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LM3450AEV120V30: TRUNCATED AC SOURCE

David Rogers56
Prodigy 20 points
Part Number: LM3450AEV120V30
Other Parts Discussed in Thread: LM3450, LM3409

IN THE PRESENT APPLICATION, THE LM3450 EVALUATION BOARD WILL "SEE" AN 120VAC INPUT WHICH WILL BE 'TRUNCATED' DOWN  TO 15V rms.  THE SOURCE AC WAVEFORM  IS FROM A TRIAC CONTROLLER. THIS IS USED TO DIM A SET OF PARALLEL LED DRIVERS. THE DOCUMENTATION I HAVE FOR THE LM3450 BOARD DOES  APPEAR  ALLOW FOR AC INPUTS OTHER THAN THE USUAL SINE WAVE FORM.

IF THE LM3450 IS NOT SUITABLE HERE, PLEASE RECOMMEND  A LED DRIVER PRODUCT WHICH IS DIMMABLE  BY THE AC INPUT

DAVID W ROGERS PE

  • 0
    TI__Mastermind 28655 points
    Hello David,

    I'd like to understand what you are trying to do before answering. Are you using the LM3450 as a triac dim decoder only?
    I'm not sure I understand "THE LM3450 EVALUATION BOARD WILL "SEE" AN 120VAC INPUT WHICH WILL BE 'TRUNCATED' DOWN TO 15V rms."
    Are you looking to use this as an isolated power supply that can determine dim angle from a triac dimmer?

    The EVM is a critical conduction mode PFC flyback that generates a voltage rail via the LM3450. It also decodes the triac firing angle and can add needed holding current if necessary This rail is used to drive the LM3409, a buck converter, as a current source to the LEDs. The triac dim decoding is sent across the isolation boundary to control dim level.

    Regards,
  • 0 in reply to Irwin Nederbragt
    Prodigy 20 points

    My client is proposing  to build an airport runway lighting product  consisting of 20 LED drivers {LM3450/LM3409} in parallel. The issue is that for the various dimming steps all the parallel LED lamps must deliver the same lumens (+/-3%).  The dimming step information is conveyed to each lamp via the conduction angle of the AC source power. This AC source a is Triac controlled  AC power supply which will be varied from 120Vrms down to 35Vrms  by truncating  the 120V mains power.

    The issue is how will the LM3450  behave 'seeing' an AC voltage chopped by a Triac control.

     

    Dave Rogers PE

  • 0 in reply to David Rogers56
    TI__Mastermind 28655 points

    Hello Dave,

    The LM3450 is intended to run from a triac chopped AC waveform.  If this dimming method is similar to how a triac dimmer for an incandescent for home use it should work fine, that's what it was designed for.  I assume the output of this is similar to the waveform 24b in the datasheet?  There is also no transformer/isolation for the AC source?

    Are the 20 drivers located close to the triac cut AC source or are these placed far away (along a runway) length?  The reason I ask is long wiring may add enough inductance that may have to be dealt with in your application.  The LM3450 is trying to decode when a rising edge of a dimmer happens and when the zero cross happens, if there ends up being ringing due to inductance it may cause issues with decoding.

    Do you know the power rating of the triac dimmer?  This too can cause issues due to holding current requirements and correct damping of the triac rising edge.  Do you have a power level for each LED driver?  Output voltage and current?  Or is that open to the final design?

    Regards,