Bidirectional DC-DC Converters (BDC) are commonly used in various applications due to the rapid growth of systems with the ability of bidirectional energy transfer among two DC buses. Along with the traditional applications such as DC motor drives, they have attracted a lot of applications in the area of the energy storage systems such as Hybrid Vehicles, Renewable energy storage systems, Uninterruptable power supplies, Fuel cell storage systems, battery chargers, battery charger/discharger for satellites and auxiliary power supplies etc.,. As a result, a DC-DC converter is always required to exchange energy between storage device and the rest of system. Such a converter should handle a bidirectional power flow capability in all the operating modes with flexible control. Thus the modeling and the control of bidirectional DC-DC converters is an important issue. In order to overcome these problems, solutions are proposed in this research work. This chapter discusses about the design of bidirectional converters.
1.1 BIDIRECTIONAL CONVERTERS
The basic model of bidirectional DC-DC converters circuit structure is illustrated in Figure 1.1. According to the position of the energy storage device, the BDC can be classified into two types namely buck mode and boost mode converter. Under the buck mode of operation, the energy storage device is sited on the high voltage side whereas in the boost mode of operation, it is sited on the low voltage side.
Figure 1.1 Illustration of bidirectional power flow
Basically they are divided into two types namely,
? Non-isolated Bidirectional DC-DC Converters and
? Isolated Bidirectional DC-DC Converters.
Non-isolated Bidirectional DC-DC Converters
A non-isolated bidirectional DC-DC converter represented in Figure.1.2 is derived from the unidirectional DC-DC converters. In this mode, the conduction capability (unidirectional) of the conventional converters is exchanged by means of switches which has bidirectional conduction.
Thus the diodes existing in the conventional buck and boost converter circuits do not have the bidirectional power flow capability. This limitation in the conventional boost and buck converter circuits can be overcome by introducing a Power MOSFET or an IGBT having an anti-parallel diode across them. These devices acts as a bidirectional switch and allows current conduction in either directions. At the same time, the switching operation of these devices can be controlled.
Figure 1.2 Basic Structure of NBDC
The operation of the NBDC is as follows. The inductor present in this converter plays a vital role in energy transfer. During every cycle of switching, it is charged by the active switch located at the source side for the time period, Ton=DT,
T – switching period
D – duty cycle.
This energy stored in the inductor is discharged to load during Toff = (1-D) T. Synchronous rectification technique can be employed so as to improve the efficiency of the converter by adding more features to the configuration.
Isolated Bidirectional DC-DC Converters (IDBC)
The structure of an IBDC is presented in Figure 1.3. It comprises two high-frequency switching DC-AC converters. In order to maintain galvanic isolation between these two sources, a high-frequency transformer is instigated. If there exist a large voltage ratio between two sources, the transformer is utilized to maintain voltage matching between them. As energy transfer in either direction is required for the system, each DC-AC converter must also have bidirectional energy transfer capability.
Figure 1.3 Basic structure of an IBDC
Thus the summary of DC-DC converter topology so far developed is illustrated in this section.
Poon and Pong (1996) formulated a DC–DC converter based on Zero Voltage Switching (ZVS) technique. Amplitude modulated square wave is implemented as pulse generator for the converter. Thus the proposed converter results in simple arrangement and in-built ZVS characteristics.
Robert Watson et al (1996) designed a ZVS active clamp fly back converter and is operated with unidirectional magnetizing current. From the analysis, it is concluded that the designed fly back is more attractive. It is relatively very simple when compared with the other topologies implemented for low power applications. Apart from that, the incorporation of the active clamp circuit with the fly back topology enables the recycling of transformer leakage energy which in turn reduces the voltage stress of the switching devices.
Marcelo Lobo Heldwein et al. (2000) focussed on the design of clamping circuit for the ZVS PWM asymmetrical half bridge DC–DC converter. This clamping circuit helps in the reduction of oscillations which is caused due to the reverse recovery of the output diodes. Hence efficiency of the converter is increased.
Hua Bai et al discussed about the short-time-scale transient procedures in an isolated bidirectional DC–DC converter along with phase shift control. In this study, the effects of dead band are analyzed under steady-state and transient condition. The analysis of variations occurred in the current due to phase-shift errors at the boundary conditions were studied through both simulation and experimental results.
Nagaraj et al designed a new step-up isolated DC-DC converter. This converter is designed with a reduced number of switching devices. Hence this converter provides better reliability and high efficiency.
Premananda Pany et al proposed a new isolated DC DC conveter topology which is suitable for electric vehicle applications so that it should be operated in three different modes namely: acceleration mode, normal (steady-state) mode and braking (regenerative) mode. However the designed converter performs well, it is bulky and costly to implement.
A non-isolated high step-up DC-DC converter using ZVS obtained with boost integration technique and their light-load frequency modulation control integrates a bidirectional boost converter with a series output module as a parallel-input and series-output configuration (Hyun-Wook Seong et al. 2008). The bidirectional isolated DC-DC converter controlled by phase-shift duty cycle obtains the resonant switching condition with minimum number of switches mentioned by Z. Zhang et al. (2009).
A soft switched dual half bridge bidirectional DC-DC converter proposed by Hamid et al. (2011) with the soft switching range improves the efficiency of the converter.
In order to reduce the switching loss, a new non-isolated ZVS bidirectional DC-DC converter with minimum auxiliary elements was proposed by Delshad et al. (2017).A bidirectional DC-DC converter for low power applications with a half-bridge on the primary and a current-fed push-pull on the secondary side of a high frequency isolation transformer was presented by M. Jain et al. (2000).
Based on the application requirements, the design of bidirectional converter can be categorized into various types. An averaged model medium power IBDC was developed by Hui Li et al. (2003). This can be utilized for medium power applications. In order to reduce both conduction and switching loss in a converter, PWM plus phase-shift controlled BDC with ZVS range was proposed by Dehong Xu et al. (2004). A novel NBDC with excellent features such as low switching loss, operating with continuous inductor current and constant switching frequency irrespective of power flow direction was designed by Pritam Das et al. (2009). A two stage DC-DC converters with an input series connection is proposed to reduce the stress of the component (Qian et al. 2010).
A novel integrated bidirectional AC-DC charger and DC-DC converter for hybrid/plug-in-hybrid conversions formulated by Y. J. Lee et al. (2009) has high switching losses due to increase in number of switches. In order to overcome this problem, two different topologies such as an independent topology and a combination topology is utilized to drive inverter of the motor. These are implemented in HEV applications.
A multi power-port topology with high gain, wide variations in load, less ripple in the output current and competency of parallel battery energy storage is designed by Tanmoy Bhattacharya et al. 2009.
The converters are also designed according to the type of batteries used in the system. For lithium-ion battery application, a BDC based on time delay control has been designed by Y. X. Wang et al. (2014).
A control technology proposed by Jian Min Wang et al. (2011) minimizes the conduction losses in the converter thereby increases the system efficiency.
The medium power isolated bidirectional DC-DC converters is investigated for various applications such as interfacing RES to utility grid, HEV and UPS systems by Hamid et al. 2011.
Zhe Zhang et al. (2012) designed a new BDC applicable for HEV. In this topology, the converter switching pulses are controlled by phase-shift and duty cycle.
D. Urciuoli et al. (2006) designed three-phase BDC for hybrid electric vehicles.
The optimal design and control of a 5kW PWM pulse-phase shift control bidirectional DC-DC converter proposed by Lei Shi et al. (2006).
Rong-Jong Wai et al 2007 proposed a high efficiency DC-DC converter with high voltage gain and reduced switch stress. In this work, a three winding coupled inductor is implemented to obtain a high voltage gain. The reverse recovery current of the output diode is reduced using coupled inductor.
Research was also carried on the comprehensive procedure for testing, modeling and control design of a fuel cell hybrid vehicle (M. Joong Kim et al. 2005 and L. Serrao et al. 2008). All these above discussed converters possess problem in stability control of the system. To solve this stability problem, the designed system should be operated in controlled mode. Hence Jonathan et al. (2008) proposed a superior ultra-capacitor voltage control algorithm for attaining high efficiency at the converter output. At the same time, a hierarchical predictive control strategy is designed to optimize simultaneously both the power utilization and oxygen control in a hybrid proton exchange membrane of a fuel cell/ultra-capacitor (Chen et al. 2009).
The converter topologies with the capability of bidirectional power flow and electrical isolation between the primary and secondary sides through a single transformer for fuel cell applications was evaluated (K. Wang et al. 1998). A DC-DC converter structure which fulfills the requirements for very low input and high output voltage ratings was proposed by K. H. Edelmoser et al. (2004). An isolated full bridge converter in which ZVS and ZCS are achieved by adding active clamping circuits to improve the performance of the bidirectional converter studied by Rongyuan Li et al. (2004). A new converter topology proposed by Huang-Jen Chiu et al. (2006) which achieves bidirectional power flow capability, soft switching without any additional devices, easy control, improved efficiency and reliability are the special features of the proposed converter.
A hard switched bidirectional flyback converter was modified into soft switched by adding an additional circuit explained by Henry Shu-Hung Chung (2004). A half bridge topology with advantages such as reduction in physical size, increase in power density and lower wiring costs was proposed (Jess Brown 2006). The operating principles and different switching modes of the dual H-bridge based DC-DC converter were analyzed (C. Mi et al. 2008). Watanabe et al. (1995) implemented and tested the buck/boost DC-DC converter using nMOSFETs. Dancy et al. (1997) designed an ultra-low power control circuit for PWM converters. Boudreaux et al. (1997) designed and simulated the converter controlled by an 8-bit microcontroller. Tarun Gupta (1997) proposed a DC-DC converter controlled by an 8-bit microcontroller which regulates the output voltage of buck/boost converters to a desired value without steady-state oscillations, despite a change in input or output.
Milan and Yungtaek Jang (1999) designed an active snubber for the boost converter to improve its performance by reducing the reverse recovery problem in the boost converter with a minimum number of components. Aleksandar et al. (2001) explained the design of a digital PWM controller for a buck converter operating at 1MHz. So et al. (1995) implemented a digital controller using a fixed point signal processor and also demonstrated the controller capability in regulating high speed switching converters. Patella et al. (2003) designed a high frequency digital PWM converter. Qi feng et al. (2006) analyzed and designed a microcontroller-based digital controller developed for a buck converter.
Chok-You Chon (2007) developed a hardware setup for a boost DC-DC converter and the non-linear controller using analog circuits. Li Peng et al. (2007) developed the double pulse width-modulation techniques which overcome the problem with microprocessor-based high frequency (PWM) converters, which had the modulating resolution limitation caused by hardware timers limited-time resolution. Philip (2007) explained the three generations of digital control; First-generation digital control: Digital processing outside a control loop, in a management or supervisory role. Second-generation digital control: digital processing inside a control loop. Third-generation digital control: digital processing was responsible for the moment-by-moment direct action of active switching devices in a converter. This control methods are also discussed which optimize the losses and takes the advantage of non-linear control methods for power converters.
Jianhua Geng (2007) presented the sliding mode control theory and its application on the buck converter, and also brought forward a novel method of designing the sliding mode controller. The new method considered the system and designed the controller in three-dimensional space. This method considered the information of all elements in the buck circuit adequately. Mariko Shirazi et al. (2008) demonstrated the effectiveness of testing fully automated capability of frequency measurement in digital PWM controllers with relatively low cost.
Lee et al. (2000) presented a converter consisting of two interleaved and inter-coupled boost converter cells which was modeled to match the current sharing between two converters. This converter is designed to have a high performance of current sharing characteristics and low output current/voltage ripple. De Oliveira Stein et al. (2002) investigated a power factor correction based on boost converters in Current Conduction Mode (CCM) with a Zero Current Transition (ZCT) circuit. During the turn off position of the main switches, the use of auxiliary circuits provides ZCT thereby minimizing the related turn-off losses.
Jain et al. (2004) presented a Zero Voltage Transition (ZVT) boost converter with an additional circuit provided for soft switching. This helps in subsequent reduction in conduction losses. Yungtaek et al. (2007) revealed a two-inductor, interleaved power factor correction boost converter with voltage-doubler characteristics. Gang Yao et al. (2007) presented a two interleaved boost converters with two simple additional circuits for soft switching. This additional circuit is connected in parallel to the main switches which realizes ZCS during turn-on and ZVS during turn-off of the main switches. It had considerably reduced the switching loss and thereby increased the efficiency.
Li and He (2007) presented an interleaved winding coupled boost converter, to obtain ZCS turn-on condition and also high voltage gain. Silva et al. (2008) explained a soft-switching converter with high voltage gain. This converter proposed is suitable for renewable energy systems. Dong Wang et al. (2008) analyzed a converter with coupled inductors and switched capacitors. The voltage double function is realized by switched capacitors which in turn increases the voltage gain and at the same time greatly reduces the switch voltage stress. Therefore, this topology proposes high efficiency. Huber et al. (2008) thoroughly analyzed four open-loop interleaving methods for boost converters. Hsieh et al. (2009) illustrated a soft-switching converter composed of two parallel elementary boost conversion units and a resonant inductor. This converter is capable of switching on both the power switches at zero voltage condition.
Kosai et al. (2009) designed analytically a magnetically coupled converter under continuous inductor current mode operation. The front-end inductors in the converter are coupled magnetically in order to improve its electrical performance. Pan et al. (2009) analyzed a two-phase interleaved boost converter with low stress and automatic capability of current sharing used for single-phase power factor sharing capability. Kosai et al. (2009) described and validated the analytical design relationships for boost converters using coupled inductors. Gallo et al. (2010) illustrated a passive soft switching lossless snubber circuit applied to the converter. The main switches are turned on and off under null current and null voltage conditions, respectively due to an additional circuit. This results in high efficiency over the entire load range.
Barbosa (2011) described an efficient ZVT based converter in which the soft switching technique helps to reduce the switching losses that could reduce the amplitude of net ripple. This in turn increases the ripple frequency of the whole converter system. Chen et al. (2012) demonstrated a BDC with ZVS and ZCS characteristics. Tseng et al. (2009) presented an interleaved boost converter with coupled inductor for Photovoltaic power system. The converter separately adopts coupled inductor and interleaved manner to achieve high step-up voltage ratio and powering capability and the same was implemented by Jung et al. (2011).
Rahavi et al. (2012) proposed a BDC topology for renewable energy applications. Sravan et al. (2012) illustrated an analytical analysis and design of an auxiliary inductor that is used to reduce the switching loss and switching stress of the converter in grid connected systems. Anushya and Raja Prabhu (2012) experimentally verified the modeling and simulation of an interleaved converter fed DC drive system. The topology is used to increase the efficiency of the AC-DC converter and reduces the switching losses by adopting a resonant soft switching method. The application of BDC in power conversion is proposed by Chen et al. (2017).
Mohammadi and Farzanehfard (2012) presented a design of zero voltage conversion bi-directional converters using coupled inductors and two additional switches. The voltage and current stresses on the switching devices is minimized with the help of additional switches.
Changchien et al (2010) formulated a converter comprises multi-winding coupled inductor with a voltage doubler circuit. This converter makes use of energy stored in the leakage inductor and hence the voltage stress on the switching devices are diminished and thus the efficiency is improved.
Chen.S.M et al. (2011) proposed a quadratic boost converter with the coupled inductor which achieves high voltage gain with suitable duty ratio and small voltage stress on the switch. Also, the energy stored in a leakage inductor can be reused. Highest efficiency has been derived while setting the low ON-state resistance RDS and employing soft switching on the switch.
Mahesh et al proposed a converter that employs a coupled inductor with same winding turns in the primary and secondary sides. The primary and secondary windings of the coupled inductor were operated in parallel charge and series discharge to achieve high step-up voltage gain in step-up mode. The primary and secondary windings of the coupled inductor were operated in series charge and parallel discharge to achieve high step-down voltage gain in step-down mode. Therefore the proposed converter has higher step-up and step-down voltage gains than the conventional bidirectional DC-DC boost/buck converter. The average value of the switch current in the proposed converter was less than the conventional bidirectional boost / buck converter, under same electrical specifications for the proposed converter and the conventional bidirectional boost / buck converter.
Thus from the above survey, it is clearly observed that the switching device of these converters undergoes high voltage stresses. Hence to get rid this said problem, this work proposed a design of cascaded non isolated bidirectional dc-dc converter with switched coupled inductor. This proposed topology with ZVS has the competency to decrease switching losses when compared with the conventional converters. These topologies may be able to achieve higher efficiency at higher switching frequencies.
Various kinds of bidirectional converters implemented in practical application has less voltage gain and a high inductor rating. Among those design of bidirectional converters, the bidirectional dc-dc converter along with coupled inductor decreases the cost and also improves efficiency of the system. It also enhances the performance of the system. So far numerous topologies for bidirectional converters with a coupled inductor have been reported in literature survey, they possess some drawbacks such as low voltage gain etc. Hence, the main focus of this thesis is to design and analyze an efficient coupled inductor based non-isolated bidirectional dc-dc converter to fulfill the requirements of soft switching and voltage gain improvement. In order to achieve this, a new Cascaded clamp circuit has been developed so as to improve the voltage gain and regulation of the converter.
Emerging applications driven by low voltage level power sources, like photovoltaic, batteries and fuel cells require static power converters for appropriate energy conversion and conditioning to supply the requirements of the load system. Theoretically, the conventional boost and buck-boost converters are the simplest non-isolated topologies for voltage step-up. However, these converters typically operate under extreme duty ratio and undergo severe losses to achieve high voltage gain. To override this problem, this work presents, analysis and design issues of the cascaded non-isolated Bidirectional DC-DC converter with a switched coupled inductor. The proposed innovative solution achieves significant performance improvement compared to other recently proposed topologies.
It is evident that the conventional bi-directional converter suffers from
• More voltage stress due to hard switching mechanism,
• High energy loss due to leakage inductance and less step-up ratio.
• Power loss occurs due to thermal resistance and junction temperature of the power devices used. The output voltage of the converter is decreased due to increasing the junction temperature of the MOSFETs. The continues charging and discharging process is also given more temperature rise in the battery. The output voltage gets variations due to change in thermal resistance in the system. This thermal resistance variation increases the loss and reduces the system efficiency and reliability.
• Open loop system has no self-regulation (voltage regulation) or control action over the output value changes.
• The output voltage is not maintained constant due to an absence of feedback system in an open loop system. As a result, the system stability is disturbed, or system performance is affected. So the closed-loop system needs to maintain the constant output and system stability.
CONTRIBUTIONS OF THE THESIS
Thus the main contributions of this research work is summarized as below:
• To propose a new Switched coupled inductor based cascaded non isolated bi-directional dc-dc converter to overcome the drawbacks of the conventional converters and to improve voltage regulation. The voltage gain of the projected system is improved with the help of switched coupled inductor. The cascaded arrangement is utilized to improve the voltage regulation of the converter.
• To incorporate different types of both conventional and artificial intelligent technique based controllers such as PI controller, fuzzy logic controller and ANFIS and compare their effectiveness. The control system operation was simulated using the MATLAB/Simulink power system toolbox and the performance of this approach is validated with laboratory low voltage prototype to verify the efficiency of this method.
• To analyze the stability of non-isolated bi-directional DC-DC converter under input and load disturbance conditions. In case of open loop system, the stability analysis is carried out using a state-space averaging method and time domain analysis. In case of closed loop system, Time domain and frequency analysis are implemented.
• To carryout thermal analysis to maintain the stable output voltage.
ORGANIZATION OF THESIS
Chapter 1 presents the general overview of Bidirectional converters, background and motivation behind the research. Finally, contributions and organization of the thesis are outlined.
Chapter 2 gives a design and development of proposed non isolated Bidirectional DC-DC converter, modes of operation, performance evaluation and soft switching.
Chapter 3 addresses the performance and implementation of the PID, Fuzzy Logic, Neuro and ANFIS controllers to the proposed system.
Chapter 4 deals with optimization technique based controllers such as BBO, GWO, hybrid GWOGA and its implementation to the proposed system.
Chapter 5 elucidates the stability analysis of non-isolated bidirectional DC-DC converter in open loop and closed loop using MATLAB with the help of root locus and bode plot.
Chapter 6 focuses on the effects of the thermal resistance on the output voltage of proposed converter both in open and closed loop control.
Chapter 7 furnishes the summary of conclusions obtained from the present research along with the suggestions and future scope of the present research.