Swarna Fina Tecgh (1).docx

DC TO DC CONVERTER CHAPTER 1 1.1

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.

1.2

PRINCIPLE OF STEP-DOWN CHOPPER

Fig 1.1:Step-down chopper with resistive load Above figure 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. SAMSKRUTI COLLEGE OF ENGINEERING & TECHNOLOGY

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DC TO DC CONVERTER

Fig 1.2: Step-down choppers –output voltage and current waveforms Vdc = average value of output or load voltage Idc = average value of output or load current tON = time interval for which SCR conducts tOFF = time interval for which SCR is OFF. T + tON + tOFF 1/T f = = period of switching or chopping period frequency of chopper switching or chopping frequency. Average output voltage

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DC TO DC CONVERTER

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DC TO DC CONVERTER

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DC TO DC CONVERTER CHAPTER 2 METHODS OF CONTROL The output dc voltage can be varied by the following methods. nPulse width modulation control or constant frequency operation. Variable frequency control.

2.1

PULSE WIDTH MODULATION

In pulse width modulation the pulse width (ton) 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, tON . Figure shows the output voltage waveforms for different ON times.

Fig 2.1: Pulse Width Modulation Control

2.2

VARIABLE FREQUENCY CONTROL In this method of control, chopping frequency f is varied keeping either constant This

method is also known as frequency modulation. Above Figure shows the output voltage waveforms for a constant chopping period T. tON and variable 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 current may become discontinuous. SAMSKRUTI COLLEGE OF ENGINEERING & TECHNOLOGY

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DC TO DC CONVERTER

Fig 2.2: Output Voltage Waveforms for Time Ratio Control

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DC TO DC CONVERTER CHAPTER 3 3.1

STEP-DOWN CHOPPER WITH R-L LOAD Below Figure 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 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 shows the output current and output voltage waveforms for a continuous current and discontinuous current operation.

Fig 3.1: Step Down Chopper with R-L Load

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DC TO DC CONVERTER

Fig 3.2: Output Voltage and Load Current Waveforms (Continuous Current) When the current exceeds current reduces to Imin . Imax the chopper is turned-off and it is turned-on when EXPRESSIONS

FOR LOAD CURRENT

iO FOR CONTINUOUS

CURRENT

OPERATION WHEN CHOPPER IS ON (0
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DC TO DC CONVERTER

When Chopper is OFF (0 < t < tOFF ) i0

R L E Voltage equation for the circuit shown in figure is

Taking Laplace transform

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DC TO DC CONVERTER Redefining time origin we have at t = 0 , initial current iO

Taking Inverse Laplace Transform

The expression is valid for

, i.e., during the period chopper is OFF. At the instant

the chopper is turned ON or at the end of the off period, the load current is

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DC TO DC CONVERTER

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DC TO DC CONVERTER

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DC TO DC CONVERTER

3.2 I

PRINCIPLE OF STEP-UP CHOPPER L

D +

+ V

Fig.3.3: Step-up Chopper Figure shows a step-up chopper to obtain a load voltage VO 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, tON. When the chopper is off, the inductor current I is forced to flow through the diode D and load for a period, SAMSKRUTI COLLEGE OF ENGINEERING & TECHNOLOGY

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DC TO DC CONVERTER 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 a continuous output voltage VO . Diode D prevents any current flow from capacitor to the source. Step up choppers are used for regenerative braking of dc motors.

3.3

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 = V .I.Ton , where tON = ON period of chopper.

When Chopper is OFF (energy is supplied by inductor to load) Voltage across L < VO –V Energy supplied by inductor L =(VO -V )ItOFF , where Chopper. tOFF = OFF period of

Neglecting losses, energy stored in inductor L = energy supplied by inductor L

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DC TO DC CONVERTER Therefore

3.4

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.

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DC TO DC CONVERTER CHAPTER 4 CLASSIFICATION OF CHOPPERS Choppers are classified as follows Class A Chopper Class B Chopper Class C Chopper Class D Chopper Class E Chopper

4.1 Class A Chopper v0

i0 + Chopper L O

FWD

A D

V

v0 V



i0

Fig 4.1: class A 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 .  Class A Chopper is a step-down chopper in which power always flows form source to load. SAMSKRUTI COLLEGE OF ENGINEERING & TECHNOLOGY

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DC TO DC CONVERTER  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. ig Thyristor gate pulse t Output current

i0 CH ON FWD Conducts

Output voltage

t

v0 tON T

fig 4.2: Output Voltage And Current Wave Forms

4.2 Class B Chopper D

i v0

0

R

L v0

V Chopper

E

i0

Fig 4.3: class B chopper

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DC TO DC CONVERTER

 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 o 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.  Class B Chopper is a step-up chopper.

ig

Thyristor gate pulse

tOFF i0

tON

t

T Output current t

Imax

D conducts Chopper conducts

Imin

Output voltage

v0

fig 4.4: Output Voltage And Current Wave Forms

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DC TO DC CONVERTER 4.3 Class C Chopper CH1

D1 v0

i0

+ R

V CH2

L v0

D2 Chopper

E

 i0

Fig Fig 4.5: class C 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. SAMSKRUTI COLLEGE OF ENGINEERING & TECHNOLOGY

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DC TO DC CONVERTER

ig1

Gate pulse of CH1 t

ig2

Gate pulse of CH2 t

i0 Output current t D1

CH1 ON

D2

CH2 ON

D1

CH1 ON

D2

CH2 ON

V0

Output voltage

t

Fig 4.6: Output Voltage And Current Wave Forms

4.4 Class D Chopper

v0

CH1 R i0 + V

D1

L

E

D2



v0

CH2

i0

Fig 4.7: class D chopper

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DC TO DC CONVERTER  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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DC TO DC CONVERTER

Fig 4.8: output voltage and current wave forms

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DC TO DC CONVERTER 4.5 CLASS E CHOPPER

Fig 4.9: class E chopper

FOUR QUADRANT OPERATION

Fig 4.10:Four quadrant operation

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DC TO DC CONVERTER

•

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

4.6

EFFECT OF SOURCE & LOAD INDUCTANCE •

The source inductance should be as small as possible to limit the transient voltage.

•

Also source inductance may cause commutation problem for the chopper.

•

Usually an input filter is used to overcome the problem of source inductance.

•

The load ripple current is inversely proportional to load inductance and chopping frequency.

•

Peak load current depends on load inductance.

•

To limit the load ripple current, a smoothing inductor is connected in series with the load.

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DC TO DC CONVERTER CHAPTER 5 EXAMPLES ON STEP UP AND DOWN CHOPPERS 5.1 EXAMPLE1: A dc chopper has a resistive load of R=10Q and input voltage of V = 200 V. When chopper is ON, its voltage drop is 2 V and the chopping frequency is 1 kHz. If the duty cycle is 60%, determine 10R Average output voltage RMS value of output voltage Effective input resistance of chopper Chopper efficiency.

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DC TO DC CONVERTER

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DC TO DC CONVERTER

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DC TO DC CONVERTER

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DC TO DC CONVERTER 5.2

EXAMPLE 2;

An 80 V battery supplies RL load through a DC chopper. The load has a freewheeling diode across it is composed of 0.4 H in series with 5Ω resistor. Load current, due to improper selection of frequency of chopping, varies widely between 9A and 10.2A. (a) Find the average load voltage, current and the duty cycle of the chopper. (b) What is the operating frequency f ? (c) Find the ripple current to maximum current ratio. Solution:

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DC TO DC CONVERTER

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DC TO DC CONVERTER 5.3

EXAMPLE 3: A transistor dc chopper circuit (Buck converter) is supplied with power form an ideal battery of 100 V. The load voltage waveform consists of rectangular pulses of duration 1 ms in an overall cycle time of 2.5 ms. Calculate, for resistive load of 10 Ω.

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DC TO DC CONVERTER CHAPTER 6 SIMULATION RESULT OF STEP DOWN CONVERTER

Fig 6.1 : buck converter sub system

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DC TO DC CONVERTER

Fig 6.2: Input Voltage, Output Voltage and Inductor Current waveforms

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DC TO DC CONVERTER CHAPTER 7 ADVANTAGES, APPLICATIONS 7.1

ADVANTAGES  DC TO DC CONVERTERS ARE used to provide smooth control and output  High efficiency  It has fast response and regeneration  To provide efficient control on dc motor operation  It has long life and less maintenance due to absence of moving parts

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DC TO DC CONVERTER 7.2

APPLICATIONS  Dc to dc converters (choppers) are used for dc motor control (battery supplied vehicles)  Solar and wind energy conservation  It is also used in electric cars  Airplanes and space ships where on board regulated dc power supplies are required  Chopper circuits are used as power supplies in computers commercial electronics and electronic instruments

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DC TO DC CONVERTER CHAPTER 8 CONCLUSION, FUTURE SCOPE 8.1 CONCLUSION A soft switched DC-DC self buck converter is analyzed, designed, simulated and implemented. The experimental resonant waveforms of this converter are confirmed with the theoretical and simulated waveforms. A simple control method using dSpic microcontroller, the pulse signal is generated which increase the power packing density of the converter. The variation in the output voltage, gain and efficiency for varying load and duty cycle are plotted. The maximum efficiency of this converter is found to be 90% at k=0.6. To double the output voltage, a soft switched voltage re-lift DC-DC converter is also analyzed, designed, simulated and implemented. The converter has been simulated using MATLAB software. The simulated waveforms are compared with the theoretical waves and they both coincide with each other. The experimented resonant waveforms of the converter are confirmed with the simulated and theoretical resonant waveforms. The variation in the output voltage, gain and efficiency for varying load and duty cycle are plotted. The maximum efficiency of this converter is found to be 80% at k=0.7. The above topologies have voltage gain which varies in the arithmetic manner. To increase the voltage gain in a geometric progression, super buck and double buck DC-DC converters are considered. A high gain soft switched super buck DC-DC converter is analyzed, designed, simulated and implemented. The experimented resonant waveforms are confirmed to the theoretical and simulated resonant waveforms of the converter. The voltage regulation of the converter is achieved using PI controller. The dSpic microcontroller is used for realizing control algorithm and for generating the gating pulse. The experimental results of regulated output voltage for supply voltage disturbance and load variations are given. The maximum efficiency of the converter is found to be 93% at rated load.

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DC TO DC CONVERTER 8.2 FUTURE SCOPE Currently, high gain DC-DC converters are widely used in aerospace and industrial applications. This necessitates further research in digitized high gain DC-DC converters. Newer resonant topologies that can overcome the limitations will be of significance. It is difficult to design a high performance linear controller which depends on the mathematical model of soft switched converters. Hence, there is a need for the design and development of flexible digital controller which can accommodate large variation in the load and supply voltage. Digital control of converters plays a vital role in computer hardware and industrial applications. A newer approach for digitized soft switching in DC-DC converters and non-linear control of DCDC converters would be interest.

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DC TO DC CONVERTER REFERENCE [1] Ned Mohan, Undeland and Robbin, ‘Power Electronics: Converters, Application and Design’, John Wiley and sons Inc., Newyork,2006. [2] Rashid M.H., ‘Power Electronics- Circuits, Device and Applications’, Prentice Hall India, New Delhi, 2009. [3] Datasheets 1N5820 http://www.farnell.com/datasheets/107972.pdf IRFZ14 http://www.irf.com/product-info/datasheets/data/irfz14.pdf Capacitor http://www.farnell.com/datasheets/1558295.pdf

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