Need help designing 220W boost converter from 12V to 74V - Page 1
Forum section: electronics / beginners

author: Zipdox
author_rank: Regular Contributor
timestamp: March 22, 2021, 08:11:42 pm
content: I'm currently working on my second iteration of a high power LED flashlight and I decided to go with an LED from Bridgelux that requires up to 74V to run. That falls outside the range of the boost converters I currently have on hand.With this in mind I hopped on Ebay to see what I could find in terms of boost converters. I founda boost converter suitable for the joband ordered it, but it's rather bulky and I'd rather use something more elegant. So I decided I want to build my own boost converter.Here are the requirements for the boost converter:Input Voltage: 12VOutput Voltage: 74VOutput Current: 3AI looked at the boost converters I already have on hand and saw that they used TI's TL494 PWM driver IC. I looked in the datasheet and it seemed like a rather easy IC to work with, so I decided I want to use it.I'm already familiar with the concept and operating principle of boost converters so, I did some research into the calculations that I have to do to decide what components to pick. I found an application report from TI calledBasic Calculation of a Boost Converter's Power Stage.I followed it and here are my calculations:Code:Switching Freq: 150kHzEstimated inductor ripple currentILr=0.3*3 (74)/(12)=5.55Right inductanceL=(12*(74-12))/(ILr*150000)=0.000894Duty cycleD=1-((12*0.8)/(74))=0.870Maximum switch current (let's suppose a 25A MOSFET)IL=(12*D)/(150000*L)=0.0779Imax=(25-((IL)/(2)))*(1-D)=3.24With these parameters in mind, what sort of real world parameters should I go for when picking components?

author: Benta
author_rank: Super Contributor
timestamp: March 22, 2021, 08:21:18 pm
content: You won't get the TL494 to run nicely at 150 kHz. It's too old and slow. 40...50 kHz is realistic.

author: Zipdox
author_rank: Regular Contributor
timestamp: March 22, 2021, 11:56:23 pm
content: The datasheet says it's perfectly capable of running at 150kHz. One of my commercial boost converters runs ar 150kHz too.

author: Benta
author_rank: Super Contributor
timestamp: March 23, 2021, 12:05:55 am
content: Good. Then try. It's your design. My experiences with TL494 are different.

author: Circlotron
author_rank: Super Contributor
timestamp: March 23, 2021, 12:49:53 am
content: I've used a SG3525 quite successfully at 100kHz so that might be a good alternative. Not so terribly different from a TL494.

author: mvs
author_rank: Frequent Contributor
timestamp: March 23, 2021, 07:07:16 am
content: Quote from: Zipdox on March 22, 2021, 08:11:42 pmYou can not get 3A output with such low switch current, no way. Inductor should have much lower inductance too.Check your math.

author: mvs
author_rank: Frequent Contributor
timestamp: March 23, 2021, 08:13:40 am
content: Quote from: Zipdox on March 22, 2021, 08:11:42 pmTL494 was not designed for boost topology in mind, outputs are also not well suited to drive MOSFET gates.In commercial boost converters you may find UC3843 or even more modern parts.https://www.ti.com/product/UC3843

author: Zipdox
author_rank: Regular Contributor
timestamp: March 23, 2021, 01:26:52 pm
content: The commercial boost converter used an additional 2 transistors to drive the gate of the MOSFET. Anyway, is it perhaps a better idea to use a dedicated boosting LED driver IC, I'm thinking of the HV9912?

author: drussell
author_rank: Super Contributor
timestamp: March 23, 2021, 02:21:56 pm
content: 20-30 years ago I built lots of car amplifier power supplies with both the SG3524 and TL494 to get dual ±35 - ±50 volt rails from the 11-15V car supply with no issues. I usually ran them about 100-120 kHz, worked just fine with nothing exotic, just hand-wound salvaged-torroidal-core transformers, a bunch of paralleled mosfets and simple diodes.(This was about the time that the early things like HexFets first started appearing with their dwindling RDS(on) making arrays of paralleled mosfets less necessary. It would even easier with today's internally-massively-parallel, ridiculously-low RDS(on), ah how things progress...)Here's a shot of the remnants of one of the TL494-driven power supply prototypes for a high-current 1000W (bridged, at extremely low impedance) amplifier for competition:

author: Zipdox
author_rank: Regular Contributor
timestamp: March 23, 2021, 09:16:04 pm
content: Quote from: drussell on March 23, 2021, 02:21:56 pmOh yeah, using a low RDS(on) MOSFET is a must for my application as the efficiency needs to be good, as it will be battery powered.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 23, 2021, 10:22:41 pm
content: Ditch the 494. Or, pledge to use it in average current mode control -- this requires an external error amp, so on top of the controller itself needing more pins, it's more parts too.Use UC3843. A circuit like this will do:Slope compensation should be added for the inductor shown, and probably the best you'll find among commercial offerings for inductors, will also need it. This is easily added per the datasheet/appnote. Also the CT can be commercially sourced, probably a 1:100 in SMT or THT, and with the 2.2 ohm burden resistor reduced proportionally. But also scaled up proportionally for the total power level.And, better capacitors and transistors exist now, so choose those. You'll need a higher voltage diode of course; schottky are available up to 200V pretty nicely these days, should be good enough.And, I'm referring to the top half of the circuit, don't mind the bottom half which is just a power oscillator really. It was originally drawn for a magnetic amplifier supply, but just the +30 is what matters here.The VFB divider will decrease of course. Oh, also change the 0.01 between pins 1-2 to an R+C, values TBD by transient response testing. Typically 1-10nF and 100k are a good starting point. The 0.0047 at the VFB divider is also open to change.Also, if you're hand winding, preferably the boost inductor should be tapped. Tap to drain, far end to rectifier diode, tap at say 1/4th the full length. The SB540 snubber diode may still be desirable (in which case, return the 10 ohm to +V, and adjust its value for best results).Finally, for LED outputs, consider replacing the upper VFB resistor with a zener (overvoltage limit, rated for some modest factor above the required output voltage, say 90-100V), and use a shunt resistor in the LED ground return.Hmm... man, this was even drawn with 3842, which doesn't start until 16V (UVLO), so would never start at a 12V input. I should probably stop referencing this old schematic, it has so many things that need to be changed... but there are so many things that need to change for a specific use case anyway, so... :shrug:?Tim

author: Zipdox
author_rank: Regular Contributor
timestamp: March 23, 2021, 11:38:53 pm
content: Quote from: T3sl4co1l on March 23, 2021, 10:22:41 pmI just came across an informativeform someone that used a UC3842/UC3843 so I think I'll go with one of those. I also found aboost converter calculatorfrom Adafruit that I think will come in handy.As for the circuit you posted, I'd rather use a shunt instead of a current transformer.Quote from: T3sl4co1l on March 23, 2021, 10:22:41 pmOh yeah, I had thought about making an isolated boost converter with a ratio in the transformer, but it never occurred to me that I could just tap the inductor. Thanks.I'll see if I can cook up a schematic tomorrow and ask for feedback.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 24, 2021, 12:11:27 am
content: Quote from: Zipdox on March 23, 2021, 11:38:53 pmNote that that calculator gives, I think, BCM (boundary conduction mode). By minimum, they mean it's in CCM for higher values. This is not quite what you want for the 3843 family: inductance must be either somewhat lower than this (forcing DCM), or somewhat higher (no more than about double) with slope compensation. The control method isnot suitablefor arbitrarily large inductors. This limits the power level, partly by gate drive capability (it's good for transistors up to, oh say, 50nC gate charge, give or take Fsw), and partly by inductor core losses (which are worse in DCM, but reduced in CCM, particularly at low ripple fractions -- but it is actually the ripple fraction that is a problem with the control).In peak current mode control appnotes, you'll often see the restriction that D < 0.5, but this is merely a convenience of the setup. For some reason, none of them make the connection that D = Vin / Vout at BCM/CCM, and that this is the critical value. (They always do a setup at Vout = 2Vin for boost, or Vout = Vin and Ns/Np = 1 for flyback, for simplicity; so naturally, CCM appears at D > 0.5.) In other words, it goes screwy as it enters CCM. It's CCM that's the problem, not some arbitrary duty cycle value.What's actually going on, is actually quite beautiful and deep (it's a chaotic system, an implementation of the logistic map function), but that's not important here; look it up if you're curious!Tim

author: Zipdox
author_rank: Regular Contributor
timestamp: March 24, 2021, 12:16:01 pm
content: Ok I'm reading through some datasheets and circuits online and I noticed that all of the circuits take current feedback from the MOSFET, not the load. Is it gonna be a problem to get the UC3842 to take current feedback from the load instead of the MOSFET?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 24, 2021, 12:39:03 pm
content: Like this:https://www.eevblog.com/forum/projects/uc3843-led-driver-with-dimming/Many other tips there as well.Tim

author: Zipdox
author_rank: Regular Contributor
timestamp: March 24, 2021, 01:07:57 pm
content: Quote from: T3sl4co1l on March 24, 2021, 12:39:03 pmOooh that might be very useful yes, thanks.

author: Zipdox
author_rank: Regular Contributor
timestamp: March 24, 2021, 08:53:45 pm
content: Is there not another fast PWM controller with 2 error amplifier inputs? The UC384x is just too restrictive to work with in my opinion.

author: mariush
author_rank: Super Contributor
timestamp: March 24, 2021, 09:48:51 pm
content: As this is a personal project, I would encourage you to decide on a different input voltage.You say it's a flashlight so let's assume it will be powered by batteries. if it's lithium batteries, just make a 5-6s battery pack and buy a proper charger for such pack.Another potential solution... consider buying a micro inverter, which will be designed to boost 12v to 110v or thereabouts.. you will probably be able to tweak it to output less than 110v and then just rectify to DC and use a buck led driver to regulate the current. Probably less efficient but saves you the hassle of designing a boost regulator will difficult specs.example 24$ 200w inverter:https://www.amazon.com/Foval-Power-Inverter-Converter-Charger/dp/B074KWP2HJ/40$ for an energizer branded one :https://www.amazon.com/dp/B08VQVLV7H/45$ for tripplite with cup like design :https://www.amazon.com/Tripp-Lite-Inverter-Charging-PV200CUSB/dp/B010R7ELLY/

author: Zipdox
author_rank: Regular Contributor
timestamp: March 24, 2021, 10:00:43 pm
content: No that's not gonna work at all. The LED needs to be current limited. It's also a massive hack.I took a better look at theboost converter I purchased from eBay, and similar ones from AliExpress, and saw that they all use a UC3842A with another IC whose marking I can only tell starts with "LM" so I'll assume it's an opamp.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 24, 2021, 10:29:23 pm
content: Quote from: Zipdox on March 24, 2021, 08:53:45 pmWhat two error amps do you need? What restrictions are you finding?Tim

author: Zipdox
author_rank: Regular Contributor
timestamp: March 24, 2021, 11:24:31 pm
content: Quote from: T3sl4co1l on March 24, 2021, 10:29:23 pmOne for voltage and one for current.Trying to use the UC3843 in my case is gonna be a big undertaking and will require a circuit too complex for my level of experience. I don't think I'd be able to pull it off.The schematic in the thread you linked uses quite a large shunt which is not feasible for be as I have a high input current. This is a problem because ISENSE requires a signal of 1V, or some funky stuff needs to be done to the COMP pin. All in all, too much hassle that can probably be avoided by using a different chip. I'll look into the SG3525 tommorrow.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 25, 2021, 12:44:52 am
content: Quote from: Zipdox on March 24, 2021, 11:24:31 pmUse one for both? I'm guessing you don't need accurate voltage limiting, for which a zener does a fine job.Or if this is a CC/CV type supply application (output characteristic has not been specified; I've been assuming this only needs to drive LEDs in the usual way), then two error amps can be used, or one with a limiter circuit around it -- more complex, but just another block in the chain and it doesn't interact with anything else.QuoteWell, ISENSE can be biased up, with a resistor from VREF to ISENSE, then a resistor from ISENSE to shunt. Alternately, the resistor can be supplied from an emitter follower from RtCt (see the appnote), which introduces slope compensation -- recommended at this power level.The load can also use a smaller shunt, whether by simply biasing up the VFB pin, or by amplifying the shunt signal with a separate opamp e.g. TLV2371.It's just a few more components, added in sensible locations, not really... complex?If sheer complexity is the issue, well, only finitely many things need to be checked, it's not at all insurmountable... but that does leave quite a wide margin, doesn't it? It's simply the nature of the beast, that there are more considerations to keep track of, compared to say an old school linear regulator. Or a purchased module, assuming one can be found of the required ratings.Voltage mode controllers like TL494 and SG3525, need to use the internal error amp for current mode control, to make them palatable. This gives a constant input current characteristic, so a second error amp must be added, to regulate load voltage or current. On the upside, the latter can be as simple as a TL431, which is best thought of as a peculiar three-terminal op-amp.Plain voltage mode control, is a disaster waiting to happen. All the TL494, etc. appnotes use it, unfortunately, as they come from a very old line of appnotes, for a very old line of parts; the appnotes have been maintained, but not improved, over the years. These chips can be used for current mode control just fine -- but the appnotes will not tell you how to do it, or only cover hacks like peak current mode control using an SCR or a handful of transistors. Which again, aren't preferable, as peak current mode control doesn't have the benefit of low ripple fraction.Average current mode control, is nice as it allows an arbitrarily low ripple fraction, saving precious core losses. It is somewhat more difficult in the boost configuration: a high side current sense amp, or Hall effect sensor, should be used. Or low side sensing, if common grounds are not needed.I would call this more complex -- there is one more op-amp in the loop. This is why I suggested 3843 first. It's not much more complexity, and it may indeed give better results at this power level, thanks to the lower ripple fraction. Just don't fall for the appnotes telling you all about voltage mode and none of its problems...I don't have a schematic handy, using a TL494 in this way, but I have one of the discrete equivalent. You'll have to read the TL494 (or other) block diagram carefully to see that this is equivalent, but it is essentially what they are doing.https://www.seventransistorlabs.com/Images/Flashlight2Sch.pnghttps://www.seventransistorlabs.com/Images/Flashlight2_Schematic.pngNote the low side input current sense, as the source is isolated (a battery). Power architecture is classic boost, with overvoltage limit and load current regulation. Two error amps (IC3) are cascaded, one for tracking the load, one for tracking inductor current. IC3B would be internal to the controller, IC3A would be added. This prevents inductor current from ever reaching an unsafe level -- under startup or fault conditions, PWM will not simply increase without bound, instead the worst it can do is saturate (i.e., IC3A output is between 0 and BATT), and that only results in a well-defined input current draw which the transistor and inductor are designed to handle.Tim

author: Zipdox
author_rank: Regular Contributor
timestamp: March 25, 2021, 03:50:44 pm
content: The boost converter I used for my last LED does low side current sensing with a shunt I think.https://www.aliexpress.com/item/1005001408105481.htmlI have several and one of them is broken so I'm gonna strip all of the components and remove the soldermask to see how it works.

author: Picuino
author_rank: Super Contributor
timestamp: March 25, 2021, 08:11:53 pm
content: Your calculations have errors.I_max = IL/2 + I_out / (1-D) = IL/2 + 3 / 0.13 = IL/2 + 23 AmpsAt least you will have 23 peak amps, plus coil ripple.Edit:To raise the voltage so much and with such a high power perhaps you should consider using the push-pull topologyhttps://en.wikipedia.org/wiki/Push%E2%80%93pull_converterhttp://electronics-diy.com/electronic_schematic.php?id=883https://www.researchgate.net/figure/Schematic-of-SG3525-based-Push-Pull-SMPS-In-fig3-SG3525-PWM-controller-is-shown-which-is_fig5_302895464

author: Zipdox
author_rank: Regular Contributor
timestamp: March 25, 2021, 08:46:39 pm
content: Quote from: Picuino on March 25, 2021, 08:11:53 pmSomebody already suggested using a tapped inductor.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 25, 2021, 11:27:46 pm
content: Nahh, I meant like this,(without the half wave doubler at the output)Push-pull is more desirable for low input voltages, because the current is shared between two paths, relaxing requirements for switch capacity and stray inductance.It's less desirable for high voltages, but 74V at 3A is no problem. Downside, it's more complicated. Not really, or not by much, and it needs all the same considerations (like current mode control) that any other topology needs. The biggest change is you're never going to find an off-the-shelf transformer in that rating. Whereas the single stage boost, you can use whatever off the shelf inductor, or at worst, rewind one of comparable rating, adding the tap as needed.And the transformer needs to be wound in a particular way, to keep the switches happy. It's not enough to just slap some wire loosely on a core, the arrangement matters too.So, rather than explain how to do all of that, on top of all the things I've already explained -- I would perhaps suggest the boost over push-pull. But yeah, this is a place where it makes more sense overall.Tim

author: Zipdox
author_rank: Regular Contributor
timestamp: March 26, 2021, 05:35:36 pm
content: Alright here are the traces of the commercial booster. The dark spots are torn off traces. I will try to create a reverse engineered schematic.

author: Zipdox
author_rank: Regular Contributor
timestamp: March 26, 2021, 08:26:51 pm
content: Ok so I hacked the boost converter by adding extra windings to the inductor, replacing a voltage feedback resistor and swaping the output capacitors for higher voltage ones. Note that the extra windings start from the MOSFET connection and connect directly to the schottky diode, which has been disconnected from the MOSFET so that it doesn't see the high output voltage.

author: Zipdox
author_rank: Regular Contributor
timestamp: March 26, 2021, 08:37:13 pm
content: Photo of setup

author: Zipdox
author_rank: Regular Contributor
timestamp: March 27, 2021, 10:37:30 pm
content: Marked all the components

author: Zipdox
author_rank: Regular Contributor
timestamp: October 01, 2021, 07:20:40 am
content: I just wanted to provide an update to my project.I posted the result to reddit:https://www.reddit.com/r/flashlight/comments/oib6rh/diy_140w_flashlight_with_bridgelux_led_and_custom/And the schematic:https://oshwlab.com/zipdox/led-boost-converter-driver-ina155ua