Power-converter IC targets dimmable LEDs, needs no optoisolator
By Margery Conner, Technical Editor -- EDN, November 12, 2009
The large installed base of dimmable-lighting switches dictates that most new lighting technologies, including LED lights, should support dimming, and the myriad disparate specifications for dimming switches requires these lights to meet a wide range of characteristics. Meeting both of those requirements, iWatt’s new iW3610 ac/dc digital-power controller for dimmable LEDs includes intelligent wall-dimmer detection for both leading- and trailing-edge dimmers. It also detects an unsupported dimmer and turns off the light rather than self-destructing. The IC supports dimming from 2 to 100% and has an optimized dimming frequency of 900 Hz, so there is no visible flickering over the entire range.
LED lighting that gets its power directly from ac-line voltage requires some form of isolation from the ac mains. An optoisolator typically performs this task, especially in low-cost, high-volume products. However, LED lights place a high premium on long life, and, because optoisolators can degrade with time, they can become a failure point in devices that must operate for 10 or more years. The 3610 tackles this problem by using primary-side regulation, eliminating the need for an optocoupler and sense resistor, reducing bill-of-material costs, and improving reliability for dimmable LED lights of 40W or lower wattage. Another version, the iW3620, is similar to the 3610, but does not support dimming.
The devices’ ac/dc-power-control ICs use digital topology that’s transparent to designers and requires no programming or GUI (graphical-user-interface) development. The iW3610’s switching frequency as high as 200 kHz means that designers can use smaller capacitors and transformers, and the devices comply with EMI (electromagnetic-interference) standard EN55015B. The 3620 supports an internal switching frequency of 130 kHz and sells for 38 cents (1000); the 3610 sells for 98 cents (1000).
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The reasons that some insist on isolating LED lighting are two: some LED only fixtures use non-isolated LEDs soldered to the fixture, which serves as a heat sink. So isolating these types of installations makes a lot of sense. The second case is everything else, where you have a bunch of people paralyzed by fear, and backed up by legal departments who live in fear of the predatory creatures called lawyers, who feel that somebody might touch a wire, since LEDs are low voltage devices, never considering that the 120 volt AC line, while not high voltage, can be fatal, is powering the whole thing. These folks are the same reason that your lawnmower has 29 safety devices that make it much less convenient to use.
So those are the reasons, and the heat-sinking one is a good reason.
William Ketel - 2009-14-12 16:39:00 PST -
It simplifies the design on the secondary side if you have isolation. You can reduce the isolation to human and to ground or other materials nearby if isolation exists.
It is a leagal demand in many countries.
Bernt O Hellstrom - 2009-25-11 04:45:00 PST -
I will never figure this out. Consider the Welsbach mantle. Transforms low grade heat into whicte light, and hot is as hot does.
With LEDS, they need heat sinking, Who will be the first to make a limelight that's a heat proof semiconductor?
Michael Wicnienski - 2009-24-11 12:15:00 PST -
Rene, I checked back with iWatt, and this is what they said about the need for isolation:
"The US Department of Energy's Energy Star program does not mandate the use of isolated or non-isolated LED drivers for offline LED lamps. The LED lamp manufacturers are free to use isolated or non-isolated LED driver designs. [However, a] heat sink is often used to take away heat from the LEDs. The heat sink is generally made of metal and is exposed to the user. If the LED driver is not isolated then the heat sink must be insulated from the mains voltage for reasons of safety. Adding insulation can reduce the effectiveness of the heat sink because the heat sink is not mated directly to the heat generating LEDs. Moreover, if the heat sink is floating, i.e not electrically grounded then it can radiate RF noise resulting in high EMI. As such, non-isolated LED driver design can make thermal management and EMI control quite complex and challenging. If the LED driver is isolated using a transformer then the heat sink can be thermally mated to the LED substrate on the low voltage secondary side and can also be grounded to reduce EMI. Isolated LED drivers can in effect improve thermal efficiency, reduce EMI and reduce the cost and complexity of LED lamp designs that use heat sink."
Margery Conner - 2009-24-11 09:57:00 PST -
CAn anyone explain why LED lighting needs isolation from the AC line? Incandescents require no such isolation; why burden LEDs with this?
Rene Jaeger - 2009-18-11 13:52:00 PST
