question about buck step down regulator

Discussion in 'The Projects Forum' started by ashade, Sep 19, 2006.

  1. ashade

    Thread Starter New Member

    Sep 19, 2006
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    I'm designing a buck step down regulator to go from 180Vdc to 14Vdc. I Im using a NPN transistor as the switch. As the square wave oscillator, Im using the LM555 timer. My question is: how will I saturate the transistor if the LM555 can output at best a 15V peak wave?
     
  2. Papabravo

    Expert

    Feb 24, 2006
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    You won't, unless the emitter of the switch is grounded. Why do you need a SMPS for such a drop in voltage? Can you post a schematic? You also need to say how much current you want out.
     
  3. ashade

    Thread Starter New Member

    Sep 19, 2006
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    well I was asked to design a switch mode power supply wich gets 127V rms in and outputs regulated 14V dc. It should be able to provide 15A of output DC current. I don't know how exactly to do that. I thought of converting from AC to DC and then apply a step down circuit working at high frequency and PWM modulation (that's the reason of the buck converter).
     
  4. n9352527

    AAC Fanatic!

    Oct 14, 2005
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    You'd probably better off with a flyback with an isolated transformer or a forward converter instead of a buck. You'd have the advantage of isolation between line voltage and the output.
     
  5. ashade

    Thread Starter New Member

    Sep 19, 2006
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    yes you're right. I'll use the half-bridge push pull configuration. I heard that it should need a bridge driver circuit... can someone explain me what is that used for and a simple way to make it by myself (without any ICs)?
     
  6. Papabravo

    Expert

    Feb 24, 2006
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    If you have an AC Input and require a DC output then what you want to do is use an AC tranformer to get the input down to a more manageable range. For example:

    You start with a transformer that takes 127VAC RMS in and puts out say 12.6 VAC RMS. The transfomer secondary should be rated for 20 to 25 Amps. That's a big piece of iron. Run the output of the secondary winding to a bridge rectifier and a filter capacitor. Now you can either have a linear 14V, 15 Amp output or you can design a buck converter with a 14V, 15A output.

    Regardless of the approach you will need to be very careful doing the layout, controlling losses, handling that much current on a PC board. If you do a buck converter you will need a great deal of luck finding an inductor that will handle the peak current without saturating. Personally, I think such a buck converter is completely beyond what I perceive to be your current level of knowledge and capability, but you need to discover this on your own.

    An LTC1775 will get you to 10 Amps.
     
  7. ashade

    Thread Starter New Member

    Sep 19, 2006
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    I'M NOT USING A BUCK CONVERTER ANYMORE..
    I'll use the half-bridge push pull configuration. I heard that it should need a bridge driver circuit... can someone explain me what is that used for and a simple way to make it by myself (without any ICs)?
     
  8. Papabravo

    Expert

    Feb 24, 2006
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    I don't think you want to do that -- Dave!

    HAL, 2001 A Space Odyssey
     
  9. beenthere

    Retired Moderator

    Apr 20, 2004
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    Hi,

    I don't think you can make such an animal without ic's. It's necessary to have a tight feedback loop and controller to get it to go.

    Linear Technology's site has many application guides and lots of ps controller chips. You might want to look over some of their material for a guide. They're awfully happy with themselves, but their stuff works.
     
  10. Papabravo

    Expert

    Feb 24, 2006
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    Amen to that! Even a digital wonk like me can get a SMPS to work first time out of the box.
     
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