Power Elec- Proj2

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Assignment on

Buck converter

Submitted to

Dr. RAJNEESH KUMAR

By

Lokesh 2008A3PS074P

Ekansh Nayal 2008A3PS143P

Savil Gupta 2008A3PS199P

Ishan Mishra 2008A3PS111P

Aman Sehgal 2008A8PS252P

In partial fulfillment of course EEEC461

POWER ELECTRONICS

April 25, 2011


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Abstract-A buck converter is simply is

a particular type of power converter with

an average output DC voltage smallerthan the input DC voltage. This type of

circuit is used to step-down a source

voltage to a higher, regulated voltage,

allowing one power supply to provide

different driving voltages. Its main

application is in regulated dc power

supplies and dc motor speed control. The

purpose of this document is for the reader

to become familiar with the function and

implementation of a buck converter. A

basic design will be discussed along with

waveforms of voltages and current

generated during its operation.

Keywords Buck Converter, Switched-Mode dc-dcconversion, Pulse Width Modulation, RippleVoltage, Snubbers.

I. INTRODUCTIONA buck converter is part of a subset of DC-

DC converters called switch-mode

converters. The circuits belonging to this

class, including boost, flyback, buck-

boost, and push-pull converters are very

similar. They generally perform the

conversion by applying a DC voltage

across an inductor or transformer for aperiod of time (usually in the 100 kHz to 5

MHz range) which causes current to flowthrough it and store energy magnetically,

then switching this voltage off and causing

the stored energy to be transferred to the

voltage output in a controlled manner.

The basic buck converter circuit consists

of only a switch (typically a transistor), a

diode, an inductor, and a capacitor.Conceptually, the basic circuit constitutes

a step-down converter for a purely

resistive load. The specific connections areshown in Figure 1.

Fig 1: Step-down dc-dc Converter

II. DISCUSSIONBy varying the duty ratio (= ton/Ts) of the

switch, Vo can be controlled. Another

important observation is that the average

output voltage Vo varies linearly with the

control voltage as in the case of linear

amplifiers. In actual applications, the

foregoing circuit has two drawbacks:-

1. In practice the load is inductive, evenwith resistive load there is always

some stray inductance.

2. Output voltage fluctuates between 0and Vd which is eliminated by

using a Low Pass Filter consisting

of inductor and capacitor.

Diode solves the problem of stored inductive

energy. The filter capacitor at the output is

assumed to be very large since constantinstantaneous voltage is required at the output.

Fig 2: Switched Mode dc-dc Conversion


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Figure 3 shows the waveforms for the

continuous-conduction mode of operation

where the inductor current flows continuously.

When the switch is on for a time ton, the

switch conducts the inductor current and

diode becomes reverse-biased. This results

in a positive voltage VL=Vd-Vo across

inductor. This voltage causes a linear

increase in inductor current. When the

switch is turned off, because of the

inductive energy storage, IL continues to

flow.

Fig 3: Circuit states assuming ILflowing

continuously (a) Switch On (b) Switch Off

Neglecting the power losses associated with all

the circuit elements, the input power Pd equals

the output power Po. Therefore

Vd/Vo=1/D proving that in continuous

conduction mode, the step-down

converter is equivalent to a dc

transformer where the turns ratio can be

controlled electronically in a range of 0-1

by controlling the duty ratio of the switch.

Fig 4: Current at the boundary of

continuous-discontinuous conduction (a)

current waveform (b) ILB versus D keeping

Vd constant.

Being at the boundary of continuous-

discontinuous mode, by definition the

inductor current IL goes to zero at the end

of the off period. At this boundary, the

average inductor current, where the

subscript B refers to the boundary is

ILB=DTs (Vd-Vo)/2L. Therefore, during an

operating condition (with a given set of Ts,

Vd, Vo, L and D), if the average output

current becomes less than the ILB value,

the IL will become discontinuous as shown

in figure 5.

Fig 5: Discontinuous condition in step-

down converter.


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III. CALCULATIONSThe different values used are

Input current = output current =1.3 A

Output Voltage = 10.28A

D = 0.42

Input Voltage = 0.34V and f= 11.2 kHz

1. Inductor Voltage

. of the switch

3. Drain current

4. Output Voltage Ripple

5. Upper graph , lower graph


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6. PWM output

IV. CONCLUSIONBuck converters are used to lower the dc

voltage to user-demanded required voltages.

The parasitic elements in a step-up converter

are due to the losses associated with the

same inductor, the capacitor, the switch and

the diode. Unlike the ideal characteristic, in

practice, Vo/Vd declines as the duty ratio

approaches unity. Without the use of

snubbers, the power dissipated during turn on

and turn off can be quite high and can lead to

damage of devices. With the use of snubbers,

the current or voltage waveforms are pulled

for a longer time hence the overlap is reduced

and hence lesser heat is dissipated. The

increase in efficiency isnt significant but it

makes our circuits safer to operate.

REFERENCES

1. Ned Mohan, Tore M. Undeland, William p.

Robbins Power Electronics converters,applications and design, Third edition.

2. Analog Electronics, L.K. Maheshwari and

MMS Anand.

3. Power Electronics Lab Manual, BITS

Pilani.

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