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    If I see PWM dimming LED is flickering, what is happenning?

    PWM dimming frequency is 200Hz.
    I change duty cycle from 10% to 1% with 10 steps equally separated, such as 10%, 9%, 8%,.....1%, within 3s.
    It looks flickering in the run, maybe especially at the middle of this run.

    What may happen? jitter of duty cycle, or overshoot of current in LED?

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    Re: If I see PWM dimming LED is flickering, what is happenning?

    Show the circuit.



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    Re: If I see PWM dimming LED is flickering, what is happenning?

    And better still, show us some waveforms.
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    *Re: If I see PWM dimming LED is flickering, what is happenning?

    I am testing it and I have no schematic of it.....

    Click image for larger version. 

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    In the run, I can get the output current waveform like this at around 5% duty time slice.
    Does this wave peak can cause the flickering?
    Last edited by KlausST; 22nd March 2017 at 15:41. Reason: merged



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    Re: If I see PWM dimming LED is flickering, what is happenning?

    Hi,

    No code, no schematic, no waveform.... How can we help?

    Klaus



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    Re: If I see PWM dimming LED is flickering, what is happenning?

    Ok your dimming period is 5ms
    The on time for 5% is 250us.
    Is your led driver capable of getting back up to regulation level in 250us?
    Is it a hysteretic led driver.
    Does it have a frequency compensated feedback loop?……if so it might be slower unless you have a controller which “freezes” the error voltage during the pwm off times…and hopefully you have a series fet to switch the leds totally off during the dead time, so that there is no need for the feedback loop to get back up into regulation when the leds come back on.
    But yes the low duties often cause problems……and your strange current variation looks odd.

    doe it look like flickering when you go from say 55% to 50%?


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    Re: If I see PWM dimming LED is flickering, what is happenning?

    Undervoltage lockout being tickled by weak mains,
    poor socket connection or a PFC front end problem?

    Thermal protection toggling due to a die or board level
    heat issue?

    Control loop instability?

    Noise issue w/ current mode control at low duty cycles
    is often a problem, with the turnoff edge coming close
    to turnon the switch "ringing tail" may not have
    quenched and can make multimodal pulse widths?


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