17V to 12V buck converter

Discussion in 'The Projects Forum' started by electrica, Feb 18, 2014.

  1. electrica

    Thread Starter New Member

    Feb 14, 2014
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    Hello

    I am trying to simulate a buck converter in Proteus which is required to convert 17V to 12V approx. Based on the simulation circuit, I will be implementing the hardware. I used a schematic which I found in one of the threads here.

    Keeping the Pulse width of the PWM signal at about 5% I'm able to obtain about 12V output but the current is too low. Considering that I need to supply to a battery for charging it,a decent level of current is needed which I'm not able to get.
    Plus if I connect a load of no matter how many ohms, the output gets reduced considerably.

    Any help would be much appreciated.
    Thank you
     
  2. crutschow

    Expert

    Mar 14, 2008
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    Can't really help without a schematic with all the part designations.

    What is a "decent level of current"?
     
  3. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    This is why I am in favor of building things and not living in the virtual world of simulation.

    You can't possibly get 12V out from a buck fed from 17V with a 5% duty cycle. Any sim that does is wrong. The operating duty cycle is ROUGHLY given by Vout/Vin, which any student finds out two minutes after breadboarding a buck converter. Your running duty cycle will be roughly 70% but actually a little more to make up for conduction and diode losses.
     
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  4. crutschow

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    Ah, another simulation Luddite. ;) But you can get 12V out of a buck converter at a 5% duty-cycle if the output load current is so low that the inductor is operating in the dis-continuous mode. This is likely what the simulation is showing (and what will also occur in the real world). If the output is given a sufficient load so the inductor is in the continuous mode (never goes to zero current), then it should give an output of 12V at a ≈70% duty cycle as you noted.
     
  5. bountyhunter

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    Sep 7, 2009
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    You proved my point: the sim is leading the user down a useless path because the "designer" does not even understand the basic operation of a buck converter. Nobody in their right mind would design a buck to run discontinuous mode for normal operation because it is so ineffecient.

    If I have little patience for this, it's because 20 seconds on Google can turn up a half dozen web sites with the basics on switching converters.

    http://en.wikipedia.org/wiki/Buck_converter
     
  6. crutschow

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    It would seem that it proves the opposite. The sim is actually showing the user that the circuit won't work as he wants before he ever builds the real circuit. That's one of the purposes of the sim, to show the designer his design errors before actual fabrication. And few of us design the perfect circuit the first time (perhaps you're the exception). :rolleyes:
     
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  7. bountyhunter

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    Sep 7, 2009
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    If the sim is showing some kind of stable operation at 5% duty cycle, that is completely mythical. When switchers (real circuits) start running in discon mode, the output gets choppy and they start dropping pulses. You don't see a steady pulse train, you see pulses of varying widths and some pulses missing because the feedback signal coming into the switch IC is choppy. I have seen it a thousand times. Anybody who has built one knows this. The extra noise on the output is a known disadvantage of running discon mode.

    The op's initail statement of:

    suggests to me the sim is feeding him baloney and he doesn't realize it or realize why. Hence my suggestion to take five minutes and build one.
     
  8. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    NO, that's why I build them up instead of taking a sim's word for it.

    Did you seriously not read the first post:

    Based on the BREADBOARD, he should be implementing hardware.
     
  9. electrica

    Thread Starter New Member

    Feb 14, 2014
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    By decent I meant...to charge the battery i need at least about 5A whereas I'm getting only few microamperes. Basically it fluctuates a lot and I cant see a proper value.
    By making the duty cycle 70% and adding a load of 220 ohms I got about 12V. Thanks a lot for you help.
     
  10. electrica

    Thread Starter New Member

    Feb 14, 2014
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  11. crutschow

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    I see the disconnect in our discussion. We are taking about two different types of circuits. What you say is true for a closed loop circuit, but he is running it open loop with a fixed duty-cycle, thus making it stable. So there's no baloney from the sim side in this case.

    But I certainly agree that he should do a breadboard after the simulation and before implementing in the final hardware.

    P.S. If you can build an actual circuit in five minutes you take the Gold for the fastest assembler on record, especially if that includes getting the parts together. :rolleyes:
     
  12. crutschow

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  13. #12

    Expert

    Nov 30, 2010
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    Now I need to look up, "Luddite":(

    That's twice today I had to use a dictionary because of this site!:mad:

    Aha! I'm a neo-Luddite.:D

    "A neo-Luddite is a Luddite in the Internet age."
     
    Last edited: Feb 19, 2014
  14. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    Actually, if I was desiring to learn and get a finished (WORKING) design I would just get an eval board for any of a wide number of simple switcher products already built up and test them. That way I could tweak a few components and have the working design done in a few minutes. You can get them on ebay or from TI and a bunch of other companies.

    The point is that would lead to a working design.

    Here you go:

    http://www.ebay.com/itm/INBOARD-LM2...711?pt=LH_DefaultDomain_0&hash=item3f132d3ddf

    $18 and all the work is done for you.


    IMHO, that means we are knee deep in baloney. Running it open loop means it is not a regulator at all and can not provide a fixed output voltage. The output voltage will just vary with the input voltage. This point up the biggest problem with sims: if the user doesn't understand even the basics of the circuit, it's garbage in garbage out.
     
  15. crutschow

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    I don't really follow your logic. If he doesn't understand the basics then the real circuit will also be knee (perhaps waist) deep in baloney with garbage-in garbage-out. You can't blame that on the sim since it's not generating the baloney, it's the design of the circuit. ;)
     
  16. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    I've noticed, so I'll spell it out clearer: somebody who thinks you can run a 17 - 12V buck at 5% duty cycle does not have even the most rudimentary understanding of how a buck works. Using a sim and building from that will result in catastrophe. How do I know? I spent the last 20 years cleaning up the smoldering wreckage from such endeavors.. I am trying to guide the OP to a path which could lead in a working design.

    here is why I recommended what I did: spend the $18 and buy a working board. Spend the whole ten minutes it will take to read the data sheet for the switcher part LM2576. Spend another ten minutes to connect the board and use a scope to look at the waveforms and the result will be the user will then understand the basic operation of the circuit. He will then have the capability of doing the design. Or even look at wikipedia:

    http://en.wikipedia.org/wiki/Buck_converter

    The danger of this is that if we let engineers actually do this simple procedure, they will no longer need the sim and then where would we be.....

    Or, he could just keep wasting time running sims and crank out the first rev of hardware. I'm sure they have at least one engineer who fixes those disasters. I used to be that guy.

    done, done, done
     
    Last edited: Feb 19, 2014
  17. crutschow

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    You've certainly made it quite clear that you vehemently hate simulators, and think anyone using them is just wasting their time. Well, you are welcome to your opinion, but that doesn't make simulations useless. It's obvious your having to deal with numerous customer idiots who didn't understand circuit design or simulations and tried to stupidly go from simulations directly to final hardware has seriously jaded your viewpoint.

    But giving the op a working board is no more likely to help him understand it's operation them having him simulate that working board. Actually the simulated board is much easier to probe to see all the currents (which are generally difficult to measure in real life) and voltages, and experiment by easily changing part values to see their effect on circuit operation.

    I fully realize I can't change your opinion about sims since you seem to be impervious to anything that suggests simulations have any use. But in my personal design experience simulations have been very useful and have helped me identify errors in designs before they were built, refine my design, and identify the source of operational anomalies in circuits that have been built by others. I'm just trying to convey to others that your negative view on simulations is not the whole story.

    done..
     
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  18. ScottWang

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    Aug 23, 2012
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    I had the experience twice about the simulation and doing the real experiment, but they were different, I was doing the real experiment and the netizen was used the simulation software, the problem was that the netizen were old engineers not a newbie.

    I also don't want to encourage the newbie of EE just using the simulator to learning the EE, if they can do both, then it will be better, it means that they should also using solder or breadboard to do the experiment not just simulation software, I didn't means that the simulation software is bad, it is a tool when the learner know more the basic theories, and doing more and more the practical experiments, the tool will help the learner save the time and see the waveform.
     
  19. ronv

    AAC Fanatic!

    Nov 12, 2008
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    I love the simulator, but I'm not a math freak and my wife doesn't like all my stuff on the kitchen table. :)
     
  20. ScottWang

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    Aug 23, 2012
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    What's on the kitchen table?
     
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