Scholarly article on topic 'Self-powered High Efficiency Coupled Inductor Boost Converter for Photovoltaic Energy Conversion'

Self-powered High Efficiency Coupled Inductor Boost Converter for Photovoltaic Energy Conversion Academic research paper on "Materials engineering"

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Energy Procedia
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Keywords
{"Coupled inductor" / "DC/DC converter" / "High step-up" / "Distributed generation (DG) system" / "Bus HVDC(High Voltage Direct Current)"}

Abstract of research paper on Materials engineering, author of scientific article — The Vinh Nguyen, Pierre Petit, Fabrice Maufay, Michel Aillerie, Ali Jafaar, et al.

Abstract The increase of the demand in efficient energy converters is a very promising line of research for small renewable energy generators and systems. It involves a maximum optimization of electronic structures using the most recent dedicated and improved components. A simple and efficient DC/DC converter related to smart grids applications connected to High Voltage Direct Current (HVDC) bus presents many advantages compared to classical distribution system on AC grids. This study is particularly orientated to the distributed energy converters for photovoltaic (mono-, poly-crystalline, amorphous) or wind production. The HVDC bus reduces the energy transport cost and is a protection way against vandalism, but the main advantage of this architecture is related to the possibility of connecting several mismatched generators without disturbing the other ones. In this paper a specific analysis was done on a magnetic coupled inductor boost DC/DC converter in order to determine the coupling factor influence. It is well known that high voltages can be obtained by using a specific transformer or coupled coils. In fact, the coupling factor decreases with the insulation voltage and fabrication constraints. Based on comparisons of simulation results with a discrete analysis, this work analyses the influence of the magnetic coupling coefficient on the coupling coils operations and on the efficiency of the converter. This study which can allow a more useful way to develop new similar DC/DC converter systems.

Academic research paper on topic "Self-powered High Efficiency Coupled Inductor Boost Converter for Photovoltaic Energy Conversion"

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Energy Procedia 36 (2013) 650 - 656

TerraGreen 13 International Conference 2013 - Advancements in Renewable Energy

and Clean Environment

Self-powered high efficiency coupled inductor boost converter for photovoltaic energy conversion

The Vinh Nguyena,b*, Pierre Petita,b, Fabrice Maufaya,b, Michel Ailleriea,b, Ali Jafaara,b,c,d, and Jean-Pierre Charlesa,b

a Lorraine University, LMOPS-EA 4423, 57070 Metz, France b Supelec, LMOPS, 57070 Metz, France c Lebanese University, Faculty of Sciences II, CEA-LPSE, Jdeidet El Mten, Lebanon d National Council for Scientific Research, CNRSL, Beirut, Lebanon

Abstract

The increase of the demand in efficient energy converters is a very promising line of research for small renewable energy generators and systems. It involves a maximum optimization of electronic structures using the most recent dedicated and improved components. A simple and efficient DC/DC converter related to smart grids applications connected to High Voltage Direct Current (HVDC) bus presents many advantages compared to classical distribution system on AC grids. This study is particularly orientated to the distributed energy converters for photovoltaic (mono-, poly-crystalline, amorphous) or wind production. The HVDC bus reduces the energy transport cost and is a protection way against vandalism, but the main advantage of this architecture is related to the possibility of connecting several mismatched generators without disturbing the other ones.

In this paper a specific analysis was done on a magnetic coupled inductor boost DC/DC converter in order to determine the coupling factor influence. It is well known that high voltages can be obtained by using a specific transformer or coupled coils. In fact, the coupling factor decreases with the insulation voltage and fabrication constraints. Based on comparisons of simulation results with a discrete analysis, this work analyses the influence of the magnetic coupling coefficient on the coupling coils operations and on the efficiency of the converter. This study which can allow a more useful way to develop new similar DC/DC converter systems.

© 2013 The Authors. Published by Elsevier Ltd.

Selection and/or peer-review under responsibility of the TerraGreen Academy

Keywords: Coupled inductor, DC/DC converter, high step-up, Distributed generation (DG) system, bus HVDC(High Voltage Direct Current)

* Also at Lorraine University, IUT de Thionville-Yutz, 57970 Yutz; Tel.: +33-387-378-565; fax: +33-387-378-559 E-mail address: vinhnt@qui.edu.vn, aillerie@metz.supelec.fr

1876-6102 © 2013 The Authors. Published by Elsevier Ltd.

Selection and/or peer-review under responsibility of the TerraGreen Academy

doi:10.1016/j.egypro.2013.07.075

1. Introduction

In energetically non-connected applications such as isolated individual houses or villages, planes, embedded systems, the DC/DC converters take a great place in the energy conversion process, when renewable energy sources are considered. Especially in low and medium capacity systems, size and mass are important issues to focus on lately. The circuit configuration and control system when used in power range uninterruptible power supply presents a lot of drawbacks. One of the biggest problems is the limited capacity of the power valves and semiconductors, passive components as filtering capacitors and high current inductors.

Nomenclature

C, Co Capacitors (nF)

Dc, Do Diodes

EMI Electro-Magnetic Interference

HVDC High Voltage Direct Current

L1, L2 Principal inductors (|uH)

Lfb Lf2 Leakage inductor (|uH)

M MOSFET switch

MCB Magnetically Coupled Boost

R1, R2 Resistor (Q)

High step-up DC/DC converters are now widely used in many applications. For example, in a sustainable energy system, with photovoltaic arrays as low voltage sources, the converter boosts low voltage to high voltage, HVDC, which can be directly used or converted in an AC utility voltage [1]. The HVDC final or intermediate bus decreases the energy transport cost and is a protection way against vandalism, but the main advantage of this architecture is related to the possibility of connecting several mismatched generators without disturbing the other ones. Thus, the high step-up DC/DC converter needs high voltage gain, high efficiency, and small volume [2]. Theoretically, a conventional boost converter can be adopted to provide high step-up voltage gain with an extremely high duty ratio. In practice, the step-up voltage gain is limited by the characteristics of the power switch, the rectifier diode, and by the equivalent series resistances of the inductors and capacitors constituting the conversion stage. Also, the extremely high duty-ratio operation may result in a serious reverse-recovery problem, low efficiency, and electro-magnetic interference (EMI) problems [3,4]. Some converters such as flyback, forward, push-pull, half- or full-bridge can adjust the turns ratio of a transformer to achieve high step-up voltage gain. However, the main switch of these converters will suffer of high voltage spike and high power dissipation caused by the leakage inductor of the transformer. To improve these drawbacks, passive elements such as diode and capacitor circuits are introduced. However, the cost will be increased due to the extra power switch and volume [5].

To achieve large high step-up gain, a suggested architecture for the converters consists of using the secondary side of the coupled inductor as a flyback and a forward type. [10]

Also, many converters using the coupled-inductor technique are proposed to achieve high step-up gain. Several converters that combine the output-voltage stacking to increase voltage gain are proposed

[11]. The boost-sepic converter with the coupled-inductor and output stacking techniques has been proposed [12]. Converters with the coupled-inductor technique increase the voltage gain by adjusting the number of turn's ratio and adding extra winding stages. In this paper a specific analysis was done on a magnetic coupled inductor boost DC/DC converter (MCB converter) in order to determine the coupling factor influence as the coupling factor decreases with the insulation voltage and fabrication constraints [13,14].

Based on comparisons of simulation results with a discrete analysis, this work analyses the influence of the magnetic coupling coefficient on the coupling coils operations and on the efficiency of the converter. This study can allow a more useful way to develop new similar DC/DC converter systems. Finally, to obtain a high voltage gain and high efficiency, this paper proposes a parameters optimization affecting directly the recovery voltage on capacitor C in the DC/DC converter self powered MCB. Special analysis of the effects of coupling coefficient to the parameters related to the performance of the DC/DC converter is done.

2. Modeling of a self-powered DC/DC MCB converter

2.1. General description of converter

The circuit sketch of a high-efficiency self-powered high-step-up DC/DC converter is depicted in Fig.1, which contains five parts: (1) a DC input source is obtained from the photovoltaic panel or panels providing a dc energy to (2) a primary dynamic stage including the switch M (usually a MOSFET) and, via a (3) passive recovery circuit, is coupled to the (4) secondary-stage by the inductors; finally a filtering circuit (5) delivers the adapted voltage to the load or the HVDC bus.

A tracker implemented in a controller self-powered by the source determines the maximum of the available power from the source that can be delivered to the output.

Fig. 1. Self-powered high efficiency coupled inductor boost converter for photovoltaic energy conversion.

We have tested and present in the following, an analysis of the signals shape occurring in the operating mode of the converter and a possible technical solution for the self-powered circuit that could be implemented in a DC/DC converter for renewable energy production.

2.2. Simulation results of the overall converter behaviour

In the schema reported in Fig.1, leakage inductors are not represented but exist at each stage of the converter, usually designed by Lfl and Le at the primary and secondary stages, of the coupling inductor, respectively. Figure 2 shows the topological mode of this converter in one complete switching cycle Ts with a and a' the duration of the first and second mode.

The explanation of each mode is as follow:

Mode 1 [to, t1]: During this time interval, the switch M is turned on while the diodes Dc, Do are turned off. The primary-side current of the coupled inductor iLf1 increases linearly. The magnetizing inductor Lm1 stores its energy from the dc source Vin. Due to the leakage inductor Lf1, the secondary-side current of the coupled inductor iL2 equals zero. The output voltage equals the voltage bus HVDC.

Vcontrol

l0 tj t2 . t

Mode 1 Mode 2 Mode 3 Mode 4

Fig. 2. Key waveforms of the converter MCB at operation.

Mode 2 [ti, t2]: During this time interval, the switch M is turned off while diodes Dc, Do are turned on. The primary-side current of the coupled inductor iLf1 linearly decreases. The energies of the leakage inductor Lf1 and magnetizing inductor Lm1 are released to the clamp capacitor C. The primary and secondary windings of the coupled inductor, dc sources Vm, transfer their energies to the output HVDC bus. This operating mode ends when capacitor C starts to discharge at t = t2.

Mode 3 [t2, t3]: During this time interval, the switch M is turned off and diodes Dc, Do are turned on. The primary-side current of the coupled inductor iLf1 is continuously linearly decreasing but at a slower rate than mode 2. The primary-side and secondary-side windings of the coupled inductor, DC sources Vm, and clamp capacitor C transfer their energies to the HVDC bus.

Mode 4 [t3, t4]: The switch M and Dc are turned off and Do is turned on. The primary-side current of the coupling inductor equals zero. The secondary-side current equals the current of the capacitor C. Clamp capacitor C transfers their energy to the HVDC bus.

The voltage on the clamp capacitor C in a self-powered high efficiency coupled inductor boost converter can be modelled, during the charge and discharge processes with the various parameters influencing the efficiency of the converter by the calculation and simulation results. We give an equation (1) should say the relationship of stable voltage on the capacitor C with the coefficients and parameters to testing and selection for optimal conversion.

Equations and formulae should be typed and numbered consecutively with Arabic numerals in parentheses on the right hand side of the page (if referred to explicitly in the text),

Vc«ab= fct{k,m,a,Vjn)

In this equation, k is the coupling coefficient of the transformer, m the transformation ratio and a, already defined as the duration of the first mode of the total cycle, i.e. the duty cycle of the switch M. Thus, in order to optimize the energy transfer efficiency, we have done a complete rewriting of Eq. 1 (details of this equation is out of the scope of the present contribution and will be published in a further contribution) and analysed the influence of these parameters of the converter as function of the input voltage.

Fig. 3. Voltage on the clamped capacitor C versus the coupling coefficient k of coupling inductors of the converter, for a variable input voltage Vjn = (15-^40)V. For this simulation, the transformation ratio is fixed at a standard value m=7, a=0.5.

We have chosen to present, in Fig. 3 the influence of the coupling coefficient k on the voltage on the clamped capacitor when a variable input voltage Vm=(15^40)V is considered and for a transformation ratio fixed at a standard value m=7. This consideration is related to the coupling quality inductors. It is a key factor in the efficiency of the converter as its value induces huge changes in the voltage Vc and, as a consequence in the global behaviour of the converter. Therefore it directly affects the voltage on the switch M. As an example, for a fixed value of the input voltage, Vin=20V, the value of k in the quasilinear considered range (0.8^0.99) induces a change of Vc of around 150 V.

2.3. The self-powered circuit

The self-powered circuit is shown in Figure 4. The main component is a LM2576 regulator ideally suited for easy and convenient design of a step-down switching regulator. This circuit is designed for driving a 3.0A load with excellent line and load regulation.

+5V DC supply for the measure and controller 6

in Vout-<>

Ground

Fig. 4. Circuit of Self-powered assuming the electrical alimentation of a high efficiency coupled inductor boost converter for

photovoltaic energy conversion driver of the switch.

These regulators are available for fixed output voltages equal to 3.3V, 5.0V, 12V, 15V, and an adjustable output version. Since the LM2576 converter is a switch-mode power supply, which is designed in this application for driving the switch (MOSFET) of the converter, its efficiency needs to be significantly higher in comparison with popular three-terminal linear regulators, especially it regulates with ouput voltage constant when input generator voltages always changes. The second component is a LM7805 regulator ideally designed for applications as linear ballast regulator delivering a 5V output voltage. In the present application, this component supplies the power of the microcontroller driving the switch of the converter, and managing all the embedded system.

3. Conclusion

We have pointed out the advantages related to the integration in a renewable energy production system of a high-efficiency self-powered high-step-up DC/DC converter based on magnetic coupled inductor boost architecture. In this solution, the MOSFET used as a switch is subject to a low voltage and can be selected from a family of components with lower voltage allowing an increase of the overall performance of the converter. We have shown that it increases the energy transfer by reducing the duty cycle duration, which is an additional factor of improvement. In the framework of integration of such a converter in a renewable energy production system, for photovoltaic energy conversion, we have

presented a technical solution for a circuit of self-powered assuming the electrical alimentation of the coupled inductor boost converter. This study opens the way of new developments of efficient and low cost DC/DC converters and can allow a more useful way to develop new similar DC/DC converter systems.

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