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LED Flicker

T

TKM

Jan 1, 1970
0
Working to mount a group of fixtures for an LED lighting fixture design
competition recently, I noticed what appears to be a wide variety of designs
for the LED power supplies and drivers. Some are 2-piece units (and rather
heavy -- transformers, I suppose); others are small, obviously electronic
and similar to computer and small electronic device drivers.

What's important though is that some of the drivers cause the LEDs to
flicker -- strobe is maybe a better word. There's a definite effect on the
quality of the LED light. Now that we've gotten rid of flicker from
fluorescent lighting, are we going to go through the same problems with
LEDs?

Who knows something about the types of driver circuits that are out there
and can reference or discuss what lighting specifiers can do to avoid
problems.

Thanks,

Terry McGowan
 
V

Victor Roberts

Jan 1, 1970
0
Working to mount a group of fixtures for an LED lighting fixture design
competition recently, I noticed what appears to be a wide variety of designs
for the LED power supplies and drivers. Some are 2-piece units (and rather
heavy -- transformers, I suppose); others are small, obviously electronic
and similar to computer and small electronic device drivers.

What's important though is that some of the drivers cause the LEDs to
flicker -- strobe is maybe a better word. There's a definite effect on the
quality of the LED light. Now that we've gotten rid of flicker from
fluorescent lighting, are we going to go through the same problems with
LEDs?

Who knows something about the types of driver circuits that are out there
and can reference or discuss what lighting specifiers can do to avoid
problems.

Terry - there are good reasons other than flicker reduction
to filter the rectified DC voltage in a fluorescent lamp
ballast. The key one is that fluorescent lamps will
operate at lower efficacy when they are run from a 120 Hz
modulated HF power source. (The efficacy loss is about 5%
for an F32T8 lamp.)

The 120 Hz modulation is eliminated or greatly reduced by
using a large electrolytic capacitor as an energy storage
device after the rectifier portion of the DC power supply of
the ballast. This capacitor introduces three problems:

1) It is often the largest component in the ballast.

2) It has the shortest life and sets the life of the
ballast.

3) It creates line current distortion that in turn creates
low input power factor. The user than has to either live
with this power factor or pay more money and give up perhaps
1% in efficacy to have a high power factor front end added
to the ballast.

Based on this issues it is no wonder that LED driver
manufacturers do not want to add the energy storage
capacitor unless it is necessary.

I suspect the only thing that light specifiers can do is to
require that the flicker index be below some value. Then
they need to measure the luminaires to make certain they
comply.

It would be interesting to see if Energy Star is going to
have a flicker requirement in the new LED luminaire spec. I
suspect not.

--
Vic Roberts
http://www.RobertsResearchInc.com
To reply via e-mail:
replace xxx with vdr in the Reply to: address
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This information is provided for educational purposes only.
It may not be used in any publication or posted on any Web
site without written permission.
 
T

TKM

Jan 1, 1970
0
Victor Roberts said:
Terry - there are good reasons other than flicker reduction
to filter the rectified DC voltage in a fluorescent lamp
ballast. The key one is that fluorescent lamps will
operate at lower efficacy when they are run from a 120 Hz
modulated HF power source. (The efficacy loss is about 5%
for an F32T8 lamp.)

The 120 Hz modulation is eliminated or greatly reduced by
using a large electrolytic capacitor as an energy storage
device after the rectifier portion of the DC power supply of
the ballast. This capacitor introduces three problems:

1) It is often the largest component in the ballast.

2) It has the shortest life and sets the life of the
ballast.

3) It creates line current distortion that in turn creates
low input power factor. The user than has to either live
with this power factor or pay more money and give up perhaps
1% in efficacy to have a high power factor front end added
to the ballast.

Based on this issues it is no wonder that LED driver
manufacturers do not want to add the energy storage
capacitor unless it is necessary.

I suspect the only thing that light specifiers can do is to
require that the flicker index be below some value. Then
they need to measure the luminaires to make certain they
comply.

It would be interesting to see if Energy Star is going to
have a flicker requirement in the new LED luminaire spec. I
suspect not.

Right. A check of V1.0, Draft 2 of the Energy Star Program Requirements for
Solid State Lighting Luminaires indicates that there is no requirement
involving flicker.

Terry McGowan
 
Probably the simplest one is a half-wave rectifier that uses a diode to
"throw away" the negative half of the AC. This will cause 60 Hz flicker
that can be stopped somewhat with a really massive filter capacitor.

In other words, if this is what you have coming in
(use a fixed width font):

/\ /\ /\
\/ \/ \/

and you have a half-wave rectifier with no capacitor, this is what you
get coming out:

/\__/\__/\__

With a big capacitor, this might be improved to something like

/\--/\--/\--

i.e., not dropping completely to zero between the peaks.

The next simplest is a full-wave rectifier that "inverts" the negative
half of the AC. This will cause 120 Hz flicker that can be stopped with
a less massive filter capacitor. You might have

/\/\/\/\/\/\

with no filter capacitor and something like

/-^-^-^-^-^-

with a capacitor - i.e., not dropping much at all between the peaks.

These first two (full and half wave rectifiers) are probably most common
with "linear" power supplies that use a transformer operating at the
line frequency - the "heavy" ones. A "switching" power supply, with the
transfomer operating at line frequency - like most desktop and laptop
computers (the "light" ones), will _probably_ have less flicker in
general for a couple of reasons. One is that with the "peaks" of
current coming much closer together (anywhere from a few kHz on up, as
opposed to 50 to 120 Hz), they can be filtered with a much smaller and
cheaper capacitor. Another is that since the the manufacturer has
decided to spend a little more money on the switching supply (vs. a
linear supply) in the first place, they are probably a little more
inclined to include adequate filtering. It's entirely possible to build
a cheap, crappy switching power supply that causes lots of flicker in
the output, though.

Another possibility is to use a capacitor (instead of a transformer) to
reduce line voltage to something suitable for powering a LED. This
would also create a relatively lightweight supply. I am not sure what
the waveforms end up looking like for this option so I can't comment
much.
I suspect the only thing that light specifiers can do is to require that
the flicker index be below some value. Then they need to measure the
luminaires to make certain they comply.

Vic's suggestion is probably the only way to know for sure. A first
step might be to specify "full-wave rectification" and/or "switching
power supply", as those choices are slightly more likely to reduce the
amount of flicker you see. But, since it's possible to build a linear
power supply with full-wave rectification or a switching power supply
that cause the LEDs to flicker like mad, measuring is the only way to
know for sure.

If the connections between the power supply and the LEDs are accessible,
you can put an oscilloscope on them to see what's going on. You don't
need a fancy oscilloscope with lots of bandwidth for this - 1 MHz would
probably be fine. You _do_ need to have either a portable (non-line-
powered) scope, or understand how to use an isolation transformer to
prevent fireworks, to do this safely. This may not help much in the
specifying process but it might be interesting just to see how different
manufacturers do it.

I wonder if the phosphor on white LEDs tends to act as an additional
"capacitor" to filter out any flicker from the power supply.

Matt Roberds
 
J

JohnR66

Jan 1, 1970
0
I wonder if the phosphor on white LEDs tends to act as an additional
"capacitor" to filter out any flicker from the power supply.

Matt Roberds
No. The phosphor in white LEDs responds very fast. You can see the flicker
clearly.
John
 
V

Victor Roberts

Jan 1, 1970
0
Vic's suggestion is probably the only way to know for sure. A first
step might be to specify "full-wave rectification" and/or "switching
power supply", as those choices are slightly more likely to reduce the
amount of flicker you see. But, since it's possible to build a linear
power supply with full-wave rectification or a switching power supply
that cause the LEDs to flicker like mad, measuring is the only way to
know for sure.

I don't think that either of these is sufficient. Full wave
rectification only converts the 60 Hz flicker into 120 Hz
flicker. I do agree that a smaller capacitor can be used
with a full wave circuit, but that does not guarantee that
the designer used a large enough value. Use of a switching
power supply also does not provide a sufficient guarantee.
If the connections between the power supply and the LEDs are accessible,
you can put an oscilloscope on them to see what's going on. You don't
need a fancy oscilloscope with lots of bandwidth for this - 1 MHz would
probably be fine. You _do_ need to have either a portable (non-line-
powered) scope, or understand how to use an isolation transformer to
prevent fireworks, to do this safely. This may not help much in the
specifying process but it might be interesting just to see how different
manufacturers do it.

Why take the risk and bother of opening the system to
measure the supply voltage, when it is so easy to measure
the flicker in the light output, which is what the user
cares about anyway?
I wonder if the phosphor on white LEDs tends to act as an additional
"capacitor" to filter out any flicker from the power supply.

Phosphor decay has a very small effect at 120 Hz.

--
Vic Roberts
http://www.RobertsResearchInc.com
To reply via e-mail:
replace xxx with vdr in the Reply to: address
or use e-mail address listed at the Web site.

This information is provided for educational purposes only.
It may not be used in any publication or posted on any Web
site without written permission.
 
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