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LM3447: Doubt regarding the hold current circuit in LM3447

Mathews Boby
Prodigy 10 points
Part Number: LM3447
Other Parts Discussed in Thread: TPS92075

Hello,

I have a doubt regarding the "Hold Current" circuit in the phase dimmable PFC flyback controller: LM3447 (or any similar phase dimmable PFC flyback controllers).

Datasheet says that the controller operates in DCM. So when the primary switch (MOSFET) turns OFF, the input current (current drawn from the mains) goes to zero. So if used with a phase dimmer, this would result in zero current through the triac (inside the dimmer) and would lead to turn-OFF of the triac and hence misfiring; unless the "Hold Current" circuit inside the LM3447 gets activated and draws the holding current.

So my question is whether the "Hold Current" circuit gets activated in every PWM cycle? If not, then how the holding current is maintained from the mains when the primary switch turns-OFF?

Regards,

Mathews

  • 0
    TI__Mastermind 25500 points

    Hello Mathews,

    I have forwarded this to the right person, but please have some patience as it is the holiday weekend. You may not get a response until next week.

    Regards,

    Clint

  • 0
    TI__Mastermind 28655 points

    Hello Mathews,

    When designing a flyback LED driver an EMI filter will be implemented.  The EMI filter takes the switched current and filters it to DC at the input so a discontinuous mode flyback will still look like a DC current draw at the input (with 60 Hz ripple for power factor correction). 

    Also note that even if there was no EMI filter the internal components of most triac dimmers will not respond to the switching frequency of a flyback converter.  From much testing I've seen the cut-off frequency around 10-12 KHz so below that you can get misfires but above the triac dimmer remains conducting (assuming sinusoidal current).  This was noted during design of the damper element for when the triac fired.

    Regards,

  • 0 in reply to Irwin Nederbragt
    TI__Mastermind 28655 points
    Hello Iwin,
    Thanks for your reply. So you say that the high frequency filter makes the input current continuous and the input current will look like a sine wave. Seems convincing.
    Then, is it possible to use a buck converter topology with EMI filter for PFC application?
    Regards,
    Mathews Boby

    Yes, it is possible to use a buck converter topology with EMI filter for PFC applications. There is a limit to how high the LED stack can be while maintaining 0.9 power factor if that is your goal since a buck converter will not draw current from the AC source when it is below the LED stack voltage. The TPS92075 is a buck converter that is setup for this.

    Regards,