DC converter problem

Discussion in 'The Projects Forum' started by Jimbo-m, Feb 17, 2009.

  1. Jimbo-m

    Thread Starter New Member

    Feb 17, 2009
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    Hello there,

    I have made a DC-DC converter which (my aim was) steps 72V to 12V. I have used a 555 IC to drive a MOSFET (IRF540N). I have made the circuit in LTspice first and it works! (24khz and 17% duty cycle). I added an inductance (0.01mh real world value for a wire and a bulb?!?) and resistance (6ohm) to see how it works under load... I added a resistor to the gate (0-1kpot) to slow down the rate of change and limit spikes.

    I the 'real world' however I have exploded 3 bulbs and showered myself in glass... exciting!!! (viva la safety goggles).

    I have since reduced the duty cycle to a point where the 12V bulbs no longer explode and tested it with my multi meter. I have now got 6.9VDC and 14.3VAC... Why is there so much AC?
     
  2. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Can you post a schematic ?
    Do you have a capacitor on the output ?

    Greetings,
    Bertus
     
  3. Jimbo-m

    Thread Starter New Member

    Feb 17, 2009
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    Umm... No Capacitor. I have not electronics training. Just a 555 book and a soldering iron.

    I accidentally destroyed my IRF540N when I shorted the circuit out with my butter fingers... Oops. I am now using a 12N604a4 IGBT as the IRF540N was out of stock.
     
  4. bertus

    Administrator

    Apr 5, 2008
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  5. Jimbo-m

    Thread Starter New Member

    Feb 17, 2009
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    Right... So having skimmed through some of that I now have a splitting head ache and no idea where to put the capacitor...

    There is some fairly complex stuff there; feedback loops to maintain a steady voltage, noise reduction etc...

    A totally steady voltage and noise doesn't really matter: I am only going to be running vehicle lights and the horn. 12VAC would not be a problem but I also need to run a 5V DC regulator (LM2940). I don't think they like AC... Do they?
     
  6. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    When you use this part of the schematic, you will have a capacitor on the DC.

    [​IMG]

    The coil stores the energy and gives it to the capacitor.
    The switch shown is the FET or IGBT you use.

    Greetings,
    Bertus
     
  7. Jimbo-m

    Thread Starter New Member

    Feb 17, 2009
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    Thanks, I'll have to meditate on it... c'mon brain get me through this and I'll get back to killing you with beer.
     
  8. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    I hope you do realize that for the above circuit +Vout will be higher than +Vin because the circuit configuration is a boost converter.

    I thought the OP is trying to reduce 72V to 12V.:confused:
     
  9. bertus

    Administrator

    Apr 5, 2008
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    Ok, Sorry.

    You need a buck converter.
    That has a transistor (switch) at the power supply side.
    Also described on that page.
    Or take a look at the LM5005.
    http://www.national.com/pf/LM/LM5005.html

    Greetings,
    Bertus
     
    Last edited: Feb 17, 2009
  10. Jimbo-m

    Thread Starter New Member

    Feb 17, 2009
    9
    0
    After staring at it long enough the penny has dropped... Thanks brain :D

    However... I am using N-Channel switch, which won't operate on the power supply side... (no positive Volts at gate to emitter/source).

    Is there a way to do it with an N-channel switch?
     
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