: White LED lifetime issues.
Apptech email (remove spam text)
> 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
> A common thing to do is a PWM drive. This is often hailed
> as a way to
> life with very little loss of light. In some cases the
> even suggest you
> 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
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.
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.
- 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.
See also: www.piclist.com/techref/displays.htm?key=led
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