Off-line LED drivers

[Fabio_78]

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.

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!

 

[[email protected]]

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.

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.

 

[josephkk]

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.

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.

?-)

 

[Tim Williams]

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

 

[[email protected]]

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.

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.

What i am not clear on is just how the rest of the string continued when
any single luminaire failed.

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.