help about DC-AC inverter

Discussion in 'General Electronics Chat' started by the kid, May 3, 2016.

  1. the kid

    Thread Starter Member

    Jan 4, 2015
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    1
    Hello everybody
    I'm done making an inverter circuit to convert 12VDC to 220 VAC.
    My project consists of two components are DC-DC and DC-AC. Below is a DC-DC circuit to convert voltage from 12VDC to 310 VDC. I made this diagram from an online project. But I do not understand the method to stabilize the output voltage for diagram no output filter inductor. Other people told me that, the output voltage is stabilized by the resistors R933, R833, R733 and R233. But in my opinion this is not a form of SMPS power supply because there is no filter chokes. anyone can help me explain the method 310 VDC voltage stability in output and not the role of the resistors R933, R833, R733 and R233 in schematic?
    Thank you everyone
    [​IMG]
     
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  2. Roderick Young

    Member

    Feb 22, 2015
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    168
    I am not sure I exactly understand your question, but will try to explain. R733, R833, and R933 can be thought of as one resistor. Most likely, the designer used three of them in order to dissipate the power, as there will be about 300 volts across them. Together with R233, the previously mentioned resistors form a voltage divider, so a fixed percentage of the output voltage appears on the left side of R133. For example, let's say that the percentage is 1%. Then if the output is 300V, there will be 3V on the left side of R133, and since the input draws hardly any current, there will be 3V on the right side, too, at the -IN of U27. Let's further say that VREF is also 3V, fed into +IN of U27.. Now, if the output voltage falls to 290V, then there will be 2.9V going into -IN, and U27 knows to generate longer pulses, and therefore more current, in driving T133. So the output voltage rises as T133 puts out more power. Now what if the output voltage goes to 301V? There will be 3.01V at the -IN, and 3V on the +IN of U27. The IC then knows to scale back the power output so that the voltage drops back into exactly 300V. The stability is achieved by this feedback mechanism, not by a filter capacitor or inductor.
     
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  3. Lestraveled

    Well-Known Member

    May 19, 2014
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    It is most definitely a SMPS. The inductive element is the transformer (T133).

    As Roderick said, the output (300V) is scaled down to a voltage that the Switch mode controller (U27) wants to see. U27 compares the scaled voltage to a voltage reference and the difference tells the controller to increase the pulse width or the decrease the pulse width. The resistors (R733, 833, 933 and 233) set the output voltage that the switch mode controller will regulate to.
     
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  4. Papabravo

    Expert

    Feb 24, 2006
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    Is it fair to claim that this is a 12V to 310V DC-DC Converter when the center tap of the transformer shows +24V? Where exactly is this 24V supposed to come from?
     
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  5. the kid

    Thread Starter Member

    Jan 4, 2015
    59
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    as you said, T133 operate as inductive. Why?
    ithink this schematic is push pull converter?
    sory for my english
     
  6. the kid

    Thread Starter Member

    Jan 4, 2015
    59
    1
    sorry, I m missing that point
     
  7. Papabravo

    Expert

    Feb 24, 2006
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    The design seems to require a +24V DC Power Supply to connect to the center tap of the transformer, and a +12V Power supply for the controller chip. Where do they come from?
    In the title you imply that you have designed a 12V to 310V DC-DC converter THAT REQUIRES ANOTHER SUPPLY VOLTAGE. How can you be missing that point? Maybe you didn't do the design and are just in marketing, or maybe it is just a careless typographical error.
     
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  8. Lestraveled

    Well-Known Member

    May 19, 2014
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    Yes, this is a push pull converter and the turns ratio of the transformer will give you the voltage increase.
     
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  9. the kid

    Thread Starter Member

    Jan 4, 2015
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    Thank you, but i dont see a inductor at output. How to filte dc voltage without inductor. What if i insert an inductior bettwen brige diodes and filter capacitor?
     
  10. Lestraveled

    Well-Known Member

    May 19, 2014
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    An inductor is not required to filter the DC output.

    Why do you think an inductor is required??
     
  11. the kid

    Thread Starter Member

    Jan 4, 2015
    59
    1
    I m not sure, but i had researched about push pull SMPS. If there is not an inductor, formular
    Vout =n* D* Vin
    is not correrct.
    in which : n is the ration bettwen secondary to primary.
    D is duty cycle
    Vin: Input voltage
     
  12. the kid

    Thread Starter Member

    Jan 4, 2015
    59
    1
    Can anyone help me expplaining why not have the inductor that still regulate output voltage? Thank u everyone
     
  13. Roderick Young

    Member

    Feb 22, 2015
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    Possibly you were looking at a forward converter, like this https://en.wikipedia.org/wiki/Forward_converter ? The formula is for maximum possible output voltage in that case.

    The extra inductor after the transformer helps to maintain current flow, but even if you remove it, the output voltage will still be regulated. It all depends on the needs of your load. In some cases, it's not terribly important to have precise voltage regulation. If your ultimate goal is to make a DC-AC inverter, that may be one of those cases. Line voltage in the US could vary from 110 volts to 130 volts, and products are made to tolerate that. Where the nominal line voltage is 220, I suspect it's the same way.
     
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  14. neonstrobe

    Member

    May 15, 2009
    13
    1
    I think you are confusing a switching converter (forward/flyback etc) which need an inductor with a simple push-pull converter. Many old type converters simply used a transformer as a transformer: A.C. in (in this case a square wave) gives A.C. out which can be at a higher or lower voltage. I would not class this as an SMPS in the sense that it does not fall into the general SMPS class (forward/flyback/resonant etc etc). The output voltage can be controlled only by controlling the input voltage (perhaps the 24V but there is no indication of a controlled input) or the run-time. NOrmally, transformers will output the voltage that their turns ratio is designed for. BUt in square wave switching, flyback voltages can create high spikes, so the output voltage could rise if lightly loaded. It might be that the control system simply shuts down operation until the voltage falls again: just on-off, basically.
     
  15. tcmtech

    Well-Known Member

    Nov 4, 2013
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    Now to really make you head spin you can modulate your PWM signals to each of the two switching device banks in a way to make the output of the HF transformer so that once it goes through a simple LC filter it appears to be a nice standard 50 - 60 Hz sine wave and eliminate the second DC to AC part of the circuit altogether. :cool:
     
  16. the kid

    Thread Starter Member

    Jan 4, 2015
    59
    1
    thank you very much. but I have a question that is if the R233 is 47 K, the output voltage can be changed to other values?
     
  17. Lestraveled

    Well-Known Member

    May 19, 2014
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    Yes, with changing the resistor ratio, you can regulate to almost any voltage.
     
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  18. the kid

    Thread Starter Member

    Jan 4, 2015
    59
    1
    Thank you very much. now i see the problem.
     
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