need help understanding PWM

Discussion in 'The Projects Forum' started by raffter, Mar 1, 2011.

  1. raffter

    Thread Starter Active Member

    Feb 28, 2008
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    Hi guys,

    here was a simple way of getting a pwm with varying pulse width

    http://forum.allaboutcircuits.com/showthread.php?t=49467

    now I have read from somewhere that trilevel pwm is better than bilevel

    though I dont quiet understand how the switching "sequence" is done.based on the attached diagram, how is the positive half cycle switched? is it like A=PULSES and C=just a POSITIVE of the sine reference? and on the negative cycle, B=PULSES and D just a POSITIVE(negative cycle) of the sine ref?

    pls note that the fullbridge is single supply only.

    thanks
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    Last edited: Mar 1, 2011
  3. raffter

    Thread Starter Active Member

    Feb 28, 2008
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    hii.. my browser cant seem to open the site?
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    That's pretty much it, except due to the way that many high and low side drivers work, switch C would be on and A PWM'ed, then for the negative part B would be on, and D PWM'ed.

    [​IMG]

    In your schematic, B and C are the low-side switches; A and D are the high-side switches. Drivers such as the IRL2110 use a "boost cap" that has to be constantly charged back up by having the low side switch turned on. Drivers like that one are pretty difficult to understand at first.
     
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  5. raffter

    Thread Starter Active Member

    Feb 28, 2008
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    nice! :) but would it be fine also IF for the low side (sine) , B=pwm'd and D=on?
     
  6. bertus

    Administrator

    Apr 5, 2008
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    raffter likes this.
  7. raffter

    Thread Starter Active Member

    Feb 28, 2008
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    thanks bertus,
    lots of reading to do.

    @sgt.
    HOW about if the roles are switched(lower switches be the PWMs and upper switches=on/off)? that way the bootstrap caps are sufficiently charged<by the switching of the lower switches>. would this be a better option?

    attached is a simulation. still unsure if these are the right signals... I have just added a comparator to have a square output from my sine ref
     
    Last edited: Mar 1, 2011
  8. SgtWookie

    Expert

    Jul 17, 2007
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    Your output waveform:

    [​IMG]

    You asked:
    If you can get it to work, go ahead.
    Drivers like the IR2110 require the high-side MOSFET to be PWM'ed in order to keep the cap charged.

    In the meantime, have a look at Linear Technology's datasheet for the LT1336 Half-H-bridge gate driver that has a boost regulator:
    http://www.linear.com/product/LT1336
    On page 15, there's an example Class D amplifier, which is similar to what you seem to be attempting to do.
     
    Last edited: Mar 1, 2011
    raffter likes this.
  9. raffter

    Thread Starter Active Member

    Feb 28, 2008
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    I forgot to ask about this.

    is this correct?

    PWM ->> A = D
    SQ ->> C (inverter) B
     
  10. SgtWookie

    Expert

    Jul 17, 2007
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    A and B must never be on at the same time.
    C and D must never be on at the same time.
    It would be OK for B and C to be on at the same time, but then you couldn't turn A or D on without shorting the supply.

    A typical sequence would go something like this:
    1) All MOSFETs are turned off.
    2) MOSFET B is turned ON.
    3) PWM is applied to MOSFET D's gate.
    4) PWM is removed from MOSFET D's gate, and MOSFET B is turned OFF.
    5) A small amount of "dead time" passes where all MOSFETs are turned off.
    6) MOSFET C is turned ON.
    7) PWM is applied to MOSFET A's gate.
    8) PWM is removed from MOSFET A's gate, and MOSFET C is turned OFF.
    9) A small amount of "dead time" passes where all MOSFETs are turned off.
    10) Go back to step 2.

    You cannot apply PWM to both A and D at the same time, or you will short the supply across the high and low side.
    You should not use an inverter between MOSFET B and C's gate signals; as there must be some "dead time" to prevent shorting the bridge on one side.

    If the PWM is still applied to the gate while the low-side MOSFET is off, the boost capacitor will discharge and the high-side MOSFET will either become disabled (if the driver is so equipped) or will burn up due to insufficient Vgs (voltage on the gate using the ground terminal as a reference point.)

    Logic level enhancement mode MOSFETs have Rds(on) specified when Vgs is 4.5v to 5v.
    Standard enhancement mode MOSFETs have Rds(on) specified with Vgs=10v.
     
  11. raffter

    Thread Starter Active Member

    Feb 28, 2008
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    thanks Sgt.

    a bit complicated eh. oh well, back to simulating.. :)
     
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