Project
| # | Title | Team Members | TA | Documents | Sponsor |
|---|---|---|---|---|---|
| 53 | Line Operated Variable Voltage Power Supply |
Cesar Mejia Feroze Butt Kevin Funkhouser |
Matthew Qi | design_document1.pdf final_paper2.pdf photo1.heic presentation1.pptx proposal1.pdf video video |
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| Team Members: - Kevin Funkhouser (ktf3) - Cesar Mejia (cmejia6) - Feroze Butt (fabutt2) # **Problem**: Many low-cost bench power supplies are noisy with bad accuracy and are sometimes unsafe. Even the top positive reviews of Amazon’s best-selling bench power supply (Kungber 30V 10A) have serious complaints about its performance, such as drift, inaccurate display readings, excessive voltage error, and outright failure. Low-cost bench supplies with poor power factors inject harmonics into the power lines, and their switching noise and other distortions can disrupt precision circuits. Additionally, basic protections are sometimes lacking, leading to unsafe conditions that could damage the supply, the circuit, or you. # Solution: We intend to build a line operated variable voltage power supply that is relatively low cost. To provide for low cost, isolation, and high efficiency, isolated switched mode conversion will be used in order to correct the power factor, as well as to ultimately transform the voltage from line levels down to the selected voltages while providing for the output error specifications. Its subsystems will include the input rectification & power factor correction, isolated switched mode conversion, feedback & control, and thermal & overcurrent protection. Its output should be adjustable from 5V to 25V at 50W, at less than 1000ppm total deviation under a static full load. # Solution Components: The solution components are pretty general for now because we haven’t zeroed in on all of the exact circuits we would like to use, which often informs the component selection. With the exception of the switching transformer and case & hardware, we have found a plethora of suitable devices for each list item. There are more details on possible topologies and selections under each of the subsections. EMI filter Rectifier diodes Power inductors and capacitors PFC boost controller IC SMPS control IC Precision voltage reference & divider elements Switching FETs & diodes Heat sinks Case & hardware Switching transformer Thermal sensor Current sense IC MCU & associated hardware General jellybean circuit elements and connectors # Subsystem 1: Input Rectification and Power Factor Correction The current proposed system entails an EMI filter followed by a full bridge rectifier. The rectified output is then boosted to a 250VDC intermediate bus, which is later switched down to output levels. There are a variety of chips that can control a boost converter for PFC use, such as the UCC28180. The EMI filter will likely consist of simple protection TVS or zener diodes and a ferrite. It is of note that there will be a fuse and a power switch “upstream” of this subsystem. This circuit should meet IEC 61000-3-2 standards for line harmonic current. # Subsystem 2: Isolated DC-DC Converter This regulated converter will transform from 250VDC down to the desired voltage. This converter must be galvanically isolated, i.e. uses a transformer or coupled inductors. The topology could possibly be a bridge, forward, or flyback converter. A possible control chip for this application is the NCP1252. A fortified output filter, with possible topologies of a capacitance multiplier or a noise clipper circuit, will provide thorough output regulation and good transient response. # Subsystem 3: Feedback and Control The switching signals for the converters must be generated using feedback from both the 250V bus and the output. Some of these signals must be isolated, likely by means of optocouplers. Control signal generation can be MCU or analog control chip based, though for the PFC and DC-DC modules we plan to choose analog (or digital controllers that act like analog) as they are both good and common these days. For display processing and input control, we plan to use an MCU to encode the sensed current and voltage for display on a simple screen such as the seven-segment. The MCU we will select depends on whether we decide to originate the voltage reference through digital means (e.g. a digital potentiometer dividing a precision reference) or analog means, as this may or may not require an ADC onboard. Though we currently do not plan to use any digital control schemes for switching logic, protection, or feedback, this is a very powerful tool we could integrate, if necessary, at a later time using a hybrid digital logic/analog control scheme. # Subsystem 4: Protection There must be a method to measure, determine, and protect against thermal overload conditions via a thermal sensor directly coupled to the limiting semiconductors. There also must be a current sense and response to overcurrent conditions, both transient and steady state. Current and heat sensing and managing are well detailed issues with many possible solutions. We believe the selection of particular topologies and devices should come after more deliberation on the exact converter topologies so that the failsafes respect the failure modes of the system. Additionally, it must be said that the first line of defense against overcurrent and overheating conditions is solid thermal management, proper device selection, and judicious board and circuit design. # Subsystem 5: Switches and Fuses and Dials This instrument will need to be operable. It will need to have a case with adequate venting and heatsink capabilities, a three prong plug with strain relief and perhaps a ferrite bead, a fused input, and a power switch. Shielding will be investigated. It will also need to have simple displays to tell real time voltage and current conditions. There must be a dial or buttons to select voltage, and banana plugs or other connectors for power, ground, and earth. Other indicators must include a power good LED as well as a power bad LED. # Criteria for Success: - Power Factor >0.9 & IEC 61000-3-2 standards for harmonic current - Deliver 5V - 25V at 50W with 0.1% accuracy to a static load across the whole voltage range - Able to detect and correct a thermal overload - Able to detect and correct a current overload, including short circuit protection - Galvanic isolation - User ability to select a voltage - Display the current and voltage supplied - Costs < $200 |
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