**1. Introduction**

DC-DC converters play an essential role in too many different applications, including renewable energy systems [1,2], hybrid or fully electric vehicles (EVs) systems [3,4], microgrids in power systems [5,6], and voltage regulation applications [7,8]. These converters are

**Citation:** Mahafzah, K.A.; Al-Shetwi, A.Q.; Hannan, M.A.; Babu, T.S.; Nwulu, N. A New Cuk-Based DC-DC Converter with Improved Efficiency and Lower Rated Voltage of Coupling Capacitor. *Sustainability* **2023**, *15*, 8515. https://doi.org/ 10.3390/su15118515

Academic Editor: J. C. Hernandez

Received: 27 April 2023 Revised: 19 May 2023 Accepted: 22 May 2023 Published: 24 May 2023

**Copyright:** © 2023 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/).

mainly divided into two types: First, a linear converter depends on a linear passive device such as a series or shunt resistance to regulate the output voltage. This converter is a very simple converter with low noise in its output voltage. However, using passive elements deteriorates the converter efficiency due to heat generation. Additionally, it is used as a step-down converter only [9–11]. Second, switching converters: these converters are the most common ones. The output of these converters is regulated by using a semi-conductorcontrolled switch (at least one switch is used). The presence of controlled switches allows for either step-up or step-down of the output voltage and even enables inversion of the output voltage polarity. Although the use of controlled switches increases complexity and output noise, it improves the overall efficiency of these converters [12–20].

Switching converters can be categorized as either hard-switching or soft-switching resonant converters. The hard-switching converters could be non-isolated or isolated DC-DC converters. The non-isolated converters include Buck, Boost, SEPIC, Buck–Boost, and Cuk converters. These topologies typically consist of a single controlled semiconductor switch, a single diode, one or two inductors, and a low-pass filter [12–20]. In contrast, the nonisolated DC-DC converters employ galvanic isolation equipment such as a transformer-like flyback converter [21] and a forward converter [22]. Hard-switching converters suffer from high switching losses, which limits their ability to achieve a high-efficiency range [23,24]. To address this drawback, soft switching converters have been introduced, significantly reducing switching losses. These converters cover the zero current switching (ZVS) [25] and zero voltage switching (ZCS) [25] converters. More details are illustrated in Figure 1.

Due to the rapid development of renewable energy resources, DC-DC converters with inverted output voltage are commonly used in hybrid solar and wind systems. These converters serve the purpose of providing a constant voltage source when the solar energy or wind speed falls below the desired limits [26]. Additionally, another significant application that requires inverted output voltage is in electric vehicles, which involve two energy storage devices: a power supply with high energy storage and a rechargeable energy storage system that enables two-directional power capability [3]. As a result, converters with inverted output voltage, such as the Buck–Boost converter and the Cuk converter, are highly preferred for these applications [27].

**Figure 1.** Classification of DC-DC Converters with the location of the Mahafzah converter [28–31].

The Buck–Boost converter can either step-up or step-down the output voltage using a low number of components. Additionally, it offers high efficiency with a low-duty cycle, making it suitable for many applications [13]. However, it cannot achieve high gain without

compromising the converter's efficiency. Moreover, the absence of isolation in the converter can lead to instability in certain applications [13]. Furthermore, when the switch is open, the stored energy in the output side inductor (*L*2) is transferred back to the supply, which can be undesirable and restrict the converter's usability [16].

The Cuk converter is utilized for both stepping up and stepping down the output voltage. It consists of two inductors that help reduce the ripple in the input/output currents. In addition, this converter has a continuous input/output current. Furthermore, in the Cuk converter, when the switch is closed, the coupling capacitor supplies energy to both the output side inductor *L*<sup>2</sup> and the load simultaneously. Yet, unlike the Buck–Boost converter, when the switch is opened, the energy stored in *L*<sup>2</sup> is transferred to the load [16]. Despite these advantages, the Cuk converter does have some drawbacks; for example, the compensation circuit may be added to stabilize the converter, which reduces its response.

According to the authors' best knowledge and after a careful review of the DC-DC converters presented in review papers [28–31], the proposed configuration is not yet presented in the literature. Therefore, this paper proposes a new converter that is designed and verified experimentally and by simulation. The outcomes of this configuration enhance the efficiency and reduce the coupling capacitor voltage rating. Table 1 compares different DC-DC converters with their limitations.


**Table 1.** Different DC-DC converters topologies with their limitations.

The main contribution of this paper is the introduction of a new DC-DC converter that offers higher efficiency, a lower rated voltage of coupling capacity, and cost reduction as compared to Cuk converters. Another advantage of the new configuration (Figure 2) is that it utilizes the same components as the well-known Cuk converter but in a different arrangement. Additionally, the proposed converter demonstrates improved efficiency compared to the Cuk converter under similar operating conditions, reaching approximately 88% at rated conditions. Furthermore, the voltage of the coupling capacitor is reduced to (*V*<sup>m</sup> = ±100 V) compared to (*V*<sup>m</sup> = ±500 V) in the Cuck converter. A design example is presented to validate the functionality of the proposed converter, which is suitable for hybrid renewable energy systems and electric vehicle applications. Moreover, a low voltage–low power prototype of 12/−18 V, 3.24 W is established to verify the operation of the proposed converter, showing a close match between measurements and simulations.

The survey above highlighted the two main types of converters: linear converters, which utilize passive elements and have simplicity but lower efficiency, and switching converters, which employ semiconductor-controlled switches for improved efficiency but higher complexity and noise. The survey also discussed the limitations of hard-switching converters and the advantages of soft-switching converters. However, despite the extensive review, the proposed configuration of the Mahafzah converter, which offers higher efficiency, reduced coupling capacitor voltage rating, and cost reduction compared to Cuk

converters, has not been presented in the existing literature. This research gap motivates the introduction of the new converter and its experimental and simulation verification, addressing the need for an improved DC-DC converter design in hybrid renewable energy systems and electric vehicle applications.

**Figure 2.** The proposed Mahafzah converter.

The rest of this paper is organized as follows: Section 1 discusses the operating modes, duty cycle, and voltage gain of the proposed converter. A design example and parameters selection is presented in Section 2. Section 3 illustrates the simulation results based on the calculations in the previous section. Section 4 provides experimental results of a low voltage–low power prototype. Finally, the paper is concluded in Section 5.
