There is a demand for low-power high-voltage-gain transformerless DC-DC converters. It is difficult to achieve this kind of conversion with good efficiency through a buck converter. The two-stage converter is proven to be an effective solution for such applications. In this paper, a series of exponential step-down switched-capacitor (ESC) converters that achieve the benefits of high-voltage-gain conversion, but comprise fewer switches than other switched-capacitor converter topologies, is proposed. The ESC converters are applicable as first-stage converters in the two-stage power conversion solution. Experimental results validating the idea are provided. © 2013 IEEE.published_or_final_versio

Tan, SC

Wong, SC

Xiong, S

There is a demand for low-power high-voltage-gain transformerless DC-DC converters. It is difficult to achieve this kind of conversion with good efficiency through a buck converter. The two-stage converter is proven to be an effective solution for such applications. In this paper, a series of exponential step-down switched-capacitor (ESC) converters that achieve the benefits of high-voltage-gain conversion, but comprise fewer switches than other switched-capacitor converter topologies, is proposed. The ESC converters are applicable as first-stage converters in the two-stage power conversion solution. Experimental results validating the idea are provided.Department of Electronic and Information EngineeringRefereed conference pape

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A series of exponential step-down switched-capacitor converters and their applications in two-stage converters

There is a demand for low-power high-voltage-gain transformerless DC-DC converters. It is difficult to achieve this kind of conversion with good efficiency through a buck converter. The two-stage converter is proven to be an effective solution for such applications. In this paper, a series of exponential step-down switched-capacitor (ESC) converters that achieve the benefits of high-voltage-gain conversion, but comprise fewer switches than other switched-capacitor converter topologies, is proposed. The ESC converters are applicable as first-stage converters in the two-stage power conversion solution. Experimental results validating the idea are provided. © 2013 IEEE.link_to_subscribed_fulltex

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Title A series of exponential step-down switched-capacitorconverters and their applications in two-stage convertersAuthor(s) Xiong, S; Wong, SC; Tan, SCCitationThe 2013 IEEE International Symposium on Circuits andSystems (ISCAS 2013), Beijing, China, 19-23 May 2013. In IEEEInternational Symposium on Circuits and Systems Proceedings,2013, p. 701-704Issued Date 2013URL http://hdl.handle.net/10722/196301Rights Creative Commons: Attribution 3.0 Hong Kong License1A Series of Exponential Step-DownSwitched-Capacitor Converters and TheirApplications in Two-Stage ConvertersSong Xiong, Siu-Chung Wong Senior Member, IEEE, and Siew-Chong Tan Senior Member, IEEEAbstract—There is a demand for low-power high-voltage-gaintransformerless DC–DC converters. It is difﬁcult to achieve thiskind of conversion with good efﬁciency through a buck converter.The two-stage converter is proven to be an effective solution forsuch applications. In this paper, a series of exponential step-downswitched-capacitor (ESC) converters that achieve the beneﬁts ofhigh-voltage-gain conversion, but comprise fewer switches thanother switched-capacitor converter topologies, is proposed. TheESC converters are applicable as ﬁrst-stage converters in the two-stage power conversion solution. Experimental results validatingthe idea are provided.I. INTRODUCTIONIt is common to use high-voltage-gain DC–DC convertersin modern electronic products. A current application of suchconverters is targeted at converting a DC bus supply voltageof 12 V or higher into an ultra-low voltage level of less than1 V [1]. At present, for such power conversions, the industryis using the buck-based topology, which is low in cost andsimple in implementation [2]. As space and weight are limitedin such electronic products, the buck converters are designedto operate at a high switching frequency to reduce the sizeof their magnetic components. To achieve a high-voltage-gainconversion, the buck converters are operated with a very smallduty ratio (D = VoutVin ), which limits the switching frequencyand complicates the control implementation [3]. Besides, theconverters inherit a deteriorated dynamic performance andpoor efﬁciency due to the short turn-on time and the longfreewheeling time of the switching actions. Converters withtransformers would be a better choice for high-voltage-gainconversion if size and cost are not of concerns.Nevertheless, conﬁning to the size and weight requirements,an alternative candidate is the switched-capacitor (SC) DC–DC converters, which are known for their advantages of smallsize and light weight [4], [5]. As they comprise only powerswitches and capacitors, they are highly suitable for integratedcircuit (IC) implementation which can be easily integrated inelectronic products [4]. For these reasons, SC converters havebeen widely considered for the high-voltage-gain applications.However, SC converters cannot achieve a high efﬁciency ifthey are used for the purpose of voltage regulation, whichlimits their usefulness to voltage transformation applications[5].The limitations of the single-stage converters have spurredthe interests in developing a new kind of high-voltage-gaintwo-stage DC–DC converters [3], [6], [7], of which the con-verter is made up of a ﬁrst-stage SC converter, followed bya second-stage buck converter. For this two-stage SC-buckconverter, the ﬁrst-stage SC converter steps down the inputvoltage to a low level (i.e., step-down voltage transformation)while the second-stage buck converter mainly performs thevoltage regulation. The overall efﬁciency of this two-stageconverter can be higher than a single buck converter for thereason that the ﬁrst-stage SC converter provides a much lowerinput voltage for the buck and it runs at a higher efﬁciencystate. It is obvious that if the energy saved by the second-stage buck converter is higher than the energy losses incurredby the ﬁrst-stage converter, then the overall efﬁciency can beimproved.Even though existing SC converter products can achievean efﬁciency as high as 98% (e.g. LTC1044, MAX1044,SI7660, GS7660 etc.), the rated powers are always very low. Inaddition, most SC converter topologies require a large numberof switches in their circuitries when the voltage conversionis high [6]–[8]. This complicates their implementation andincrease their cost. It is desired to have SC converters withfewer switches that can still attain high-voltage-gain conver-sion with high efﬁciency. In this paper, a series of exponentialstep-down switched-capacitor (ESC) converters which satisfysuch requirements is proposed.II. A SERIES OF ESC CONVERTERSA. Second-Order ESC Converter: Topology and OperationFig. 1 shows the topology of the second-order step-downESC converter and its timing control diagram. This convertersteps down the input voltage to a quarter of its amplitude,that is Vo = 14Vin. Comparing with the basic series-charging-parallel-discharging SC converter, this converter uses fewerswitches.By neglecting the states in which very short dead-timeoccur, the number of operating states can be reduced to fouras shown in Fig.1(b). Fig. 2 gives the circuit representationof these states. In State 1 (Fig. 2(a)), ﬂying capacitor Cf1is discharging while capacitor Cf2 is charging. In State 2(Fig. 2(b)), both capacitors Cf1 and Cf2 are charging. InState 3 (Fig. 2(c)), Cf1 is charging and Cf2 is discharging.In State 4 (Fig. 2(d)), both Cf1 and Cf2 discharges. From thetiming diagram, the ESC converter is operated in the followingsequence: State 1 - State 2 - State 3 - State 4 - State 1 and soon.978-1-4673-5762-3/13/$31.00 ©2013 IEEE 7012				 ΦΦΦΦΦΦΦΦ(a) Topology      	  ΦΦΦΦ(b) Timing diagramFig. 1. Proposed ESC converter and its timing diagram.				 ΦΦΦΦΦΦΦΦ(a) State 1				 ΦΦΦΦΦΦΦΦ(b) State 2				 ΦΦΦΦΦΦΦΦ(c) State 3				 ΦΦΦΦΦΦΦΦ(d) State 4Fig. 2. Operating states of the proposed ESC converter.B. Generalization of ESC converterA comparison of the proposed topology in Fig. 1 with thatgiven in [9] shows their difference being the additional struc-ture given in Fig. 3(c). By adopting this structure (Fig. 3(c)), aseries of higher-order ESC converters can be developed. Fig. 4shows a third-order ESC converter.1) Topologies of High-Order ESC converter: Figs. 3(a)and 3(b) give the basic structure of an ESC converter andits timing diagram. This structure gives an output voltage halfof the input voltage. Here, Cf1 will parallel with C11 duringState 1, of which Cf1 will be charged to the voltage levelof C11. In State 2, Cf1 is parallelled with C10, of whichCf1 discharges to C10 and output load. As a whole, Cf1automatically equalizes the voltage of the capacitors C10 andC11, which is half the input voltage. Figs. 3(c) and 3(d) arethe extension structure of the ESC converters and its timingdiagram. The operation of the extension structure is similar tothe basic structure.If the basic structure is cascaded with the extension struc-ture, a second-order ESC converter as given in Fig. 1(a) isresulted. Likewise, a third-order ESC converter (output voltagebeing one eighth of its input voltage) can be derived by furthercascading the second-order ESC converter with an additionalextension structure, as given in Fig. 4. Similarly, a n-orderESC converter can be derived using the same approach. Itsoutput voltage isVo =12n· Vin. (1)2) Switching Intervals of High-Order ESC converter: Thetiming diagram is similar to that of the second-order ESCconverter (see Fig. 1(b)). The switching interval is T6 forthe third-order ESC converter and T2·n for an n-order ESCconverter.				ΦΦΦΦ(a) Basic structure   ΦΦ(b) timing diagram of (a) 	  	 		ΦΦΦΦ(c) Extension structure  ΦΦ(d) Timing diagram of (c)Fig. 3. Structures of the generalized ESC converter.III. ESC-BUCK CONVERTERA series of ESC converters with ﬁxed conversion ratio of 12nhave been introduced. The output voltage of these converterschanges with the input voltage. If the required output voltage isnot an exact multiple of 12n of the input voltage, the converteris inapplicable. This lack of voltage regulation function limitstheir applications. It has been demonstrated that a two-stageconverter can give a better efﬁciency than a single buckconverter in high-voltage-gain conversion operations [3], [6].Here, the ESC converter ﬁts well for this application.In this paper, it is proposed that the ESC converter is usedas the ﬁrst-stage converter of the two-stage converter. Fig. 57023						Fig. 4. Third-order ESC converter.shows a topology of a second-order ESC-buck converter. Theﬁrst stage is a second-order ESC converter which steps downthe input voltage to 1/4, while the second-stage buck converterregulates the output voltage.				 	 	ΦΦΦΦΦΦΦΦΦΦFig. 5. Second-order ESC-Buck converter.IV. EXPERIMENTAL RESULTSExperimental prototypes are constructed for the veriﬁcationof the proposed ESC-buck converter. The parameters of thebuck converter are shown in Table I. The ESC converter is builtwith discrete elements using parameters shown in Table II.Experiments are performed on three topologies of the con-verters. The ﬁrst set of experiments are performed on theTABLE IMAIN PARAMETERS OF THE TWO BUCK CONVERTERSIC Type MAX8655Input Voltage 4.5–25 VOutput Voltage 1.2 VSwitching Frequency 400 kHzFilter Inductor 0.82 μHOutput Capacitor 4× 47 μFTABLE IIPARAMETERS OF THE ESC CONVERTER USED FOR MAX8655ESC Type First-Order Second-OrderFlying Capacitor Cf1 6× 47 μF 6× 47 μFFlying Capacitor Cf2 NA 6× 47 μFCapacitor C10 1× 47 μF 1× 47 μFCapacitor C11 1× 47 μF 1× 47 μFCapacitor C12 NA 1× 47 μFSwitch NTMFS4897 NTMFS4897commercially available buck converter IC MAX8655. Thesecond set of experiments are performed on a ﬁrst-order ESC-buck converter (i.e., ﬁrst-order ESC converter as the ﬁrst-stage converter and MAX8655 as the second-stage converter).The third set of experiments are performed on a second-orderESC-buck converter (i.e., second-order ESC converter plusMAX8655). In all cases, the same buck converter settingsare employed. The efﬁciency measurement given in this workis the overall system efﬁciency which includes the powerconsumed by the drivers and controller circuit of the ESCconverter.Fig. 6 shows the efﬁciency curves of the buck IC MAX8655.It can be seen that the efﬁciency for the buck converter ishigh at a low input voltage of 5 V and 10 V. Its efﬁciencyis signiﬁcantly lower for the same output power when inputvoltage is 20 V. Fig. 7(a) gives the efﬁciency comparison of theﬁrst-order and second-order ESC-buck converters. The second-order ESC-buck converter has a higher efﬁciency than the ﬁrst-order ESC converter when the input voltage is higher than26 V.Fig. 7(b) shows that at an input voltage of 20 V, both theﬁrst-order and second-order ESC-buck converters give a higherefﬁciency than the MAX8655 buck converter operating on itsown. This validates the use of ESC converters as the ﬁrst-stageconverter of a two-stage converter solution for high-voltage-gain applications. In this case, the ﬁrst-order and second-orderESC-buck converter each excels at different load conditions.For output power higher than 6.5 W, the former gives a higherefﬁciency than the latter. For output power lower than 6.5 W,the latter gives a higher efﬁciency. However, as shown inFig. 7(c), when both ESC converters are operated with an evenhigher input voltage of 30 V, which is over the input voltagerating of MAX8655, the efﬁciency of the second-order ESC-buck converter is higher than that of the ﬁrst-order ESC-buckconverter for the entire speciﬁed load range.V. CONCLUSIONSThis paper presents a series of exponential switched-capacitor (ESC) converter topologies which can exponentiallystep-down the voltage input with fewer switches than con-70342 4 6 8 10 12 14 16 18 205658606264666870727476788082848688909294Output power (W)Efficiency (%)20Vin10Vin5VinFig. 6. Efﬁciency curves of MAX8655.ventional switched-capacitor converters. While the prospect ofthe ESC converter is very good for voltage transformationapplication, without regulation capability, its application islimited. Knowing that there are numerous buck type ICs inthe market which perform very well under normal conditionbut suffers from a poor efﬁciency when the input voltage ishigh, it is proposed in this paper that for high-voltage-gainconversion functions, the ESC converter is placed betweenthe input power source and the buck IC to form a two-stageconverter. Experimental results of the ESC-buck convertersshow that a higher efﬁciency compared with the single buckIC converter can be achieved.REFERENCES[1] [online] http://www.itrs.net/Links/2005ITRS/Home2005.htm[2] J. J. Sun, Y. Qiu, M. Xu, and F. C. Lee, “High-frequency dynamiccurrent sharing analyses for multiphase buck VRs,” IEEE Trans. onPow. Electron., vol. 22, no. 6, pp. 2424–2431, Nov. 2007.[3] R. Guo, Z. Liang, and A. Huang, “A multi-modes charge-pump basedhigh efﬁciency wide input range DC–DC converter,” in IEEE EnergyConvers. Congr. and Expo. (ECCE), pp. 2706–2712, Sep. 2010.[4] J. Liu, Z. Chen and Z. Du, “Switched capacitor DC–DC convertersenable electronic products to become more compact”, in ProceedingsInter. Conf. on Soft. Eng. (ICSE), pp. 234–237, Nov. 1996.[5] S. V. Cheong, H. S. H. Chung, and A. Ioinovici, “Inductorless DC–DCconverter with high power density,” IEEE Trans. on Ind. Electron., vol.41, no. 2, pp. 208–215, Apr. 1994.[6] S. Xiong, S. C. Tan and S. C. Wong, “Analysis and Design of a High-Voltage-Gain Hybrid Switched-Capacitor Buck Converter” IEEE Trans.Cir. Sys. I, Regul. Pap., vol. 59, no. 5, pp. 1132–1141, May. 2012.[7] Y. H. Chang, “Variable-conversion-ratio switched-capacitor-voltage-multiplier/divider DC–DC converter,” IEEE Trans. on Cir. and Sys.–I:Regular Papers, vol. 58, pp. 1944–1957, Aug. 2011.[8] W. Qian, D. Cao, J. G. Cintron-Rivera, M. Gebben, D. Wey and F.Z. Peng, “A Switched-Capacitor DC-DC Converter With High VoltageGain and Reduced Component Rating and Count” IEEE Trans. Indust.Appli., vol. 48, no. 4, pp. 1397–1406, July/Aug. 2012.[9] T. Umeno, K. Takahashi, I. Oota, F. Ueno, and T. Inoue, “New switched-capacitor DC–DC converter with low input current ripple and itshybridization,” in Proceedings 33rd IEEE Midwest Symp. on Cir. andSys. (MWSCS), pp. 1091–1094, Aug. 1990.10 12 14 16 18 20 22 24 26 28 308284Input voltage (V)Efficiency (%)2nd−order ESC−buck1st−order ESC−buck(a) Efﬁciency versus input voltage with 10 A output.2 4 6 8 10 12 14 16 18 20565860626466687072747678808284868890Output power (W)Efficiency (%)Buck2nd−order ESC−buck (30k)1st−order ESC−buck (30k)(b) Efﬁciency versus output power at an input voltage of 20 V.0 2 4 6 8 10 12 14 16 18 2066687072747678808284Output power (W)Efficiency (%)2nd−order ESC−buck (30k)1st−order ESC−buck (30k)(c) Efﬁciency versus output power at an input voltage of 30 V.Fig. 7. Experimental results of ESC-buck converter using MAX8655operating at a regulated output voltage of 1.2 V. The ESC converter operatesat a switching frequency of 30 kHz.704

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A series of exponential step-down switched-capacitor converters and their applications in two-stage converters

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