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Trasformer Desig Difficulties of Curret Resoat Coverter for High Power Desity ad Wide Iput ltage Rage Toshiyuki Zaitsu Embedded System Research Ceter Omro Corporatio Kyoto, Japa toshiyuki_zaitsu@omro.

2 Trasformer Desig Difficulties of Curret Resoat Coverter for High Power Desity ad Wide Iput ltage Rage Toshiyuki Zaitsu Embedded System Research Ceter Omro Corporatio Kyoto, Japa Seiya Abe Life Sciece of Systems Egieerig Kyushu Istitute of Techology Kitakyushu, Japa Abstract A curret resoat coverter icludig wider iput voltage rage ad higher power desity is required for various applicatios. However, i our study, it has bee foud that the drastic icreasig of the trasformer widig loss becomes a severe problem. This paper clarifies the mechaism of trasformer widig loss occurrece ad proposes the differet resoat mode operatio which solves the problem of the huge trasformer loss. Keywords Curret resoat coverter; High power desity; Wide iput voltage; Trasformer loss I. INTRODUCTION A Curret resoat coverter such as LLC coverter is oe of the most popular topology as soft-switchig DC-DC coverter, ad it is widely used for cosumer ad idustrial applicatios because it has followig good performace[1-4]; (1) ZVS (Zero ltage Switchig) capability of primary side switches at tur o time. () ZCS (Zero Curret Switchig) capability of the secodary side diodes at tur off time. (3) The output voltage cotrol capability by resoat frequecy characteristic. (4) Small size by usig itegrated trasformer. Moreover, the sychroous rectificatio techique of LLC coverter has bee itroduced to the market i order to achieve higher efficiecy. As a result, LLC coverters become very popular i may applicatios [5-9]. O the other had, a PFC (Power Factor Collectio) coverter as a pre-regulator is geerally used for LLC coverter i those applicatios. Therefore, LLC coverters ca operate easily with arrow iput voltage rage. Recetly, LLC coverters are expected for DC-DC coverters without preregulator ad smaller size. The wider iput voltage rage capability ad higher power desity are ecessary to satisfy both requiremets. However, as show i Fig.1, those requiremets cause a severe problem of huge trasformer widig loss by three reasos as below. 1. Large cupper loss by fier widig.. Large magetizig loss by small magetizig iductace. 3. Large eddy curret loss by wide air gap betwee trasformer cores. Those fatal drawbacks ihibit the realizatio of LLC coverter with wide iput voltage rage ad high power desity. This paper clarifies the mechaism of trasformer loss occurrece i the covetioal LLC coverter. Moreover, LC series resoat mode (LC mode) operatio is proposed i order to reduce the trasformer loss. The evaluatio board is implemeted with the specificatio of 1V-4V iput voltage rage ad 1V/3A output to compare with LLC mode ad LC mode operatio. As a result, the trasformer widig loss is dramatically reduced with LC mode operatio. Large coversio ratio Small flux desity Larger turs ratio L large High Power Desity High Freq. fsw, Fier litz wire 5A/mm High curret 1A/mm desity 1W/cc 1W/cc Smaller Tras. Wide air gap Air gap Dilemma Triple Problems for Trasformer widig Large cupper loss by fier widig Large magetizig loss by small Lm Large eddy curret loss by wide air gap Fig. 1. Triple trouble for trasformer widigs Tras. Widig Larger Eddy Curret Wide Iput ltage Rage Narrow widig widow Vds ILr Vi_max Vi_mi Steep shape of voltage characteristic Idiode Small : Lm Large I Lm Small Lm ILm Larger Magetizig Curret Over 15deg.

3 II. MECHANISM OF LARGE WINDING LOSS OCCURENCE Figure shows a covetioal LLC coverter. There is o smoothig iductor at output side, ad the resoat iductor is icluded i the itegrated trasformer. So, the LLC coverter ca achieve small size. O the other had, the trasformer is the most importat elemet to decide the size of the LLC coverter. I the commercialized LLC coverter, the trasformer accouts for -3% of coverter circuit. Therefore, miiaturizatio of trasformer is oe of the most importat techology to make high power desity LLC coverter. I order to desig optimal for wide iput voltage ad high power desity, it is ecessary to grasp the frequecy characteristics of LLC coverter i detail. The frequecy characteristics of LLC coverter are give by usig well kow FHA(Fudametal Harmoics Aalysis) techique [11-16], ad the output voltage is derived as followig equatio Eq. (1).-(4). Figure 3 shows a geeral frequecy characteristic of the output voltage, ad the optimal operatig regio to achievig primary side ZVS ad secodary side ZCS. I the covetioal LLC coverters, the frequecy characteristic ca accept the gradual curve due to arrow iput voltage rage with the pre-regulator as show i Fig.3. Vi = V i Q 1 Q 1 1 f sr f f sr 1+ 1 Q + L f f L r C r ir L m :1:1 im Fig.. Covetioal LLC coverter. D D 1 id1 (1) C o R L + v o - L f L m = () Lr sr 1 = (3) π LC π Lr Q = (4) 8 C RL r r r However, realizatio of wide iput voltage rage capability ad high power desity have some problems metioed above. I order to solve those issues, the mechaism of loss occurrece should be clarified. Here, the trasformer widig loss occurrece mechaism is discussed uder the specificatio of Vi=1V-4V, =1V, Io=3A, fsw=5khz, ad PC95PQ65 (TDK) core is used for the trasformer targetig 1W/cc power desity coverter. A. Large cupper loss As show i Eq (5), the turs-ratio is decided by the output voltage value usig Vi_max at frequecy of. Therefore, the large umber of trasformer turs-ratio is eeded to satisfy ZVS & ZCS operatio for wide iput voltage rage. LLC V = V i _ max o (5) O the other had, the high switchig frequecy is eeded i order to achieve the high power desity. Hece, the small size trasformer core for high power desity ad large tursratio for wide iput voltage rage lead to fier widigs as show i Fig.1. This results i larger copper loss. B. Large mgetizig loss As show i Fig.3, geeral LLC coverters with arrow iput voltage desig (L=5, Q=.5) does ot reach to desired output voltage whe the iput voltage is low. I order to achieve the desired output voltage at the miimum iput voltage Vi_mi, the output voltage characteristic has to be steep shape as show i Fig.4 [17, 18], ad the small magetizig iductace achieves this steep shape. Figure 5 shows the frequecy characteristic of steep shape by small magetizig iductace with Vi_max ad Vi_mi. However, this small iductace results i a large magetizig curret loss. Output ltage (V) ZVS ZCS V = i _ max _ LLC Output ltage (V) Small Lm fpr Frequecy (Hz) Frequecy (Hz) Fig. 4. frequecy characteristic with small Lm. Fig. 3. Covetioal LLC coverter frequecy characteristic

Figure 6 shows the magetizig curret ad magetizig curret loss agaist iput voltage ratio (Vi_max/Vi_mi) whe the umber of widig is =. Whe the iput voltage ratio is 4.

However, the wide air gap leads to large leakage flux, ad the large eddy curret flows through the trasformer widigs. Therefore, the ac equivalet resistace of trasformer widig becomes larger.

4 Figure 6 shows the magetizig curret ad magetizig curret loss agaist iput voltage ratio (Vi_max/Vi_mi) whe the umber of widig is =. Whe the iput voltage ratio is 4., the magetizig curret loss reaches to W which is ot acceptable. C. Large eddy curret loss I order to realize small magetizig iductace Lm, a wider air gap betwee the trasformer cores is eeded. However, the wide air gap leads to large leakage flux, ad the large eddy curret flows through the trasformer widigs. Therefore, the ac equivalet resistace of trasformer widig becomes larger. Figure 7 shows the required air gap agaist the iput voltage ratio. Over 7mm air gap is eeded whe the iput voltage ratio is 4.. Figure 8 shows the electromagetic field simulatio result with 1.mm air gap as a example. It is foud out the curret cocetratio occurs by the ifluece of the eddy curret. III. SOLUTION OF TRIPLE PROBLEMS I order to overcome metioed above loss trouble of the trasformer widigs, followig specificatios are desired i the trasformer. <1 > Small turs-ratio for small copper loss <> Large magetizig iductace for small magetizig curret loss <3> Narrow air gap for small eddy curret loss The solutio for the problems metioed above ca be figured out by oly the operatig mode chage of LLC coverter. It ca improve dramatically ot oly trasformer widig loss but also trasformer desig difficulty. Figure 9 shows the operatig coditio of proposed LC series resoat mode (LC mode) operatio. As show i Fig. 3 ad 5, the operatio frequecy is lower tha series resoat frequecy, ad magetizig curret cotributes to the output voltage. The triple problems occur i covetioal LLC mode operatio as metioed above. By chagig the operatio frequecy higher tha, this coverter operates as LC mode as show i Fig.9. I this case, there is o ifluece of magetizig iductace. Magetizig Curret Ipk(A) Ipeak(fop) Gap Iput ltage ratio Fig. 7. Required air gap of trasformer core. Air Gap (mm) Tras. Widig ZVS Output ltage (V) Vi_max Vi_mi Fig. 8. Electromagetic field simulatio result of eddy curret. Frequecy (Hz) ZVS Fig. 5. frequecy charateristic of steep shcape by small iductace. Magetizig curret peak (A) 5 18 Peak of magetizig curret 16 Magetizig curret loss Iput ltage ratio Magetizig curret loss (W) Output ltage (V) Fig. 6. Magetizig curert ad loss vs iout voltage ratio. Fig. 9. frequecy charactersitc of LC mode oepratio.

Therefore, larger magetizig iductace is acceptable which ca reduce the air gap betwee the trasformer cores. This results i the low magetizig curret loss ad eddy curret loss.

LC V = V i _ mi o (6) IV. EXPERIMENTAL RESULTS I order to evaluate metioed above discussios, the prototype evaluatio board is implemeted for both of LLC ad LC mode operatio.

5 Therefore, larger magetizig iductace is acceptable which ca reduce the air gap betwee the trasformer cores. This results i the low magetizig curret loss ad eddy curret loss. Also, the copper loss of trasformer widigs should be reduced due to the small trasformer turs-ratio LC. Because LC is decided by miimum iput voltage i LC mode operatio as show i Eq. (6). LC V = V i _ mi o (6) IV. EXPERIMENTAL RESULTS I order to evaluate metioed above discussios, the prototype evaluatio board is implemeted for both of LLC ad LC mode operatio. The circuit parameters ad specificatios are show i Table 1. The desig specificatios are Vi=V-4V, =1V ad Io=3A for feasible costructio. Output ltage (V) TABLE I. Descriptio LC mode LLC mode _LC Fig. 1. Operatig coditio for both mode. CIRCUIT PARAMETERS AND SPESIFICATIONS Symbol fop LLC _LLC Test coditio Frequecy (Hz) Value Iput ltage Vi V Output Coditio / Io 1V/15A Resoat Iductace Lr 11uH 8.4uH Magetizig Iductace Lm 18uH - Resoat Capacitor Cr 4.7F 47F Turs Ratio 8 Primary Widig Litz wire φ.1*6 φ.1*1 Secodary Widig Cu plate T:.3mm W:.5mm Air Gap 3.5mm - Resoat Frequecy 7kHz 5kHz Switchig Frequecy fsw 51kHz 45kHz LC I order to compare fairly for both coverters, the test coditio is set to Vi=V, =1V with operatig frequecy is aroud 5kHz. As show i Fig.1, i order to realize those coditio, the series resoat frequecy of LLC coverter, _LLC, is set to aroud 7kHz so that LLC coverter ca operate aroud 5kHz. Ad, the series resoat frequecy of the proposed LC mode coverter, _LC, is set to aroud 5kHz so that it ca operate aroud 5kHz as well to get =1V. PC95PQ65 (TDK) is used as the trasformer core. The turs ratio _LLC of the trasformer is for LLC mode coverter, ad the turs-ratio _LC is 8 for LC mode coverter, respectively. Figure 11 shows the key waveforms ad the temperature distributio of covetioal LLC mode desig board. Vds(Q) Temperature (deg) 1App V (a) Key waveforms im (b) Trasformer widig temperature (c) Temperature Fig. 11. Covetioal LLC coverter. ir (5A/div) Time (m)

Figure 1 shows the key waveforms ad the temperature distributio of the proposed LC operatio desig board. As show i Fig.

6 As show i Fig. 11 (a) the resoat curret ir is domiated by the magetizig curret im which is aroud 1Ap-p. The widig temperature is icreased to 176deg, especially the hot spot occurs aroud the air gap of core as show i Fig.11 (c). Figure 1 shows the key waveforms ad the temperature distributio of the proposed LC operatio desig board. As show i Fig.1 (a) the resoat curret ir is dramatically reduced to 4Ap-p, ad the maximum temperature is reduced to aroud 76deg as well as show i Fig.1 (b) ad (c). V. CONCLUSIONS This paper ivestigate the mechaism of trasformer widig loss occurrece for LLC coverter with high power desity ad wide iput voltage rage, ad the solutio for the loss problems is proposed. As a result, it is clarified that the mechaism of triple problems of trasformer widigs (magetizig loss, eddy curret loss, copper loss). The solutio for the problems is figured out by oly the operatig mode chage of LLC coverter, which ca dramatically improve ot oly trasformer desig difficulty but also trasformer widig loss. REFERENCES Temperature (deg) Vds(Q) 4App V (a) Key waveforms (b) Trasformer widig temperature (c) Temperature ir (A/div) Time (m) [1] Yaju Zhag, Dehog Xu, Mi Che, Yu Ha, Zhog Du, "LLC Resoat Coverter for 48V-.9V VRM," PESC'4, pp , 4. [] Y. Zhag, D. Xu, K. Mio, K. Sasagawa, "1MHz-1kW LLC Resoat Coverter with Itegrated Magetics," PESC'7, pp , 7. [3] K. Morita, "Novel Ultra Low-oise soft-switch-mode Power Supply," INTELEC'98, pp , [4] F. Musavi, M. Craciu, M. Edigto, W. Eberle, W. G. Duford,"Practical Desig Cosideratios for a LLC Multi-Resoat DC-DC Coverter i Battery Chargig Applicatios," APEC'1, pp , 1 [5] C. Zhao, L. Bao-hog, J. Cao, Y. Che, X. Wu, Z. Qia, "A Novel Primary Curret Detectig Cocept for Sychroous Rectified LLC Resoat Coverter," ECCE'9, pp , 9 [6] D. Fu, Y. Liu, F. C. Lee, M.g Xu, "A Improved Novel Drivig Scheme of Sychroous Rectifiers for LLC Resoat Coverters,"APEC'8, pp , 8 [7] W. Feg, P. Mattavelli, F. C. Lee, D. Fu, "LLC Coverters with Automatic Resoat Frequecy Trackig Based o Sychroous Rectifier (SR) Gate Drivig Sigals,"APEC'11, pp. 1-5, 11 [8] G. Zhag, J. Zhag, C. Zhao, X. Wu, Z. Qia, "LLC Resoat DC/DC Coverter with Curret-Drive Sychroized ltage-doubler Rectifier,"ECCE'9, pp , 9 [9] S. Abe, T. Zaitsu, J. Yamamoto, S. Ueda, S. Yag, M. Shoyama, T. Niomiya, "Sesig-less Drive of Sychroous Rectifier for LLC Resoat Coverter," INTELEC'1, sessio., 1 [1] X. Fag, H. Hu, L. Che, A. Amirahmadi, J. She, I. Batarseh," Operatio Aalysis ad Numerical Approximatio for the LLC DC-DC Coverter," APEC'1, pp , 1 [11] H. Huag,"FHA-Based ltage Gai Fuctio with Harmoic Compesatio for LLC Resoat Coverter," APEC'1, pp , 1 [1] J. F. Lazar ad R. Martielli, "Steady-State Aalysis of the LLC Series Resoat Coverter," APEC '1, pp , 1. [13] Ashoka K. S. Bhat,"A Geeralized Steady-State Aalysis of Resoat Coverters Usig Two-Port Model ad Fourier-Series Approach," IEEE Tras. o P. E., vol. 13, No. 1, pp , 1998 [14] R. L. Steigerwald, "A Compariso of Half-Bridge Resoat Coverter Topology," IEEE Tras. O PE, l. 3, No., pp , [15] V. rperia, "High-Q Approximatio i The Small-Sgal Aalysis of Resoat Coverters," PESC'85, pp , 1985 [16] V. rperia, S. Cuk, " A Complete DC Aalysis of The Series Resoat Coverter," PESC'8, pp. 85-1, 198 [17] B. Lu, W. Liu, Y. Liag, F. C. Lee, J. D. va Wyk,"Optimal Desig Methodology for LLC Resoat Coverter," APEC'6, pp , 6 [18] T. Liu, Z. Zhou, A. Xiog, J. Zeg, J, Yig,"A Novel Precise Desig Method for LLC Series Resoat Coverter," INTELEC'6, pp. 1-6, 6 Fig. 1. Propsosed LLC coverter with LC mode operatio.

doi: info:doi/ /epe17ecceeurope

doi: info:doi/ /epe17ecceeurope doi: info:doi/10.3919/epe17ecceeurope.017.8099085 Solution of Triple Problems in Transformer Windings for Current Resonant Converter with High Power Density and Wide Input ltage Range Seiya Abe (1), Toshiyuki