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LED driver

Discussion in 'LEDs and Optoelectronics' started by rob_croxford, Sep 19, 2012.

  1. rob_croxford

    rob_croxford

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    Aug 3, 2010
    Hi all,

    I am currently working on an LED driver to run 2 strings of series LED's in parallel. The output power is required to be 18W (36V @ 500mA).

    I have decided to use a fairchild driver chip that allows for feedback to be recided via an opto isolator to control current and check for over voltage conditions.

    The following circuit is proposed by fairchild for use with their fl6961 driver chip.

    http://www.fairchildsemi.com/an/AN/AN-9737.pdf

    I am stuck at how to change this version (24V @ 700mA) to the required output of my driver.

    The reason for this is that i cannot get my head arround the control loop back into the driver. It appears that it works almost like a conventional constant current source by simply comparing the output with a voltage reference that is used to set the required current level and then adjusting the PWM out of the chip to drive the primary side of the transformer. But i am unsure of how this information is passed through the opto as it would need to be in some kind of switching format.

    any help is greatly appreciate.

    All the best

    Rob
     
    Last edited: Sep 20, 2012
  2. BobK

    BobK

    7,599
    1,641
    Jan 5, 2010
    Designing a mains powered switching power supply is no easy task, not one I would take on. Unless you have the experience to do this, you would be much better off either buying the switcher or using a transformer.

    Bob
     
  3. rob_croxford

    rob_croxford

    262
    0
    Aug 3, 2010
    I have a fair amount of experiance in the design of switchmode PS and DC/DC converters when it comes to controlling a motor however i have generally used a Micro to develop a "loose" control loop.

    For this project i wanted to keep the cost down by utilising an off the shelf driver chip as oposed to developing a digital controller.

    The rectification stage up to the output is fairly strait forward. I am just unsure of how this design regulates the current. Without knowing this i cant make any intellegent changes to the design to get the required power output!! It is very frustrating! lol. Especially when the datasheet offers no real design tools.

    Regards,

    Rob
     
  4. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,266
    Nov 28, 2011
    The optocoupler operates at DC. Increasing the current into the LED causes the controller chip to reduce the amount of energy it feeds into the transformer.

    In the application circuit, there are three sources of this "back off" signal, diode ORed together. I'm not happy with the way these signals are generated - especially the lack of DC feedback around the two op-amps, but I'm prepared to accept that the designer of that circuit knows more about it than I do :)

    So it's just a continuous DC control loop.
     
  5. rob_croxford

    rob_croxford

    262
    0
    Aug 3, 2010
    thanks for the replys :)

    I Have a feeling i am missing something vital (and more than likely relativly simple!!)

    So the current is sensed via the sense resistor. The voltage generated (Vsense) is then compared to a voltage reference (in this case 2.5V) and the output of the comparator generates and error signal depening on what level the measured value is compared to what is required.

    How is this information sent back via an opto? surely the opto must recieve either Digital inputs or at least a switching signal as variable voltages cannot be transfered. Well as far as i am aware they cant anyway.

    Thanks again :)
     
  6. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,266
    Nov 28, 2011
    Yes, it's passed through the optocoupler as a "variable voltage". Optocouplers have a linear region. Actually it's not very linear, but it is at least analogue.
     
  7. rob_croxford

    rob_croxford

    262
    0
    Aug 3, 2010
    Ahh ok i did not know that Opto's could pass variable voltages. This makes slightly more sense to me now.

    So just to recap. The Constant current control is the dominant signal for generall operation sending a signal that is proportional to the output of the controller which in turn generates an error signal when compared to a set point value.

    The constant voltage control only comes into play if there is an over voltage condition at which point the comparator swings high and shuts off the driver.

    Sound correct??

    Oh and i did mean LED's and not resistors!!! ;)
     
  8. rob_croxford

    rob_croxford

    262
    0
    Aug 3, 2010
    Update for anyone also looking at LED driver topologies.

    The feedback of this circuit is actually fairly clever (and simple). The current is sensed (as previously mentioned) and compared to a set point. If the sensed level is above that of the set point the capacitor in the feedback loop of the CC opamp forces the output to opamp VCC (or just bellow). This signal tells the driver chip to essentially shut down. Once the output current gets back to the set point the output drops to zero telling the controller to continue driving. This is a continous loop and care should be taken when choosing cap values for feedback as the capactitor controlls the Rate of change of the output when compared with the input.

    The over voltage protection works exactly as mentioned above - simple comparator circuit.

    Both feedback outputs are Or'd together using diodes and that is basically it.

    Tuneable parameters are: Current sense resistor, Voltage sense divider and OpAmp feedback arrangement.

    Also for any other power output than that of the original circuit a new transformer will be needed. Whats even more interesting is how this circuit powers the secondary side of the transformer with DC at its primary. A little light reading for anyone whos interested ;)

    Most is in the datasheet and a little bit of inductor theory.

    Cheers everyone!
     
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