The ides is that they can be driven by switching in parts of the string at various points in the rectified input sinewave, which leaves a small (low wasted power) voltage drop across the current control circuit.
It also means that the current waveform follows the voltage waveform (in a steppy way), helping power factor.
However, it is a naturally flickery way of driving leds, simply because some of the leds are going on and off at 100Hz (or 120Hz).
Now Seoul has introduced MicroDriver, a driver series for its own Acrich leds, claiming: “measuring just 38 x 28 x 20.5, giving lighting designers the ability to develop ultra-thin and novel luminaires with flicker-free operation.”
Sadly the flicker-free bits turns out not to be true, but the firm has made an effort to cut flicker, with an electrolytic capacitor in there to spread energy through the half cycle.
Data sheets are not yet available on the page linked to above, but in the application examples that can be found from the same page.
For example, in this 230V 1,430lm 80 CRI 3,000K application, flicker is described at <11%, and measured at 5.75% without dimmer. I can’t find figures for flicker with dimming in the app note, but the waveform plots show that at least the output is not cutting to zero twice a cycle. Efficacy here is a nice 115 lm/W.
Ah ha, this 120V 1,190 lm 80CRI 4,000K app note shows 3.5% flicker with no dimmer, 5.3% with a triac dimmer turned up full, 4.8% dimmed to half output flux and 15.5% flicker dimmed to one fifth flux.
So the “low flicker” claim on the product page is justified for some of the dimming range, and I am not qualified to judge its follow-up: “The MicroDriver has significantly low flicker enabling easy [California] Title 24 flicker compliance.”
The 10 model Series delivers outputs across 8-24W at either 120 or 230V for outputs from 900 to 2,400 lm, estimates the firm, and is aimed at designs such as wall sconces, vanity lights, downlights, and flush-mounted lighting fixtures.
“The smaller size facilitates the conversion of these applications to LED light sources, which was not previously possible due to bulky conventional LED drivers, making halogen lamp replacement possible without the need for a large volume recess for the driver, or a reduction in light output,” said Seoul.
Interestingly, it has decided to waterproof them – rating is IP66.
Other ratings are Class B EMI and 2.5kV surge.