Use of LED driven over-current in a strobe lamp

Question

I'd like a strobe lamp using RGB LEDs as it's light source. I want to pulse the LEDs with very short duration pulses (ideally microseconds or less) at around 100 Hz.

The total on time for the LEDs per second is likely to be less than 1/1000 of a second. If the LEDs are driven at nominal power, the total light output will be low and the resulting illumination will be very poor. I'm interested in the idea of driving very short pulses through the LEDs that are constant power, but with a current well over the nominal. Ideally, 10x or even 100x over nominal.

A thread here: High Current Pulse on LED suggests that a few times over nominal current for short pulses is probably okay, but I think they're talking about longer pulses than I am imagining.

Could anyone comment on whether the LEDs are likely to survive long enough to be useful? I don't mind a drastically reduced total life. As long as they'll survive for a few tens of hours of usage (total on time probably less than an hour), that's fine.

Answer 1

For a practical answer to the question, destructive testing of at least one LED, preferably a few, will be required.

Broadly:

LEDs are primarily destroyed by heat, not so much by current. Depending on the internal construction of the LED and its short-term thermal dissipation performance, an LED could conceivably survive 100x its rated current. Equally, if the thermal off-take from the junction is not quick enough, an LED could well be destroyed by as little as 5x the rated current.

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Given the desired pulse duration mentioned in the question, I just tried the following:

I have a cheap no-name 20 mA red LED being pulsed at 0.8 Amperes at 12 Volts, with pulse duration 5 microseconds, duty cycle 1/256 (0.39%). It has not blown up in the last 15 minutes, in fact the leads are not even discernibly warm. It is not very brightly lit, though - which might be partly because of droop in switching waveforms.

For similar LED overdrive requirements, an in-house rule of thumb I follow is to derate the average power rating of the LED by 10% for every 100% increase of drive current over nominal. I believe this is overly conservative, but I have had success with as much as 30x nominal current for "camera flash" type applications using white Piranha LEDs.

Would this exceeding of rated values be considered acceptable engineering? Not by a long shot.

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Update:

1. Subsequent to the test with the red LED described above, the PWM frequency was reduced such that each "on" pulse became 20 microseconds, from the previous 4.88 microseconds, keeping the duty cycle the same as before.

   The result was true destructive testing: The LED exploded spectacularly, the top half has still not been found.

   Hypothesis: With the pulse duration being comparable to the LED's rise time, the LED does not really light up much, nor does it exhibit expected thermal catastrophic effects.

2. While retaining the 20 microsecond pulse duration and 0.39% duty cycle, current limiting was introduced, systematically increasing allowed current from 50 mA to beyond 400 mA. The LED survives up to a point and is much brighter than in the 4.88 microsecond case throughout.

   Beyond around 350 mA, the LED dies, magic smoke comes out, i.e. it transforms to SED (Smoke Emitting, Dead).

   Conclusions:

   • Average power is not the only factor contributing to destruction (or survival), keeping pulses too short simply does not allow the LED to turn on enough to matter
   • With 20 microsecond pulses, the 20 mA LED survives approximately 17.5 times its nominal current rating before destruction
   • I need to buy more LEDs.

Answer 2

The amount that an LED can be over driven is highly dependent on the design. Every LED has a maximum temperature that can be reached before failure for each material involved.

Maximum continuous current is usually limited by the encapsulation, the lens material that protects the diode. This kind of failure either melts, or turns the lens opaque (usually yellow, then brown). Maximum continuous current can be increased by reducing the heat produced (increasing efficiency) or effective thermal conduction. This is how high power LEDs are made.

Maximum pulse current is usually determined by the current carrying materials. The conductors have such little mass that they overheat quickly and fail catastrophically (i.e. Amindo Gosh's answer with the exploding LED). The conducting path overheated and failed because it did not have enough mass to handle the current surge. Even if the LED has a low thermal resistance and can handle large continuous current, it may not be able to handle much more than that in pulsed current.

An LED can be thought of a chain of thermal capacitors and resistors (resistors in series with bypass capacitors). The diode has a low capacitance but also a low thermal resistance. It can drain away heat quickly but it can't handle surges. The encapsulation has a high capacitance but also a high thermal resistance. It handles surges but can't handle large continuous current.

Also in regard to LED turn on time. This is most likely limited by your control circuitry not the LED. I am only familiar with CREE XLAMP LEDs which have a transition time of about 10 nanoseconds.

Answer 3

Interesting work from Anindo on 20mA LEDs, which I always understood could be overdriven for short duty cycles, though I never knew how much. I figured maybe 10:1, 40:1 may be pushing it!

However this may not carry forward as well to the newer high performance LEDs which are already being run harder, with careful thermal design.

This high power LED from HP (cough, Avago) for example has explicit "absolute max" ratings for "peak pulsing current" of 2.4A for InGaN, 1.5A for AlInGaP diodes, only about 3.5x and 2x the rated 700ma current. Page 6 of this device's datasheet has what you want : pulse current vs duration graphs for different duty cycles.

A brief review of other high power LED datasheets showed one (350ma design current) with a 1.2A "absolute max" with the interesting proviso that it should not reach this current for 60 seconds cumulative over the entire life of the product.

So it apparently varies a lot with different makes and models of high power LED.

Answer 4

It is common for the Absolute Maximum Ratings section of an LED spec to specify a current which is higher than the continuous operating current which would be permitted for the device. If you exceed that specified maximum current even for a nanosecond, then as far as the manufacturer is concerned, all bets are off.

In practice, it's pretty likely that even if the Absolute Maximum Rating specifies 500mA, one could but 1A through the part for 10us, once every second for a year, without damaging anything. On the other hand, it's also likely that putting 1A through the part for 10us may not generate much more light than would be obtained if one put through 500mA for 10us. No matter how much power one puts into an LED, there's a limit as to how much light it will generate via its intended means (i.e. by means other than going up in flames). Since any power one puts in that doesn't get converted into light will get converted into heat, there is a point beyond which increased peak current will adversely affect the lifetime of the part far more than it will affect the amount of light generated.

Answer 5

It may be that if the power is not higher than the rating of the LED it could be calculated easily if the ratio of pulse rate to duty cycle does not exceed the 100% duty at say 20mA power, that is if the way the power is used to convert to light is linear. If it is not linear then it would be a a curve of some sort and use calculus to find the curve to find the point where it exceeds the design parameters. There may be a point of course where the heat can not be taken away fast enough and then interferes with the electron to photon conversion. Therefore if a heat-sink could be more directly linked physically to the interior of the LED it could be more readily (carry away heat) heat-sinked, or actively cooled off. this would make the LED much less power efficient but the LED could then be driven with more current for different applications like stroboscopic, pulse modulations, etc. . As well the waveband output of an LED is somewhat monochromatic but will change its waveband with temperature so could be a way to tune the waveband of the LED for monochromater applications if the change in illumination changes corrected for and calibrated. There is probably an apparent brightness seen by the eye as being more efficient or less having nothing to do with the quantum efficiency of the LED but more to do with the quantum conversion of retinal chemistry and size of pupil and persistence of vision, and therefore there should be an optimal power pulse conversion for this apparent illumination to the eye.
In any case the current interaction should become non-linear at some point and destroy the LED. Maybe cool the LED by circulating some cooled oil around it with silver or gold heat sinks on the leads or drench in liquid nitrogen. Seems that good electron conductors are good materials to carry heat away and gold is chemically more stable than silver although expensive.