A direct high step-down DC/DC converter using cascade ring-type converters

Eguchi, Kei; Shibata, Akira; Harada, Yujiro Article A direct high step-down DC/DC converter using cascade ring-type converters Energy Reports Provided in Cooperation with: Elsevier Suggested Citation: Eguchi, Kei; Shibata, Akira; Harada, Yujiro (2020) : A direct high step-down DC/DC converter using cascade ring-type converters, Energy Reports, ISSN 2352-4847, Elsevier, Amsterdam, Vol. 6, Iss. 2, pp. 119-124, https://doi.org/10.1016/j.egyr.2019.11.051 This Version is available at: https://hdl.handle.net/10419/243867 Standard-Nutzungsbedingungen: Die Dokumente auf EconStor dürfen zu eigenen wissenschaftlichen Zwecken und zum Privatgebrauch gespeichert und kopiert werden. Sie dürfen die Dokumente nicht für öffentliche oder kommerzielle Zwecke vervielfältigen, öffentlich ausstellen, öffentlich zugänglich machen, vertreiben oder anderweitig nutzen. Sofern die Verfasser die Dokumente unter Open-Content-Lizenzen (insbesondere CC-Lizenzen) zur Verfügung gestellt haben sollten, gelten abweichend von diesen Nutzungsbedingungen die in der dort genannten Lizenz gewährten Nutzungsrechte. Terms of use: Documents in EconStor may be saved and copied for your personal and scholarly purposes. You are not to copy documents for public or commercial purposes, to exhibit the documents publicly, to make them publicly available on the internet, or to distribute or otherwise use the documents in public. If the documents have been made available under an Open Content Licence (especially Creative Commons Licences), you may exercise further usage rights as specified in the indicated licence. https://creativecommons.org/licenses/by-nc-nd/4.0/ Available online at www.sciencedirect.com ScienceDirect Energy Reports 6 (2020) 119–124 www.elsevier.com/locate/egyr The 6th International Conference on Power and Energy Systems Engineering (CPESE 2019), September 20–23, 2019, Okinawa, Japan A direct high step-down DC/DC converter using cascade ring-type converters Kei Eguchia,∗, Akira Shibataa, Yujiro Haradab aFukuoka Institute of Technology, 3-30-1 Wajirohigashi, Higashi-ku, Fukuoka, 811-0295, Japan bNational Institute of Technology, Kurume College, 1-1-1, Komorino, Kurume, Fukuoka, Japan Received 2 October 2019; accepted 22 November 2019 Abstract For data center applications, we present a direct high step-down DC/DC converter designed by using switched capacitor (SC) techniques. To achieve high step-down conversion, the proposed converter connects a novel cascade ring-type converter to a traditional buck converter. Unlike existing high step-down SC DC/DC converters, such as ladder-type SC converter, the proposed cascade ring-type converter provides (1/4)2×stepped-down voltage by using four-phase clock pulses, where a 48 V input is converted to 3V. In the buck converter, the output of the cascade ring-type converter is converted to 3DV (0<D<1). Hence, the proposed converter can achieve the (D/16)×voltage gain by converting the 48 V input twice. Furthermore, small size and light weight are offered by the proposed converter with a single inductor. The performance of the proposed converter is evaluated by theoretical analysis, computer simulations, and bread board experiments. The evaluation results demonstrate that the proposed converter outperforms the conventional ladder-type SC converter in the point of power efficiency and output voltage. c 2019 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 6th International Conference on Power and Energy Systems Engineering (CPESE 2019). Keywords: DC/DC converters; Step-down converters; Cascade connections; Ring-type converters; Data center applications 1. Introduction For 48 V data center architecture, a high step-down dc/dc converter is receiving many researchers’ attention. To achieve high step-down gain, several attempts have already been undertaken in past studies. For example, Saadatizadeh et al. [1] proposed two new transformerless high step-down dc/dc converters and [2] suggested a highstep-down dc–dc converter using coupled-inductors. However, these converters are bulky, because several inductors are necessary to design these power converters. On the other hand, some researchers developed an inductor-less step-down dc/dc converter, such as exponential step-up/step-down type switched-capacitor (SC) converter proposed ∗Corresponding author. E-mail address: [email protected] (K. Eguchi). https://doi.org/10.1016/j.egyr.2019.11.051 2352-4847/ c 2019 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/ licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 6th International Conference on Power and Energy Systems Engineering (CPESE 2019). 120 K. Eguchi, A. Shibata and Y. Harada / Energy Reports 6 (2020) 119–124 by [3], Dickson-type adder/subtractor dc/dc converter suggested by Eguchi et al. [4], and step-up/step-down k (=2, 3, . . . )-Fibonacci dc–dc converter developed by Eguchi et al. [5]. However, the output regulation of these converters is poor. To overcome these problems, Lazzarin et al. [6] proposed a high step-down converter by combining the buck converter with the Ladder-type SC converter. By using a single inductor and cascade-connection of Ladder-type cells, Lazzarin’s converter achieves not only small size but also flexible output regulation. However, there is still room for improvement in power efficiency. In this paper, we present a direct high step-down SC dc/dc converter for data center applications. In the proposed converter, a novel cascade ring-type converter and a traditional buck converter are connected in series. Unlike existing high step-down SC dc/dc converters, such as ladder-type SC converter [6], the proposed cascade ring-type converter provides (1/4)2×stepped-down voltage by using four-phase clock pulses, where a 48 V input is converted to 3 V. In the buck converter, the output of the cascade ring-type converter is converted to 3DV (0 <D<1). Hence, the proposed converter can achieve the (D/16)×voltage gain by converting the 48 V input twice. Furthermore, small size and light weight are offered by the proposed converter with a single inductor. To evaluate the performance of the proposed converter, theoretical analysis, computer simulations, and bread board experiments are conducted. 2. Circuit configuration The circuit configuration of the proposed high step-down converter is illustrated in Fig. 1. As Fig. 1 shows, the proposed converter is composed of a cascade ring-type converter and a buck converter. By driving the transistor switches, S1, ..., S9, cyclically by four-phase clock pulses, the proposed high step-down converter generates the following output voltage: Vout =(D/16)×Vin,(0<D<1)(1) where Vin is an input voltage and Dis the duty factor of the buck converter. In Eq. (1), the D×voltage gain is achieved by the buck converter. On the other hand, the (1/16)×voltage gain is realized by the cascade ring-type converter. By connecting from C1to C4in series, the voltage of the capacitor C1, ..., C4becomes Vin /4 in the 1st block. After that, each voltage of C1, ..., C4is converted again in the 2nd block. Since the 2nd converter has the same structure as the 1st converter, the voltage of the capacitor C5, ..., C8becomes Vin/16. Therefore, the cascade ring-type converter generates (1/16)Vin. Fig. 1. Proposed high step-down converter. 3. Theoretical analysis In this section, the characteristics of the proposed converter is analyzed theoretically by using the four-terminal equivalent model [7]. To save space, we discuss the theoretical analysis of the cascade ring-type converter only, because the theoretical analysis of the buck converter is well-known. To simplify the theoretical analysis, the theoretical analysis is conducted under the conditions that 1. On-resistance of all transistor switches Sj(j=1, K. Eguchi, A. Shibata and Y. Harada / Energy Reports 6 (2020) 119–124 121 . . . , 8) is Ron; 2. Parasitic elements are negligibly small; and 3. Time constant is much larger than the period of clock pulses, T. In a steady state, the differential value of electric charges, ∆qk Ti, in Ck((i=1, . . . , 4) and (k=1, . . . , 9)) satisfies 4 ∑ i=1 ∆qk Ti=0,(2) where T= 4 ∑ i=1 Tiand T1=T2=T3=T4=T/4.(3) Because the overall change in ∆qk Tiis zero in the cycle T. Using Kirchhoff’s current law, we have the relation between the electric charge of Vin in State-Ti,∆qTi,vin , and the electric charge of the output terminal Voin State-Ti, ∆qTi,voas follows: ∆qTi,vin =∆qr Ti,where r={i+3i f i =1 i−1i f i >1.(4) ∆qTi,vo=∆q9 Ti+∆qi+4 Ti−∆qs Ti,where s={i+5i f i ≤3 i+1i f i =4.(5) ∆qa Ti=∆qb Ti=∆qc Ti, where a={i+1i f i ≤3 i−3i f i =4,b={i+2i f i ≤2 i−2i f i >2,and c={i+3i f i =1 i−1i f i >1.(6) ∆qa+4 Ti=∆qb+4 Ti=∆qc+4 Ti.(7) ∆qa Ti=∆qi Ti=∆qc+4 Ti.(8) Using Eqs. (4) and (5), the average input/output currents, Iin and Iout , can be expressed as Iin = ∆qvin T=1 T 4 ∑ i=1 ∆qTi,vin and Io= ∆qvo T=1 T 4 ∑ i=1 ∆qTi,vo.(9) Substituting Eqs. (2)–(8) into Eq. (9), we get the relation of Iin and Iout as follows: ∆qvin = − 1 16 ∆qvo,Iin = − 1 16 Io,and m1= − 1 16,(10) because the cascade ring-type converter has a symmetrical structure. Next, the consumed energy of the cascade ring-type converter is discussed to obtain the internal resistance RSC1. Since the cascade ring-type converter has a symmetrical structure, the total consumed energy WTcan be expressed as WT= 4 ∑ i=1 WT i =4WT1 =4×⎧ ⎪ ⎨ ⎪ ⎩ Ron (∆q1 T1)2 T1 +4Ron (∆q2 T1)2 T1 +4Ron (∆q2 T1−∆q1 T1)2 T1 +Ron (∆q5 T1)2 T1 +Ron (∆q5 T1+∆q1 T1−∆q2 T1)2 T1⎫ ⎪ ⎬ ⎪ ⎭ =(477 256)Ron (∆qVo)2 T.(11) 122 K. Eguchi, A. Shibata and Y. Harada / Energy Reports 6 (2020) 119–124 From Eq. (11), we have RSC1as (477/256)Ron, because WTof Fig. 2 can be expressed as WT≜RSC (∆qVout )2 T.(12) Therefore, the equivalent circuit of the cascade ring-type converter is expressed as [Vin Iin ]=[1/m10 0m1][1RSC1 0 1 ][Vo −Io].(13) Fig. 2. Simulated power efficiency as a function of output power. As the reference [6] shows, the conventional high step-down converter is designed by connecting the laddertype converter and the buck converter in series. Therefore, the total performance of the high step-down converters depends on the performance of the converter blocks, namely, cascade ring-type converter and ladder-type converter. Table 1 shows the performance comparison between the proposed cascade ring-type converter and the conventional ladder-type converter. As Table 1 shows, the power efficiency and output voltage of the proposed cascade ring-type converter is higher than that of the conventional ladder-type converter, because the proposed cascade ring-type converter has small internal resistance RSC1. Therefore, the proposed high step-down converter outperforms the conventional high step-down converter. Table 1. Performance comparison between the proposed cascade ring-type converter and the conventional ladder-type converter. Converter name Gain RSC1Power efficiency Output voltage Vo Cascade ring-type converter 1/16 (477/256)Ron RL/{RL+(477/256)Ron }Vin ×RL/[16{RL+(477/256)Ron}] Ladder-type converter [6] 1/16 (85/32)Ron RL/{RL+(85/32)Ron }Vin ×RL/[16{RL+(85/32)Ron}] 4. Simulations To clarify the effectiveness of the proposed converter, the characteristics of the proposed converter are compared with that of the conventional ladder-type SC converter [6] through simulation program with integrated circuit emphasis (SPICE) simulations. The SPICE simulations were conducted under the conditions that Vin =48 V, Ron =0.1,C1= · · · = C9=33 µF, Cout =10 µF, L =10 µH, and f=500 kHz. Fig. 2 demonstrates the simulated power efficiency, and Fig. 3 depicts the simulated output voltage. As Fig. 2 shows, the proposed converter can achieve higher power efficiency than the conventional converter. Concretely, the proposed converter can achieve 83% efficiency when D=0.7 and the output power Pout =3 W. When D=0.4 and Pout =2 W, the proposed converter can achieve 73% efficiency. Furthermore, these simulation results agree well with the theoretical results shown in Table 1. Of course, the output power of the proposed converter can be increased by connecting the proposed converter in parallel. On the other hand, as Fig. 3 shows, the proposed converter has flexibility of voltage gain by controlling the duty factor D. From these simulation results, we can confirm the effectiveness of the proposed converter. 5. Experiments To confirm the feasibility of the proposed topology, the experimental circuit of the proposed converter was built with commercially available ICs, namely, photo MOS relays AQV 212, darlington sink drivers TD62004APG, a K. Eguchi, A. Shibata and Y. Harada / Energy Reports 6 (2020) 119–124 123 Fig. 3. Simulated output voltage as a function of output power. microcontroller PIC, and electrolytic capacitors, on a breadboard. Fig. 4 demonstrates the measured output voltage, where the input voltage Vin =24 V, C1= · · · = C9=33 µF, Cout =100 µF, L=100 mH, f=40 Hz, and the output load RL=100 k.AsFig. 4 shows, the experimental circuit can achieve high step-down gain. Concretely, the voltage gain is about 1/53 when D=0.4. On the other hand, the voltage gain is about 1/19 when D=0.7. Hence, we can confirm the feasibility of the proposed topology from these experimental results. Fig. 4. Measured output voltage: (a) D=0.4 V; (b) D=0.7 V. 6. Conclusion For data center applications, a direct high step-down dc/dc converter using cascade ring-type converters has been proposed in this paper. By comparing the conventional ladder-type SC converter, the effectiveness of the proposed converter was clarified through theoretical analysis, computer simulations, and bread board experiments. The comparison results showed that 1. The proposed converter can achieve higher power efficiency than the conventional converter. Concretely, the proposed converter can achieve 83% efficiency when D=0.7 and Pout =3 W. On the other hand, when D=0.4 and Pout =2 W, the proposed converter can achieve 73% efficiency. These simulation results agreed well with the theoretical results; and 2. The feasibility of the proposed topology was confirmed by experiments. The experimental circuit achieved high step-down gain, namely, 1/53×and 1/19×, from a 24 V input. In a future study, we are going to investigate the characteristics of the proposed converter through experiments. References [1] Saadatizadeh Zahra, Heris C Pedram, Sabahi Mehran, Babaei Ebrahim. Two new transformerless high step-down DC–DC converters. IET Power Electron 2016;12(5):1205–19. [2] Hajiheidari Marziyeh, Farzanehfard Hosein, Adib Ehsan. High-step-down dc-dc converter with continuous output current using coupled-inductors. IEEE Trans Power Electron (Early Access) 2019;1. [3] Iwanaga Tamiya, Terada Shinya, Eguchi Kei, Oota Ichirou. Exponential step-up/step-down type switched-capacitor power supply with variable conversion ratio. WSEAS Trans Circuits Syst 2016;15(19):165–70. [4] Eguchi Kei, Julsereewong Prasit, Julsereewong Amphawan, Fujimoto Kuniaki, Sasaki Hirofumi. Dickson-type adder/subtractor dc-dc converter realizing step-up/step-down conversion. Int J Innovative Comput Inf Control 2013;9(1):123–38. 124 K. Eguchi, A. Shibata and Y. Harada / Energy Reports 6 (2020) 119–124 [5] Eguchi Kei, Hirata Satsuki, Shimoji Motoki, Zhu Hongbing. Design of a step-up/step-down k (=2, 3, . . . )-Fibonacci dc-dc converter designed by switched-capacitor techniques. In: Proc. the 5th international conference on intelligent networks and intelligent systems. 2012, p. 170–3. [6] Lazzarin B Telles, Bharatiraja Chokkalingam, Lehman Brad. Direct 48V to 1V step-down dc-dc converter. In: Proc. 2018 IEEE international conference on power electronics, drives and energy systems. 2018, p. 1–6. [7] Do Wanglok, Fujisaki Haruka, Asadi Farzin, Eguchi Kei. A cross-connected charge pump for energy harvesting applications. Int J Innovative Comput Inf Control 2019;15(3):2335–42.