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LED Driver Eschews Inductors

There's a new LED driver available from TI that uses an interesting approach for driving the usual stack of high-brightness LEDs from the 120 V or 240 V power line. First, what's pretty much like what we've seen before: It works with phase-cut dimmers. It has a high power factor, low harmonic distortion, and low current-ripple. It can work with loads up to 70 W.

What's unusual about the device is that it needs no inductors. EMI generation is very low. The capacitors that it uses are not outrageously large (i.e., the product of voltage and capacitance won't bust your budget). The trick here is that you use multiple ICs stacked in series with their corresponding stack of LEDs.

As you can see, any one IC is not subjected to the full line voltage, so it can be built on a lower voltage process. That keeps the price for the device quite reasonable (23 cents each in lots of 1,000 pieces). You get best performance with double the LED stack length per IC as you move up the chain towards the supply rail. The TI data sheet gives this brief explanation:

For the 120V application shown in [the figure above], the highest efficiency is obtained by using a high-voltage total LED stack to reduce losses in the linear regulator FET. The best current sharing efficiency between stacks can be achieved by using the lowest voltage stack at the bottom and making each stack voltage above 2 times the voltage of the stack below it. In this example 20V LEDs are used. This effectively gives the lowest stack a total of 20V, the middle stack a total of 40V, and the upper stack a total of 80V.

Note that it's not all ICs, diodes, and capacitors here. You do need to add some extra active circuitry to function as a constant-current regulator. Power dissipation could be an issue, but as long as the stack is tall enough, the power dissipation in that power FET (Q1, below) is reasonably low.

A variation on the circuit works for 240 V applications as well.

TI has an Eval system available if you want to jump in with minimal soldering.

I may order one just to see how well it works with a dimmer (often the stumbling block for LED lamps). Have you done any design with high-power LEDs, especially for power-line applications? Did you encounter any problems?

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— Brad Albing, Editor-in-Chief, Planet Analog and Integration Nation Circle me on Google+

5 comments on “LED Driver Eschews Inductors

  1. Davidled
    November 18, 2013

    Diagram indicates only 200V diode to prevent the reverse current feeding into IC Chip. Wire insulation with ground would be considered in the high voltage environment.  Ground isolation would be recommended in the high voltage and low voltage, if other IC or IC is integrated.  Also I wonder that a dimmer could be controlled in TI driver.

  2. samicksha
    November 19, 2013

    I agree you daej, LED driver responds to the ever-changing needs of the LED, through constant amount of power to the LED, as its electrical properties change with temperature.

  3. RedDerek
    November 19, 2013

    @DaeJ – there are a couple different dimmer topologies out there. The common one is just triggering a triac at some time during the half-cycle of the input. For some of the digital dimmers, I have found that a dimmable LED never really goes off at the minimum output unless the ac is completely turned off (ie, the triac is not even fired at all).

    In one lightbulb set, I actually have left one incadescent bulb in place to ensure no light is generated because the incadescent sucks up any current the LED circuit would want to use.

  4. Brad_Albing
    November 23, 2013

    @DaeJ — You should only need a 200V diode at each IC since each IC in the string will see no more than that voltage. Now as you've noted, when designing and building devices that run directly from the power line (mains), care must be taken regarding insulation. There are IEC and UL standards that control the amount of insulation needed and the accessibility of conductors with high voltage present (when there is no isolation from the mains).

    These devices should work with convential triac dimmers – tho' you could also make them work with other dimming control methods, such as 0 to 10V control.

  5. Brad_Albing
    November 23, 2013

    @RedDerek – I used that trick also to make sure the LEDs don't “ghost.”

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