100w LED Boost converter - Page 1
Forum section: electronics / projects

author: pyrohaz
author_rank: Regular Contributor
timestamp: January 19, 2015, 12:16:57 am
content: Hello all!A couple of weeks back, I bought one of those dock off 100w LEDs off eBay. It was only today that I decided I'd test it and see if it actually works. After building a quick dirty boost converter (full explanation at my blog:http://hsel.co.uk/2015/01/19/100w-leds-and-boost-converters/), I've concluded that the LED works and even running it at 10W, it is extremely bright! So, I want to design a proper boost converter to run it at its full power. I've already sorted out the heatsinking and its the actual electronics that I need to get sorted now. I've had a go at designing a boost converter and it works perfect in simulation. I do however want to know if anybody can recommend better ways of achieving the same end result.I decided to go for a current controlled boost converter where the current through the LED controls the boosting. I'll attach the schematic and await your recommendations! I'd preferably like to make it using easily bought components and I don't fancy winding my own magnetics either.Thanks,Harris

author: Yansi
author_rank: Super Contributor
timestamp: January 19, 2015, 12:45:22 am
content: This not even hardly a proper boost converter. Have you tried to simulate that? The topology is a nonsense. You have fixed switching frequency and duty cycle and only enabling or disabling it by a comparator, with no hysteresis or any linear feedback.I really don't know, why everyone just slaps NE555 in any power DC/DC converter. If you want a properly designed boost converter, go for some proper controller IC, like TL494, which is easily obtainable, cheap and at least will work well.There are several ways how to make a boost converter for LED, I'll just suggest with a little sketch. I hope, it will help. Sorry for the DaveCAD drawing, but it is fast and does the business.Now you have at least a proper starting point with cheap and salvagable components, no expensive stuff. The choke could be wound on an iron powder toroidal core, also salvageable from a PC ATX supply junk.Note that designing a boost converter about 100W is not as funny and simple as it looks, power electronics just don't excuse mistakes, it just blows right in ur face, if done improperly.//Do not take the schematic too seriously, it is not a fully done work. The choke must be calculated, caps chosen to withstand the current ripple, overvoltage protection must be added, etc...

author: void_error
author_rank: Frequent Contributor
timestamp: January 19, 2015, 08:04:40 am
content: You know, you could limit the output voltage using the second error amp within the TL494...

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 19, 2015, 09:11:31 am
content: Getting better, but active current limiting (at the switch) would be even better: like this one,http://seventransistorlabs.com/Images/Mag_Amp_PSU.pngconfigured for LED current rather than output voltage regulation of course, or even something likehttp://seventransistorlabs.com/Images/LED_Light.pngwhich is fully line powered and includes overvoltage and thermal protection.Tim

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 19, 2015, 09:16:37 am
content: Quote from: Yansi on January 19, 2015, 12:45:22 amIt's linear, in the sense that, the feedback still needs to be compensated, and stable in that, while the comparator would like to operate in a hysteretic mode, which works for a buck (if not the greatest method... *cough* MC34063...), that's impossible here (the transistor switches on, and stays on waiting for the output to change but never does...), the gating forces it to generate voltage or not, keeping it from, at least immediately, exploding...But yes, 555 is basically the worst choice for anything SMPS.Tim

author: pyrohaz
author_rank: Regular Contributor
timestamp: January 19, 2015, 05:15:57 pm
content: Quote from: Yansi on January 19, 2015, 12:45:22 amThanks for your comments! I have simulated it and I get the results I was actually expecting to get. Firstly, a lot of switch mode regulators are fixed frequency aren't they? I know that most vary their duty cycle to supply the load with varying levels of power, in this case, the simulation shows that the duty cycle gets modified by gating it on and off, this produces the varying levels of power. The TL494 is a good shout though! Do you know anywhere I could learn about designing a boost converter using it?Quote from: T3sl4co1l on January 19, 2015, 09:11:31 amHi Tim, thanks for your input too! Is the current limiting achieved by the first transformer (the 1:150 one)? I see that the half rectified output of that goes into the Isense of your controller. Do you get any problems with using paralleled diodes? I understand their in the same packages but still.Your LED circuit certainly looks complex!Quote from: T3sl4co1l on January 19, 2015, 09:16:37 amWould I be better off replacing my simple comparator for a schmitt trigger with well chosen trip points? The transient response of the circuit isn't actually that bad in simulation (obviously different in real life...). I'll include a few pictures simulating various parts.I chose the free-running frequency and duty cycle to never exceed the maximum specification of the inductor. The off time is long enough to ensure the regulator will always run discontinuously too.

author: Yansi
author_rank: Super Contributor
timestamp: January 19, 2015, 05:23:01 pm
content: Discontinuous mode for the booster? Thats wrong engineering. It must run continuousmode currents with only small ripple (under 30%). Otherwise, you will burn the inductor, if you use iron powder core.Do you want better and proper solution, or do you just want us to confirm your design with 555 is good? (No, it is not.)For protection against saturating the core (for iron powder, it is unprobable it will happen), you should use currentmode controller, as someone has already suggested.

author: pyrohaz
author_rank: Regular Contributor
timestamp: January 19, 2015, 05:37:30 pm
content: Quote from: Yansi on January 19, 2015, 05:23:01 pmHi Yansi,How come a boost converter has to run in continuous mode at all times? I've seen a few TI data sheets which mention running boost converters in discontinuous mode.I did ask if you could direct me to some material on designing boost converters with the TL494, I'd still appreciate some!I merely designed the 555 based boost converter as I don't have experience with any other non-fully integrated controllers. Life is much easier when it comes to slapping together a single IC with a few caps, resistors and an inductor with help of a reference design.Cheers!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 19, 2015, 06:02:50 pm
content: In fact, continuous (CCM) is the worst option for the circuits I mentioned, because the peak mode control is hopeless in that regime.DCM is undesirable at high power (over maybe 50W?) because of the very high ripple currents subjected to the input and output filter caps, and the inductor. CCM is undesirable because of control method (ideally, you would use an average current mode control, which means a high side current sense) and hard turn-on switching (schottky diodes are preferable, but even then, a snubber is still desirable).Conventional powdered iron cores are unsuitable, but powder composites types are often better performing than ferrite cored inductors these days, go figure.Paralleling diodes in the same package isn't usually a problem; even if they used separate dies, they are much better thermally coupled (and probably matched) than two diodes side by side. Often, they use a monolithic dual diode which doesn't mind at all, either way.For converter design, check out appnotes for the most popular controllers, like TL494 and UC3842. There's a lot to learn that you aren't going to find in appnotes, and spending some time breadboarding things and seeing firsthand why some choices are bad (like voltage mode control) and others are tantamount (like current mode control) will be very valuable.Tim

author: Yansi
author_rank: Super Contributor
timestamp: January 19, 2015, 06:38:02 pm
content: Boost converter with iron powder core must run in CCM, otherwise the power loss inside the core is too high, the core would almost melt.Powder cores don't like high magnetic flux swing.DCM is used for offline switchers with ferrite cores. Because with DCM the overall size of the transformer is smaller and the ferrite amterial is not bothered by high magnetic flux swings. The limit is only the sharp saturation, which occurs at about 0.35-0.4T (depends on material) and must be avoided.Powder cores can withstand high flux densities, like 1T or more (like normal iron transformers), but do not like high swing in flux density. The swing of the flux density must be limited, so CCM is the only suitable for iron powder cores.________T3sl4co1l: I haven't suggested the CCM for your circuits. What do you mean "composite types" exactly?For lowering the current ripples, I'd suggest to experiment with two-phase booster, made from reverse connected TL494. There are some better methods how to do that, but mostly not as cheap as this "junkbox solution".

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 19, 2015, 06:52:11 pm
content: Quote from: Yansi on January 19, 2015, 06:38:02 pmBlah blah blah...Just squawking it over and over doesn't make it true!Example,I made this years ago, and though it gets rather warm, it does work. A mix #52 powder core is about the second worst choice for this, but it manages to operate because of the low power level. On the upside, no RC damping is necessary on the transformer...QuoteFrom a similar era, I also built this,(afraid I don't have a larger pic handy)which as you can see is a ferrite cored transformer, running near 1MHz, and I think in the >0.1T range.It, too, got hot. About as hot as the other one.Hmm, 1.63 mm^2 is surely a typo, 16.3 would make more sense. That puts Bmax around 20mT?When it comes to magnetics, there are no absolute rules. Please do not make statements as if they are such!Please do explore some resources, like Coilcraft's website. They have a coil loss calculator that runs against their entire database! Enter some figures for DCM and CCM and see what types of parts pop up. I think you'll be pleasantly surprised!QuoteComposite, molded, pressed: resin bonded powder, formed to shape, encapsulating the winding.The losses are much lower than the old fashioned powdered iron toroids, and often better than ferrites, especially comparing to MnZn ferrites at higher frequencies (500kHz up into the low MHz).Tim

author: Yansi
author_rank: Super Contributor
timestamp: January 19, 2015, 06:59:29 pm
content: 150mA 12V and getting the coil warm is nice indication, that you did it wrong. I think it shouldn't even be warm. No blah blah. That is no power, that is fun power.Try DCM with the iron powder core (like #26) at 100-300W and then tell me, how many seconds did the core lasted, before it smoked out. Mix like no 26 is for sure intolerant for high swing of flux density, isn't it?I agree, I shouldn't make my statement as general rules, it can get misleading.By the way, one curious question though: Why do you have on the gate the dual winding, one part coupled through resistance, the other through capacitance?//one of my last offline switchers, which I haven't had time to complete...

author: Psi
author_rank: Super Contributor
timestamp: January 20, 2015, 02:18:59 am
content: Be aware that most of those 100w LEDs cheap on aliexpress or eBay are rejects.They work, but your results will be mixed. First one I bought ran for two weeks as my main room lighting before series strings in the lad started to die one by one. I ordered 5 more, with the intent to run more of them at a lower power, however i was sent 10. (I think the seller knew some were duds). When testing them at just enough power to start getting light i found some are really bad with bright spots while others are more or less fine. Out of the 10 3 were good 3 were OK, 3 were bad and one was terrible. I put one of the good one in my room lighting rig and its been going good for 5 months

author: pyrohaz
author_rank: Regular Contributor
timestamp: January 20, 2015, 06:05:47 pm
content: Quote from: AcHmed99 on January 20, 2015, 01:01:28 amI would do but I do think its quite nice to learn the effort involved in designing such a beast! If it was a proper product that I'd be selling for people to buy, I definitely wouldn't do a "ricky" solution like this.Quote from: Psi on January 20, 2015, 02:18:59 amThanks for the heads up! It seems mental that you can get a beast of an LED for ~£4. I'll keep the reliability in mind!Quote from: T3sl4co1l on January 19, 2015, 09:16:37 amI had a go at designing the same controller around the MC34063 controller and I was pleasantly surprised to see that it pretty much has the same operation mode as the one I designed (most likely unintentionally...) where it seems to just gate the clock. Simulating it in LTSpice pretty much produces the same results as mine too. Are many controllers designed around clock gating or are most linear in the sense of the duty cycle is proportional to the output power currently required?I have got to ask as well Tim, how do you know what is required for your really intricate designs?! I look at some of your schematics and obviously they work but I find it really hard to decode what each transistor is doing in the circuit etc.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 20, 2015, 08:36:01 pm
content: Quote from: pyrohaz on January 20, 2015, 06:05:47 pmCertainly, the result has to be duty cycle control. Whether that's achieved by something as crude as clock gating, or something more nuanced like peak or average current mode control, doesn't matter over the long term.Over the short term, it does matter how well those pulses are controlled, because excessive pulse lengths cause extra peak current in the components and cost efficiency, and if the pulse widths are constantly bouncing around, those variations show up as output ripple, requiring more inductance and capacitance for a given output noise level.The difference between "short" and "long" is basically why designs with the 34063 have about three times the inductor as the rest of the components in the circuit, and I mean volume per volume, for a THT build! The oscillation frequency is already fairly low by modern standards, and the rate at which it is controlled is even worse, hence the inductor must be oversized by that much.An example of "medium term" variation might be chaotic operation of the peak current mode controller: when inductor current doesn't return to zero each cycle, the duty cycle will bounce back and forth erratically -- in fact, it is chaos (mathematically speaking!). The inductor emits an audible hissing sound, and the output becomes much noisier (in the sense of true random noise, rather than periodic switching transients). The output voltage and peak switch current are still controlled -- these are guaranteed by the design -- so in the grand scheme of things, it can't be said to be truly unstable, but it's a decidedly suboptimal operating condition!QuoteEh... they're not usually too intricate, and the ones that are, are only by repeated iteration and contemplation. I tend to go for high level design, which tends to cost in parts count. My Theremin project is probably the best example of that, using at least several transistors in each stage, with each stage built in its own isolated section; the total count is around 50 transistors right now.My discrete switching circuits I think are neat, but still largely high level, if you know what to look for. Timer, switching, current sense, reference.If you want to see intricate... look up some old radio or TV schematics. The average TV set from the tube-hybrid-early solid state era (1960s) are all perfect examples of this: not one tube or transistor is wasted; connections go back and forth all over the place! AGC this, amplifier that; they're a mess to follow. They also regularly used single side PCBs, and wasted few jumper wires in the process.Tim