Off-line LED drivers

A better approach for the industry would to standardize some constant_currents_ (DC) such as 20/50/100/350/1000 mA so that any light panel from any manufacturer rated for a specific constant current could be plugged into a series string, just like christmas tree lights.

In a system driven by a constant current, each load would have the same current, but the voltage could be different, depending of the power levels needed. This is very similar as the Pxxnn series tubes used in old European TVs, in which the filaments were in series across the 220 Vac mains (with some inrush current limiting).

For larger LED panels it would be mechanically feasible to use multiple constant current generators with separate strings with two or more of those standard currents through the sockets. The panels would only have pins for the required current and the socket would short circuit the unused current lines, maintaining loop continuity. Some open/short circuit detection at each constant current source could take out unused current loops.

Wouldn't replacing the output voltage feedback divider at pin 1 with a high side load (LED string) and 100 ohm to ground and feeding the voltage across the resistor to the pin 1 (feedback) produce a 20 mA constant current source ? This should be able to drive a string of 100 LEDs.

Thus only one switching transistor and one inductor would be needed for both PFC as well as constant current regulation.

Of course, with such highly integrated chips, this might cause havoc e.g. in startup functions.

Hello, in my answer I was thinking to isolated designs with lowish forward voltage led strings, here the pfc would run around 400V, isolating the leds from the dissipator will kill the thermal resistance.

If I understand you are thinking to use a pfc booster to drive the leds directly with a current feedback. I think that can be done but the output current will follow input voltage, at least the driver is designed to work this way, and it's needed to have a reasonable power factor of the system.

So I think you need a storage capacitor to lessen the flickering of the leds. This problem is similar to single stage pfc-flyback, you have only one degree of freedom, and you need a compromise between current ripple in the leds (that generates flickering but also reduces somewhat the luminous efficacy) and filter capacitance. About the particular implementation in the L6561/2, i'm not that experienced with feedback loops analysis, as I understand these pfc devices have a slow contol loop for output voltage regulation (slow enough to average the voltage over multiple line cycles) and fast "emergency" limiter for line transients. The first probably can be used for average current control in led string.

This approach could be useful with some recent high voltage led arrays, that promise high efficacy at reasonable cost.

By the way, reading you answer on another message of this topic, you made very good points about the figures reported by datasheets about efficacy, current and temperature of the leds. The manufacurers tend to write the best numbers for efficacy, but usually these are obtaind for best bins, low currents and impossible junction temperatures, unless you live in polar regions :) Fortunately some manufacturers are starting to specify efficacy at 85°C Tj.

Ciao!

Some serious manufacturers these days specify significant parameters at reasonable values, e.g. usable junction temperatures and at current levels that might be usable at above 50000 hours.

In the old days, while some manufacturers did have fine charts for various parameters vs. junction temperature or vs. forward current, but some parameters specified at Imax and others at Tj=25 C, you needed quite a few iterations on a spreadsheet to get the forward voltage, power dissipation and junction temperature calculations to stabilize :-), in order to get a stable operating point for the optical parameters.

source

Interesting idea. Been used before, in many locations around the world series street lighting has been used. There would be a high voltage constant current transformer sourcing many luminaries along the roadway. What i am not clear on is just how the rest of the string continued when any single luminaire failed.

?-)

Antifuse, of course :-)

Same reason x-mas light strings don't (always) go out when a bulb fails -- they do that by wrapping the (probably molybdenum) terminals with anodized aluminum; when the bulb burns out, the anodizing breaks down (at 100V, say) and shorts out.

I've seen some of the hardware they use in this area. Lots of old circuits with CC transformers, usually a few kilovolts total loop at 10A or so. Each light only drops what it needs, usually 200V or so.

Tim

Series street lights have been used since the 1880's. In one Thomson-Houston system about 30 arc lamps were connected in series and powered by a 10 kW 1500 Vdc dynamo. The nominal loop current was 6 A, so each lamp consumed 300 W, but due to the low efficiency (only a few lm/W), not much light was produced.

Carbon arc lights needs constant electrode adjustment done with electromagnets. These electromagnets were involved in starting the lamp as at least some cases also bypassed failed lamps.