06195022

390 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 28, NO. 1, JANUARY 2013

Step-up AC Voltage Regulatorswith High-Frequency Link

Daolian Chen, Senior Member, IEEE, and Yanhui Chen

AbstractA circuit configuration and a circuit topological fam-ily of step-up ac voltage regulators with high-frequency link areproposed. This kind of circuit topology is composed of input LC fil-ter, energy-storage inductor, input cycloconverter, high-frequencytransformer, output cycloconverter and output filtering capacitor.The regulators can convert an unstable sinusoidal voltage with highTHD to a stable sinusoidal voltage with the same frequency and lowdistortion. Operating principle, normalized output characteristics,phase-shifting control strategy, methods to suppress magnetic satu-ration of the energy-storage inductor at start-up, and voltage spikecaused by the high-frequency HF transformer leakage inductanceare proposed and fully investigated. The results from theoreticalanalysis and principle experiment indicate that the proposed regu-lators have advantages of high-frequency galvanic isolation, simpletopology, two-stage power conversions, bidirectional power flow,high conversion efficiency, and high reliability during short-circuitconditions of the load.

Index TermsAC voltage regulators, cycloconverter, high-frequency link (HFL), phase-shifting control strategy, step-up.

I. INTRODUCTION

S TEP-UP converter has smaller input pulsating current andlower EMI compared with step-down and step-up/step-down converters [1]. Step-up converter is suitable for low-inputcurrent ripple, high reliability under short-circuit load, and largeoutput capability field.

Recent researches on step-up converter were limited to dcdc [2] and acdc converters [3]. Researches on acac converterswere mainly limited to nonisolated acac converters and acdcac converters. The nonisolated acac converters, such as thedirect matrix converter [4], the pulse-width modulation (PWM)ac chopper [5], and the soft switched ac-link buckboost acac converter [6] have the features of single or two stages powerconversion, low-voltage transmission ratio no need of intermedi-ate dc energy-storage component, etc. The acdcac converters,such as the indirect matrix converter [7], the acdcac con-verter with low-frequency link [8], and the acdcac converter

Manuscript received December 6, 2011; revised February 29, 2012;accepted April 20, 2012. Date of current version September 11, 2012. This workwas supported by National Nature Science Foundation of China under Grant50577031. Recommended for publication by Associate Editor P.-T. Cheng.

The authors are with Power Electronics and Drives Institute, FuzhouUniversity, Fuzhou 350108, China (e-mail: [email protected];[email protected]).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TPEL.2012.2197829

with high-frequency link (HFL), have the features of two orthree stages power conversion, low or high voltage transmis-sion ratio, high line power factor (LPF), and intermediate dcenergy-storage component, etc. A step-up/step-down ac voltageregulator with HFL was proposed and investigated in [9]. In [9],the kind of ac voltage regulator has the features of simple cir-cuit, discontinuous current mode (DCM), and is suitable for lowcapability field.

A circuit configuration and a circuit topological family ofstep-up ac voltage regulators with HFL are proposed and fullyinvestigated in this paper. The proposed ac regulators are tar-geted at a new type of regulated sinusoidal ac power supplies,and electronic transformers in which isolation and/or bidirec-tional power flow are needed.

II. CIRCUIT CONFIGURATION AND CIRCUITTOPOLOGICAL FAMILY

A. Circuit Configuration

As shown in Fig. 1, the circuit configuration of step-up acvoltage regulators with HFL is composed of input LC filter,energy-storage inductor, input cycloconverter, high-frequency(HF) transformer, output cycloconverter and output filtering ca-pacitor. The ac voltage regulators can convert one kind of sinu-soidal voltage with unstable magnitude and high total harmonicdistortion (THD) to another kind of sinusoidal voltage with thesame frequency, stable magnitude and low distortion.

When energy is transferred from source to load, the low-frequency (LF) sinusoidal inductance current iL with HF rippleis modulated into bipolarity three-state HF pulse current i1 bythe input cycloconverter. After galvanic isolation and currentmatching of the HF transformer, i1 is demodulated into unipo-larity three-state LF pulse current by the output cycloconverter,then filtered into high-quality sinusoidal voltage uo . High LPFare obtained by inductor L and filter at input side.

B. Circuit Topological Family

As shown in Fig. 2, the circuit topological family of thestep-up ac voltage regulators with HFL contains seven circuittopologies, namely single-forward mode, push pull-full wavemode, push pull-full bridge mode, half bridge-full wave mode,half bridge-full bridge mode, full bridge-full wave mode, andfull bridge-full bridge mode.

III. CONTROL PRINCIPLE

Taking full bridge-full bridge mode circuit topology as anexample, shown in Fig. 2(g), the instantaneous voltage feedbackstrategy with phase-shifted control is introduced in the proposed

0885-8993/$31.00 2012 IEEE

CHEN AND CHEN: STEP-UP AC VOLTAGE REGULATORS WITH HIGH-FREQUENCY LINK 391

Fig. 1. Circuit configuration of the step-up ac voltage regulators with HFL.

Fig. 2. Circuit topological family of the step-up ac voltage regulators with HFL. (a) Single forward mode. (b) Push pull-full wave mode. (c) Push pull-full bridgemode. (d) Half bridge-full wave mode. (e) Half bridge-full bridge mode. (f) Full bridge-full wave mode. (g) Full bridge-full bridge mode.


Fig. 3. Instantaneous voltage feedback strategy with phase-shifted control of the proposed regulator. (a) Principle waveforms. (b) Control block diagram.

regulator. When the input voltage or the load varies, the outputvoltage will keep stable by adjusting the duty cycle D.

In Fig. 3, the error voltage signal ue is obtained from thevoltage amplifier by comparing the feedback signal uof of outputvoltage with the reference signal ur synchronized with inputvoltage ui . The PWM signal uk 3 is generated by comparing theabsolute value of ue with the saw-tooth wave uc . The signaluk 0 is obtained by comparing ur with zero. The signal uk 1 isobtained from uc by frequency divider circuit at trailing edge.The signal uk 2 is the logical negation of uk 1 .

Ts is HF switching period, and Ton is the conduction time ofS1 (S 1) in Ts . There is 180

difference between the two drivingsignals of S1 (S 1), S2 (S

2) in the input cycloconverter. The duty

cycle of S1 (S 1) and S2 (S2) are both greater than 0.5. The

common conduction time Tcom in Ts /2 is Tcom = (Ts /2)/180.Thus, the duty cycle of the converters is

D = Tcom/ (Ts/2) = /180 (1)

where (0 0 and the current of storageinductance i L > 0, the proposed regulator has fourteen operat-

ing states, as shown in Fig. 4(a). The seven operation states inhalf-switching period are shown in Fig. 4(b)(h).

t = t0t1 : Switches in the input cycloconverter are on statebefore t0 . The energy-storage inductor is magnetized by theinput voltage ui and the load ZL is supplied by the output filteringcapacitance Cf . At t0 , S2a , S2b , S 2a , S

2b are turned OFF with

zero-voltage switching (ZVS). C2a , C 2a are charged by IL untilthe drainsource voltages of S2a , S 2a reach UoN1 /N2 at t1 .

t = t1t2 : At t1 , D3a and D3a turn ON and Llk , C2a , and C2a

begin to resonant; iL lk , uC 2a , and uC 2a are given by

iL1 k = IL IL cos 1 (t t1) (2)uC 2 a = uC2 a = UoN1/N2 + |z1 | IL sin1 (t t1) (3)

where, 1 = 1/

2L1kC1 , |z2 | =

Llk/(2C1). At t2 , iL1 k =IL , UC 2 a = UC2 a = UoN1/N2 + |z1 | IL = U1 . After t1 , ZVSof S3a , S 3a can be achieved.

t = t2t3 : Llk , C2a , C2b , C 2a , and C2b begin to resonant at t2 .

iL lk , uC 2a , uC 2a , uC 2b , and uC 2b can be represented as

iL1 k = IL + (|z1 | / |z2 |)IL sin 2 (t t2) (4)uC2 a = uC 2 a = [|z1 | IL cos 2 (t t2) |z1 | IL ] /2 + U1 (5)uC2 b = uC 2 b = [|z1 | IL cos 2 (t t2) |z1 | IL ]/2 (6)

where, 2 = 1/

LlkC1 , |z2 | =

Llk/C1 .The LC damped resonance will fade away at t3 for the ex-

istence of internal resistance in the circuit. The current of theleakage inductor is equal to iL simultaneously.

t = t3t4 : iL flows through S1a , S1b (D1b ), S 1a , S1b (D

1b ), and

the current of second winding flows through S3b , S3a (D3a ), S 3b ,and S 3a (D

3a ). Energy is transferred from the energy-storage

inductor to the load.t = t4t5 : S3a and S 3a are turned off with ZVS at t4 . iL flows

through S1a , S1b (D1b ), S 1a , S1b (D

1b ), the current of the second


Fig. 4. Seven operating states in half-switching period. (a) Steady principle waveforms. (b) t0 t1 . (c) t1 t2 . (d) t2 t3 . (e) t3 t4 . (f) t4 t5 . (g) t5 t6 . (h) t6 t7 .

winding flows through S3b , D3a , S 3b , D3a . Energy is transferred

from the energy-storage inductor to the load.t = t5t6 : S2a , S2b , S 2a , and S

2b are turned ON with ZCS at t5 .

The energy-storage inductor is magnetized by the input voltage.The current of the second winding flows through S3b , D3a , S 3b ,D3a , and iL lk and is expressed as

iLl k = IL (UoN1/N 2)(t t5)/Llk (7)

where iL lk is decreasing linearly until iL lk reaches zero at t6 .t = t6t7 : From t6 , the energy-storage inductor is magnetized

by ui and ZL is supplied by Cf .Another seven operating stages (t = t7t14) are similar to the

former (t = t0t7). The next switching cycle starts at t14 .On the other three conditions, namely uo > 0 and iL < 0,

uo < 0 and iL > 0, uo < 0 and iL < 0, the converter stillhave fourteen operating states in one switching period, similarto the situations of uo > 0 and iL > 0.

Fig. 5. Equivalent circuits in one switching period. (a) DTs . (b) (1D)Ts .

B. Switching State Equivalent Circuits

The equivalent circuits of the regulators at continuous cur-rent mode (CCM) in one switching period are shown in Fig. 5,where r is internal resistance of the regulators, and it includesthe winding resistance of the HF transformer, the leakage resis-tance, the on-resistance of the power switches and the parasiticresistance of the filtering inductor.


Fig. 6. Energy-storage inductance current waveforms in switching periods.

The state equations of the equivalent circuit shown in Fig. 5(a)and (b) are, respectively, given by

LdiL/dt = ui riL (8a)Cf duo/dt = uo/RL (8b)LdiL/dt = ui riL uoN1/N2 (9a)

Cf duo/dt = iLN1/N2 uo/RL. (9b)Equation (8) multiplied by D plus (9) multiplied by (1D),letting diL /dt = 0, duo /dt = 0, the steady expressions of statevariable can be obtained as

IL = Ui/[(N1/N2)2 (1 D)2 RL + r] (10a)

Uo = UiN2/N1/{1 D + r/[(N1/N2)2(1 D)RL ]}.(10b)

C. Steady Output Characteristics

The proposed regulators can operate at four quadrants of theuoio coordinate plane. Therefore, there is no DCM but CCMof inductance current. In one Ts , the proposed regulators areequivalent to an isolated bidirectional boost dcdc converter.The initial value of iL has three initial value mode, that is greaterthan, equal to, or less than zero, as shown in Fig. 6. It shouldbe pointed out that there are three kinds of situations of currentless than zero, shown as curves 3, 4, and 5.

Because the regulators only work in CCM mode, the outputcharacteristic in ideal condition (r = 0) is given by

Uo = UiN2/ [N1 (1 D)]. (11)If IL min = 0, there is an equation as follows:

Ui = LiL (DTs) /DTs (t = 0 DTs). (12)The critical load current IG at zero initial inductance current canbe derived by (11) and (12), that is

IG = Io = N1iL (DTs) (1 D) Ts/(2N2Ts)= N1UiD (1 D) Ts/(2N2L). (13)

IG is maximal when D = 1/2 from (13). The max. value is

IG max = N1UiTs/ (8N2L). (14)

From (13) and (14), the ideal output characteristic of the regu-lators at IL min = 0 is deduced to

IG = 4IG maxD (1 D) . (15)

Fig. 7. Normalized output characteristics of the proposed regulators.

The normalized output characteristics of the regulators areshown in Fig. 7. It can operate at four quadrants. The 1st and3rd quadrants, as well as the 2nd and 4th quadrants are cor-responding to positive and negative directions of power flow,respectively. Take the 1st quadrant as an example, curve A,determined by (15), represents the ideal output characteristicwhen iLmin = 0. The left and right curves of A represent theoutput characteristics at negative and positive values of iLmin ,respectively. The solid and dashed lines represent the ideal andactual output characteristics, determined by (11) and (10b),respectively.

D. Magnetizing States of HF Transformer

The magnetizing states of HF transformer of the proposedregulators are shown in Fig. 8, where uy represents the polaritysignal of uo , and md represents magnetizing direction.

The results from analysis demonstrate: 1) the magnetizingstates (MS) have nothing to do with uy under the same condition;2) the MS of magnetic core in Ts is symmetrically bidirectionmagnetized approximately; 3) the output voltage uo is very smallat the vicinity of zero, so even magnetization with same directioncannot lead to severe magnetic bias; 4) the HF transformercan realize magnetic reset and be bidirectional symmetricallymagnetized in one or half period of the output voltage, whetheruy synchronize with driving signals or not; 5) the MS of HFtransformer are similar, but the initial magnetic density B0 at t= 0 lies on the 3rd quadrant or the 1st quadrant, respectively,when uy is leading or lagging driving signals.

V. CONSIDERATION OF SEVERAL KEY PROBLEMS

A. Magnetic Saturation of Energy-Storage Inductor forStart-Up

When the regulators start up, there is ui uoN1 /N2 for severalTs , the energy-storage inductor will soon become saturated.

An input voltage zero-crossing start-up circuit is proposed toavoid the magnetic saturation when the regulator starts to work,


Fig. 8. Magnetizing states of the high-frequency transformer in the proposed regulators. (a) uy synchronizing with the drive signal of S3 and uy unutilized tocontrol. (b) uy synchronizing with the drive signal of S3 and uy utilized to control. (c) uy leading the drive signal of S3 and uy unutilized to control. (d) uy laggingthe drive signal of S3 and uy utilized to control.

Fig. 9. Input voltage zero-crossing start-up circuit.

as shown in Fig. 9. The start-up signal is used for controllingall power switches. When the input voltage reaches to zero,the regulator begins to start softly and smoothly since the inputvoltage and the output capacitor voltage are both zero.

B. The Suppressing of Spike Voltage

A high transient voltage spike is generated by the leakageinductance Llk . To suppress this spike, a RC snubber circuit oran active-clamped circuit is proposed, as shown in Fig. 10.

CC in Fig. 10(b) is designed based on the resonance of Llkand CC . The dead time between SC 1 turn-off and S2(S 2) turn-on is so small that it can be ignored. SC 1 will turn OFF withZCS if resonance period Tr can satisfy the condition of 3Tr /4

= (1D)Ts /2. The clamped capacitance is derived from

CC = (1 D)2 (Ts/2)2 /(92L1k ). (16)

VI. EXPERIMENTAL STUDY

The designed prototype: full bridge-full bridge mode circuittopology, input voltage Ui = 220 V 20% 50 Hz ac, out-put voltage Uo = 220 V 50 Hz ac, normalized power S =1 kVA, switching frequency fs = 50 kHz, load power factor is0.75 to 1.0 to +0.75, HF transformer N1 /N2 = 28/10, energy-storage inductor L = 700 H, Cf = 6.6 F/400 V, input fil-tering inductor Li = 30 H, Ci = 6.6 F/400 V, IGBT deviceHGTG10N120BND for power switches of input cycloconverter,MOSFET device IRFP460 for power switches of output cyclo-converter, IC chip UC3879 for control circuit.

The experimental waveforms of the proposed regulator at nor-malized input voltage and normalized resistive load are shownin Fig. 11. The experimental results demonstrate that: 1) the pri-mary and the secondary voltages of transformer are both bipolarthree-state HF ac pulses, as shown in Fig. 11(a) and (b); 2) thedriving signal of S1a is HF PWM waveform, the collectoremitter voltage of S1a is HF pulse waveform, whose positivehalf cycles envelope is LF period sinusoid and negative half islow level, as shown in Fig. 11(c); 3) the driving signal of S3a ishigh level in positive half-LF period, and its negative half period


Fig. 10. Full bridge step-up HFLs ac voltage regulators with RC snubber circuit or active clamped circuit. (a) RC snubber circuit. (b) Active clamped circuit.

Fig. 11. Experimental waveforms of the proposed regulator. (a) Primary voltage of HF transformer uN 1 and its expanded waveform. (b) Secondary voltage ofHF transformer uN 2 and its expanded waveform. (c) Collectoremitter voltage & driving signal of S1a and its expanded waveforms. (d) Drainsource voltageand drive signal of S3a and its expanded waveforms. (e) Collectoremitter voltage and drive signal of SC 1 and its expanded waveforms. (f) Voltage of clampedcapacitor. (g) Amplified error voltage ue . (h) Output voltage uo at Ui = 176 Vrm s . (i) Output voltage uo at Ui = 220 Vrm s .

is HF PWM waveform, the drainsource voltage of S3a is pulsewaveform of three-level (uo , uo /2, 0) in negative half-period,and its envelop is half-cycle LF sinusoid, while zero in positivehalf cycle, as shown in Fig. 11(d); 4) the drive signal of SC 1 isHF PWM waveform (100 kHz) in positive half LF period and itsnegative half period is high level, the collectoremitter voltageof SC 1 is HF pulse wave(100 kHz) in negative half period, and

its envelope is half LF period sinusoid (uo > 0), while zero inpositive half cycle (uo < 0), as shown in Fig. 11(e); 5) the voltageof clamped capacitance is a LF sinusoidal wave with 220 Vrms ,as shown in Fig. 11(f); 6) the error voltage is approximatelyan LF square waveform, as shown in Fig. 11(g); 7) one kindof sinusoidal voltage with unstable magnitude and high THDto another kind of sinusoidal voltage with the same frequency,


stable magnitude and low distortion, as shown in Fig. 11(h) and(i). The developed prototype has the excellent performances:the load power 1 kVA, Ui = 176264 V 50 Hz ac, Uo = 2202 V, Fo = 50 Hz, the line power factor at the different load0.720.99, the output voltage THD 1.8%.

The voltage transmission ratio of the proposed regulators isgreater than that of nonisolated ac regulators [4][7]. The LPF ofthe proposed regulators is higher than that of acdcac converterwith diode rectifier. The weight and bulk of the proposed regu-lators are less than those of acdcac converter with LF link [8].The proposed regulators have higher efficiency and lower costthan acdcac converter with HFL. The proposed regulatorshave higher reliability than acdcac converter with interme-diate dc filtering capacitor [8]. The proposed regulators havelarger output capacity, higher efficiency, and higher LPF thanstep-up/step-down ac voltage regulators with HFL [9]. How-ever, the disadvantages of the proposed regulators are nonidealLPF and additional clamped circuit. Therefore, the proposedregulators are suitable for acac regulation fields of high powerdensity, high voltage transmission ratio, high conversion effi-ciency, higher LPF, and large output capability.

VII. CONCLUSION

The proposed ac voltage regulators with HFL can convertan unstable sinusoidal voltage with THD to a stable sinusoidalvoltage with the same frequency and low distortion. This circuittopological family contains seven circuits.

The instantaneous voltage feedback strategy with phase-shifted control is introduced in the proposed regulators. Thereare fourteen operating stages in Ts . The output features in fourquadrants and the MS of the HF transformer of the proposed reg-ulators are derived. The automatic input voltage zero-crossingstart-up circuit can avoid the magnetic saturation when the reg-ulators start to work. The active clamped circuit can effectivelysuppress the voltage spike caused by the leakage inductanceof the HF transformer. The circuit topological family, controlstrategy, and theoretical analysis are verified by the principleexperimental results. The regulators which have excellent per-formance can be used for a new type of regulated sinusoidalac power supplies, the electronic transformers of which needisolation and/or bidirectional power flow.

REFERENCES

[1] M.-K. Nguyen, Y.-C. Lim, and Y.-J. Kim, A modified single-phase quasi-Z-source AC-AC converte, IEEE Trans. Power Electron., vol. 27, no. 1,pp. 201210, Jan. 2012.

[2] H.-H. Huang, C.-Y. Hsieh, J.-Y. Liao, and K.-H. Chen, Adaptive droopresistance technique for adaptive voltage positioning in Boost DCDCconverters, IEEE Trans. Power Electron., vol. 26, no. 7, pp. 19201932,Jul. 2011.

[3] T.-S. Lee and J.-H. Liu, Modeling and control of a three-phase four-switch PWM voltage-source rectifier in d-q synchronous frame, IEEETrans. Power Electron., vol. 26, no. 9, pp. 24762489, Sep. 2011.

[4] M. Rivera, C. Rojas, J. Rodriguez, P. Wheeler, B. Wu, and J. Espinoza,Predictive current control with input filter resonance mitigation for adirect matrix converter, IEEE Trans. Power Electron., vol. 26, no. 10,pp. 27942803, Oct. 2011.

[5] C.-M. Wang, C.-H. Lin, C.-H. Su, and S.-Y. Chang, A novel single-phasesoft-switching AC chopper without auxiliary switches, IEEE Trans.Power Electron., vol. 26, no. 7, pp. 20412048, Jul. 2011.

[6] H. A. Toliyat, A. Balakrishnan, M. Amirabadi, and W. Alexander, Softswitched ac-link AC/AC and AC/DC buck-boost converter, in Proc.IEEE Power Electron. Spec. Conf., Rhodes, Greece, 2008, pp. 41684176.

[7] G. T. Chiang and J.-I. Itoh, DC/DC Boost converter functionality ina three-phase indirect matrix converter, IEEE Trans. Power Electron.,vol. 26, no. 5, pp. 15991607, May 2011.

[8] T. Zhou and B. Francois, Energy management and power control of ahybrid active wind generator for distributed power generation and gridintegration, IEEE Trans. Ind. Electron., vol. 58, no. 1, pp. 95104, Jan.2011.

[9] D. Chen, Novel current mode AC-AC converters with high frequency aclink, IEEE Trans. Ind. Electron., vol. 55, no. 1, pp. 3037, Jan. 2008.

Daolian Chen (SM08) was born in Fujian, China, in1964. He received the B.S., M.S., Ph.D. degrees andPostdoctoral Certification from the Department ofElectrical Engineering, Nanjing University of Aero-nautics and Astronautics (NUAA), Nanjing, China,in 1986, 1989, 1998, and 2001, respectively.

He was a Teaching Assistant, Lecturer, AssociatedProfessor, and Professor at the Department of Elec-trical Engineering, NUAA, in 1989, 1991, 1996, and2002, respectively. He has been a Minjiang Scholar inthe College of Electrical Engineering, Fuzhou Uni-

versity, Fuzhou, China, since 2005. He has published three works and morethan 100 technical papers. He is the holder of 13 invention patents. His researchinterests include power electronics conversion, renewable energy source gener-ating.

He received one National and three Province Class Reward Productions ofScience and Technology.

Yanhui Chen was born in Henan, China, in 1980. Shereceived the M.S. degree from the College of Electri-cal Engineering, Wuhan University, Wuhan, China,in 2004. She is currently a Ph.D. student in electricalengineering in Fuzhou University, Fuzhou, China.

She was a Lecturer in the college of ElectricalEngineering, Fuzhou University, Fuzhou, China, in2007. She has published almost ten technical papers.She is the holder of three invention patents. Her re-search interests include HF power conversion.

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