>> SMD withstanding a small surge? (sanity check) - Page 1
Forum section: electronics / beginners

author: 3roomlab
author_rank: Frequent Contributor
timestamp: January 09, 2015, 11:30:41 am
content: hello folksthe question is how do we estimate a ratio for (general passive) components to withstand a certain amount of power surge? (or where to start the ratio? 1:5 ? 1:10? some diodes are specced with surge handling of 1:50)example, if a 2512 SMD resistor is rated for 1 watt (1ohm example) ... if we discharge a 1 time 1000uF 10v thru it, it "eats" the 10watts in less than a second and it doesnt blow it up (no doubt internally it is having a bad "stomach ache"). how do we actually determine say, at what max repeated rate of such pulses it can perform such surges? no doubt a device with more thermal mass could handle more shock surges, i assume 1 could by estimation relate this surge vs thermal mass as a good guide?on a related issue is discharging a higher working voltage into higher power LEDS, eg ... maybe 10v 1000uF into a generic 1w LED to blink it. what could be a good way to estimate its handling capacity? a 1 in 10 on:off rate? as in discharging "10w" of power into a "1w" device but using only 1:10 timing ratio or more?attached example trace of blue = capacitor charging to 50v, red = discharge into a 5ohm load. 100ms per div. at 1:10 duty cycle, should we assume that all/most components can withstand such a power surge?

author: KJDS
author_rank: Super Contributor
timestamp: January 09, 2015, 12:14:08 pm
content: First read the datasheet, here's an example.http://rohmfs.rohm.com/en/products/databook/datasheet/passive/resistor/chip_resistor/mcr_g.pdfIf the datasheet test conditions are not close enough to your case, then you'll need a full thermal analysis.

author: amyk
author_rank: Super Contributor
timestamp: January 09, 2015, 01:53:54 pm
content: Quote from: 3roomlab on January 09, 2015, 11:30:41 amFirst of all the peak power will be ~100W (the peak current will be closer to 10A), and secondly the voltage will decrease as the cap discharges so what you really want to know is how muchenergywas dissipated in the resistor in total. 1000uF at 10V is 50mJ.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 09, 2015, 02:33:31 pm
content: Yeah, energy limit. Although, mind that thermal diffusion tends to be, well, diffusion, which goes as t^-0.5 or so, rather than a simple pole that goes as t^-1. The consequence is, while you'd like to be able to say it can tolerate infinite energy for unlimited time (i.e., dissipate some constant power forever) and constant energy at small time scales (for t << tau, the power goes up proportionally as pulse width goes down, so 1kW for 1ms is the same as 10kW for 100us and etc.), the energy capacity still varies with time, even for very short pulses, so that the energy per pulse is not as high as you would expect (maybe it's only 1kW at 1ms and 10kW at 10us, not 100us!).There are also hard limits due to electric breakdown (excessive voltage drop), skin effect (very fast dI/dt does not flow through the entire resistive material thickness or width at once), and electromigration (excessive current density). These are sometimes provided in the datasheet, usually only as worst case permissible voltage or current ratings.There are, of course, special purpose resistors made for very high pulse capacity. There are regular enameled chip type resistors, which don't seem to be any different from the regular kind (so they might just be spec'd higher -- a cheap way to make an extra cent or two selling resistors?), and there are bulk resistance parts (metal foil and cermet block kinds). The most impressive I've seen are toroid hockey puck types which are advertised for kilojoules in the same size as a 50W power resistor.Tim

author: 3roomlab
author_rank: Frequent Contributor
timestamp: January 10, 2015, 02:31:59 am
content: Quote from: KJDS on January 09, 2015, 12:14:08 pmi see, this brand spec is limit at 2.5x overload @ 2s@ amyk, ah yes if im understanding correctly. if 10w is dissipated in 1s = 10w, 10w in 100ms = 100w peak power. in 10ms = 1kwmmm ok 1/2.C.V^2 = energywhich means watts = energy/time (if 50mJ over 10ms) = 0.05/0.01 = 5watts per spike (2x over the overload of Rohm spec sheet)@ roughly 4Hz = 40watts in 2seconds ? <-- does this compute sound logical ?if to play with "numbers", is it logical therefore to stretch out the operation to 0.25Hz, so that 5w is now in between 4s bursts? --> so that in theory, it sort of does fall inside the 2.5x overload spec?thanks for the tips folks

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 10, 2015, 03:08:21 am
content: Yes, BUT like I said, energy withstand is related to thermal diffusion in these parts, so it may be more like 100W at 10ms than at 100ms, and so on.

author: 3roomlab
author_rank: Frequent Contributor
timestamp: January 10, 2015, 03:32:32 am
content: thanks for the tipi get the rough picture

author: 3roomlab
author_rank: Frequent Contributor
timestamp: January 10, 2015, 09:39:28 am
content: need some help with sanity checkdischarging capacitive energy into LEDcapacitor 100uF charged to 50v = 1/2 x 100uF x 2500 = 0.125Jassume peak power discharged in 10ms burst = 0.125 / 0.01 = 12.5watts (bursting @ 2Hz)stringing a series of LED with total power capacity of 12watts, each LED cell is rated for about 2.7v (12 cells = 32.4v)somehow i feel that the 100uF is too small, but it does seem practically within spec (or so it seem to me?)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 10, 2015, 02:16:28 pm
content: Cap discharging into LED to what end? As a strobe?The trouble with LEDs is, you don't gain much intensity at high current, so it's not worth trying to drive them more than a few times ratings. So, you might as well design the circuit for continuous operation, except you can use a too-large filter capacitor that averages over the pulses (with only a modest voltage drop) so the average supply current consumption remains fairly low.Put another way: worry more about the LEDs (and other semiconductors) than the resistor(s). You don't usually gain much from using any semiconductor in pulsed operation, because the active part is relatively expensive, so it's already designed to be near or at the limits of useful operation. At high currents, LED efficiency drops off; BJT hFE falls, and Vce(sat) and switching times rise; MOSFETs are limited by Rds(on) and supply voltage; etc.By and large, the only advantage that you get from pulsed operation is reduced thermal capacity. In other words, you can't save a thin cent on semiconductor ratings, but you can save on heatsinking.The problem with using LEDs as strobes is, you want to design for a certain maximum pulse width, usually a few milliseconds, so that everything illuminated by the flash appears momentarily frozen. You don't want a long pulse (10s or 100s of ms), because that reduces the speed something needs to move at to appear to, well, move -- it breaks the illusion, and is less useful if this gets used for more than entertainment (e.g., automotive timing indicator, inspecting rotating machinery). So the pulse width is your hard limit.To maximize brightness, you need to maximize power during the pulse. If you're limited by how much peak power you can deliver (like because it's not making more light, and damaging the LEDs, to go higher), then you can only make the pulse more square.The reason traditional photoflash methods use capacitive discharge is, because they're very well suited to it. Once you turn a xenon tube into plasma, it tends to stay that way. It has no control electrode. So, if you can give it a push, then happily wait for it to run its course as it fucks shit up (typical peak current is in the 100s of amperes -- the sheer force of magnetic fields against wires is part of the 'pop' sound characteristic of a flash circuit!), you don't have to worry about timing and control circuitry, it just keeps going until the cap is empty.But you don't get the same thing with LEDs, they simply conduct as current is applied. You can discharge a cap into them, all the same, but after the initial peak, it tails off very, very slowly. It's not like a plasma discharge nearly shorting it out and gulping down the power. The LEDs will never turn off as such, and will leave plenty of energy in the capacitor, since it's only going to discharge to a bit less than Vf.So, like I said, you might as well use an overly large capacitor, at a nominal voltage, and drive it with a mostly constant current, and implement the timing externally -- heck, a 555 would be fine here.As an aside...What's scary about that is, you know how digital cameras have flash settings?...well, it's not like they're adjusting charge on the capacitor or something, that wouldn't be nearly accurate enough. Slow, too. No, they actually have an IGBT in series with the flash tube. They turn it off after a few microseconds to milliseconds, when the scene is bright enough. For short flashes (under a ms), the intensity should be a pretty flat and square pulse. Using it like it's an LED, but a whole hell of a lot more luminous! It's pretty crazy that they're able to get all that current (100A or less) through one tiny SMD chip (usually SO-8 size), with about 5V drop typical.Tim

author: 3roomlab
author_rank: Frequent Contributor
timestamp: January 10, 2015, 04:18:57 pm
content: hmmm just to explore ... what kind of limits is explorable. since its been done before here --> that kickstarter? VELA labs? ... LOL CRAZY LED FLASH !!!but no im not really going to make or replicate that ... but if it does get interesting ... i might just go "murder" some 10w COB LEDS lol ... and hopefully not burn any retina in the processbut with what VELA is doing, i dont understand how they could possibly squeeze 6v into COB package like that ... unless they really boost the voltage. from what i see, COB are suppose to be operating at least over 15v ..so i got a simulation running @ 50v ... just clocking a mosfet to dump capacitor charges into a string of LEDs... but like what you said, instead of over "peaking" the LED load ... making the pulse more squarish is the other way

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 10, 2015, 06:35:49 pm
content: 6V what? You mean because it takes 4 x AA? They probably charge a cap with a switcher. Again -- enough capacitance to hold the energy for some useful change in voltage, running the LEDs as hot as practical. Not obvious how much power they're pulsing the things at. The webpage makes absolutely no mention of luminosity, only ISO 100 at some distance. Means nothing to me...If they got custom LEDs, they could very well be large single dies, or wired in parallel. They'd still need a lot of capacitance to support the battery (which won't do much over 10A short circuit, and that's on fresh cells), but it wouldn't really be any different.Tim

author: ConKbot
author_rank: Super Contributor
timestamp: January 10, 2015, 11:33:39 pm
content: Sounds like you need a pulse forming network instead of a simple RC.http://www.ga.com/pulse-forming-networksOpen up excel, break out kirchoff's laws, and you can make an iterative/recursive calculator, and some current/voltage plots.A couple hints if you do, model the LED as a 'ideal' diode with a constant voltage, with a series resistor. Look at a few I/V points on a curve in the datasheet, to figure out the "Dynamic resistance"You could also use a PFN and IGBT/chunky FET to shut off the LED early to control your pulse width, as T3sl4co1l mentioned.

author: 3roomlab
author_rank: Frequent Contributor
timestamp: January 12, 2015, 09:43:38 am
content: Quote from: ConKbot on January 10, 2015, 11:33:39 pmthat is great, i have been thinking since there is a delay in inductor ramping up its magnetic energy, there could be a way to couple cap/inductor so that it somehow "delay" and hold the spike in for a short time ... but due to my low level theory ... i never got to find out how that could be practically applied. i tried tp simulate a single stage , type A PFN 3 days ago ... of course i didnt know it is called a PFN LOL ... ok now i know it has to work in multiple stageshmmm looks like i need some help fiddling with kirchhoff law ... how does 1 overlay it in such a way to graph that PFN curve?with random values ... simulation of PFN. instead of a needle spike ... its now a squigly

author: 3roomlab
author_rank: Frequent Contributor
timestamp: January 12, 2015, 12:57:04 pm
content: a random pot shot in the simulator (because my theory sucks lol) ... but it turned out i managed to get a small "square" wave ...

author: ConKbot
author_rank: Super Contributor
timestamp: January 12, 2015, 01:34:25 pm
content: Quote from: 3roomlab on January 12, 2015, 09:43:38 amSpice simulation should work fine. I forget that people have the luxury of installing software on their PCs, unlike when I'm at work, and everything is a 3 month processI was manually doing the simulation and time increments in excel using the basic formulas (dV/dT = I*C, dI/dT = V*L, along with using KCL and KVL ) along with guess and check for refining the values. Graphing currents vs time to see whats going on in the inductors instead of just the voltage at each node may help you figure out how to tune it.Also, I'd work in improving the load model. Fit a linear function to the If/Vf curve of the diode where it will be operating (Y=mX+b ) and use the 'm' term for series resistance, and the b term for the Vf of an ideal diode. If the software supports a 2nd order function you could refine it even more. How the diode removes energy from the LC network will affect the current just as much as tweaking values

author: 3roomlab
author_rank: Frequent Contributor
timestamp: January 13, 2015, 04:48:31 pm
content: hmmm, ok im going forward with my mucking around ... say i have a oscillator going around 1-5Hz (to trigger a string of LED blinkers). primarily, i am going to discharge a capacitor on them.if just based on this mini-RC calculator .... logically i would need a humongous inductor to correctly fit such a low Hz ... or i could just ignore the over-saturation and let the losses be ? (... in the first place, the idea of using a inductor/capacitor is to "shape" the spike into a non-spike, hopefully with least losses)now assuming a certain size of inductor works, say 1mH, in the string of PFN setup ...but when i try to put it into the mini ring calculator, at very very low Hz like 1 to 5hz ... according to the calculator, the toroid will surely saturate and there will be significant power loss.... (temp rise 124019 degrees ?... ) ... the numbers dont look right but it does tell the story of gross over saturationthe ferrite core in question is rather small, but ... does it really over saturates into so many folds? or it is simply the spike rate of change is too high?so realistically, a capacitor pulse of around 0.1+ J ... into a string of LEDs ... it looked simple to me, but just thinking about shaping the energy output really breaks my low level maths lolso the question is, if 1mH is the inductor required ... does it mean at 50v ... the size is going to be an elephant? or i just need a large core with lower mu?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 13, 2015, 05:10:58 pm
content: As you have guessed, you're doing a number of things wrong:- I don't know whose idea it was to suggest PFNs, but that's retarded. PFNs and transmission lines are used for nanosecond pulse generation.This kind of shit.The design constraints of a PFN are the desired rise time, pulse width and flatness; if you want a very sharp pulse, you need geometrically more and more stages for a given pulse length. The flatness is determined by the order and value of L and C, just as the profile of a transmission line pulse is determined by the ratios of line impedances joined together.With high speed circuitry being available for cents, there is very little need for PFNs anymore. The main uses today include sub-nanosecond pulse generation, and high voltage pulse generation.- You're putting 1-5Hz into the calculator, assuming that a sinusoidal waveform of that frequency is applied to the inductor. Which is obviously not the case, it's just a tiny pulse (a few microseconds)! You cannot do sinusoidal steady state AC analysis on this (well.. you can, but you don't want to, because you need to analyze hundreds or thousands of harmonics to get there).- The actual peak current in the inductor is determined by voltage and the PFN impedance and matching, just as the peak current from a transmission line is determined by voltage, its impedance and matching. A simple RLC circuit, where the capacitor starts at an initial  voltage V, will never develop more than I = V / sqrt(L/C) peak amperes. The maximum will occur 1/4 wave (i.e., pi*sqrt(L*C)/2) after the initial condition (i.e., closing the capacitor into the inductor).- Note the PFN will also invert if energy reflects back into it, so you want a matched  load: a constant resistance. An LED isn't very good at this, so you're probably going to get reverse bias and ringing if you try.You also have no way to vary pulse width, because it's fixed by PFN length. Have fun rebuilding it every time you want a different pulse width..Tim

author: 3roomlab
author_rank: Frequent Contributor
timestamp: January 13, 2015, 05:59:43 pm
content: hmm but the PFN did seem to be able to "massage" the shape somewhat ...

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 13, 2015, 08:49:55 pm
content: If you want to control the shape, yeah, you can construct one to do that, but... to what end? If you just want the LED on, and emitting as much light as it can... drive it with a transistor and a square wave. Simple as that.