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LM3447: Lm3447 slight flicker of the LED

Alexey Ivanov
Prodigy 30 points
Part Number: LM3447

Hello!
The project is implemented on the device LM3447. The input voltage is 230AC 50 Hz. Load 16DC, 300 mA (4,8 W). However, at low brightness levels of 10-20% there is a slight flicker of the LEDs with a frequency of about 5-15 Hz (visible to the eye). The project is a classical scheme. Calculations are conducted in accordance with the recommendations of the Ti. In addition, periodically a bright flash of LEDs occurs.
Regulation was made on the front and on the trailing edge of the sine. In order to prevent false triggering of the dimmer, an incandescent lamp with a capacity of 60 Watt 230 V is connected in parallel. Attempts to select Cff, Ccomp, Rac the results were not given.
What actions should I take, in order to completely eliminate the flicker of the LEDs?

  • 0
    TI__Mastermind 25500 points

    Hello Alexey,

    I have forwarded this to the expert on this device. He should be in tomorrow.

    Regards,

    Clint

  • 0
    TI__Mastermind 28655 points

    Hello,

    Do you have a way to look at the input current?  This is very low power for a 230 VAC design and triac dimmers (leading edge) require holding current to stay on.  The value of the EMI capacitor across the input or rectified AC takes holding current away when the rectified AC is falling which makes this problem more difficult.  Adding a 60W incandescent lamp is similar to loading the triac dimmer making sure it has enough holding current.

    If you can watch the input current see if it is shutting off too early.  If that is the case either the holding current isn't high enough, there is too much line capacitance or the dimmer requires too much holding current for an 4.8 watt design.

    If it is false triggering then the damper may not be sufficient to keep the triac dimmer on after it fires.  This is caused by the current ringing from the rising edge of the triac, if it rings negative enough it will also cause the triac to shut off.

    Page 23 in the datasheet has the calculations for CFF, Ccomp and Rac.  As well as explanations starting at page 14.

    The hold circuit can help with triac dimming issues however the more power you dissipate the larger the components/heatsink for the hold/bias MOSFET needs to be.

    Regards,

  • 0 in reply to Irwin Nederbragt
    Prodigy 30 points

    The shape of the current is shown in the figure. This is the minimum level of brightness.
    We conducted tests on another load (15W) using LM3447-PAR-230VEVM. At the minimum dimming levels, there are also minor changes in brightness visible to the eye. As if the current is floating at a frequency of 1-5 Hz. Is it possible to eliminate this phenomenon?

      

    Ch1 - rectified line voltage (100V/div) Ch2 - line current (100mA/div)

  • 0 in reply to Alexey Ivanov
    Prodigy 30 points

    One more thing. When the minimum brightness level is set, after some time the brightness is reduced to full blackout

  • 0 in reply to Alexey Ivanov
    TI__Mastermind 28655 points

    Hello Alexey,

    What is the current per division on this picture?  Was is the AC source to this (line or power source or)?  The shape doens't look exactly like I would expect for a rectified AC falling AC waveform, it should look more sinusoid.

    Flutter at low dim levels can be caused by many things.  If the circuit is working correct I would guess that the line voltage is slightly fluctuating.  If this happens the triac will fire at slightly different points and when dimmed very low the slight change will become very noticeable.  I have also seen this when test equipment is connected to the circuit when it is running from the AC directly.  Does it change significantly when placing an incandescent in parallel with it or does it still flutter?  Does the AC look more sinusoid when an incandescent is connected in parallel?

    Regards,

  • 0 in reply to Alexey Ivanov
    TI__Mastermind 28655 points

    Hello Alexey,

    This can also be caused by many things.  It is likely that the triac dimmer is misfiring making the angle detect small.  I would look at the rectified AC and input current before it goes out and after it goes out and see what is different.

    Regards,