fixed frequency vs variable frequency

Discussion in 'General Electronics Chat' started by noobuser, Jun 28, 2011.

  1. noobuser

    Thread Starter New Member

    Oct 19, 2010
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    Hi,

    I am working on a power electronic converter design and want to know what are the major advantages and disadvantages between using a fixed frequency control or a variable frequency control?

    I did a quick search but the only advantage i could find was that the variable frequency control has less harmonics than fixed frequency, but fixed frequency control may be a little simpler to implement.

    Are there any other major advanteges/disadvantages of using either or?

    thanks.
     
  2. noobuser

    Thread Starter New Member

    Oct 19, 2010
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    0
    Bump. Anybody?
     
  3. strantor

    AAC Fanatic!

    Oct 3, 2010
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    1,988
    For what?

    I don't know what you're talking about, but when you mention variable frequency, I think of variable frequency drives for electric motors.

    If that's what you're talking about then the advantage would be that with variable frequency you can run a motor at any speed, up to max. with fixed frequency you would be stuck with one speed.
     
  4. noobuser

    Thread Starter New Member

    Oct 19, 2010
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    0
    My apologies.

    I should have mentioned that I was referring to the switching frequency in a power electronic converter. For example, a pwm based converter typically a fixed switching frequency but a resonant converter may use a variable switching frequency.

    I'm just not sure what are the pros and cons of each type of are.

    Hopefully that clears things up.

    Thanks
     
  5. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    1,305
    Most modern SMPS ICs will use fixed frequency PWM for most of the time but switch to "discontinuous mode" under low current situations.

    As you might guess discontinuous mode or variable frequency mode can be better for regulation at very low currents as it can have a larger ON pulse width, which produces some benefits.

    At higher currents variable frequency is generally not used as it can have a lower frequency and higher current and voltage ripple, and larger inductor needed. So a fixed high frequency PWM is used which allows a smaller inductor and smaller filter caps and can run the inductor through a more predictable mode of operation.

    If your variable frequency is from a resonant system it probably won't vary in frequency that much and you may still get decent performance from it.
     
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