Camera flash led array control

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killertoffy
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Posted by killertoffy: Mon. Mar 2, 2009 - 12:10 PM
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I thought about splitting this into many threads, but it might be better to describe the application in full.
So sorry if it is a bit long.

At this stage, I don't know where to start, as there's so many possibilities for the project.
I want to create a controller to power leds for a "flash" application.
The duration of the flash would never exceed 50ms, time between flashes can be up to 2-3 seconds (although shorter is better)
It needs to be battery powered, not too heavy probably 4 AA cells max.
I will need around 3 amps for the leds, 6 would be nice.

I wanted to use an AVR for the timing, charge and constant current for the leds.

To provide the power, I thought about boosting voltage to a capacitor bank,
then using that charge to power the leds in serie with constant current.
(Simmilar to what a xenon camera flash does, so it keeps working even with low bat)
Question is of course how much voltage should I target...
The higher, the more in leds I can put in serie in each string, but probably boost efficiency will degrade.
Also probably depends on the caps I can find...
Can leds can survive to something like 200v ?

Lower voltage means less leds in serie and more strings, so probably more danger of blowing them because of variations in current.
(would also be safer of course)

The next thing is the constant current with the AVR.
If I want to keep the inductor small, then I need high speed pwm to control current.
Is it realistic to control the current with an AVR ADC over a sense resistor ?
That is using a higher PWM frequency than the AVR ADC monitors current to reduce L size ?
Or should I think about using an external opamp with a comparator maybe ?

Maybe I can share the inductor for charge/discharge to decrease the weight, but haven't found any similar design yet.

If you have any advice, recommendations or ideas, you're welcome ;-)

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Learning and Information, General Electronics
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jayjay1974
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Posted by jayjay1974: Mon. Mar 2, 2009 - 03:30 PM
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Never exceed the absolute maximum ratings of the LEDs, 200V from a cap over a single LED will likely result in an explosion. But I guess you mean 200V over a string of LEDs, that should be no problem as long the maximum current is limited.

You could use the AVR for PWM and there are a few models that have high speed PWM modules ideally suited for this. But LEDs are very non-linear devices, just a couple of tenth volts of difference will have a huge effect on the current they take, and the PWM resolution is limited to 8 bits. This low resolution means you cannot really accurately control the current.

Is this for a photography application?

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killertoffy
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Posted by killertoffy: Mon. Mar 2, 2009 - 10:23 PM
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jayjay1974 wrote:
Never exceed the absolute maximum ratings of the LEDs, 200V from a cap over a single LED will likely result in an explosion.

The thing is that I've never seen a maximum voltage specified for a led in any data sheet... only current.

Quote:
But I guess you mean 200V over a string of LEDs, that should be no problem as long the maximum current is limited.

I guess it all depends on how many leds there are ;-)
The more "headroom" voltage I have above the Vf of the leds, the longer I will be able to keep the current constant, even if the cap voltage is dropping.
That's why I thought about using a high voltage.

Quote:
You could use the AVR for PWM and there are a few models that have high speed PWM modules ideally suited for this. But LEDs are very non-linear devices, just a couple of tenth volts of difference will have a huge effect on the current they take, and the PWM resolution is limited to 8 bits. This low resolution means you cannot really accurately control the current.

But if I use a controlled current scheme, I don't really mind voltage changes or am I wrong ?

Quote:
Is this for a photography application?

Indeed, it is.

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killertoffy
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Posted by killertoffy: Mon. Mar 2, 2009 - 10:24 PM
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jayjay1974 wrote:
Never exceed the absolute maximum ratings of the LEDs, 200V from a cap over a single LED will likely result in an explosion.

The thing is that I've never seen a maximum voltage specified for a led in any data sheet... only current.

Quote:
But I guess you mean 200V over a string of LEDs, that should be no problem as long the maximum current is limited.

I guess it all depends on how many leds there are ;-)
The more "headroom" voltage I have above the Vf of the leds, the longer I will be able to keep the current constant, even if the cap voltage is dropping.
That's why I thought about using a high voltage.

Quote:
You could use the AVR for PWM and there are a few models that have high speed PWM modules ideally suited for this. But LEDs are very non-linear devices, just a couple of tenth volts of difference will have a huge effect on the current they take, and the PWM resolution is limited to 8 bits. This low resolution means you cannot really accurately control the current.

But if I use a controlled current scheme, I don't really mind voltage changes or am I wrong ?

Quote:
Is this for a photography application?

Indeed, it is.

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