Optimization of Impedance-Source Galvanically Isolated DC-DC Converters with Reduced Number of Switches

In this study, a new type of Z-source converter comprising galvanic isolation transformers is introduced based on classical non-isolated Z-source converters, which are promising devices for renewable applications. Compared to the typology of conventional converters with isolation transformers, the proposed converter topology has a simpler structure with fewer power switches and lower costs. Considering the lower number of switching devices required in the proposed typology, these efficiency and reliability of the presented converters are higher than those of the conventional ones. As only one switch is used in the developed structure, voltage stresses on the Z-source converter decreases in each period, which maintains voltage stress low and steady across the switch. Another desirable feature of the proposed topology is reduced converter size and volume with a maximum theoretical and practical efficiency of 97 to 94.05%, respectively. Moreover, these converters need low input voltage and provide the required amount of output voltage in small duty cycles. The theoretical and operational analysis of the designed converter and its comparison with other counterpart converters (e.g., isolated Z-source converters) is presented. The simulation and experimental results confirm the applicability of the proposed system.


I. INTRODUCTION
Because of the ever-growing demand for renewable energy resources, such as photovoltaic systems, wind turbines, and fuel cells, considerable attention has been directed to such resources. Therefore, the need for high voltage (HV) DC-DC power supplies is now highly tangible. In general, conventional DC-DC topologies are categorized into with and without transformer topologies. Although the technology of these typologies has been widely investigated and recognized, they have a fairly large size, impose high costs, and yield low efficiency owing to their single structure in complex applications [1], [2]. To eliminate these problems and boost their efficiency, DC-DC converters with HV gains should be utilized [3]; therefore, Z-source (ZS) networks have been considered.
The concept of ZS networks was first introduced by Peng in 2003 [4]. This network refers to a one-stage increment/decrement converter that has the potential to augment voltage through a capacitive inductive impedance network located between the source and the inverter. In this converter, no additional DC-DC solvents are required. In addition, as voltage sources are not allowed for use in this type of inverter, short-circuit intervals are placed in the legs of the inverter for voltage increment. Because short circuit intervals are permitted to be used in the legs of the converter as part of the power conversion process, this converter is not sensitive to noise switching. Furthermore, among the advantages of ZS networks are one-step conversion, low number of components, high efficiency, low energy loss, high reliability, and low costs [5]. Provided the characteristics of ZS networks, it has widely been simulated in converters with transformers in modern power systems, such as HVDC networks, smart networks, photovoltaic power plants, wind power plants, and electric vehicle batteries. In these simulations, transformers may be integrated into isolated DC-DC power converters [6]. Therefore, an appropriate power converter is required to connect DC microgrid systems with loads [7] so that a relatively high voltage can be generated using the DC-DC converter [8], as illustrated in Fig. 1.
By locating ZS networks in converters with isolation transformers, achieving higher performance is feasible in approximately all types of energy conversion, including AC-AC, DC-DC, AC-DC, and DC-AC. Among them, DC-AC energy conversion, and thus the topology of DC-AC converters has garnered a considerable attention. Moreover, ZS-based converters have several drawbacks, such as discontinuous input current, limited boost factor 1/(1-2D), where D represents the duty cycle in a short circuit [9]. In addition, D must be increased to achieve an HV gain; however, it causes HV tensions in network components [10]. Therefore, a variety of topologies for ZS-based DC-DC converters have been proposed using magnetic connection methods (e.g., methods that use transformers) to eliminate this problem, optimize voltage gain, and reduce voltage stress [11]. The current study primarily concentrates on ZS-based DC-DC converters with isolation transformers. These converters are capable of enhancing voltage gain by adjusting the turn ratio of transformers [12], and have higher reliability and safety compared to non-isolated converters [13]. However, the switches in these converters might undergo intense voltage pressures owing to the leakage inductance of transformers [14]. To overcome this problem, a resistive capacitive diode is required [15]; nonetheless, it boosts power loss and declines efficiency. Another solution is to utilize active-clamp circuits [16], which eliminates voltage spike without increasing power loss. However, another power switch is required, which imposes higher costs and enhances control complexity. Moreover, the use of high-frequency transformers with complicated configurations in isolated DC-DC converters increases the costs, size, and weight of the converters [14], [15], [17], [18].
Another less-recognized method to solve the problems associated with the use of converters with isolation transformers is to reduce the number of active and inactive components of these converters. Hence, the size, weight, and related costs decrease. It is also possible to highly benefit from the desirable characteristics of isolation transformers. Furthermore, unnecessary active elements, such as insulated gate bipolar transistor (IGBT) or metal-oxidesemiconductor field-effect transistor (MOSFET) with high power and frequency require the addition of heat-sink and drive circuits, which promotes the control complexity of these converters [19].
This study introduces an optimized structure for Z-source DC-DC converters based on galvanically isolation transformers with a reduced number of switches and diodes as an alternative to conventional isolated Z-source DC-DC converters. In the proposed topology, by reducing voltage stress on the elements of the Z-source network, a fewer number of constituents (e.g., switches and diodes) are needed, voltage tension on active elements decreases, and using double-winding isolation transformers becomes feasible. Therefore, the proposed converter exhibits a high efficiency concerning its expenses, size, and weight. The simulation analysis is conducted using the MATLAB software and several experiments are performed to evaluate the characteristics of the proposed converter.
Photovoltaic panels, fuel cells, and wind turbines on a small scale (20 to 150 V) should be adjusted by dc-dc converters in order for their low voltage to reach higher dc-link voltage (between 200 and 600 V) [20]. The proposed impedance-source isolated dc-dc converter with an output voltage of 206 V is located in the range of 200 to 600 V before being connected to the network of 110, 230, and 400 V AC [21], [22].Therefore, the proposed converter is a promising device for photovoltaic panels, fuel cells, and wind turbines.

II. CONVENTIONAL DC-DC CONVERTERS WITH ISOLATION TRANSFORMERS
Conventional DC-DC converters are mainly divided into nonisolated and isolated types, as illustrated in Fig. 2. Non-isolated typologies include Boost-Buck, Boost, Cuk, Zeta, and SEPIC types. Among isolated typologies are forward, flyback, push-pull, full-bridge, and half-bridge circuits. To connect renewable systems to the power grid with a low input voltage, one of the most promising options is to use DC-DC converters with isolation transformers and a DC bus at the high voltage side [23]- [26]. However, owing to their single, they have large sizes, impose elevated costs, and reduce efficiency structure in complex applications.

III. PROPOSED TOPOLOGY AND ITS OPERATING PRINCIPLES
Wang et al, presented isolated DC-DC converters based on Zsource networks for use in renewable energy systems [19]. Fig. 3 illustrates that the structure of these converters is comprised of four main parts: 1) z-source impedance network, 2) single-phase fullbridge inverter, 3) step-up isolation transformer, and 4) single-phase voltage doubler rectifier (VDR). These converters consist of many switches in their structure, which occupy a large system space, increase converter weight, and consequently boost costs. In addition, the switches undergo an intense pressure of HV because of the leakage inductance of transformers. Therefore, all switches exhibit significant energy losses, and thus the quality of output power decreases on the primary side of the transformer. Using many switches at high power and frequency also necessitates the addition of heat-sink and drive circuits, which increases the control complexity of these converters. Consequently, some measures have been offered in the present study to eliminate these problems.

FIGURE 3. Conventional isolated DC-DC converter based on Z-source.
The proposed Z-source converter consists of a unique impedance network and a galvanically isolation transformer, as depicted in Fig. 4. It contains a Z-source network comprising inductors (L1, L2), capacitors (C1, C2), and diodes (D1), a power switch (S), a galvanically isolation transformer (T), VDRs consisting of diodes D2 and D3, capacitors C3, C4, and Cf, and a blocking capacitor in series with the primary winding of the transformer. The blocking capacitor is required to isolate the DC component from the input voltage of the transformer so as to hinder its saturation.
The Z-source converter alters voltage by switching the power switch (S). To change voltage, the converter directs it toward the isolation transformer, and ultimately, the required DC voltage is obtained via VDR modifications. Moreover, D1 is used to prevent the discharge of the capacitor onto the DC source. Therefore, the presence of this diode in series with the DC source causes discontinuities in the source current.

A. OPERATING PRINCIPLES OF THE PROPOSED CIRCUIT
In terms of the structure of the proposed converter, if the impedance network is symmetrical, we can write: where V C and V L represent the average voltage value at both ends of capacitors and instantaneous voltage value on both sides of inductors, respectively.
Siwakoti et al. discussed that the impedance source converter has the shoot-through transfer and non-shoot-through transfer operating modes [10]. Fig. 5 indicates the equivalent circuits of the Z-source converter.

FIGURE 5. Equivalent circuits of the Z-source DC-DC converter in the (a) shoot-through transfer mode, and (b) non-shoot-through transfer mode.
Considering Equations (1) and (2) and Fig. 5(a), the following relationships are obtained: where vi and vpn represent the input and output voltages of impedance sources, respectively.
According to Equations (1) and (2) and Fig. 5(b), the relationships can be expressed as follows: In a stable system, the average voltage of the two ends of an inductor in one switching period (T sw = T sh + T nsh ) is equal to zero based on the principle of voltage balance, the following equation is formed: where v L.av represents the average voltage value at both ends of the inductor. To calculate the second integral in Equation (9), a new time origin is considered. By substituting the values of v L from Equations (4) and (7) into Equation (9), the result can be expressed as follows: Based on Equations (5) and (8), the average value of the DC-link voltage between the power switches (S) (i.e., v pn.av ) can be computed as Using Equation (11), the peak output voltage of the Zsource impudence can be expressed as follows: where, B is the boost factor induced by the shoot-through mode and is greater than or equal to 1 (B≥ 1), therefore: where D represents the shoot-through duty cycle, which satisfies 0≤D≤0.5. The shoot-through duty cycle is equal to From Equation (13), shoot-through duty cycle can be measured as follows: The converter performance highly depends on the voltage of the capacitor in the Z-source network. From Equations (2) and (13), the average voltage at both ends of the capacitor (VC) is obtained as follows: By substituting Equations (13) and (14) into Equation (15), the voltage of the capacitor in the Z-source network is computed as follows: As illustrated in Fig. 7, the boost factor can be controlled by straight lines as follows: where V Ca is the peak value of the triangular carrier signal. Therefore, as angles a 1 and a 2 are equal, D can be estimated as follows: By substituting Equation (19) into Equation (13), the boost factor can be calculated as follows: Based on Equations (19) and (20), if V Ca is constant and V P increases, D and B reduce. According to Equation (15), V C decreases under these conditions.
Capacitor voltage functions as a voltage stress on the capacitor and can be estimated using Equations (16) and (20) using. Therefore, V P cannot be less than approaches infinity, which indicates the HV stress on the capacitors of the impedance network Ellabban et al, proved that the DC voltage is a discontinuous unipolar pulse [27]. Hence, the DC voltage, as the input voltage on the primary side of the transformer, can alter the input voltage because n = N tp /N ts . In addition, the output voltage of VDR can gain the required DC voltage on the secondary side of the transformer: From Equations (11) and (22), we can write Therefore, the DC voltage gain is calculated as follows: If the capacitors and inductors of the impudence network are appropriately determined, the proposed converter can function effectively in a wide range of voltages and loads despite discontinuous input voltage [28]. Compared to conventional Z-source DC-DC converters, the proposed converter elevates output DC voltage as the number of active elements are reduced. Furthermore, the secondary winding of the isolation transformer should have two more rotations to maintain the DC voltage gain. Therefore, the introduced converter achieves higher efficiency, simpler operation, and lower cost than those of the conventional Z-source converters.
Operating principle of the proposed structure described in Fig. 6 contains two switch modes that occur during the switching period (T): First mode [t1‫؛‬ t2]: S is on and D1 is off. Capacitors (C1, C2) are discharged and inductors (L1, L2) start being charged. Therefore, D3 conducts the current, and D2 is reverse biased. In addition, C4 is charged, whereas C3 discharges load current.

B. CONTROL STRATEGY OF THE PULSE WIDTH MODULATION (PWM)
The performance of isolated Z-source converters can be controlled using various schemes of PWM, including simple boost, maximum boost, and constant boost controls [4], [5]In this study, the simple control boost scheme is employed as a control strategy. The simple boost control scheme for the proposed converter involves the comparison of a triangular wave signal with an adjustable DC reference, called V P . Consequently, duty cycle of the switching pulse can be changed to fabricate the required and desirable converter. Fig.  7 shows the control strategy of the proposed structure. These results show that despite the discontinuous input current, IL1 and IL2 operate in the continuous conduction mode (CCM).

IV. DESIGN OF IMPEDANCE NETWORK CAPACITORS AND INDUCTORS A. INDUCTOR
To determine the values of inductances, induction capacity should be relatively small. If induction capacity is large, power and system losses increase, which leads to the discontinuity of alternating current and increases the complexity of the control system. Furthermore, a moderately small induction capacity prevents inductor current ripple [14].
During the short circuit, the inductors are charged by the capacitors; thus, inductor current ripple can be determined as follows: From Equations (14), (16), and (25), the following equation is achieved: Therefore, ∆i L increases with an increasing T sw or V in ; however, it decreases with an increasing L. If B = 1 or D = 0, then ∆i L = 0. By substituting Equation (20) into Equation (26), inductor current ripple is calculated as follows: Equation (27) indicates that variations in V P have a significant effect on ∆i L . Therefore, if V P = V Ca 2 or D = 0.5, ∆i L approaches infinity.

B. CAPACITOR
During the short circuit mode, capacitors are discharged. Thus, capacitor voltage ripple can be computed as follows: where I L and I are the average inductor and load currents, respectively.
Considering equations (28) Table. 1 summarizes the main topologies of the galvanically isolated Z-source DC-DC converters. In addition to the converter introduced in [19], six other converters with improved structures are compared with the introduced converter in terms of the number of passive and semiconductive components and maximum duty cycle of switches. The advantages and disadvantages of each converter is listed in Table.1. Many studies have focused on increasing DC voltage in the boost mode and neglected reducing passive and active elements. Therefore, the proposed topology has not only high output DC voltage but also the least number of active components in comparison with the those of the other converters.

VI. SIMULATION AND EXPERIMENTAL RESULTS
Performance of the designed converter is evaluated through simulations in MATLAB/SIMULINK and testing the built prototype. The ideal methods of non-active components (inductors, capacitors, and an isolation transformer) are utilized to simplify the analysis. The simulation parameters of the proposed device and the specifications of the semiconductor components are presented in Tables. 2 and 3, respectively.

A. SIMULATION RESULTS
Conventional isolated Z-source DC-DC converters with four MOSFET switches, require a spacious system space, have high weights, and thus impose substantial costs. On the contrary, the proposed converter with only one MOSFET switch not only has the benefits of conventional isolated Z-source DC-DC converters but also decreases costs and required space and weight. Furthermore, reducing the number of switching devices enhances the efficiency and reliability of the converter [29].
The simulation results in Figs Fig. 10(b). Moreover, the highest voltage stress (reverse-bias voltage) of diode D1 is -107 V, as indicated in Fig. 10(c). Similarly, Figs. 10(d)-(e) depict the voltage waveforms of the transformer on the primary side (VTR, pr = 107 V) and secondary side (VTR, sec = 207 V). Compared to the structures of conventional isolated Zsource DC-DC converters displayed in Fig. 3 and improved isolated Z-source DC-DC converters presented in Table. 1, typology of the proposed isolated Z-source converter consists of only one switch, which increases its simplicity, and yet yields a high output voltage gain. Therefore, the presented converter can be utilized in various applications.

B. EXPERIMENTAL RESULTS
A 200 W prototype converter was assembled to perform experimental tests on the proposed topology. The prototype and its technical specifications are displayed in Fig. 4 and Table. 2, respectively. The experimental parameters match the simulated parameters presented in Subsection VI. A.
The Z-source network comprises a coupled inductor that is made of ferrite material N87 with a magnetizing inductance of 24 μH, and is wound on an EFD25 core. The isolation transformer is wound on the ETD34 core with a magnetizing and primary-side leakage inductances of 35.5 and 0.65 μH, respectively. Capacitors of the Z-source network are operated with electrolytic multilayered capacitors connected in parallel. Each capacitor of the Z-source network includes 3 EKZN101ELL151MJ25S capacitors operated at 100 V with a capacity of 150 μF. Moreover, two EGXF251ELL510MK25S capacitors with a capacity of 51 μF and voltage of 250 V are connected in series to form the VDR. All diodes are Silicon (Si)based or Silicon carbide (SiC)-based Schottky diodes. Si-and SiC-based Schottky diodes are placed on the input (low voltage) and the output (HV) sides, respectively. The types and electrical specifications of semiconductors are presented in Table. 3. The prototype is constructed using SPP20N60C3 MOSFET (600 V and 20.7 A). For the half-and full-bridge methods, two and four MOSFET switches are needed, respectively, whereas the proposed typology requires only one MOSFET switch.
The The experimental results slightly differ from the simulation results, which can primarily be attributed to the leakage inductance of the coupled inductors in the Z-source network, leakage inductance of the transformer, equivalent series resistance (ESR) of each component, voltage drop owing to the parasitic parameters of passive and active components (particularly VDR diodes in the proposed typology), parasitic resistance of inductance ( r L ), ESR of capacitors ( r C ), and equivalent resistance of the drain-source switch (r DS ).    Fig. 14 (a). The maximum theoretical efficiency of the proposed converter is 97% under 200% of the nominal load and is higher than 90% under 100% of the nominal load. Nonetheless, the maximum experimental efficiency is 94.05%. The efficiency is greater than 86.15% under 100% of the nominal load and decreases by 2.95% under 200% of the nominal load. This reduction can be ascribed to the leakage inductance of coupled inductors in the Z-source network, leakage inductance of the transformer, ESR, parasitic parameters of passive and active components, and diodes of VDR. Furthermore, Fig. 14(b) illustrates the voltage gain curves of the proposed converter with respect to the duty cycle. Fig. 14(b) illustrates that the theoretical and experimental voltage gain of the introduced converter is considerably high for duty cycles in the range of 0.3-0.4.

VII. CONCLUSION
The performance of the improved galvanically isolated Z-source DC-DC converter was compared with that of conventional isolated Zsource DC-DC converters. The advantages of the proposed converter are as follows: a) generates desirable output voltage just by altering the voltage ratio at the same duty cycles; b) maintains voltage stress down; thus, the number of required switches reduces, production cost and size of the converter decreases, and efficiency and reliability increase; c) has a simple controller; hence, no disturbing signals exist; and, d) although the implementation of VDR on the secondary side of the converter reduces the voltage drop, it promotes the efficiency of the rectifier and stabilizes the performance of the converter. Consequently, the proposed converter is a promising candidate for interfacing an input DC source with a nonharmonic DC bus or a DC microgrid owing to its excellent specifications. Theoretical results were verifies using the experimental tests. The measurements on the fabricated prototype proved that it can control a power of 200 W with an efficiency of 94.05%, and provide an output voltage of 200 V.