Final Critique Modified Sine Wave generator

Discussion in 'The Projects Forum' started by Marcus2012, Mar 20, 2015.

  1. Marcus2012

    Thread Starter Member

    Feb 22, 2015
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    Hey everyone

    Just a last check (I promise :)) now I've selected components (all except one). I was hoping you wouldn't mind just giving it a quick glance please. It's all ok on logic functions but a few things have me concerned, mainly regarding the H-bridge.

    1) I have matched P (high side) and N-channel (low side) power MOSFETs as much as possible but they are in the TO-220A package rather than the TO-247 so I was just wondering if this package would be ok with an appropriate heat sink (with fan) at 12V 40A or is it just asking to blow up as the package is limited.
    2) The flyback diodes, I'm having trouble finding anything with the forward voltage and current I need so does anyone have any suggestions there?
    3) Anything you see that you think should be of concern please speak up :)

    This won't be running at higher than 20kHz so the propagation delays, rise/fall times etc are all acceptable and I have a theoretical bottleneck of 250kHz with these drivers.

    As usual thanks for any and all help and your time guys :D

    ================================================
    Edit : Link to two previous topics, please click on below -- Scott Wang.

    1. CMOS 7555 abstable output @ 800 KHz - trangular wave instead of square, slew rate problem?

    2. MOSFET selection and setup for modified sine wave generator.

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    Last edited by a moderator: Mar 22, 2015
  2. Marcus2012

    Thread Starter Member

    Feb 22, 2015
    349
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    EDIT: I think I've misunderstood the diode characteristics. I have never needed diode with such high ratings before :(. Forward voltage (Vf) refers to the voltage drop across the diode right? If this is correct I can never seem to find a maximum forward voltage limit in the datasheets but its it basically just the same value as the Maximum repetitive reverse voltage?
     
  3. ScottWang

    Moderator

    Aug 23, 2012
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    I'm not sure why you want to post three threads for the same projects?
     
  4. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Yes.
    No. Vf is typically ~0.6-0.7V for any silicon diode, less for Schottky and germanium diodes, and increases slightly with current. The Vf limit is not normally something you need to consider.
     
    Marcus2012 likes this.
  5. Marcus2012

    Thread Starter Member

    Feb 22, 2015
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    So I can ask different questions after applying different things, I don't want to basically lie in the title after leading the original topic off on several tangents. But regardless of that if it's a problem let me know I won't do it again sorry.
     
    Last edited: Mar 20, 2015
  6. Marcus2012

    Thread Starter Member

    Feb 22, 2015
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    Thank you for clearing that up for me :D As it's only 12V it should be fine I hope I'll get one for a higher current rating though I think (about 80A) just in case of any peaks from an inductive load.
     
  7. ScottWang

    Moderator

    Aug 23, 2012
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    Any threads and the contents will be as the database in this site, the discussing of contents will be the reference for every members, if you can keeping a complete contents then when the members searching them that will be more valuable.

    The members may frowned at the multiple threads on the same topic and lazy to answer or solve the problems for the other members, I asked that there is nothing personal in the comment, you can imagine that if many members do as you do then the forums will be appear a lots of title or threads as the Twin and Triplets, and then who will get the benefit, will you hope that to happen?

    We all learn from observing when people do something right and when they do something wrong.

    I made two links to linking to your other threads and close two previous threads, so other members can easy to get to the same topic, I hope you won't do it again, thank you.
     
  8. Marcus2012

    Thread Starter Member

    Feb 22, 2015
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    Ok thanks for that but tbh at this point I think I've realised the only thing holding me back from the build is lack of confidence
     
  9. ScottWang

    Moderator

    Aug 23, 2012
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    This forum welcome every members join the discussing just following the rules, some stuffs maybe not write into the rules, but the members have a common sense not to affecting any activities of other members, I'm one of member, so I still need to follow the rules, if you doing as a normal person do then you will enjoy here and get a great help, someday you may help some other new members ... :)
     
  10. crutschow

    Expert

    Mar 14, 2008
    13,013
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    An inductor can generate voltage peaks above the supply when the supply is abruptly stopped but the current peak will never be higher than the maximum current through it.
    Thus the diode current rating need be no higher than the maximum inductor current.
     
  11. Marcus2012

    Thread Starter Member

    Feb 22, 2015
    349
    26

    Great thank you I wasn't sure on that, this is great and means I don't need some obscenely big diodes lol :D

    I may go with this one then as a flyback diode

    NTST30100SG DIODE, SCHOTTKY, 100V, 30A, TO-220-3

    I think it would be quite good for the application and has a peak current of 60A, 100V Vrrm and a very low forward voltage drop. Does that seem ok?
     
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