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12Vdc – 120Vac Emergency Power System

12Vdc – 120Vac Emergency Power System. Jim Mosley TA: Wayne Weaver. Introduction. ac power is taken for granted Most dc powered communication systems are charged by ac systems Back-up systems are rarely capable of extended operation

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12Vdc – 120Vac Emergency Power System

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  1. 12Vdc – 120Vac Emergency Power System Jim Mosley TA: Wayne Weaver

  2. Introduction • ac power is taken for granted • Most dc powered communication systems are charged by ac systems • Back-up systems are rarely capable of extended operation • Economical alternative to stand-alone ac generation is needed

  3. Batteries • First option to be suggested • Good source for clean dc power • Limited amount of energy storage • Usually charged by an ac source

  4. Charging • Typical ac powered charger • Alternative power such as solar and wind • Alternators and generators

  5. Power Source • All methods of recharging a battery require a power source • Source must be: Reliable Available at all times Maintenance free Not an expensive “just in case” item

  6. People Power • People are always around • Reliable, although intermittent • Not sitting in storage waiting to be used • Don’t have a shelf life

  7. Power Transfer Next time I’m taking the bus! • Person + Bicycle + Alternator = Charged Battery + Tired Person

  8. Muscle to Electrons • Modified commercial bike stands • Home-made stands

  9. What is feasible? • Typical person has sustainable output of around 100 watts • Power directly from a human powered source is too intermittent to be reliable for most electrical devices • Main power source would be the battery with a person recharging the battery

  10. Complexity of Simplicity • Design standardization is required for use by the general public • There are many different alternators, each with their own mounting and wiring quirks • There are just as many different bicycles, each with their own gearing, tire dimensions, and crank lengths

  11. Determining the range of operation • Bench test several different alternators to find any similarities in operation • Efficiency • Minimum speed required to output at least 35 watts • Torque requirements at 35 watts • Calculate normal operating speeds from pulley dimension

  12. Determining the range of operation • Record data from a variety of bicycles to find a common gear ratio • Perform tests to determine comfortable range of cycling • Perform tests to estimate the power a human can comfortably produce

  13. Data • Only one suitable alternator was found • Machine shop was unable to complete mount in time • Bicycle data was collected, but not analyzed • 100-120 watts is practical

  14. dc Voltage to ac Voltage • How to get the “readily available” dc source to power ac chargers and emergency communication equipment • Converter is necessary

  15. Converter Components • Two components are needed • Push-pull forward converter to step up 13.4Vdc to 120Vdc • Inverter to produce 120V square wave

  16. Push-Pull Forward Converter • To achieve a high gain necessary, the push-pull forward converter uses a dc bus with MOSFETs Q1 and Q2 switching at 50kHz to apply an ac current across the high frequency transformer T1 • The diodes rectify the signal back to dc while L1 and C2 help to clean up the signal

  17. Inverter • MOSFETs T1, T4 provide the positive pulse of the output while T2, T3 provide the negative pulse • Deadtime between the switching events eliminates the current spikes that would result from the short circuit

  18. Unitrode UC2526 PWM Modulator

  19. Factors in Choosing the Unitrode Chip • Low supply current • Soft-start • Over-current protection • Under-voltage protection • Thermal protection • Shut-down input for other external protective circuits

  20. Testing the Unitrode 2526A • After several attempts to operate the converter with the Unitrode chip, it was replaced with the TL494 PWM modulator • TL494 has less features, but was chosen because of extensive use in the Dept • Lessons learned implementing the TL494 provided potential solutions for applying the UC2526A

  21. Key Requirements for PWM control • Error amplifiers are non-inverting • Un-used amplifier inputs should not be left floating • Reference input should be kept 2V below Vref • Oscillator frequency easily adjusted with an RC circuit

  22. TL494 Operation • 50 kHz oscillator signal used by comparitor

  23. TL 494 Outputs at Different Feedback Voltages

  24. MIC4424 MOSFET Driver • To protect the output of the TL494, a line driver was used • Higher current capacity • Cheaper and easier to replace • Two inputs and two outputs so only one chip is needed

  25. Other Converter Components • MOSFETs were chosen to meet voltage and current requirements • Center-tapped transformer wound to provide the widest operating range • Large capacitor on supply to reduce switching noise • High current diodes to rectify the output • Large capacitor to smooth the output voltage

  26. Output Waveforms • MOSFET gate signal and output voltage before the diodes • MOSFET gate signal and output voltage after the diodes and capacitor

  27. Inverter Components • MC78L00 voltage regulator to provide 5Vdc control power • LM555 timer for 50% duty cycle 60Hz oscillator • SN74LS75 latch to provide complimentary outputs • IR2113 high/low side MOSFET driver

  28. LM555 • The versatile LM555 timer has been a reliable industry work-horse for many years • Simple RC circuit sets frequency

  29. Difficulties Implementing LM555 • Original design incorporated an 74LS14 Schmitt trigger inverter to provide complimentary inputs to the MOSFET driver • Unable to achieve dead-time at MOSFET driver due to only one rising edge from the LM555 • SN74LS75 latch with complimentary outputs used to provide two outputs

  30. Solving Dead-time Issues • IR2113 has Schmitt trigger input • Dead-time easily controlled by RC circuit

  31. Difficulties in Inverter Operation • Dead-time • During testing, line driver was configured for low/low side operation • Jumper was not removed to allow high/low side operation • By-pass caps not installed to reduce switching noise

  32. Tests Performed • Operation with varying input voltages • Effect on small ac charger output • Efficiency

  33. Commercial “brick” with dc Output • Output waveform on inverter • Output waveform on commercial power • Voltage spikes are more pronounced

  34. Commercial “brick” with ac Output • Output wave form on inverter • Output wave form on commercial power

  35. Efficiency • Preliminary results show an overall efficiency of 3% at no load and 56% at full load • The converter is more efficient with a no load efficiency of 53% and 58% at full load • The main reason for the difference is that the converter requires a minimum load to operate, therefore the actual output is 0 at no load, but the converter is still consuming power

  36. Future Modification • Safety features such as: • Floating the input and referencing the output to earth ground • Overcurrent protection • Short-circuit protection • Under-voltage shutdown

  37. Credits • Professor Swenson • Professor Chapman • Wayne Weaver • Brett Nee • Jonathan Kimball • Dustin Kramer

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