I'm trying to get less than 100ns pulses at >10A, preferably up to
20-30A for 10-50ns into Cree XP-E colored LEDs.
My first attempt at fast pulsing these has been to parallel 4 of TC4422A MOSFET drivers.
With 18V power supply, I can get 10-12A pulses. Below about 100ns they can't produce full output amplitude. I can get to about 50ns pulses from the LED, but with much weaker output than >100ns.
My second approach has been to use a flyback topology, with the LED tied across the inductor in the drain circuit of a NMOS. The LED gets the inductor current driven into it when the NMOS turns off.
I have simulated both CCM and DCM. It seems CCM works better, giving nicer shaped pulses than letting all the inductor energy dump in the LED.
But it seems with this circuit, the main challenge to getting it to work in reality is to drive the NMOS gate fast enough. Also, level shifting from a 5V logic level trigger input to the gate driver's input is needed. I have been looking into a simple complimentary BJT push-pull pair, but not sure what to do about level translation at speed.
Thanks for any comments.
I'm reading with interest the thread "Super duper hype fast FET driver?"
That's limited in the speed it can slam the LEDs on, since the current in the L is all the current that's available. OTOH, the LED turnoff will be hard.
If cost is not a big deal, look at the IXYS/DEI gate driver chips. Heck, they may be able to drive the LEDs all by themselves. Interesting packaging.
The people who sell mega-laser driver boxes tend to use special flex/foil transmission lines to connect to the loads, just a few ohms impedance. A 50 ohm line, or a 100 ohm twisted pair, can make trouble at these speeds and currents.
Please do, preferably later in the day. I have a cute and cheap way to drive mosfet gates really fast and hard. I'd post it publically, but Thompson would steal it, Fields would whine about what it really means, and Sloman would use it as an excuse to bombast about climate change.
I have not done a systematic study of this, but have begun considering how I might go about it.
My limited experience based on bench testing and deploying about 4-5 LED plus driver assemblies based on the TC4422A to several laboratories doing high speed Schlieren imaging is that:
I have never blown an LED with 1us or less pulses at up to 10-12A.
I have put nearly the rated average thermal load of about 5W into the LEDs with continuous pulses in the 6-9A range and up to 10us, and they live (for at least minutes-hours).
I have blown up several blue LEDs packaged by LEDengin (I believe Cree die, but not sure) when pulsing over 100us at high currents, perhaps 5-6A.
I have recently acquired a bunch of top bin Cree XP-E green, red, and royal blue emitters. Also some Lumileds Rebels in cyan and royal blue.
My first thought at how to test them would be to normalize for average power. For ex., at 1W of average power, ramp up current for a given pulse duration (while simultaneously reducing duty cycle to keep power constant) then see when it breaks. How to present the data I haven't quite worked out.
Also, some long term experiments are in order, to see if at some peak current level and thermal load, does the output gradually drop rather than catastrophic failure. If so, can time constants be derived.
One major problem with this kind of testing is that the next batch of dice from the same maker may behave quite different. Even for a given experiment, it would probably be necessary to break at least a small handful at each parameter set to get some stats.
Also, this testing would best be performed with a scientific grade pulser, like the ones used for large laser diode bars. I think some colleagues have these, so I could try to borrow one.
But my present setup is designed to be cheap and quick to get something running in the lab.
I'm not sure if my manager will go for having me make this into a research project to characterize LEDs. At least they gave me the freedom to have developed the drivers somewhat in the background, and now that they are popular, I can spend more time on that part.
It is remarkable that the radiance of these LEDs is now competitive with a Xe short arc lamp, over narrow wavelength bands.
We had previously used Xe short arcs for Schlieren rather than a laser, because the laser's coherence and thus speckle degrades image quality. But the short arc's white light causes severe chromatic aberration in our optical engines with quartz cylinder liners. Two options are to put a bandpass filter in front of the Xe lamp, or find another light source.
The BP filters have only about 50% throughput. Thus, even though the Xe radiance is much higher over its full spectrum, or even just the visible, the LED actually wins out over a narrow range of 10-30nm!
Plus the LED can be pulsed at very high rep rates. I can get 5MHz with full modulation from the TC4422As. Though we are doing only about
50-100kHz imaging right now.
The push to lower pulse durations is because when looking at fuel injection sprays right near the tip, the velocity may be 600m/s. So even 200-300ns pulses typical of high rep-rate DPSS lasers blur the motion.
The LEDs have won their place in our labs now, but we still seek to coax as many photons from them as we can. So that's why I hammer them with very high currents. I can get about 4-6x the light output when driving them at 10-12A vs. the rated 1A CW. What I really hope for is to be able to get into the 10-20W peak power range. So far the best LEDs put out about 1.1W at 700mA. I'm hoping these will produce 10W or better with 20-30A pulses.
In the cases where they fail, it's just 15 minutes and about $10 to replace them!
I have not done it with the newer leds, but about 15 years ago I was pulsing smaller IR leds well outside of their specifications like that. (Younger and more foolish...). They worked fine for a while, but died after a year or so in service. So any damage may not be visible on short timescales.
Fortunately in our case, we have the "factory" in house, so for this admittedly fringe application, blowing and frequently replacing the LEDs is no problem.
These LEDs are displacing a subset of applications for >$100000 laser systems, so at their typical price, cost is no object. We could afford to replace them every experiment, even.
The long term longevity issue though, really does throw a wrench in the works of any plans to commercialize an application that depends on massive overdriving.
I've looked for commercial products similar to what we are doing and found only a company called Visual Instrumentation. Interestingly, they are serving a similar market to us, providing high speed lighting to auto makers doing crash testing. But they aren't overdriving.
There is some documentation on this subject from Cree as of late:
But, it can happen. Pulsed current can, for instance, cause exploding wires (the tiny leadwires going to the die). When possible, try to keep to the manufacturer's recommended limits. The difference in a 20A drive and 10A drive is only gonna be three dB in light output, after all.
Yes it is fun, because I can blast pretty colored light all over my lab and don't have to worry about laser safety rules, goggles, etc.
I haven't measured the capacitance yet. Maybe I'll see if the intern and I can figure out how to do that this week before I run out of him on Thursday. Then again that will distract us even further from my summer priority task of building the F2812+Spartan3e system...
Why is it limited in speed? My understanding is that the inductor will slew as fast as possible to keep current flowing, so the only limiting factor is capacitance. A rough guess at risetime, with even 10nF and to reach 12V on the LED with a 20A inductor current is only 6ns.
The plan was not to make a particularly efficient circuit, but just to supply a 20-30A constant current source (voltage limited to about 2-3V) to the flyback circuit, with hopefully low enough Rl + Ron resistance to keep on-state power dissipation at a few watts.
My LTspice sim, even when I add an extra nF to the NMOS drain (a wild guess at the LED cap.), can make 20ns pulses easily.
The IXD_630 has me drooling. The only drawback is UVLO. Oh well. I can still do CW drive with the current limiting of a lab supply. I think this chip will be the answer to my dreams. Thanks for the tip! Oh, and no, cost is no big deal :-)
I've also been aware of the DEI fast laser diode drivers previously. I'll have to give them another read. If anything I should see if I can buy one and learn how it works. The really fast ones I'm very curious about.
That's an incredible bummer: we have several products that use Spartan
All our new designs are Altera. Xilinx is just a mess.
Right, the capacitance is the limit. With a flyback, there's no tricks available to boost the leading edge current.
One other possibility would be to use a Zetex avalanche transistor. 60 amps or so at 200 volts, 2 ns risetime, from a SOT23 transistor! Do the old radar charged-line trick. That would work great as long as the pulse rate is low.
Drat. Every time I hear something about Xilinx on these boards, it's not good and makes me want to consider bailing.
We've got some really good people with extensive Altera experience in-house, including a new hire that worked there and knows the guts inside and out.
I would be happy to bail if I didn't have some existing Xilinx projects deployed. But the thought of having to continue to work with two tool sets is not pleasing.
Plus there are many cheap dev. boards for Xilinx. Argh! Now I have to re-think this all over again...
Which at 10-30A is going to give enough speed anyway in these apps.
I ordered some of the Directed Energy drivers today, the "PCO-7110 40A
4ns Fixed Pulse Width" and the PCO-7120 12ns and up, 50A driver.
These should be fun to play with, and I can learn a lot about how they work.
I think the PCO-7120 will be suitable for our situations where we need well under 100 ns.
For stuff that only needs 50ns and up, I think the 30A gate driver will be worth making a custom board.
I'm also considering something really crazy: for flood illumination where I can afford a large source area, a 1x2" matrix of about 32 LEDs driven in groups of 2 to 4 by something like the IXD_609 in DFN on the back side of a little tiny 1x2" board. Won't be able to dissipate heat well, but it could pump about 60-120W of optical radiation at low duty cycles into an engine head window.
I'll have to look into that. Can an avalanche transistor be triggered reliably?
I discussed with the intern about this, but I don't think he got this question answered yet.
Or, you can ditch the LED and use a proper gas discharge lamp. At least, it'll turn ON quick enough (the OFF time depends on gas mixture, and fill pressure). Very high power (multiple kilowatts) single lamps are available, but NOT in LED technology.