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    LED drive circuit design

    Hi ALL:
    I need your advice for designing an string of White LEDs driver circuit using TI UCC28810 (LED light controller).

    16 White LEDs with a spec of 10.5V at 350mA and the power is 3.67W for each LED. Total power dissipation by the LED string would be 16x3.67~59Watts. With 20% derating the power is~70W. If the efficiency of the driver system is considered as 85% (very conservative) then the total power requirement ~82Watts.

    In the design, the first stage is a Flyback PFC stage and the second stage is a low-side buck stage. Both are operating at critical conduction mode. The LED current would be provided by the low-side buck converter.
    Now my questions:
    (1) In terms of the PFC performance or better power management, which one would be the most suitable among the following options to connect the LEDs in the load?
    1 rows of 16 LEDs
    2 rows of 8 LEDs
    4 rows of 4 LEDs
    8 rows of 2 LEDs

    Note: Each LED would have 10.5V across it and 350mA is flowing..

    (2) What's the class 1 and class 2 power specification? How can I incorporate the class 1 and class 2 specs into this system.

    I need your advice.
    Please help.
    thanks,
    Sattar

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  2. #2
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    Re: LED drive circuit design

    Hi,

    Who would of thought powering LEDs could be so complicated.
    Looking at the spec sheet Figure 26 shows a configuration for powering 30 LEDs
    in series. I think 16 LEDs in series would be the most efficient configuration
    because it is only one current loop to monitor and control.

    scanman



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    LED drive circuit design

    16LEDs will be best for efficiency.but the input power would be higher due to Vf for each LED would be around 3.0V~3.5V, then input voltage will be 48V.
    considering LED driver IC power consumption, the input voltage for LED driver would be higher than 50V.



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  4. #4
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    LED drive circuit design

    Hi Scanman:
    Did you consider the fact that when all 16 LEDs are in series in one current line, if one LED fails then the LED string is cut off ~ results of discontinuation of lighting from other LEDs in the series?
    What are problems of putting two rows of 8 LEDs?
    thanks,
    Sattar


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  5. #5
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    Re: LED drive circuit design

    Sattar,

    It is a question of available current ~ 700ma to run 2 rows of 8 LEDS.
    The example in Fig 26 shows current of 900ma so it looks doable.
    The two rows is probably the best tradeoff configuration for efficiency and
    all or nothing failure mode.

    scanman



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    Re: LED drive circuit design

    Scanman, Doubletao:
    Thank you for your comments/suggestions.

    Any other ides from any one else?

    Sattar



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    Re: LED drive circuit design

    Class 1 must have their chassis connected to earth ground whereas Class 2 is designed is such a way that it does not require safety connection to earth ground.

    Each LED has 3~3.5V drop. If you need to meet SELV (operating output voltage below 60V), then you are better off having at least two outputs with 8 leds in each output string which gives approximately 28V on each string. One can also run all 16 leds on one string (approx. 56V) but you would have to make sure that the output voltage never goes above 60V since you need 56V and you are cutting it very close to the SELV threshold.

    HB Leds have built-in circuit whereby opening any leds would bypass the failed led so the other leds in the string are still working. You will have to use current mode control to run leds since the brightness of the led is directly related to the current flowing through it. The voltage drop across a led is non-linear.

    I am not sure about the use of flyback topology for PFC. Since you already have a buck dc-dc down converter, you would be better of using a boost converter for PFC. A boost converter is easier to design. Having a flyback topology makes it more complicated but can be done. Also, using a single row, you can eliminate the use of low side buck converter for LED current making it a 2 stage converter.



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