INTRODUCTION :

Recently, the demand for dc/dc converters with high voltage gain has increased. The energy

shortage and the atmosphere pollution have led to more researches on the renewable and green

energy sources such as the solar arrays and the fuel cells. Moreover, the power systems based on

battery sources and supercapacitors have been increased. Unfortunately, the output voltages of these

sources are relatively low. Therefore, the step-up power conversion is required in these systems.

Besides the step-up function, the demands such as low current ripple, high efficiency, fast dynamics,

light weight, and high power density have also increased for various applications. Input current

ripple is an important factor in a high step-up dc/dc converter. Especially in the fuel cell systems,

reducing the input current ripple is very important because the large current ripple shortens fuel

cell’s lifetime as well as decreases performances. Therefore, current-fed converters are commonly

used due to their ability to reduce the current ripple

In applications that require a voltage step-up function and a continuous input current, a

continuous-conduction-mode (CCM) boost converter is often used due to its advantages such as

continuous input current and simple structure. However, it has a limited voltage gain due to its

parasitic components. Moreover, the reverse-recovery problem of the output diode degrades the

system’s performances. At the moment when the switch turns on, the reverse-recovery phenomenon

of the output diode of the boost converter is provoked. The switch is submitted to a high current

change rate and a high peak of reverse-recovery current. The parasitic inductance that exists in the

current loop causes a ringing of the parasitic voltage, and then, it increases

.

Fig. 1. Circuit diagram of the proposed dc/dc converter

the voltage stresses of the switch and the output diode. These effects significantly contribute to

increase switching losses and electromagnetic interference. The reverse-recovery problem of the

output diodes is another important factor in dc/dc converters with high voltage gain. In order to

overcome these problems, various topologies have been introduced. In orderto extend the voltage

gain, the boost converters with coupled inductors are proposed in and Their voltage gains are

extended, but they lose a continuous input current characteristic and the efficiency is degraded

Fig. 2. Key waveforms of the proposed converter.

due to hard switchings of power switches. For a continuous input current, current-fed step-up

converters are proposed in . They provide high voltage gain and galvanic isolation. However, the

additional snubbers are required to reduce the voltage stresses of switches. In order to increase the

efficiency and power conversion density, a soft-switching technique is required in dc/dc converters.

A soft-switching dc/dc converter with high voltage gain, which is shown in Fig. 1, is proposed. A

CCM boost cell provides a continuous input current. To increase the voltage gain, the output of the

coupled inductor cell is laid on the top of the output of the CCM boost cell. Therefore, the high

voltage gain is obtained without high turn ratio of the coupled inductor, and the voltage stresses of

the switches are confined to the output voltage of the CCM boost cell. A zero-voltage-switching

(ZVS).

operation of the power switches reduces the switching loss during the switching transition

and improves the overall efficiency. The theoretical analysis is verified by a 200 W experimental

prototype with 24-to-360 V conversion.

Direct current

Direct current  (DC) is the unidirectional flow of electric charge. Direct current

is produced by sources such as batteries, thermocouples, solar cells, and commutator-type electric

machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also

flow through semiconductors, insulators, or even through a vacuum as in electron or ion beams.

The electric charge flows in a constant direction, distinguishing it from alternating current (AC).

A term formerly used for direct current was galvanic current.

Direct current may be obtained from an alternating current supply by use of a

current-switching arrangement called a rectifier, which contains electronic elements (usually) or

electromechanical elements (historically) that allow current to flow only in one direction. Direct

current may be made into alternating current with an inverter or a motor-generator set.

Types of current

The first commercial electric power transmission (developed by Thomas Edison in

the late nineteenth century) used direct current. Because of the significant advantages of alternating

current over direct current in transforming and transmission, electric power distribution is nearly all

alternating current today. In the mid 1950s, HVDC transmission was developed, and is now an

option instead of long-distance high voltage alternating current systems. For applications requiring

direct current, such as third rail power systems, alternating current is distributed to a substation,

which utilizes a rectifier to convert the power to direct current. See War of Currents.

Types of direct current

Direct current is used to charge batteries, and in nearly all electronic systems, as

the power supply. Very large quantities of direct-current power are used in production

of aluminum and other electrochemical processes. Direct current is used for

some railway propulsion, especially in urban areas. High-voltage direct current is used to transmit

large amounts of power from remote generation sites or to interconnect alternating current power

grids.

Definition

Within  electrical engineering, the term DC is used to refer to power systems that

use only one polarity of voltage or current, and to refer to the constant, zero-frequency, or slowly

varying local mean value of a voltage or current. For example, the voltage across a DC voltage

source is constant as is the current through a DC current source. The DC solution of an electric

circuit is the solution where all voltages and currents are constant. It can be shown that

any stationary voltage or current waveform can be decomposed into a sum of a DC component and a

zero-mean time-varying component; the DC component is defined to be the expected value, or the

average value of the voltage or current over all time.

Although DC stands for "direct current", DC often refers to "constant polarity".

Under this definition, DC voltages can vary in time, as seen in the raw output of a rectifier or the

fluctuating voice signal on a telephone line.

Some forms of DC (such as that produced by a voltage regulator) have almost no

variations in voltage, but may still have variations in output power and current

Applications of direct current

Direct-current installations usually have different types of  sockets, switches,

and fixtures, mostly due to the low voltages used, from those suitable for alternating current. It is

usually important with a direct-current appliance not to reverse polarity unless the device has a diode

bridge to correct for this (most battery-powered devices do not).

DC is commonly found in many low-voltage applications, especially where these

are powered by batteries, which can produce only DC, or solar power systems, since solar cells can

produce only DC. Most automotive applications use DC, although the alternator is an AC device

which uses a rectifier to produce DC. Most electronic circuits require a DC power supply.

Applications using fuel cells (mixing hydrogen and oxygen together with a catalyst to produce

electricity and water as byproducts) also produce only DC.

Many telephones connect to a twisted pair of wires, and internally separate the AC

component of the voltage between the two wires (the audio signal) from the DC component of the

voltage between the two wires (used to power the phone).

Telephone exchange communication equipment, such as DSLAM, uses standard -

48V DC power supply. The negative polarity is achieved by grounding the positive terminal of

power supply system and the battery bank. This is done to prevent electrolysis depositions.

Electric charge 

Electric charge is a physical property of matter that causes it to experience

a force when near other electrically charged matter. Electric charge comes in two types,

called positive and negative. Two positively charged substances, or objects, experience a mutual

repulsive force, as do two negatively charged objects. Positively charged objects and negatively

charged objects experience an attractive force. The SI unit of electric charge is the coulomb (C),

although in electrical engineering it is also common to use the ampere-hour (Ah). The study of how

charged substances interact is classical electrodynamics, which is accurate insofar as quantum

effects can be ignored.

The  electric charge is a fundamental conserved property of some subatomic

particles, which determines their electromagnetic interaction. Electrically charged matter is

influenced by, and produces, electromagnetic fields. The interaction between a moving charge and an

electromagnetic field is the source of the electromagnetic force, which is one of the four fundamental

forces (See also: magnetic field).

Twentieth-century  experiments demonstrated that electric charge is quantized; that

is, it comes in multiples of individual small units called the elementary charge, e, approximately

equal to 1.602×10−19 coulombs (except for particles called quarks, which have charges that are

multiples of ⅓e). The proton has a charge of e, and the electron has a charge of −e. The study of

charged particles, and how their interactions are mediated by photons, is quantum electrodynamics.

Solar cell

A solar cell (also called photovoltaic cell or photoelectric cell) is a solid state electrical

device that converts the energy of light directly into electricity by the photovoltaic effect.

Assemblies of solar cells are used to make solar modules which are used to capture

energy from sunlight. When multiple modules are assembled together (such as prior to installation on

a pole-mounted tracker system), the resulting integrated group of modules all oriented in one plane is

referred to in the solar industry as a solar panel. The electrical energy generated from solar modules,

referred to as solar power, is an example of solar energy.

Photo voltaics is the field of technology and research related to the practical application

of photovoltaic cells in producing electricity from light, though it is often used specifically to refer to

the generation of electricity from sunlight.

Cells are described as photovoltaic cells when the light source is not necessarily sunlight (lamplight,

artificial light, etc.). These are used for detecting light or other electromagnetic radiation near the

visible range, for example infrared detectors, or measurement of light intensity

Battery (electricity)

An electrical  battery is one or more electrochemical cells that convert stored

chemical energy into electrical energy. Since the invention of the first battery (or "voltaic pile") in

1800 by Alessandro Volta and especially since the technically improved Daniell cell in 1836,

batteries have become a common power source for many household and industrial applications.

According to a 2005 estimate, the worldwide battery industry generates US$48 billion in sales each

year, with 6% annual growth.

There are two types of batteries:  primary batteries (disposable batteries), which

are designed to be used once and discarded, and secondary batteries (rechargeable batteries), which

are designed to be recharged and used multiple times. Batteries come in many sizes, from miniature

cells used to power hearing aids and wristwatches to battery banks the size of rooms that provide

standby power for telephone exchanges and computer data centers

Thermocouple

A thermocouple is a device consisting of two different conductors (usually metal

alloys) that produce a voltage, proportional to a temperature difference, between either end of the

two conductors. Thermocouples are a widely used type of temperature sensor for measurement and

control and can also be used to convert a temperature gradient into electricity. They are

inexpensive, interchangeable, are supplied with standard connectors, and can measure a wide range

of temperatures. In contrast to most other methods of temperature measurement, thermocouples are

self powered and require no external form of excitation. The main limitation with thermocouples is

accuracy and system errors of less than one degree Celsius (C) can be difficult to achieve.

Any junction of dissimilar metals will produce an electric potential related to

temperature. Thermocouples for practical measurement of temperature are junctions of

specific alloys which have a predictable and repeatable relationship between temperature and

voltage. Different alloys are used for different temperature ranges. Properties such as resistance to

corrosion may also be important when choosing a type of thermocouple. Where the measurement

point is far from the measuring instrument, the intermediate connection can be made by extension

wires which are less costly than the materials used to make the sensor. Thermocouples are usually

standardized against a reference temperature of 0 degrees Celsius; practical instruments use

electronic methods of cold-junction compensation to adjust for varying temperature at the instrument

terminals. Electronic instruments can also compensate for the varying characteristics of the

thermocouple, and so improve the precision and accuracy of measurements.

Thermocouples are widely used in science and industry; applications include

temperature measurement for kilns, gas turbine exhaust, diesel engines, and other industrial

processes.

Dynamo

A  dynamo (from the Greek word dynamis; meaning power), originally another

name for an electrical generator, generally means a generator that producesdirect current with the use

of a commutator. Dynamos were the first electrical generators capable of delivering power for

industry, and the foundation upon which many other later electric-power conversion devices were

based, including the electric motor, the alternating-current alternator, and the rotary converter.

Today, the simpler alternator dominates large scale power generation, for efficiency, reliability and

cost reasons. A dynamo has the disadvantages of a mechanical commutator. Also, converting

alternating to direct current using power rectification devices (hollow state or more recently solid

state) is effective and usually economic.

The word still has some regional usage as a replacement for the word  generator. A

small electrical generator built into the hub of a bicycle wheel to power lights is called a  Hub

dynamo, although these are invariably AC devices.

High voltage direct current (HVDC)

A high-voltage, direct current (HVDC) electric power transmission system

uses direct current for the bulk transmission of electrical power, in contrast with the more

common alternating current systems. For long-distance transmission, HVDC systems may be less

expensive and suffer lower electrical losses. For underwater power cables, HVDC avoids the heavy

currents required by the cable capacitance. For shorter distances, the higher cost of DC conversion

equipment compared to an AC system may still be warranted, due to other benefits of direct current

links. HVDC allows power transmission between unsynchronized AC distribution systems, and can

increase system stability by preventing cascading failures from propagating from one part of a wider

power transmission grid to another.

The modern form of HVDC transmission uses technology developed extensively in

the 1930s in Sweden at ASEA. Early commercial installations included one in the Soviet Union in

1951 between Moscow and Kashira, and a 10–20 MW system between Gotland and mainland

Sweden in 1954.[1] The longest HVDC link in the world is currently the Xiangjiaba-Shanghai

2,071 km (1,287 mi) 6400 MW link connecting the Xiangjiaba Dam to Shanghai, in the People's

Republic of China. In 2012, the longest HVDC link will be the Rio Madeira link connecting the

Amazonas to the São Paulo area where the length of the DC line is over 2,500 km (1,600 mi)

High voltage (in either AC or DC electrical power transmission applications) is used

for electric power transmission to reduce the energy lost in the resistance of the wires. For a given

quantity of power transmitted and size of conductor, doubling the voltage will deliver the same

power at only half the current. Since the power lost as heat in the wires is proportional to the square

of the current, but does not depend in any major way on the voltage delivered by the power line,

doubling the voltage in a power system reduces the line-loss loss per unit of electrical power

delivered by a factor of 4. Power loss in transmission lines can also be reduced by reducing

resistance, for example by increasing the diameter of the conductor; but larger conductors are heavier

and more expensive.

High voltages cannot easily be used for lighting and motors, and so transmission-

level voltages must be reduced to values compatible with end-use equipment. Transformers are used

to change the voltage level in alternating current (AC) transmission circuits. The competition

between the direct current (DC) of Thomas Edison and the AC of Nikola Tesla and George

Westinghouse was known as the War of Currents, with AC becoming dominant.

Practical manipulation of high power high voltage DC became possible with the

development of high power electronic rectifier devices such as mercury arc valves and, more

recently starting in the 1970s, high power semiconductor devices such as high power thyristors and

21st century high power variants such as integrated gate-commutated thyristors (IGCTs), MOS

controlled thyristors (MCTs) and insulated-gate bipolar transistors (IGBT).

Soft Switching

A soft switch is a central device in a telecommunications network which

connects telephone calls from one phone line to another, typically via the internet, entirely by means

of software running on a general-purpose computer system. Most landline calls are routed

by purpose-built hardware, formerly using physical switchboards, but soft switches are the dominant

21st century trend.

Although the term soft switch technically refers to any such device, it is more

conventionally applied to a device that handles IP-to-IP phone calls, while the phrase "access server"

or "media gateway" is used to refer to devices that either originate or terminate traditional "land line"

(hard wired) phone calls. In practice, such devices can often do both. As a practical distinction,

a Skype-to-Skype phone call is entirely IP (internet) based, and so uses a soft switch somewhere in

the middle connecting the calling party with the called party. In contrast, access servers might take a

mobile call or a call originating from a traditional phone line, convert it to IP traffic, then send it over

the internet to another such device, which terminates the call by reversing the process and converting

the IP call back toISDN digital or analog/PSTN format, and connecting it to a destination phone

number.

A soft switch is typically used to control connections at the junction point

between circuit-switched and packet-switched networks. A single device containing both the

switching logic and the switching fabric can be used for this purpose; however, modern technology

has led to a preference for decomposing this device into a Call Agent and a Media Gateway.

The Call Agent takes care of functions such as billing, call routing, signaling, call services

and the like, supplying the functional logic to accomplish these telephony meta-tasks. A call agent

may control several different media gateways in geographically dispersed areas via a TCP/IP link.

The Media Gateway connects different types of digital media stream together to create an

end-to-end path for the media (voice and data) in the call. It may have interfaces to connect to

traditional PSTN networks, such as DS1 or DS3 ports (E1 or STM1 in the case of non-US networks).

It may also have interfaces to connect to ATM and IP networks, and the most modern systems will

have Ethernet interfaces to connect VoIP calls. The call agent will instruct the media gateway to

connect media streams between these interfaces to connect the call - all transparently to the end-

users.

The soft switch generally resides in a building owned by the  telephone company called

a central office. The central office will have telephone trunks to carry calls to other offices owned by

the telecommunication company and to other telecommunication companies via the PSTN.

Looking towards the end users from the switch, the Media Gateway may be connected to

several access devices. These access devices can range from small Analog Telephone Adaptors

(ATA) which provide just one RJ11 telephone jack to an Integrated Access Device (IAD)

or PBX which may provide several hundred telephone connections.

Typically the larger access devices will be located in a building owned by the

telecommunication company near to the customers they serve. Each end user can be connected to the

IAD by a simple pair of copper wires.

The medium-sized devices and PBXs are most commonly used by business that locates them

on their own premises, and single-line devices are mostly found at private residences.

At the turn of the 21st century with IP Multimedia Subsystem (or IMS), the Soft switch

element is represented by the Media Gateway Controller (MGC) element, and the term "Soft switch"

is rarely used in the IMS context. Rather, it is called and AGCF (Access Gateway Control Function)

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.1. 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 dv/dt and di/dt 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. Typical converter switching frequency was limited to a few tens

of kilo-Hertz (typically 20kHz to 50kHz) 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 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. 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 100kHz to 500kHz). 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. 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 500kHz 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.

Fig.1 Typical switching trajectories of power switches.

Fig.2. Typical switching waveforms of (a) hard-switched and (b) soft-switched devices

Classification

Resonant Switch

Prior to the availability of fully controllable power switches, thyristors were the major

power devices used in power electronic circuits. Each thyristor requires a commutation circuit,

which usually consists of a LC resonant circuit, for forcing the current to zero in the turn-off

process. This mechanism is in fact a type of zero-current turn-off process. With the recent

advancement in semiconductor technology, the voltage and current handling capability, and the

switching speed of fully controllable switches have significantly been improved. In many high

power applications, controllable switches such as GTOs and IGBTs have replaced thyristors.

However, the use of resonant circuit for achieving zero-current-switching (ZCS) and/or zero-

voltage-switching (ZVS) has also emerged as a new technology for power converters. The concept

of resonant switch that replaces conventional power switch is introduced in this section.

A resonant switch is a sub-circuit comprising a semiconductor switch S and resonant

elements, Lr and Cr. The switch S can be implemented by a unidirectional or bidirectional switch,

which determines the operation mode of the resonant switch. Two types of resonant switches,

including zero-current (ZC) resonant switch and zero-voltage (ZV) resonant switches, are shown in

Fig.3 and Fig.4, respectively.

Fig.3 Zero-current (ZC) resonant switch.

Fig.4 Zero-voltage (ZV) resonant switch.

ZC resonant switch

In a ZC resonant switch, an inductor Lr 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 Lr and Cr. 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.

ZV resonant switch

In a ZV resonant switch, a capacitor Cr 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 Cr 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.

Quasi-resonant Converters

Quasi-resonant converters (QRCs) can be considered as a hybrid of resonant and PWM

converters. The underlying principle is to replace the power switch in PWM converters with the

resonant switch. A large family of conventional converter circuits can be transformed into their

resonant converter counterparts. The switch current and/or voltage waveforms are forced to

oscillate in a quasi-sinusoidal manner, so that ZCS and/or ZVS can be achieved. Both ZCS-QRCs and

ZVS-QRCs have half-wave and full-wave mode of operations.

ZCS-QRCs

A ZCS-QRC designed for half-wave operation is illustrated with a buck type dc-dc converter.

The schematic is shown in Fig.5(a). It is formed by replacing the power switch in conventional PWM

buck converter with the ZC resonant switch in Fig.3(a). The circuit waveforms in steady state are

shown in Fig.5(b). The output filter inductor Lf is sufficiently large so that its current is

approximately constant. Prior to turning the switch on, the output current Io freewheels through

the output diode Df. The resonant capacitor voltage VCr equals zero. At t0, the switch is turned on

with ZCS. A quasi-sinusoidal current IS flows through Lr and Cr, the output filter, and the load. S is

then softly commutated at t2 with ZCS again. During and after the gate pulse, the resonant

capacitor voltage VCr rises and then decays at a rate depending on the output current. Output

voltage regulation is achieved by controlling the switching frequency. Operation and characteristics

of the converter depend mainly on the design of the resonant circuit Lr - Cr. The following

parameters are defined: voltage conversion ratio M, characteristic impedance Zr, resonant

frequency fr, normalized load resistance r, normalized switching frequency .

M =V oV i (1a)

Zr = √ LrC r (1b)

f r =1

2 π √Lr C r (1c)

r =RLZr (1d)

γ =f sf r (1e)

It can be seen from the waveforms that if Io > Vi / Zr, IS will not come back to zero naturally

and the switch will have to be forced off, thus resulting in turn-off losses. The relationships

between M and at different r are shown in Fig.5(c). It can be seen that M is sensitive to the load

variation. At light load conditions, the unused energy is stored in Cr, leading to an increase in the

output voltage. Thus, the switching frequency has to be controlled, in order to regulate the output

voltage.

(a) Schematic diagram.

(b) Circuits waveforms.

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

g

M

0.5

1

r=2510

(c) Relationship between M and .

Fig.5 Half-wave, quasi-resonant buck converter with ZCS.

If an anti-parallel diode is connected across the switch, the converter will be operating in

full-wave mode. The circuit schematic is shown in Fig.6(a). The circuit waveforms in steady state

are shown in Fig.6(b). The operation is similar to the one in half-wave mode. However, the inductor

current is allowed to reverse through the anti-parallel diode and the duration for the resonant

stage is lengthened. This permits excess energy in the resonant circuit at light loads to be

transferred back to the voltage source Vi. This significantly reduces the dependence of Vo on the

output load. The relationships between M and at different r are shown in Fig.6(c). It can be seen

that M is insensitive to load variation.

(a) Schematic diagram.

(b) Circuit waveforms.

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

g

M

r=1-10

(c) Relationship between M and .

Fig.6 Full-wave, quasi-resonant buck converter with ZCS.

By replacing the switch in the conventional converters, a family of QRC with ZCS is shown

in Fig.7.

Fig.7 A family of quasi-resonant converter with ZCS.

ZVS-QRC

In these converters, the resonant capacitor provides a zero-voltage condition for the switch

to turn on and off. A quasi-resonant buck converter designed for half-wave operation is shown in

Fig.8(a) - using a ZV resonant switch in Fig.4(b). The steady-state circuit waveforms are shown in

Fig.8(b). Basic relations of ZVS-QRCs are given in Equations (1a-1e). When the switch S is turned on,

it carries the output current Io. The supply voltage Vi reverse-biases the diode Df. When the switch

is zero-voltage (ZV) turned off, the output current starts to flow through the resonant capacitor Cr.

When the resonant capacitor voltage VCr is equal to Vi, Df turns on. This starts the resonant stage.

When VCr equals zero, the anti-parallel diode turns on. The resonant capacitor is shorted and the

source voltage is applied to the resonant inductor Lr. The resonant inductor current ILr increases

linearly until it reaches Io. Then Df turns off. In order to achieve ZVS, S should be triggered during

the time when the anti-parallel diode conducts. It can be seen from the waveforms that the peak

amplitude of the resonant capacitor voltage should be greater or equal to the input voltage (i.e., Io

Zr > Vin). From Fig.8(c), it can be seen that the voltage conversion ratio is load-sensitive. In order to

regulate the output voltage for different loads r, the switching frequency should also be changed

accordingly.

(a) Schematic diagram.

(b) Circuit waveforms.

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

g

M 0.5

0.2

0.1

0.8

0.9

(c) Relationship between M and .

Fig.8 Half-wave, quasi-resonant buck converter with ZVS.

ZVS converters can be operated in full-wave mode. The circuit schematic is shown in

Fig.9(a). The circuit waveforms in steady state are shown in Fig.9(b). The operation is similar to

half-wave mode of operation, except that VCr can swing between positive and negative voltages.

The relationships between M and at different r are shown in Fig.9(c).

(a) Schematic diagram.

(b) Circuit waveforms.

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

g

M0.5

0.2

0.1

0.8

0.9 0.9

1

0.2

0.5

0.8

(c) Relationship between M and .

Fig.9 Full-wave, quasi-resonant buck converter with ZVS.

Comparing Fig.8(c) with Fig.9(c), it can be seen that M is load-insensitive in full-wave mode.

This is a desirable feature. However, as the series diode limits the direction of the switch current,

energy will be stored in the output capacitance of the switch and will dissipate in the switch during

turn-on. Hence, the full-wave mode has the problem of capacitive turn-on loss, and is less practical

in high frequency operation. In practice, ZVS-QRCs are usually operated in half-wave mode rather

than full-wave mode.

By replacing the ZV resonant switch in the conventional converters, various ZVS-QRCs can

be derived. They are shown in Fig.10.

Fig.10 A family of quasi-resonant converter with ZVS.

Comparisons between ZCS and ZVS

ZCS can eliminate the switching losses at turn-off and reduce the switching losses at turn-on.

As a relatively large capacitor is connected across the output diode during resonance, the converter

operation becomes insensitive to the diode’s junction capacitance. The major limitations associated

with ZCS when power mosfets are used are the capacitive turn-on losses. Thus, the switching loss is

proportional to the switching frequency. During turn-on, considerable rate of change of voltage can

be coupled to the gate drive circuit through the Miller capacitor, thus increasing switching loss and

noise. Another limitation is that the switches are under high current stress, resulting in high

conduction loss. It should be noted that ZCS is particularly effective in reducing switching loss for

power devices (such as IGBT) with large tail current in the turn-off process.

ZVS eliminates the capacitive turn-on loss. It is suitable for high-frequency operation. For

single-ended configuration, the switches could suffer from excessive voltage stress, which is

proportional to the load. It will be shown in Section 15.5 that the maximum voltage across switches

in half-bridge and full-bridge configurations is clamped to the input voltage.

For both ZCS and ZVS, output regulation of the resonant converters can be achieved by

variable frequency control. ZCS operates with constant on-time control, while ZVS operates with

constant off-time control. With a wide input and load range, both techniques have to operate with

a wide switching frequency range, making it not easy to design resonant converters optimally.

Control Circuits for Resonant Converters

Since the 1985s, various control integrated circuits (ICs) for resonant converters have been

developed. Some common ICs for different converters are described as in this section.

QRCs

Output regulations in many resonant-type converters, such as QRCs, are achieved by

controlling the switching frequency. ZCS applications require controlled switch-on times while ZVS

applications require controlled switch-off times. The fundamental control blocks in the IC include an

error amplifier, voltage-controlled-oscillator (VCO), one shot generator with a zero wave-crossing

detection comparator, and an output stage to drive the active switch. Typical ICs include UC1861-

UC1864 for ZVS applications and UC 1865-UC 1868 for ZCS applications. Fig.11 shows the

controller block diagram of UC 1864.

Fig.11 Controller block diagram of UC1864

(Courtesy of Unitrode Corp. and Texas Instruments).

The maximum and minimum switching frequencies (i.e., fmax and fmin) are controlled by the

resistors Range and Rmin and the capacitor Cvco. fmax and fmin can be expressed as

f max = 3 . 6(Range // Rmin ) CVCO and

f min = 3.6Rmin CVCO (2)

The frequency range f is then equal to

Δf = f max − f min = 3.6Range CVCO (3)

The frequency range of the ICs is from 10kHz to 1MHz. The output frequency of the

oscillator is controlled by the error amplifier (E/A) output. An example of a ZVS-MR forward

converter is shown in Fig.12.

Fig.12 ZV-MR Forward Converter.

(Courtesy of Unitrode Corp. and Texas Instruments)

DC-DC Converter

 DC-to-DC converters are used in a wide variety of applications. They are used in

small things like laptop chargers up to large industrial applications like transporting large amounts of

power from one location to another. DC/DC converters are used in most mobile devices (mobile

phones, PDA etc.) to maintain the voltage at a fixed value whatever the voltage level of the battery

is. These converters are also used for electronic isolation and power factor correction. For the smaller

devices, DC-DC converters are used after the AC-DC converter to convert the high voltage to a

lower voltage for charging the device. They are also used in small devices to regulate the voltage to

the device from the battery. The power range of DC-DC converters is from a few watts up to Giga

watts.

A DC-to-DC converter is an electronic circuit which converts a source of direct

current (DC) from one voltage level to another. It is a class of power converter.

DC to DC converters are important in portable electronic devices such

as cellular phones and laptop computers, which are supplied with power from batteries primarily.

Such electronic devices often contain several sub-circuits, each with its own voltage level

requirement different from that supplied by the battery or an external supply (sometimes higher or

lower than the supply voltage). Additionally, the battery voltage declines as its stored power is

drained. Switched DC to DC converters offer a method to increase voltage from a partially lowered

battery voltage thereby saving space instead of using multiple batteries to accomplish the same thing.

Most DC to DC converters also regulate the output. Some exceptions include high-

efficiency LED power sources, which are a kind of DC to DC converter that regulates the current

through the LEDs, and simple charge pumps which double or triple the input voltage

Conversion methods

Electronic

Linear

In electronics, a linear regulator is a component used to maintain a steady voltage. The

resistance of the regulator varies in accordance with the load resulting in a constant output voltage.

In contrast, the switching regulator is nothing more than just a simple switch. This switch goes on

and off at a fixed rate usually between 50 kHz to 100 kHz as set by the circuit. The regulating device

is made to act like a variable resistor, continuously adjusting a voltage divider network to maintain a

constant output voltage. The primary advantage of a switching regulator over linear regulator  is very

high efficiency, a lot less heat and smaller size

Linear regulators are used for low power systems. They are not very efficient since they

dissipate the excess energy they receive from the source as heat. They simply act as a regulated

voltage drop from the source voltage to the desired output voltage. For example if it’s a 5V regulator

and 9V is supplied to it, it will have an output of 5V and a 4V voltage drop across it. These

regulators are useful in small systems since you can get the voltage regulation needed in 1

component. Linear regulators are good for applications that need a low noise reliable voltage supply,

often they should be used with a heat sink to get rid of the heat they are dissipating. They are easy to

use and not very expensive. When using them you must consider the low efficiency they offer .

The transistor (or other device) is used as one half of a potential divider to establish the

regulated output voltage. The output voltage is compared to a reference voltage to produce a control

signal to the transistor which will drive its gate or base. With negative feedback and good choice

of compensation, the output voltage is kept reasonably constant. Linear regulators are often

inefficient: since the transistor is acting like a resistor, it will waste electrical energy by converting it

to heat. In fact, the power loss due to heating in the transistor is the current times the voltage dropped

across the transistor. The same function can often be performed much more efficiently by

a switched-mode power supply, but a linear regulator may be preferred for light loads or where the

desired output voltage approaches the source voltage. In these cases, the linear regulator may

dissipate less power than a switcher. The linear regulator also has the advantage of not requiring

magnetic devices (inductors or transformers) which can be relatively expensive or bulky, being often

of simpler design, and being quieter.

Linear regulators exist in two basic forms: series regulators and shunt regulators.

Series regulators are the more common form. The series regulator works by providing a path

from the supply voltage to the load through a variable resistance (the main transistor is in the

"top half" of the voltage divider). The power dissipated by the regulating device is equal to the

power supply output current times the voltage drop in the regulating device.

The shunt regulator works by providing a path from the supply voltage to ground through a

variable resistance (the main transistor is in the "bottom half" of the voltage divider). The current

through the shunt regulator is diverted away from the load and flows uselessly to ground, making

this form even less efficient than the series regulator. It is, however, simpler, sometimes

consisting of just a voltage-reference diode, and is used in very low-powered circuits where the

wasted current is too small to be of concern. This form is very common for voltage reference

circuits.

All linear regulators require an input voltage at least some minimum amount higher

than the desired output voltage. That minimum amount is called the dropout voltage. For example, a

common regulator such as the 7805 has an output voltage of 5V, but can only maintain this if the

input voltage remains above about 7V, before the output voltage begins sagging below the rated

output. Its dropout voltage is therefore 7V - 5V = 2V. When the supply voltage is less than about 2V

above the desired output voltage, as is the case in low-voltage microprocessor power supplies, so-

called low dropout regulators (LDOs) must be used.

When one wants an output voltage higher than the available input voltage, no linear

regulator will work (not even an LDO). In this situation, a switching regulator must be used.

Linear regulators are practical if the current is low, the power dissipated being small,

although it may still be a large fraction of the total power consumed. They are often used as part of a

simple regulated power supply for higher currents: a transformer generates a voltage which, when

rectified, is a little higher than that needed to bias the linear regulator. The linear regulator drops the

excess voltage, reducing hum-generating ripple current and providing a constant output voltage

independent of normal fluctuations of the unregulated input voltage from the transformer/bridge

rectifier circuit and of the load current.

Linear regulators are inexpensive, reliable if good heat sinks are used and much

simpler than switching regulators. As part of a power supply they may require a transformer, which

is larger for a given power level than that required by a switch-mode power supply. Linear regulators

can provide a very low-noise output voltage, and are very suitable for powering noise-sensitive low-

power analog and radio frequency circuits. A popular design approach is to use an LDO, Low Drop-

out Regulator, that provides a local "point of load" DC supply to a low power circuit.

Switched-mode conversion

Electronic switch-mode DC to DC converters convert one DC voltage level to another,

by storing the input energy temporarily and then releasing that energy to the output at a different

voltage. The storage may be in either magnetic field storage components (inductors, transformers) or

electric field storage components (capacitors). This conversion method is more power efficient (often

75% to 98%) than linear voltage regulation (which dissipates unwanted power as heat). This

efficiency is beneficial to increasing the running time of battery operated devices. The efficiency has

increased since the late 1980s due to the use of power FETs, which are able to switch at high

frequency more efficiently than power bipolar transistors, which incur more switching losses and

require a more complicated drive circuit. Another important innovation in DC-DC converters is the

use of synchronous rectification replacing the flywheel diode with a power FET with low "on

resistance", thereby reducing switching losses.

Most DC-to-DC converters are designed to move power in only one direction, from

the input to the output. However, all switching regulator topologies can be made bi-directional by

replacing all diodes with independently controlled active rectification. A bi-directional converter can

move power in either direction, which is useful in applications requiring regenerative braking.

Drawbacks of switching converters include complexity, electronic noise ( EMI / RFI)

and to some extent cost, although this has come down with advances in chip design.

DC-to-DC converters are now available as integrated circuits needing minimal

additional components. They are also available as a complete hybrid circuit component, ready for use

within an electronic assembly.

Magnetic

In these DC-to-DC converters, energy is periodically stored into and released from

a magnetic field in an inductor or a transformer, typically in the range from 300 kHz to 10 MHz. By

adjusting the duty cycle of the charging voltage (that is, the ratio of on/off time), the amount of

power transferred can be controlled. Usually, this is applied to control the output voltage, though it

could be applied to control the input current, the output current, or maintain a constant power.

Transformer-based converters may provide isolation between the input and the output. In general, the

term "DC-to-DC converter" refers to one of these switching converters. These circuits are the heart

of a switched-mode power supply. Many topologies exist. This table shows the most common.

.

Forward

Energy goes from the input, through the

magnetics and to the load, simultaneously

Flyback

Energy goes from the input and stored in the

magnetics

Later, it is released from the magnetics to the load

No transformer

Non-isolated

Step-down (Buck) - The output voltage is lower than

the input voltage, and of the same polarity

Non-inverting: The output voltage is the same polarity

as the input

Step-up (Boost) - The output voltage is higher

than the input voltage

SEPIC - The output voltage can be lower or

higher than the input

Inverting: the output voltage is of the opposite polarity

as the input

Inverting (Buck-Boost)

Ćuk - Output current is continuous

True Buck-Boost - The output voltage is the same polarity as the input and can be lower or higher

Split-Pi (Boost-Buck) - Allows bidirectional voltage conversion with the output voltage the same polarity as the input

and can be lower or higher.

Cuk (Cuk) - Allows bidirectional voltage conversion with the output voltage

of inverted polarity.

With transformer

May be

isolated

Half bridge - 2 transistors drive

Full bridge - 4 transistor drive

Flyback - 1 or 2 transistor drive

In addition, each topology may be:

Hard switched - transistors switch quickly while exposed to both full voltage and full current

Resonant - an LC circuit shapes the voltage across the transistor and current through it so that

the transistor switches when either the voltage or the current is zero

Magnetic DC-to-DC converters may be operated in two modes, according to the current in its main

magnetic component (inductor or transformer):

Continuous - the current fluctuates but never goes down to zero

Discontinuous - the current fluctuates during the cycle, going down to zero at or before the end

of each cycle

A converter may be designed to operate in continuous mode at high power, and in discontinuous

mode at low power.

The Half bridge and Flyback topologies are similar in that energy stored in the magnetic

core needs to be dissipated so that the core does not saturate. Power transmission in a flyback circuit

is limited by the amount of energy that can be stored in the core, while forward circuits are usually

limited by the I/V characteristics of the switches.

Although MOSFET switches can tolerate simultaneous full current and voltage (although

thermal stress and electro migration can shorten the MTBF), bipolar switches generally can't so

require the use of a snubber  (or two).

Capacitive

A charge pump is a kind of DC to DC converter that uses capacitors as energy storage

elements to create either a higher or lower voltage power source. Charge pump circuits are capable

of high efficiencies, sometimes as high as 90–95% while being electrically simple circuits.

Charge pumps use some form of switching device(s) to control the connection of voltages

to the capacitor. For instance, a two-stage cycle can be used to generate a higher pulsed voltage from

a lower-voltage supply. In the first stage of the cycle, a capacitor is connected across the supply,

charging it to that same voltage. In the second stage of the cycle, the circuit is reconfigured so that

the capacitor is in series with the supply to the load. Ignoring leakage effects, this effectively

provides double the supply voltage to the load (the sum of the original supply and the capacitor). The

pulsing nature of the higher voltage output is typically smoothed by the use of an output capacitor.

An external or secondary circuit drives the switching, typically at tens of  kilohertz up to

several megahertz. The high frequency minimizes the amount of capacitance required as less charge

needs to be stored and dumped in a shorter cycle. The capacitor used as the charge pump is typically

known as the "flying capacitor".

Another way to explain the operation of a charge pump is to consider it as the combination

of a DC to AC converter (the switches) followed by a voltage multiplier.

The voltage is load-dependent; higher loads result in lower average voltages.

Charge pumps can double voltages, triple voltages, halve voltages, invert voltages,

fractionally multiply or scale voltages such as x3/2, x4/3, x2/3, etc. and generate arbitrary voltages,

depending on the controller and circuit topology.

The term 'charge pump' is also used in phase-locked loop (PLL) circuits. This is a

completely different application. In a PLL the phase difference between the reference signal (often

from a crystal oscillator) and the output signal is translated into two signals – UP and DN. The two

signals control switches to steer current into or out of a capacitor, causing the voltage across the

capacitor to increase or decrease. In each cycle, the time during which the switch is turned on is

proportional to the phase difference, hence the charge delivered is dependent on the phase difference

also. The voltage on the capacitor is used to tune a voltage-controlled oscillator (VCO), generating

the desired output signal frequency. The use of a charge pump naturally adds a pole at the origin in

the loop transfer function of the PLL, since the charge-pump current is driven into a capacitor to

generate a voltage (V=I/(sC)). The additional pole at the origin is desirable because when

considering the closed-loop transfer function of the PLL, this pole at the origin integrates the error

signal and causes the system to track the input with one more order. The charge pump in a PLL

design is constructed in integrated-circuit (IC) technology, consisting of pull-up, pull-down

transistors and on-chip capacitors. A resistor is also added to stabilize the closed-loop PLL

Electrochemical

A further means of DC to DC conversion in the kiloWatt to many MegaWatts range is

presented by using redox flow batteries such as the vanadium redox battery, although this technique

has not been applied commercially to date.

Switching mode power supply

A switched-mode power supply (switching-mode power supply, SMPS, or

simply switcher) is an electronic power supply that incorporates a switching regulator in order to be

highly efficient in the conversion of electrical power. Like other types of power supplies, an SMPS

transfers power from a source like the electrical power grid to a load (such as a personal computer)

while converting voltage and current characteristics. An SMPS is usually employed to efficiently

provide a regulated output voltage, typically at a level different from the input voltage.

Unlike a linear power supply, the pass transistor of a switching mode supply continually

switches between low-dissipation, full-on and full-off states, and spends very little time in the high

dissipation transitions (which minimizes wasted energy). Ideally, a switched-mode power supply

dissipates no power. Voltage regulation is achieved by varying the ratio of on-to-off time. In

contrast, a linear power supply regulates the output voltage by continually dissipating power in the

pass transistor. This higher power conversion efficiency is an important advantage of a switched-

mode power supply. Switched-mode power supplies may also be substantially smaller and lighter

than a linear supply due to the smaller transformer size and weight.

Switching regulators are used as replacements for the linear regulators when higher

efficiency, smaller size or lighter weights are required. They are, however, more complicated, their

switching currents can cause electrical noise problems if not carefully suppressed, and simple

designs may have a poor power factor.

 Switched-mode power supplies are used for a lot of applications since the efficiency is

high (up to 98%).A large advantage to these converters, is that they are smaller than the linear

regulators due to the high switching frequencies requires smaller inductors and capacitors, which

reduces the volume and mass of a product. This converts a DC voltage to another DC voltage, the

voltage can either be stepped up or down. These converters use inductors and capacitors to store the

energy to be released later for either higher or lower voltage output. All switched mode power

supplies have a switch to control the duty cycle. The duty cycle is used to control the output power of

the system, the switching is done at high frequencies. The switching at high frequencies can cause

electromagnetic interference which needs to be limited in some applications such as medical

instruments. Since the late 1980s bipolar transistors and FETs have been used as switches in a lot of

applications, the FETs have a very low “on resistance” which gives the good efficiencies. Thyristors

are used as switches for HVDC systems, because they can transmit power up to megawatts. Most

switched mode power supplies, supply power in one direction, although there are some bi-directional

power supplies. Unlike the linear regulators the switched mode power supplies don’t create the same

amount of heat when in use and they have a good efficiency, but they are more complex to use they

can create electrical noise and are relatively more expensive. There are several different systems for

switched mode power supplies here are a few of the most common:

Buck converter

Step down voltage. The voltage is determined by the duty cycle of the switch.The output voltage

is Vout=Vin*D(Duty cycle).

Buck converter

A buck converter is a step-down DC to DC converter. Its design is similar to the step-

up boost converter, and like the boost converter it is a switched-mode power supply that uses two

switches (a transistor and a diode), an inductor and a capacitor.

The simplest way to reduce the voltage of a DC supply is to use a  linear

regulator (such as a 7805), but linear regulators waste energy as they operate by dissipating excess

power as heat. Buck converters, on the other hand, can be remarkably efficient (95% or higher for

integrated circuits), making them useful for tasks such as converting the 12–24 V typical battery

voltage in a laptop down to the few volts needed by the processor

Theory of operation

Continuous mode

A buck converter operates in continuous mode if the current through the inductor (IL) never falls to

zero during the commutation cycle. In this mode, the operating principle is described by the plots in

Evolution of the voltages and currents with time in an ideal buck converter operating in continuous

mode.

When the switch pictured above is closed (On-state, top of figure 2), the voltage across the

inductor is  . The current through the inductor rises linearly. As the diode is

reverse-biased by the voltage source V, no current flows through it;

When the switch is opened (off state, bottom of figure 2), the diode is forward biased. The

voltage across the inductor is   (neglecting diode drop). Current IL decreases.

The energy stored in inductor L is

Therefore, it can be seen that the energy stored in L increases during On-time (as IL increases)

and then decreases during the Off-state. L is used to transfer energy from the input to the output

of the converter

Discontinuous mode

In some cases, the amount of energy required by the load is small enough to be transferred in a time

lower than the whole commutation period. In this case, the current through the inductor falls to zero

during part of the period. The only difference in the principle described above is that the inductor is

completely discharged at the end of the commutation cycle (see figure 5). This has, however, some

effect on the previous equations.

Evolution of the voltages and currents with time in an ideal buck converter operating in

discontinuous mode.

We still consider that the converter operates in steady state. Therefore, the energy in the inductor is

the same at the beginning and at the end of the cycle (in the case of discontinuous mode, it is zero).

This means that the average value of the inductor voltage (VL) is zero; i.e., that the area of the yellow

and orange rectangles in figure 5 are the same. This yields:

So the value of δ is:

The output current delivered to the load ( ) is constant, as we consider that the output

capacitor is large enough to maintain a constant voltage across its terminals during a

commutation cycle. This implies that the current flowing through the capacitor has a zero

average value. Therefore, we have :

Boost converter

The voltage is determined by the duty cycle of the switch. The output voltage is Vout=Vin/(1-

D).

Boost converter

A boost converter (step-up converter) is a power converter with an output DC

voltage greater than its input DC voltage. It is a class of switching-mode power supply

(SMPS) containing at least two semiconductor switches (a diode and a transistor) and at least

one energy storage element. Filters made of capacitors (sometimes in combination

with inductors) are normally added to the output of the converter to reduce output voltage ripple.

Buck-boost converter

Step voltage either up or down and inverts it with respect to the input voltage. The

voltage is determined by the duty cycle of the switch. If the duty cycle is less than 50% Vo<Vi,

and if it is duty cycle is greater than 50% Vo>Vi, but both are negative. Vo = -Vi*(D/(1-D)).

Buck-Boost converter

The buck–boost converter is a type of DC-to-DC converter that has an output voltage

magnitude that is either greater than or less than the input voltage magnitude.

Two different topologies are called buck–boost converter. Both of them can produce a

range of output voltages, from an output voltage much larger (in absolute magnitude) than the input

voltage, down to almost zero.

The inverting topology

The output voltage is of the opposite polarity as the input. This is

a switched-mode power supply with a similar circuit topology to the boost converter and

the buck converter. The output voltage is adjustable based on the duty cycle of the switching

transistor. One possible drawback of this converter is that the switch does not have a terminal

at ground; this complicates the driving circuitry. Neither drawback is of any consequence if

the power supply is isolated from the load circuit (if, for example, the supply is a battery) as

the supply and diode polarity can simply be reversed. The switch can be on either the ground

side or the supply side.

A buck (step-down) converter followed by a boost (step-up) converter

The output voltage is of the same polarity as the input, and can be lower or

higher than the input. Such a non-inverting buck-boost converter may use a single inductor

that is used as both the buck inductor and the boost inductor.

Advantages and disadvantages of (SMPS)

The main advantage of this method is greater efficiency because the switching

transistor dissipates little power when it is outside of its active region (i.e., when the transistor acts

like a switch and either has a negligible voltage drop across it or a negligible current through it).

Other advantages include smaller size and lighter weight (from the elimination of low frequency

transformers which have a high weight) and lower heat generation due to higher efficiency.

Disadvantages include greater complexity, the generation of high-amplitude, high-frequency energy

that the low-pass filter must block to avoid electromagnetic interference (EMI), a ripple voltage at

the switching frequency and the harmonic frequencies thereof.

Very low cost SMPSs may couple electrical switching noise back onto the mains

power line, causing interference with A/V equipment connected to the same phase. Non-power-

factor-correctedSMPSs also cause harmonic distortion

Operation of SMPS

Block diagram of a mains operated AC/DC SMPS with output voltage regulation

Input rectifier stage

If the SMPS has an AC input, then the first stage is to convert the input to DC. This is

called rectification. The rectifier circuit can be configured as a voltage doubler by the addition of a

switch operated either manually or automatically. This is a feature of larger supplies to permit

operation from nominally 120 V or 240 V supplies. The rectifier produces an unregulated DC

voltage which is then sent to a large filter capacitor. The current drawn from the mains supply by this

rectifier circuit occurs in short pulses around the AC voltage peaks. These pulses have significant

high frequency energy which reduces the power factor. Special control techniques can be employed

by the SMPS to force the average input current to follow the sinusoidal shape of the AC input

voltage, correcting the power factor. An SMPS with a DC input does not require this stage. An

SMPS designed for AC input can often be run from a DC supply (for230 V AC this would be 330 V

DC), as the DC passes through the rectifier stage unchanged. It's however advisable to consult the

manual before trying this, though most supplies are quite capable of such operation even though

nothing is mentioned in the documentation. However, this type of use may be harmful to the rectifier

stage as it will only use half of diodes in the rectifier for the full load. This may result in overheating

of these components, and cause them to fail prematurely.[15]

If an input range switch is used, the rectifier stage is usually configured to operate as

a voltage doubler when operating on the low voltage (~120 V AC) range and as a straight rectifier

when operating on the high voltage (~240 V AC) range. If an input range switch is not used, then a

full-wave rectifier is usually used and the downstream inverter stage is simply designed to be flexible

enough to accept the wide range of DC voltages that will be produced by the rectifier stage. In

higher-power SMPSs, some form of automatic range switching may be used.

AC, half-wave and full-wave rectified signals.

Inverter stage

This section refers to the block marked chopper in the block diagram.

The inverter stage converts DC, whether directly from the input or from the rectifier

stage described above, to AC by running it through a power oscillator, whose output transformer is

very small with few windings at a frequency of tens or hundreds of kilohertz. The frequency is

usually chosen to be above 20 kHz, to make it inaudible to humans. The output voltage is optically

coupled to the input and thus very tightly controlled. The switching is implemented as a multistage

(to achieve high gain) MOSFET amplifier. MOSFETs are a type of transistor with a low on-

resistance and a high current-handling capacity.

Voltage converter and output rectifier

If the output is required to be isolated from the input, as is usually the case in mains

power supplies, the inverted AC is used to drive the primary winding of a high-

frequency transformer. This converts the voltage up or down to the required output level on its

secondary winding. The output transformer in the block diagram serves this purpose.

If a DC output is required, the AC output from the transformer is rectified. For output

voltages above ten volts or so, ordinary silicon diodes are commonly used. For lower

voltages, Schottky diodes are commonly used as the rectifier elements; they have the advantages of

faster recovery times than silicon diodes (allowing low-loss operation at higher frequencies) and a

lower voltage drop when conducting. For even lower output voltages, MOSFETs may be used

as synchronous rectifiers; compared to Schottky diodes, these have even lower conducting state

voltage drops.

The rectified output is then smoothed by a filter consisting of  inductors and capacitors.

For higher switching frequencies, components with lower capacitance and inductance are needed.

Simpler, non-isolated power supplies contain an inductor instead of a transformer.

This type includes boost converters, buck converters, and the buck-boost converters. These belong

to the simplest class of single input, single output converters which use one inductor and one active

switch. The buck converter reduces the input voltage in direct proportion to the ratio of conductive

time to the total switching period, called the duty cycle. For example an ideal buck converter with a

10 V input operating at a 50% duty cycle will produce an average output voltage of 5 V. A feedback

control loop is employed to regulate the output voltage by varying the duty cycle to compensate for

variations in input voltage. The output voltage of a boost converter is always greater than the input

voltage and the buck-boost output voltage is inverted but can be greater than, equal to, or less than

the magnitude of its input voltage. There are many variations and extensions to this class of

converters but these three form the basis of almost all isolated and non-isolated DC to DC

converters. By adding a second inductor the Ćuk and SEPIC converters can be implemented, or, by

adding additional active switches, various bridge converters can be realised.

Other types of SMPSs use a  capacitor-diode voltage multiplier instead of inductors

and transformers. These are mostly used for generating high voltages at low currents (Cockcroft-

Walton generator). The low voltage variant is called charge pump.

Voltage

Voltage, otherwise known as electrical potential difference or electric

tension (denoted ∆V and measured in volts, or joules per coulomb) is the potential difference

between two points — or the difference in electric potential energy per unit charge between two

points. Voltage is equal to the work which would have to be done, per unit charge, against a

static electric field to move the charge between two points. A voltage may represent either a source

of energy (electromotive force), or it may represent lost or stored energy (potential drop).

A voltmeter can be used to measure the voltage (or potential difference) between two points in a

system; usually a common reference potential such as the ground of the system is used as one of the

points. Voltage can be caused by static electric fields, by electric current through a magnetic field,

by time-varying magnetic fields, or a combination of all three.

Definition

The voltage between two ends of a path is the total energy required to move a small

electric charge along that path, divided by the magnitude of the charge. Mathematically this is

expressed as the line integral of the electric field and the time rate of change of magnetic field along

that path. In the general case, both a static (unchanging) electric field and a dynamic (time-varying)

electromagnetic field must be included in determining the voltage between two points.

Historically this quantity has also been called "tension" and "pressure". Pressure is now obsolete but

tension is still used, for example within the phrase "high tension" (HT) which is commonly used in

thermionic valve (vacuum tube) based electronics.

Voltage is defined so that negatively-charged objects are pulled towards higher

voltages, while positively-charged objects are pulled towards lower voltages. Therefore,

the conventional current in a wire or resistor always flows from higher voltage to lower voltage.

Current can flow from lower voltage to higher voltage, but only when a source of energy is present

to "push" it against the opposing electric field. For example, inside a battery, chemical reactions

inside the battery provide the energy needed for current to flow from the negative to the positive

terminal.

Applications

Specifying a voltage measurement requires explicit or implicit specification of the

points across which the voltage is measured. When using a voltmeter to measure potential difference,

one electrical lead of the voltmeter must be connected to the first point, one to the second point.

A common use of the term "voltage" is in describing the voltage dropped across an

electrical device (such as a resistor). The voltage drop across the device can be understood as the

difference between measurements at each terminal of the device with respect to a common reference

point (or ground). The voltage drop is the difference between the two readings. Two points in an

electric circuit that are connected by an ideal conductor without resistance and not within a

changing magnetic field, have a voltage of zero. Any two points with the same potential may be

connected by a conductor and no current will flow between them.

Addition of voltages

The voltage between A and C is the sum of the voltage between A and B and the

voltage between B and C. The various voltages in a circuit can be computed using Kirchhoff's circuit

laws.

When talking about alternating current (AC) there is a difference between

instantaneous voltage and average voltage. Instantaneous voltages can be added for direct

current (DC) and AC, but average voltages can be meaningfully added only when they apply to

signals that all have the same frequency and phase.

Voltage gain

In electronics, gain is a measure of the ability of a circuit (often an amplifier) to increase

the power or amplitude of a signal from the input to the output. It is usually defined as the

mean ratio of the signal output of a system to the signal input of the same system. It may also be

defined on a logarithmic scale, in terms of the decimal logarithm of the same ratio ("dB gain"). A

gain greater than one (zero dB), that is, amplification, is the defining property of an active

component or circuit, while a passive circuit will have a gain of less than one.

Thus, the term gain on its own is ambiguous. For example, "a gain of five" may imply

that either the voltage, current or the power is increased by a factor of five, although most often this

will mean a voltage gain of five for audio and general purpose amplifiers, especially operational

amplifiers, but a power gain for radio frequency amplifiers, and for directional aerials will refer to a

signal power change compared with a simple dipole. Furthermore, the term gain is also applied in

systems such as sensors where the input and output have different units; in such cases the gain units

must be specified, as in "5 microvolts per photon" for the responsivity of a photosensor. The "gain"

of a bipolar transistor normally refers to forward current transfer ratio, either hFE ("Beta", the static

ratio of Ic divided by Ibat some operating point), or sometimes hfe (the small-signal current gain, the

slope of the graph of Ic against Ib at a point).

In laser physics, gain may refer to the increment of power along the beam propagation in

a gain medium, and its dimension is m−1 (inverse meter) or 1/meter.

Voltage gain

When power gain is calculated using voltage instead of power, making the

substitution (P=V 2/R), the formula is:

In many cases, the input and output impedances are equal, so the above equation can be

simplified to:

and then the 20 log rule:

This simplified formula is used to calculate a voltage gain in decibels, and is equivalent

to a power gain only if the impedances at input and output are equal.

Amplitude

Amplitude is the magnitude of change in the oscillating variable with

each oscillation within an oscillating system. For example, sound waves in air are oscillations

in atmospheric pressure and their amplitudes are proportional to the change in pressure during one

oscillation. If a variable undergoes regular oscillations, and a graph of the system is drawn with the

oscillating variable as the vertical axis and time as the horizontal axis, the amplitude is visually

represented by the vertical distance between the extrema of the curve and the equilibrium value.

In older texts the phase is sometimes very confusingly called the amplitude.[1]

Peak-to-peak amplitude

Peak-to-peak amplitude is the change between peak (highest amplitude value) and

trough (lowest amplitude value, which can be negative). With appropriate circuitry, peak-to-peak

amplitudes can be measured by meters or by viewing the waveform on an oscilloscope. Peak-to-peak

is a straightforward measurement on an oscilloscope, the peaks of the waveform being easily

identified and measured against the graticule. This remains a common way of specifying amplitude,

but sometimes other measures of amplitude are more appropriate.

Peak amplitude

In audio system measurements, telecommunications and other areas where

the measurand is a signal that swings above and below a zero value but is not sinusoidal, peak

amplitude is often used. This is the maximum absolute value of the signal.

Semi-amplitude

Semi-amplitude means half the peak-to-peak amplitude.  It is the most widely used

measure of orbital amplitude in astronomy and the measurement of small semi-amplitudes of nearby

stars is important in the search for exoplanets. For a sine wave, peak amplitude and semi-amplitude

are the same.

Some scientists use "amplitude" or "peak amplitude" to mean semi-amplitude, that is, half the peak-

to-peak amplitude.

Root mean square amplitude

Root mean square (RMS) amplitude is used especially in electrical engineering:

the RMS is defined as the square root of the mean over time of the square of the vertical distance of

the graph from the rest state.

For complex waveforms, especially non-repeating signals like noise, the RMS

amplitude is usually used because it is both unambiguous and has physical significance. For

example, the average powertransmitted by an acoustic or electromagnetic wave or by an electrical

signal is proportional to the square of the RMS amplitude (and not, in general, to the square of the

peak amplitude).

For  alternating current electrical power, the universal practice is to specify RMS

values of a sinusoidal waveform. One property of root mean square voltages and currents is that they

produce the same heating effect as DC in a given resistance.

A sinusoidal curve

1 = Peak amplitude ( ),

2 = Peak-to-peak amplitude ( ),

3 = RMS amplitude ( ),

4 = Wave period (not an amplitude)

The peak-to-peak voltage of a sine wave is about 2.8 times the RMS value. The peak-

to-peak value is used, for example, when choosing rectifiers for power supplies, or when estimating

the maximum voltage insulation must withstand. Some common voltmeters are calibrated for RMS

amplitude, but respond to the average value of a rectified waveform. Many digital voltmeters and all

moving coil meters are in this category. The RMS calibration is only correct for a sine wave input

since the ratio between peak, average and RMS values is dependent on waveform. If the wave shape

being measured is greatly different from a sine wave, the relationship between RMS and average

value changes. True RMS-responding meters were used in radio frequency measurements, where

instruments measured the heating effect in a resistor to measure current. The advent

of microprocessor controlled meters capable of calculating RMS bysampling the waveform has made

true RMS measurement commonplace.

Ambiguity

In general, the use of peak amplitude is simple and unambiguous only for

symmetric periodic waves, like a sine wave, a square wave, or a triangular wave. For an asymmetric

wave (periodic pulses in one direction, for example), the peak amplitude becomes ambiguous. This is

because the value is different depending on whether the maximum positive signal is measured

relative to the mean, the maximum negative signal is measured relative to the mean, or the maximum

positive signal is measured relative to the maximum negative signal (the peak-to-peak amplitude)

and then divided by two. In electrical engineering, the usual solution to this ambiguity is to measure

the amplitude from a defined reference potential (such as ground or 0 V). Strictly speaking, this is no

longer amplitude since there is the possibility that a constant (DC component) is included in the

measurement.

Pulse amplitude

In telecommunication, pulse amplitude is the magnitude of a pulse parameter, such as

the voltage level, current level, field intensity, or power level. Pulse amplitude is measured with

respect to a specified reference and therefore should be modified by qualifiers, such as "average",

"instantaneous", "peak", or "root-mean-square".

Pulse amplitude also applies to the amplitude of  frequency- and phase-

modulated waveform envelopes.

Rectifier

A  rectifier is an electrical device that converts alternating current (AC), which

periodically reverses direction, to direct current (DC), which flows in only one direction. The process

is known as rectification. Physically, rectifiers take a number of forms, including vacuum

tube diodes, mercury-arc valves, solid-state diodes, silicon-controlled rectifiers and other silicon-

based semiconductor switches. Historically, even synchronous electromechanical switches and

motors have been used. Early radio receivers, called crystal radios, used a "cat's whisker" of fine

wire pressing on a crystal of galena (lead sulfide) to serve as a point-contact rectifier or "crystal

detector".

Rectifiers have many uses, but are often found serving as components of

DC power supplies and high-voltage direct current power transmission systems. Rectification may

serve in roles other than to generate direct current for use as a source of power. As

noted, detectors of radio signals serve as rectifiers. In gas heating systems flame rectification is used

to detect presence of flame.

The simple process of rectification produces a type of DC characterized by

pulsating voltages and currents (although still unidirectional). Depending upon the type of end-use,

this type of DC current may then be further modified into the type of relatively constant voltage DC

characteristically produced by such sources as batteries and solar cells.

A device which performs the opposite function (converting DC to AC) is known as an inverter.

Rectifier devices

Before the development of silicon semiconductor rectifiers, vacuum tube diodes

and copper(I) oxide or selenium rectifier stacks were used. High power rectifiers, such as are used

in high-voltage direct current power transmission, now uniformly employ silicon semiconductor

devices of various types. These are thyristors or other controlled switching solid-state switches which

effectively function as diodes to pass current in only one direction.

Half-wave rectification

In half wave rectification, either the positive or negative half of the AC wave is

passed, while the other half is blocked. Because only one half of the input waveform reaches the

output, it is very inefficient if used for power transfer. Half-wave rectification can be achieved with a

single diode in a one-phase supply, or with three diodes in a three-phase supply. Half wave rectifiers

yield a unidirectional but pulsating direct current.

Half wave rectifier

The output DC voltage of a half wave rectifier can be calculated with the following two ideal

equations:

Full-wave rectification

A full-wave rectifier converts the whole of the input waveform to one of constant

polarity (positive or negative) at its output. Full-wave rectification converts both polarities of the

input waveform to DC (direct current), and is more efficient. However, in a circuit with a non-center

tapped transformer, four diodes are required instead of the one needed for half-wave rectification

(see semiconductors and diode). Four diodes arranged this way are called a diode bridge or bridge

rectifier.

Full wave rectifier

For single-phase AC, if the transformer is center-tapped, then two diodes back-to-back

(i.e. anodes-to-anode or cathode-to-cathode) can form a full-wave rectifier. Twice as many windings

are required on the transformer secondary to obtain the same output voltage compared to the bridge

rectifier