4 microsecond high power pulses through LED - Page 1
Forum section: electronics / beginners

author: mribble
author_rank: Regular Contributor
timestamp: January 31, 2018, 04:19:07 pm
content: So I'm designing a circuit to blink some LEDs for 1-4 microsecond durations. I've read via several papers that you can overdrive LEDs by about 50-100 times beyond standard max power for short durations like this, but I need to do some testing. So I made a test PCB to try different capacitors, current limiting resistors, and LEDs.I've attached a drawing of the relevant parts of my circuit. I also just realized it's missing a current limiting 1 ohm resistor above between 30V and the LED.The problem I'm seeing is the shown in the attached scope trace. The yellow line is current of the 1 ohm resistor. Each box represents 10A, so the average triggering current is about 15A. The cyan line is voltage across the LED. Each box represents 10V so the average triggering voltage is about 10V. On the x axis each box represents 500 ns so the total trigger time is about 4 us as expected.I see 2 problems.One thing that concerns me is the ringing. Do you guys know what would be causing that? Is it possible that it's just my sloppy wiring and it will go away when I get everything on a nice better PCB? (Remember it's a test pcb so I didn't minimize trace length because I wanted to try different components, but the traces are quite wide where they need to be)The other thing that concerns me is the downward slope of voltage and the upward slope of current during triggering. This didn't show up when I tested charging the cap to 20V, but is is easy to see at 30v on the cap (I decided I wanted to try and understand this before trying higher voltages). The total average power in this 30V test is about 3x the 20V test (150W vs 50W).I checked and I'm confident the dropping voltage isn't that the cap is being discharged so it's something else.Here are the parts I'm using. I know the voltages and currents for these parts are well beyond what I need right now, but I was planning to test with much higher voltages.* Cap is 220uF and 250V. ESR is rated at under 1 ohm, but I don't know how they measured that (my understanding is ESR can be pretty different depending on how it's measured).https://www.mouser.com/productdetail/cornell-dubilier-cde/slpx221m250a1p3?qs=sGAEpiMZZMvwFf0viD3Y3ROGmxq83YAGBlZdwq9XP7Y%3D* Cree 5W (10V) LED -https://www.mouser.com/productdetail/cree-inc/mhbawt-0000-000c0bd250e?qs=sGAEpiMZZMu4Prknbu83y3sOCTnhdZkGJ0P2fVkJFMdF%252b96cyz53ew%3D%3D* Big mosfet -https://www.mouser.com/productdetail/infineon-technologies/irfs4227trlpbf?qs=sGAEpiMZZMshyDBzk1%2FWi5%252bqVgN3%252bWS8ZsC7Yl6o%2FTc%3D* Schottky diode -https://www.mouser.com/productdetail/stmicroelectronics/stps4s200s?qs=sGAEpiMZZMtQ8nqTKtFS%2FM1SZ0THNZ7sE6Rykgl7LP%252bZ3tc7dz%252bcbQ%3D%3DA big thanks for any help you folks can provide!

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: January 31, 2018, 04:45:45 pm
content: A parallel resistor across the LED may help with ringing. LEDs aren't designed with low inductance in mind so it may be worth trying a few differnt ones to see which works best.Note that 100x power will not give anything remotely like 100x brightness as LED efficiency drops at higher currentsWith white LEDs there may be a point where the phosphor saturates, so you won't get any more.

author: mribble
author_rank: Regular Contributor
timestamp: January 31, 2018, 05:05:58 pm
content: What kind of resistor across the LED would you suggest trying? Just a starting point for resistance is what I'm looking for.I'm aware that 100x power won't give 100x brightness. I've got a setup to measure power consumed by the LED and measure brightness so I'm going to graph that to the point the LED dies and then pick a point that seems to work.Would it be an option to just ignore the ringing I'm seeing? I means if it's doesn't damage anything I don't think I have an issue with it...Thanks!The bigger problem right now is why does my voltage drop off in that scope trace. I don't really want to test higher voltages until I get that figured out.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: January 31, 2018, 05:14:31 pm
content: Quote from: mribble on January 31, 2018, 05:05:58 pmIf nothing else it will help get a cleaner off-time. Value maybe to take a few % of youre "on" current.Also, there may be an advantage to biasing the LED to a "just off" voltage with a resistor divider, to get a faster turn-on as teh stray capacitance will already be partly charged.Also, how are you probing it? You need to have a very short ground to minimise inductance, using the ring at the probe tip

author: mribble
author_rank: Regular Contributor
timestamp: January 31, 2018, 05:29:50 pm
content: Quote from: mikeselectricstuff on January 31, 2018, 05:14:31 pmI'm just using my scope. If by "ring at the probe tip" you mean that standard 3-ish inch ground clamp on cheap rigol scopes then I'm doing that. If you mean something more excotic then I'm not doing it and you'll need to explain it to me.

author: tggzzz
author_rank: Super Contributor
timestamp: January 31, 2018, 05:38:45 pm
content: Don't forget the standard equation for changing currents and inductors: V = Ldi/dtwhere V is the induced voltage.Work out your change in current, di, and the time in which it changes, dt. Then assume as a starting point that a wire's inductance is 1nH/mm.Also inductor + capacitor => resonant circuit, resonant frequency f =1/2*pi*sqrt(LC)So, given how you have constructed your circuit, what induced voltage and resonant frequency would you expect?

author: Zero999
author_rank: Super Contributor
timestamp: January 31, 2018, 05:59:26 pm
content: The ringing is due to the inductance, resonating with parasitic capacitance. The LED doesn't appear to be too inductive, going by how it's packaged.Yes, a resistor across the LED will cut down on the ringing, as well as improve the off time. You may also want to consider another MOSFET, in parallel with the LED, forming a half bridge, to improve the off time further. Keep the MOSFET and capacitor, as close to the LED, as possible.

author: Dubbie
author_rank: Unknown
timestamp: January 31, 2018, 06:23:10 pm
content: Using the ground lead on the scope will show ringing that isn’t really there. Take it off and use the little ground probe spring instead.

author: mribble
author_rank: Regular Contributor
timestamp: January 31, 2018, 06:39:14 pm
content: Thanks for all the help so far!tggzzz, thanks for that equation. I have never had to worry about that sort of thing before (I'm just a software guy). After reading up on it I think I understand it now. However, I don't know the inductance of my PCB traces and wires. That said, I know I can reduce those things in my next PCB design. I suppose I could figure out the inductance by looking at the voltage (and I know di/dt), but there really isn't a point. I think I just need to design the pcb with minimum inductice on this critical loop. Basically do what Hero999 suggested of getting the cap, led and mosfet as close as possible.I tried putting a 220 ohm resistor in parallel with the LED and it didn't seem to help noticeable. I guess I should try a smaller value. Can I just use a small wattage resistor here since it will only be pulsed on for 4 microseconds or less? I don't actually know how much current is going to go through the resistor since my mental model of an LED is just the ideal theoretical LED and in that case no current will go through the resistor.I don't really want to go the route of adding an extra mosfet on the LED. The big reason here is because I am just testing the baby version of this circuit right now. The next PCB I make will be the full version which has 36 LEDs (6 strings of 6 leds with the cap running at around 150V and peak currents around 120A). For that circuit adding all the extra mosfets would be an issue. However, I also think the ringing in that circuit will be even worse since the di/dt will be bigger.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 31, 2018, 08:40:24 pm
content: You, uh, can't probe the voltage across the LED, unless you have a differential probe. All the probe grounds are the same common ground.Tim

author: tggzzz
author_rank: Super Contributor
timestamp: January 31, 2018, 08:42:42 pm
content: Quote from: mribble on January 31, 2018, 06:39:14 pmUntil you have a better figure, assume 1nH/mm - and do the calculations!

author: kony
author_rank: Regular Contributor
timestamp: January 31, 2018, 08:54:24 pm
content: Curious about usecase - what is this source for?

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: January 31, 2018, 08:55:09 pm
content: Quote from: mribble on January 31, 2018, 05:29:50 pmYou need to keep the ground lead as short as possible - no longer than 10mm. The ringing you are seeing may be entirely the result of using a long ground lead.This is what I mean about using the tip ring :https://youtu.be/-siuk4p4dO0?t=1083What you are most interested in is the current through the LED, the easiest way to measure this would be to connect a low value resistor, maybe 10mR in the ground connection to the MOSFET, and scope across this with a x1 probe, to give a 10mV per amp signal

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: January 31, 2018, 08:57:08 pm
content: Quote from: T3sl4co1l on January 31, 2018, 08:40:24 pmthe PSU does not need to be grounded - with a floating PSU you can ground the scope to the +ve rail, but make sure the PSU ground really isn't connected to mains earth.

author: mribble
author_rank: Regular Contributor
timestamp: January 31, 2018, 09:44:36 pm
content: Quote from: T3sl4co1l on January 31, 2018, 08:40:24 pmThis explains some strangness I was seeing. From your description I understand why I can't probe across multiple sources, but just probing only the LED seems to trigger the system and I don't understand why. If I'm only using one probe it shouldn't matter if the scope sees ground as the point between the LED and mosfet, right? There is something fundamental about the scope's probe I still don't understand...Quote from: kony on January 31, 2018, 08:54:24 pmThe use case is a microsecond flash for photograph. It is fast enough to freeze bullets in mid-air. Previously I worked with airgap flashes, but this should be a much cheaper and safer (relatively speaking) way to do it. There are also several other advantages that I won't get into unless you're really interested. These first tests already proved to me that I can get enough light so I am pretty sure I can get this working with some more effort now that I'm sure the photons are there.Quote from: mikeselectricstuff on January 31, 2018, 08:55:09 pmRight, I already have a 1 ohm 5W resistor to limit current and have been measuring current through it. The most important thing is current like you said, but voltage is also useful since I need to know what voltage to apply to power a string of these. I think I have that voltage measurement, but as I mentioned above it's acting kind of odd when I probe the LED voltage. Anyways current through the resistor measure fine so that's what I focused on. I made a spring tip like you suggested and that didn't seem to impact the ringing (might of helped some, but not enough for me to easily notice). I suspect the issue is my PCB has about 200 mm of traces between the critical components and another 100mm of ground plane (not sure if the ground plane portion of the loop counts.Quote from: tggzzz on January 31, 2018, 08:42:42 pmI did that and I estimate I have 200mm of traces in the critical loop and another 100 mm of ground plane. Not sure if I should include the ground plane or not so I'll do it both ways. I estimate my di/dt current change is 100 million amps per second (20A in 0.2 microseconds). So that gives me an induced voltage of 30V which is in the range of what I'm seeing on the scope. For resonance frequency I using some really rough number I get 9nS which again is within an order of magnitude of what I'm seeing on the scope.I think this is likely my issue. Now that I know how important this is for this circuit I'll make sure I spend time optimizing my next PCB for all this. I basically want the thickest, shortest traces. What I'm not sure about is if the ground plane under all this help this kind of circuit and how bad vias are?Quote from: mikeselectricstuff on January 31, 2018, 08:57:08 pmAh, this explains the strangeness I was seeing when trying to measure the measure the led voltage with my scope. I totally forgot about this. I can switch to a floating power supply (batteries) if I want to measure it in the future.Thanks everyone, you have all been super helpful to an ignorant software guy trying to struggle though this stuff for the first time!! I keep thinking I must know enough about electronics now for my IOT devices, and then I keep finding some new use case that takes me totally out of my EE comfort zone.

author: mribble
author_rank: Regular Contributor
timestamp: January 31, 2018, 09:51:03 pm
content: Actually, I just realized this answers my question about the ringing. Does anyone know what would be causing problem #2 in my first post? The downward slope of voltage on the scope. Is that someone a measurement error from my scope because of the ground issue? I kind of doubt that because I also see current sloping up and that measurement should be ok. Is this somehow related to my ringing problem?

author: kony
author_rank: Regular Contributor
timestamp: January 31, 2018, 10:14:47 pm
content: Yes, I'd actually like to hear more detailed explanation about the usecases. Similar fast pulse LED sources are used in older methods for volumetric flow recording, stumbled upon that recently and hence I was curious if it is anything related.

author: tggzzz
author_rank: Super Contributor
timestamp: January 31, 2018, 10:37:49 pm
content: Quote from: mribble on January 31, 2018, 09:44:36 pmI suspected it might. BTW time is measured in seconds(s) not Siemens (S)!QuoteYou are on the right track (ho ho). Ground planes will definitely help, vias won't be a major issue at these frequencies, but do be sure there are no gaps in the groundplane. The rule of thumb is that in a ground plane the return current "likes" to follow underneath the signal current, so don't route critical signals across gaps in the groundplane.When you have time, google for "Bogotin's rules of thumb" on EDN. They provide a succinct glimpse into what lies ahead.QuoteIt is a pleasure to help someone that thinks, listens, does the work, understands, and makesnewmistakes next time around.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: January 31, 2018, 10:52:33 pm
content: QuoteYou should be aiming to get all the current paths as short as possible, and minimise loop area - tens, not hundreds of mm.For a large array, you'll probably be better off with multiple caps and mosfets. This will also give flexibility to do things like sequentially firing different sections.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: January 31, 2018, 10:53:35 pm
content: BTW I thought this sounded familar :https://www.kickstarter.com/projects/vela/vela-one-the-worlds-first-high-speed-led-flash

author: DrGeoff
author_rank: Frequent Contributor
timestamp: January 31, 2018, 11:04:30 pm
content: Check di/dt for the LED you are going to use. These figures may not even be published for a LED.However a short duration high current pulse through a semiconductor can punch a hole through the junction due to carrier mobility restrictions.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 31, 2018, 11:14:30 pm
content: Quote from: mribble on January 31, 2018, 09:44:36 pmTo a first degree -- if you have an isolated supply, or more basically, no other ground connections to the circuit -- yes, you can clip the probe to the switched node.BUT, you'll still get a waveform similar to what's pictured, because there is no such thing as perfect (as in radio frequency) isolation. The PS, and scope (if ground is lifted), have capacitance to ground (or mains), so you get a long loop, from probe to scope cord to power supply cord to supply leads, which has a modest inductance (some uH). The capacitance and inductance resonate, and you get ringing, and currents induced in the circuit that wouldn't otherwise be there (your measurement disturbs the circuit -- no, it's not just for quantum mechanics!).The preferred way is to keep grounds...groundly. The +30V and 0V nodes will have little AC on them, so are a good reference to work from. (One or the other, not both, of course!) The probe tip then can be clipped onto whatever circuit node is of interest.Note that, if you ground to +30V, all signals are "below" ground, so you will see negative voltages. Just make the mental sign-flip (or tick the "invert" option on the input channel menu, if you have one?).So that solves the 2nd order problem (when is isolation, not isolation? -- capacitance!).You're still not completely free of ground noise, because there could be some crossed paths yet, at high frequency. Remember that grounding to the switching node. puts switching voltage on that huge loop? Well, any segment of that loop has a fraction of the full loop's inductance. That is to say, a loop is a single inductor, which is equivalent to many small inductors in series. Inductors in series means you have a voltage divider among them. So, even the few inches of probe ground clip, has a nonzero voltage drop! Or even the few cm of traces in the circuit, or the few mm of lead length on the components (and the bond wires inside them).You probably won't have to worry about extremely low probing inductance now (the spring clip Mike's talking about) -- but that is definitely the preferred method.By the way, the probe itself has some impedance, which draws current from the circuit, and that current returns through the probe ground (clip or spring, whichever you're using). Which, since that ground is a wire of nonzero length, it will also have some inductance, and some voltage drop at high frequencies. That voltage, in turn, still appears across the big loop (probe cable, mains cable, back to the supply), so you can still experience ringing from that path, even though your probing technique is otherwise quite good, and your ground point is otherwise quiet. In this case, you already have one intended current path: the path through the probe ground clip. You can reduce the current through other paths (the large loop) by increasing the impedance of those paths. How? Add ferrite beads to the probe cord.Note this is a 3rd order consideration -- usually not significant, but it is nice to understand what's going on, especially when you're working with frequencies high enough (risetimes short enough) that you need a spring clip and whatnot.(Basically, if you don't see a difference from clipping a ferrite bead on the probe cord, it's probably weak enough not to matter, and the 2nd order approximation will be fine.)Tim

author: khs
author_rank: Regular Contributor
timestamp: January 31, 2018, 11:20:06 pm
content: First I would not care about ringing.First I would take a look at the optical output of the LED with a fast photodiode because the datasheet of the LED tells nothing about the optical risetime/falltime - if I'm not wrong.There are not only the (optical) rise and fall times of the LED behind the phosphor, there may be a 'lifetime' of the electrons of the phosphor too. This can cause an additional slow down of the fall times.Maybe it works with your LED. Many it works not. I would not be sure.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 31, 2018, 11:30:04 pm
content: Quote from: khs on January 31, 2018, 11:20:06 pmI think -- suspect? -- that the phosphors typically used have a modest lifetime (microseconds or less), so it should work out pretty okay here. There may be quite a lot of "drool" (a 1/f tail, or a fraction at much longer time constants?), I don't know.I know they're not good enough for that "LiFi" thing some people were talking about -- that uses higher frequency modulation (10s of ns) and is only sensible through a blue filter (the blue LED shines through a yellow phosphor to make white light, so the blue spectral line is not delayed or anything).If you needed absolute fastest (fractional us), you could make an array of RGB LEDs (no phosphor -- also, higher overall efficiency I think?). You'd have serious problems with power density though (LEDs aren't very useful at peak currents over 10-100x rated current), for which you'd probably have to fall back on the air spark anyway. Fortunately, there aren't many non-military things that move that quickly..?Tim

author: mribble
author_rank: Regular Contributor
timestamp: February 01, 2018, 12:12:09 am
content: tggzzz, those Bogotin's rules of thumb" on EDN look pretty dense, but I'll definiently work my way through them before I do my next pcb layout for this project.Quote from: kony on January 31, 2018, 10:14:47 pmHere is a blog post on airgap flashes I posted years ago.http://www.glacialwanderer.com/hobbyrobotics/?p=490They work and generate more light, but their biggest downside is the physics they use to force the discharge of the spark is charging a quartz tube and that process is not very exact. I've tried to optimize it in the past after talking to some people at MIT and the best I could get was about 2 microseconds of variance. In most of my cases that is fine for a single flash, but it leads to noticeable ghosting when trying to use 2 or more flashes, which is pretty common for me. My actual use great examples of use cases for high speed photography. I've heard of volumetric flow use cases you mentioned, but I've never done those. I have done Schlieren photography in the past. But mainly I just like doing things that look cool like little explosions or bullets hitting things. I just feel this type of photography looks beautiful, and it's a fun challenge to freeze the moment in time that you want. I also make a high speed triggering system called the Camera Axe (https://www.dreamingrobots.com/pages/camera-axe). I make all my designs (including this led flash if I get it working) open source and sell a small number of them on my store for those who don't want to make the items themselves.Quote from: mikeselectricstuff on January 31, 2018, 10:53:35 pmYes, I actually own two of those. They are pretty good, but have some major downsides for the type of photography I do. Some of the big issues with it are that it's too big to fit where I want it to and it has no optics so it wastes the vast majority of its light. Another issue is they cost $1200. I figure I can make a smaller version that does what I want for a fraction of that cost.Quote from: DrGeoff on January 31, 2018, 11:04:30 pmThat spec isn't in my LED datasheet, but it's interesting that some LEDs might have the data. I'll do a little looking to see if I can find that on any LED datasheets. That said, I am going well outside the specs. However, like I said there are are handful of papers where universities go well beyond the specs of LEDs and say as long as your pulses are short they should work fine. I've done some testing and have 10K flashes on an LED at 50x it's rated wattage in 4 microsecond bursts and the LED is still working fine matching what these papers found. Even Cree published a paper saying if you want to overdrive our LEDs for short durations you do so at your own risk, but it will probably work.http://www.cree.com/led-components/media/documents/XLampPulsedCurrent.pdfTim, that was a lot of info about measurements. Normally this high frequency stuff doesn't matter to me, but it sure bit me here and at least your 2nd order seems applicable to me. My take away is to keep grounds being ground if I can.Quote from: khs on January 31, 2018, 11:20:06 pmGood point. I actually already tested that awhile ago to confirm the phosphors didn't have an afterglow, which I've read some old while LEDs had. I did not see any ringing in the output of the LED. The output of the LED looked like a nice square wave on my high speed light diode sensor. But I did this testing at 20V (which had the ringing, but not the voltage drop off). I will test again at 30V to see if the output of the light is actually dropping. If anything changes with the 30V test I'll give an update, but I'm kind of expecting to see a nice square light output there too.T3sl4co1l, my findings agree with your comments about phosphors. There is a tiny amount of tail, but it was less than 0.1 us. Since I only care about relatively long1us durations this doesn't matter to me.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: February 01, 2018, 12:20:35 am
content: Bear in mind different manufacturers may well use different phosphors - even shining a UV LED light at a random bunch of white LEDs produces significantly different amounts and colours of fluorescence, so may be worth trying some different types.RGB might be interesting but combining all colours homgeneously might be tricky. Sequencing the colours might make for some interesting shots as you will be able to tell what order things happen in. RGB is certainly nowhere near as efficient than white, and the spectrum will be spikier.I wonder if there might be some mileage in blue laser + phosphor.Maybe even blue laser remotely, firing at a local phosphor so you could get it really close for macro shots?Lasers will probably be less tolerant of overdriving though due to optical damage.

author: mribble
author_rank: Regular Contributor
timestamp: February 01, 2018, 01:24:28 am
content: Mike, I've thought about RGB LEDs and even just separate strips of them. While like you I initially thought the color spectrum would be worse, I've read it's actually better. I'm sure if you had a certain type of object it would be worse, but most photographers say they are generally better. Another interesting thing in this area are that high end cell phones actually use 2 different white leds (one cooler and one warmer) and can control the temperature of the flash. Anyways, with all those cool options I still think for this initial version I'm just going with the fast simple option.I did the photodiode measurement tests and I do see some ringing at this new 30V level. It's not as bad as it was on the current limiting resistor, but it is certainly there. This was with no probes attached so now I know there is some ringing all the time. At this point I think I'll just try a better pcb layout and see if fixes the ringing in the led. See attached photos of the light output.

author: mribble
author_rank: Regular Contributor
timestamp: February 02, 2018, 08:29:36 pm
content: I think I found most of the source of my ringing. I was using a wire 5W 1 ohm wire wound resistor to limit LED current for these pluses. When I removed that the small amount of ringing on the light output is gone. I didn't have a good place to measure voltage directly in the circuit anymore so I don't know if a little remained (I'm guessing it did and am working on a better PCB layout).Anyways I plan to go to a metal oxide 3W resistor since those are supposed to have much less inductance. Does anyone have a suggestion for a better approach?Also is there a rule of thumb about how much you can overdrive a resistor if it's just for a few microseconds?

author: alsetalokin4017
author_rank: Super Contributor
timestamp: February 02, 2018, 10:31:40 pm
content: I was going to mention that your 5W resistor was likely wirewound and thus had significant inductance which would contribute to the ringing.Why not try an actual non-inductive resistor here? There are several types you might try. I like the Ohmite precision Ayrton-Perry types:https://www.digikey.com/catalog/en/partgroup/wn-series/6251

author: mribble
author_rank: Regular Contributor
timestamp: February 02, 2018, 10:43:16 pm
content: Thanks for the link. I'll look into these and other non-inductive resistors. The issue I see with those is they are expensive and I need 4 per unit. On my next version I'll test the light output with these and a cheaper metal oxide resistor to see if they make a difference.I still am wondering if I could use something like a 2W resistor for 20V at 5A if it is just on for 4 microseconds.

author: kony
author_rank: Regular Contributor
timestamp: February 02, 2018, 10:55:17 pm
content: Carbon composition resistors handle peak transients very well (used commonly in input protection due to that property), you don't really care that much about tad higher noise and worse accuracy/tempco.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 02, 2018, 11:35:01 pm
content: Carbon comp is an anachronism.Carbon, metal and metal oxide film types are all excellent. Wirewounds have more inductance, unless otherwise stated.Regardless of type, expect on the order of 1-2 nH per mm of component length. This works for wires just as well as resistors, capacitors and so on!Ringing can also be dampened by adding an R+C across the switch, C >= 2 * Coss and R = sqrt(L/C), where Coss is the switch output capacitance (MOSFET Coss parameter; note that this is voltage dependent, so pick a voltage typical of the ringing), and L is the stray inductance (if you don't have a number, estimate it from the ringing and Coss). The resistor doesn't need to be big here, since it's only consuming energy peaks, not continuous duty.Tim

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: February 02, 2018, 11:48:29 pm
content: Multiple surface mount resistors in parallel would probably give the lowest inductance

author: mribble
author_rank: Regular Contributor
timestamp: February 02, 2018, 11:53:54 pm
content: Quote from: T3sl4co1l on February 02, 2018, 11:35:01 pmThanks for the info. A little ringing doesn't matter to me and it should be greatly reduced with my new pcb and when I stop using wire wound resistors so I think I'll try skipping the RC across the switch.That said, you mention resistors don't need to be big since it's just for peak. Are there any rules of thumb. Let's say I have 5A and 20V for 5 us. Do I need a 1/2 watt, 1W, 2W, or 5W resistor?I'm happy to use surface mount resistors, but again I need to have an idea of what wattage I need to handle for these pluses.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 03, 2018, 12:09:27 am
content: Quote from: mribble on February 02, 2018, 11:53:54 pmWhat's pulse rep rate / duty cycle?5A * 20V * 5us = 500uJ, about a flea fart. An SMT chip resistor might poof, but I don't think the average 1/2W resistor would even mind.Strictly speaking, you need to shop for pulse-rated resistors, and find where on their power(time) curve 5us and 100W lies.Informally, resistors usually go about as t^1/3 below the dominant (thermal) time constant. 1/2 exponent would make more sense (thermal diffusion), or proportional (fixed energy), but, eh, for whatever reason, that seems to be the most common pattern. Most power resistors are rated for a 5x or 10x overload for 5 seconds. Combining these two properties, we might expect a 6-decade faster pulse to have 2-decade more power. So a 100W pulse could be handled by a 1W resistor (no overload condition needed) or a 0.2W resistor (5x overload 5s). Resistors quite this small, I would worry that their time constants will be much shorter, so erring on the side of 1W would be better.This assumes a pulse rep rate much longer than the longest thermal time constant, i.e., so that the resistor temperature returns to ambient before the next hit.For a one-shot flash, I'm guessing this will be sufficient? If you're instead doing (or "also considering", since it's trivial once you've built the hardware) a stroboscope, then the temp may not return to zero between pulses, and you have to worry about peak loading on top of the baseline temperature.It seems likely that, for resistors over 1W, you'll have no problem with peak loading, so you can use average power just as well. For example, 2W resistors would get you a duty cycle up to 2% (a pulse rep rate of 5kHz, and more if you reduce the pulse width / intensity).Tim

author: tggzzz
author_rank: Super Contributor
timestamp: February 03, 2018, 12:12:34 am
content: Quote from: mribble on February 02, 2018, 11:53:54 pmRead the data sheet, looking for pulse power. If not listed, find a data sheet that does list it!

author: mribble
author_rank: Regular Contributor
timestamp: February 03, 2018, 01:17:02 am
content: You guys are awesome! I tend to sort resistors by lowest costs so I guess it isn't surprising the ones I've looked at don't list pulse ratings. I did a little looking at some more name brand SMD resistors (like ones from Bourns) and they have pulse ratings. There are cheap 2 watt resistors from Bourns that handle 600W for 100uS. They are thick film and have much shorter leads so should be low inductance from what I've read. I think I'm set.Tim, since you asked my duty cycles are low. The worst case will be 5 us on then off for 100us (repeat 5 times so total on time is 25 us), then off for second. Most of the time it will just be a single 1 us pulse and then off for minutes.

author: mribble
author_rank: Regular Contributor
timestamp: February 03, 2018, 07:03:44 pm
content: Thanks for all the great feedback. I've incorporated all the great suggestions into a new pcb. I'm attaching the schematic, board files (zip because svg files aren't allowed), and the gerber files. If you have any more feedback based on this info let me know.