**An Original Transformer and Switched-Capacitor (T & SC)-Based Extension for DC-DC Boost Converter for High-Voltage/Low-Current Renewable Energy Applications: Hardware Implementation of a New T & SC Boost Converter**


Received: 25 September 2017; Accepted: 6 November 2017; Published: 29 March 2018

**Abstract:** In this article a new Transformer and Switched Capacitor-based Boost Converter (T & SC-BC) is proposed for high-voltage/low-current renewable energy applications. The proposed T & SC-BC is an original extension for DC-DC boost converter which is designed by utilizing a transformer and switched capacitor (T & SC). Photovoltaic (PV) energy is a fast emergent segment among the renewable energy systems. The proposed T & SC-BC combines the features of the conventional boost converter and T & SC to achieve a high voltage conversion ratio. A Maximum Power Point Tracking (MPPT) controller is compulsory and necessary in a PV system to extract maximum power. Thus, a photovoltaic MPPT control mechanism also articulated for the proposed T & SC-BC. The voltage conversion ratio (*Vo*/*Vin*) of proposed converter is (1 + *k*)/(1 − *D*) where, *k* is the turns ratio of the transformer and *D* is the duty cycle (thus, the converter provides 9.26, 13.88, 50/3 voltage conversion ratios at 78.4 duty cycle with *k* = 1, 2, 2.6, respectively). The conspicuous features of proposed T & SC-BC are: (i) a high voltage conversion ratio (*Vo*/*Vin*); (ii) continuous input current (*Iin*); (iii) single switch topology; (iv) single input source; (v) low drain to source voltage (*VDS*) rating of control switch; (vi) a single inductor and a single untapped transformer are used. Moreover, the proposed T & SC-BC topology was compared with recently addressed DC-DC converters in terms of number of components, cost, voltage conversion ratio, ripples, efficiency and power range. Simulation and experimental results are provided which validate the functionality, design and concept of the proposed approach.

**Keywords:** DC-DC boost converter; transformer; switched capacitor; maximum power point tracking; renewable energy; high-voltage; low current

#### **1. Introduction**

In recent years the importance of using renewable energies has grown significantly due to the fact that the usage of fossil fuels such as oil, coal, and gas results in environmental pollution and serious

greenhouse effects which have a huge influence on the world [1,2]. The demands for energy and power converters have been increasing for the last several decades, due to the greater industrialization, rising population and increased living standards of society [3–9]. The International Energy Agency (IEA) has anticipated that the developing nations are raising their energy utilization at a quicker pace than developed ones and will need to nearly the double their present installed generation facility by the year 2020 for fulfill their energy requirements. The total energy consumption of the world from 1980 to 2025 is depicted in Figure 1a [10–12]. The IEA has also details that more than 1.3 billion people in the developing nations are living with insufficient or without any access to electricity because of the unavailability of electric grid in these regions and other constraints [10–12]. Thus, there is a need for new sources of energy that are cheaper and sustainable with less carbon emissions [13,14]. Photovoltaic energy is considered as a reliable, promising and favorable source and it has various advantages such as being pollution free, long life, low maintenance, etc. [15–18]. In Figure 1b the global cumulated PV capacity (in Gigawatts) from 1996 to 2012, and an estimation (E) by 2020 is shown. It is observed that PV is a fast emergent segment among renewable energy systems [19]. To increase the effectiveness and efficiency of power conditioning to tracking Maximum Power Point (MPP) plays an important role in increasing the conversion efficiency. A PV system has non-linear P-V and I-V characteristics and the generated power depends on the environmental conditions such as solar irradiation and temperature [20–22]. The power of a PV system is higher at the knee point of the P-V characteristic curve, as the MPP keeps on changing according to the varying irradiation levels, a MPPT method is used to track the MPP of the system. The PV grid-connected power system in domestic applications is becoming a fast-rising segment in the PV market [23–26]. Unfortunately, the output voltage of the PV arrays or panels is relatively low. Thus, PV series-connected configurations are generally used in order to satisfy the demand and the high bus voltage requirements of the half and full-bridge, multilevel inverters (MLI) needed to transfer energy to electric grid. This type of system suffers from partial shading, reduced PV module efficiency, and mismatched MPPT control [27,28]. Thus boost converters with high voltage conversion ratio and inverters are required to feed energy to the electric grid. In practice DC-DC converters with high efficiency with low input voltage, high input current, high output voltage and high voltage conversion ratio provide a practicable solution for photovoltaic systems to transfer photovoltaic energy to the electric grid via inverters [29,30].

**Figure 1.** (**a**) Radar plot of energy consumption in quadrillion from 1980 to 2030; (**b**) Radar plot of global cumulated photovoltaic capacity (in GW) from 1996 to 2020.

Additionally, traditional boost converters are not a practicable solution to achieve high conversion ratios due to the leakage resistance of the inductor, high switch stress and the performance of a boost converter is deteriorated by the high duty cycle of the power switch and thus, not able to achieve conversion ratios of more than four. To operate the DC-DC converters for getting high output voltage without using high duty cycles for power semiconductor controlled switches, isolated converters can be employed which contain transformers, coupled inductors, etc. [29–37], but the usage of such a large number of magnetic components increases the size of the circuit as well as the leakage reactance of the converter and also produces electromagnetic interference which reduces the converter function ability and efficiency. Recently many boost converter topologies are addressed by extending the power circuit of the traditional boost converter. In [38], a single switch *n*-stage Cascaded Boost Converter (*n*-stage CBC) is discussed to achieve high voltage but it requires a large number of inductors, diodes and capacitors. Figure 2a depicts the power circuit of a single switch *n*-stage Cascaded Boost Converter (single switch *n*-stage CBC). In [38], a single switch boost converter with a voltage multiplier extension to achieve high voltage is discussed, but it requires a large number of diodes and capacitor circuitry. Figure 2b depicts the power circuit of a single switch boost converter with voltage multiplier. In [39], a new coupled inductor-based step-up converter is discussed with a large pump. The voltage can be easily achieved by modifying the turns-ratio of coupled inductors but the leakage energy induces high voltage stress and switching losses. The power circuit of coupled inductor based step-up converter is shown in Figure 2c. In [40], a new Quadratic Boost Converter (QBC) with coupled inductor in second boost converter is proposed and shown in Figure 2d. This QBC achieves high step-up voltage gain with an appropriate duty ratio and low voltage stress on the power switch.

**Figure 2.** Recently addressed DC-DC power converter circuit (**a**) single switch *n*-stage Cascaded Boost Converter (*n*-stage CBC) (**b**) single switch boost converter with voltage multiplier (**c**) coupled inductor based step-up converter and (**d**) Quadratic Boost Converter (QBC) with coupled inductor.

In [41], a DC-DC non-inverting Nx Interleaved Multilevel Boost Converter (Nx IMBC) is proposed to achieve a high voltage conversion ratio (*Vo*/*Vin*) and to reduce voltage/current ripple. Nx-IMBC provides N times more voltage conversion ratio compared to traditional boost converters but requires large number of diodes and capacitors. In [42,43], DC-DC non-inverting 2Nx and 4Nx Interleaved Boost converters (2Nx IMBC and 4Nx IMBC) are proposed to achieve high voltage conversion ratio (*Vo*/*Vin*) and to reduce voltage/current ripple. 2Nx IMBC and 4Nx IMBC provide 2N and 4N times more voltage conversion ratio compared to traditional boost converters, but also require large numbers of diodes and capacitors. To achieve a high inverting voltage conversion ratio, a new inverting Nx and 2Nx Multilevel Boost Converter (MBC) was proposed for renewable energy applications [44]. In [45,46], a new family of DC-DC converters called "X-Y Converter Family" was proposed to achieve high conversion ratios for renewable applications. These converters are well suited to achieve high voltage, but need more number of devices, thus increasing the size and cost of the converter.

In this article, a new T & SC-BC is proposed for high-voltage/low-current renewable applications to overcome the drawback of recently addressed converters. The power circuit and block diagram of proposed converter scheme system is shown in Figure 3. The proposed T & SC-BC is an original extension for DC-DC boost converters which is based on T & SC. The proposed T & SC-BC converter combines the features of a conventional boost converter and T & SC [47] to achieve high voltage conversion ratios. In a PV cell model the current source is basically associated in parallel with the reversed diode and also has series and parallel resistance as shown in Figure 3. Resistance in series (*RS*) is due to barrier in the pathway of flow of electrons from *n* to *p* junction and resistance in shunt (*RSh*) is due to the leakage current [48]. The relation between the currents of a PV cell is shown in Equation (1):

$$\begin{aligned} I &= I\_{ph} - I\_d - I\_{Sh\prime} I\_d = I\_o (\varepsilon^{\frac{V + IR\_\xi}{nV\_T}} - 1), I\_{Sh} = \frac{V + IR\_\xi}{R\_{Sh}}\\ V\_T &= \frac{kT}{q}, V\_T = 0.0259V \text{ at } T = 25 \text{ }^\circ \text{C} \end{aligned} \tag{1}$$

where *Iph* is the photocurrent generated by the cell, *Id* is the current flowing through the diode of the solar cell, *ISh* is the shunt current flowing through the shunt resistance (*RSh*), *I* is the output current or current flowing through the series resistance (*RS*), *V* is the voltage at the output terminal of the cell, *Io* is the reverse saturation current, *n* is the diode ideal factor, *VT* is the thermal voltage, *k* is the Boltzmann constant and *T* is the absolute temperature. When irradiance strikes the flat surface of a PV module or cell, an electrical field is produced within the cell. In the presence of an electric field, these charges can create a current that can be used in a peripheral circuit. This current depends on the concentration and intensity of the incident solar radiation. The higher the level of the light intensity, the more electrons can be allowed to run free from the flat surface, and the more current is created. All the time it is necessary to track MPP due to deviation of hotness and irradiation of the array [49,50]. MPPT techniques to track MPP have been addressed and published over years of research [50–53]. Every MPPT method has its own merits and demerits like required sensors, complexity, cost, range of effectiveness, convergence speed, correct tracking when the irradiation and temperature change. The Perturb & Observe (P & O) algorithm and Incremental Conductance algorithm is the most popular and simple methods to track MPP. TheP&O algorithm is depends on hill climbing concept hence also called "hill-climbing P & O Method". This is one of most used algorithms due to its simplicity and ease of implementation and low cost [49,53]. It operates with a cyclic perturbation (increase or decrease) of the array terminal voltage and by comparing the PV power of last perturbation. If the power increases the perturbation goes with the same direction otherwise it will goes with the reverse direction. In this method, the sign of the last perturbation and the sign of the last increment in the power are used to decide what the next perturbation should be [53]. The concept of the P & O MPPT algorithm is shown in Figure 4a–c with the P-V and I-V characteristics. To extract the maximum power, aP&O MPPT control mechanism is used to locate the MPP for the proposed T & SC-BC.

**Figure 3.** Power circuit and block diagram of proposed converter system: Transformer and Switch Capacitor Based Boost Converter (T & SC-BC) with Maximum Power Point Tracking (MPPT) for high-voltage/low-current renewable applications.

**Figure 4.** Concept of Maximum Power Point Tracking for proposed T & SC-BC (**a**) Perturb and Observed algorithm (**b**) PV and IV characteristics of photovoltaic cell and (**c**) Concept to track Maximum Power Point (MPP) to extract maximum power.

The conspicuous features of proposed T & SC-BC are:


Simulation and experimental result are provided which validate the functionality, design and concept of the proposed approach.
