Analysis and Design of High Effeiceny ZCS Buck (PWM) Converter in battery charger


Master's Thesis, 2014
70 Pages, Grade: Master degree

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Table of Contents

Abstract

Chapter 1: Introduction
1.1 Objective
1.2 Scope
1.3 Methology
1.4 Problem Satement
1.5 Background
1.6 Proposed Project
1.7 Outline of the Thesis

Chapter 2: Literature and Review
2.1Battery Charger System & Capacity Demand
2.2 Methods of Battery Charger System & its Topologies
2.3 Battery Charger Systems Functional Components
2.4 Resonant Converter
2.4.1 ZC Resonant Switch
2.4.2 ZV Resonant Switch
2.5 Switching Techniques
2.5.1 Hard switching and Soft Switching Techniques
2.5.2 Classifications
2.5.3 ZCS Tecniques
2.5.4 Why Choice for ZCS
2.5.5 Comparisons Between ZCS and ZVS
2.6 DC-DC Buck Converter
2.6.1 Buck Converter Operation
2.7 Quasi-Resonant Converters
2.7.1 ZCS- Quasi-Resonant Buck Converters
2.7.2 ZVS- Quasi-Resonant Buck Converters

Chapter 3: Circuit Analysis Description of Novel Battery Charger
3.1 ZCS Resonant Buck Converter
3.2 A Novel Buck ZCS PWM Converter for Battery Chargers
3.3 Operation Principle
3.4 Mode of Operation
3.5 The Output Voltage Gain
3.6 Normalized Voltage Gain

Chapter 4: Design Consideration & Calculation
4.1Design Methology
4.2 Practical Calculations of Novel Charger
4.3 Duty Cycle of Novel Charger

Chapter 5: Simulation and Experiment Results

Chapter 6: Concolusion

Aknowledgment

Reference

Appendix-A

List of Figuers

Figure 1-1: Research Methology of Project

Figure 1-2: Block Diagram for the Proposed Novel Battery Charge

Figure 2-1: Battery Capacities of Various Battery-Powered Devices

Figure 2-2: Structure of a multi- piece battery charger system. The efficiency calculation is made over a 24 hour charge and maintenance period and a 0.2 C discharge for the battery. (Prepared for California Energy Commission Contract by EPRI Solution Ltd.,)

Figure 2-3: Switch Mode Battery Charger Power Visibility

Figure 2-4 Power Tool Efficeincy Comparison

Figure 2-5 Zero-current (ZC) resonant switch

Figure 2-6 Zero-voltage (ZV) resonant switch

Figure 2-7 Typical switching trajectories of power switches

Figure 2-8 Typical switching waveforms of (a) hard-switched and (b) soft-switched devices

Figure 2-9 Classification of resonant Type DC-DC Converteres

Figure 2-10 Buck Converter with Load Resistor

Figure 2-11 Step down Switch mode Power supply

Figure 2-12 Buck Converter Topology

Figure 2-13 Half-wave, quasi-resonant buck converter with ZCS

Figure 2-14 Full-wave, quasi-resonant buck converter with ZCS

Figure 2-15 A family of quasi-resonant converter with ZCS

Figure 2-16 Half-wave, quasi-resonant buck converter with ZVS

Figure 2-17 Full-wave, quasi-resonant buck converter with ZVS

Figure 2-18 A family of quasi-resonant converter with ZVS

Figure 3-1 Traditional ZCS Resonant Buck Converter

Figure 3-2: Proposed a Novel ZCS PWM Converter dc-dc Buck for Battery Charger

Figure 3-3: Key waveforms of the proposed novel charger

Figure 3-4 Equivalent Circuit of ZCS PWM Converter dc-dc Buck

Figure 3-5 The Mode I equivalent circuit of the novel charger

Figure 3-6 The Mode II equivalent circuit of the novel charger

Figure 3-7 The Mode III equivalent circuit of the novel charger k

Figure 3-8 The Mode IV equivalent circuit of the novel charger

Figure 3-9 The Mode V equivalent circuit of the novel charger

Figure 4-1 Normalized Load Characteristics curve (Versus M and fns) for novel battery charger

Figure 4-2 Practical Circuit Protype of Novel Battery Charger

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Figure 5-6 Voltage Curve during the Charging Period

Figure 5-7 Charging Current during the charging period

Figure 5-8 The Charging efficiency Varition Curve of the Novel Charger during the charging period

ACRONYM LIST

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Chapter 1: INTRODUCTION

This chapter briefs the overview of novel battery charger system including project background, objectives, scopes, methodology and outline of the thesis. It explains the description of the battery charger system, methods, topologies, ZCS resonant dc-dc buck converter and whole block diagram of propose novel battery charger. At the end, outline of this thesis is given in this chapter.

1.1 Objective

The main objective of this thesis is to develop a novel high-efficiency battery charger with ZCS PWM buck topology which has simple circuit structure, low switching losses, easy control and high charging efficiencies. Beside, this project is about to investigate the action of efficient performance in charging shaping to gain the high output charging efficiency of the battery charger [45], [53].

1.2 Scope

The scopes of this project are:

- Study of Battery Charger
- Study the operation of buck converter.
- Study the operation of PWM step down techniques [53].
- Design the buck converter power stage circuit.
- Design the PWM controller stage circuit.
- Simulation of ZCS PWM buck converter by using MATLAB software’s
- Testing and calibration of the completed ZCS PWM buck converter with battery to confirm the actual response with the theoretical predictions [45]
- Observation of various voltage, current waveforms through inductor, capacitor of the converter
- published a Research article in International journal according to the project work

1.3 Methodology

This chapter discusses the research methodology and procedures as well as equipments and software’s in the entire and intial work process. The methodology describes how the flows of the project work and procedures the project topic was divided into different phases-sections and how to the work involved in each phase. The work schedule topic mentioned about the use of Gantt charts for the project schedules and the equipment and software topic describes about the equipments and software’s used when the project was carried out.

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Figure 1-1: Research Methology of Project

Fig.1-1 above shows that the first step in this project was focused on research study of design the circuit diagram and components value for buck converter configuration. During this step, the components value was calculated using established equations and formulas. Then models of buck converter and its PWM controller were built and simulated using MATLAB software’s. The output voltage and out put current of a noval battery charger ZCS PWM resonant buck converter were analyzed and compared with the earlier theoretical predictions [53].

1.4 Problem Statement

Today’s technologies have shown a drastic extreme changing in all section due to its developments and achievements. Many systems had been created for this purpose. In the battery industries, there are lot of battery charger that been developed to drive a good charging process [42]. However there are still many chargers that are not suitable to use that may damage the battery itself or the user. A bad charging process may shorten the lifetime of the battery and more dangerous is the battery may explode [42]. In order to achieving high efficiency in battery charger, append the traditional battery charger with the technique of ZCS ( Zero-Current- Switching) resonant buck topology which delivered the efficient performance in charging shaping. Accordingly, a ZCS converter with a wide input voltage or load range has a wide frequency range. This work presents a ZCS PWM converter for battery charger to solve the foregoing problems [1].

1.5 Background

Batteries are extremely convenient energy devices that continuously switch between charge and discharge modes and can be used repeatedly many times [2], [3]. Batteries cause little environmental pollution and are much more widespread than conventional dry cells that must be disposed of when depleted. Batteries are employed everywhere in daily life, in energy storage, lighting, household appliances and portable electronic devices. They also provide power for vehicles, boats, ships and airplanes. Saving electric energy is important [43]. Although traditional buck converters can meet the requirements of simplicity of structure, high efficiency and reduced voltage, they can be operated only with low-frequency switching. The switching frequency of switches must be increased to reduce the circuit volume, producing excessive switching losses. However, such an increase also substantially reduces the overall efficiency of the buck converter. Accordingly, a buck converter is at best only a highly efficient buck charger for use in low-frequency switching and is not suitable for high-frequency switching. Conventional buck converters are sufficient for some industrial applications. However, the stringency of requirements associated with high-frequency switching is increasing with rapid industrial development. Resonant approaches for turning switches on or off under zero-voltage or zero-current conditions are then applied to minimise the overall efficiency reduction that is associated with severe switching losses that are caused by high-frequency switching in traditional buck converters. The buck zero-current-switching (ZCS) converter is adopted to charge the battery, clearly increasing the switching frequency of the switches [50]. The resonant approach permits the use of switches under zero-current conditions, not only reducing the circuit volume, but also preserving the high efficiency of a traditional buck converter. However, the circuit that charges the battery still raises a difficulty: since only one switch is used, a high charging current increases the circuit resonance. Under the enlarged resonance, the switch components have a shorter service life because they have to endure higher temperatures. Additionally, the battery charges the resonant capacitor, whereas the buck ZCS converter charges the battery before the switches are turned on. The electric charge that remains on the resonant capacitor is present before the switches are turned on, and the remnant electricity affects the accuracy of the circuit resonance. The charge mode significantly affects the battery life and capacity. Charging methods are utilised to return energy to a battery so that it can continue to function as a normal power supply to a load [52]. The conventional battery charger with a large volume suffers from power dissipation during charging and is well known to have poor efficiency [51]. Choosing the best charging method is essential to increase the charging efficiency and extend the battery life. The traditional hardswitching pulse-width-modulated (PWM) converters are still extensively used in battery chargers. However, the traditional hard-switching PWM rectangular voltage and current waveforms cause turnon and turn-off losses that limit the operating frequency. The inability of traditional hard-switching PWM battery chargers to operate efficiently at very high frequencies imposes a limit on the size of the reactive components of the charger circuits, thereby on power density. Seeking chargers that are capable of operating at higher frequencies, power electronic engineers have begun to develop charger topologies that shape either the sinusoidal current or the sinusoidal voltage waveform, markedly reducing the switching losses. The underlying idea is to use a resonant circuit. Unlike traditional hard-switching PWM battery chargers, resonant chargers can be designed to be small and low weight. Numerous resonant schemes have been proposed to improve the switching behaviour of the battery chargers [53]. Among various resonant topologies, the zero-voltage-switching (ZVS) and ZCS can be applied to eliminate switching losses. The voltage of the power switch in a ZVS battery charger is set to zero prior to turn on, eliminating turn-on switching losses and the Miller effect. However, the traditional ZVS procedure has many limitations. First, the power switch in a ZVS converter suffers from high voltagenstress which is proportional to the load range. Hence, a high voltage power switch accompanied by high onresistance and large input capacitance must be used, substantially increasing conduction losses and gate driver losses. Second, a wide switching frequency range is required for a ZVS converter to operate with wide input voltage and load ranges. The wide frequency range makes difficult the optimisation of the input filter, the output filter, the control circuit and the driver circuit. However, the ZCS method eliminates the voltage and current overlap by forcing the power switch current to zero before the switch voltage rises. Therefore the ZCS converter is deemed more effective than ZVS in reducing switching losses. For high-frequency applications, the ZCS converter is the most commonly used. However, one of the major limitations of the ZCS converter is that high circulating energy is produced by the resonant inductor, which is in series with the power switch. The second shortcoming is severe parasitic ringing on the power switch. The third shortcoming is a variable frequency operation, since the ZCS converters operate with constant on-time control. The proposed battery charger adds an auxiliary switch to the resonant loop, dominating the resonance time accurately. In the ZCS PWM converter, resonance does not occur between the inductor and the capacitor until the auxiliary switch is turned on; then, the inductor begins to resonate with the capacitor. After the resonance brings the inductor current to zero, the main power switch is turned off with ZCS. Hence, the function of the auxiliary switch is to hold off the resonance for a period. The on-time of the power switch can be varied by controlling this hold-off period, enabling the ZCS PWM converter to regulate the output while the circuit is operated at a fixed switching frequency.

1.6 Proposed Project

In recent years, with the enhancement of power electronics technology and control strategies in power electronics devices coupled with the increasing demand of high efficiency in battery charger system has invoked enormous attention from the research scholars around the world [61], [62]. Battery charger system technology is currently being incorporated in urban industrial areas to maintain with these demands lot of work is on towards. Therefore, many battery chargers with different ratings and functionalities are being developed for high output efficiency since few years [51], [52], [54]. The battery charger usually works to globalize the energy saving and to serve in fast transportation systems. The use of battery charger brings convince life solution during the traveling from urban to rural areas. Many techniques were fetched out by the scientists since battery charger device was developed for renewable energy generation, electronic communication power supplies, electric vehicles, UPS or an uninterruptible power supplies, PV systems and portable electronics products. Many charging methods have been developed to improve the battery charger efficiency in the last few decades. In order to achieving high efficiency in battery charger, append the traditional battery charger with the technique of ZCS ( Zero-Current- Switching) resonant dc-dc buck topology which delivered the efficient performance in charging shaping [4],[5],[6],[7].

This work looks at the issues which associates ZCS PWM (Zero-Current-Switching Pulse width Modulation) converter, buck topology with the battery charger. This paper develops a novel high-efficiency battery charger with ZCS PWM buck topology which has simple circuit structure, low switching losses, easy control and high charging efficiencies [1],[8]. Zero Current Switching resonant buck converter is analyzed and mode of operation is also studied. Various waveforms & charging curve period were noted down during the piratical examine using MATLAB software. The curve of charging efficiency during the charging period shows 89% charging output efficiency of novel proposed prototype [45]. The proposed novel charger not only provides the advantages of both hard switching and resonant converters but also further yields a constant-frequency control, reducing the resonant time. The switch components in the novel charger are all operated at zero current, yielding high charging efficiency and large switching loss reduction.

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Figure 1-2: Block Diagram for the Proposed Novel Battery Charge

This paper uses a buck ZCS PWM converter in battery chargers [53]. Fig. 1-2 displays the basic configuration of the developed charger circuit, including (1) a dc source, (2) a PWM control circuit to control the switching of switches in the buck converter, and (3) a buck ZCS resonant converter. In addition to having safe operation and small-sized circuit volume, the developed novel charger has a very high efficiency, and controls the resonance time without restraint at a constant frequency.

1.7 Outline of the thesis

This thesis consists of 5 chapters. In the first chapter, it discusses and describes the methodology, equipments of project and block diagram. It gives introduction, background, project objects, scopes and outline thesis. In Chapter 2, literature reviews and theories on Battery chargering system, their topologies, campacity demand and resonant, power converter, ZCS switching converter are discussed & PWM technique and step down buck operations and are discussed while introduction about buck converters, ZCS resonant buck converter design, A Novel Buck ZCS PWM Converter for Battery Chargers design & control scheme and strategies, modes of operation, normalized voltage gain as well as output voltage gain are discussed in chapter 3. In the chapter 4, deign methology of novel battery charger, practical calculations of noval charger and duty cycle of novel chager are appened in detailed. Chapter 5, Topic covers discussion of simulation and results. It gives the result information of software with observation of waveforms novel battery charger. The suggestions and conclusions of efficiency of charger obtained upon successfully completing this research work are given. Finally, the last part in the thesis provides the conclusions, references, Acknowledgement and appendices appened in the project as well. The appendics of thesis is mentioned the master degree publication upon the thesis is written on the behalf.

Chapter 2: LITERATURE & REVIEW

2.1. Battery Charger System & Capacity Demand

Today’s most modern electrical appliances receive their power directly right away the utility grid. Many devices are being developed everyday which requires electrical power from the batteries in order to achieve large mobility and greater convenience [42].

The battery charger system utilizes the battery by working to recharge the battery when its energy has been drained. The uses rechargeable batteries include everything from low-power cell phones to high-power industrial fork lifts, and other construction equipment. Many of these products are used everyday around-the-clock commonly in offices, schools, and universities, urban and civilian areas [9], [10]. In fig. 2-1 shows that the Battery Capacities of Various Battery-Powered Devices which are used in different rate of watt per hours level in cell phones, laptops, power tools, forklifts and golf crafts etc.[11].

The term “battery charger systems” refers collectively to battery chargers coupled with their batteries. Battery charger systems include, but are not limited to:

- Electronic devices with a battery that are normally charged from ac line voltage through an internal or external power supply and a dedicated battery charger;
- The battery and battery charger components of devices that are designed to run on battery power during part or all of their duty cycle (such as many portable appliances and commercial material handling equipment);
- Dedicated battery systems primarily designed for electrical or emergency backup (such as emergency egress lighting and uninterruptible power supply (UPS) systems);
- Devices whose primary function is to charge batteries, along with the batteries they are designed to charge [51], [53]. These units include chargers for power tool batteries and chargers for automotive, AA, AAA, C, D, or 9 volt rechargeable batteries, as well as chargers for batteries used in motive equipment, including golf carts, electric material handling equipment, lift-trucks, airport electric ground support equipment (EGSE), port cargo handling equipment, tow tractors, personnel carriers, sweepers and scrubbers [41], [44].

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Figure 2-1: Battery Capacities of Various Battery-Powered Devices

A battery charger system is a system which uses energy drawn from the grid, stores it in an electric battery, and releases it to power device. While engineers are used modern techniques to usually design the battery charger systems, which maximize the energy efficiency of their devices to make certain long functioning & operation time between charging; however they often neglect how much energy is used in the conversion process of ac electrical power into dc electrical power stored in the battery from the utility grid [42]. Apparently, energy savings can be possible if the conversion losses are reduced which associated with the charging batteries in battery-powered products & output voltage can be controlled via switching frequency [59], [60]. We can achieve these savings using different techniques including battery charger topology that is readily available today and is being employed in existing products [57]. The same technique and topology is discussed in this paper which increases the efficient performance in charging shaping of novel battery charger. Batteries cannot be charged simply by connecting them to a standard wall outlet. A series of power conversion steps must be performed to shape the high-voltage ac electricity from the utility into low-voltage dc electricity that can be accepted by the battery, and the charging process must be controlled so that the battery receives the appropriate amount of current. Battery chargers accomplish all this through three functions: 1) reducing voltage from the utility level to the lower voltage at which batteries operate, 2) rectifying ac electricity into dc electricity, and 3) controlling the low-voltage dc current into the battery. The first two stages are functions typically incorporated into ac-dc power supplies.1 The addition of the third stage – controlling the rate of charge of the battery with charge control circuitry – is typically what distinguishes a battery charger from an ac-dc power supply [57], [59], [60]. These power supply and charge control circuitry subsystems are illustrated in Fig. 2-2.

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Figure 2-2: Structure of a multi- piece battery charger system. The efficiency calculation is made over a 24 hour charge and maintenance period and a 0.2 C discharge for the battery. (Prepared for California Energy Commission Contract by EPRI Solution Ltd.,) [11].

Some battery chargers, which we call multi-piece chargers, are simply conventional power supplies with additional control circuitry added to the output. Multi-piece chargers are cheap to design and build, being specified from pre-designed, off-the-shelf components, but tend to be relatively inefficient. They are commonly used in smaller, less expensive products. The power supply, often an external power supply, may be an ac-dc power supply with a regulated output voltage, or an ac-ac power supply with an ac output. The output from this kind of power supply is changed to regulated dc current through the charge control circuitry. Single-piece battery chargers are integrated devices custom-designed for a specific application. In such devices, the power supply and charge control circuitry are integrated [59].

Methods of Battery Charger System & its Topologies

Methods of efficiency improvements in battery charger systems in use today have substantially lower possibilities due to a lack of cognitive skills in the charger and battery which commonly consume more electricity than the product they powern [42]. The energy savings are achieved in millions of battery charger systems that are presently in operation worldwide by reducing inefficiencies in charger and battery [47], [48], [49]. Battery charger systems work in three modes of operation. In charge mode of operation, the battery is accumulating the charge while the maintenance mode of operation occurs when battery is fully charged and charger is only started to supply energy to undermine the natural discharge. No-battery mode of operation shows that the battery has been physically disconnected from the charger [9], [10]. Each of three modes has inefficiencies associated with it. Depending on how the product is used, wasted energy may be largely associated with a particular mode. For instance, a UPS system spends the majority of its time in maintenance mode, and experiences active charge relatively rarely. In this case, wasted energy depends largely on maintenance mode efficiency. Cellular phones, on the other hand, are usually drained and fully recharged every day, so that they spend a significant time in active charge mode. In many cases, the chargers for cellular phones are left plugged in even if the phone is not present, so that no-battery mode may also be important. The 24-hour charging and maintenance period used in testing is intended to capture inefficiencies in both active charge and maintenance modes; separate no-battery mode testing is required to measure inefficiencies when a battery is not attached.

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Figure 2-3: Switch Mode Battery Charger Power Visibility

There are lots of methods which are recognized to achieve the higher efficiency in battery charger systems, including:

- Higher voltage systems
- Switch mode power supplies
- Synchronous rectification
- Improved semiconductor switches
- Lithium-ion batteries [52], [59]
- Charge and discharge at lower current rate
- Off-grid charger when no battery is present.

Table 2-1: Efficiency improvements in charger topologies

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Table.1 show that the efficiencies of normal and improved range are measured less than 15%, comparable systems with overall efficiencies of 65% or greater are technically feasible in charger topologies for battery charger system [47], [48], [49]. The linear and switch mode chargers are analogous to linear and switch mode power supplies with the exception that the charger topologies also incorporate charge control circuitry on their outputs. Most multi- or single-piece chargers are either linear or switch mode chargers. These two categories are found commonly in consumer applications, particularly in the residential public sector. Ferro-resonant and SCR (silicon controlled rectifier) battery chargers form a large percentage of the chargers utilized in developed industrial applications [11]. This paper provides basic idea about the method of use of switch mode power supplies such as dc-dc converters are considered as they can achieve higher efficiency in battery charger scheme.

2.3 Battery Charger Systems Functional Components

All battery charger systems have three functional components:

A power supply (either internal or external) that converts high voltage ac (either single phase or three phase) to low voltage dc;

- Charge control to regulate electric current going to the battery during charge and battery maintenance modes [48], [49];
- A battery that stores energy for the end use product [43].

Table 2-1: Form Factor Configuration of Battery Charger

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Four basic charge control designs and four general chemistries are found in the marketplace today (Table 2 and Table 3). Details about these designs can be found in table 3 and table 4.

Table 2-2: Summary of Battery Charge Control Designs

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Table 2-3: Chemical characteristics of most common battery chargers found in today’s marketplace

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These electrical components can be housed in a variety of ways, and one cannot determine the efficiency of the charger by examining these external housings that are also known as form factors. The four different battery charger configurations (Table 1) are:

1. Power supply, charge control circuitry, each in separate housings.
2. Power supply and charge control circuitry in one housing, battery in separate housing.
3. Charge control circuitry and battery in one housing, power supply in separate housing.
4. Power supply, charge control circuitry, and battery all in the same housing.

Differences among charge control designs are evident, even when comparing nearly identical products. Figure 2-4, below, shows test results of two, 7 volt lithium ion power tools available commercially. The charger on the left is a linear design, which is 24% efficient over a 24-hour charge and maintenance cycle. The charger on the right is switch mode design and is nearly twice as efficient over the same 24-hour period with significantly less energy used in battery maintenance and no battery modes [65].

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Figure 2-4: Power Tool Efficeincy Comparison

Table 2-4: Tool Charger comparison chart

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2.4 Resonant Converter

The converters which employ ZC and/or ZV switching technique are usually called resonant converters [35]. The resonant converters were investigated in early 1980s as they can achieve very low switching loss thus enabling the resonant topologies to work with a high switching frequency. In these converters, some form of L-C resonance is used, which is why they are known as resonant converters [12]. Resonant converters are repelled or driven with constant pulse duration at a variable frequency to maintain control over output voltage. The pulse duration is required to be equal to half of the time of resonant period for switching at the zero current or voltage crossing points. The resonant converters contain the serial or parallel connections of inductors and capacitors to enable the switch to achieve the ZCS & ZVS under resonance conditions, the result effects switching losses, switching stress and EMI problems [13], [14], [15], [1]. The switching resonant converter controls the output voltage through switching frequency, and generally can be sub-classified in ZCS Converter and ZVS Converter [14]. There are many variations that can be placed at the primary or secondary side of the transformer and alternatively called serial or parallel resonant circuit which indicates whether it is required to turn off the transistor when current or voltage is zero. So far these are distinguished as ZVS and ZCS resonant converters [35]. Resonant converts are combination of converter topologies or switching strategies that in consequence produce zero voltage and/or zero current switching.

2.4.1 ZC resonant switch

In a ZC resonant switch, an inductor is connected in series with a power switch S in order to achieve zero-current-switching (ZCS). If the switch S is a unidirectional switch, the switch current is allowed to resonate in the positive half cycle only. The resonant switch is said to operate in half-wave mode. If a diode is connected in anti-parallel with the unidirectional switch, the switch current can flow in both directions. In this case, the resonant switch can operate in full-wave mode. At turn-on, the switch current will rise slowly from zero. It will then oscillate, because of the resonance between. Finally, the switch can be commutated at the next zero current duration. The objective of this type of switch is to shape the switch current waveform during conduction time in order to create a zero-current condition for the switch to turn off [64][65].

2.4.2 ZV resonant switch

In a ZV resonant switch, a capacitor [Abbildung in dieser Leseprobe nicht enthalten] is connected in parallel with the switch S for achieving zero-voltage-switching (ZVS). If the switch S is a unidirectional switch, the voltage across the capacitor [Abbildung in dieser Leseprobe nicht enthalten] can oscillate freely in both positive and negative half-cycle. Thus, the resonant switch can operate in full-wave mode. If a diode is connected in anti-parallel with the unidirectional switch, the resonant capacitor voltage is clamped by the diode to zero during the negative half-cycle. The resonant switch will then operate in half-wave mode. The objective of a ZV switch is to use the resonant circuit to shape the switch voltage waveform during the off time in order to create a zero-voltage condition for the switch to turn on.

2.5 Switching Techniques

In switching technique, the mainly research carried out thus for pertains to hard switching and soft switching techniques. Hard switching technique relates to the stressful switching behaviors of power electronics devices whilst soft switching techniques are applied to eliminate the harmful effects of hard switching [56]. Therefore soft switching techniques are more significantly developed and are normally applied to reduce the problems of switching losses in dc-dc power converters operating with high switching frequency [13], [14], [15]. Generally there are two types of techniques known as Zero-current switching (ZCS) and Zero-Voltage Switching (ZVS) which are called conventionally employed soft-switching methods [16].When the switch current is reached to zero at the switching instants, it is usually known as Zero-Current switching (ZCS) and if certainly the switch voltage is reached to zero at switching instants, it is usually known as Zero-voltage switching (ZVS). The main difference between the two is to do with when the switching occurs [8].

2.5.1 Hard switching and Soft Switching Techniques

In the 1970’s, conventional PWM power converters were operated in a switched mode operation. Power switches have to cut off the load current within the turn-on and turn-off times under the hard switching conditions. Hard switching refers to the stressful switching behavior of the power electronic devices. The switching trajectory of a hard-switched power device is shown in Fig. 2-7 During the turn-on and turn-off processes, the power device has to withstand high voltage and current simultaneously, resulting in high switching losses and stress. Dissipative passive snubbers are usually added to the power circuits so that the of the power devices could be reduced, and the switching loss and stress be diverted to the passive snubber circuits. However, the switching loss is proportional to the switching frequency, thus limiting the maximum switching frequency of the power converters [56]. Typical converter switching frequency was limited to a few tens of kilo-Hertz (typically 20 kHz to 50 kHz) in early 1980’s. The stray inductive and capacitive components in the power circuits and power devices still cause considerable transient effects, which in turn give rise to electromagnetic interference (EMI) problems. Fig. 2-8 shows ideal switching waveforms and typical practical waveforms of the switch voltage. The transient ringing effects are major causes of EMI.

In the 1980’s, lots of research efforts were diverted towards the use of resonant converters. The concept was to incorporate resonant tanks in the converters to create oscillatory (usually sinusoidal) voltage and/or current waveforms so that zero voltage switching (ZVS) or zero current switching (ZCS) conditions can be created for the power switches [55], [56]. The reduction of switching loss and the continual improvement of power switches allow the switching frequency of the resonant converters to reach hundreds of kilo-Hertz (typically 100 kHz to 500 kHz). Consequently, magnetic sizes can be reduced and the power density of the converters increased. Various forms of resonant converters have been proposed and developed. However, most of the resonant converters suffer several problems. When compared with the conventional PWM converters, the resonant current and voltage of resonant converters have high peak values, leading to higher conduction loss and higher V and I ratings requirements for the power devices. Also, many resonant converters require frequency modulation (FM) for output regulation. Variable switching frequency operation makes the filter design and control more complicated.

In late 1980’s and throughout 1990’s, further improvements have been made in converter technology. New generations of soft-switched converters that combine the advantages of conventional PWM converters and resonant converters have been developed [56]. These soft-switched converters have switching waveforms similar to those of conventional PWM converters except that the rising and falling edges of the waveforms are ‘smoothed’ with no transient spikes. Unlike the resonant converters, new soft-switched converters usually utilize the resonance in a controlled manner. Resonance is allowed to occur just before and during the turn-on and turn-off processes so as to create ZVS and ZCS conditions. Other than that, they behave just like conventional PWM converters. With simple modifications, many customized control integrated control (IC) circuits designed for conventional converters can be employed for soft-switched converters. Because the switching loss and stress have been reduced, soft-switched converter can be operated at the very high frequency (typically 500 kHz to a few Mega-Hertz). Soft-switching converters also provide an effective solution to suppress EMI and have been applied to DC-DC, AC-DC and DC-AC converters. This chapter covers the basic technology of resonant and soft-switching converters. Various forms of soft-switching techniques such as ZVS, ZCS, voltage clamping, zero transition methods etc. are addressed. The emphasis is placed on the basic operating principle and practicality of the converters without using much mathematical analysis [62],[63].

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Figure 2-7: Typical switching trajectories of power switches

Abbildung in dieser Leseprobe nicht enthalten

Figure 2-8: Typical switching waveforms of (a) hard-switched and (b) soft-switched devices

2.5.2 ZCS Techniques

The ZCS is a type of soft switching technique which was first proposed by F C Y Lee al (1987) [17]. Reducing stress on the switching components is a major incentive for resonant operation; and we need to understand ways through which that might be fulfilled. The simplest approach and the one to which most of this paper presents ZCS operation of a converter switch must be such that involves the current flowing through the switch being induced to rise gradually just after the switch is turned-on so that it has a ZCS turn-on. The switch current must also be induced to descend gradually just before the switch is turned-off so that it can have a ZCS turn-off. The ZCS turn-on feature of a converter switch can be made certain by simply connecting an inductor in series as the current flowing through an inductor cannot change immediately. Connecting an inductor in series with a switch also ensures that the current flowing through the other devices in the converter is gradually drawn back so that they can turn-off with ZCS. The ZCS turn-off of a converter switch can be made certain by providing another path for the current to flow through, just before the switch is turned off. Since the switch has a relatively small voltage drop, the other path must be at a lower voltage potential so that current can be turned away from the switch [18].

70 of 70 pages

Details

Title
Analysis and Design of High Effeiceny ZCS Buck (PWM) Converter in battery charger
College
Hohai University  (College of Energy and Electrical Engineering)
Course
Power Electronics
Grade
Master degree
Author
Year
2014
Pages
70
Catalog Number
V266788
ISBN (Book)
9783656579199
File size
2333 KB
Language
English
Notes
This study employs a buck ZCS (PWM) converter and develops a novel soft-switching approach for charger batteries. This thesis presents technique for battery charger to achieve efficient performance in charging shaping, minimum low switching losses and reduction in circuit volume. The operation of circuit charger is switched with the operation of zero-current-switching, resonant components and appends the topology of dc-dc buck. The proposed novel dc-dc battery charger has advantages with the simplicity, low cost, high efficiency.
Tags
Battery Charger, ZCS, PWM Resonant Converter, dc-dc Buck
Quote paper
Engineer Irfan Jamil (Author), 2014, Analysis and Design of High Effeiceny ZCS Buck (PWM) Converter in battery charger, Munich, GRIN Verlag, https://www.grin.com/document/266788

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