I knew it would happen at some point. I killed some 30x30 mil LED chips in this stock 10W green package. The remaining good ones still light off DC:
Actually two others in the left string also appear at higher currents; before the "event," they all had similar forward voltages and would all light about equally at a few mA.
I built this new flash circuit using a NTD5867NL MOSFET driven by the NCP81074B gate driver IC. The circuit has the same topology as I used before - this time, I substituted a 100 milliohm 1206 for the current sense resistor, and started with the 40uF photoflash capacitor. The idea with this test was to see how high I could get the current at reasonable voltages. The MOSFET is rated for 60V Vdss, so I figured 50 might be a decent de-rating for tests.
With the 40uF cap in the circuit, I tested the pulses up to 50V. This is the voltage across the 100-milliohm sense resistor as measured with my soldered-on 2x Z0 probe. I believe that ringing is really present in the circuit, since the Z0 probe terminates the coax at both ends, and has shown clean pulses before. Accounting for Rsense and the 2x probe, the cursor measurement is at around 23A. With a 50V supply, this isn't great.
The optical pulse, as measured with my biased photodiode looks good, shape-wise:
the cyan trace seems to confirm the ringing issue - the light is ringing, too! The optical pulse would be fine for a camera exposure, though, capturing a clean 1.086 us instant.
I wasn't satisfied with 23A - that's less than 8A for each LED chip, and I had pumped 12 or so into the smaller ones before. I knew the 40uF capacitor had a high ESR limiting the current, so I replaced it with a 100uF photoflash unit, expecting a smaller ESR but perhaps a larger ESL. With the 100 uF capacitor in place, I turned up the voltage, eventually reaching the 50V mark. Before I could measure the current or take a screenshot on the scope, though, the current waveform began to droop - pulse by pulse (about 1Hz), the waveform fell little by little until it came to rest at less than half of what it had been. At the same time, some of the LEDs stopped working.
My suspicion is that the LEDs were killed by overstress transients caused by the ringing. I had recently found the paper Pulsed operation of high-power light emitting diodes for imaging flow velocimetry by Willert, et al. In their driver circuit, using a similar MOSFET/capacitor arrangement, they place a BYT 01-400 rectifier diode reversed across the LED. They state that:
Diode D1 protects the LED from potentially damaging reverse currents that arise during the rapid switching transients of the circuit.
sounds like what I've just experienced. I'm not sure about their choice of diode, though - that rectifier diode has a reverse recovery time of 55ns, which sounds too slow. Maybe the LED-killing transients are at the end of the pulse? You can see the red curve above spike negative when the LED is turned off. At the end of the pulse, you don't care about the trr of the protection diode.
My next step is to replace this LED with a fresh one and add a protection diode - but which one? Jack Smith has done some great measurements on forward and reverse recovery time in various diodes. His studies indicate that even the lumbering 1N4007 with it's 3us trr switches on in a few nanoseconds to eat transients. Luckily, I have more 1N4007s than I will ever use.
I am a little concerned by the long bond wires in this LED package, though. Maybe it would be better to go with the 5W LEDs that have (4) 30x30 mil chips in a smaller package? I'll have to try some things out.
I replaced the first LED and added a 1N4007 diode. This one lasted long enough to make some interesting measurements before failing in the same way after about 400 flashes. After failure, some of the LED chips only partly glow. Notice the ones in the lower left:
While it was still working, I was able to grab some data about the crazy transients in this circuit. The ringing during the pulse is reduced by the protection diode, but I finally got a look at the transients after the falling edge:
The forward current pulse is around 27A with a 50V supply, but look at that ringing at the end! Zooming in, we get a better look:
That first cycle peaks at 40A! I can see how that could fry LEDs - but it should be going through the protection diode now - unless the 1N4007 just isn't suitable here. Another possibility is adding a RC (or RCD) snubber to damp these oscillations, which look like around 70 MHz.
Even though the LED didn't survive, the optical pulse shape verifies the ringing during the pulse has been cleaned up:
I'll try another protection diode.