Design Lab 2: Design of a switched-mode power supply
Objective
In this lab, we will design a step-down DC-DC voltage converter based on the Buck topology, as shown in Fig. 1, for a switched-mode power supply. We will first build a simulation model in Matlab Simulink to simulate the performance of the converter, and finalise the component values. After the simulation, we will implement the design with discrete components on a strip board, and measure its performance using lab instruments. The design specifications to achieve are as follows:
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Input voltage
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Vs = 24 V
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Minimum switching frequency
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Fmin = 50 kHz
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Output voltage
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Vout = 5 V
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Output current
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Io = 50 mA
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Output ripple voltage
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Vripple p-p = 25 mV
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Simulink settings: Simulation time = 0.01 s. Use ideal switches and disable Snubbers, On resistance and forward voltage drop in switching devices. Try both the Continuous and Discrete solver options. What difference do you see? In the Discrete option, consider what sample time is appropriate.
Fig. 1 Simulink model of the Buck converter
Task 1
Plot the simulated inductor current (IL) and output voltage (Vout) waveforms. Compare the results with theoretical waveforms shown in page 3 of your SMPS lecture notes.
Fig. 2 Simulated current of the inductor L and output voltage
Task 2
Now replace the passive LC components with realistic models to represent their physical performance. Discuss what impact the parasitic of the LC components have on the waveforms of the Buck converter.
Fig. 3 Parasitic resistances added to the inductor and capacitor of the DC-DC Buck converter
Task 3
Use MOSFET and diode model parameters provided by any suitable manufacturer of your choice. Insert these into the Buck converter model. Disable the use ideal switching devices in the Solver option. Simulate and compare the simulated waveforms with the ideal model case.
Fig. 4 MOSFET and Diode model included for the DC-DC Buck converter
Task 4
Modify the duty cycle of the pulse width modulated signal for the pulse generator and/or adjust the passive L C components to achieve the original design specifications of the Buck converter.
Determine the efficiency of your Buck switched-mode power supply final design.
Task 5
Implement your finalised design with passive LC components, an n-channel MOSFET and a Schottky diode on a strip board. Use the power supply unit in the lab to provide the 24 V supply voltage, and the signal generator as the pulse generator. Use an oscilloscope to measure the voltage across RL. Determine the efficiency of your final design. How does it compare to your simulation results, and why?
Please refer to the datasheets of your chosen components for your analysis. There is a list of available components in the appendix of this document. Each component has an order code from either Farnell or RS. You can find the datasheet by searching the order code on Farnell or RS websites:
https://uk.farnell.com/
https://uk.rs-online.com/web/
Reports
. There are two reports and a strip board implementation required for all students:
- Report 1 on Task 1 – 4;
- Report 2 on Task 5.
- Strip board implemented in Task 5.
. Each report should be no more than three pages
. Submit your reports to Moodle.
. In Report 1:
- write a brief introduction (not more than 500 words) about the simulations and Matlab Simulink
- Show clearly your Simulink model schematics, parameters used and your simulated results in high quality figures
. In Report 2:
- Clearly list the parameters and the part numbers of the components you use for the
circuit implementation. Refer to your Simulink schematic of each component
. In Both reports, there should be a discussion section. For Report 1, answer the questions in Tasks 2, 3, and 4. For Report 2, discuss the impact from the real component parameters on the output, and your adjustment to compensate.
. Please clearly label your name on the stripboard.
Appendix: Component List
Note: Components may be from different suppliers. Check on their website using the order code in the table to find the appropriate datasheets.
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Item
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Farnell Order code
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IRLB8748PBF – Power MOSFET, N Channel
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3155154
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Inductor, Coilcraft, 680 uH
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2526926 / 2211735
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Inductor, Panasonic, 820 uH
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2211717
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Inductor, Coilcraft, 1000 uH
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3291051
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Capacitor, tantalum, 16V, 4.7uF
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9708324
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Capacitor, tantalum, 16V, 6.8uF
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9708332
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Capacitor, tantalum, 16V, 10uF
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9708340
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Capacitor, tantalum, 16V, 22uF
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9708359
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Capacitor, tantalum, 16V, 33uF
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1100471
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Diode, IN4003
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2723902
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