https://doi.org/10.6113/JPE.2019.19.5.1069

ISSN(Print): 1598-2092 / ISSN(Online): 2093-4718

Two-Inductor Non-Isolated DC-DC Converter with High Step-Up Voltage Gain

Sze Sing Lee^*, Bing Chu^**, Chee Shen Lim^*, and Kyo-Beum Lee^†

^*Dept. of Electronics and Computer Science, University of Southampton Malaysia, Iskandar Puteri, Malaysia

^**Dept. of Electronics and Computer Science, University of Southampton, Southampton, ENG, UK

^†Dept. of Electrical and Computer Engineering, Ajou University, Suwon, Korea

Abstract

In this paper, an alternative non-isolated DC-DC converter with a high voltage boosting capability is proposed. Two inductors are used and one of them has its flux linkage increases during its charging period to achieve a high step-up voltage gain. Among the three integrated capacitors, one portrays the partial characteristic of the switched-capacitor technique, while the other two are connected in series across the load. With the two switches controlled using the same duty cycle, the proposed topology demonstrates the merits of a higher and wider range of step-up voltage gain when compared with recent topologies. In addition, a reduction in loss is induced and a higher efficiency is ensured with all the voltage stresses constrained within the output voltage. Operation of the proposed converter is analyzed and validated through experimental results obtained with a prototype.

Key words: DC-DC converter, High step-up gain, Non-isolated, Two-inductor

Manuscript received Dec. 6, 2018; accepted Apr. 17, 2019

Recommended for publication by Associate Editor Younghoon Cho.

^†Corresponding Author: kyl@ajou.ac.kr Tel: +82-31-219-2376, Ajou University

^*Dept. of Electron. & Comput. Sci., Univ. of Southampton Malaysia, Malaysia

^**Dept. of Electron. & Comput. Sci., Univ. of Southampton, Southampton, UK

Ⅰ. INTRODUCTION

There is an ongoing paradigm shift to renewable energy sources to resolve the ever increasing world energy crisis for a sustainable future. The low output voltages [1] and intermittency characteristics [2], [3] of renewable energy sources can be alleviated by various step-up DC-DC converters, where a comprehensive review is summarized in [4].

Two-inductor non-isolated step-up converters have been gaining popularity in recent years owing to their simpler operation and higher step-up voltage gain when compared to conventional boost converters. A simple topology with two active switches is presented in [5]. Its two inductors are charged in parallel during the switched-on period and discharged in series during the switched-off period to achieve a high step-up voltage gain. On the other hand, the authors of [6] advocate an improved three-level-boost (TLB) converter by integrating an additional inductor into a conventional TLB converter to double the voltage gain. In [7], a combination of a conventional boost converter and a diode-capacitor based single ended primary inductor converter (SEPIC) is presented to increase the step-up voltage gain. Meanwhile, the topologies in [5] and [7] benefit from reduced voltage stresses when compared with conventional boost converters, and the topology in [6] suffers from a voltage stress as high as its output voltage on one of its switches.

Interestingly, the authors of [8] proposed a different way to produce a high voltage gain by introducing two different charging duty-cycles to both of its inductors. A power MOSFET and a diode are added to the topology in [5], where the added switch suffers from a voltage stress equal to its high output voltage. From the analysis in Fig.1, it is obvious that the voltage gain is proportional to λ₂ for a given duty cycle. Considering the volt-seconds balance of the inductor voltage during steady-state operation, λ₂ is equal to λ₁. A lower charging voltage during D₂T_s results in a reduction of λ₁. Therefore, a decrement in D₂ leads to an increment in the voltage gain. In the case when D₂ = 0, the maximum voltage gain is achieved and the topology is equivalent to that in [5]. The increased component count and controller complexity, when compared to topology in [5], make it a less attractive alternative for practical applications. Other techniques using coupled inductor [9] or DC transformer (DCX) [10] are also recently explored.

Fig. 1. Inductor with two charging duty-cycles [8].

The goal of this paper is to accomplish a novel topology with a high voltage gain by increasing λ₁. While keeping all of the voltage stresses within the output voltage, the established topology exhibits a higher and wider voltage gain than existing two-inductor non-isolated converters. This paper is organized as follows. Section II presents the proposed topology and a comparative analysis with recent two-inductor topologies. Section III discusses simulation results and section IV presents experimental results obtained with a prototype. Finally, section V draws some conclusions.

Ⅱ. PROPOSED NON-ISOLATED DC-DC CONVERTER

The proposed converter topology is constituted by two switches S₁ and S₂, a capacitor C, two inductors L₁ and L₂, three diodes D₁, D₂ and D₃, and two output capacitors C_O1 and C_O2, as depicted in Fig. 2.

Fig. 2. Circuit diagram of the proposed non-isolated DC-DC converter.

The two series-connected output capacitors are used to enhance the step-up gain, while C is used to absorb the energy from L₁ before it is transferred to C_O2 and L₂. Note that the incorporation of C is inspired by the switched- capacitor technique. Compared with topologies where only one output capacitor involved, the adoption of two output capacitors efficiently reduces the voltage stress on each capacitor. Key waveforms and equivalent circuits for the continuous inductor current are illustrated in Fig. 3 and Fig. 4, respectively. Note that the two switches are controlled by the same duty cycle, while L₂ is charged to the sum voltage across C and V_dc in order to increase λ₁ for further voltage boosting.

Fig. 3. Key waveforms of the proposed non-isolated DC-DC converter.

Fig. 4. Equivalent circuits of the proposed non-isolated DC-DC converter with a continuous inductor current. (a) Mode 1. (b) Mode 2.

(a)

(b)

Considering the volt-seconds balance of the inductors during steady-state operation, the following equations can be derived:

                         (1)

                       (2)

where V_dc is the input voltage, D is the duty-cycle, V_C is the average voltage of C, and V_O1 is the average voltage of C_O1. Referring to mode 1, shown in Fig. 4(a), the average voltage of C_O2 is equal to the sum of V_dc and V_C. Thus, it is written as:

                        (3)

The total average voltage across C_O1 and C_O2 is given as:

                   (4)

The average inductor currents flowing through L₁ and L₂ are:

                   (5)

                            (6)

where I_dc is the average input current, I_O is the average output current, I_L1 is the average current of L₁, and I_L2 is the average current of L₂. Considering the boundary condition for the inductor current, the minimum inductances for both of the inductors are given by:

        (7)

                  (8)

where f_S is the switching frequency, and L_1,min and L_2,min are the respective minimum inductances for L₁ and L₂, which guarantee continuous current.

The step-up voltage gains, as a function of duty-cycles for various DC-DC converters, are compared in Fig. 5. Existing two-inductor converters demonstrate better step-up capabilities when benchmarked with a conventional boost converter. However, the proposed topology offers the highest and widest step-up voltage gain among all of them. In addition, Fig. 6 shows that the voltage stresses of all the switches and diodes of the proposed topology are less than the output voltage. This guarantees the added benefits of low power loss and high efficiency.

Fig. 5. Voltage gain comparison with recently presented two- inductor non-isolated DC-DC converters.

Fig. 6. Voltage stresses on the switches and diodes.

Ⅲ. SIMULATION RESULTS

To verify the operation of the proposed topology, simulations were conducted. The inductor current, capacitor voltage and output voltage are depicted in Fig. 7. A voltage boost gain of 10 is achieved at D = 0.61. In addition, the voltage across each of the capacitors is in good agreement with the theoretical analysis in (1)-(3).

Fig. 7. Simulated waveforms of the proposed DC-DC converter.

Ⅳ. EXPERIMENTAL RESULTS

The experimental prototype depicted in Fig. 8 was tested to further verify the operation of the proposed DC-DC converter. The specifications of the experimental prototype are summarized in Table I. The limited bandwidth of the current probe restricts the operating switching frequency to 10 kHz. Open-loop control was implemented by using a function generator to generate a PWM signal. The duty-cycle was manually adjusted to produce an output voltage of 200V.

Fig. 8. Experimental prototype.

TABLE I Prototype Specifications and Component Ratings

Parameter	Value
Input voltage, Vdc	20V – 60V
Output voltage, VO	200V
Switching frequency, fS	10kHz
Capacitor (C, CO1, CO2)	2700μF
Inductor (L1, L2)	3mH
Power MOSFET (S1, S2)	C2M0080120D
Diode (D1, D2, D3)	C3D08065A

Fig. 9 shows waveforms captured when the input voltage was 20V. The duty-cycle was set to approximately 0.62 to generate an output voltage of 200V. The measured step-up voltage gain of 10 is slightly less than its theoretical value of 10.6. Both of the inductor currents are continuous with a steeper slope on i _L2 during the charging period (DT_S). This indicates that L₂ is charged with a higher voltage to increase λ₁. This is done to enhance the voltage boosting capability. This experiment was repeated at an input voltage of 60V with the duty-cycle was set to 0.1. Fig. 10 reveals that the measured and calculated step-up gains are approximately the same, i.e. 3.33 and 3.35, respectively.

Fig. 9. Experimental waveforms at an input voltage of 20V.

Fig. 10. Experimental waveforms at an input voltage of 60V.

Experiments were then conducted to study the efficiency of the prototype at different input voltages and different output powers, as shown in Fig. 11. Considering a fixed output power of 100W, it can be seen that the efficiency increased with the input voltage, and that it was recorded well above 95% for an input voltage beyond 30V. The measured efficiencies for output power ranges from 100W to 130W, on the other hand, centers around 95% for an input voltage of either 60V or 30V.

Fig. 11. Efficiency of the experimental prototype.

Ⅴ. CONCLUSION

This paper establishes a new two-inductor non-isolated DC-DC converter that features a high voltage boosting capability, a wide voltage gain range, and low voltage stress across switches and diodes. It is superior to recent topologies in view of its higher and wider range of step-up voltage gain. Good agreement between the theoretical analysis and experiments verified the conceptual validity and operation of the proposed topology. The peak efficiency of the proposed topology exceeds 95%. Therefore, it could be an interesting alternative for DC-DC power conversion systems.

ACKNOWLEDGMENT

This work was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government (MOTIE) (No.20182410105160, Demonstration and Development of ESS Solution Connected with Renewable Energy against with the weather condition of Middle East Region).

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Sze Sing Lee received his B.Eng. (Hons.) and Ph.D. degrees in Electrical Engineering from the Universiti Sains Malaysia, Penang, Malaysia, in 2010 and 2013, respectively. He is presently working as an Assistant Professor at the University of Southampton Malaysia, Iskandar Puteri, Malaysia. From September 2018 to February 2019, he was a Visiting Research Professor at Ajou University, Suwon, South Korea. His current research interests include power converter/inverter topologies and their control strategies. Dr. Lee received an International Scholar Exchange Fellowship from Korea Foundation for Advanced Studies in 2018.

Bing Chu received his B.Eng degree in Automation and his M.Sc degree in Control Science and Technology from Tsinghua University, Beijing, China, in 2004 and 2007, respectively. He received his Ph.D. degree in Automatic Control and Systems Engineering from the University of Sheffield, Sheffield, ENG, UK. He is presently working as an Associate Professor at the University of Southampton, Southampton, ENG, UK. Before joining the University of Southampton in 2012, he was a Postdoctoral Researcher at the University of Oxford, Oxford, ENG, UK, from 2010 to 2012. He was a recipient of a number of awards including the best paper prize of the 2012 United Kingdom Automatic Control Council International Conference on Control, a Certificate of Merit for 2010 IET Control and Automation Doctoral Dissertation Prize, and an Automation Doctoral Dissertation Prize. His current research interests include iterative learning and repetitive control, the analysis and control of large-scale networked systems and applied optimization theory, and their applications to robotics, power electronics and next generation healthcare.

Chee Shen Lim received his B.Eng (Hons.) degree in Electrical Engineering from the University of Malaya, Kuala Lumpur, Malaysia, in 2009; and a joint-university Ph.D. degree from the University of Malaya and Liverpool John Moores University, Liverpool, ENG, UK, in 2013. From 2009 to 2013, he worked as a Research Assistant at the Power Energy Dedicated Advanced Centre, University of Malaya. From 2013 to 2015, he was a Research Scientist at the Experimental Power Grid Centre of the Agency for Science, Technology and Research, Singapore. He is presently working as an Assistant Professor at the University of Southampton Malaysia, Iskandar Puteri, Malaysia. His current research interests include advanced model predictive control design, multiphase motor drives, grid-connected converter control and microgrid hierarchical control.

Kyo-Beum Lee received his B.S. and M.S. degrees in Electrical and Electronic Engineering from Ajou University, Suwon, South Korea, in 1997 and 1999, respectively. He received his Ph.D. degree in Electrical Engineering from Korea University, Seoul, South Korea, in 2003. From 2003 to 2006, he was with the Institute of Energy Technology, Aalborg University, Aalborg, Denmark. From 2006 to 2007, he was with the Division of Electronics and Information Engineering, Chonbuk National University, Jeonju, South Korea. In 2007, he joined the Department of Electrical and Computer Engineering, Ajou University. His current research interests include electric machine drives, renewable power generation and electric vehicle applications. Dr. Lee is an Associate Editor for the IEEE Transactions on Power Electronics, the IEEE Transactions on Industrial Electronics, the Journal of Power Electronics, and the Journal of Electrical Engineering and Technology.