F
Fabio_78
- Jan 1, 1970
- 0
Wouldn't replacing the output voltage feedback divider at pin 1 with a
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!
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!