24/7/1437

dc-dc

converter Power electronics devices

Supervisor doctor: MOHAMMED T. LAZIM

Professor of Electrical and Electronics Engineering

Electrical Engineering Department

Philadelphia University, Jordan

Student name:moaz mh zafer al hosny

Student number:201310175

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DC CHOPPERS INTRODUCTION

A chopper is a static device which is used to obtain a variable dc voltage from a

constant dc voltage source. A chopper is also known as dc-to-dc converter. The

thyristor converter offers greater efficiency, faster response, lower maintenance,

smaller size and smooth control. Choppers are widely used in trolley cars, battery

operated vehicles, traction motor control, control of large number of dc motors,

etc….. They are also used in regenerative braking of dc motors to return energy back

to supply and also as dc voltage regulators.

Choppers are of two types

• Step-down choppers

• Step-up choppers.

In step-down choppers, the output voltage will be less than the input voltage

whereas in step-up choppers output voltage will be more than the input voltage.

Classification of Choppers:

(a) Depending upon the direction of the output current and voltage, the

converters can be classified into five classes namely

Class A [One-quadrant Operation]

Class B [One-quadrant Operation]

Class C [Two-quadrant Operation]

Class D [Two-quadrant Operation]

Class E [Four-quadrant Operation]

(b) Based on the output voltage of the output, the choppers are classified as

(i) Step-Down Chopper

In this case the average output voltage is less than the input voltage. It is also known

as step down converter

(ii) Step-Up Chopper

Here the average output voltage is more than the input voltage. It is also known as

step up converter

(iii) Step-Up/Down Chopper

This type of converter produces an output voltage that is either lower or higher than

the input voltage

(c) Depending upon the power loss occurred during turn ON/OFF of the switching

device, the choppers are classified into two categories namely

(i) Hard switched Converter

Here the power loss is high during the switching (ON to OFF and OFF to ON) as a

result of the non zero voltage and current on the power switches.

(ii) Soft switched or resonant converters

In this type of choppers, the power loss is low at the time of switching as a result of

zero voltage and/or zero current on the switches.

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PRINCIPLE OF STEP-DOWN CHOPPER

Figure 2.1 shows a step-down chopper with resistive load. The thyristor in the

circuit acts as a switch. When thyristor is ON, supply voltage appears across the load

and

when thyristor is OFF, the voltage across the load will be zero. The output voltage and

current waveforms are as shown in figure 2.2.

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The output voltage can be varied by varying the duty cycle.

METHODS OF CONTROL:

pulse width modulation

In pulse width modulation the pulse width of the output waveform is varied

keeping chopping frequency ‘f’ and hence chopping period ‘T’ constant. Therefore

output voltage is varied by varying the ON time, Figure 2.3 shows the output

voltage waveforms for different ON times.

VARIABLE FREQUENCY CONTROL

In this method of control, chopping frequency f is varied keeping either ton or

Toff constant. This method is also known as frequency modulation.

Figure 2.4 shows the output voltage waveforms for a constant ton and variable

chopping period T.

In frequency modulation to obtain full output voltage, range frequency has to be

varied over a wide range. This method produces harmonics in the output and for large

toff load current may become discontinuous.

STEP-DOWN CHOPPER WITH R-L LOAD

Figure 2.5 shows a step-down chopper with R-L load and free wheeling diode.

When chopper is ON, the supply is connected across the load. Current flows from the

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supply to the load. When chopper is OFF, the load current io continues to flow in the

same direction through the free-wheeling diode due to the energy stored in the

inductor L. The load current can be continuous or discontinuous depending on the

values of L and

duty cycle, d. For a continuous current operation the load current is assumed to vary

between two limits imin and Imax .

Figure 2.6 shows the output current and output voltage waveforms for a continuous

current and discontinuous current operation.

When the current exceeds Imax the chopper is turned-off and it is turned-on when

current reduces to I min .

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PRINCIPLE OF STEP-UP CHOPPER

Figure 2.13 shows a step-up chopper to obtain a load voltage O V higher than the input

voltage V. The values of L and C are chosen depending upon the requirement of

output voltage and current. When the chopper is ON, the inductor L is connected

across the supply. The inductor current ‘I’ rises and the inductor stores energy during

the ON time of the chopper, ON t . When the chopper is off, the inductor current I is

forced to flow through the diode D and load for a period, TOFF . The current tends to

decrease resulting in reversing the polarity of induced EMF in L. Therefore voltage

across load is given by

If a large capacitor ‘C’ is connected across the load then the capacitor will provide

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a continuous output voltage O V . Diode D prevents any current flow from capacitor to

the source. Step up choppers are used for regenerative braking of dc motors.

EXPRESSION FOR OUTPUT VOLTAGE

Assume the average inductor current to be I during ON and OFF time of Chopper.

When Chopper is ON

Voltage across inductor L =V

Therefore energy stored in inductor

PERFORMANCE PARAMETERS

The thyristor requires a certain minimum time to turn ON and turn OFF. Hence

duty cycle d can be varied only between a minimum and a maximum value, limiting

the minimum and maximum value of the output voltage. Ripple in the load current

depends inversely on the chopping frequency, f. Therefore to reduce the load ripple

current, frequency should be as high as possible.

CLASSIFICATION OF CHOPPERS

Choppers are classified as follows

• Class A Chopper

• Class B Chopper

• Class C Chopper

• Class D Chopper

• Class E Chopper

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• Class A Chopper is a first quadrant chopper • When chopper is ON, supply voltage V is connected across the load. • When chopper is OFF, vO = 0 and the load current continues to flow in the same direction through the FWD. • The average values of output voltage and current are always positive. Class A Chopper is a first quadrant chopper • When chopper is ON, supply voltage V is connected across the load. • When chopper is OFF, vO = 0 and the load current continues to flow in the same direction through the FWD. • The average values of output voltage and current are always positive. • Class A Chopper is a step-down chopper in which power always flows form source to load. • It is used to control the speed of dc motor. • The output current equations obtained

in step down chopper with R-L load can be used to study the performance of Class A Chopper.

Class B [One-quadrant Operation]

• Class B Chopper is a step-up chopper

• When chopper is ON, E drives a current

through L and R in a direction opposite to

that shown in figure.

• During the ON period of the chopper, the

inductance L stores energy.

• When Chopper is OFF, diode D

conducts, and part of the energy stored in

inductor L is returned to the supply.

• Average output voltage is positive.

Average output current is negative.

• Therefore Class B Chopper operates in

second quadrant.

• In this chopper, power flows from load to

source.

• Class B Chopper is used for regenerative

braking of dc motor.

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Class C [Two-quadrant Operation] • Class C Chopper can be used as a step-up or step-down chopper • Class C Chopper is a combination of Class A and Class B Choppers. • For first quadrant operation, CH1 is ON or D2 conducts. • For second quadrant operation, CH2 is ON or D1 conducts. • When CH1 is ON, the load current is positive. • The output voltage is equal to ‘V’ & the load receives power from the source. • When CH1 is turned OFF, energy stored in inductance L forces current to flow through the diode D2 and the output voltage is zero. • Current continues to flow in positive direction. • When CH2 is triggered, the voltage E forces current to flow in opposite direction through L and CH2 . • The output voltage is zero. • On turning OFF CH2 , the energy stored in the inductance drives current through diode D1 and the supply • Output voltage is V, the input current becomes negative and power flows from load to source.

• Average output voltage is positive • Average output current can take both positive and negative values. • Choppers CH1 & CH2 should not be turned ON simultaneously as it would result in short circuiting the supply. • Class C Chopper can be used both for dc motor control and regenerative braking of dc motor.

Class D Chopper [Two-quadrant

Operation]

• Class D is a two quadrant chopper.

• When both CH1 and CH2 are triggered

simultaneously, the output voltage vO = V

and output current flows through the load.

• When CH1 and CH2 are turned OFF, the

load current continues to flow in the same

direction through load, D1 and D2 , due to

the energy stored in the inductor L.

• Output voltage vO = – V.

• Average load voltage is positive if

chopper ON time is more than the OFF

time

• Average output voltage becomes

negative if tON < tOFF .

• Hence the direction of load current is

always positive but load voltage can be

positive or negative.

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Class E Chopper [Four-quadrant Operation

• Class E is a four quadrant chopper

• When CH1 and CH4 are triggered, output current iO flows in positive direction through

CH1 and CH4, and with output voltage vO = V.

• This gives the first quadrant operation.

• When both CH1 and CH4 are OFF, the energy stored in the inductor L drives iO through D2

and D3 in the same direction, but output voltage vO = -V.

• Therefore the chopper operates in the fourth quadrant.

• When CH2 and CH3 are triggered, the load current iO flows in opposite direction & output

voltage vO = -V.

• Since both iO and vO are negative, the chopper operates in third quadrant.

• When both CH2 and CH3 are OFF, the load current iO continues to flow in the same

direction D1 and D4 and the output voltage vO = V.

• Therefore the chopper operates in second quadrant as vO is positive but iO is negative.

Application of Power Electronics in Renewable Energy Systems Nowadays, the electrical power generation from renewable energy sources has

become a focal point in research because of environmental problems and a

perceived of traditional energy sources in the near future. Since last decade,

researchers have been working on electrical systems for variable speed wind

turbines. The main advantages of variable speed are noise reduction, maximum

power tracking, and proper controlled torque and in this manner, the possibility to

damp resonance and avoid speeds causing resonance. Several electrical systems

have been presented to connect the wind turbine with variable speed and frequency

to the constant voltage and frequency of the network. The main aspects of these

topologies are increased efficiency and robustness, a decrease in the size and

maintenance of the system and eventually reduction of whole system expense.

On the other hand, grid connected PV systems, mostly single-phase PV systems

and their contribution to clean power generation, is recognized more and more

worldwide. The main advantages of PV system are long lifetime, high efficiency and

good environmental condition. The most important issues for grid connected PV to

gain wide acceptance are reliability and low cost. There are two approaches to

achieve high voltage and high efficiency, one is to connect the cells in series to

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generate high voltage DC and use high voltage DC to an AC inverter circuit.

However, this configuration needs high voltage rate devices for the inverter. Another

approach is to use low voltage devices for the inverter and then step up the voltage

using transformers. This can increase losses and cost of system. Using

transformerless concepts are advantageous with regard to their high efficiency and

the resulting benefits of reduction in cost, size, weight and complexity of the inverter.

Another renewable energy source is FC which are considered attractive for

Distributed Generation (DG) applications. Fuel cells are electrochemical devices that

convert the chemical energy of fuel and oxidant directly to electrical energy and heat.

In fuel cell powered applications, a fuel cell (low power) will supply the system, then

a DC-DC converter is used to boost the low voltage of the fuel cell to make a high

voltage DC link. A DC-AC inverter is used to obtain AC voltage to feed the load.

List of applications:

1- Electric vehicles

2- Variable Speed Wind Turbine Systems

Power conversion in WT systems using back-to-back configuration

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Power conversion in WT systems using rectifier and step-up converter 3- Photovoltaic and Fuel Cell Systems

Power conversion in transformerless PV systems

Multilevel converter intransformerless PV systems

SIMULATION RESULTS OF DC-DC CONVERTERS on PSpice

Circuit for the simulation of buck converter

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References: 1- Dc Choppers from nptle university. 2- http://www.completepowerelectronics.com/converters/chopper/ 3- RASHID_Power_Electronics_Handbook. 4- Mit open course for power electronics.