Linear Power Supply
Group Members:
| EDIRISINGHE E.A.D.D.D. | | JANANDITH W.A.O | | ABEYSINGHE D.U | | HEWAGAMAGE K.L.N |
Abstract
This project report outlines the comprehensive design and implementation of a 10V linear power supply featuring a maximum output current of 10A and integrated short circuit protection. The report covers the theoretical foundations of linear power supplies, emphasizing the selection and integration of components tailored to achieve both precision and high current output. Special attention is given to addressing thermal considerations, ensuring the stability of the power supply under varying load conditions. The implementation details cover the construction of the circuit, with a focus on layout and safety measures. Performance evaluation demonstrates the power supply’s ability to provide a stable 10V output at 10A while effectively protecting against short circuits.The report concludes by discussing potential enhancements for future development.
The linear power supply ensures a consistent output voltage unaffected by varying loads. Power supplies play a crucial role in delivering continuous voltage and current to connected devices. When designing a power supply, factors such as efficiency, load, line regulation, and short circuit protection are taken into account. This project involves the creation of a voltage regulator from scratch, intended to power a high-load (100W) with a 15V input voltage. The primary objective is to construct a linear power supply capable of delivering a stable 10V output with a maximum current of 10A, incorporating circuit protection to prevent short circuits and over-current, all while optimizing power supply efficiency. The operational principle of a linear power supply entails transforming the input voltage, rectifying and filtering it, and then regulating it to provide a consistent and accurate output voltage for electronic devices.
Rectification involves the conversion of AC voltage into DC voltage. Initially, the AC voltage from the power source undergoes a reduction in the transformer, lowering it to a suitable level (230V rms -15V rms) for the power supply. Then AC voltage is directed to a diode bridge comprising four diodes arranged in a specific configuration.
From the output of the rectifier, we can get a pulsating DC voltage V_out,
V_out = V_in(peak) - 2V_d
V_out = 15V - 1.1V = 20.11V
PIV = V_in(peak) - V_d = 20.51V
Since PIV of BR3510 bridge rectifier is 1000V, it is sufficient for this design.
Voltage regulation mechanism is used to provide a constant voltage at the output despite the variation of the load. Here we have used TIP 142 darlington power transistor (Q1). The regulation is improved and ripple voltage reduced by the addition of a pre-regulator circuit. This additional circuitry consists of Q2, R6, R2 and Z1. It will provide a constant current of reduced ripple to the collector of Q3 and hence to the base of Q1. The zener diode Z1 will hold the base voltage of Q2 fixed, and the negative feedback voltage developed across R6 will tend to keep the collector current constant. Q2 and Q3 are selected as a complementary pair. Overall negative feedback is applied through Q3, R13, R8 and R14. Biasing current for D4 flows through R9, R10, R12. It’s noise is reduce by the filters implemented by C2 and C3 capacitors.
In PSU, smoothing technique is employed to mitigate ripple voltage or noise in the DC output voltage. This approach incorporates a capacitor filter to smooth the pulsating DC voltage generated by the rectification circuit, resulting in a more consistent and stable DC voltage output. The capacitor filter comprises capacitors connected in parallel with the load circuit. In the peaks of the pulsating DC voltage produced by the rectification circuit, the capacitor accumulates charge up to the peak voltage. Conversely, during the declining phase of the pulsating voltage, the capacitor discharges its stored charge, ensuring a continuous and steady current flow to the load circuit. The amount of smoothing or ripple reduction provided by the capacitor filter is dependent on the value of the capacitor. Which is calculated as follows,
V_ripple(pp) < V_out - 10V = 10.11V
V_ripple(pp) =
R_(min) = = = 1 Ohm
10.11 >
c > 19.89 mF
From calculation we have found that 19.89 mF is required for expected ripple reduction. But during prototyping we have discovered that 14.1 mF is sufficient for ripple reduction as well as maintaining smaller transient current. Therefore we have used three 4.7 mF capacitors parallel to the output. Additional 4.7mF capacitor is placed parallel to the regulatory circuit in order to reduce the ripple in the input of regulatory circuit.
Over - voltage protection is implemented using a varistor at the input of the circuit. This varistor is capable of protecting the circuit at surges. For over - current protection we have used 12A Fuse. It is capable of withstanding a maximum current of 12A. Hence, the fuse will burn prevent any harm to the circuit when the input current exceeds 12A. Additionally, we have implemented a current limiter using BC 547 transistor (Q4) and 0.22 Ohm resisters. After testing we have discovered that 0.044 Ohm is required for current limiter to function while maintaining a minimal voltage drop. Therefore five 0.22 Ohm 5W resistors are connected parallel to each other.
In our design, bridge rectifier and the darlington transistor generate the majority of the heat. Due to the design of the bridge rectifier, passive cooling is sufficient to maintain it under operation temperature. On the other hand, the darlington transistor require active cooling solution. Therefore a cooling fan integrated with a heat sink is used.
Bridge Rectifier - BR3510 bridge rectifier was chosen as it has voltage range of 50V to 1000V as well as 35A current. It’s 1000V surge rating as well as 400A surge current ensures the protection against surges. Finally it’s superior thermal design ensures that passive cooling is sufficient to maintain it under operating temperature.
Darlington Transistor - TIP 142 Darlington Complementary Silicon Power Transistor is used for regulatory circuitry. It is capable of operating in the range of 60 to 100 V and 125W load. Due to it’s high gain ( hE**F = 1000 ) regulatory circuit has managed to attain high efficiency. Also it’s packaging makes it easier to mount a heat sink.
Smoothing Capacitors - As the circuit require a capacitance of 14.1mF, we decided to use three electrolytic capacitors, each with a capacitance of 4.7mF. We specifically chose low ESR variant in order to minimize the voltage drop of the output at high load.
Low resistance resistors - As the current limiter circuit require a resistance of 0.44 Ohm, we used five 0.22 Ohm 5W resister. 0.22 Ohm was the smallest available value in the local market. It’s power rating ensures that it can withstand high current.
Variable resistors - We have use 1k and 10k trimmers to fine-tune the output after soldering. Due to compact size of these trimmers, they were able to fit into the PCB without taking up excessive space.
Rest of the components were selected based on required specifications, availability in local market and cost.
For the PCB, we have used 2 layer 2 Oz copper design where top layer is used for power lines and signal lines and the bottom layer was used for ground lines. Minimum trace width of 3.6mm is maintained for all power lines. Copper polygons are used in top and bottom layers.They provide a lower resistance path for current flow, reducing power loss and voltage drop. Adequate clearance and spacing between components and traces are maintained, in order to prevent shorts and interference as well as to meet production capabilities. PCB was designed using Altium and produced by JLC PCB.
Enclosure was designed in order to have sufficient space for PCB and the cooling mechanisms. Air intake vent was placed at the back of the enclosure and two exhaust vents were placed at the sides. Power input and the fuse were placed at the back. Power output, power switch and indicator LED were placed at the front. 0.5mm sheet metal was used make light weight enclosure while maintaining structural integrity. Two part design was made in order to provide access to interior easily.
Initial testing was done in the Multisim simulation. Here we managed to achive output voltage of 10V for a load of 2 Ohm and peak to peak noise of 4 mV.
Hardware testing was done on a bread board and we have manage to draw maxium current of 1A while maintaining 10V output. Average peak to peak ripple was 42 mV.
Efficiency of the PSU can be further enhanced by integrating a dynamic cooling system. It will control the speed of cooling fan according to the system temperature. Also a LCD display can be mounted in the front provide information such as current and voltage to user.
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