i L - i Lk is L,ZV which is ZV current for lower switches when the upper switch is turned off and can be expressed as, Fig. 5. Key waveforms of the generalized multi-phase C-C converter t decreases as P increases and does not depend on N. The negative peak value of the transformer leakage inductor current can be obtained by, V N ( ) V = + O lk, pk RO NP P + Lk fs ( ( ) ) t decreases as P increases and does not depend on N. (4) L, ZV = Lk, + pk ( L, av ΔL ) VO N V VO = + ( ) RO NP P ( + ( ) ) Lk fs V RO N P V + (5) L f The positive peak of i L - i Lk is U,ZV which is ZV current for upper switches when the lower switch is turned off and can be expressed as, U, ZV = Lk, pk + ( L, av + ΔL ) VO N ( ) V VO = + + RO NP P ( + ( ) ) Lk fs V RO N P V + (6) L f To ensure the ZV turn on of upper switch U the following condition should be satisfied, V L > C ( ) k U, ZV os, tot (7) n fact, the condition of equation (7) can easily be satisfied, and ZV of upper switch U can be achieved over the whole load range. To ensure the ZV turn on of lower switch L the following condition should be satisfied, Output Power [W] ZV region nput Voltage [V] (a) (b) Fig. 6. ZV currents and ZV ranges of lower and upper switches as the function of the input voltage and output power when N=3 and P= (a) lower switch (b) upper switch (V : 35~55V, V O = 400V, P O : 00W~5kW, N /N P =, L k=4uh) 78 Authorized licensed use limited to: eoul National University of Technology. ownloaded on August 7, 009 at 03:46 from EEE Xplore. Restrictions apply.

4 V L > C ( ) k L, ZV os, tot (8) The equation (8) may not be satisfied under the conditions of small transformer leakage inductance, large input filter inductance, and/or heavy load (Refer to equation (5)). ncreasing transformer leakage inductance to enlarge the ZV region makes the duty cycle loss large, resulting in increased turn ratio. nstead, in order to enlarge the ZV region, the input inductance can be decreased so that L,ZV can be increased. However, decreasing the input filter inductance increases the current rating of the power devices, and therefore the input filter inductance should be properly chosen considering a trade-off between the ZV region and the device current ratings. Using equations (5), (6), (7) and (8), the ZV currents and ZV ranges of lower and upper switches as the function of input voltage and output power are plotted, respectively, as shown in Fig. 6. As shown in Fig. 6(a), the ZV current of the lower switch tends to increase as the output power increases and decrease as the input voltage increases. This means that the ZV turn-on of the lower switch can be more easily achieved under the condition of higher output power and lower input voltage. t is noted that the ZV range of the lower switch becomes broader for smaller total output capacitance C os,tot =C oss,l +C oss,u of MOFETs. For example, if MOFETs with total output capacitance C os,tot of.5nf are selected in this example, the ZV turn-on of the lower switch can be achieved with output power which is greater than 000W at input voltage of 40V(ee Fig. 6(a)). The ZV current of the upper switch tends to increase as the output power and input voltage increase. t should be noted from Fig. 6(b) that the ZV turn-on of the upper switch can be achieved in the overall input voltage and output power ranges.. nterleaving Effect Each leg of the multi-phase converter is switched with a phase difference of /(N P). The ripple frequency of the input and input capacitor currents becomes N P times of the switching frequency of the main switch. The rms current of the input and input capacitor also decrease as N and P increases. The ripple frequency of the output capacitor current becomes P times of the switching frequency of the main switch. The rms current of the output capacitor decreases as P increases. ue to the interleaved operation the weight and volume of input capacitors, output capacitor and input inductors are significantly reduced. The interleaving effect on the input inductor and output capacitor of the proposed converter is obvious and has been mentioned in many literatures [-4]. The interleaving effect on the input capacitors C U and C L differs from that of the input inductor and output capacitor. The capacitor rms currents are calculated and plotted in Fig.7 as a function of input voltage and N. t tends to decrease as N increases in general. The capacitor current as a function of P is not shown in this paper and it also tends to decrease as P increases. E. Voltage Conversion Ratio The ideal voltage conversion ratio of the proposed converter can be obtained by, VO N N = (9) V N Basically, as N increases the voltage conversion ratio linearly increases. Considering the effect of voltage drop across the leakage inductance of the transformer the actual voltage conversion ratio can be obtained by, VO ( ) = V (( ) + ) Lk fs N ( ) NP P + N R P N N P O (0) n Fig. 8, the actual voltage conversion ratio is plotted as a function of duty ratio with different N and P. t can be seen that as P increases the voltage conversion ratio Capacitor RM current [A] (a) (b) Fig. 7. RM current of input capacitor as a function of input voltage and N when P= (a) C L (b) C U (V : 35~55V, V O=400V, P O: 00W~5kW, L k=4uh) 783 Authorized licensed use limited to: eoul National University of Technology. ownloaded on August 7, 009 at 03:46 from EEE Xplore. Restrictions apply.

5 Fig. 8. Voltage conversion ratio as a function of duty ratio with different N and P (V =35V, V O=400V, L k=4uh, f =50kHz, N /N P=, P O = 5kW) also slightly increases (theoretically, it converges to the ideal voltage conversion ratio as P increases to infinity) since the effect of the voltage drop across the leakage inductance on the voltage conversion ratio becomes smaller. n general, the duty cycles of the conventional voltage-fed and current-fed converters based on push-pull, half-bridge, or full-bridge topologies are restricted to smaller than 0.5 or larger than 0.5, respectively. However, the duty cycle of the proposed converter based on the BHB cell ranges from 0 to, resulting in no use of an additional clamping circuit as well as improved dynamic characteristics. t should be noted that no additional start-up circuits is also required for the proposed converter since there is no restriction on the duty cycle. t should also be noted that the rectifier diodes of the proposed converter are turned off with ZC, as shown in Figs. 5 and 0, surge voltage associated with the diode reverse recovery is trivial, and therefore snubber circuit does not necessitate.. EGN EXAMPLE n this section a design example of the proposed converter is presented considering the following specifications: P O = 5kW, V O = 400V, V : 35~55V, L k = 4uH, f = 50kHz s = 5% V O = 3% TABLE TRANFORMER TURN RATO AN VOLTAGE RATNG OF THE WTCH AN OE (V : 35~55V, V O = 400V, P O = 5KW) N Range of Range of witch iode N /N P N /N P V PK V PK 3.0 ~ ~ ~ ~ ~ ~ ~ ~ P TABLE CURRENT RATNG OF THE WTCH AN OE WTH N=3 (V : 35~55V, V O =400V, P O =5KW) Lower witch L,rms Upper witch U,rms Upper iode U,av ( U,PK) Lower iode L,av ( L,PK) (58) 4.0(58) (9) 7.0(9) (9.3) 4.7(9.3) (4.5) 3.5(4.5) large peak current rating of the components. Table lists the usable range of the transformer turn ratio for several cases of N for a specified duty cycle range of 0.3 < < 0.7. A proper transformer turn ratio is chosen within the usable range, considering the actual duty cycle range which also affects the voltage and current rating of the switch and diode. t should be noted that the peak voltage rating of the main switch is calculated to be V,min /(- max ) which does not depend on the output voltage, N and P. n this example, N is chosen to be 3 so that the switch voltage rating is 89V, and therefore the MOFETs with lower R ds(on) can be chosen for the proposed converter, resulting in reduced conduction losses. The peak diode voltage rating is 33V, a third of output voltage, due to the series connection of the three voltage doublers. A schottky diode of voltage rating of 70V with lower reverse recovery loss and forward voltage drop can be used, and the losses associated with rectifier diodes can be significantly reduced. With N= 3 and several cases of P, the current rating of the main switch and rectifier diode can also be calculated using the current waveform in Fig. 5. Table lists the current rating of the switch and diode for several cases of P when N=3. n this example P is chosen to be, and the MOFET XFH0N5(50V, The usable range of the transformer turn ratio can be calculated, using equation (9), as follows, V MAX N V < < V N N V N O O MN, MN P, MAX () The duty cycle of each switch of the proposed converter, theoretically, ranges from 0 to due to the switching method based on asymmetrical PWM. However, the operating duty cycle should be limited, say 0.3 < < 0.7, since too small or large duty cycles cause Fig. 9. Circuit diagram of the proposed converter with N=3 and P= 784 Authorized licensed use limited to: eoul National University of Technology. ownloaded on August 7, 009 at 03:46 from EEE Xplore. Restrictions apply.

6 TABLE RATNG OF PAVE COMPONENT WTH N=3, P= (V : 35~55V, V O= 400V, P O=5KW, = 5%, V O= 3%) nput filter inductor L,L,L 3 nput capacitor C U, C L Output capacitor C OU, C OL esign item nductance rms( pk) Capacitance rms( pk) Capacitance rms ( pk) Value uh 48.45A(64.65A) 0uF 56.9A (.69A) 6.8uF 7.7A(39.A) Fig. 0. Key waveforms of the proposed converter with N=3 and P= 0A) and the schottky diode TP3070C(70V, 30A) are selected, which satisfy both voltage ratings in Table and current ratings in Table. The circuit diagram of the proposed multi-phase converter with N=3 and P= is shown in Fig. 9. The proposed converter with N=3 and P= consists of three filter inductors, six MOFET switches, two input capacitors at the low voltage side and three series-connected voltage doubler rectifiers at the high voltage side. Three high frequency transformers are employed for step-up and isolation. Three voltage doubler rectifiers are connected in series at the output so that the diode voltage rating becomes one third of the output voltage. The proposed converter also employs six output capacitors, but total energy volume of the capacitors are smaller since the capacitor peak voltage is much smaller. ince the ideal voltage conversion ratio of the proposed converter is three times that of the conventional BHB converter the required turn ratio has greatly reduced, N P : N = : in this example, which reduces the number of turns of the transformer winding, leading to reduced copper loss and leakage inductance of the transformer. The ratings of the passive components such as the input inductor, the input capacitor, and the output capacitor according to the design specification are calculated and listed in Table. Fig. 0 shows key waveforms of the proposed converter with N=3 and P=. The three switch legs are interleaved with 0 phase shift, and the upper and lower switches of each leg are operated with asymmetrical complementary switching to regulate the output voltage. The converter has six operating modes within each operating one third cycle. Fig. shows the equivalent circuits of the 6 operating modes. At the beginning of Mode 3, the output capacitor of upper switch U, is discharged by U,ZV, summation of the input filter inductor current and the transformer leakage inductor current at t 3, which is determined by (7). At the beginning of Mode 6, the output capacitor of lower switch L, is discharged by L,ZV, difference between the transformer leakage inductor current and the input filter inductor Fig.. Operation modes of the proposed converter with N=3 and P= 785 Authorized licensed use limited to: eoul National University of Technology. ownloaded on August 7, 009 at 03:46 from EEE Xplore. Restrictions apply.

7 (a) (c) (d) Fig.. Experimental waveforms (a) inductor currents (b) input current (c) voltage and current waveforms of upper switch (d) voltage and current waveforms of lower switch current at t 6, which is determined by (8). V. EXPERMENTAL REULT A prototype of the proposed multi-phase converter with N=3 and P= (Fig. 9) has been built to demonstrate the operating principles. The proposed converter is experimented under the following system parameters. P O =kw, V O =360V, V :~4V, f =50kHz N P :N =5:5, L k =uh, L =L =L 3 =8uH, C U =C L =30uF, C OU =C OL =uf Experimental waveforms for the proposed scheme are shown in Fig.. Fig. (a) shows three interleaved inductor current waveforms and Fig. (b) shows the input current. t can be seen from Fig. (c) and (d) that both upper switch and lower switch are being turned on at zero voltage. V. CONCLUON This paper proposes a generalized multi-phase C-C converter using the BHB cell and voltage doubler for parallel and/or series connection to increase the output voltage and/or the output power. The proposed converter has the following features: significantly reduced transformer turn ratio, ZV turn-on of switches and ZC turn-off of diodes, no additional clamping and start-up circuits required, high component availability and easy thermal distribution, flexibility in device selection resulting in optimized design. Therefore, the proposed multi-phase converter is suitable for high voltage and high power applications. A way of determining the optimum circuit configuration for given output voltage and power level has been presented. Experimental results have also been provided to validate the proposed concept. (b) REFERENCE [] G. M. ivan and R.W. A. A. e oncker, A three phase soft switched high-power density dc/dc converter for high power applications, EEE Trans. nd. Applicat., vol. 7, no., pp , Jan./Feb. 99. [] emercil. Oliveira Jr. and vo Barbi, "A Three-Phase ZV PWM C/C Converter with Asymmetrical uty Cycle for High Power Applications," in Proc. EEE PEC003, vol., pp [3].V.G Oliveira, vo Barbi, "A three-phase step-up C-C converter with a three-phase high frequency transformer," in Proc. EEE E, vol., pp June. 0-3, 005 [4] Hanju Cha, Prasad Enjeti, A Novel Three-Phase High Power Current-Fed C/C Converter with Active Clamp for Fuel Cells, in Proc. EEE PEC007, June. 7-, 007, pp [5] Hyungjoon Kim, Changwoo Yoon, ewan Choi, A three-phase ZVZC C-C converter for fuel cell applications, in Proc. EEE PEC08, 5-9 June 008, pp [6] Jih-heng Lai, "A high-performance V6 converter for fuel cell power conditioning system," in Proc. EEE VPPC, pp.7, ept [7] Khairy Fathy, Hyun Woo Lee, Tomokazu Mishima and Mutsuo Nakaoka, Boost-Half Bridge ingle Power tage PWM C-C Converter for mall cale Fuel Cell tack, in Proc. EEE PECon, 8-9 Nov. 006, pp [8] J. Zeng, J. Ying, and Q. Zhang, A novel dc dc ZV converter for battery input application, in Proc. EEE Appl. Power Electron. Conf. Expo (APEC), Mar. 00, vol., pp [9] Chong-Eun Kim, Gun-Woo Moon, ang-kyoo Han, Voltage oubler Rectified Boost-ntegrated Half Bridge (VRBHB) Converter for igital Car Audio Amplifiers, EEE Trans. Power electronics., vol., pp ,Nov. 007 [0] H.Watanabe and H. Matsuo, A novel high-efficient C-C converter with V/0A dc output, in Proc. EEE nt. Telecommun. Energy Conf. (NTELEC), ept./oct. 00, pp []. Choi, B. Lee,. Choi, C. Won,. Yoo, "A Novel Power Conversion Circuit for Cost-Effective Battery-Fuel Cell Hybrid ystems," Journal of Power ources, ec. 005, Vol. 5, pp [] B. estraz, Y. Louvrier, A. Rufer, High Efficient nterleaved Multi-channel dc/dc Converter edicated to Mobile Applications, in Proc. EEE ndustry Applications Conference, st A Annual Meeting, Oct. 006, Vol. 5, pp [3] M. Gerber, J.A. Ferreira,.W. Hofsajer, N.eliger, nterleaving optimization in synchronous rectified C/C converters, in Proc. EEE Power Electronics pecialists Conference 004., PEC 04, June 004, Vol:6, pp [4] Chin Chang, M.A. Knights, nterleaving technique in distributed power conversion systems, EEE Trans Circuits and ystems,vol: 4, ssue: 5, pp. 45 5, May 995 [5] Hyungjoon Kim, Changwoo Yoon, ewan Choi, An improved current-fed ZV isolated boost converter for fuel cell applications, in Proc. EEE APEC, 4-8 Feb. 008, pp Authorized licensed use limited to: eoul National University of Technology. ownloaded on August 7, 009 at 03:46 from EEE Xplore. Restrictions apply.