Research Article Modified High Gain DC-DC Converter with APICs for Microgrid

This paper proposes a nonisolated high voltage gain DC-DC converter. The active–passive inductor cells (APICs) makes the converter topology expandable, and it also helps in providing a high gain quality at lower duty cycles. The proposed converter is suitable for high gain applications mainly necessitating microgrid as well as integration of renewable energy sources into the existing grid infrastructure. The merits of the converter can be attributed to the ability to achieve high gain and low current stress and switching losses on power electronic switches. The proposed converter is analyzed in both continuous and discontinuous modes of operation, the efficiency is calculated and simulation results substantiate the operation of the converter.


Introduction
tIntegration of renewable energy sources into the grid infrastructure and expanding focus on microgrids and their economic as well as environmental signi cance have compelled a demand for new topologies in converters for high gain applications. is is mainly caused by the low-output voltage derived from these sources, which have always stood as the foremost limitation in their utilization. An extendable converter using APICs is presented [1]. e DC-DC converter introduced is based on the technique that uses three states of switching [2]. e converter is analyzed theoretically and is also veri ed experimentally through a laboratory prototype. Several other topologies are reviewed to understand their limitations and also to prove the advantages of the proposed topology. e conventional boost converters do not have the ability to provide high gains owing to the design structure with power switches, diodes, the presence of inductors, and capacitors [3]. e proposed structure is based on parallel charging of inductors during switch ON period and series discharging during switch OFF period. e converter provides high step-up voltage within a shorter range of duty ratio. e converter is based on active switched inductors and passive switched capacitors. Apart from the ability to achieve high gain the topology also achieves high e ciency and reduces voltage stress along with the number of elements in the circuit [4]. e family of transformerless active switched inductor and switched capacitor Cuk converter with high gain ability provides low voltage and current stress. Semiconductor devices with low voltage ratings are used to curtail the conduction losses [5]. Applications like grid-connected PV systems require high step-up voltage. e employment of traditional boost converters in such scenarios are limited. e paper introduces hybrid switched inductor converters to achieve high gain conversion [6].
A hybrid transformerless converter designed with two inductor boost converter consisting of a voltage multiplier and switched capacitor cells is proposed by Andrade et al. [7]. Improved voltage gain and efficiency, low current and voltage stress, and the ease of operation are some of the key features of the structure. In the two inductor high gain converter topology the difference between two inductor current can be compensated by increasing the inductor current of the corresponding path. is is obtained through increasing the turn-on time. e suggested method has no effect on voltage transfer gain of the converter [8]. Switching structures designed with two capacitors and two-three diodes or two inductors and two-three diodes are described.
ese can be classified into ldquostep-downrdquo or ldquostep-up.rdquo. ese structures when combined with converters such as buck, boost, buck-boost, Cuk, zeta, and sepic converters give a step up function. ese converters can increase or decrease the gain more than the classical converters [9].
High gain DC-DC converter integrates voltage RE-Lift, and Super RE-Lift Luo converters have provided sufficient solar energy to the grids. High voltage gain and high power efficiency are benefits of the topology alongside high efficiency. e output voltage is increased in geometric progression. e voltage gain produced by most of the converter structures is narrowed down by the losses [10]. is possesses a limitation in the integration of renewable energy sources using these converters. Melo de Andrade et al. targets series connection of DC-DC converters to prepare an assessment based on Cuk and SEPIC converter topologies [11]. Das and Agarwal studied an expandable high gain converter for microgrid applications. High gain quality is obtained at lower duty cycles themselves. High efficiency, low stress on switches, and minimum output voltage ripple (MOVR) are other benefits [12].
A new series DC-DC converters "XL converters" are suggested by Sayed et al. e main application is renewable energy conversion. e topology is implemented using two buck-boost converters combined with different switched reactive networks [13]. is converter structure offers the feature of high negative voltage at moderate duty ratios. It does not include a transformer and has only a single power control switch. Bidirectional DC-DC converters play a vital role in applications involving increased voltage gain ratios. Asha et al. presented a nonextendable bidirectional buckboost converter.
is topology does not require coupled inductors [14]. e topology has stages of operation and the voltage gain can be derived for n times. Kanimozhi et al. proposed a boost converter based on a new coupled inductor. e structure can be realized for battery charging in photovoltaic systems fed PMDC motor. e closed-loop control is also achieved and maximum possible energy from the PV panels is extorted through the method of single-stage conversion [15].
Improving the output voltage level obtained from various distributed energy sources is very crucial to determine a common voltage throughout the DC link of a particular microgrid. Mahajan et al. intended to eliminate the limitation of the conventional boost converters for this function.
e recommended topology provides a quadratic voltage gain. e converter has been validated through comparison with existing topologies and also through loss analysis [16]. e converter proposed by Hossein Hosseini et al. is appropriate for applications involving hybrid electric vehicles. e exceptional aspects include reduction in voltage stress, switching, and conduction losses [17]. e method adopted is that of bipolar switching, to benefit from decreased losses. e converter discussed in Sundaram et al. is nonisolated and maintains a high gain [18]. Low input current ripple makes it a good choice for realizing MPPT in photovoltaic systems. Further assessment of the converter is aided with steady-state analysis, which also justifies the efficacy of the converter.
A DC-DC converter with high gain and without isolation is established on dual coupled inductors by Ahmad et al. achieves high gain via the dual coupled inductors. e topology is consistent with applications involving distributed energy resources. A regenerative snubber circuit is combined to recycle the energy leaked.
is converter is promising in transformerless grid-connected PV systems [19]. A magnetically coupled single switch converter implemented by Kanimozhi et al. [20] attains a high voltage ratios on the basis of the inverse operating principle. e applications of the converter are renewable-based energy systems. To prevent core saturation a regenerative circuit is added to the topology [21] that examines a bidirectional DC-DC converter. It consists of coupled inductor and a voltage doubler cell. Extended output voltage is received through a dual active half-bridge circuit. ZVS is utilized for lowering the switching losses. e losses in the semiconductor devices are calculated to compute the losses in the converter. e conduction losses are calculated by multiplying the voltage and current values across individual devices, measured through voltage and current measurement units from the simulation of the converter.
Forouzesh et al. proposed a modular switched capacitor converter topology. e topology is modified from the original Dickson converter. To achieve dynamic variation in gain, two different methods are carried out [22]. e prototype and the equivalent circuits are analyzed with experimental results. e converter design is convenient for green energy-related applications. e structure has been analyzed and the results are presented. Furthermore, the power conversion efficiency has a scope of improvement [23]. A large gain step-up converter is achieved with an asymmetric voltage multiplier network. e converter is designed to mitigate the problem of reverse recovery in the diode of the leakage inductor [24].

Proposed Converter
e topology of the proposed converter is depicted in Figure 1. For the analysis, all the inductors are given the same inductance values. e converter is analyzed in both continuous conduction mode (CCM) and discontinuous conduction mode (DCM) of operation. e circuit diagram of the converter for n � 1 is given in Figure 2. e theoretical waveforms for CCM and DCM operation are given in Figure 3. Figure 4 represents the operational modes of the structure and the corresponding circuit diagrams. (1) e inductor current is as follows:

Analysis of the Converter
In this interval, the inductors get charged therefore the inductor current increases and reaches its maximum value at t DT. Applying t DT in equation (2) the maximum inductor current can be derived as follows: e capacitor is discharged by the interval end, and the capacitor voltage decreases to V CV .

Time Interval of T OFF .
All the switches are OFF. e voltage across the inductor can be given as follows: n is the number of APICs. For this interval the current across the inductors is as follows: At the end of this interval, the inductor current will be equal to I LV , since the current decreases as a result of the inductor discharging. Applying t (1 − D)T in equation (4), the minimum inductor current is as follows: As the current across inductors and the capacitor decreases at this interval, inductor provides the load current along with charging the capacitor. (1) and (3), the voltage gain can be derived as follows:

Voltage Gain Calculation. Considering equations
e converter is designed considering minimum output voltage ripple. Least OVR is obtained in CCM operation. e value of the inductor is calculated as given in Table 1. e voltage gain as well as the output ripple of the converter is independent of the inductance value.

Voltage Stress and Current Stress.
e voltage stress across the switches and the diodes depends upon the voltage gain and the number of APICs and can be formulated as a function of the same. Since the ripple factor of the converter is considerably low (∼2%), the voltage and current stress of the converter are reduced.   At this time interval, the analysis is the same as that of CCM, therefore the maximum current through the inductor is given as follows:

Time Interval of T OFF .
At this time interval, the capacitor current equals the load current. e capacitor voltage decreases as the capacitor discharges. e maximum inductor current is as follows: 3.3. Critical Inductance. e minimum inductor current is given as follows: Applying I LV � 0 (10), the critical inductance between CCM and DCM can be obtained as follows:

Design Considerations
e converter components are designed to obtain suitable operation with less output voltage ripple. e critical inductance is derived as a function of V i and R. For a particular value of inductance the converter can operate in either CCM or DCM mode, as given in Table 1. e output voltage of the converter is less in CCM operation than that of DCM [25]. e converter is specified for renewable energy integration with the grid in microgrids. In these cases the quality of the waveform is crucial. erefore maintaining MOVR is a very important condition considering these aspects. e converter is suitable for such applications.

Simulation Results
e simulation results are discussed to validate the converter topology. e converter parameters are verified for n � 1. e converter parameters are consolidated in Table 2. e critical inductance value obtained is L C � 11.3 μH. e simulation is implemented in both modes of operation. e CCM analysis is performed for L � 20 μH and DCM analysis is performed for L � 5 μH. Figures 5 and 6 show the output waveforms attained for CCM. e output waveforms obtained for DCM are presented in Figures 7 and 8. It can be inferred from the waveforms that voltage stress is highest in the diode at the output side. An output voltage ripple (OVR) of 2% is produced. e MOVR does not depend upon inductance value.
For further analysis, the theoretical and simulation results are compared. e efficiency as well as the voltage gain of the topology are improved with the increase in n (number of APICs). e efficiency reduces with an increase in the number of passive elements. e capacitive filter of the converter is calculated based on minimum output voltage ripple. e ripple factor is considered to be 2% and the C value is calculated from the following equation: where C is the capacitance in farad, R is resistance, r is the ripple factor, and f is the switching frequency.

Comparison
A conventional boost converter and an extendable high gain converter is used for comparing the topology. e comparison among different parameters of the same is given in Table 3. e output voltage and the corresponding voltage gain increases with an increase in 'n' APICs. is converter is appropriate for application in microgrids for integrating renewable energy systems into the grid. Reasonably high values of voltage gain can be obtained at lower values of duty ratios. e number of inductors and passive elements in the topology is lesser than other extendable topologies. Considering the overall losses contributed by the semiconductor devices, major proportion (approximately 98%) is contributed by the total diodes used in the design. But with respect to the topology used for comparison, the losses are further reduced since the number of diodes in the design are less by a number of 5 and the inductors by a number of 2.
erefore the losses are reduced. Small inductance values are sufficient.
All the inductors in the topology are considered to be of the same value to make the analysis of the converter simpler. Voltage gain of the converter depends upon the number of     Mathematical Problems in Engineering inductors in the circuit, which is derived assuming same inductors. If the inductors of different inductance values are considered, then analysis of the converter will be different. But even in such cases the final result of the main parameters will be similar. e proposed converter efficiency all though higher than the topology used for comparison is less than that of the conventional boost converter. However the comparison of efficiency is not approved due to certain factors of consideration. Furthermore, the variation in voltage gain and the efficiency with respect to duty cycle that are plotted and compared for n � 1 and n � 2 are shown in Figure 9. From the plot of efficiency versus duty cycle it can be validated that for a typical value of duty cycle the efficiency of the converter is improved with the increase in n.

Conclusion
An extendable high gain DC-DC converter has been proposed for microgrid applications. Since the topology involves active-passive inductor cells the topology can be extended to provide improved voltage gain. e voltage stress on semiconductor devices is low, thereby reducing the losses. e inductors required are of small size. e proposed topology can achieve high gains at lower values of the duty cycle, which is difficult to accomplish from conventional boost converters. e topology is compared with conferred literature structures and the conventional boost converter. e converter elements are designed based on MOVR. e simulation results validate the merits of the proposed converter.

Data Availability
e data used to support the findings of this study are included in the article. However, the reader may contact the corresponding author for more details on the data.

Conflicts of Interest
e authors declare that there are no conflicts of interest regarding the publication of this paper.