High frequency PWM dimming of LEDs?

Discussion in 'The Projects Forum' started by eem2am, Jan 12, 2013.

  1. eem2am

    Thread Starter New Member

    Jul 22, 2012
    9
    0
    Hello,

    We are wanting to do high frequency PWM dimming of the output of current transformers (which feed LED lamps) hooked up to a twisted pair bus.
    Do you think the “gap toothy” resulting waveform (see below) will be a cause of any problems?

    (This is a different question on a subject discussed elsewhere here)

    Block diagram………………..
    [​IMG]

    ….you can see the “shorting FETs which dim by stopping current flowing to the LEDs when the FETs are ON.

    We do NOT want to do low frequency PWM dimming, (~200Hz) because the low frequency of this dimming causes problems for the current source. (-it means the current source is switching from high power to low power output which causes problems)

    Therefore, we wish to do dimming at a much higher frequency (83.3KHz). But do you think that the resulting current waveform (as below, when D is not equal to 1) is going to mean problems?


    LED lamp current with no dimming (D = 1)
    [​IMG]


    LED lamp current with dimming with D = 0.083
    http://i48.tinypic.com/3ctas.jpg


    LED lamp current with dimming with D = 0.33
    [​IMG]

    LED lamp current with dimming with D = 0.92
    [​IMG]


    (Also, how would you reduce dissipation in the current source by correcting the power factor in the twisted pair?)
     
    Last edited: Jan 12, 2013
  2. wayneh

    Expert

    Sep 9, 2010
    12,154
    3,061
    I don't think it'll be a problem. We'll see if someone else has a different idea.
     
  3. eem2am

    Thread Starter New Member

    Jul 22, 2012
    9
    0
    Last edited: Jan 12, 2013
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