XIII International PhD Workshop OWD 2011, October Single-Stage DC-AC Converter Based On Two DC-DC Converters

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1 XIII International Ph Workshop OW 20, October 20 Single-Stage C-AC Converter Base On Two C-C Converters Tine Konjeic, University of Maribor ( , prof. Miro Milanovič, University of Maribor) Abstract This paper presents a basic operation principle an experimental testing of a single-stage C-AC converter that is base on two boost C-C converters. Its main attribute is the ability to generate an AC output voltage higher than the C input. In orer to test an verify the basic operation principle of the converter, an experimental prototype was esigne an built in the laboratory. An openloop control scheme was implemente an experimental tests were carrie out. As open-loop control cannot assure reliable performance an aaptation to a variable operation point, a closeloop control scheme was propose, implemente an experimentally teste as well.. Introuction A single-stage C-AC converter base on two C-C converters has the ability to generate AC output voltage that is lower or higher than the C input voltage. For this kin of power conversion, multi-stage converters are mostly use. The first conversion stage is usually represente by a C-C converter, which amplifies the C voltage to a esire level. In the secon conversion stage, this voltage is converte into alternating voltage, using a conventional full-brige inverter. As every conversion stage prouces a certain amount of loss, it is expecte that the use of a single-stage converter will lea to achieving higher converter efficiency. The main avantage of a single-stage C-AC converter is the fact that it can execute C voltage amplification an C-AC conversion in a single conversion stage []. While single-stage conversion theoretically enables achieving high converter efficiency, its simple structure (Fig.) assures small size, light weight an low prouction costs [2, 3]. Among avantages, the 346 ic R i C 2 v C S S 2 i O L il i L 2 L 2 Cin S 3 S 4 C2 v2 Fig.. Basic circuit of a single-stage C-AC converter. buck-boost features of the converter must be mentione as well. They assure efficient operation in a wie range of C input voltages an aaptability to various loas. In comparison with a conventional multi-stage converter, the isavantage of the iscusse converter is in system control. The control scheme for multi-stage converters is less complex, as C- C conversion an C-AC conversion stages are separate an therefore controlle separately. In case of single-stage converters, both conversion stages are joine an consequently it is impossible to control them separately. Many control strategies for the single-stage C- AC converter have been propose. The most promising of them are sliing moe control [4], aaptive control [5] an ouble-loop cascae control of inuctor current an output voltage [6, 7]. Sliing moe control achieves goo results merely in steay state an has some rawbacks relate to the complex theoretical backgroun, the variable switching frequency an the absence of inuctor current control. With implementation of aaptive control strategy variable switching frequency is eliminate plus better responses to loa changes are achieve. Similar to sliing moe control, aaptive control has a complex theoretical backgroun which is har to implement. The best results with the above mentione control strategies are achieve with the use of ouble-loop cascae control of inuctor current an output voltage, which assures stable an efficient operation uring transient states, such as abrupt loa changes an short circuits. The latter

2 control strategy is aitionally explaine in this paper. In the following sections, the basic operation principle of single-stage C-AC converter is presente. A prototype was esigne an built in the laboratory in orer to test the operation of the converter. For this reason, open-loop control as well as close-loop control were esigne, implemente an evaluate on the basis of experimental test results. C v 2 C v 2. Operation principle Basic circuit of a single-stage C-AC converter (Fig.) consists of two biirectional boost C-C converters, which are connecte to a mutual C power source. A basic requirement for proper operation of the converter is the complete symmetry of both C-C converters in the circuit. The propose single-stage C-AC converter achieves C-AC conversion by connecting the loa ifferentially across the output of both C-C converters, while moulating their output voltages sinusoially []. Both C-C converters have to be moulate such that they generate the same unipolar C-biase sine wave output voltages, with a phase shift equal to 80 between them (Fig.2). A phase shift of 80 has to be present in orer to imize the voltage excursion across the loa. Since the converter is symmetrical an the loa is expose to the ifference between voltages v an v 2, the C component U C is eliminate. As a result, there is a bipolar AC voltage v O, with ouble amplitue regaring to voltages v an v 2, generate at the converter output. Following the presumption that all the components in the circuit are ieal an all the currents are continuous, the following equation for a boost C-C converter can be written to escribe voltage conitions in one leg of propose converter: =, () where is C-C converter output voltage, input voltage an uty cycle. While single- Fig.2. The principle of generating AC output voltage: output voltage of the first converter leg v (upper), output voltage of the secon converter leg v 2 (mile), AC output voltage of the converter (lower). stage C-AC converter is mae of two such converters, whose output voltage is 80 out of phase, the output voltage O of the propose converter can be efine as: O = 2 =. (2) On the basis of (2), the voltage gain of the iscusse converter is obtaine: O 2 = ( ). (3) From (3), it is evient that zero output voltage is obtaine while = 0.5. If the uty cycle is varie aroun this point, AC voltage can be generate at the output of converter. 3. Experimental prototype In orer to test the operation of a single-stage C-AC converter, an experimental prototype was esigne an built in the laboratory. The prototype was esigne for the power of kw, C input voltage up to 00, AC output voltage 230 RMS with frequency of 50 Hz an switching frequency of 25 khz. In the phase of esign, much attention was given to achieving the symmetry of both converter legs an choosing suitable active an passive components of the circuit. The latter were chosen on the basis of analytical calculations an simulation results. Taking voltage an current criteria into account, IGBT transistors (600, 45 A) with antiparallel ioes were selecte as the most suitable for the application. 347

3 v i 40m 38m i O 76.m -76.5m Fig.3. Experimental prototype of a single-stage C-AC converter. Maximum allowe current ripple, energy that has to be store in the inuctor over one switching perio an operation outsie of saturation area serve as criteria for inuctor esign. The require inuctance ( L = L2 = L = 300 µh) of inuctors in the converter circuit was calculate as: C Ts L, (4) I L IL IL where an I L are uty cycle an inuctor current at a chosen operation point, T s switching perio an IL IL the imum allowe current ripple. For the inuctor, a core of Microsil was use. With the imum magnetic flux ensity B =,6 T an an air gap of 0.2 mm, it assures that the inuctor never reaches saturation uring the operation of the converter. The require capacitance ( C = C2 = C = 50 µf) of output capacitors was calculate at nominal operation point of the converter an imum allowe output voltage ripple O =.5 % as: C T s, (5) ( ˆ O ) where is the imum uty cycle an R the resistance of the loa at nominal operating point. For the implementation of converter control an protection functions, a igital signal processor ezsp TMS320F282 was use. R Fig.4. Input voltage v an current i an output voltage u O an current i O by open-loop control (v :0 /iv, i : 2,5 A/iv, :20 /iv, i O:0,5 A/iv). 4. Control esign an experimental testing of the converter 4. Open-loop control In orer to verify the escribe operation principle of the converter, an open-loop control system was esigne an implemente. For generation of transistor gate rive signals pulsewith moulation was use. With the presumption of ieal components in the converter circuit, functions for esire altering of uty cycle were erive:,4 2,3 = K + + t ( ˆ ) sin( ω ) ( ) = K + t ( ˆ ) sin ( ω ) ( ), (6), (7) where,4 is the instantaneous value of uty cycle for switches in the first converter leg (S, S 4), 2,3 the instantaneous value of uty cycle for switches in the secon converter leg (S 2, S 3), K the parameter for setting the value of C an ˆ the parameter for setting the amplitue of the voltage across each output capacitor. Experimental tests were carrie out at input voltage = 22, loa with resistance R = 00 Ω, switching frequency f s = 20 khz an parameters K =.2, ˆ = 2. The test results (Fig.4) have confirme that a single-stage C-AC converter has the ability to generate an AC output voltage higher than its C input. As a result of the nonlinear characteristics of real components an the presumption of ieal components in the uty cycle function, some 348

4 I O ref ICref IL ref L I I C srcc + sc I O I Lref Lref L sl + R L I L Fig.5. ouble-loop cascae control scheme: outer control loop for the control of voltage across output capacitors (upper) an inner control loop for the control of inuctor current (lower). eviations from a clear sine wave have occurre in output voltage an current. 4.2 Close-loop control To eliminate eviations from a clear sinusoial shape an assure stable an efficient operation uring transient states cause by a change of loa, close-loop control has to be applie. For its implementation, a ouble-loop cascae control was chosen. The control scheme (Fig.5) consists of outer control loop for the control of voltage across output capacitors an of inner control loop for the control of inuctor current. On the basis of ifference between reference an measure C-C converter output voltage, the outer loop generates reference inuctor current I Lref. This value is use as an input parameter for the inner loop, which generates the uty cycle ( ). For each converter leg, the following current an voltage equations can be written: ( ) i i = i, (8) C O L ( ) v v = v, (9) L where v an i C are the output capacitor voltage an current, v L an i L the inuctor voltage an current, v the input voltage, i O the output current an the uty cycle. Currents through the output capacitor an voltage across the inuctor of one C-C converter can be escribe as: 349 ic v i + C RCC C t = t, (0) i v R i L t L L = L L +, () where R L an R C are resistances of inuctor an capacitor. In orer to erive transfer functions of both systems escribe with (0) an (), a Laplace transform was performe. With capacitor current ( i C) an inuctor voltage ( v L) chosen as control variables, the following transfer functions can be written: (s) I (s) sr C + sc C =, (2) C I L(s) = (s) sl + R L L. (3) Each transfer function represents a simplifie moel of the system that has to be controlle. Controllers for both control loops were selecte on this basis. A lea-lag controller was chosen for the outer control loop, as it is the most appropriate to control the system escribe with (2). Since (3) represents a first-orer system, a PI controller was chosen for the inner control loop.

5 m 240m v i clear sine wave output current an voltage. In case of close-loop control, the uty cycle is generate on the basis of feeback information about current system state. Therefore all the nonlinearities of components in the circuit an loa changes can be compensate m 76.m Fig.6. Input voltage v an current i an output voltage u O an current i O by close-loop control (v :0 /iv, i : 2,5 A/iv, :20 /iv, i O:0,5 A/iv). Parameters of both controllers were set with the use of SISO esign Tool in Matlab software. In orer to assure proper operation of the control system, controller outputs are limite. Bounary values are calculate on the basis of current state of the system. The control of the single-stage C-AC converter is achieve by implementing the previously escribe ouble-loop cascae control scheme on both C-C converters an riving their output voltages with proper C biase sinusoial references ( vref, v 2ref ) [6]. These references serve as input parameters for outer control loop an are efine as: vo vref = C + sin( ωt), (4) 2 vo v2ref = C sin( ωt). (5) 2 The opposite sign of alternating components in (4) an (5) assures phase shift of 80, which is require for achieving imum amplitue of AC voltage across the loa. C components C in (4) an (5) are present for preventing C-C converter from operation outsie boost regime. They have the same values in both cases, thus they can be mutually eliminate with ifferential connection of the loa. The propose close-loop control scheme was implemente on the experimental prototype of a single-stage C-AC converter. Experimental tests were carrie out at input voltage = 22 an an abrupt loa change from R = 50 Ω to R = 00 Ω. Test results (Fig.6) have shown that converter output voltage maintains the same value after the loa change an that the transient state, which is seen in output current, ens in less than 0 ms. In contrast with open-loop control, close-loop control assures i O 5. Conclusion This paper eals with a single-stage C-AC converter. An open- as well as close-loop control strategy was propose, esigne an implemente in orer to verify the converter operation. The problem of the open-loop control is reflecte in eforme shape of output voltage an current, which eviate from esire sinusoial shape. The reason for this is in erivation of moulation functions, where it is practically impossible to inclue all the nonlinearities of real components in the converter circuit. The isavantage of open-loop control is also in inaequate system responses to loa changes, ue to the lack of feeback information about the current system state. Mentione problems can be solve with the use of a close-loop control. ouble-loop cascae control of inuctor current an output voltage, which was presente in this article, has shown goo results in the way of achieving clear sinusoial shape of output current an voltage as well as assuring reliable operation of the system uring transient states cause by abrupt loa changes. Acknowlegement I woul like to thank M. Roic, M. Truntic an C. Restrepo for their support an many helpful iscussions throughout the research. Bibliography [] R.O. Cáceres, I. Barbi, "A Boost C-AC Converter: Analysis, esign, an Experimentation", IEEE Transactions on Power Electronics, vol. 4, no., Jan [2] T. Konjeic, "Enostopenjski C-AC Pretvornik za Solarne Sisteme", iploma thesis, University of Maribor, Faculty of electrical engineering an computer science, ec [3] S. Menaka, S. Muraliharan, "esign an performance analysis of novel boost C-AC converter," 3r International Conference on Electronics Computer Technology (ICECT) 20, vol.2, 8-0 Apr. 20. [4] R.O. Cáceres, I. Barbi, "Sliing Moe Controller for the Boost Inverter", Power Electronics Congress (CIEP '96), Cuernavaca, Mexico, Sep

6 [5] C. Albea, C. Canuas-e-Wit, F. Gorillo, "Aaptive Control of the Boost C-AC Converter", 6th IEEE International Conference on Control Applications, Singapore, Oct [6] P. Sanchis, A. Ursæa, E. Gubía, L. Marroyo, "Boost C-AC Inverter: A New Control Strategy", IEEE Transactions on Power Electronics, vol. 20, no. 2, Mar [7] B. Kalaivani,. Kumar Chinnaiyan, J. Jerome, "A Novel Control Strategy for the Boost C- AC Inverter", Proceeings of Inia International Conference on Power Electronics 2006 (IICPE '06), Inia, ec Author: Ph stuent Tine Konjeic University of Maribor Smetanova ulica Maribor tel. (+386) tine.konjeic@uni-mb.si 35