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Edge or center aligned PWM...

Discussion in 'General Electronics Chat' started by smanches, Jun 29, 2009.

  1. smanches

    smanches New Member

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    Do you think it matters if you use edge aligned PWM or center aligned?

    Center aligned makes it a bit easier to do deadtime timing, but I'm wondering if I'm missing something.
     
  2. dknguyen

    dknguyen Well-Known Member

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    It only makes a difference if you have multiple PWM signals. As for deadtime, many MCUs already have this built-in so the difficulty of implementing it is the same.

    I think center-aligned produces less noise if I am not mistaken because the transitions are staggered rather than occuring all at once.
     
    Last edited: Jun 29, 2009
  3. smanches

    smanches New Member

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    I'm in the process of making a synchronous buck converter without uC. I was using a sawtooth wave as input to a comparator, but this made it hard to put in deadtime on both sides of the PWM wave.

    Then it dawned on me that if I used a triangle wave, I could change the comparator reference voltage just a bit and instant deadtime on both sides. Just never played with center-aligned before.
     
    Last edited: Jun 29, 2009
  4. dave

    Dave New Member

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  5. dknguyen

    dknguyen Well-Known Member

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    Well I guess in that case center-aligned would make a difference even if there was only one PWM signal since the issue is about the actual processing of the PWM waveform than the final result (which would be the same whether or not you use edge or center aligned since you onyl have one PWM waveform).
     
    Last edited: Jun 29, 2009
  6. smanches

    smanches New Member

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    I'm thinking this would also work for an H-Bridge too, but only for the signals of the high side mosfet and it's complimentary low-side. The timing between the two sides would still have to be handled manually.

    Or would it? I'm not sure why I can't think through this...
     
  7. Mr RB

    Mr RB Well-Known Member

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    Umm trick question?

    If the frequency is the same then both waveforms are absolutely identical; edge aligned and centre aligned. The one and only variable is the duty cycle.
     
  8. smanches

    smanches New Member

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    I finally found some references to it. Center-aligned is preferred for multi-phase ACIM and BLDC motor controls. I guess they generate less EMI since the turn on and turn off times are slightly different on each phase. On left-aligned PWM, all the phases turn on at the same time which causes a large current spike and it's associated harmonics.

    I had always seen the option to select between the two on MC controller ICs, but didn't know why you would want to choose. Now I know. :)
     
    Last edited: Jun 30, 2009
  9. OutToLunch

    OutToLunch New Member

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    for a voltage mode application, it should not make a difference. If you were going to use current mode, then trailing edge modulation would be required for the current signal to ramp up.

    If you are concerned about dead times to eliminate cross conduction, you should let the FET drivers handle that.
     
  10. smanches

    smanches New Member

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    What do you mean by voltage mode and current mode?

    Can you point out some drivers that handle the dead time? I've never seen that before. Dead time is almost always controlled by the PWM generator.

    BTW, I'm building my own drivers for this application, just simple totem poles. It's a learning experience on how to do switching supplies using only simple analog components.
     
  11. Mr RB

    Mr RB Well-Known Member

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    You said you are making a synchronous buck converter, then it turned into multi-phase??? :eek:
     
  12. dknguyen

    dknguyen Well-Known Member

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  13. OutToLunch

    OutToLunch New Member

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    voltage mode just means that you are using the output voltage as the control input to the control loop while in current mode it is the current information.

    Typically any IC that creates PWM outputs for power regulation was designed to be paired with a driver. It is the driver that has the deadtime control. Fro power controller ICs that have the drivers integrated, the dead time control is still going to be in the driver section of the IC.

    Most of the analog IC companies that are in the power business will have drivers that incorporate a deadtime.
     
  14. smanches

    smanches New Member

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    At first I thought you just meant voltage regulated or current regulated, but after reading the paper DK provided, I understand what your saying now. They are both voltage regulation methods, just one used the output voltage as the feedback, the other uses the switch current as feedback. Does that seem right?

    My original question was generic as I just wanted to know peoples experiences with both. I gave a specific example of what I'm working on right now that lead to my question, but the question was still generic in nature. I then was able to confirm what DK said about center-aligned reducing EMI, and that it is mostly useful in multi-phase motor applications. Hope that clears up your confusion.
     

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