Yes, yes, I know, 'prefectly'. So what does the LED data sheet say
about the conditions you are subjecting it to? For example, here is a
data sheet that specifies maximum PEAK current ("Limits of Safe
Operation") at 25°C of 150mA (only 5 x the maximum DC current) for
pulses of < 10 *microseconds*).
http://www.lumex.com/pdf/SSL-LX5063ID.pdf
It should be derated above 25°C, obviously.
I would like to add that LED datasheets tend to be conservative in terms
of short high current pulses.
At your own risk, you must do "sufficient reliability testing", but I
give good odds of success with situations that the datasheet does not say
is good.
Slight caution - the LED may have a design change not reflected in the
datasheet and such design change could reduce ability to survive
conditions that the datasheet never supported. Those who abuse LEDs are
"on their own", although so far I have yet to blow an LED with hours of
low-duty-cycle pulsing with peak current that according to the datasheet
"should blow the LEDs sky-high" (my words).
I would like to add:
Look at the datasheet for both peak current and maximum pulse duration
and also duty cycle.
So far, I have yet to blow an LED with higher peak current as long as
pulse duration is reduced in inverse-square relation to increase in peak
current according to the datasheet, the average power dissipation does not
exceed what the datasheet supports as allowable (even with higher voltage
drop at higher current), and the RMS current does not exceed what is known
safe.
As in: For most LEDs rated 30 mA max continuous, 100 mA max peak at 10%
duty cycle and for 10 milliseconds on-time (cycle period 100 milliseconds):
RMS current tends to be allowed to be up to 32 mA or so according to the
datasheet, though not documented when peak current gets past 100 mA.
I would extrapolate to pulse duration at current past 100 mA being 10
milliseconds times square of ratio of 100 mA to actual pulse current, and
allowable duty cycle 10% times is square of ratio of 100 mA to actual
pulse current.
So far, I have yet to bloe LEDs with such conditions with pulsing to an
amp or so. However, I do warn that doing anything outside the datasheet's
box is at your own risk!
One more thing - LEDs are nonlinear. Most white, blue, and
non-yellowish-green ones are more efficient at lower peak currents!
Same story at least with "low current red" - as in most red ones of a
30-plus-year-old chemistry but having nominal peak wavelength of 690, 697
or 700 nm (believe me, their bandwidth is on the high side for
phosphorless LEDs).
Most high brightness and high efficiency ones other than the above tend
to have efficiency maximized when peak current is in or near the range of
from half of maximum rated continuous current to double maximum rated
continuous current.
One more thing with pulsing LEDs: Watch for higher voltage drop, and do
not exceed maximum rated power dissipation (I find that is usually easy to
obey with lower average current when peak current is high and a maximum
RMS current is obeyed).
RMS is not 1.11 times average for most pulsed waveforms - for
rectangular pulses, RMS is peak times square root of duty ratio. For
example, 1 amp at duty ratio of .1% or 1/1,000 has RMS current of 31.6 mA.
- Don Klipstein (
[email protected])
