Ultra Short, Ultra Fast LED Flash - Page 1
Forum section: electronics / projects

author: JAndrew
author_rank: Contributor
timestamp: March 05, 2019, 11:06:50 pm
content: At a previous job, I did some high speed stop action photography. (See Attached Pictures)The way this is accomplished is to use a dark room, with a long exposure on the camera, and to control the exposure with a short duration flash. The shorter the duration of flash the faster an object can move and still capture a clear image. Its important that the flash is bright, short and abrupt. We used Air Gap flash that generated a flash duration of 500ns. Hack-a-day explains how to build one but ours was a commercial one. (Uses high voltage caps, something I'm not real comfortable toying with)I'd like to try and take photos like this on my own. I could go out and purchase the commercial unit for $1000+ or I figured I'd see if I could build it using RGB LED's. The reason I'm using RGB LED's is the phosphorus layer on a "White" LED Glows for a unspecified amount of time after the blue LED shines on it. This may extend the length of the flash beyond my target of 500ns.The issues I'm having is trying to build a driver for the LED's. I plan on triggering it with a 5v high pulse driven from an ardiuno or some other simple electronic trigger. My thought was to use 25 of these 3 Watt LED for a total of 75 Watts:https://www.superbrightleds.com/moreinfo/high-powered/vollong-3w-rgb-high-power-led/899/2214/Any thoughts on how I might go about building a driver for this?EDIT:The direction that I think I am heading in, is to purchase some power MOSFET's. Probably one for each leg of the LED's. The larger power MOSFET's would be driven by a single smaller MOSFET. As far as switching goes it seems like MOSFETs are fast and can handle higher amps and voltages without being expensive.Thanks in advance,Jay

author: dmendesf
author_rank: Frequent Contributor
timestamp: March 06, 2019, 03:51:12 am
content: You should take a look at this:https://www.analog.com/en/analog-dialogue/articles/led-driver-for-high-power-machine-vision-flash.htmlEnviado de meu moto g(6) plus usando o Tapatalk

author: David Hess
author_rank: Super Contributor
timestamp: March 06, 2019, 04:02:05 am
content: Many years ago I designed a low power LED flasher like that which easily generated pulses below 500 nanoseconds. It used +5 volt logic to drive the emitter if an NPN transistor with the base tied to the +5 volt supply and operating as a high voltage cascode so transistor drive was ideal and the resistance between the emitter and gate output controlled the current very precisely.Scaling this up would not be difficult with a power MOSFET on the low side and NPN RF power transistor as the cascode. The advantage of running at higher voltages is that unavoidable parasitic inductance has less of an effect. In practice the cascode is probably not required although it makes current regulation more precise.If you operate with such short pulse widths, then the power can be much higher although this makes the power switching more difficult. If the pulse width is only 10s of nanoseconds, then a charged transmission line and bipolar avalanche transistor might be worth doing; the voltage with that many LEDs in series is high enough. But my guess is that you do not need to go below 200 nanoseconds.

author: StillTrying
author_rank: Super Contributor
timestamp: March 06, 2019, 09:49:45 am
content: There are a few threads on fast LED flashes on here.https://www.eevblog.com/forum/beginners/4-microsecond-high-power-pulses-through-led/msg1414506/#msg1414506https://www.eevblog.com/forum/beginners/microsecond-led-flash-ringing-issue/https://www.eevblog.com/forum/chat/20w-halogen-bulb-viewed-by-a-photodiode/msg1751210/#msg1751210If you go by the half light point the rise and fall times of medium-white LEDs is quite fast at about 100ns.

author: Zero999
author_rank: Super Contributor
timestamp: March 06, 2019, 10:16:29 am
content: I've done this before. The requirement was for a 1µs. Phosphor LEDs are variable. Some are fast and others are slow.Attached is a schematic of the test set-up I used to drive LEDs with 1µs pulses of hundreds of amps, hence the low value current limiting resistor. It's possible to overdrive LEDs by a factor of 10, for short pulse lengths. Note transformers are used to drive the MOSFET and measure the current, to avoid large currents passing through the grounds of the test equipment.

author: ogden
author_rank: Super Contributor
timestamp: March 06, 2019, 10:55:39 am
content: Any constant current *switching* regulator will be too slow to produce speeds you need. Flash capacitor bank with current-limiting resistors and solid state switch (MOSFET or IGBT) is way to go. For design phase I would not use 5$ LEDs but cheapest I can find at given rating, initially use shorter string at lower voltage. You may find design shown in following post relevant. IMHO few tweaks needed and you are ready to go:https://www.eevblog.com/forum/beginners/4-microsecond-high-power-pulses-through-led/msg1417075/#msg1417075

author: Siwastaja
author_rank: Super Contributor
timestamp: March 06, 2019, 01:28:04 pm
content: Most LEDs have rise and fall times below 30 ns, usually below 20 ns. Similarly, fairly easy to find a power MOSFET that can switch in 20ns or even faster. Hence, you requirements are easy.Constant current driver may pose a problem - the inductor slows down its response time. Some special design is required.Although not perfectly optimal, the overall easiest solution, by far, is to use a bog standard regulated voltage supply and a plain old series resistor - the only downside is, to maintain some current regulation, you need to drop some meaningful voltage over the resistor, let's say at least 20-30% of the total supply voltage. So it's not as efficient as a constant current drive, but can be acceptably efficient. For even faster switching, you can use a small bypass capacitor parallel to the series resistor, but this shouldn't make a difference with your fairly slow pulses. Even without the small capacitor, you are already at full LED power after about 20-30ns, which is a small percentage of your 500ns pulse.Do beware of stray inductance. Keep the physical loop as small as humanly possible. If you have a lot of LEDs, make the LED series layout a tight, thin "U shape" so that the opposing currents go as close to each other as possible, and Vdd and GND are right next to each other (preferably sandwitched on a multilayer PCB):VDD-Res-LED-LED-LED-LED-|GND-FET-LED-LED-LED-LED-|Spray ceramic capacitors (snubbed with a larger electrolytic) between your Vdd and GND.If your duty cycles are high, you'll have two opposing constraints, thermal design would benefit from loose layout, while you need tight layout for low inductance.

author: StillTrying
author_rank: Super Contributor
timestamp: March 06, 2019, 02:26:29 pm
content: Quote from: JAndrew on March 05, 2019, 11:06:50 pmYou could try wiring up just one or two of the RGB LEDs with 1 to 3 Watts worth, and use its light in a darkened room to takea normalphoto of the objects and look at the exposure time. It'll be a slow 1/5 sec-ish, but will give you some idea of how much more light is needed for a 1us exposure. You'll find for a 1us exposure you'll need to multiply the light/power by about 50,000.Quote from: Siwastaja on March 06, 2019, 01:28:04 pmLOL.

author: ogden
author_rank: Super Contributor
timestamp: March 06, 2019, 02:37:06 pm
content: Quote from: Siwastaja on March 06, 2019, 01:28:04 pm20-30% of waste power?Please don't... Somebody may take your joke as an advise. Example: Osram 24V strips uses 7 white LED's in series.

author: JAndrew
author_rank: Contributor
timestamp: March 06, 2019, 03:52:44 pm
content: Thanks for all the replies. I've got a lot of reading and studying to do.Duty cycle would be pretty low. At most we would have 1 shot ever few minutes. I thought about over powering the LED's and wondered how well they would handle it for such a short duration and if I would need any sort of passive or active cooling. From the sounds of it, I don't think I will.The machine vision is something I hadn't even considered. I know high speed flash is used for inspection on turbine and other high speed equipment. I should study a few of those circuits.

author: Zero999
author_rank: Super Contributor
timestamp: March 06, 2019, 04:15:07 pm
content: Quote from: ogden on March 06, 2019, 02:37:06 pmFor short pulses, at a low duty cycle, the efficiency is often unimportant, as the total power is very low. If the LED is being over-driven the resistance of the bond wires starts to become significant at high currents and an additional series resistor is unnecessary. I've recently tested a white COB LED, the Cree® XLamp® CMA3090, with a nominal forward voltage of 45V and maximum current rating of 3.6A, with 1µs 105V pulses, no resistor, and the bond wires limited the current to 43A: well over the absolute maximum rating but it ran with a 100Hz repetition rate for an hour or so, with no damage or measurable reduction in the light output. Note that although the instantaneous power was 4.5kW, the duty cycle was only 0.01% so the total power was just 0.45W. The efficiency is probably much lower than non-pulsed operation but who cares if it's only on for just enough time to take a picture?https://www.cree.com/led-components/media/documents/ds-CMA3090.pdfI agree about not using a conventional SMPS, as it will be too slow to regulate.If the delay isn't important or can be factored into the timing, one possibility is to charge up an inductor to the required current, then dump it through the LED. If the inductance and current are known variables, then the energy per pulse can be accurately controlled.

author: ajb
author_rank: Super Contributor
timestamp: March 06, 2019, 07:31:19 pm
content: Many switching constant current LED drivers support PWM dimming by using a transistor to short the LED(s), so you could do the reverse here. Establish your target current in the inductor, then open the switch across the LEDs for your desired duration, then close it again. If the pulse is long enough and the response of the switching converter is fast enough the converter will pick up the load to maintain the LED current, otherwise you can just use a big enough inductor to sustain your required pulse.

author: ogden
author_rank: Super Contributor
timestamp: March 06, 2019, 09:07:09 pm
content: I do not see inductor as better solution than simple resistor. I would just make 3-channel system with individually programmable voltages and timing for each color, obviously with (inrush) current limiting resistors to drop like 3..5% of voltage on each string. Not for god's sake 30%. It is not needed to increase voltage specs of whole system just because someone here in the forum said that ballast resistor shall drop 30% of the voltage which for high power LED's shall be considered as insanity.

author: Zero999
author_rank: Super Contributor
timestamp: March 07, 2019, 11:21:34 am
content: I agree 30% voltage drop is quite a lot.The big advantage of using an inductor is the voltage can be boosted, so a lower voltage can be used to drive a long string of LEDs. It's also good if the LED has a much lower forward voltage than the power supply, so a power wasting resistor can be avoided. Note that in boost configuration there needs to be something to protect against open circuits, otherwise the high voltage spike will destroy the switching transistor.I agree that it's better if the power supply voltage can be set to a suitable value and the current limited with a resistor.As I said above: it's very important to keep the grounds for the logic and power separate: use an isolated MOSFET driver or pulse transformer for the transistor and another transformer or Hall effect sensor to measure the current.I experimented with several different pulse and current transformers. Keep the burden, the secondary side resistor, as low as possible to avoid core saturation and improve accuracy. It's fine to overdrive current transformers, with low duty cycles, as long as the burden resistor is proportionally reduced to avoid core saturation. I've had quite a lot of success with winding my own on ferrite ring cores, but I found off the shelf ones work quite well. In the schematic I posted previously, I used the Murata 54100C with both primary windings in parallel and a 1R terminating resistor for a 10mV/A output. At 250A it worked perfectly fine and gave accurate measurements, when compared against a resistor.https://docs-emea.rs-online.com/webdocs/0eb1/0900766b80eb1ad9.pdf

author: Marco
author_rank: Super Contributor
timestamp: March 07, 2019, 12:14:47 pm
content: With RGB LEDs you'll need resistors for current limiting for white balance, even if they can take just dumping the full voltage in.Personally I'd try 74LVC1G123 to generate the pulse, MCP14A0301 gate driver, 12 V power supply, random low voltage low rds MOSFET, resistors for current limiting and balancing, a lot of LEDS&resistors in parallel, feed the power&signal to the gate driver/mosfet/flasher section through two common mode chokes. If it generates too much noise you can always go the GDT route later.

author: ogden
author_rank: Super Contributor
timestamp: March 07, 2019, 01:30:55 pm
content: Quote from: Marco on March 07, 2019, 12:14:47 pmNo, no, no. Better use (inductive switching) converter to pump energy into (flash) capacitor (48V or even 100V), to *avoid* lot of LEDs in parallel, but use single or few (long) strings. White balance using programmable voltage.

author: Marco
author_rank: Super Contributor
timestamp: March 07, 2019, 01:41:04 pm
content: He only wants 500 ns pulse width. Avalanche transistors, massive overvoltage to overcome inductance, worrying about a little trace length are all overkill.Meanwhile low voltage has the advantage it can't kill.

author: Zero999
author_rank: Super Contributor
timestamp: March 07, 2019, 02:23:30 pm
content: One thing to note is the LED(s) should have a reverse parallel diode. I didn't include one in my schematic, because the LEDs I tested all have one built-in, which is very common for high current LEDs.

author: Seekonk
author_rank: Super Contributor
timestamp: March 07, 2019, 04:17:42 pm
content: Quote from: ogden on March 06, 2019, 02:37:06 pmResistors, we don't need no stinking resistors. I hit the LED right from the FET at about four times the rated voltage.

author: schmitt trigger
author_rank: Super Contributor
timestamp: March 07, 2019, 04:19:00 pm
content: Quote from: JAndrew on March 05, 2019, 11:06:50 pmDoc Edgerton himself would be proud of this photo

author: JAndrew
author_rank: Contributor
timestamp: March 08, 2019, 01:15:01 am
content: Quote from: schmitt trigger on March 07, 2019, 04:19:00 pmThanks man, I’ve got a collection of similar photos, this is probably the one I am most proud of.Sent from my iPhone using Tapatalk

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 08, 2019, 01:18:18 am
content: The biggest problem is getting enough peak power. LEDs aren't good very far beyond ratings, unfortunately. (The efficiency tanks, and then eventually you get accelerated damage, and outright failure.)Tim

author: Giaime
author_rank: Regular Contributor
timestamp: March 08, 2019, 08:24:06 am
content: Quote from: ajb on March 06, 2019, 07:31:19 pmBeen there, done that. This is the best way. Use Ti parts like LM3409 or TPS92641, TPS92515, etc, all support shunt PWM dimming. You can get 300-500ns pulses like that if you control the circuit inductance carefully.

author: ogden
author_rank: Super Contributor
timestamp: March 08, 2019, 09:45:11 am
content: Quote from: Giaime on March 08, 2019, 08:24:06 amWhy, why do you people suggest such?First one, LM3409. Datasheet states that buck regulator frequency above 1MHz is hard to achieve. This means that one buck regulator pulse alone is longer than required, not to mention that first pulse most likely will not reach nominal current. Following waveform from LM3409 DS clearly shows problem why buck regulator and inductor-based approach as such is not the best choice (to say it politely) for this application:

author: StillTrying
author_rank: Super Contributor
timestamp: March 08, 2019, 10:08:46 am
content: Quote from: ogden on March 08, 2019, 09:45:11 amYes, I think a charged cap is the only way the energy can be stored for many minutes while waiting for the trigger, storing it in an inductor adds all kinds of timing problems.But the biggest problem is still getting the LED light up to 10s of kW for very short exposures.This ~5.7kW for 4us one mentioned above is about the best I've seen anywhere on the web.https://www.eevblog.com/forum/beginners/4-microsecond-high-power-pulses-through-led/

author: Zero999
author_rank: Super Contributor
timestamp: March 08, 2019, 12:07:16 pm
content: Some oscillograms.A low current test: 3A, but the LXM2-PH01-0070 is only rated to 700mA continuous. The signal generator waveform isn't great because I used it to drive the gate drive transformer, without a MOSFET driver IC. The photodiode was about 100mm from the LED.A high current test: 60A through the CMA3090, which is only rated to 3.6A continuously. The photodiode distance was increased to 200mm, otherwise it saturated. I managed to increase the voltage to 210V, giving a 120A pulses, before the LED failed open circuit.

author: Siwastaja
author_rank: Super Contributor
timestamp: March 08, 2019, 12:22:57 pm
content: Quote from: ogden on March 06, 2019, 09:07:09 pmI said: at least 20-30%for good current regulation(against Vf variations through temperature and unit variations). This may or may not be important. For a low-duty special effect equipment, a 20-30% efficiency drop might not be a problem at all. For a general purpose LED lighting fixture, it would be an environmental disaster IMHO, so I understand where you are coming from.Yeah, don't follow my advice blindly, do your own analysis. 20-30% drop was based on my own work on time-of-flight imaging where exposure consistency does matter.Of course, if current regulation is unimportant, a no-resistor-at-all, limited-by-bondwires approach is often just fine. Or as you say, a few percent of the supply voltage. If you just need light for some photographic trick photo and have enough dynamic range in the imager, and are not expecting predictable exposure within a few percent, and your duty cycle is low and you are confident you are not killing the LEDs on unpredictable overcurrent, by all means reduce the waste power in the series resistor.You may even be able to choose the FET so that its Rds(on) temp coeff compensates (at least partially) for the LED Vf temp coeff, regulating the current! This can be a neat trick.You may evenwantthe current to increase with increasing temperature (within reason; kind of controlled/anticipated "thermal runaway"), because LEDs produce fewer photons per coulomb at high temperatures, so you may want to increase the current with temperature to keep the light output constant. If you want this, then use a resistor "too small".Of course, all of this babble is just about finetuning a simplistic, easy approach which, by definition, will be inaccurate in light output. I'm assuming this is good enough for the OP.As a general note, I have seen that the very short pulse handling capabilities of LEDs often tend to be better than specified, but the efficiency does go down, so increasing current a lot has diminishing returns.

author: ogden
author_rank: Super Contributor
timestamp: March 08, 2019, 01:13:23 pm
content: Quote from: Siwastaja on March 08, 2019, 12:22:57 pmGood idea indeed, but you don't have any control over results *and* you shall thermally connect FET to LED. "Standard" and more controllable approach would be to measure LED temperature, set voltage accordingly (assuming that LED V/I curve is known, measured in advance).

author: ajb
author_rank: Super Contributor
timestamp: March 08, 2019, 03:42:50 pm
content: Quote from: ogden on March 08, 2019, 09:45:11 amYou've missed the boat. The point is not to turn the switching regulator on at the start if the pulse and off at the end, that wouldn't work well at all, as you've mentioned. The point is to turn the converter on ahead of time, while the LEDs are shorted by the PWM transistor. This will establish the desired current in the inductor, then when you want a pulse of light, you turn the PWM transistor off, which will allow the inductor current to flow through the LEDs instead. This way the current is directly controlled, rather than relying on a regulated voltage and a resistor (which becomes subject to all sorts of other variables that will affect the actual current through the LEDs).This is a fairly conventional topology for PWM dimming with constant current switching converters, and many have built-in support for it, like in the example below. This shows the dimming transistor being controlled through the switching converter; this is often done to allow the converter to adapt its switching behavior or even apply power saving behaviors when the dimming transistor is active, but driving the dimming transistor externally may be better for this application.

author: Zero999
author_rank: Super Contributor
timestamp: March 08, 2019, 04:05:27 pm
content: Quote from: ajb on March 08, 2019, 03:42:50 pmYes, that will work and I did hint on using it in posts10and13. The main downside is the inductor needs to be charged up before the pulse, which takes time. It's not a problem if there's an advance warning before flashing the LED, but it's no good if it has to trigger as quickly as possible.

author: Marco
author_rank: Super Contributor
timestamp: March 08, 2019, 04:27:06 pm
content: Lets do some numbers, lets say you want 10 kW for 1 us. That's 0.01 Joule, lets round that up to 0.1 Joule to keep the current approximately constant for the pulse ... also a relatively safe amount of energy.Lets say you use 100V string of LEDs (LED filaments would work for that) so you need 100A of current, so you need 20 uH of inductance, of course with a SRF>>1 MHz. That's a spicy inductor. Compare that to storing 0.1 Joule at 100V in a capacitor, that's a 2 dollar electrolytic capacitor in singles.

author: ogden
author_rank: Super Contributor
timestamp: March 08, 2019, 07:43:38 pm
content: Quote from: ajb on March 08, 2019, 03:42:50 pmYes, kind of.Did read only first datasheet.QuoteI am afraid that this is where you are missing the boat. When load is shorted, regulator provides let's say 0.1V @ 10A = 1W power. When shunt is open then regulator shall suddenly jump to 20V @ 10A = 200W power (or whatever is operating voltage of the LED's). Voltage of inductor cannot jump instantly, it is against it's formula V = L(di/dt). Sure/agreed - stopping/starting converter is even slower. Inductor can't compete with capacitor no matter how you twist it. I did not find any real, measured LED current PWM shunt waveforms for this case. If you know any or can measure - please provide, would be nice to see.

author: ajb
author_rank: Super Contributor
timestamp: March 08, 2019, 09:21:41 pm
content: Quote from: ogden on March 08, 2019, 07:43:38 pmYou're neglecting the energy stored in the inductor, and that the defining characteristic of an inductor is that it resists changes in current, which is the whole point. We're literally talking about a boost converter minus the output capacitance and with a larger inductor. Boost converters clearly work, including constant (ish) current LED boost drivers, so there's no reason that this won't too. Of course there's plenty of room left to argue about the practicalities, costs, and other relative design tradeoffs of the inductor vs capacitor based solutions, and a lot of that comes down to how closely you need/want to regulate the LED current, and how much energy is involved. The OP said 75W * 500ns, while for some reason Marco has decided to make the problem much harder by assumed 10kW for 1us, which is 266 times as much energy. (I have no idea how much energy is actually required to get the results the OP is after, but chances are it's somewhere between 75W and 10kW...)

author: ogden
author_rank: Super Contributor
timestamp: March 08, 2019, 09:35:04 pm
content: Quote from: ajb on March 08, 2019, 09:21:41 pmOh, suddenly it's me who neglects energy stored in the inductor.Look, unless you show real-world shunt-PWM waveform that disproves what I say, we do not have any further progress in this discussion.

author: Marco
author_rank: Super Contributor
timestamp: March 08, 2019, 09:46:59 pm
content: In retrospect I decided that 75W isn't really a replacement for an open air spark gap ... it's not going to be enough to expose a normal camera.He's going to need some serious power, keeping the energy to supply that power circulating in an inductor is awkward. For a couple ms it's not so bad, but when you want to do it continuously to be ready for a trigger you need so much copper to keep the losses down. Meanwhile a capacitor can just sit there minding its own business doing nothing.

author: StillTrying
author_rank: Super Contributor
timestamp: March 08, 2019, 10:01:14 pm
content: LED or xenon, 10s of kW is needed for a small close-up 1us exposure.

author: Marco
author_rank: Super Contributor
timestamp: March 08, 2019, 11:37:38 pm
content: Unless it's some kind of special high pressure Xenon lamp it will have lousy fall off.

author: JAndrew
author_rank: Contributor
timestamp: March 09, 2019, 12:23:48 am
content: 75 Watts is a starting point, it may not be enough light.If I can get a 500ns 75w flash then scaling up as needed is probably doable.Xenon Lamps are too slow on the fall off and will create motion blur.Historically units like these were used.https://www.ietlabs.com/genrad-1538-a-stroboscope.htmlI can pick one up on EBay for ~$300 dollars but the light isn’t as bright as a air-gap. Maybe it’s worth it to try and pick up a couple of these old units rather then to try and cobble together my own.Sent from my iPhone using Tapatalk

author: Marco
author_rank: Super Contributor
timestamp: March 09, 2019, 04:39:55 am
content: Quote from: JAndrew on March 09, 2019, 12:23:48 amIn that case I'd try to drive one of those 12V 10W COBs with 24V, that should just about get you there electrically (lil higher). Use a MCP14A0302 with a simple ~500p/1K high pass filter on the input to create your pulse from a 5V step (unless the Arduino can generate a 500ns pulse on its own). Something like BUK7M45-40E to drive the COB.

author: Siwastaja
author_rank: Super Contributor
timestamp: March 09, 2019, 08:25:41 am
content: Quote from: ogden on March 08, 2019, 01:13:23 pmThey tend to be somewhat thermally coupled, because the inductance needs to be minimized as well, so they are close. Of course it isn't exact.

author: Wolfram
author_rank: Frequent Contributor
timestamp: March 09, 2019, 01:17:46 pm
content: It probably makes sense to get an idea of exactly how much light you need, this is one of the most important design specifications and it affects a lot of design choices. A few exposures using the planned camera system, and a charged electrolytic dumped into an LED through a resistor, will already tell you if your planned 75 W for 500 ns is a realistic starting point.

author: JAndrew
author_rank: Contributor
timestamp: March 09, 2019, 03:57:44 pm
content: When I approached the project. I broke it down into two main problems. The trigger, and the flash.The trigger is fairly easy to solve, there are various optical sensors I can use to trigger the flash as the bullet passes buy. Even put in a variable time delay.The flash and the flash duration has been the harder problem for me. Again my thinking was using power MOSFETs to control a bank of LED’s. If I needed more light I would add more banks of LED but the circuitry would all be the same. Regardless how I go, my thought would be as long as I have one circuit that works setting up several in parallel or looking at ways to overdrive the LED’s for a brief moment wouldn’t be a challenge.I tried to use an Arduino as a pulse generator. I figure that would be a pretty easy way to go. I could specify the length of the flash, time delay, and even do several flashes. However I don’t think I could ever get the program to run fast enough to switch the I/O pin that quickly. I remember playing with it, and looking at things like the Beagle Board or Raspberry Pi to see if I could do with with a faster processor. Even bought my first O-scope for this project.I suspect you’re right a 75w target is not enough. Perhaps even the number of LED’s needed to get to the right level of light are impractical. So it could be I’m going about this wrong. Perhaps I should look into what makes a “General Radio GR GenRad 1538-A Strobotac” tick. (I’m thinking aloud)I haven’t looked spec sheets for all of the different chips that have been suggested or studied everyone’s response. I’ve been tied up at work. I’m going it over this weekend so I suspect I’ll have a few questions.Thanks for all the input.Sent from my iPhone using Tapatalk

author: Marco
author_rank: Super Contributor
timestamp: March 09, 2019, 04:46:21 pm
content: I suspect than LEDs are 10-100x more efficient than sparkgaps though, so the difference might not be that large.

author: StillTrying
author_rank: Super Contributor
timestamp: March 09, 2019, 04:58:09 pm
content: Quote from: JAndrew on March 09, 2019, 03:57:44 pmIt's a smallish 800v tube, 0.5us to 5us, I've not found a matching data sheet.Triggering is easy compared with getting the light level up!

author: Wolfram
author_rank: Frequent Contributor
timestamp: March 09, 2019, 05:30:34 pm
content: Quote from: Marco on March 09, 2019, 04:46:21 pmIt's hard to find good sources for the luminous efficacy of an air spark, but it seems to be in the range of a lumen-second per joule. A non-overdriven LED will therefore be roughly hundred times more efficient, but 75 W is also about five orders of magnitude below the peak power of a normal air gap flash. The total light output for an LED driven at 75 W for 500 us will therefore be about a thousandth of that of an air-gap flash like the EG&G Microflash 549.

author: Marco
author_rank: Super Contributor
timestamp: March 09, 2019, 05:43:14 pm
content: Looking around the web I'd guestimate a LED spotlight overdriven to 100W will do about 100K Candela. So to achieve the same intensity as the Microflash you'd need about 50kW of overdriven power.

author: JAndrew
author_rank: Contributor
timestamp: March 09, 2019, 05:47:54 pm
content: Quote from: Marco on March 09, 2019, 05:43:14 pmHmmmm, my first impression is that that’s beginning to touch on the realm of impractical.Sent from my iPhone using Tapatalk

author: dmendesf
author_rank: Frequent Contributor
timestamp: March 09, 2019, 05:51:52 pm
content: You need to use an Arduino timer to drive the pulse (probably in pwm mode, but I'm not sure... I know pics better than avr). This way you will have very fine control of the pulse width (with steps of 1/f, so about a hundred ns).Enviado de meu moto g(6) plus usando o Tapatalk

author: NiHaoMike
author_rank: Super Contributor
timestamp: March 09, 2019, 06:52:59 pm
content: Some modern xenon flashes use IGBTs to allow switching off the tube at an arbitrary time. 500ns is definitely on the fast side for IGBTs, but maybe it can be done with MOSFETs? Apparently, using a higher voltage but lower capacitance also helps for shortening the duration of xenon flashes.Where LEDs excel is for video lighting in order to reduce motion blur, where the operating frequency is much too high for xenon. Unfortunately, few consumer/prosumer cameras have a sync output, but maybe it could be possible to use the vsync from the HDMI output, then use an adjustable delay circuit to sync the LEDs to exactly the right time?

author: JAndrew
author_rank: Contributor
timestamp: March 09, 2019, 08:16:07 pm
content: This is the unit we used to take the pictures.http://prismscience.com/spot.phpTaken Directly from the specification page.SPOT: SpecificationsHoused in a 6.7" H 6.7 H 8" enclosure (Fig 1), and weighing in at 10 lbs, SPOT is a compact submicrosecond flash system operating from 120 VAC power. Three trigger inputs are included:· Manual — push button· Switch closure — �" phone jack· Pulse (e.g. TTL) — BNCThe stored energy of approximately 8 J is released in a short flash of less than 500 ns. Fig. 2 illustrates FWHM and FW1/3M times for full visible spectrum and for 520 nm line filter. The light collected by a 6" fresnel lens forms a relatively uniform "spot, " Fig. 3. The peak intensity of 69 million lumens delivers a pulse having 28.5 beam candlepower-seconds. At 40" from SPOT, the beam diameter is nominally 20" to 24"; however, beam spreads from 14" to 40" can selected during manufacture. A typical camera setting is f/5.6 to f/8 at 200 ASA for objects illuminated from 3 feet.The design can be adapted to create different output characteristics for specific applications, such as a 2.7 J pulse with 200 ns FWHM for extremely fast motion, or point sources for shadowgraphs or schlieren images.I'm referencing this as a comparison for what I'd like to try an emulate. The one point I see that I hadnt looked at before was the lumens 69Mil. Trying to drive that sort of output from LED's even if it is for a brief period doesn't seem feasible.

author: Marco
author_rank: Super Contributor
timestamp: March 09, 2019, 08:40:34 pm
content: Quote from: JAndrew on March 09, 2019, 05:47:54 pmThat's only if you want to get that bright, you can make do with a lot less light. Look at the Vela One, I'd estimate they are doing closer to 5 kW, 200-300 Watts worth of COBs.

author: StillTrying
author_rank: Super Contributor
timestamp: March 09, 2019, 08:49:50 pm
content: Quote from: JAndrew on March 09, 2019, 08:16:07 pm16 MW.

author: JAndrew
author_rank: Contributor
timestamp: March 09, 2019, 08:50:55 pm
content: Huh....ok, so I'm not completely out to lunch... so they have something pretty neat but it's unobtainable. I think I remember seeing this a few years ago, and it looked great but it was not available then either.So from the description they are using 9 5000lm COB LEDs over driven by 2000% to produce 1mil lumen. This is all driven off a four AA batteries.What sort of wizardry do they have going on?

author: JAndrew
author_rank: Contributor
timestamp: March 09, 2019, 08:59:22 pm
content: Looking around for LED COBshttps://www.ebay.com/itm/100W-Cold-White-High-Power-9000-10000LM-LED-light-Lamp-COB-Chip-35DI-/391802735671?_trksid=p2349526.m4383.l4275.c1#viTabs_0Pretty inexpensive for 10,000LM, but if I drive these by 1000% and get 10 of them. Then I could match the 1mil Lum. However the power requirement would be pretty high as well. Input current under normal operation is 3000mA and voltage is 32-34vdc.This isn't something I could drive with 4 AA's.

author: ogden
author_rank: Super Contributor
timestamp: March 09, 2019, 09:14:24 pm
content: Quote from: JAndrew on March 09, 2019, 08:59:22 pm10K lm? - Scam. With such specs he can put OSRAM out of business, yet they are doing wellYou may indeed user those to start with, but do not expect to get even close to specified light output.QuoteIt is easily achievable with 4AA's - in pulse, from charged flash capacitor. After all this is how Xenon flashes/strobes work for decades and those guys are doing exactly that:https://www.vela.io/vela-one-high-speed-flash

author: JAndrew
author_rank: Contributor
timestamp: March 09, 2019, 09:25:46 pm
content: If that is the case how hard would it be to adapt a typical flash camera circuit, to drive an LED?Sent from my iPhone using Tapatalk

author: ogden
author_rank: Super Contributor
timestamp: March 09, 2019, 09:33:53 pm
content: Quote from: JAndrew on March 09, 2019, 09:25:46 pmIn theory - yes. You may replace xenon tube with (250V...300V) LED string, modify trigger circuit. I am not sure about power capability - existing capacitor or bank will be enough or you need to add extra (capacitance).  How hard - it depends on flash, it's circuit complexity.

author: Marco
author_rank: Super Contributor
timestamp: March 09, 2019, 09:52:46 pm
content: The new'ish 12V COBs seem to have better construction than those older ones.

author: JAndrew
author_rank: Contributor
timestamp: March 09, 2019, 11:09:27 pm
content: Trying not to reinvent the wheel, so I'm looking at flash circuit kits that I might just adapt to overdrive an LED.Here's one I am looking at.https://www.xenonflashtubes.com/flash-driver-modules/51-5v-optical-flash-trigger-photography-kit-diy_131.html#/30-flash_energy-10j_wsI'm interested mainly in just the charge and trigger. Looks like it can be trigger pretty simply, and It's in expensive enough that I could set up a few units to trigger simultaneously. I suspect the 1000uF 450v isn't enough and will likely need to replace it with something larger.I'll couple it with this 100w LED on COB that is supposedly rated for 9,000lm @ 28-36vdc@3000maeBay auction: #https://www.ebay.com/itm/10W-20W-30W-50W-70W-100W-High-Power-LED-Chip-COB-SMD-Bulb-DIY-Lamp-Light/263961952554?var=&hash=item3d755bc12a&enc=AQADAAAC8FjVrDbVsZ8oH%2F8PNHtt9VX4%2Fw7FZcmMuqsX8uaFEduV1JjTNcqxLENQ%2BAoiD9Vd%2Fo0t6JaYcgeiBJwDWg916eTMBW0ftEcw49m0a6WTi8bnXqMs1rT8%2FhEPw2EnUesyxZ4yaCmm3nWNnCBC77obZ03VtmDnTIEKS9Ra8EDyFGcD7BFW9%2BgJGn%2FX6rG1%2B%2F9YdGQAzZlDbBJ1KwsnmICW4EUSMkH4GvW%2BKjbiczw%2BnjyxtaHSdG9uBPK2AQHTlF9t6GJBJy19FBbuaz6d3Zneu2yCtAyqRKyR70YKKgJHYejwmZ7ID0t84YQJ8IhtgciwaCYRHvux4bD4%2B4UyzSPsDxVVVj8Hkp0nwaOKSUB383V29nAKRqyx6YYHi2wj%2FbLTYOsukafpYjDurl26JoRFx%2BnDIR12YoaoYkZvRUzQKvd%2Bg3L%2BQVGhqwQPrLGnpDiaDpxC6wnVJt95niIQ7ziF15W4hAKdaNra1Crqjdn2yEvljBVDCMP9GqL5IBxu08zjbPeeYTykPBuc3wXgVuwhXfqtV1cGCZr0MfxlOjRePdUwRXlNxOuuaH5Qps9lN1vZP4OuHqhBP%2BiaeiUc8o7NbyhS2bIpjagJX6xsc0WQRVLm4j2G6KHpCyICn7yBEwF3%2FuHWNFQPY3UhPf486TYHTxPeW5Y2dtGAI0oDlLt7Mf%2B45njYlojDp4VpRXER%2BkpRQiTJgiPzms2SLZnCOnVTVeBX0XlgFk41SY73Nujg3lAY9KkrpuAOyOtjwVJRH%2FvQR9ZwXTmswdeLyPa%2FBfUa7aznGxThaMRHONOhyZZYiotqe5%2Fjizn21%2BFlMxHaHZTiHVpgCDD1zw2pF9ZGY27SRDDllEVoa1d3Gzd0EPeV2jJ8A%2FweAWzGG5fTA3jiQEqXV42WQr9fNzu5kVWcHaEmQkKGEtChwYJm%2F13TJG7S6oJw7RbqoRUlxyPe6ZGsQnkINsEKSPQ3M%2Ft69st%2Bgs2ochwkyH0bU4ZsfMGUwzeLvMnA&checksum=26396195255498a2b9072f3042a086822bfd2b40e7f2From there I should be able to use the photo transistor in the circuit to measure the flash duration. At this point I'm into the project for under $40 and should be able to prove out the concept

author: Marco
author_rank: Super Contributor
timestamp: March 09, 2019, 11:23:36 pm
content: Just drive them with a MOSFET, the capacitive discharge method is for xenon tubes ... it's a bit silly for a LED.

author: ogden
author_rank: Super Contributor
timestamp: March 09, 2019, 11:39:44 pm
content: Quote from: JAndrew on March 09, 2019, 11:09:27 pmOnly thing you need from that flash is charge stored in the capacitor. Xenon flash trigger is no use anyway, LED needs high voltage transistor driven from high power gate driver (refer to 4ns LED flash circuit posted here). You may even leave that flash thing unmodified and operable if needed, just tap capacitor for your LED project. COB's you mention looks good as well (for starters), hopefully their bonding wires will not explode when overdriven. Flash voltage is specified 310V, so you need ~4 COB LED's in series for 200% power, then you "improve" it further until it explodes.Be careful. Hopefully you know how to handle high voltages, 1000uF@300V (~50 joules of energy) is deadly.Quote from: Marco on March 09, 2019, 11:23:36 pmRight. LED shall be connected to capacitor and after 500ns transistor shall be switched off. You do not leave LED after trigger connected to cap forever, such way it is impossible to get short pulses.

author: Marco
author_rank: Super Contributor
timestamp: March 09, 2019, 11:54:49 pm
content: That's why the 12V COBs are nice, if you use the 70W version which Big Clive reviewed then 36V should get you roundabout 20x the current. Nice low voltage.You might need one driver+MOSFET per COB, but the voltage stay safe.

author: JAndrew
author_rank: Contributor
timestamp: March 10, 2019, 12:00:59 am
content: I wanna make sure I'm understanding the idea.We use the charging circuit, but the rest of the circuit isn't needed since we're not driving a Xenon flash. Instead we tap in a MOSFET rated for medium voltage, and trigger the MOSFET (in this case we have a logic level MOSFET driving the larger MOSFET) to flash the LEDs.QuoteWhich one was this one exactly?QuoteI was hoping to stay away from high energy sources. It looks like it's going to be unavoidable. I'll be brushing up on the safety aspects of this project before embarking on it.

author: Marco
author_rank: Super Contributor
timestamp: March 10, 2019, 12:04:25 am
content: You need high energy, but you don't need high voltage.

author: ogden
author_rank: Super Contributor
timestamp: March 10, 2019, 12:28:32 am
content: Quote from: JAndrew on March 10, 2019, 12:00:59 amYou use MOSFET driver IC which have high current push-pull drive capability. Single mosfet driving other mosfet's gate will not be fast enough. You need both - *fast* turn-on and turn-off of the main MOSFET. Sorry, I did mean 4us flash, not 4ns. There's circuit in this post:https://www.eevblog.com/forum/beginners/4-microsecond-high-power-pulses-through-led/msg1417075/#msg1417075QuoteHigh voltage is avoidable, you just need bigger wires, capacitors and transistors because currents are higherAs Marco suggested - to avoid high voltage you may use 12V COB's in parallel, then you mau get away with safe <50VDC voltage. Which way to go - up to you. We (forum members) are not your parents.

author: JAndrew
author_rank: Contributor
timestamp: March 10, 2019, 01:24:42 am
content: I appreciate all the input.QuoteDefinitely a worthwhile read through. The schematics he posts at the end to show his completed projected aren't in a format that I am used to reading.EDIT:I've studied his schematics and everything he's doing makes sense. I am assuming the length of the pulse is being controlled by the software on the MC.I get it now on the MOSFET Driver, I haven't used one before, but it makes sense. All looks very doable. I wish he posted an update on the how the project turned out. I checked out his YouTube channel but that video looked to be the last he had posted on the topic.

author: Zero999
author_rank: Super Contributor
timestamp: March 10, 2019, 11:39:04 am
content: Quote from: JAndrew on March 05, 2019, 11:06:50 pmYou mean phosphor, not phosphorous, which is flammable.One problem I've had with over-driving LEDs is colour shift, especially with green LEDs. I haven't noticed it with red and didn't test blue, but green LEDs shift towards the blue end of the spectrum, overdriven. This effect is clearly visible, when I tried drivinga green LED at ten times the rated current, but at a low duty cycle its colour drifted from a bright green to turquoise, almost cyan hue. It wasn't a problem with any of the phosphor LEDs I tried.Some phosphor LEDs have a significant afterglow, others don't. The green phosphor LEDs I tested were very fast. No measurable afterglow, with instant on and off times. The yellow ones were the worst, with a slow on time and lots of afterglow. If I get the time, I'll update with part numbers and an oscillogram.See the paper linked below about LED pulsed power. At the start of my tests I used a similar circuit, with the MOSFET driving the low side and the current sensor in series with the source, but I abandoned it in favour of using transformers for both driving the LED and current measurement, as the galvanically isolating the test equipment gave better, more reliable readings.https://core.ac.uk/download/pdf/30991250.pdf

author: Marco
author_rank: Super Contributor
timestamp: March 10, 2019, 03:49:56 pm
content: Quote from: JAndrew on March 10, 2019, 01:24:42 amMicrocontroller control only really makes sense if the micro triggers both the chain of events you want to photograph as well as the flash.If you are say pulling the trigger on a gun with a string it makes much more sense to just keep everything analogue and trigger on sound or a light gate, no micro necessary.

author: Sceadwian
author_rank: Unknown
timestamp: March 10, 2019, 04:12:02 pm
content: Is this even possible with white LEDs? I thought the phosphor delay was WAY more than the nanosecond range?

author: Zero999
author_rank: Super Contributor
timestamp: March 10, 2019, 05:12:21 pm
content: Quote from: Sceadwian on March 10, 2019, 04:12:02 pmIt depends on how fast and the LED. I posted an oscillogram of the CMA3090 being pulsed at 1µshere. As you can see there some delay in the on and off time, but not much. As I said in my previous post, some phosphor LEDs are worse, others are better.What's odd about phosphor based LEDs is, although the initial decay is fairly fast (even the slow yellow one I tested had a time constant in the µs range) a very faint afterglow persists for a long time after the LED is turned off. This is no problem for a photoflash application, because it's several orders of magnitude too small to cause a problem: it's just about detectable, with dark adapted eyes, in a very dark room. I've seen it for myself. I ruled out persistence of vision and charged capacitors by closing my eyes, exposing the LED to a backlight (UVA) whilst my eyes were still closed, turning off the UV source and looking at the faint afterglow from the LED. I have only done this with white LEDs and don't have the equipment to make quantitive tests, otherwise I would.

author: JAndrew
author_rank: Contributor
timestamp: March 10, 2019, 06:20:47 pm
content: Quote from: Zero999 on March 06, 2019, 10:16:29 amI'm working on digesting all the idea's that have been suggested here. Zero999, I'm just making sure my flea brain is understanding the diagram, you use an inductor to drive the LEDs rather then using a Flash Capacitor.I appreciate the lead on the LED you were testing. Vela mentions they spent a lot of time trying to find the right LED's for the job. I suspect this gets back to the issue of how long the phosphor stays excited. I'm fairly confident they are using white LED's, the images of the unit show the LED's and the yellowish tint characteristic of white LED's is clearly visible. This would simplify the circuit, as we wouldn't need to worry about white balancing the RGB LED's. Since LED's MFG's aren't typically concerned about the switching time of a White LED, I can image there is a lot of variation, possibly even from lot to lot.Quote from: Marco on March 10, 2019, 03:49:56 pmAs far as triggers go, I was planning on using an IR optical sensor. I've put one together before, and have had fairly good success with it. As the bullet passes by the "shadow" creates a signal. The trouble with using this directly is that the duration of this pulse is dependent on bullet velocity and bullet length. Fast short bullets would create pulses that would likely be far less then 500ns, while long slow bullets would create a pulse that would last far longer then 500ns. (These type of sensors are commonly used in high end chronographs)So we would want the MC to trigger on the pulse, with a programmable delay (just a few microseconds) before sending the trigger for the flash. By having a short delay, we can dial in the position of the bullet in the frame without physically moving the set up. I know I've tried to use an Ardiuno to create a short 500ns square wave but I seem to remember that it struggled. I don't think the code was fast enough to switch the I/O pin that quickly. It will be another problem I'll need to work through.To Marco's comment of using low voltage and high amps, I was thinking on that last night. A 10s (37v) Lipo battery rated at 4000mAh could provide a pretty good jolt of energy across an LED. With the duration being so low, I don't think there would be any lasting damage to the Lipo and it would be good for quite some time. There may be other reasons that this wouldn't work, but I was brainstorming.

author: ogden
author_rank: Super Contributor
timestamp: March 10, 2019, 06:24:57 pm
content: Quote from: Marco on March 10, 2019, 03:49:56 pmRight. Attiny and software definitely is not up-to task for external trigger application. It possibly could be done in *hardware* of other MCU - like advanced timer of stm32 clocked at 72MHz. Using such one can make adjustable delay and pulse length with 14ns error. If programmable delay is not needed then just pulse length has 14ns error.

author: Sceadwian
author_rank: Unknown
timestamp: March 10, 2019, 07:03:35 pm
content: Yeah I missed some of those earlier posts thanks, some good info there. I've noticed the phosphor afterglow myself apparently it's normal. The action/reaction of phosphors has never been anything I"ve looked into, get into chemistry and some atomic physics that have a bit more nuance than I care to get intoThanks for the graphs though! Very helpful.Quote from: Zero999 on March 10, 2019, 05:12:21 pm

author: Marco
author_rank: Super Contributor
timestamp: March 10, 2019, 07:07:55 pm
content: Quote from: JAndrew on March 10, 2019, 06:20:47 pmDo an analogue delay, a current source charges up a capacitor, you compare it against an adjustable threshold with a fast comparator.QuoteNope.The amount of energy you're burning is tiny, so you just use electrolytic capacitors (they actually have pretty low inductance). It's advantageous to keep the powersupply which charges those capacitors low current, because if something does go wrong by any chance that limits the continuous current which can flow so everything can't blow up. Unlike what happens if you short that LiPo stack.None of this is entirely trivial, it's quite a complex project. I think I'd design it with a comparator on the input with adjustable threshold and feedback to the input with a diode (so once triggered it stays on, until you arm it again with a physical button, add a signal LED so you can see if it's armed). This will turn your trigger signal into a step signal. The step signal goes into your pulse generator with adjustable delay and pulse duration, a fairly complex circuit ... too complex to describe in text. That then goes to the drivers/LEDs/MOSFETs. You need 2 boost converters. One boost converter for the MOSFET drivers, one boost converter to charge the flash capacitors. Also one linear regulator for the trigger section.All in all, a lot of components.

author: JAndrew
author_rank: Contributor
timestamp: March 10, 2019, 08:36:29 pm
content: QuoteI definitely see that. I've learned quite a bit in the last 48-72hrs just going through everyone's responses. Honestly its likely a little above my current level of understanding. If I am going to make this work I'm going to have to spend some time on smaller and slower circuits. Particularly looking at/recreating the examples of what has already been done.I appreciate everyone's input, I definitely have a better sense of where to start, and how to eventually get there.To be clear, you can still take some pretty awesome pictures with flash duration of 2-4 microseconds. I just set a target, based on the specs of the equipment I had used prior.

author: Zero999
author_rank: Super Contributor
timestamp: March 10, 2019, 09:01:56 pm
content: Quote from: JAndrew on March 10, 2019, 06:20:47 pmNo, C1 stores all the energy. None of the inductors are used for energy storage. T2 is a pulse transformer which drives the MOSFET. T1 is a toroidal current transformer, which measures the current through the LED. The current in the secondary is divided by the turns ratio. In the above schematic, the current through the secondary and R3 and R6 (the termination resistor at the oscilloscope end to match the 50Ω cable) is1/100of that in the single turn primary. I used several different transformers, depending on the current. Firstly a small 4mm ferrite ring with a 10 turn secondary and a 0.1Ohm resistor for R3, which was fine for up to 5A or so, then the Murata 54100C (the one in the above schematic with both primary windings in parallel (datasheet linked below) and eventually a 10mm ferrite ring with 21 turns and 2R2 for R3. The reason for 21 turns was R1||R6 = 50||2Ω2 = 21, giving 0.1V per Amp in the primary.https://docs-emea.rs-online.com/webdocs/0eb1/0900766b80eb1ad9.pdfhttps://www.grainger.com/content/supplylink-what-is-a-current-transformerQuoteI have used a Vela strobe before for another test a few years ago. Unfortunately it failed to meet our requirements. It was bright enough, the problem was there was a huge seemingly random delay 8µs to 24µs, if I remember rightly, between the trigger pulse and the strobe firing. It used lots of LEDs in series and a microcontroller-based control system, which I believe was responsible for its downfall. If the variable triggering delay isn't an issue for you, it's a perfectly good strobe, otherwise steer clear of it.QuoteI would not recommend using an MCU to control the triggering, certainly not directly, without any additional protection. If there's a problem with the firmware, it could blow up all the LEDs. A CPLD or even discrete glue logic gates are ideal. Whatever you decide, another layer of hardware to protect against too longer pulses, at too higher repetition rate, is a good idea. A high pass filter can be used to limit the maximum pulse length and a monostable multivibrator to control the minimum delay between pulses. Connect both to an AND function, before the MOSFET driver: MCP14A0301 looks good because it has an enable function, so the AND isn't required. I'd go for the 74HC123 for the monostable. It needs to be negative edge triggered with an inverting output, so the MOSFET driver is enabled, when the input goes high, then disabled, after the input goes low and triggers the monostable.

author: Giaime
author_rank: Regular Contributor
timestamp: March 11, 2019, 08:07:11 am
content: Quote from: ogden on March 08, 2019, 09:35:04 pmHere you can see a measurement on one of the systems I work with. This is exactly what we're discussing: a DC/DC already running from some time in short circuit, suddenly the shunt FET is turned off (you can see it's VDS - green - rising). This is a 1200ns pulse (digital1 is uC PWM) but you can clearly see it can be cut down to 500ns.The limit here is the risetime of the current-voltage on the output node, largely due to the interaction of DC/DC current and node capacitance. Here for EMC reasons there was also 1nF additional capacitance at the output, if you skip this you can go faster. Also higher DC/DC currents helps a lot, here it's only 400mA.I have a lot of experience with this kind of systems, if you or the OP needs some more info I'm here.

author: ogden
author_rank: Super Contributor
timestamp: March 11, 2019, 08:50:53 am
content: Quote from: Giaime on March 11, 2019, 08:07:11 amOne scope screen is worth thousand wordsThanx! I am really surprised that it works so well. Until now regarded shunt PWM as snake oil, especially when saw na'ive simulated waveforms in the TI datasheet. What is specs of inductor and LED used, what is frequency of the converter while shunt is short/open?

author: StillTrying
author_rank: Super Contributor
timestamp: March 11, 2019, 10:15:53 am
content: I can't see how this inductor storage works for 1 very big triggered LED flash.The single ~1us pulse applied to the LEDs needs to be around 30A to 100A at 35V to 120V.

author: ogden
author_rank: Super Contributor
timestamp: March 11, 2019, 10:40:14 am
content: Quote from: StillTrying on March 11, 2019, 10:15:53 amIt does not. We already discussed here that capacitor is much better "tank" for LED flash energy storage. I was arguing that shunt PWM cannot achieve speeds required for application, yet here we have proof that I was wrong. Yes, "shunt PWM" is not suitable for LED Flash, yet I (personally) do not see any problem to discuss it here.

author: mzzj
author_rank: Super Contributor
timestamp: March 11, 2019, 11:27:43 am
content: Quote from: ogden on March 09, 2019, 09:14:24 pm10k Lm out of 100W COB is hardly any sort of state-of-the art nowadays so its certainly possible even for low cost chinese COB.Probably the chinese are bit "optimistic" and you get only 8kLm and lifetime is certainly a mystery vs. more expensive brand name product.

author: Siwastaja
author_rank: Super Contributor
timestamp: March 11, 2019, 12:23:11 pm
content: Quote from: ogden on March 11, 2019, 08:50:53 amShunt switching, driven from a constant current supply, is widely used in 3D Time-of-flight designs, with pulses down to around 10ns. I designed mine using a voltage source and series resistor instead, encouraged by our chip supplier's reference design with such a simple topology, but OTOH I'm only modulating at 20MHz (hence, 25 ns pulses with 25ns offtime). Works well enough for us, although we do see a drop in differential "payload" signal amplitude at 20MHz, but I'm almost certain this is limited by the LEDs, more than by the switching topology.

author: Zero999
author_rank: Super Contributor
timestamp: March 11, 2019, 01:50:35 pm
content: Quote from: mzzj on March 11, 2019, 11:27:43 amYes, the CMA3090 I tested is rated for 12k Lm at the maximum rated drive current of 3.6A. Over-driving does increase the intensity, but it's not linear, especially at higher currents, which result in diminishing returns. According to the results of my experiment, with an LED drive of 35A, the photodiode current was 5.35 times, that when the LED was driven at the rated current, which seems a reasonable and is way below the 120A which was destructive. You could have an array of 16 LEDs, each driven at 36A to give 1M Lm.

author: Marco
author_rank: Super Contributor
timestamp: March 11, 2019, 02:08:24 pm
content: Quote from: Siwastaja on March 11, 2019, 12:23:11 pmWith much lower pulse energy. Inductive energy storage has multiple problems as you try to scale it up in energy. Bandwidth, inductor cost and standby power consumption. Bandwidth isn't really the problem here, inductor cost and standby power consumption are nearly insurmountable problems. You can either have a couple bucks worth of electrolytics which can run on AA batteries, or you can have a couple 100 bucks worth of inductors which require a mains connection and active cooling.Get back to this when we get room temperature superconductors.

author: David Hess
author_rank: Super Contributor
timestamp: March 11, 2019, 03:20:58 pm
content: Quote from: Marco on March 11, 2019, 02:08:24 pmIf a fixed pulse width is acceptable, then a charge line or for longer pulse widths, a lumped delay line made from inductors and capacitors can be used instead of an inductor.The way to do it in this application though is a switched current source or switched voltage source and ballast resistor.

author: Marco
author_rank: Super Contributor
timestamp: March 11, 2019, 04:02:59 pm
content: With a switched current source with ballast you retain the problem of the amount of circulating current.If you go the low voltage COB way, you could easily be talking about 1000A of current. That current isn't started a ms or so before the trigger either. Much of the time in these photography setups the events are kicked off by slow humans, so count on that current to be circulating for minutes rather than milliseconds. Even if you went for the very complex solution of circulating the current through a regenerative power supply so you don't have to burn the full Vs*I, keeping a 1000A running through some fast MOSFETs is going to burn way too much power to run this on a couple of AA batteries.It's not worth it, he's not looking for single digit ns rise/fall times or 1% precision regulation of the current. Just dump some voltage on and take it off again with a MOSFET, good enough.

author: David Hess
author_rank: Super Contributor
timestamp: March 11, 2019, 04:34:01 pm
content: Quote from: Marco on March 11, 2019, 04:02:59 pmSorry, in this case I meant driving a current into a cascode transistor (emitter/source switching) which then handles the voltage compliance. An actual switched current source could be much faster but would also dissipate the peak power continuously.The advantage is that the switching transistor can be much smaller for lower capacitance and higher speed and the fixed voltage across the resistor yields a more accurate and regulated output current.

author: Marco
author_rank: Super Contributor
timestamp: March 11, 2019, 07:31:14 pm
content: I guess a little overvoltage with active current regulation makes sense given the likely high parasitic inductance of a COB (anyone measured it?). I don't really see the need for a cascode though, a simple source resistor with a BJT to pull down the gate should be enough to get decent regulation. Or just a source resistor period for that matter.

author: Zero999
author_rank: Super Contributor
timestamp: March 12, 2019, 05:06:35 pm
content: If a triangle wave will do, an inductor can be used to limit the current. I did a quick simulation in LTSpice and a 50nH series inductor with a freewheeling diode could theoretically limit the current to 90A into the Luminous devices PT-121-B, which is similar to one of the LEDs I've tested. 50nH is small and theoretically could be an air core.