Content Page Introduction Performance Methodology Design and Build Design Specification Specification Design Circuit Buck Converter Topology Calculation & component Selection
Component Cost Details
Maltab Simulation References
Appendixes Appendixes
Introduction The main objective is to design a switch-mode, none isolated, dc-dc converter which can supply supply 3V at 2A nomin nominal al (6W) for powerin powering g a solar solar model-car. model-car. Currentl Currently y cost is the most most important issue, issue, followed by size. size. The specifications specifications for the converter converter are as follows: First, First, inpu inputt range range is +10V +10V to +20V dc. Secon Secondly dly,, outpu outputt shoul should d be +3 V, with good contro controll capability. capability. Subsequently, Subsequently, maximum output output power needs to be up to 6 W, while while isolation is not required. Lastly, although although this is not listed in the assignment brief, but we tried to have a total parts cost (based on high-volume high-volume production) production) to not exceed £10. The converter converter should be capable of remote remote control (on/off) by a low-power low-power TTL or CMOS compatible compatible signal. signal. The converter design is done as a group and we have realized that there are people who have done the same project in the previous years, thus, we have tried to make our own design unique although the same components are to eventually be given to every group by the tutor. We will build a converter converter with the input range range voltage ranging from 10 V DC to 20 V DC. We then need to obtain 3V DC and and 6 W as our output voltage and maximum maximum power. Since the cost of this project must be about 10 pounds, the converter’s circuitry is very simple and highly efficiency efficiency and we hope that this project project will be helpful to the Automobile Automobile industry. To initi initiate ate the pract practica icall phase phase of the proje project ct work work we are advis advised ed to conc concent entrat rate e on the the MOSFET driver, which we have selected through our analysis and components selections. Usin Using g the the data data shee sheett for for a low low cost cost PWM PWM cont contro roll chip chip,, we used used TL49 TL494, 4, whic which h was was recommended recommended for non isolated switch mode DC-DC converter; we basically basically went through the process of setting up the PWM control chip on a breadboard breadboard and producing open-loop single and double-en double-ended ded switching switching outputs. outputs. In hoping to reach reach our goal, we will build build the buck converter with the input voltage ranging from 10 V DC - 20V DC, the output about 3 V DC, and the maximum power 6 W. This report details the initial calculations and design performed by a group of four students for a group project.
Performance •
Input voltage Vin = 15V nominal, 10V min working, 20V abs max.
•
Output voltage V out = 3Vdc.
•
Output current capability from 2A nominal, 3A maximum, maximum, 0.5A minimum. minimum.
•
Converter Vout ripple = 100mV maximum.
•
Converter Converter output current ripple less 1%.
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Converter output voltage regulation = 0.2V (ideally 2.9-3.1V), for 0.5 - 3A load and 10-20Vdc 10-20Vdc input voltage.
•
Target efficiency of > 80%.
Methodology To achieve the aims of the project, the entire project will be tackled in four separate stages: Phase one - Research and Report write up. Phase two - Design and Build circuit to perform set objectives Phase three – Test Phase four – Conclusion
Design and Build
This will cover the following steps 1. Design the circuit 2. Selection of component values from design
3.
Circuit design simulation using Simulink in MATLAB.
4. Calculation Calculation of power losses in each component and efficiency of the circuit. 5.
Build prototype using Vero board
Design Circuit
Figure 1: Buck Converter
Buck Converter Topology Figure 1 shows a basic buck topology using ideal components. The inductor serves as a current source to the output load impedance. When the transistor (MOSFET) switch is on, the inductor current increases, inducing a positive voltage drop across the inductor and a lower output supply voltage in reference to the input source voltage. When the transistor (MOSFET) switch is off, the inductor current discharges, inducing a negative voltage drop across the inductor. Because one port of the inductor is tied to ground, the other port will have ha ve a hi high gher er vo volt ltag age e le leve vel, l, wh whic ich h is th the e ta targ rget et ou outp tput ut su supp pply ly vo volt ltag age. e. Th The e ou outp tput ut capac ca pacita itance nce ac acts ts as a lo low-p w-pas ass s fil filter ter,, re reduc ducing ing ou outpu tputt vo volta ltage ge rip rippl ple e as a re resul sultt of th the e
fluctuating current through the inductor. The diode provides a current path for the inductor when the transistor (MOSFET) switch is off. The clear advantage of this method is efficiency, as minima minimall power is dissipa dissipated ted in the t he power path pa th (FE (FET T sw switc itche hes) s) wh when en th the e ou outpu tputt su supp pply ly vo volta ltage ge is su suffi fficie cient nt for the lo load ad sta state. te. Essentially,, the power converter "shuts off" when power is not needed, due to minimal switch Essentially duty-cycle.
Calculation The following calculation was carried out to find out the correct values of each component in the circuit. Vin = 15V; V0 = 3V; I0 = 2A; Step 1: calculating the Duty cycle
This is;
Therefore;
Step 2: Using the following equations
Now calculating calculating the inductor inductor value value
Value of inductor selected from the data sheet:1.2mH From the 1.2mH inductor’s datasheet (refer to appendix) we find out the maximum DC resistance: ESR
And calculate Power loss:
Calculating the Output Capacitor value ,
=
From the datasheets a 2.2
capacitor with an ESR of 58 Ω was selected (for
datasheets refer to appendix) Power of capacitor: Let’s assume that ESR=0, then
Diode Selection Estimate Diode Current: I D = (1-D) × I Load I D= (1-0.2) × 2 = 1.6A Where D = Duty cycle Max Diode Reverse Voltage = 15V Select Schottky rectifier: A 1N5820, 20V, 3 A Schottky meets requirements Power Dissipation:
Mosfet Selection and Power Calculations
From the information given a p-channel p-channel mosfet was selected from the datasheets. With the following specifications: specifications: •
VDS max =55V
•
VGS= 20V
Calculation Calculation of power loss of mosfet:
Total power loss in the system:
Therefore the efficiency achieved:
2.3: Cost of Components The above calculated components were selected and costing was done as shown below
Costs The individual costs are as follows: •
MOSFET
£0.336
RS Stock No 650-4485 International >International Rectifier
•
Capacitor
£0.398
RS Stock No 305-2385 Vishay
•
Inductor
£1.35
B82722A2302N001 Epcos
•
Diode
£0.74
1N5820DICT-ND
Vishay •
Tl494
£0.72
Total price = £3.54
Design Matlab simulation
Rs stock No 517-2986
Results of above Simulation(V Out=3V and IL=2A
References Websites •
Buck converter Component Selection http://powerelectronics.com/m http://powere lectronics.com/mag/power_g ag/power_guide uide (Accessed 27/02/12)
•
Practical Design For Buck Converters http://www1.futureelectronics.com/doc/ (Accessed 27/02/12)
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Power Mosfet selection http://www.nteinc.com/Web_pgs/MOSFET.html (Accessed 29/02/12)
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Design and Implementation of a Buck Converter http://www.scribd.com/doc/24501749 (Accessed 04/03/12)
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Power Systems- DC-DC converter Design http://powerelectronics.com/p http://powere lectronics.com/power_systems ower_systems (Accessed 24/02/12)
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SMPS www.microchip.com/smps/16-bit (Accessed 05/05/12)
Appendixes Appendixes Data sheets Mosfet P-channel 1.2mH Inductor Data sheet 2.2uF Capacitor Diode Tl494 Datasheet. 4
