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Buck Converter (DC to DC step down converter) Zafar Ahmad , Syed Afzal Shah , Hafeez Ullah, Adnan khan Department of Electrical Energy System Engineering Engineeri ng USPCAS-E , University of Engineering and Technology Peshawar
(DSPs). This circuit that we design has two parts 1. Abst Abstrac ract t — this paper is the implementation of
circuitry of buck converter (from 24 volt to 12 volt and 0.5A current). For gate pulses of MOSFET (IRF740) we used 555 timer in a-stable mode instead of PIC Microntroller which is expensive than 555 timer. MOSFET is used as a switch in this circuit and a low pass filter is used to cut off high frequencies and pass the lower frequencies. And diode is used to provide
path
for
the
inductor
current
when
MOSSFET is on and to block the current when
Pulse generator, 2.dc-dc step down converter. 1.
Pulse generator is used for the generation of pulses with the help of 555 timer. IC used in astable mode to give a full square wave to the next circuit with duty cycle of near 50%. The resistor and capacitor are used to change the duty cycle of the output pulse it can vary from 50% up to 99% and therefore the next circuit changes their output according to the pulses given by the pulse generator.
MOSFET is off. Keywords:
MOSFET
(Metal-Oxide
Semiconductor
Field-Effect Transistor).
2. Now dc to dc step down converter converter is used used to step down the 24 volts given to 12 volts with 5A
I. INTRODUCTION
P
current on output. Now in this circuit MOSFET (IRF740) is linked with the first circuit i.e. pulse
ower semiconductors are with us since 1970s and a lot of work has been done to derive the benefits regarding their ability to control both the
alternating current (AC) driven loads or direct current (DC) driven loads for both the home and industrial grades. The sensitivity level to drive these devices has made it possible to control these power controlling switches by the use of cheap and commercially available digital controllers such that Integrated Circuits, or digital signal processors
generator). When MOSFET receive the high pulse from pulse generator its becomes becomes turn on and current flow start through inductor through diode and when MOSFET receives the low pulse from the pulse generator it turns off and the diode become reversed biased and current stops through diode.
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II. EXPERIMENTAL SETUP The proposed design is implemented and fully tested Using prototype circuit boards. Heart of the system is IC 555 timer. IC is energized with 9-15 volts. Output can be enable and disabled by placing push button. Output is then used for switching of MOSFET which turns on and off with pulse changing to high and low respectively. And due to 50% duty cycle 24 volt converts to 12 volts.
A. Pulse Generator (generating firing pulses): Pulses are generated By IC 555 timer and can be varied by changing RC combination combination at IC in puts. This circuit is operating in a-stable mode. A-stable Circuit has no stable state - hence the name "a-stable". The output continually switches state between high and low without any intervention from the user, called a 'square' wave. This type of circuit could be used to give a mechanism intermittent motion by switching a motor on and off at regular intervals. It can also be used to flash lamps and LEDs, and is useful as a 'clock' pulse for other digital ICs and circuits.
Frequency of Output = 1/ [0.7*(R 3+2*R 4)*C3] where R A, R B, and C are shown in fig below Based on the components I used in this sample circuit, we can calculate the range of possible output frequencies as follows: assuming the potentiometer is turned all the way to one side and the resistance = 100kohms Frequency of Output = 1/[0.7*(100,000+2*10,000)*0.00000047] Frequency of Output =~ 25Hz t = 0.7*R B*C substituting R B = 10kOhms and C = 0.47uF you get: t =~ 3ms which can be verified in figs. f igs.
MATHS
3 Output:
both decrease. Therefore, Therefore, high switching frequencies frequencies are desirable to reduce the size of both the inductor and the capacitor. The voltage and current waveforms of the inductor is shown in fig.
B. Buck converter (Dc-Dc step down converter): This circuit output depends on the pulse generator circuit as the firing pulse width changes, output of this circuit also changes. When the MOSFET is in ON condition, the output is the same as the input, and the output is zero when the MOSFET is in OFF condition. The periodic openings and closings of the switch results in the pulsed output.
The average inductor current must be the same as the average current in the load resistor, since the average capacitor current must be zero for steady-state operation (IL = IC + IR). As there is a change in inductor current, the maximum and minimum values of the inductor current are computed as:
The diode provides path for the inductor current when the MOSFET is ON and is reverse biased when the MOSFET is OFF. To obtain a dc output, a low-pass filter circuit is inserted after the MOSFET. The input to the filter is Vin when the MOSFET is in ON state and is zero when the MOSFET is in OFF state, provided that the inductor current remains positive, keeping the diode on.
An inductor current that remains positive throughout the switching period is known as continuous current. Note that as the switching frequency increases, the minimum size of the inductor to produce continuous current and the minimum size of the capacitor to limit output ripple
MATHS
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The circuit to the given buck converter is given below
III. CONCLUSION
From this circuitry I concluded how to convert high dc voltage to a low dc voltage.
And also I learn how to generate square wave firing pulses through 555 timer. APPENDIX
Not needed. needed. ACKNOWLEDGMENT The work described in this paper is substantially supported by and supervised by Engr. Usher Wahid of Department of Electrical Engineering University of Wah.
5 EFERENCES R EFERENCES HTTP://CLARKSONUK .COM/555TIMER /OPERATION/HIFRA MES.HTML HTTP://WWW.555-TIMER -CIRCUITS.COM/OPERATINGMODES.HTML HTTPS://DOCS.GOOGLE.COM/FILE/D/0B5KPSLWFFSAR S
5YCJG4N3UY NTQ/EDIT?PLI=1 K 5Y HTTP://WWW.ENGINEERSGARAGE.COM/CONTRIBUTION/A MBHATT/UNDERSTANTING-OPERATING-MODES-OF-555TIMER -IC
Power Electronics: Circuits, Devices, and Applications by M. H. Rashid.
