Maker Pro
Maker Pro

Digital Current Control for LED array

J

John_H

Jan 1, 1970
0
My intent when posting the article wasn't to describe an efficient
system, it was to give the Jackal something to chew on. I,
personally, have no particular interest in high-powered LEDs, but if
you do, go for it! And good luck!

If you apply 15 posts toward an effort you have no interest in in order to
bait someone, I'd respectfully recommend putting the word "Professional" in
your signature in quotes from now on.
 
J

John Fields

Jan 1, 1970
0
If you apply 15 posts toward an effort you have no interest in in order to
bait someone, I'd respectfully recommend putting the word "Professional" in
your signature in quotes from now on.
 
J

John Fields

Jan 1, 1970
0
It's easy to get under $4.50 an LED and have efficiencies significantly
higher than the 30% you propose (10V supply, 3V load). The MOSFETs are
about $0.50 each, the 4-channel opamp is under $2 ($0.50 per channel), the
discrete resistors are negligible, and the cheap pots are down to $0.25
each. It isn't just a nest of wires but the supply can be made to
accommodate a very small dropout voltage for very high efficiencies. The
solution is elegant rather than brute force. I'd hate to throw away 50W of
power for 21W of light.

Most folks designing with the high-power Luxeon LumiLEDs are interested in
efficiency above convenience; otherwise incandescent lamps would be fine.

---
After reading your post again, I think you're talking pie in the
sky. Sure, you can lower the power supply voltage to get an
operating window with less loss, but you still have to waste the
energy to make the bright LEDs as dim as the dull ones to start out
with if you use the same gross power supply for them all, and since
you're not talking PWM or some reactive scheme you're talking DC, so
you're stuck. It's still inelegant brute force, but you're just
using a smaller sledgehammer.

Then you've still got the problem of making the lamps' luminance
track as they're being dimmed, so you need an array of scalable
voltage controlled current sources which will vary the current into
the lamps depending on some nebulous open-loop calibration done at
what? one point?

If you want to go with that, then you've got to admit that all your
scheme does is trim the waste, not get rid of it.
 
J

John_H

Jan 1, 1970
0
John Fields wrote:
---
After reading your post again, I think you're talking pie in the
sky. Sure, you can lower the power supply voltage to get an
operating window with less loss, but you still have to waste the
energy to make the bright LEDs as dim as the dull ones to start out
with if you use the same gross power supply for them all, and since
you're not talking PWM or some reactive scheme you're talking DC, so
you're stuck. It's still inelegant brute force, but you're just
using a smaller sledgehammer.

Then you've still got the problem of making the lamps' luminance
track as they're being dimmed, so you need an array of scalable
voltage controlled current sources which will vary the current into
the lamps depending on some nebulous open-loop calibration done at
what? one point?

If you want to go with that, then you've got to admit that all your
scheme does is trim the waste, not get rid of it.

No pies in my sky: it's engineering.

It's true that using 7 scalable voltage-controlled current sources as
series pass elements is wasteful but the waste is minor. It would be
best to have 7 voltage controlled switching current regulators but the
loss with the simpler series regulators is easier to implement with very
little additional loss. Calibration would be done at one point since
this will be a first-order match of brightness with the proportional
currents providing a close approximation to proportional luminance
values; the differences in the current/luminance curves at that point
are second-order mismatches.

The DC supply can be tuned to a voltage slightly higher than the largest
Vf LED (and series regulator) would require at 100% rated current. Done
properly, the source is a switching regulator that self-adjusts to have
only one gate drive near (rather than at) the positive rail, the rest at
lower drive voltages. If you increased the supply voltage, the gate
drive levels would simply reduce and the Vds values increase by the same
amount the supply increases. Nowhere was it specified that the supply
had to be a dumb, fixed voltage but even then a 5V supply would produce
good results in all cases.

If a relatively large current sense resistor (as current sense resistors
go) of 100 milliOhm were used (100mW is hotter than I like) the voltage
drop lost to sensing would be 100 mV. The MOSFETs in series with the
worst-case Vf LED should have a drop of about 200 mV at the gate-source
voltage provided by a rail-to-rail output op amp. The worst case for
*loss* would have all LEDs at 100% current but one of the LEDs at the
highest Vf, the rest at the lowest. But in any case the overall *power*
dissipated in each LED/regulator combination would be less than or equal
to 100% rated current at the worst Vf of the 7 LEDs plus 300 mV for the
pass elements. The only way to improve on the loss in the "more
efficient" lower Vf LEDs would be to have individual switchers for each
LED, still had for less than $4.50 each.

This is not brute force. This is engineering tradeoff. You can have a
power draw for 7 LEDs that's 7 times the power draw of the largest Vf
device with simple pass regulators (1 MOSFET, 1 OpAmp channel, a current
sense resistor and 2 set resistors for the OpAmp) presenting one voltage
that controls 7 channels. Or you can use 1A switchers per-channel for
90% efficiency overall; these still need to servo to fixed percentages.
In most cases where the Vf is similar for the multiple devices, the
efficiencies of the two approaches will differ by only a few percent.

When you suggest 30% efficiency rather than 85%-95%, you can see where
"brute force" easily loses out. The brute force approach you
recommended would produce extremely poor matching results in a system
where some LEDs have a Vf of 3.03V and others are 4.47V, the range of Vf
at 700 mA as published in the Lumileds datasheet; the current ratios
would no longer match as the system is dimmed and brightened because of
the large disparity.
 
J

John Fields

Jan 1, 1970
0
When you suggest 30% efficiency rather than 85%-95%, you can see where
"brute force" easily loses out. The brute force approach you
recommended would produce extremely poor matching results in a system
where some LEDs have a Vf of 3.03V and others are 4.47V, the range of Vf
at 700 mA as published in the Lumileds datasheet; the current ratios
would no longer match as the system is dimmed and brightened because of
the large disparity.

---
Actually, I think the brightness tracking would be OK because of the
initial "calibration". That is, with my "brute force" approach, the
differences in Vf would wash because the LEDs would all be forced to
the same brightness initially and their individual delta Vf's, from
bright to dim would, I suspect, vary little from bright to dim.
They are, after all, only diodes in conduction. If you doubt it,
build an array using my "brute force" way and report back with what
you find, OK?

In any case, as I've already stated, the system I described was to
give what's-his-face a little food for thought. Much like
describing running into a brick wall going a hundred miles an hour
isn't advocating doing it.

But, after that crack you made about my sig, I suspect you're trying
to pick a fight, so you're not going to be happy with anything I
say.

Right?
 
J

John_H

Jan 1, 1970
0
John said:
Actually, I think the brightness tracking would be OK because of the
initial "calibration". That is, with my "brute force" approach, the
differences in Vf would wash because the LEDs would all be forced to
the same brightness initially and their individual delta Vf's, from
bright to dim would, I suspect, vary little from bright to dim.
They are, after all, only diodes in conduction. If you doubt it,
build an array using my "brute force" way and report back with what
you find, OK?

In any case, as I've already stated, the system I described was to
give what's-his-face a little food for thought. Much like
describing running into a brick wall going a hundred miles an hour
isn't advocating doing it.

But, after that crack you made about my sig, I suspect you're trying
to pick a fight, so you're not going to be happy with anything I
say.

Right?


I won't be happy with unprofessional behavior. I don't pick fights
though I sometimes register my distaste for those who make the public
forum ugly. This is a "newsgroup" where people can pick fights somewhat
anonymously and get joy out of the experience while degrading the
quality of the information that *should* be exchanged in forums like
this. No posts are removed here, though by the quality of postings that
I've seen I have a greater respect for moderated information exchange
forums.

As for the LEDs: what happens at 20% brightness? if a 10V supply
provided 100% current to the 3.03V and 4.47V Vf LEDs for matched
brightness, 20% to the 4.47V LED would be 4.47V+(5.53V/5)=5.58V which
would give the other LED (5.58V-3.03V)/6.97V or 37% brightness. If the
lower Vf LED were reduced to 20%, the system voltage would be
3.03V+(6.97V/5)=4.42V. which means the other LED has zero current.

The reality is the Vf varies with forward current but it varies for all
LEDs. While the high Vf LED may not be "off" at 4.42V, it may be off at
the drive voltage required to get the low Vf LED to 20%. There will be
a first-order mismatch in the LEDs with the high-loss brute-force method
you presented to "give what's-his-face a little food for thought."

As someone who actually would like to help a few people when they have a
question that 1) I have interest in and 2) I can provide sound help with
reasonable expertise, I would like to see the discussions here be about
supplying answers to questions.
 
J

John Fields

Jan 1, 1970
0
John Fields wrote:


I won't be happy with unprofessional behavior.

---
What you're saying is that in order for you to be happy, you want
everyone to conform to what _you_ consider to be professional
behavior. Forget it, it isn't going to happen.
---
I don't pick fights though I sometimes register my distaste for
those who make the public forum ugly.

---
What you're saying is that when, in your opinion, the public forum
turns ugly then you feel justifed in criticizing whom, in your
opinion, caused it to turn ugly. That's exactly the same thing as
walking up to someone and getting in their face with, "I don't like
what you're doing and I want you to star behaving in the way I tell
you to." Sure sounds like picking a fight to me.
---
This is a "newsgroup" where people can pick fights somewhat
anonymously and get joy out of the experience while degrading the
quality of the information that *should* be exchanged in forums like
this.

---
Wrong. The quality of the information exchanged will remain
unchanged, but the culture of the newsgroup will change.

Take, for example, the quality of the technical information you've
posted so far. Good or bad, it's exactly what you posted, so its
quality hasn't changed. However, your telling me how I should be
acting is certainly going to elicit a response which will change the
mood of the discussion and to a greater or lesser extent, the
culture of the newsgroup.
---
No posts are removed here, though by the quality of postings that
I've seen I have a greater respect for moderated information exchange
forums.
 
T

theJackal

Jan 1, 1970
0
<snip>

YES! You've got it now. The only major concept that you aren't seeing is
that a constant-current series regulator that uses feedback from the current
running through the LED *won't* be affected by the other LEDs current
changes as long as the supply is properly regulated. Constant-current loads
need a supply with a voltage that can meet the LED voltage drop plus
regulator "dropout voltage" for all the LEDs, supplying enough current to
accommodate all 7 LEDs at full current.

If you stop thinking "resistors" and start thinking "regulators" everything
can come together.

The need to ramp the intensity up and down with a single control - one
dimmer for 7 matched intensities - means that the regulators must track each
other as the brightness is increased and decreased. If the 7 regulators run
from 75% to 100% rated current to match brightnesses for a particular batch
of LEDs, half-power would servo those regulators to 37.5% to 50% of rated
current proportionally.

These high-current LEDs need a transistor (I'll use n-channel MOSFETS),
current sense (small value resistor for my implementation), an amplifier
with feedback to maintain a fixed current through the sense resistor through
the gate drive voltage, and a method to calibrate the individual LEDs and
present a "master" brightness. I may use potentiometers with tap to the
opamp and rails of ground and "master brigtness voltage" or use an 8-channel
multiplying DAC: change the reference voltage and the outputs scale.

The main supply just needs to supply a voltage that will have all the
regulators properly regulating (or all the op-amps out of saturation).
My original idea was to use only passive components.

What current regulator are you using in your setup? If you use potentiometers isn't the
setup getting too complicated? If you already have a current regulator why use a MOSFET?
I would use MOSFETs to switch to different resistances but that would limit and
complicate my brightness control.


If you are really set on connecting the LEDs in parallel I'd use the MAX1916 . I'm sure
its a cheaper option. It has a current match of 0.3%. and you can vary the brightness by
changing the duty cycle of PWM signal on the enable pin.
Let me know what you think.

theJackal
 
J

John_H

Jan 1, 1970
0
My original idea was to use only passive components.

What current regulator are you using in your setup? If you use
potentiometers isn't the
setup getting too complicated? If you already have a current regulator why
use a MOSFET?
I would use MOSFETs to switch to different resistances but that would
limit and
complicate my brightness control.


If you are really set on connecting the LEDs in parallel I'd use the
MAX1916 . I'm sure
its a cheaper option. It has a current match of 0.3%. and you can vary the
brightness by
changing the duty cycle of PWM signal on the enable pin.
Let me know what you think.

theJackal

Passive components alone won't give good tracking of current with one master
control if efficiency is an issue.

The current regulator I will use is a combination of a MOSFET and an OpAmp
with current sensed through a small-value resistor (under 100 milli-ohm).
Current regulators (at the 1A level) are not something I'm aware you can buy
readily as a low-dropout device; simple 3-terminal variable voltage
regulators have a dropout of at least 1.25V (the reference voltage) when
wired up as a constant current source.

The 7 LEDs need to be configured for a "best match" at a nominal master
current level; this can be done through simple open-frame potentiometers,
swapping resistor values in and out in the OpAmp feedback, or through
voltage control that is otherwise made proportional to the master level
control such as an 8-channel multiplying DAC.

The best control over the LED intensity is direct control over the current.
Making these currents track each other based on one master intensity control
is the design goal. Different LEDs won't have an exact track of intensity
versus control voltage but will be close enough for most purposes.

I suggested the MAX6958/6959 as one example of LED control early in this
thread. The device uses PWM but is designed for regular LEDs. These
high-power LEDs would have to be driven with a MOSFET to get the milliamps
of current to change to an Amp of control per LED.

I figure making a separate regulator for each channel that can deliver a
low-dropout voltage-controlled current source is the best combination of
efficiency and cost. I haven't bought the parts yet to wire up my own 3W
Lumileds but I expect to have them lit with something other than a lab
supply within the month. I may look into per-LED switchers as an
alternative but I'd expect those to be more costly and more prone to EMI
troubles for something "slapped together" rather than detailed on well laid
out PC boards.
 
Top