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problem with PWM signal

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by maicael, Nov 1, 2013.

  1. maicael

    maicael Member

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    hello.
    Thank you as it makes a lot of sense now so an LC combo would be better.i really need to go read up a few things again.
    anyway since we are experimenting with different topologies and this one uses a push-pull config to drive a centre tap transformer.
    Our main concern now is protecting the mosfet power stage from transformer spikes and other hazards and you said something about a snubber but i read that the snubber circuit has to be carefully calculated.
    anyway i was wondering if there are any other suggestions and recommendations i dont know about in fully protecting the switching mosfets power stage.
    I know about TVS diodes but not much but checking it out.
    any ideas would be fully appreciated.
     
  2. audioguru

    audioguru Well-Known Member Most Helpful Member

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    In an output that uses a center-tapped transformer and each end of the winding is driven from a separate Mosfet then a diode cannot be used from the drain of the Mosfet to the positive supply because the drain must be allowed to normally swing to double the supply voltage due to transformer action.
     
  3. maicael

    maicael Member

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    Hello,
    Would it be ok to use a 7-0-7 transformer for a 12v input since the peak of a sine wave is 1.4141 * rms voltage so by my calculation for a 12v input we have 12/1.4141 = 8.5v so the input windings must be lower than this also especially since I am implementing low battery cut off at about 10.8v so at this voltage 10.8v/7 = 1.54...
    This would still ensure that it is still maintains the peak voltage.
     
  4. dave

    Dave New Member

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  5. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi,

    If you provide a lot of filtering and low current load you can get a higher than RMS value out as DC, but dont push it too close to the 1.4142 calculation because there is loss in the diodes too.
     
  6. maicael

    maicael Member

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    1)So it is ok to use this 7-0-7 as the main trafo that will be switched at high frequency in my push-pull design?
    2)pls kindly Explain further what you mean by a lot of filtering and low current load especially the lot of filtering part.
    Thanks.
     
  7. audioguru

    audioguru Well-Known Member Most Helpful Member

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    A 12V lead-acid battery is dead when it is about 12.2V.
    Does your circuit have voltage regulation because the transformer output will be much higher than you want when the load current is small?
     
  8. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hello again,

    The peak voltage with a 7v rms source is:
    7*1/4142=9.8994 volts, which is approximately 9.9 volts.

    Subtract 2 volts from that for the diodes, we now have:
    9.9-2=7.9 volts.

    Now the peak of a full wave rectified 60 Hertz sine occurs once every 8.333ms, so the voltage deviation estimate is:
    dv=I*0.008333/C

    where I is the DC current and C is the capacitance used as the filter.

    With a 100ma current and 1000uf we get a voltage deviation of 0.833 volts, so now the DC can fall to as low as 7 volts.

    With a 10ma current and 1000uf we get a voltage deviation of 0.0833 volts, so now the DC can fall to as low as about 7.8 volts.

    Using 3300uf would work ever better, but there will always be some ripple without a regulator. If the ripple is objectionable in the application then you need to use a regulator, or two regulators if you have plus and minus voltages.

    If the above voltage is not high enough then you need a transformer with a higher output voltage or else build a voltage doubler if the current is low enough.

    So by "more filtering" i mean add more capacitance on the output of the rectifier.
     
  9. maicael

    maicael Member

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    yes my circuit has voltage regulation as i know how important this feature is.
    you said a 12V lead acid battrery is dead at about 12.2V so does this mean this should be my cut off voltage or something like that.
     
  10. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hello,

    Not all lead acid batteries are "dead" at 12.2 volts. Some are nearly full charge. It depends on the specific unit in question, and varies depending on sealed/flooded etc.
     
  11. maicael

    maicael Member

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    hello,
    i am thinking of using RCD snubbers to protect the mosfets which will be switching at high frequency but my question is this,
    in a case where i parallel two or more mosfets, will each mosfet have its own RCD snubber network?
     
  12. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi,

    That would only be necessary if the MOSFET's are located physically far apart.
     
  13. bootaali

    bootaali New Member

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    i make a circuit of power amp ..volts are proper according to diagram.but the voice is so poor not ok not in pressure..can some body help me about this problem..some body have any idea abt this matter...???thankx
     
  14. audioguru

    audioguru Well-Known Member Most Helpful Member

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    This thread is talking about a pure sinewave inverter. You are hijacking this thread by talking about your audio amplifier.
    Please make a NEW thread to talk about your audio amplifier and post its schematic in the new thread.
     
  15. maicael

    maicael Member

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    Hello,
    Since we cannot put just a Cap at the output of the micro push pull inverter i was wondering what type of LC filter I use and how do I wind one on my own.
    The switching frequency is 25khz and output frquency is 50hz what will be its cuf off frequency and do I use an iron core inductor.
    Any valuable link as to how to make an inductor at home on my own as I may have to experiment with different values winding them myself.
    I do have some notes but you guys may have practical experiences to guide me.
    Any info will be appreciated
     

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