Fuzzy controlled modified SEPIC converter with magnetic coupling for very high static gain applications

Fuzzy controlled modified SEPIC converter with magnetic coupling for very high static gain applications Rahul P Raj 1,Rachel Rose 2 1 Master s Student, Department of Electrical Engineering,Saintgits college of engineering. 2 AssociateProfessor, Department of Electrical Engineering,Saintgits college of engineering. Abstract This paper deals with a new method for very high static gain for DC-DC converter based on the modified SEPIC converter. The main improvement is low switch voltage with higher efficiency can be achieved by using magnetic coupling and output diode clamping. Without increase the switch voltage static gain can be increased by using magnetic coupling and voltage multiplier. Voltage multiplier at the secondary side will reduce the overvoltage across output diode. Simulation result show that proposed SEPIC converter suitable for low input and high output applications. This method improve the load regulation. The overall circuit produce 94% efficiency with an output voltage equal to 300V. Index Terms Magnetic coupling, voltage multiplier, very high static gain, SEPIC converter I. INTRODUCTION Non conventional energy sources are the solar energy,wind energy have commonly used for replacing the conventional sources like coal, natural gas. The advantage of renewable energy source is its ease of availability and its free of cost. Thus the research have been done with these renewable energy source to develop it for various practical applications and its developments to connection with micro smart grid. Especially solar cell is east to integrate with boost converter topology.the major challenges dealing with the connection of solar cell is that to reduce losses, weight and volume of the overall circuit. High gain operation is necessary to maintain good performance and characteristics. The challenge dealing with the control circuit consisting of converter is the improvement of the efficiency, reduction of the cost and reliability of the circuit during the life time of the module. The first goal is to reduce the overall power consumption within the converter. This leads to minimum heat dissipation which in turn ensure minimal thermal management, cooling control and improved reliability. The second consideration pertains to produce the component count simple controller but the voltage stress across the components are high.. The basic topology of the converter part discussed in this paper is the high gain SEPIC (Single Ended Primary Inductor Converter DC-DC converter allowing the voltage at its output to be greater than or less than or equal to that at its input. By controlling the duty cycle of control transistor, SEPIC output can be controlled. A SEPIC is essentially a boost converter followed by a buck boost converter, its normally called as buck boost converter, but has advantage of having non-inverted output (the out put has the same voltage polarity at its input) to couple energy from the input to output by the use of a series capacitor, and being a capable of true turned off: when the switch is turned off, its output drop to 0V,following a moderately heavy transient dump of charge. The use conventional SEPIC converter is the most efficient way to introduce the high gain converter with reduced cost. The conventional one has a coupling capacitor that can improve the output power of the circuit. The coupling cost is higher so to get a high gain converter, a double capacitor multiplier cell is attached across the switch. This will also improve the efficiency of the switch and reduce the stress the switch. Figure 1 Block diagram The above block diagram represents the overview of the control circuit that consist of converter with a constant DC source. A DC source is given as input to the high gain SEPIC converter and the output of the IJIRT 143909 INTERNATIONAL JOURNAL OF INNOVATIVE RESEARCH IN TECHNOLOGY 39

converter is given to the resistive load. The DC voltage generated from the solar cell is generally small, this can be overcome by two methods, by multi cascading DC-DC converters and second method is to use high gain converters II. COMPARISON WITH CONVENTIONAL CONVERTER In conventional SEPIC converter high static gain with low voltage stress cannot be obtained. The boost converter is the classical non isolated step-up converter and which can be normally operate with an adequate static and dynamic performance with a duty cycle close to D=.8, resulting an output voltage around five times that of input voltage. In this scheme there are mainly three static ranges are considered. Static gain q=5 is considered as standard gain when a DC-DC converter operate, when the static gain is higher than q=10 is considered as high gain and finally static gain higher than q=20 is considered as very high static gain solution. The efficiency and power density can be improved by eliminating the power transformer. Because the power transformer which will reduce efficiency due to the transformer power loss and leakage inductance. The main technology to increase the static gain by the addition of voltage multiplier cell, switch capacitors and inductor magnetic coupling and also combination of all these three methods. In this preferred topology, magnetic coupling based on modified SEPIC converter employed. Without increase the switch voltage static gain can be equal to or greater than 20 times can be achieved. Conventional SEPIC converter is shown in fig.1. Hard switching operation is used in the conventional topology. The main problems due to the hard switching operation is the extreme switching as well as conduction loss will high, in order to overcome these disadvantage soft switching method is employed in this scheme. ZCS turn on commutation and reduce the diode reverse recovery current can be small leakage inductance is necessary. By using these overall performance and losses can be reduced. Good load regulation can be achieved by using fuzzy controlled modified SEPIC converter with magnetic coupling. During load disturbance and parameter variations Fuzzy can provide good performance and also which can be operate with noise and other disturbance of different nature. Fig.2. Conventional SEPIC converter III. ANALYSIS OF MODIFIED SEPIC CONVERTER WITH MAGNETIC COUPLING AND OPERATING SATGES The block diagram of complete setup shown in fig.3. DC supply from a PV cell given to the high gain SEPIC converter. The converter gives an output gain equal to 20. The output of the converter gives to the resistive load, the voltage sensor sense the out put voltage Fig.3 Block diagram of complete setup The output voltage sensed by the voltage sensor which is actually a voltage multiplier. Fuzzy control take the control of the out put voltage based on Fuzzy rules. These rules tune the circuit whenever there is any change in the load in order to make the output voltage constant. Whenever there is change in the load, the Fuzzy controller adjust the duty cycle based on the requirement. The controlled DC supply is given through a fixed switching frequency A. Analysis of Modified SEPIC converter Circuit IJIRT 143909 INTERNATIONAL JOURNAL OF INNOVATIVE RESEARCH IN TECHNOLOGY 40

Fig 4.Proposed system Fig 4.shows that the proposed system, with magnetic coupling and voltage clamping circuit. Overvoltage across the diode can be reduced by providing a voltage multiplier circuit at the secondary side. The multiplier consist of dioded m2 and capacitor C s2, which will act as non dissipative clamping circuit for the output diode. The main function of voltage multiplier circuit to increase the static gain as well as reduce the voltage across the diode D 0 than the output voltage. And thus stored energy in the inductor is transferred to the output. For reducing the switching losses switch turn on only at zero current. The current variation ratio( d i d t ) presented by all diode is limited due to the existence of coupling leakage inductance. Due to the leakage inductance,switch over voltage happen in isolated DC-DC converters. The energy stored in the leakage inductance must be transferred to the clamping circuit. For achieving ZCS turn on commutation and reduce diode reverse recovery problem leakage inductance is necessary in this topology. Very high static gain and better performance can be achieved by using magnetic coupling at the input side and voltage multiplier at the secondary side as discussed. Ripple current at the input side increased with respect to the input inductor winding turns ratio when the magnetic coupling is included with the input inductor. For obtaining higher efficiency mainly isolated active clamp SEPIC fly back converter are illustrated. How ever,the preferred topology present pulsating current and the active clamp technique with an additional switch increases the converter complexity. In this topology commutation losses and switch voltage can be reduced and ZCS switch turn on obtained at the resonant stage and considering the same transformer ratio very high static gain can be obtained. These are the main advantages over the previous topology. IV. MODES OF OPERATION OF SEPIC CONVERTER WITH MGNETIC COUPLING The main focus of this preferred topology to increase the converter static gain double than that of the conventional topology and also to reduce the switch voltage. To achieve this converter can be operate in CCM mode at a duty cycle greater than q=.5. Modified SEPIC converter in CCM mode has five operational stages. For theoretical analysis following assumptions are done 1)All capacitors are considered as voltage source 2) A ll semiconductors are considered as ideal. 1) Stage 1[t o t 1 ]: Switch S is conducting and the energy stored in the input inductor L 1. Through the secondary winding L 2S and dioded M2 capacitor C S2 charged. The leakage inductance decreases the current and the energy transference happen in a resonant way. The output diode is blocked and the maximum diode voltage is equal to ( V O V CM ) and the energy transference to the capacitor C S2 is completed and diode D M2 blocked, at the instant t 1. Fig 5: First operation stage 2) Stage 2 [t 1 t 2 ](fig ): at the instant t 1,the energy transference to the capacitor is completed C S2 and diode D M2 is blocked. Switch is turned off at the instant t 2 and the inductors L1 and L2 store energy and current linearly increase. Fig 6. Second operation stage IJIRT 143909 INTERNATIONAL JOURNAL OF INNOVATIVE RESEARCH IN TECHNOLOGY 41

3) Stage 3 [t 2 t 3 ](fig ) :Switch is remain in turned off at the instant t2.energy stored in the L1 inductor transferred to the capacitor CM. And also energy transference to the output through the capacitors cs1,cs2 inductor L2 and output diode d0 In this proposed topology static gain calculated by (1). Without increasing the switch voltage, static gain is increased by increasing the winding turns ratio (n). v 0 = 1 (1+n) (1) V i 1 D n represents the inductor winding turns ratio, n = N L2s N L2P (2) where n=1,static gain q=10,n=2 static gain q=15 and n=3 static gain q=20 all these three cases, at duty cycle D=.8 switch voltage become five times the input voltage. Fig 7.Third operation stage 4) Stage 4[t 3 t 4 ](fig ):Energy transference to the capacitor CM is finished and the diode DM1 is blocked during the instant t 3. The energy transferred to the output is maintained until the instant S Fig 8.Fourth operation stage 5) Stage 5[t 4 t 5 ](Fig ):Power switch is turned on at the instant t 4,the current at the output diode D0 linearly decreases and change in current is limited by leakage inductance, reducing the diode reverse recovery problems. The converter returns to first stage when the output diode blocked. Maximum switch voltage is obtained during the third operation stage an which is equal to the capacitor voltage CM. capacitor switch voltage VCM, V CM V I = 1 1 D B) Design of proposed converter Voltage input=15v; Switching frequency=24khz; Output voltage=300v Power outputp 0 =100w; (3) Using these specification, design of modified SEPIC converter with magnetic coupling and voltage multiplier are obtained: 1)Dutycycle,D: Duty cycle is calculated by using the equation (1), static gain q=20 and winding turns ratio n=2.6 ie D = 1 - V i V O (1+n) =(1-15/300) (1+2.6) =.819 (4) 2) Diode voltage vdo and switch vs:capacitor voltage equal to the diode and switch voltage, which is obtained from the equation V s = V DM 1 = V i 1 D (5) = 15. 1.819 = 82.9 V Voltage across the output diode VDO and voltage at the diode DM2 are equal, which are calculated by Fig 9:Fifth operation stage V Do = V DM 2 = V 0 - V CM = n V i 1 D (6) IJIRT 143909 INTERNATIONAL JOURNAL OF INNOVATIVE RESEARCH IN TECHNOLOGY 42

= 2.6 15. 1.819 = 215.5 V 3) Inductance L 1 and L 2P - L 2s : for current ripple at the input side i L = 5A, the inductance values are calculated by L 1 = L 2P = V i D i L f (7) From equation (4)D=.819 and f is the switching frequency=24khz, L 1 = L 2P = V i D i L f = 15.812/5 ( 24 10 3 ) =102µH (8) The average current values at the Inductance L 1 and inductance L 2 equal to the input and output current. For the magnetic coupling inductor L2S can be obtained from L1 inductance, for n=2.6 L 2S = 2.6 2 (102 10 6 ) =689.52µH (9) 4)Leakage inductance L: Leakage inductance not represented in this proposed fig 3 and also during the operation stages 1-5. Leakage inductance can be placed in series with L2P inductor or which is placed in series with L2S inductor. Leakage inductance is necessary to reduce the reverse recovery current at the output diode and also to achieve ZCS turn-on commutation. For these, small value of leakage inductance is necessary During the fifth stage of operation, when the switch is turned on,very high change in current DI/DT at the output diode and also reverse recovery current will happen, thus the commutation losses will increases. This can be limited by including a leakage inductance at the primary side. Change in current at the output diode is equal to 25A/µs. Leakage inductance value calculated by = 15 1.819 25 10 6 2.6 L r = V i 1 D n d i d t = 1.27µH (10) 6) Output current: The average current value of all diode equal to the output current. I Do = I DM 1 = I DM 2 =I o = P o V o = 100/300 =.333 A (11) 7)CapacitorsC s and C M : capacitor voltage ripple factor V C equal to 15% of capacitor voltage CM ie, V C = V i 1 D V i P 0 = 15 15 =12.4V (12) 1.819 100 Voltage across the capacitor CS1 and CM can be calculated as =.333 2.6 12.4 ( 24 10 3 ) C s1 = C M = I 0 n v c.f =2.9µF (13) V. FUZZY CONTROL FOR LOAD REGULATION A) simple fuzzy logic controller is made up by a group of rule based on the human knowledge regarding the system behavior. Fuzzy set theory is largely used in the control area with some applications to dc-dc converter system. A Fuzzy logic controller forms an important part of this controller which process the voltage error resulting from the comparison of set voltage reference and sensed voltage from the converter output. The resultant voltage error amplified and compared with a saw tooth carrier wave of fixed frequency (f)for generating the PWM pulses for controlling switch of the converter. B) Fuzzy controller design: Input for the fuzzy controller are Error (E) and change in error(ce). Change in duty cycle is the output of the controller. Error is defined as the difference between the reference voltage and actual voltage,where as the change in error is defined as the present error and previous error and output. Change in duty cycle in which could be either positive or negative is added with the existing duty cycle to determine the new duty cycle. Input to the fuzzy logic controller are the error and change in error. These two input are divided into seven group : LN: Large negative, SN: Small negative, ZE: Zero error, LP: Large positive, SP: Small positive, VLN: Very large negative, VSN: Very small negative. Instead of numerical variablefuzzy logic uses linguistic variables. Converting numerical variable into linguistic variable is known as fuzzification. Choice of membership function and choice of scaling IJIRT 143909 INTERNATIONAL JOURNAL OF INNOVATIVE RESEARCH IN TECHNOLOGY 43

factor are the two parts of Fuzzifier. The fuzzy variable error, change in error and change in duty cycle are quantized using linguistic terms LN,SNZE.LP,SP,VLN,VSN. A rule base (a set of if- Then rules), which consist a fuzzy logic quantification of the experts linguistic description of how to achieve control action. Once the rule have been established, a fuzzy logic system can be viewed as mapping from input to output. Rules can be extracted from the numerical data. The performance of the controller can be improved by adjusting membership function and rules The fuzzy operation is implemented by using 25 rules. Finally the fuzzy output is converted into real output value by the process is called defuzzification.fig.10 shows the complete simulation setup Fig 12:Input voltage Fig 13:Output current Fig 10 simulation diagram of complete setup VI. SIMULATION RESULTS For achieving the regulated out put fuzzy controlled modified SEPIC converter with magnetic coupling and voltage multiplier can be simulated by using MATLAB/SIMULINK, model. Here input voltage equal to 15 and correspondingly the out put voltage will be 300 for the variation of load. Fig 14:Output voltage TABLE I SIMULATION PARAMETERS Input voltage 15V Output voltage 300V Switching frequency 24 Duty cycle 85 Switching voltage 83V Static gain 20 Output power 105 Fig 11 Gate pulse The pulse is generated for a duty cycle of 80% and switching frequency 24 KHZ Efficiency 94 Table 1. Simulation Parameter details IJIRT 143909 INTERNATIONAL JOURNAL OF INNOVATIVE RESEARCH IN TECHNOLOGY 44

The whole system is simulated in MATLAB/SIMULINK tool. In the following graphs, which is clear that for any variation in the load the regulated voltage can be obtained Fig 15:Output voltage with load equal to 900Ω (a) Fig 16:Output voltage Load equal to 875Ω (b) Fig 17:Output voltage load equal to 925 Ω (c) Fig.12 to Fig 14 shows that for any variation in the load we can obtain the constant output voltage, ie regulated output voltage. VII. CONCLUSION This paper present to obtain very high static gain without increase in the duty cycle and switch voltage by using magnetic coupling and voltage multiplier in the modified SEPIC converter. Static gain around 20 can be achieved with 15v input. The efficiency of the proposed topology can be obtained as 94% with an input voltage equal to 15V and output voltage equal to 300V and also the output power 103W. However due to the leakage inductance commutation losses are reduced and overvoltage across the diode limited bt using voltage clamping circuit. In order to achieve regulated output modified SEPIC converter with magnetic coupling can be controlled by using the Fuzzy control REFERENCES [1] C. W. Li and X. He, Review of non-isolated high step-up DC/DC converters in photovoltaic grid-connected applications, IEEE Trans. Ind.Electron., vol. 58, no. 4, pp. 1239 1250, Apr. 2011. [2] C. S. B. Kjaer, J. K. Pedersen, and F. Blaabjerg, A review of singlephase grid-connected inverters for photovoltaic modules, IEEE Trans.Ind. Appl., vol. 41, no. 5, pp. 1292 1306, Sep. 2005. [3] D. Meneses, F. Blaabjerg, O. Garcia, and J. A. Cobos, Review and comparison of step-up transformerless topologies for photovoltaicac- Module application, IEEE Trans. Power Electron., vol. 28, no. 6, pp. 2649 2663,Jun. 2013. [4] B. Axelrod, Y. Berkovich, and A. Ioinovici, Switched- Capacitor/Switched-Inductor structures for getting transformerless hybrid DC DC PWM converters, IEEE Trans. Circuits Syst. I, Reg.Papers, vol. 55, no. 2, pp. 687 696, Mar. 2008. [5] L.-S.Yang, T.-J. Liang, and J.-F. Chen, TransformerlessDC DC converters with high step-up voltage gain, IEEE Trans. Ind. Electron., vol. 56, no. 8, pp. 3144 3152, Aug. 2009. [6] Dongbing Zhang, Designing ASepic Converter. May 2006, revised April 2013 Formerly National Semiconductor Application Note 1484, now Texas Instruments Application Report SNVA168E. [7] A. TOMASZUK_ and A. KRUPA Faculty of Electrical Engineering, Department of Automatic Control Engineering and Electronics, Bialystok IJIRT 143909 INTERNATIONAL JOURNAL OF INNOVATIVE RESEARCH IN TECHNOLOGY 45

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