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BUCK CONVERTER:

There are three basic types of dc-dc converter circuits, termed as buck, boostand buck-boost. In all these circuits, the thyristor is connected in series with load to a dc supply,

or a positive (forward) voltage is applied between anode and cathode terminals. The thyristor

turns off, when the current decreases below the holding current or a reverse (negative) voltage is

applied between anode and cathode terminals .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.It may be noted here that buck converter (dc-dc) is called as step-down chopper,

whereas boost converter (dc-dc) is a step-up chopper.

A buck converter can be used to maximize the power transfer through the use of impedance

matching. An application of this is in a "maximum power point tracker" commonly used in

photovoltaic systems

BUCK CONVERTER

FIG.1

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FIG.2

FIG.3

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FIG.4

Fig.1. Buck- converter ,Fig 2.switch on for a time duration DT ,Fig 3.switch off for a time

duration (1-D)T ,Fig 4.key waveforms.

Circuit description and operation:

Circuit description.

The three basic dc-dc converters use a pair of switches, usually one controlled (eg.

MOSFET,IGBT etc..) and one uncontrolled (ie. diode), to achieve unidirectional power flow

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from input to output. The converters also use one capacitor and one inductor to store and transfer

energy from input to output. They also filter or smooth voltage and current.

The dc-dc converters can have two distinct modes of operation: Continuous conduction

mode (CCM) and discontinuous conduction mode (DCM). In practice, a converter may operate

in both modes, which have significantly different characteristics. Therefore, a converter and its

control should be designed based on both modes of operation. However, for this course we only

consider the dc-dc converters operated in CCM.

Circuit Operation.

When the switch is on for a time duration DT, the switch conducts the inductor current and the

diode becomes reverse biased. This results in a positive voltage vL= V

g V

oacross the inductor.

This voltage causes a linear increase in the inductor current iL. When the switch is turned off,

because of the inductive energy storage, iLcontinues to flow. This current now flows through the

diode, and vL

= -Vo

for a time duration (1-D)T until the switch is turned on again.

Assumptions made about the operation of the converter are as follows:

The circuit is operating in the steady state

The circuit is operating in the CCM

The capacitor is large enough to assume a constant output voltage

The component are ideal.

Equating the integral of the inductor voltage over one time period to zero yields

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Assuming a loss less ckt,

Therefore

For a buck converter, it is obvious that

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The peak-peak output voltage ripple

vo.From the information of the capacitor current, i

c, we can obtain v

o.

Data Sheet For Buck Converter

Input Voltage=50Volts

Chopping Freqency=100Khz

S.NoDuty cycle

(in %)

Voltage

(in Volts)

Current

(in Amp)

Power

(in watts)

1 5 2.883 0.2883 0.8313

2 10 6.226 0.6226 3.876

3 20 11.94 1.194 14.25

4 30 16.79 1.679 28.18

5 40 21.12 2.112 44.59

6 50 24.68 2.468 60.93

7 60 29.6 2.96 87.6

8 70 34.58 3.458 119.6

9 80 39.56 3.956 156.5

10 90 44.53 4.453 198.3

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Buck Converter without using fuzzy control

Output Waveforms without using fuzzy logic control at duty cycle of 50%

Continuous

powergui

v+-

Terminator

Scope

Pulse

Generator

Product1

g m

D S

Mosfet1

24.68

Display6

2.468

Display4

60.93

Display1

DC Voltage Source

i+

-

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Buck Converter with using fuzzy control

Continuous

powergui

v+-

Terminator

Scope

6

Reference voltage 1

Product1

d c

PWM Block

g m

D S

Mosfet1

Fuzzy Logic

Controller

6.035

Display6

0.6035

Display4

3.642

Display1

DC Voltage Source

i+

-