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International Electrical Engineering Journal (IEEJ) Vol. 6 (2015) No.3, pp. 1822-1827 ISSN 2078-2365 http://www.ieejournal.com/ A Novel Phase Shifted DC-DC Converter with Adaptive Soft Switching to Improve Efficiency under wide Load Range Sudha Bansal, Lalit Mohan Saini [email protected] Abstract—This paper presents a novel dc-dc converter with adaptive soft switching as a means to achieve ZVS operation for all the switches. It adopts phase shift modulation features for constant frequency operation and adaptive soft switching at low load. To accomplish this task two loops are employed; inner current control loop is to check the loading condition and turn on of auxiliary switch accordingly. Outer control loop is used to improve the dynamic performance of DC-DC converter by achieving a robust output voltage against load disturbances. This paper also presents the performance of various controllers. A 1kW/100KHz dc/dc converter is simulated and analyzed. Performance of the proposed topology is evaluated at different loads i.e. static and dynamic load (DC motor). An efficiency comparison of the converter with a reported topology has also been carried out. Index Terms— DC-DC converter; phase-shifted; resonant tank; reverse recovery; resonant converter; Soft switching. I. INTRODUCTION DC-DC conversion technology has been developing very rapidly, and DC-DC converters have been widely used in industrial applications such as dc motor drives, computer systems and communication equipment. The output voltage of pulse width modulation (PWM) based DC-DC converters can be controlled by changing the duty cycle. In general, to minimize the size and weight of pulse width modulated (PWM) converters, it is required that the switching frequency must be increased [1]- [2]. However, increasing the switching frequency leads to substantial switching losses, which causes deterioration in system efficiency. Therefore, the switching losses should be reduced in the case of high switching frequency operation. Various types of resonant converters have been reported to decrease the switching losses during the transient states [3] - [5]. Numerous soft switching techniques for the switching power converters have been proposed [6] [9]. These techniques reduce the switching losses, thus enabling high frequency operation and also reduce the overall system size. Among many new techniques proposed for high frequency power conversion to reduce the switching loss in traditional PWM converters, the phase- shifted zero-voltage full-bridge pulse width modulation (ZVS FB- PWM) converters are most desirable since they reduce switching loss considerably without the penalty of a significant increase in conduction loss [10] - [12]. For power levels up to 3 kW, the full-bridge converters employ MOSFET switches and use Phase-Shift Modulation (PSM) to regulate the output voltage. The FB ZVS phase-shift DC-DC converter is preferred due to its remarkable features. In most of these converters, zero voltage switching (ZVS) is achieved by placing a snubber capacitor across each of the switches and either by inserting an inductor in series with the transformer or by inserting an inductor in parallel to the power transformer. These benefits can be summarized as: ZVS for all the bridge transistors, reduction of the conduction losses as compared with quasi-resonant converters, reduction of the electromagnetic noise, utilization of the device output capacitance and transformer leakage inductance, fixed-frequency operation[13] - [14]. However, there are some drawbacks as high circulating currents , loss of duty cycle , ZVS is lost for light loads i.e. narrow ZVS range and load-dependent dc characteristics, also, it undergoes various influences by ringing between the parasitic capacitor of rectifying diodes and the leakage inductance, which cause switching losses and switching noise. Since LLC resonant converter can offer wider zero voltage switching region and higher conversion efficiency, it recently has been widely adopted in the design of front end power supply where output voltage is relatively high and output load current is relatively low. The problem of narrow ZVS range at light load current is addressed in this paper. For this an L-C- L resonant circuit, with an auxiliary active switch is employed. The main contribution of this paper is to propose a simple adaptive soft switching converter to achieve ZVS operation for all the switches. It adopts phase shift modulation features for constant frequency operation and adaptive soft switching at low load. ZVS of active switches in the lagging leg can be achieved from zero- to full-load condition. In this paper the operation of proposed full bridge phase-shifted converter and then steady state analysis of the converter is discussed in 1822 Sudha and Lalit A Novel Phase Shifted DC-DC Converter with Adaptive Soft Switching to Improve Efficiency under wide Load Range International Electrical Engineering Journal (IEEJ) Vol. 6 (2015) No.3, pp. 1822-1827 ISSN 2078-2365 http://www.ieejournal.com/ D1 Cs2 Cs3 II. Chapter III explains the design of controller. The chapter Cs4 performance at different loading condition (static and dc D5 motor load) is analyzed in chapter IV. . Q1 C1 D1 I0 Q3 C3 D3 Lr Vi Cr C0 D2 Vg1,2 C2 D4 Q1 Vg3,4 D R2 Qa Q4 C4 Q2 Q1 Q3 Q4 Qa R0 La Ca Q2 L0 DR1 Q4 Qa Vab i1 ia t0 t1 t2 t3 t4 t5 t6 t7 t8 t9 Fig. 1. An adaptive soft switching converter and its Steady state waveform II. INSIGHT INTO THE OPERATING PRINCIPLE AND CIRCUIT DESCRIPTION Fig.1 shows the schematic of full bridge adaptive soft switching converter and its steady state waveforms. The primary size contains five switches Q1 ̴ Q4 & Qa . The phase shifted full bridge is formed by the switches Q1 ̴ Q4 with their output parasitic capacitors C1 ̴ C4, anti- parallel diodes D1 ̴ D4. Resonant circuit is formed by resonant inductor Lr, resonant capacitor Cr, parallel inductor La in series with an active switch Qa. Dr1 & Dr2 are the output rectifier diodes, Lo is the output filter inductor and Co is output filter capacitor. Vo’ is the output voltage reflected to primary, Vi is the input voltage, Qr = (Lr/Cr)1/2/RL’, Ro’ = Vo’2/Po, ω = ωs/ωr, ωr is the angular resonant frequency, ωr = 2πfr = (Lr/Cr)1/2, ωs is the angular switching frequency of converter, ωs = 2πfs. The duty cycle of auxiliary switch Qa is kept low and control pulse given to the auxiliary switch depends upon the load current. The details of this current controller are discussed in chapter III. The operation of the converter for heavy load attains ZVS for all switches but under light load, the load current is low, the ZVS of the lagging-leg switches is lost as the energy stored in the leakage inductance of the transformer is insufficient to discharge the switch and transformer capacitances. Thus, the operation of the converter for light load is discussed here. The pulse width given to switch Qa depends upon the load current when the current reduces, pulse duration increases. Inductance in the circuit increases with turn on of Qa, so as to obtain sufficient energy to discharge the switch and transformer capacitances and hence ZVS condition at low load is obtained. Under steady state operation, eight operating states in each half switching cycle are discussed. To simplify the analysis, few assumptions made are as follows: All active switches and diodes are ideal. Inductors, capacitors and transformers used in the circuit are ideal. The output filter inductor, Lo is large enough so that the ripple current is neglected. The output filter capacitance, Co is large enough so that the output voltage is constant The operation is divided into ten intervals in one cycle. Mode 1(t0-t1) : Prior to t0 switches Q1 & Q4 are conducting and power is transferred from input to the load through switches, and rectifier diode DR1. At t= t0, switch Q1 turns off with ZVS as the capacitor C1 blocks the sudden rise of the voltage across the switch. Now in this mode flow of primary current charges the output capacitance C1 of Q1 and discharges capacitor C2 of Q2. At t=t1, the voltage of C2 decreases to zero and the body diode D2 conducts naturally. This mode consists of only charge and discharge operations, it lasts a short time interval (from t0 to t1). Hence the dead time should be sufficient enough to enable this operation. In this interval the load current flows through the transformer secondary winding. Hence inductive energy provided to charge/ discharge the capacitor comes from the leakage and output inductor. VC1 VC2 Vi td t0 t1 Fig. 2: Charge/discharge Operation of Capacitor t1 t 0 t d (C1 C 4 ) Vi ip (1) Mode2 (t1- t2): At t= t1, capacitor C2 is completely discharged hence diode D2 starts conducting. Now primary current freewheels through D2, Q4. The primary current decreases from IP-PK to IP2 with slope – Vo’/Lo’. Auxiliary switch Qa starts conducting in this duration and auxiliary inductor La energies by the auxiliary current. Since La << Lo, hence most of the current passes through La and stores the 1823 Sudha and Lalit A Novel Phase Shifted DC-DC Converter with Adaptive Soft Switching to Improve Efficiency under wide Load Range International Electrical Engineering Journal (IEEJ) Vol. 6 (2015) No.3, pp. 1822-1827 ISSN 2078-2365 http://www.ieejournal.com/ energy in the inductor. The auxiliary current flowing through the switch is given by V (2) i a (t ) i a (t1 ) i (t t1 ) Le Where, ia (t1 ) is the initial current flowing at t1 . The voltage across the switch is (3) vds1 Vi cos[r (t t1 )] Where, r 1 / Ce ( La Lr ) is the resonant frequency, and Ce = C3 + Cr Mode 3 (t2- t3) : This mode starts with the turn off of the switch Q4, primary current flows through C3 & C4. The capacitor C4 begins to charge by the primary current and when voltage across C4 becomes higher than the total voltage drop value of the inductances, rectifier diode DR2 turns on and starts conducting and hence the secondary winding of the transformers is short circuited. Primary voltage changes from 0 to –Vin. The energy required to charge the capacitor C4 and discharge the capacitor C3 is provided by leakage and auxiliary inductor. 1 1 2 2 ( L r La ) I p (C 4 C3 )Vi 2 2 Where, Mode 4 (t3- t4) : At the end of the mode 3, C3 gets completely discharged, hence diode D3 starts conducting. Primary current falls rapidly with the slope of –Vi/Le. Diode D3 and Q2 conducts during the time interval t3-t4 and Q3, Q2 conduct during the time interval t4-t5. At the end of this mode the primary current reaches to a magnitude –IP1. (5) Where, Le = Llk+ Lr Mode 5 (t4 – t5): Switches Q3 & Q2 conduct. The power is transferred from input to the output circuit by the rectifier diode DR2. The current rises in the circuit and it reaches to I P PK at the end of this mode. di p dV ' V' (6) ( Llk L'o ) Vi Vo' & Co 0 i p 0 dt dt R0 From t6 – t10: same cycle repeats for the next half cycle. In summary since the stored energy in the auxiliary inductor is utilized additionally in lagging leg switching transition, all switching devices in the full bridge inverter of the proposed converter turn on & off with ZVS condition from no load to full load. In order to analyze the performance of the converter in switching transition period can be expressed as: ( Lr La ) di p (t ) dt 1 Cr i p (t ).dt 0 Refer equation 3, at t=t3, vds3= Vi and td = (t3-t2) (7) i1 .sin[r (t t2 )] Ce .r Hence, V .C . 1 . tan 1 i e r Ce ip td = (8) A. Zero Voltage Condition Due to the influence of energy stored in the output filter inductor, ZVS is achieved by the leading leg switches Q1 & Q4 for wider range of load. For the lagging leg switches the critical current is required to attain ZVS depends on the energy stored in the inductive circuit. For the particular value of inductance the critical current required to supply sufficient inductive energy to ensure ZVS can be determined. The critical current required for the converter to attain ZVS without having any auxiliary inductance is given by, Ct I p , min .Vi Llk With the incorporation of auxiliary inductor the required current will be, I p , min, a (4) vds3 is the voltage across the switch Q3. di1 (t ) Vi & dia (t t3 ) vds2 (t t3 ) dt Le dt Le Vi Vi . cos[r (t t2 )] Vi Ct .Vi Le On comparing, I Llk or Le p , min, a I I p , min Le p , min I p , min, a 2 .Llk Since, Le= Llk+ La, hence the value of auxiliary inductor required to increase the ZVS range depends on the value of ‘a’ La a 2 1 .Llk ; Where, a = I p , min,a (9) I p ,min A. The Current Ripple The effective duty cycle deff is the ratio of the voltage pulse duration from +Vi or –Vi appearing at the transformer secondary winding in a switching cycle. The DC voltage conversion ratio of the converter directly depends upon the effective duty cycle. V N o s .d , the effective duty cycle is given by, Vi Np eff d eff = do - d , where, do is the original duty cycle of the converter & d is duty cycle loss and is given by, d I p1 I p 2 Vi TS . Llk 2 B. Dead Time The dead-time between the lagging leg switches is one fourth of the resonant period (T R/4), which is calculated by utilizing the transformer leakage inductance and the total output parasitic capacitance of the lagging leg switches. The dead-time requirement for MOSFETs is short while IGBTs require larger dead-time due to the tail current. 1824 Sudha and Lalit A Novel Phase Shifted DC-DC Converter with Adaptive Soft Switching to Improve Efficiency under wide Load Range International Electrical Engineering Journal (IEEJ) Vol. 6 (2015) No.3, pp. 1822-1827 ISSN 2078-2365 http://www.ieejournal.com/ Hence, the dead-time requirement of the circuit must be determined based on the switching device characteristics. III. CONTROLLERS DESIGN A dual loop controller is designed for the system. A. Inner Loop For adaptive control of switch Qa a current controller is designed. The principle behind the operation of this controller is that it will direct the switch only in the case of current falls to the 20% of rated current. This switch remains inoperative in the case of high load current. It will help in getting zero voltage condition from zero to the rated load condition. DC/DC DC/DC CONVERTER CONVERTER Vi Switching Pulse to Qa Load Io + + - H(s) IratedController Vref + (a) Vref PI/ Fuzzy Φ Controller + +Switching Pulse To Sa Control Circuit Switching Pulse +- Power Vo Circuit Voltage Sensor to Qa (b) Fig. 3. (a) Inner Current Control loop & (b) Outer Voltage controlled loop B. Outer Loop To obtain the output voltage regulation various controllers can be used. Here in this paper two types of controllers’ i.e. PI and fuzzy controllers are discussed and later on the performance of the proposed converter using these controllers are analyzed. + - i) PI controller: In order to design a PI controller, the linearized model of the converter has to be determined. In this, the output voltage is compared with the reference to generate the error signal and this signal is given to the PI controller to obtain the desired phase- shift for the active switches. These switches are given phase – shifted pulse to control the output voltage. The value of proportional controller, kp and integral controller, ki are taken as 0.1 and 20 and are finely tuned thereafter. ii) Fuzzy logic controller : PI controller is simple to implement and easy to design, but its performance generally depend on the working point, so that the presence of parasitic elements, time-varying loads and variable supply voltages can make selection of the control parameters difficult, which ensure a proper behavior in any operating conditions. Achieving large-signal stability often calls for a reduction of the useful bandwidth, so affecting converter performance. Fuzzy control is applied to control dc–dc converters because of its simplicity, ease of design and ease of implementation. Fuzzy controllers are well suited to nonlinear time-variant systems and do not need an exact mathematical model for the system being controlled. The fuzzy logic controller determines the operating condition from the measured values and selects the appropriate control actions using the rule base created from the expert knowledge. The control will work based on two input sets: the output voltage error, e(k) = Vref - Vdc (k) (10) and the change in error variations de(k) = e(k) - e(k-l)) (11) Which are sampled at every Tsv= 5µs. The k is the actual sampling sequence. The basic control structure of a fuzzy logic system and its membership function for error, change in error and output are shown in Figure 4(a)-(d). The FLC has three functional blocks for calculation and two databases. The functional blocks in FLC are: 1) fuzzifier; 2) rule evaluator; and 3) defuzzifier. The two databases are Rule base and Database. Fuzzy logic uses linguistic variables instead of numerical variables. The process of converting a numerical variable (real number) into a linguistic variable (fuzzy number) is called fuzzification. For a given crisp input, fuzzifier finds the degree of membership in every linguistic variable. Since, there are only two overlapping memberships in this specific case, all linguistic variables except two will have zero membership. In FLC, the equivalent term is rules and they are linguistic in nature. A fuzzy logic controller so designed is used to control the converter by sending the desired control signal to the PWM signal generator. (a) (b) 1825 Sudha and Lalit A Novel Phase Shifted DC-DC Converter with Adaptive Soft Switching to Improve Efficiency under wide Load Range International Electrical Engineering Journal (IEEJ) Vol. 6 (2015) No.3, pp. 1822-1827 ISSN 2078-2365 http://www.ieejournal.com/ zero ps psm psl pm pml pl pvl pel 1 Degree of membership 0.8 0.6 0.4 0.2 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 output (c) (d) Fig. 4. (a) Internal structure of FLC (b) Membership function plot for error, (c) Membership function plot for change in error and (d) Membership function plot for output IV. RESULTS AND DISCUSSION The simulation of the 1kW, 100/50 Volt, 100 kHz full bridge phase- shifted DC/DC converter circuit is carried out. The systems transient performance is evaluated on applying sudden load changes and the results so obtained are analysed in terms of steady state error, peak overshoot, settling time. Here two types of controllers’ i.e. PI and fuzzy controllers are discussed and the performance of the proposed converter using these controllers are analyzed. (c) Fig.6.(a) With PI Controller,(b) With Fuzzy Controller &(c) Current of the DC/DC converter at 10% of load, when sudden load is increased to 10% of full load B. At full load and 50% of load The converter is initially full loaded and then the load is reduced suddenly to 50% of full load at t = 0.06 sec. The output voltage and current are measured with this sudden change at t = 0.12 sec load is again increased suddenly to full load. Fig. 6 (d). Output Voltage and Current of the DC/DC converter at 100% load, When Sudden load is reduced to 50% load at t =0.06 Sec & again increased at t=0.12 sec. Fig. 5. Transformer voltage and current waveform at the primary of the Inverter A. At 10% of full -load to 20% of full -load: In this converter is initially loaded with 20% of the full load and then load is suddenly decreased to 10% of full -load at t=0.06 sec by step decrease in load. The output voltage and current waveform for PI Controller and Fuzzy controller are shown in fig.6. C. At different values of Load Inductance, keeping resistive load constant: The behavior of converter circuit with different values of inductive load is observed. The effect of inductance variation on output voltage is shown in fig.7. (a) Fig. 7. The output Voltage at different load conditions (b) D. Zero voltage Switching: All the active switches in the converter turns on and off at zero voltage, so as to minimize switching losses. The voltage across drain to source at 5% of load and at full load is shown in fig. 8 (a) & (b) respectively. 1826 Sudha and Lalit A Novel Phase Shifted DC-DC Converter with Adaptive Soft Switching to Improve Efficiency under wide Load Range International Electrical Engineering Journal (IEEJ) Vol. 6 (2015) No.3, pp. 1822-1827 ISSN 2078-2365 http://www.ieejournal.com/ Hua G., Lee F. C., Jovanovic M. M., “An Improved Full-Bridge Zero- Voltage- Switched PWM Converter Using a Saturable Inductor,” IEEE Transaction on Power Electronics 1993, vol. 8, issue 4, pp. 530-534. [6]. Sun J., Hamada S., Yoshitsugn J., Guo B., Nakaaka M. , “Zero Voltage Soft Commutation PWM DC-DC Converter with Saturable Reactor Switch-Cascaded Diode Rectifier,” IEEE Transaction on Circuits System 1998, vol. 45, issue 4, pp.348-354. [7]. Redl R., Balogh L., Edwards D. W., “Optimum ZVS Full-Bridge DC/DC Convener with PWM Phase-Shift Control: Analysis, Design Considerations, and Experimental Results,” Proceedings of IEEE APEC conference 1994, pp.159 – 165. [8]. Valtchev S., Barges B.V., “1KW/250 kHz Full Bridge Zero Voltage Switched Phase Shift DC-DC Converter with Improved Efficiency,” Proceedings of INTELEC conference 1995, pp.803-807. [9]. Koo G., Moon G., and Youn M.Y., “New zero-voltage switching phase-shift full-bridge converter with low conduction losses”, IEEE Transaction of Industrial Electronics 2005, vol. 52, pp. 228–235. [10]. A. K. Rathore, A. K. S. Bhat and R. Oruganti, “A comparison of soft switched dc-dc converters for fuel cell to utility interface application,” IEEJ Transactions on Industry Applications, vol. 128, no. 4, pp. 450-458, 2008. [11]. A. K. Rathore, A. K. S. Bhat and R. Oruganti “Small signal modeling and closed loop control design of ZVS current-fed half-bridge L-L type dc/dc converter with active-clamp," International Journal on Computer and Electrical Engineering, vol. 2, no. 6, Dec 2010, pp. 951-959. [12]. Wu X., Zhang J., Xie X., and Qian Z., “Analysis and optimal design considerations for an improved full bridge ZVS dc-dc converter with high efficiency”, IEEE Transactions on Power Electronics 2006, vol. 21, issue 5, pp.1225-1234. [13]. Song T., Wang H., Chung H., Tapuhi S., and Ioinovici A., “ A high-voltage ZVZCS dc–dc converter with low voltage stress”, IEEE Transactions on Power Electronics 2008, vol. 23, issue 6, pp. 2630–2647. [14]. Joseph X.F., Islam N., Kumar S.P., Dominic D.A. and Prabha D.M.M. S.R., “Design and Simulation of a PI controlled Soft Switched Front end Converter for Switched Reluctance Motor,” International Journal of Computer Applications 2011, vol. 34, issue 10. [15]. S. Sunisith Lizi Joseph and M. Saritha, “Comparison of Fuzzy PID Controller with Conventional PID Controller in Controlling the Speed of a Brushless DC Motor”, International Electrical Engineering Journal (IEEJ),VOL_5 NO_12, 25 JAN, 2015 [5]. (a) (b) Fig. 8(a) & (b) Vds and Gate pulse at 5% of load & at 100% of load TABLE I. PERFORMANCE COMPARISON BETWEEN FUZZY AND PI CONTROLLER S. No. Peak overshoot Settling time (Sec) % Steady state error PI controller 0- 3.6 0- 0.08 0 - 0.3 Fuzzy Controller 1.1 - 1.2 0.02 0.02 V. CONCLUSION This paper has presented two different controllers PI and fuzzy controller to improve the dynamic response of the DC/DC boost converter against the load variation. It has been observed that the system is stable from 5% of load to full load. The system shows excellent performance when DC motor load is applied. The system is also verified on frequent change of load. The simulation result shows that the performance of converter with fuzzy controller ensures the tight regulation of output voltage effectively under the load uncertainty. The efficiency of the converter at different loading conditions is also measured and improvement in efficiency is found. . REFERENCES [1]. A. K. Rathore, A. K. S. Bhat and R. Oruganti “Analysis, design and experimental results of wide range ZVS active-clamped L-L type current-fed dc-dc converter for fuel cell to utility interface application,” IEEE Transactions on Industrial Electronics, vol 59, issue 1, Jan 2012, pp 473-485. [2]. A. K. Rathore, "Interleaved soft-switched active-clamped L-L type current-fed half-bridge dc-dc converter," International Journal on Hydrogen Energy(Elsevier), vol. 34, issue 24, Dec 2009, pp. 9802-9815. Hua G.and Lee F. C., “Soft-switching techniques in PWM converters,” IEEE Transaction of Industrial Electronics 1995; vol. 42, issue 6, pp. 595 – 603. [4]. Watson R.and Lee F. C.,”A soft-switched, full-bridge boost converter employing an active- clamp circuit”, Proceedings of IEEE PESC conference 1996 pp. 1948-1954. [3]. 1827 Sudha and Lalit A Novel Phase Shifted DC-DC Converter with Adaptive Soft Switching to Improve Efficiency under wide Load Range