A pair of dimmable, ac-dc digital pulse-width modulation (PWM) controllers for 100-V/230-V ac offline LED lighting applications illustrates the design complexity of what at first glance seems a simple task: using the chopped sine wave of a triac dimmer for incandescent lamps to pulse-modulate a constant current applied to a string of LEDs to obtain the same kind of dimming.
The design turns out to be anything but trivial. Considerations range from the obvious, such as power factor, efficiency, reliability, size, and bill-of-materials (BOM) cost, to the easily overlooked. For example, does the legacy dimmer on the wall chop the ac on the rising edge of each half cycle or the falling edge? How does the circuit tell? What if it can’t tell?
Then, there are unexpected considerations. How do you avoid provoking epileptic seizures in susceptible individuals under certain low-light conditions? Also, how do you provide hot-plug support—that is, how does the controller maintain dimming when the end user unscrews one LED bulb from the socket and screws in another without being switched on and off again?
The illustrative products are the iW3602 (3 W to 10 W) and iW3612 (3 W to 25 W) and controllers made by iWatt Inc. for incandescent replacement lamps (see the figure), either space-constrained (GU10, MR16) or the larger (A, PAR) types.
The company’s ICs have a number of interesting characteristics, such as the use of digital feedback and a non-isolated flyback topology in the ac-dc front end. The digital control loop permits fine-tuning of loop constants to match the specific customer bulbs into whose bases the control ICs will be built.
The loop also provides for a sophisticated combination PWM/pulse-frequency modulation (PFM) scheme that maintains high efficiency across the dimming range and across environmental conditions like temperature and humidity. Applying the regulation on the primary side of the ac-dc section eliminates the need for an optocoupler, the only component in the circuit that could have a shorter lifetime than the 50,000- to 100,000-hour LEDs in the replacement bulb.
On the subject of ac-dc design, power-factor correction (PFC), a requirement for European Union (EU) and Energy Star certification, has an interesting drawback in dimmable LED lighting. Essentially, the harmonics of the ac line frequency that PFC keeps off the power line are not so much eliminated as they are shifted to the dc output.
This is not a problem at high brightness levels. But at extremely low levels, the harmonics manifest themselves as a flicker that can cause health effects ranging from headaches to seizures in susceptible individuals. According to iWatt, pending legislation will deal with these issues, but the company says it has already eliminated the problem in its products.
Also, iWatt claims to have eliminated the tendency of the flyback transformer to resonate audibly at lower brightness levels in its control design. (That’s where some controllers shift from PWM to PFM; iWatt’s shift from PWM to PFM, back to PWM, and then back to PFM to hold up low-load efficiency while stopping the transformer from “singing.”)
To reduce BOM costs and make it easier to fit the control circuit into a standard Edison bulb base, switching frequency up to 200 kHz enables the use of smaller capacitors and transformers. Additionally, designers may use a smaller heatsink because the high efficiency (85% or better) reduces thermal loss.
Other features include adaptability to different kinds of dimmers, from the most primitive hardware-store triac-diacs to the most sophisticated dimmers for executive-suite conference rooms. What if the controller cannot identify the dimmer? According to iWatt, customers ask for controllers that apply no current at all. They prefer perceived “in-op” failures to a fire.
That’s what goes into a third-generation LED dimmer IC. Remarkably, what sounds like space-program design doesn’t come at mission-control cost. The iW3602 costs $0.84 and the iW3612 costs $0.98, both in 1000-piece pricing.