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Can I use a stock SIGNAL GENERATOR to drive an H bridge?

Discussion in 'Electronic Design' started by Ignoramus965, Sep 9, 2005.

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  1. Ignoramus965

    Ignoramus965 Guest

    My objective is to make a square wave inverter for high amperage DC
    for TIG welding, from a 300A CC DC welding power supply.

    I would actually be content with 200A current. AC would be used for
    aluminum only.

    I own a Wavetek 171 signal generator:

    http://www.labx.com/v2/spiderdealer2/vistaSearchDetails.cfm?LVid=2779965

    It can do up to 10 volts, supposedly, and any frequency.

    Can I simply use it to drive either MOSFETs or IGBT's that make up the
    H bridge?

    thanks

    i
     
  2. Pooh Bear

    Pooh Bear Guest

    Not directly.

    You'll need some level translation.

    If you don't know what that means then you shouldn't be attempting to make a
    welder.

    Unless you fancy welding *yourself* to something ? Terminally maybe ?

    Btw - electronics hobbyists are advised to try out *low current* stuff
    before progressing to the 'arcs and sparks' scenario !

    Graham
     
  3. Ignoramus965

    Ignoramus965 Guest

    do you refer to signal voltage level?
    I would love to hear you share your knowledge.

    i
     
  4. There's quite a bit of gate capacitance in larger MOSFETS that has to be
    quickly filled or emptied. Drivers are by no means trivial.

    In fact, the design of the driver stage is usually much more difficult
    than the output stage.


    --
    Many thanks,

    Don Lancaster
    Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
    voice: (928)428-4073 email:

    Please visit my GURU's LAIR web site at http://www.tinaja.com
     
  5. Ignoramus965

    Ignoramus965 Guest

    Don, what are your thoughts on using IGBT's for switching?

    i

    --
     
  6. Pooh Bear

    Pooh Bear Guest

    Google IR2011.

    That'll explain driving H bridges a bit. Be especially aware of the term 'high side drive'.

    Graham
     
  7. Pooh Bear

    Pooh Bear Guest

    Graham replies that they're very good these days.

    Mosfets are best at the higher frequencies though.

    IGBTs have their own little 'quirks'.

    trouble is.... you really have to have been doing this stuff for ages to pick up
    the relevant issues. For a beginner, jumping in at the deep end involves a
    cliff-like learning curve.

    Graham
     
  8. Ignoramus965

    Ignoramus965 Guest

    Thanks. I just read their spec sheet. Is there some other document
    that you have in mind?

    i
     
  9. Jon Elson

    Jon Elson Guest

    Oh, boy! Now, you're REALLY getting in deep water!

    The FETs are a lot more forgiving of shorts and linear operation, and
    at the voltages you are looking at, they work a lot better, too.

    But, still, you need perhaps 2-3 A of gate current for EACH FET in
    your circuit! If you put 5 in parallel, you need 10-15 A of gate
    current to charge the gates. Now, that is only for 50 nS or so,
    then the current in the gate drops to zero. But, if you don't
    provide this kind of current, the transistor will burn up in the
    linear region before it ever gets fully turned on.

    With IGBTs, if they are ever allowed to operate in the linear region,
    even for a hundred nS, they are destroyed! The FETs have what is
    called negative temperature coefficient, when they get hot, the
    current they will conduct drops. This allows them to distribute
    current evenly across the transistor, and across multiple transistors,
    even when in the linear region. IGBTs flatten out in the saturation
    region, but have a strong POSITIVE temp coeff. in the linear region.
    Current will "hog" to the hottest part of the transistor die in the
    linear region, and the transistor will self-destruct.

    So, you really want to avoid the IGBT.

    And, you will not be able to use your signal generator because it
    can't provide several amps. You could do a low-power test, maybe
    lighting an LED with it, to see the rise-times, etc. on your
    scope.

    For an H bridge, you need 4 different gate drives. Two of them
    can be ground-referenced, but two of them (for the "high-side"
    transistors) will have to be floating. You don't want to use
    complementary transistors, ie. P channel, as they have MUCH lower
    performance than N channel. So, using all N channel, the high side
    transistors have their source connected to the floating output
    terminal. So, the gate driver must supply gate voltage referenced
    to the floating output voltage.

    The IR 2113 and related chips can handle much of this stuff for
    you.

    One final problem is the DC welder will have a huge inductor at the
    output. Thus, the output will probably have large voltage excursions
    when the electrode touches the workpiece and the arc starts. So, you
    can't depend on getting a steady 40 V DC or whatever. When the arc
    breaks or sputters, you may have hundreds of Volts on the electrode
    cable. I think the standard practice is to move the inductor to the
    electrode cable AFTER the switching circuit. But, the design of
    the welder may make this hard to do. Certainly in AC "buzz box"
    welders, the output inductance is just built into the transformer,
    it is not a separately-connected coil. I guess they can't do that
    in a DC welder, so you may be able to change the connection there.

    Jon
     
  10. Pooh Bear

    Pooh Bear Guest

    I can think of loads ! You can't imagine how many data sources I used before venturing into this
    arena.

    You need to examine the application notes really.

    The ANs normally contain 'nuggets of wisdom' worth as much or more than gold.

    Graham
     
  11. Jon Elson

    Jon Elson Guest

    Yeah, like hanging on the cliff while the enemy is strafing you!
    Actually, that is being showered by incandescent particles of your
    expensive IGBTs, right after the cannon-like bang when they explode!
    I know, I've been there. The last inverter design I worked on, I
    had two glowing IGBTs shoot past my head on either side. After that,
    I took to powering the thing up while crouching under the table with
    ear protectors on. Once it survived power-up, I got up and looked at
    the picture on the scope with a lot less fear.

    I told all this stuff about keeping out of the linear region, etc.
    to a guy working on a hybrid electric car drive, but he had to do
    it his way! He ended up with a Ford Taurus wagon filled with batteries
    almost doing a wheelie inside a garage full of people when the IGBT
    popped!

    Jon
     
  12. Ignoramus965

    Ignoramus965 Guest

    My frequencies will be low, think 50-300 Hz or so.
    I am hoping that I can get some help from learned and esteemed experts
    here... Doubtless, I could not undertake this project alone. I hope
    that there will be people who may take enough interest to stop me if I
    do something stupid. I am not really afraid of, say, losing $80 that I
    would spend on components that blow up, although I would prefer not to
    endanger anyone's safety.

    I already did many interesting things by "standing on the shoulders of
    giants", such as repair of a diesel generator or building of a phase
    converter etc. I am saying this not to brag -- these were relatively
    simple things compared to electronic design issues that some of you
    are experts in -- but rather to underscore that projects can be brought
    to successful completion with a bit of care, open mind and good advice.

    i
     
  13. Better read the application notes really carefully too. What you don't
    know will fry expensive semiconductors, typically a few at a time.


    Best regards,
    Spehro Pefhany
     
  14. Ignoramus965

    Ignoramus965 Guest

    Thanks. I will read them closely tomorrow, when I print that document
    out.

    That said, can I get some intro on how these chips are used? Are they
    connected to DC power (for sending signals to MOSFETs), as well as to
    a signal generator that "drives" the chip? How do I tell the chip what
    to do?

    Is that big enough for a 12 kW power transmission device, or is it
    just an example?

    I apologize if this question is answered in application notes.

    i
     
  15. Pooh Bear

    Pooh Bear Guest

    Trouble is....

    Even when you do know it, you're likely to fry several by accident for good measure anyway !

    Fzzztttzzzzzzttt ! poPPPP ! BANG !!

    Graham ;-)
     
  16. Ignoramus965

    Ignoramus965 Guest

    Got it.
    That's not good...
    Very nice. I will try to read the PDF file about it to see if I can
    make sense out of it and understand how it should be wired.
    got it.

    Thanks.

    i
     
  17. Ignoramus965

    Ignoramus965 Guest

    I once tried to start a diesel generator (Onan DJE,
    http://igor.chudov.com/tmp/onan/Diesel/), sitting on a firnuture
    moving pad.

    A surprise for me was that when it started, it jumped off the pad.

    Fortunately, I forgot to disconnect chain hoist from it and it saved
    the generator as it kept it in an upright position.

    i
     
  18. Pooh Bear

    Pooh Bear Guest

    So many issues !

    When you've read a few ANs about gate drivers pop back in and ask again.

    Ohh - and you do know about how to control circulating currents in pcbs ?

    Graham
     
  19. Ignoramus965

    Ignoramus965 Guest

    Thanks. I will spend quite a while reading that stuff. That said, all
    I find are spec sheets. Is there some secret location for application
    notes, say for the IR 2113 driver?

    i
     
  20. Ignoramus965

    Ignoramus965 Guest

    scratch that, I found something

    http://www.irf.com/technical-info/appnotes/an-978.pdf

    i


    --
     
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