boost convertor

Discussion in 'Homework Help' started by arijit.india, May 14, 2014.

  1. arijit.india

    Thread Starter New Member

    Apr 23, 2014
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    I am designing a boost converter for my final year college project. The circuit seems correct to me but the problem is that the output is slowly falling after a certain time. I am using 12 volts adapter as the power source and the expected output is around 24 volts. A 555 timer is used for the pwm in the circuit. I have attached the circuit diagram please check it and let me know of all the possible changes as i have my submissions on 17th of May.
     
  2. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    I would make 555 on & off times independently adjustable so that L doesnot go beyond saturation, off time then adjusts V out. A faster D7 might help.
     
  3. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    I don't see any feedback to stabilize the output voltage level, so it will wander around as the parts warm up. The FET on resistance increases with temperature, as does its gate drive current.

    ak
     
  4. crutschow

    Expert

    Mar 14, 2008
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    As AnalogKid noted the circuit is running open loop with no feedback from output to control the PWM duty-cycle so the circuit output would not be expected to be stable. If you want a stable output you will need to add such feedback. That's not particularly easy using a 555 timer.
     
  5. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    Scratch built to power a 24V air solenoid for testing.
     
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  6. arijit.india

    Thread Starter New Member

    Apr 23, 2014
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    Bernard can you please suggest some faster schkotty diodes for use in the circuit
     
  7. arijit.india

    Thread Starter New Member

    Apr 23, 2014
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    anaog kid please suggest me some way to add the feedback so that i can make the output stable or any other way to generate the pwm.
     
  8. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    My stock of shottky diodes is limited to just two surface mounts, B835LG & CMSH3-40 BK by Central Semi - 3A @ 40 V.
    You could also replace the two 12 V zener diodes with 1M pot & make output adjustable.
    Looks like diodes between pins 6 & 7 are reversed, want on time adjustable for minimum current ; off, short- max output- controlled by reset.
     
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