> Russel,
>
> Interesting. I'm curious, you propose at the end that:
>
>> Needless to say, to have any chance of achieving rated
>> lifetimes LEDs need to be driven in a constant current 
>> mode
>
> A common thing to do is a PWM drive. This is often hailed 
> as a way to
> extend
> life with very little loss of light. In some cases the 
> even suggest you
> can
> over drive them slightly and not cause harm. Views?
>
> Bill van Dijk

The ways of achieving "constant current" can vary.
And the current is allowed to vary as long as it's 
controlled and designed.

I used the term as an accurate enough short-hand way of 
saying the following.

LEDs are rated by their current draw. The forward voltage at 
rated current varies very widely amongst LEDs of the same 
type, widely amongst LEDs in the same production batch and 
even substantially amongst LEDs "binned" (selected) to have 
nominally the same current.

At the same forward voltage (Vf) two nominally identical 
LEDs may have currents that differ by 2:1. Even more at 
extreme ends of the range.

Hard connecting a number of LEDs in parallel will cause the 
low Vf per mA LEDs to draw more to far more current than any 
high Vf companions

To achieve manufacturer's specifications it is necessary to 
control the current drawn and to therefore let the Vf assume 
whatever value is appropriate for the LED.

A common way to get equal Iled is to connect LEDs in series 
and drive them from a suitably high and variable voltage and 
to then control the string current to get the value desired.

If desired LEDs may be driven with square wave current, 
giving  peaks of light. Mean current for a square wave is 
then Ion x duty_cycle. As long as Ion meets manufacturers 
spec for amplitude and duration and repetition rate then all 
is well. This allows dimming by varying duty_cycle. This is 
superior to varying analog constant current as the LED is 
always run at the same current when on so colour is 
constant.

One common LED driving method is to charge a capacitor from 
a resistor and then to short the LED across the capacitor 
with PWM to achieve variable brightness with mean current 
variation. A problem with this method is that the LED peak 
current is undefined depending only on ILED at Vcap_max and 
stray cct resistances and cap ESR. This can lead to 
overstressed LEDs and reduced lifetimes.

SO

While LEDs can be driven in all sorts of ways, when they are 
operating their current (rather than their voltage) should 
be controlled to meet the designers will. If instead the 
voltage is controlled then current CANNOT be properly 
controlled and bad things will almost certainly happen.

Note:

- White LEDs tend to be more efficient at lower currents 
than at rated current, so PWM that uses max/off switching 
will usually produce reduced efficiency compared to driving 
with analog average current.

- If analog average current is varied to change white LED 
brightness then as the current changes the % of light which 
comes from phosphor re radiation and direct LED die output 
varies and the colour changes. How much this happens depends 
on many factors including the LED basic technology. eg a 
blue emitter/yellow reradiation phosphor will give different 
effects than an LED with UV  die and dual phosphor 
reradiation or than an RGB LED.


        Russell


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