Design and Implementation of Nonisolated High Step-Up DC-DC Converter

Department of Electrical Engineering, Sahand University of Technology, Tabriz, Iran Faculty of Electrical and Computer Engineering, University of Tabriz, 29th Bahman Blvd, Tabriz, Iran Faculty of Electrical and Computer Engineering, University of Hormozgan, Bandar-e-Abbas, Iran Electrical Engineering Department, Seraj Higher Education Institute, Tabriz, Iran R&D Department, SmartD Technologies, Montreal, Canada


Introduction
Recently, the usage of DC-DC converters has risen in electric vehicle (EV) applications, renewable energy, LED drivers, and uninterruptible power supplies system (UPS) [1,2]. Te use of high voltage gain and high-efciency DC-DC converters is recommended to increase voltage levels and efciency in renewable energy sources [3]. When designing these converters, it is essential to consider factors such as efciency, voltage gain, input current ripple, and converter volume. Conventional boost converters are not ideal for high voltage gain due to losses caused by various components [4][5][6]. Increasing the duty ratio can also lead to issues with the output diode. Tis can limit the switching frequency and system size while reducing efciency and causing electromagnetic interference (EMI). To address these issues, nonisolated high voltage gain DC-DC converters have been developed through research [7][8][9][10].
In isolated converters, extensive and costly transformers are used [11][12][13]. So, isolated converters have a higher implementation cost than nonisolated converters [14]. Because numerous applications do not need electrical isolation in a nonisolated DC-DC structure, the usage of coupled inductors equips a valuable voltage-lift technique in these topologies to achieve high step-up voltage gain [15]. Also, by adopting a coupled inductor, two coils can be implemented through one core [16]. Hence, the volume of the structure can be diminished. In [17], a closer investigation of the isolated converter has been presented. Converters employing transformers are regarded as diferent categories of the dual active bridge (DAB). Te main disadvantages of these converters are high dv/dt stresses on AC link insulation, high conduction losses, a higher number of components, an imbalance in the DC-link capacitor's voltage, and complicated voltage balancing control.
Recently, many studies have been presented in the nonisolated DC-DC converters' area. In [18], an interleaved high step-up topology is introduced for renewable energy systems using the voltage multiplier. Tis topology has a continuous input current. Nevertheless, the number of components that are used is high. Te introduced converters in [19][20][21] use a voltage multiplier circuit and coupled inductor in their structures. Tese converters sufer from a high component count and input current ripple. Another nonisolated structure is reported in [22], which has bidirectional and soft switching features. Besides, this converter utilizes a coupled inductor to attain high voltage gain. But, the number of employed power switches is high, which has a negative efect on efciency.
In [23], several structures are proposed by using one coupled inductor and two power switches. However, they have similar characteristics, such as soft switching, and provide more choices for practical application. In [24], a coupled inductor-based boost DC-DC converter is rendered that is adequate for distributed generation applications. Tis converter benefts from soft switching and has high efciency. However, the number of power utilized is high, and the voltage stress over the output diode increases by turns ratio enhancement. In [25], a nonisolated interleaved structure assisted with a coupled inductor is introduced. Tis structure has a higher number of elements in comparison to other analogous topologies. A coupled inductor-based bidirectional converter is reported in [26], which has high efciency. Nevertheless, the voltage gain of this converter is lower than other related topologies. Some nonisolated structures are reported in [27,28]. Although these kinds of topologies provide high voltage gain, they experience some problems, such as low efciency, hard switching, and a high number of used components. An interleaved transformerless high step-up DC-DC converter without auxiliary switches and low input current ripple is presented in [29] for PV-based power generation systems. Tis topology benefts from the soft switching of semiconductors, where all power switches and didoes operate under soft-switching conditions. Terefore, higher power efciency is obtained. In [30], a series LC switch capacitor multistage high voltage gain boost converter is proposed for electric vehicle (EV) usages. Te main advantages of this converter are the low number of components and the VMC stages are extendable. Thus, the voltage gain can be adjusted, resulting in reduced voltage and current stress of semiconductors. In [31], a coupled inductor-based interleaved soft-switched high step-up converter is suggested for renewable energy applications. Te voltage lift capacitors and coupled inductors are used to increase the output voltage without further increasing the duty cycle of the power switches. Due to the interleaved structure, a low ripple input current is obtained. Also, the voltage stress over power switches is reduced.
Tis article introduced a new structure of a high step-up DC-DC converter with soft switching capability on the output diode. Comprehensively, the proposed converter ofers several advantages, including a high voltage conversion ratio, a simple and compact structure, easy controllability, a low component count, and high efciency.
In this converter, a two-winding coupled inductor is employed to enhance gain of the voltage. Te recommended structure has just one low ON-state resistance (R DS-ON ) power switch that reduces the conduction power losses. Also, a noticeable beneft of this suggested topology is the ZVS and ZCS of diodes that can increase efciency. Te rest of the article presents the principle of the modes of operation, analysis of the steady-state and efciency, design procedure, and comparison between the proposed and other similar confgurations. Consequently, an experimental prototype with 12 V/150 V and a 200 W power level under a 50 kHz switching frequency is developed to verify the proposed topology's analysis and truthfulness. Te structure of this document is as follows: Section 2 presents the proposed DC-DC converter and its operating modes. Te investigation of the suggested structure in steady-state is discussed in Section 3, while Section 4 explains the efciency interpretation. Design considerations are outlined in Section 5, and comparisons with other topologies are made in Section 6. Te experimental results are presented in Section 7, followed by the conclusions in Section 8. Finally, an appendix is included in Section 9.

Proposed DC-DC Converter and Its Operating Modes
Te proposed high step-up DC-DC converter's power circuit and the primary time waveforms of this structure are demonstrated in Figures 1 and 2, respectively. As demonstrated in Figure 1, the components of the presented topology are one coupled inductor, one power switch (S), four diodes (D 1 , D 2 , D 3 , and D o ), and four capacitors (C 1 , C 2 , C 3 , and C o ). Capacitors C 2 and C 3 act as passive clamps and diminish the voltage stress over the power switch. Based on the following considerations, it can simplify the steady-state and the operation principle investigations of the proposed structure.
(1) All components of the proposed converter are ideal (2) Due to the large value of the capacitors, the ripple of the capacitor's voltage can be ignored throughout the period (T s ) (3) Te term of the transitory modes is shorter than the main operation modes' duration (4) Te coupled inductor is assumed as an ideal transformer with a turns ratio equal to N � n 2 /n 1 , leakage (L k ), and magnetizing (L m ) inductances In the rest of this section, the operation principle of the proposed high step-up topology is analyzed. According to  International Transactions on Electrical Energy Systems 2.1. Mode 1 (t 0 < t < t 1 ). In this transient mode, power switch S is turned ON. Diodes D 1 , D 2 , and D 3 are reverse biased, and diode D o is conducted. Also, capacitors C 1 , C 2 , and C 3 are discharged. One can state that the magnetic current (I L m ) begins to charge linearly by utilizing the energy stored in capacitor C 2 . Te equivalent circuit is presented in Figure 3(a). Equations (1)-(4) in this mode are . During this time interval, diode D 1 is turned On at ZVS state. Diode D o and power switch S are still turned ON. Because of the forward bias of D 1 , the voltage of capacitor C 1 is identical to the secondary side of the coupled inductor voltage. In this mode, the currents of L m and L k increase linearly. Capacitor C 1 is charged, and C 2 and C 3 are discharged. Te equivalent circuit of this mode is displayed in Figure 3(b). Based on this fgure, the energy of capacitor C 2 is moved to the load by diode D o . Te equations of this interval are

Mode 3 (t 2 < t < t 3 ).
In mode 3, power switch S and diode D 1 are still turned ON. Furthermore, diodes D 2 and D 3 are turned OFF. When t is equal to t 2 , at the state of ZCS, diode D o is turned OFF. Tus, its reverse recovery loss is alleviated. Te passing current of capacitors C 2 and C 3 are zero, and C o is discharged to the load. Magnetic current (I L m ) reaches to the maximum point. Te equivalent circuit of this status is shown in Figure 3(c). Moreover, the equations of this stage are achieved by

Mode 4 (t 3 < t < t 4 )
. When t is equal to t 3 , switch S and diode D 1 are turned OFF. Te voltage of capacitors C 2 and C 3 are equal. Moreover, diodes D 2 and D 3 are forward-biased. International Transactions on Electrical Energy Systems

International Transactions on Electrical Energy Systems
Tese capacitors are charged through D 2 and D 3 , and their currents are equal to half of the input current. Furthermore, the currents L m and L k decrease in this condition. Te equivalent circuit is depicted in Figure 3(d). Te equations of this mode are

Investigation of the Suggested Structure in Steady State
Tis section presents the voltage gain, voltage stresses of the power switches and diodes, and current equations for the proposed converter based on the assumptions made in the previous section.

Voltage Gain.
In accordance with the principle of voltsecond balance upon magnetizing inductor L m , equations (15) and (16) can be written in the step-up mode: In equation (6), the voltage of the capacitors C 2 and C 3 is Finally, the output voltage is twice the voltage of ca- Ignoring the leakage inductance L k efect on the output voltage (for k � 1), the voltage gain equation is Figure 4 displays the voltage gain variations versus the turns ratio of the coupled inductor and duty cycle. It is evident that by raising D and N, the voltage gain is enhanced. Figure 3(d), the related equation in mode 4 is

Voltage Stress. As displayed in
Tus, the voltage stress all over the power switch can be written as Te following equation is written in mode 4 Consequently, the voltage stress of the diode D 1 is obtained as In equations (19) and (23), V D1 can be rewritten as a function of the output voltage Furthermore, the voltage stress throughout the didoes D 2 , D 3 , and D o are archived as follows Te voltage stress on power switch S and didoes D 2 , D 3 , and D o are the same. Figure 5 depicts the variations of the normalized voltage stresses against D and N. According to Figure 5, the voltage stress across semiconductors is constant with duty cycle variations.

Calculation of the Current.
In this part, the peaks, root mean square (RMS), and average values of the currents are determined. Te peak currents of the semiconductors are calculated as follows: It is evident that the peak currents of diodes D 2 and D 3 are equal. Also, the peak current of diode D 1 is the maximum peak current of the proposed structure. Te average currents of the used semiconductor elements are given by International Transactions on Electrical Energy Systems According to (30), the average currents are equal for diodes D 2 , D 3 , and D o . Te RMS currents are essential for efciency analysis. By (32)-(35), the RMS currents of the power switch and the diodes obtain the following: Te capacitors and coupled inductor RMS currents are (36)

Efficiency Interpretation
Considering the parasitic resistances below, the efciency of this proposed converter can be easily specifed.
(1) R DS-ON : the resistance of the MOSFET in a turnon state (2) R( Ln1,Ln2 ): the equivalent series resistor (ESR) of the winding coupled inductors (3) R C : the ESR value of the C 1 , C 2 , C 3 , and C o (4) R D : the diodes' resistance in forwarding state Te efciency of the recommended structure can be calculated by using (37) and (38) ∆P � P S + P D + P CL + P C .
In (38), P S indicates the power loss of the switch S, which can be given by where P Cond is the conduction losses, and P Switching is the switching losses of the switch S  (40) P D shows the losses of the diodes: where P cond_D and P Forward_Volatge are the conduction and forward voltage losses of the used diodes, in order Besides, P C shows the power loss of all capacitors, which is

Design Considerations
Tis section discusses the design of diferent circuit components. Te value of the magnetizing inductor is important in determining the characteristics of the coupled inductor. Te turns ratio of the coupled inductor is also crucial as it afects switch voltage stress and duty cycle. Capacitor values are determined based on voltage ripple magnitudes. Finally, the dynamic performance subsection demonstrates the recommended structure's frequency response and control loop.

Magnetizing Inductance L m . L m can be written as
In order to achieve CCM state, the magnetizing inductance current ripple can be presumed as Terefore, the value of L m can be written as a function of the input voltage, average of output current, duty cycle, and switching frequency.

Turns Ratio (N).
Using (19), the turns ratio can be derived as follows: According to (48), the value of D infuences N.

Capacitors. Te value of all used capacitance is
that is, the voltage ripple of the capacitors are considered as follows: Using (50), it can be mentioned that the capacitor's voltage ripple is negligible. Tus, the values of the used capacitors are

Dynamic Performance.
Te dynamic performance of the proposed structure is studied using the state-space average method. Te system equations are provided. Te following presumptions are investigated in order to have a state equation, (1) All components are considered ideal (2) Te input current is continuous Te dynamic analysis of the proposed converter based on the open loop transfer function has been performed by using the Bode diagram. Te frequency response of the suggested structure, which is named G vd (control-to-output) can be obtained as follows: .01 × 10 7 s 4 − 8.9 × 10 − 6 s 3 + 2.14 × 10 17 s 2 − 314000s + 6.22 × 10 24 s 6 + 99.12s 5 + 8.88 × 10 8 s 4 + 9.01 × 10 10 s 3 + 3.34 × 10 16 s 2 + 5.66 × 10 20 s + 7.21 × 10 22 .

(52)
Te equation (52) is obtained by considering the experimental result value and element values. Te Bode diagram, which is defned as a control to output transfer function (the magnitude dB and phase frequency response) in the Laplace domain, has been depicted in Figure 6(a). In order to control the output voltage, the closed loop scheme International Transactions on Electrical Energy Systems with a PI controller is suggested, which is indicated in Figure 6(b). As seen in Figure 6

Comparison with Other Topologies
In this part, a comparative study between the proposed coupled inductor-based high step-up DC-DC converter and other converters are provided. In this approach, voltage gain, number of utilized elements, voltage stress all over the diodes and power switches, common ground, input current ripple, continuous input current, and efciency are regarded. Te comparison is presented in Table 1.
According to this Table, the voltage gain of the proposed converter is equal to 13 in D � 0.6 and N � 2, which is higher than the other introduced converters. Terefore, the proposed structure has a smaller duty cycle, and the coupled inductor turns ratio for the same voltage gain. Figure 7 shows the voltage gain comparison among the presented topologies in Table 1.
Te voltage stress of the power switch of the proposed structure is equal to 0.5 for any range of D and N, which is higher than the converters in [1,3,6,10,[12][13][14] and [16]. However, all of these converters have higher voltage stress across the diodes in comparison to the proposed structure.
Accordingly, the proposed structure has the advantages of high voltage gain, lower voltage stress on the semiconductors, and high efciency with sufcient elements over the previous topology in the feld of coupled inductor-based DC-DC. Tus, it has to be noticed that the presented high step-up converter can be employed in high voltage applications.
Te proposed high step-up and high-efciency DC-DC converter can be used in some of the abovementioned applications, such as renewable energy systems like photovoltaic (PV) and lighting systems. Te presented converter can boost the low voltage of the battery (∼12 V) to the high voltage level needed in automotive lighting devices that drive high intensity discharge (HID) lamps and LED foodlights.
Also, standalone PV-based street lighting systems are another LED technology in which the suggested DC-DC converter can be used to increase the PV panel's low voltage (<20 V) to the needed high voltage level for lighting.

Experimental Results
Tis section evaluates and verifes the operation principle, the steady state, and the efciency analysis of the proposed structure. Te experimental prototype has experimented with 200 W, 12 V input to 150 V output voltage, and 50 kHz switching frequency.
In the Appendix section, the list of the prototype circuit components and the specifcation of the experimental prototype of the presented topology are given in Tables 2 and  3, respectively. As illustrated in Figure 8, the input, output, and capacitors' (C 1 , C 2 , and C 3 ) voltages are measured. In this fgure, mathematical analysis related to the voltage of the capacitors is verifed. According to Figure 8(a), the voltage of capacitor C 1 is almost 19 V, which verifes equation (6). Te voltages of C 2 and C 3 are illustrated in Figures 8(b) and 8(c), which are equal to 75 V. Terefore, these results confrm the accuracy of equation (17). Te input and output voltages are shown in Figure 8(d). Because of the turns ratio of the coupled inductor, the high output voltage of 150 V is obtained for the low value of the duty cycle (D � 0.6). Figure 9 illustrates the voltage across the power switch S, which is about 77 V and confrms the equation (20). Tis value is much less than the converter's output voltage. Consequently, in the presented structure, a power switch with a low R DS-ON can be used, which decreases the conduction loss and improves the efciency of the converter. Te voltage and current measurements of diodes D 1 , D 2 , D 3 , and D o are depicted in Figures 10(a)-10(d). According to Figure 10(a), the value of the voltage of diode D 1 is almost 51 V, which verifes equation (24). Besides, diode D 1 is turned ON at the ZVS state. Te voltage measurement of diode D 2 is shown in Figure 10(b), which is about 77 V.   Voltage Gain [10] [13] Proposed [1] [3]= [6]= [12]=[14= [16] [5]= [7]       A comparison between the calculated and measured voltages at 200 W output power is presented in Table 4.
Te efciency curve of the presented nonisolated high step-up DC-DC converter is indicated in Figure 11. From this fgure, the maximum efciency of the proposed topology occurs at the rated output power (200 W), which is equal to 96.98%. Also, the proposed converter efciency is high using a low ON-state resistance power switch.
Technical investigation and experimental analysis outcomes verify that the recommended converter is an appropriate option for renewable energy applications like PV systems due to the high voltage gain. Also, because of the low peak voltage through the elements, the less number of the power switch and diodes, and the high output voltage, the proposed topology can be utilized for diferent power rates, Figure 12. Te implemented prototype of the proposed structure has been denoted. Output power (W) Figure 11: Te efciency curve of the proposed converter. Furthermore, the proposed converter is connected to a PV array, including three series 60 W PV panels that can produce 180 W in ideal irradiation (G � 1000 W/m 2 ). Also, between the PV array and the proposed converter, a lowpass flter (LC) is added to reduce the input current ripple. MPPT result is depicted in Figure 13. Using the P&O MPPT algorithm, the maximum power point of the PV array is tracked for two diferent irradiations. Before t � 0.1 sec, the irradiation of PV panels is 1000 W/m 2 , and the available MPP is 180 W. Te tracked MPP using the proposed converter is obtained at 174.29 W. After t � 0.1 sec, the irradiation of PV panels became 500 W/m 2 , and the available MPP is 87 W. In this condition, the tracked MPP using the proposed converter is obtained at 82.01 W.

. Conclusion
Tis article proposed a novel high step-up DC-DC converter based on the coupled inductor technique. Te proposed structure advantages can be classifed as (1) high voltage conversion ratio with high efciency, (2) simple structure, (3) lower peak voltage across the semiconductor elements, and (4) less number of components. Tis converter employs a two-winding coupled inductor to enhance the output voltage. Tis topology has just one power switch with lower ON-state resistance, reducing the switch losses. Besides, the zero-voltage switching (ZVS) and zero-current switching (ZCS) of diodes are the other advantages of the proposed structure, which can increase the overall efciency. Te principle of the operation modes analysis of the steady-state and efciency, design procedure, and comparison with other introduced similar structures are presented to verify the performance of the proposed topology. Te match between the presented experimental results and analytical analysis verifes the performance and viability of the proposed converter and its applicability for various applications. International Transactions on Electrical Energy Systems 13