**1. Introduction**

Due to the rapid development of power electronic technology, the ESS dependent on batteries or ultracapacitors has been widely employed in various scenarios, such as photovoltaic and wind energy systems, smart grids, electric vehicles, urban rail transportation systems, etc. [1]. The renewable energy (RE) sources are frequently combined with other clean energy sources. The energy source is varied by using numerous inputs in order to boost dependability, and the use of RE sources have DC voltage outputs, and each source has its unique voltage and current features. MI converters are used to decrease the quantity of converters needed to transform the voltage [2,3]. However, MO converters are required to give the necessary power to loads operating at various voltages. A multi-input–multi-output converter (MIMO) is created by a cascading connection of single-input–multi-output (SIMO) and multi-input–single-output (MISO) converters [4]. The MIMO converter provides benefits over multiple individual converters, such as fewer circuit components and conversion phases that will lead toward a more compact design and a cheaper price. In some circumstances, it could be necessary to integrate various renewable sources with varying power levels, such as photovoltaic (PV) panels, fuel cells,

**Citation:** Aljafari, B.; Ramu, S.K.; Devarajan, G.; Vairavasundaram, I. Integration of Photovoltaic-Based Transformerless High Step-Up Dual-Output–Dual-Input Converter with Low Power Losses for Energy Storage Applications. *Energies* **2022**, *15*, 5559. https://doi.org/ 10.3390/en15155559

Academic Editor: Nicu Bizon

Received: 8 July 2022 Accepted: 28 July 2022 Published: 31 July 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

batteries, and ultracapacitors, in order to provide a single load [5]. Therefore, it is important to combine the various energy sources as efficiently as feasible. A MIMO can resolve this problem by combining the advantages of each source into a single converter to provide a controlled DC output and energy storage, or it can provide the AC load after passing through a DC–AC inverter, as shown in Figure 1. verters. The input cells are typically linked in parallel at the input stage of numerous single inductor MIMO converters, but the output cells may be linked in series or in parallel [8,9]. Due to the common inductor, there is a cross-regulation issue with the output cells that are linked in parallel. Implementing the appropriate control to decrease the reciprocal coupling among output cells coupled in parallel is inherently difficult [10].

MIMOs can be classified as isolated or non-isolated. The isolated-type MIMO uses a transformer with numerous secondary circuits to interact with numerous loads. In this instance, just one of the outputs is highly restricted; the other outputs are connected to additional subordinate windings. In non-isolated MIMOs, there is no electrolytic separation between various ports [7]. Additionally, they are further divided into single and numerous inductors depending on the quantity of the inductive loads. MIMO converters provide the benefits of a lower device count, better power density, and fewer power transmission phases as compared to the traditional DC–DC system design with separate con-

**Figure 1. Figure 1.** Block diagram of MIMO converter in energy storage. Block diagram of MIMO converter in energy storage.

*Energies* **2022**, *15*, 5559 2 of 20

a DC–AC inverter, as shown in Figure 1.

propriately when relating to the MISO converter.

sources with varying power levels, such as photovoltaic (PV) panels, fuel cells, batteries, and ultracapacitors, in order to provide a single load [5]. Therefore, it is important to combine the various energy sources as efficiently as feasible. A MIMO can resolve this problem by combining the advantages of each source into a single converter to provide a controlled DC output and energy storage, or it can provide the AC load after passing through

MIMO converter offers various merits, when compared to single-input–single-output (SISO), SIMO, and MISO converters. A MIMO converter enables customers to employ them in numerous sorts of loads by enabling input from several sources. These converters often assist in combining several sources that produce DC power directly. Due to environmental concerns and the need for stability, research emphasis has recently shifted to micro grids that combine various energy storage devices and renewable energy sources. MIMO converters are essential in such situations for integrating and merging these energy sources to serve the loads when compared to SISO and SIMO converters. The MISO converter is the most often suggested method by researchers to mix various energy sources at various voltage levels [6]. These converters have the drawback of having just one source able to power the load simultaneously. By splitting the current from sources, the MIMO guarantees a seamless transition of power across loads and finally divides the power ap-

MIMO converter offers various merits, when compared to single-input–single-output (SISO), SIMO, and MISO converters. A MIMO converter enables customers to employ them in numerous sorts of loads by enabling input from several sources. These converters often assist in combining several sources that produce DC power directly. Due to environmental concerns and the need for stability, research emphasis has recently shifted to micro grids that combine various energy storage devices and renewable energy sources. MIMO converters are essential in such situations for integrating and merging these energy sources to serve the loads when compared to SISO and SIMO converters. The MISO converter is the most often suggested method by researchers to mix various energy sources at various voltage levels [6]. These converters have the drawback of having just one source able to power the load simultaneously. By splitting the current from sources, the MIMO guarantees a seamless transition of power across loads and finally divides the power appropriately when relating to the MISO converter.

MIMOs can be classified as isolated or non-isolated. The isolated-type MIMO uses a transformer with numerous secondary circuits to interact with numerous loads. In this instance, just one of the outputs is highly restricted; the other outputs are connected to additional subordinate windings. In non-isolated MIMOs, there is no electrolytic separation between various ports [7]. Additionally, they are further divided into single and numerous inductors depending on the quantity of the inductive loads. MIMO converters provide the benefits of a lower device count, better power density, and fewer power transmission phases as compared to the traditional DC–DC system design with separate converters. The input cells are typically linked in parallel at the input stage of numerous single inductor MIMO converters, but the output cells may be linked in series or in parallel [8,9]. Due to the common inductor, there is a cross-regulation issue with the output cells that are linked in parallel. Implementing the appropriate control to decrease the reciprocal coupling among output cells coupled in parallel is inherently difficult [10].

The charge sharing method for creating numerous outputs is described in [11]. The authors suggested that even in the CCM, the outputs are load-dependent. A MIMO

converter's setup and its transactional and analytical challenges are addressed in [12]. The functionality of this converter family depends on a time-multiplexing technique [13]. There were a lot of cross-regulation issues, since the outputs of this specific type of MPC are all linked to the same switch node. The dynamic responsiveness of the converters discussed before is typically insufficient in terms of efficiency. A DC–DC converter with resonant MI and MO has been developed [14]. To ensure zero-voltage switching (ZVS) including all power switches during both turn-on and turn-off, the control system must adhere to a certain PWM technique. To address the cross-regulation issue, a model predictive control-based digital control system has been presented [15].

In [16], a unique construction for MIMO DC–DC boost converters is put forth, one that features a high switching frequency without a transformer, a constant current, a high voltage gain without a large duty cycle, and a high voltage gain. A unique nonisolated SEPIC-based multi-input structure is discussed in [17] to address the issues with intermittency of discontinuous power supply. The authors of [18] introduce a new doubleinput architecture built from a boost and Y-source DC–DC converter [19]. A deadbeat-based solution that incorporates input current and output voltage regulation has been suggested to enhance the performance of regulation [20]. These techniques demand some quite challenging computations. A limitless number of input devices cannot be integrated in a large portion of the previous research work because the time-multiplexing regulation cannot be ignored [21]. Furthermore, the shared inductor and the cross-regulation issue cannot be solved when the outputs are connected with one another. In some circumstances, this limitation exponentially increases the related control complexity. There are various uses, including solar power supply, energy storage, and Internet-of-Things (IoT) devices, despite the non-common ground caused by the series assembly type [22,23]. The design technique is rigorously followed to obtain the controller's optimal performance. In this research, a dsPIC30F4013 controller was used to execute the proposed converter.

A switched capacitor-based MIMO converter with a high voltage gain and an easy control scheme is given in [24]. But the number of components is very high. A boost converter using MIs and MOs has been proposed in [25], and it is appropriate for combining various energy sources in EVs. The inability to use two sources together is a drawback of this arrangement. In [26], a non-isolated MIMO converter with a high gain and no linked inductor was constructed. With this converter, the voltage increase ratio is greater. However, compared to the proposed converter, it has less voltage gain. The MI step-up converter is reported in [27] for integrating several energy sources. However, the voltage gain of this converter is less than the proposed converter. Hence in this paper, non-isolated structure, high voltage gain, minimum number of components are considered to design the proposed converter.

This paper focuses on analyzing and proposing a new MIMO boost converter with high efficiency and constant input and output currents. There is no requirement for time-multiplexing, and the accompanying control technique is fairly simple. In addition, adjustable additions and deletions of inputs and outputs are possible without altering the control strategy. The proposed converter enables the two inputs to feed the load simultaneously with a continuous input current. As a function, this arrangement is very generic for expansion, has uncomplicated control, is highly flexible, and has no crossregulation issue.

The structure of this paper is as follows: The operation of the proposed converter is provided in Section 2. The steps in the proposed converter's design are described in Section 3. The simulation and experimental results of the proposed converter are discussed in Section 4. A comparison of the results obtained to previously released works is shown in Section 5. The paper's conclusion is the focus of Section 6.

#### **2. Operation of the Proposed Converter**

In its most basic form, an MI converter is a circuit layout that incorporates several input voltage sources with various voltage levels and delivers an output DC load. If a

DC–DC converter is synthesized using a pulsing voltage source, or a pulsating current source relies on the characteristics of circuits and systems [28], these modules must be linked in parallel to provide the load in the event of pulsing current sources, such as boost converters. This paper studies and analyses the converter's MI and MO configurations, which are depicted in Figure 2. DC converter is synthesized using a pulsing voltage source, or a pulsating current source relies on the characteristics of circuits and systems [28], these modules must be linked in parallel to provide the load in the event of pulsing current sources, such as boost converters. This paper studies and analyses the converter's MI and MO configurations, which are depicted in Figure 2.

In its most basic form, an MI converter is a circuit layout that incorporates several input voltage sources with various voltage levels and delivers an output DC load. If a DC–

*Energies* **2022**, *15*, 5559 4 of 20

**2. Operation of the Proposed Converter** 

**Figure 2.** Circuit diagram of proposed converter. **Figure 2.** Circuit diagram of proposed converter.

Each of the pulsing current sources is made up of inductors (La and Lb), diodes (D1 and D2) and power switches (S1 and S2). They are each linked to various voltage sources at various voltage levels, notably Vinput1–Vinput2. Additionally, regardless of the switching strategy, the capacitors C1 and C3 and the inductor Lc are to guarantee the output current's continuance. The proposed converter's working principal is explained below, with discussions of theoretical voltage and current studies of each CCM and discontinuous conduction mode (DCM). Each of the pulsing current sources is made up of inductors (L<sup>a</sup> and Lb), diodes (D<sup>1</sup> and D2) and power switches (S<sup>1</sup> and S2). They are each linked to various voltage sources at various voltage levels, notably Vinput1–Vinput2. Additionally, regardless of the switching strategy, the capacitors C<sup>1</sup> and C<sup>3</sup> and the inductor L<sup>c</sup> are to guarantee the output current's continuance. The proposed converter's working principal is explained below, with discussions of theoretical voltage and current studies of each CCM and discontinuous conduction mode (DCM).

#### *2.1. Mode1 Operation 2.1. Mode 1 Operation*

Lc.

The schematic diagram of Mode 1 is represented in Figure 3. In Mode 1, switch S1 conducts, while switch S2 remains non-conducting at the beginning of this time period. The corresponding time period is represented as Ton. Since the inductor La's current rises proportionately as a result of being closely linked to the first voltage input (Vinput1), the amount of energy that can be stored in La also grows. Diode D1 is reverse-biased. As a result, the output load's currents are provided by the inductors Lb and Lc via the capacitors C1 and C3. To do this, there is a steady reduction in the stored energy of C1, C3, Lb, and The schematic diagram of Mode 1 is represented in Figure 3. In Mode 1, switch S<sup>1</sup> conducts, while switch S<sup>2</sup> remains non-conducting at the beginning of this time period. The corresponding time period is represented as Ton. Since the inductor La's current rises proportionately as a result of being closely linked to the first voltage input (Vinput1), the amount of energy that can be stored in L<sup>a</sup> also grows. Diode D<sup>1</sup> is reverse-biased. As a result, the output load's currents are provided by the inductors L<sup>b</sup> and Lc via the capacitors C<sup>1</sup> and C3. To do this, there is a steady reduction in the stored energy of C1, C3, Lb, and Lc.

**Figure 3.** Mode 1 operation (**----** Current flow direction). **Figure 3.** Mode 1 operation (**—-** Current flow direction). **Figure 3.** Mode 1 operation (**----** Current flow direction).

#### *2.2. Mode 2 Operation 2.2. Mode 2 Operation 2.2. Mode 2 Operation*

The schematic diagram of Mode 2 is represented in Figure 4. In this state, switch S1 is turned off, while switch S2 is active. The period of time is now represented as Toff. In this mode, the circuit is supplied by input source Vinput2, which are both present. Reverse bias is present in diode D1. As a result, the capacitors C1 and C3 and the inductors Lb and Lc, are linked. La's current is thereafter sharply decreased from its peak level to bottom value as a result of the energy stored in La being recycled to inductor Lc and capacitor C3. As a consequence, from its lowest value to its highest value, the current passing through Lb and Lc is sharply increased. The capacitors C1 and C3's voltages are increasing. The key waveforms under the CCM condition are displayed in Figure 5. The schematic diagram of Mode 2 is represented in Figure 4. In this state, switch S<sup>1</sup> is turned off, while switch S<sup>2</sup> is active. The period of time is now represented as Toff. In this mode, the circuit is supplied by input source Vinput2, which are both present. Reverse bias is present in diode D1. As a result, the capacitors C<sup>1</sup> and C<sup>3</sup> and the inductors L<sup>b</sup> and Lc, are linked. La's current is thereafter sharply decreased from its peak level to bottom value as a result of the energy stored in L<sup>a</sup> being recycled to inductor L<sup>c</sup> and capacitor C3. As a consequence, from its lowest value to its highest value, the current passing through L<sup>b</sup> and L<sup>c</sup> is sharply increased. The capacitors C<sup>1</sup> and C3's voltages are increasing. The key waveforms under the CCM condition are displayed in Figure 5. The schematic diagram of Mode 2 is represented in Figure 4. In this state, switch S1 is turned off, while switch S2 is active. The period of time is now represented as Toff. In this mode, the circuit is supplied by input source Vinput2, which are both present. Reverse bias is present in diode D1. As a result, the capacitors C1 and C3 and the inductors Lb and Lc, are linked. La's current is thereafter sharply decreased from its peak level to bottom value as a result of the energy stored in La being recycled to inductor Lc and capacitor C3. As a consequence, from its lowest value to its highest value, the current passing through Lb and Lc is sharply increased. The capacitors C1 and C3's voltages are increasing. The key waveforms under the CCM condition are displayed in Figure 5.

**Figure 4.** Mode 2 operation (**----** Current flow direction). **Figure 4.** Mode 2 operation (**----** Current flow direction). **Figure 4.** Mode 2 operation (**—-** Current flow direction).

**Figure 5.** Mode 3 operation (**----** Current flow direction). **Figure 5.** Mode 3 operation (**—-** Current flow direction).

#### *2.3. Mode 3 Operation 2.3. Mode 3 Operation*

The schematic diagram of Mode 3 is represented in Figure 5. In this state, both switches S1 and S2 are turned off. The stored energy in Lb is released through D2 for charging C2, and the energy stored in La is supplied to R1 at the same time. Currents going through La and Lb are gradually decreased. The key waveforms are displayed in Figure 6. The schematic diagram of Mode 3 is represented in Figure 5. In this state, both switches S<sup>1</sup> and S<sup>2</sup> are turned off. The stored energy in L<sup>b</sup> is released through D<sup>2</sup> for charging C2, and the energy stored in L<sup>a</sup> is supplied to R<sup>1</sup> at the same time. Currents going through L<sup>a</sup> and L<sup>b</sup> are gradually decreased. The key waveforms are displayed in Figure 6.

**Figure 6.** The proposed converter's theoretical operational waveform. **Figure 6.** The proposed converter's theoretical operational waveform.

#### **3. The Proposed MIMO Converter's Design Process 3. The Proposed MIMO Converter's Design Process**

The basic control strategy on the input side involves controlling the input sources while employing the matched input cells as controllable sources. The steady-state flow analyses are only computed in the CCM since the converter operation is chosen in CCM. Under the CCM scenario, the optimum voltage gain of the proposed converter is first determined, followed by the average current of the power components. The equipment's current stress predictions are determined in the last phase. The basic control strategy on the input side involves controlling the input sources while employing the matched input cells as controllable sources. The steady-state flow analyses are only computed in the CCM since the converter operation is chosen in CCM. Under the CCM scenario, the optimum voltage gain of the proposed converter is first determined, followed by the average current of the power components. The equipment's current stress predictions are determined in the last phase.

It is possible to regulate the output cells separately. Direct duty cycle control is achieved using the switches S1 and S2 of the output voltage source mode (VSM) cell if a controllable output current is needed [29]. Voltage-programmed regulation can be used when a controlled output voltage is necessary. We show the MI and MO types of the It is possible to regulate the output cells separately. Direct duty cycle control is achieved using the switches S<sup>1</sup> and S<sup>2</sup> of the output voltage source mode (VSM) cell if a controllable output current is needed [29]. Voltage-programmed regulation can be used when a controlled output voltage is necessary. We show the MI and MO types of

the proposed converter here with an experiment, where one output provides a constant current via direct duty cycle control, while the second output delivers constant voltage via voltage-programmed regulation. Here, the MI and MO types of the proposed converter are presented, where the first output offers a fixed current, and the second output delivers a fixed voltage.

#### *3.1. Voltage Gain*

The output voltages of the converters are,

$$V\_{Output1} = V\_{c1} = \frac{V\_{input1}}{1 - D\_1} = \frac{5}{1 - 0.79} = 23.80\text{ V} \tag{1}$$

$$V\_{Output2} = V\_{c3} = \frac{V\_{input2}}{1 - D\_2} = \frac{12}{1 - 0.67} = 36.36\text{ V} \tag{2}$$

This configuration's conversion ratio can be written as [30],

$$\mathcal{G}\_1 = \frac{V\_{output1}}{V\_{input1}} = \frac{1}{1 - D\_1} \tag{3}$$

$$\mathbf{G\_2} = \frac{V\_{output2}}{V\_{input2}} = \frac{1}{1 - D\_2} \tag{4}$$

The load current can be represented as,

$$I\_0 = \frac{V\_{output}}{R} = (1 - D\_1)I\_{La} + (1 - D\_2)I\_{Lb} \tag{5}$$

The inductor current *ILc* can be written as,

$$I\_{L\mathfrak{c}} = D\_1 I\_{La} + D\_2 I\_{Lb} \tag{6}$$

#### *3.2. Power Consumed by Each Source*

The closed-loop regulation of the output current of all sources, the inductor current ILa, is kept constant. A closed-loop control is also used to adjust the output voltage of the second output. The output and input currents of the VSM cell are directly related to one another. As a result, fixed duty cycle regulation can be used to produce a constant current at the first output. Here, we employ a basic duty cycle regulation to retain the steady output current. Considering that the duty cycle *D*<sup>1</sup> of switch S<sup>1</sup> and the duty cycle *D*<sup>2</sup> of switch S<sup>2</sup> are equal, the power produced from the input sources through the associated input cell during ideal circumstances can be stated as,

$$P\_{input1} = V\_{input1} D\_1 I\_{La} \tag{7}$$

$$P\_{input2} = V\_{input2} D\_2 I\_{Lb} \tag{8}$$

where *ILa* and *ILb* are the steady-state inductor current, and *Pinput*<sup>1</sup> and *Pinput*<sup>1</sup> are the power supplied from the input sources *Vinput*<sup>1</sup> and *Vinput*<sup>2</sup> , respectively. The connection shown below can therefore be obtained:

$$\frac{P\_{input1}}{P\_{input2}} = \frac{V\_{input1}}{V\_{input2}}\tag{9}$$

According to the calculations provided, every input source's power consumption is related to its associated voltage when the fixed current regulation is used. It is a result of the switches on the respective input cells having the same duty cycle.

It is important to remember that when there are more than two input cells, some of them are regulated to supply a fixed current, while other inputs are directly controlled by

duty cycle regulation. The functionality of this setup will not be impacted if any of these input cells are removed. Power supplied over the first current of input would instantly rise whenever the power supplied through the second cell diminishes. The direct connection between the input and output currents remains constant at the same moment. In this technique, the input voltage providing the second input cell, i.e., *Vinput*<sup>2</sup> , is subjected to a designing restraint to guarantee appropriate operation. Particularly, *Vinput*<sup>2</sup> needs to be lower than the power needed mostly by the output loads.

The necessary power through the loads Po can be represented as follows:

$$P\_o = \left(1 - D\_1\right)^2 I\_{La}^2 R\_1 + \left(1 - D\_2\right)^2 I\_{Lb}^2 R\_2 \tag{10}$$

where *R*<sup>1</sup> and *R*<sup>2</sup> are the respective loads of the two output cells.
