Designing 200W boost converter for bridgelux LED with UC3843 and LM358 - Page 1
Forum section: electronics / beginners

author: Zipdox
author_rank: Regular Contributor
timestamp: May 12, 2021, 09:09:32 pm
content: I'm designing a boost converter for the Bridgelux BXRC-50G10K1-C, which I intend to drive at max 3A which would be around 72.1V. I don't know how to select the oscillator resistor and capacitor because I don't know what an appropriate frequency would be to drive the converter at. I also don't have a clue what to do with the COMP pin.The opamp connected to Isense is configured for a max of 40A peak inductor current, which ideally isn't reached during normal operation.My questions are, what do I do with the COMP pin and what R and C should I use for the oscillator? Also please leave any problems you see in the schematic as well as any recommendations.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 12, 2021, 10:22:34 pm
content: A number of things:0. What is VCC range?1. +5V isn't enough, you need 9V minimum, preferably 12V.2. U1 VCC isn't powered so nothing happens (or something bad happens forcing 5V into VREF?).3. Why LM358? The 3843 is perfectly capable of running all by itself.4. If the purpose is to reduce shunt resistor losses, you will need MUCH higher bandwidth than the pitiful LM358 can deliver. At least if you want to run at anything higher than like, 20kHz.(If an external amp is used, it can be powered from VREF -- check the 3843's maximum load current spec.)5. What is L1, is it tapped? What ratio, 50%?6. D1 needs to be rated 200V or more. Note L1 multiplies the down-swing as well as the up-swing.7. Is that really enough capacitance for this much power level? Those better be some high current types (polymer?). Impedance or ESR may still not be low enough to reduce ripple to acceptable levels (depends on LED ESR).8. Why so many resistors on VFB? Simply design the sense amp for 2.5V output at desired output current.9. COMP will be a R+C from COMP to VFB. Keep the resistor between sense amp output and VFB, 100k is fine. Typical starting values for the RC will be 100k and 10nF, and adjust from there to optimize step response.10. Likewise, RtCt will be a resistor from VREF to RtCt, and a cap to GND. Typical values 10k and 2.2nF for a frequency of, uh... low 100s kHz I think offhand, but check with the datasheet formula to be sure.Frequency is determined by component ratings, especially L1.Also, no VCC bypass cap?Oh wait, I see what happened at the 3843 VCC, the VCC and GND symbols areshorted togetherand the cap is sitting sideways from them! Maybe these are connected correctly after all, I have no idea, but they're completely wrong graphically at least!11. You may find it helpful to add a snubber to Q1 or D1. For Q1, probably a dV/dt (rate) snubber will do, and it will be working against Q1 C_oss and L1 leakage inductance (when Q1 turns off). For D1, probably an RC damper will do, working against D1 C_JO and L1 leakage (when Q1 turns on).Since leakage is involved and L1 is currently undefined, the selection and rating of the snubber may vary. The above assumes L1 is well behaved (k > 0.98 say?).12. You can avoid using current sense amps, by simply biasing up the sense node. Make a resistor divider from VREF to VFB to LED-. Select values such that, at the desired V(LED-), VFB = 2.5V, and the Thevenin equivalent resistance at VFB is the same as R11. Or to scale the Thevenin equivalent, change the COMP R+C proportionally (R prop., C inv.).Likewise for ISENSE, a divider from VREF to ISENSE to shunt can be used, in this case targeting 1.0V at maximum-nominal-peak current. Or even better, add an emitter follower from RtCt (say MMBT3904, base to RtCt, collector to VREF), and wire the divider from emitter to ISENSE to shunt. This has the effect of pulling up V(ISENSE) while also adding timer slope -- permitting operation in mild CCM, likely mandatory for stable operation at this power level.13. You may also consider a current transformer for Q1, which can have much lower voltage drop than a shunt, without any pull-up hackery. (You'll still need the pullup from emitter follower, if slope compensation is required.) This may be a cost adder, YMMV.14. Oh, and there is no voltage limiting shown. Add a e.g. 100V zener diode from the output, to a pulldown resistor to GND, then a signal diode and series resistor from the new node to VFB. This way, if output voltage exceeds ~103V, the zener pulls up on the node, and the diode pulls VFB up from there. (Do not use a zener directly from output to VFB: it could deliver fatal current to the controller, in transient or runaway conditions. The combination of extra pull-down and series diode limits current flow while achieving the same control function.)Alternately, consider a newer controller, LM3481 for example; there are many to choose from. Key features include: wider supply voltage, stronger output drive, internal slope compensation, lower shunt voltage, faster clock frequency, overload/fault modes (e.g. frequency or current foldback, PGOOD or FLT signals, etc.), and more.Tim

author: Zipdox
author_rank: Regular Contributor
timestamp: May 12, 2021, 10:58:10 pm
content: My bad, I was in a hurry with posting this as I had something to do.1. 12V-24V, from a 4S LiPo or a 24V PSU.2. VCC is just the supply voltage, U1 creates +5V and U2 receives it.3. Because the UC3843 current sense works with 1V internal comparator. The voltage from the sense resistor is too low. Picking a higher sense resistor value would result in huge power loss so I need an op-amp to amplify it. Same goes for VREF but 2.5V.4. What do you recommend I use instead? I think I could get away with using the LM358 for the LED current feedback since it doesn't need to be fast at all.5. Yes it's a tapped inductor.6. I didn't really think of that, good observation. Would using a regular non-tapped inductor be possible with this voltage ratio?7. Should I add ceramics?8. I want it to be dimmable. I suppose I can make the gain of the op-amp adjustable. I copied parts of the circuit fromthis thead.9. I don't really understand the purpose of this. Could you perhaps link to an application note or something?10. Seems reasonable. I'll see if I can find the equation in the datasheet.Yes I still have to add input caps.Oops, VCC to GND is a silly mistake.11. I intend to minimize parasitics as much as possible, I'll see if I need a snubber.12. I think the voltage on the sense resistors is far too low for this. The resistor tolerance is too big. Should I just treat VFB and COMP as a regular op-amp and use the Gain=R2/R1+1 equation?I have no idea what all this means, sorry. I prefer a simple solution. If there's an IC that does the job I'll gladly buy it.13. I think this would be too complicated.14. Yeah that's also something I worried about. If the output goes open circuit it would go beserk. Is there some way of combining voltage and current control onto VFB?I might consider a newer controller yes, provided it makes my design easier. You got any good suggestions?Thank your for your feedback!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 13, 2021, 01:20:07 am
content: First stop after the datasheet should always be the app notes:https://www.ti.com/lit/an/slua143/slua143.pdfQuote from: Zipdox on May 12, 2021, 10:58:10 pmOk, you'll need a regulator to deal with the high voltage range. The 3843 can handle voltage just fine, it's the MOSFET -- and drive power -- that can't.Like on the quad LED supply I showed in the linked thread, the 3842s run around 18V and get rather hot, even into fairly modest size MOSFETs. And those are only about 50W/ch.QuoteI don't have an option offhand, but you'll need a low voltage (5V) RRIO or single-supply amp with GBW > 20MHz and input offset < 1mV. Probably this will be a CMOS type. Also acceptable is a high voltage (>=16V) single-supply type"Single supply" means V_OL very near to, and V_ICM extending below, VEE, while the upper limit (and V_OH) doesn't matter. Distributors rarely track this (or accurately), you need to read datasheets to be sure.QuoteIt's not terrible. It does put more peak current into the rectifier and output filter caps.QuoteProbably? Depends what mix of capacitors you have/need. Have you shopped for any yet?QuoteVariable gain..? Nah, that's hard and nonlinear, just vary the offset. Same as we discussed in that thread.Quotehttps://www.ti.com/lit/an/slva662/slva662.pdfI don't get why all the 3843 datasheets/appnotes show R||C, it's stupid. Well, I know why theyalldo, they're all copying the original (Unitrode). A resistor between COMP and VFB causes a "squishy" output -- poor regulation, i.e., output parameter depends on error amp output. That's literally the meaning of such a feedback resistor.The capacitor breaks that path, giving excellent DC regulation, and the resistor provides phase margin.QuoteL1 design is critical, guessing it's hand wound unless it's just on the edge of what those like uh, Wurth has some dual flat-winding things probably about the right size, and should have adequately low leakage.Selection of coupled inductors is quite poor, especially in large sizes. FYI, you'll probably want to avoid toroidal types, like, these have a ton of leakage:https://www.digikey.com/en/products/detail/pulse-electronics-power/P0596NL/2266694note that they aren't even polite enough to provide a k or LL spec.QuoteWell, use 1% resistors obviously, and I don't have a problem with tweaking say 0.5V up to 2.5V. Which is about half range (2.5V is about halfway between 0.5V and 5.0V) so the error amp sees 0.25V of range. That's not grotesque.If you can't handle 3A at 0.5V = 1.5W, not even a percentage point against overall efficiency... you probably have bigger problems than this.QuoteJust follow along with a pad and paper and the description..?I can never tell if my descriptions are actually terrible, or if I always end up talking to people with aphantasia. Text is a poor medium for circuits, granted, but surely there is some middle ground between those extremes.Since I almost never get feedback on these matters, I tend to write these lengthy posts for my own edification, or in the hopes that someone does eventually find them useful (if not always the OP).Anyway, they show the connection in the appnote, Fig.32, with the 2N2222, 15k and 470Ω. It's a lot of words for just three components.QuoteThey also show the CT connection in the same figure, though it's complicated by the, heh that must be a flip-flop, so they're doing a full wave alternating thing. Just imagine that's not there and it's flyback into one half of the transformer, and the CT is in series with it as normal.QuoteYes! I wrote about both.HTH,Tim

author: Zipdox
author_rank: Regular Contributor
timestamp: May 13, 2021, 01:40:20 pm
content: Would the INA240 be suitable for inductor Isense at say, 100kHz?

author: Zipdox
author_rank: Regular Contributor
timestamp: May 13, 2021, 05:39:41 pm
content: I revised the schematic. Have a look.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 13, 2021, 07:27:11 pm
content: Nix R3 and C3, or seedatasheet9.1.1 for how to do it.Yes, INA240 should be fine. Questionable at 100kHz, a bit slower preferably. Hmm, why not use the -A1 and not need a resistor divider (R13-14)?R12 goes in series with C15, not in parallel...Note also LM358 isn't RRIO, its output will saturate at about 3.5V. Recommend TLV2372 or similar. Or, it's actually better to just skip the 358 and wire the pot/divider to R5... Note that the resistance (2k and 2k) is pretty low, that's up to 2.5mA drawn from VREF. Mind the output current, it's not a 7805 in there.So, that should be pretty good on topology.Component selection and other specs will drive the power components (snubber, bypass caps, etc.).Tim

author: Zipdox
author_rank: Regular Contributor
timestamp: May 13, 2021, 07:48:30 pm
content: Quote from: T3sl4co1l on May 13, 2021, 07:27:11 pmSo I should use a resistor at each side of the shunt and one capacitor between the IN pins? Also, I read that the resistors should be 10 ohms or less, should I abide by this?Quote from: T3sl4co1l on May 13, 2021, 07:27:11 pmI'll reduce the frequency a bit then.Quote from: T3sl4co1l on May 13, 2021, 07:27:11 pmBecause I want to have headroom to adjust the current in the future if I need to.Quote from: T3sl4co1l on May 13, 2021, 07:27:11 pmThe other LED driver thread did it like this, but I'll change it.Quote from: T3sl4co1l on May 13, 2021, 07:27:11 pmIs the TLV2372 suitable to replace the INA240 as well? Then I'd only need one op-amp.Quote from: T3sl4co1l on May 13, 2021, 07:27:11 pmMaybe I can run the op-amp off of the 12V rail instead.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 13, 2021, 08:30:22 pm
content: Quote from: Zipdox on May 13, 2021, 07:48:30 pmYes.QuoteYes and you'll note the number of times I insisted upon exactly this change, to no effect...QuoteNo. Like I said, shop for a faster amp.Oh, it's also available in SOT-23-5 singles (2371), FYI.QuoteYes, that also works.Tim

author: Zipdox
author_rank: Regular Contributor
timestamp: May 13, 2021, 09:14:48 pm
content: Quote from: T3sl4co1l on May 13, 2021, 08:30:22 pmDoes doing this negate the need for a rail-to-rail opamp? That would be ideal.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 13, 2021, 09:40:38 pm
content: Yes.

author: Zipdox
author_rank: Regular Contributor
timestamp: May 14, 2021, 03:35:49 pm
content: I used net labels to make it more readable. Also added overvoltage protection.Lowered the frequency a bit too.Is it ready to be turned into a PCB?

author: Zipdox
author_rank: Regular Contributor
timestamp: May 14, 2021, 05:01:03 pm
content: Oops I seem to have made the same +12V to GND mistake again at U2

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 14, 2021, 09:07:07 pm
content: R16 supply can still be +5V... or was this to mitigate 5V load? Why not use a 10k or higher pot?D1 circuit looks like this,PCB ready? How the heck should I know, you haven't mentioned a single component selection as yet... you're gonna have a hard time if several of those componentsdon't exist, or need to be rated much higher than the few parts you have here now (like the bypass caps).Oh weird, how did Q2 get to be Q, typo?You can avoid the accidental short circuits, and potentially overlapping texts, by expanding the circuit out like this. Leave space between components, and run wires to meet them.Tim

author: Zipdox
author_rank: Regular Contributor
timestamp: May 14, 2021, 10:42:44 pm
content: Yes R16 is to mitigate the load. All of the components I either have or are available on TME, that's how I chose them. Q2 was the default label, I'll change it to a D.With PCB ready I meant that there aren't any mistakes and I can start designing the board layout.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 15, 2021, 12:12:23 am
content: I mean like the inductor and supply/output capacitors, those aren't exactly general purpose "anything will do" y'know...Tim

author: Zipdox
author_rank: Regular Contributor
timestamp: May 15, 2021, 12:35:16 pm
content: I think I'll steal the toroid from an old boost converter and hand wind it, I bought an LCR meter so I can measure the inductance. I'll wind it a full circle twice instead of one tap per side, to increase coupling.These are the caps I have in mind:https://www.tme.eu/nl/en/details/eeefc1v331p/smd-low-imped-electrolytic-capacitors/panasonic/https://www.tme.eu/nl/en/details/ucd2a470mnq1ms/smd-low-imped-electrolytic-capacitors/nichicon/

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 15, 2021, 09:36:34 pm
content: The 330uF is rated 670mA at 100kHz, 105C, at best maybe 1.4-2x more if operated in low ambient (<40C?). And the 47uF are only rated 500mA. They will not run cool here!Tim

author: Zipdox
author_rank: Regular Contributor
timestamp: May 15, 2021, 10:02:51 pm
content: Let's say these for input:https://www.tme.eu/nl/en/details/eeefp1v331ap/smd-low-imped-electrolytic-capacitors/panasonic/I can't find anything good for output. Should I just compensate with big ceramics?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 15, 2021, 11:16:53 pm
content: Those are still only 1.19A.Have you calculated how much RMS ripple the input and output will require?Tim

author: Zipdox
author_rank: Regular Contributor
timestamp: May 17, 2021, 06:31:26 pm
content: You mean ripple voltage? I think it's not really critical, since it's an LED and not more circuitry. For 0.5V ripple I got 77uF usingthis calculator.

author: TimNJ
author_rank: Super Contributor
timestamp: May 18, 2021, 02:19:02 am
content: No, ripple current through (into and out of) the output capacitors. Each switch-mode topology has it’s own governing equation for RMS ripple current in the output capacitors. For topologies which can be operated in DCM or CCM, the ripple current equation changes depending which mode you are operating in. DCM higher ripple current than CCM.During each switching cycle current charges up the capacitor and then the load discharges it. All capacitors have non zero ESR. The capacitor will dissipate some power by I(ripple)^2 * ESR. For this reason, electrolytic cap manufacturers give a ripple current rating which represents the ripple current the capacitor can handle at its rated temperature to assure it will meet its stated life. You need a reasonable amount of capacitance to keep the control loop stable. I’d argue that the primary design goal is not the capacitance value, but ESR/ripple current rating. This will lead you to a narrow set of options, assuming you have some space constraints or otherwise. Then go and confirm that the control loop is happy with your selection of capacitance. If not, maybe you need a re-think on a higher level. This is the way I’ve done it..doesn’t mean it’s a perfect approach.