Dc/dc transformer

Discussion in 'The Projects Forum' started by Madstudio, Nov 2, 2014.

  1. Madstudio

    Thread Starter New Member

    Nov 1, 2014
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    Hey guys. I try to make simple dc/dc converter . I tried with buck converter chips but all of them are regulators. I would like to have fixed ratio converter, or when the Vin changes Vout follows the same by choosen ratio. I presume that I need fixed duty PWM to achieve this, but it looks pretty complex and "current hungry". My voltages are 7-40 Vin , Vout@36v=7v/125ma. Input current should be lower than 40ma. Can you advice what type of circuit can achieve this?
    Cheers. Milan
     
  2. ronv

    AAC Fanatic!

    Nov 12, 2008
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    So you want 7 volts out with 35 volts in and about 1.5 volts out with 7 volts in?
    How well regulated?
     
  3. #12

    Expert

    Nov 30, 2010
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    Basic concept: Divide the input voltage by 5. Compare that to the output voltage. Give that result to the "voltage satisfied" pin of the buck regulator.
     
  4. Madstudio

    Thread Starter New Member

    Nov 1, 2014
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    Yes, something like that. Ratio 5.14:1. . Sorry, how you mean "regulated"? If you ask for ratio tollerance, then it's not critical, most important is that input current never goes more than 40ma and with voltage @vin 36v is Vout 7v +\- 1%. With lower Vin values tollerance can be less acurate. I hope that I described well enaugh :/
     
  5. Madstudio

    Thread Starter New Member

    Nov 1, 2014
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    Very interesting :) i have to try this with LM2597 which is right now on my bench. Thanks
     
  6. #12

    Expert

    Nov 30, 2010
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    Looks like 61% efficiency required. Entirely possible.

    No guarantees that you can finish this circuit, but I am trying to present a concept. If I seem to be suffering from cranio-rectal inversion, the other denizens here at AAC will surely point it out for us. We have a very bright crowd here.
     
  7. Madstudio

    Thread Starter New Member

    Nov 1, 2014
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    Thanks again. i will post my result A.S.A.P :)
     
  8. Madstudio

    Thread Starter New Member

    Nov 1, 2014
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    hey guys, I have one more question before i burn something ;) Do I miss something or simple opamp comparator will do the job on the #12 idea circuit? If yes, then do I have to pay attention on CM voltages. The worst scenario can be, 7v at -in and 7V at +in. since I can supply the opamp with single +15v do I play in "opamp safe area"?
     
  9. #12

    Expert

    Nov 30, 2010
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    Yes, common mode voltage limits are a requirement of op-amps. Choose the right chip or your switching regulator dies a horrible death.

    I have to confess, fast switchers are outside my area of expertise. For instance, this one needs a feed-forward capacitor that I have no idea how to calculate for a variable supply and the noise radiation depends heavily on the current loop area of the regulator board. A comparator is faster than most op-amps, but you need to arrange the circuit to add positive voltage to pin 4 to stop the voltage rise. An op-amp can be as simple as a differential pair and a voltage reference ( drawing attached) or you might find Op-amp IC's that outperform anything you can build with one transistor at a time.

    I'm experienced enough to see the concept but I'm not educated enough to design the circuit. I'd hope for MikeML or Crutschow to try to get this to run on a simulator program (LTSpice, maybe) but it's really out of my league.
     
  10. Madstudio

    Thread Starter New Member

    Nov 1, 2014
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    Here's my sim attempt with Tina
     
  11. Madstudio

    Thread Starter New Member

    Nov 1, 2014
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    Oops, sorry, I uploaded the wrong jpg. here is the right one ;) comparator is LM2903
     
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