Terrible Idea Boost Converter To Be Tested On Gocart - Page 1
Forum section: electronics / projects

author: i_am_fubar
author_rank: Regular Contributor
timestamp: December 24, 2019, 07:58:41 am
content: I got an 1800W treadmill motor a while ago and decided to make a go cart with it.It's a 180v motor.I've run it off 4 SLA's, but at 48v, the speed is less that impressive. Plus, I think the batteries are almost dead, so really not as fun as I hoped.Sooooooooooo, I've decided to build a boost converter / motor driver to run treadmill motors (upto 200v) from not 4, not 3, not 2 but 1 SLA battery (probably a big ol' leasure battery).... Just dealing with the fact it'll pull 150A. It's freaking me out a bitHave got prototype working on the bench without load, but wondering if there are any other similar designs out there I could get inspired from?

author: richard.cs
author_rank: Super Contributor
timestamp: December 24, 2019, 10:23:23 am
content: Boosting a 12 V battery up to 180 V for a motor can probably be made to work but for that sort of power levels will almost certainly cost more than buying a lower voltage motor or a higher voltage battery pack. A 150 A input boost converter just isn't that sensible but if you really wanted to then you should look at schematics for a 2 kW inverter minus the output chopper (or just buy one and modify it).I have a kart project at the moment using a car dynamo as the motor (in seperately excited mode) with a 48 or 60 V battery (not settled yet). This is a very cheap way of getting a brushed motor capable of around a kilowatt.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 25, 2019, 01:26:12 am
content: Just one? It's not going to run long, that's for sure; that's typical current draw for a starter motor!Consider using several channels in parallel, and using push-pull forward converters. This reduces the amount of reactive power required, so you can use smaller inductor, capacitors and transistors, and layout is easier and efficiency is better.Transformers can be scrapped from car audio amps, rewinding the secondary for the desired output voltage. The power transistors and controller (usually a TL494) can be reused. You will need to add an output filter choke, current sensor (can be a shunt resistor or Hall effect sensor), gate drivers and rectifiers; snubbers are a likely addition as well.The channels can be phase-shifted between each other, to help cancel out some ripple, so you don't need as large capacitors at the input and output. (The caps from the audio amps would be fine for input, and caps from off-line PSUs would be fine for output.) You then need one TL494 controller per channel, and a phase shift / clock oscillator to run them all (which I haven't figured out a minimal method for TL494s just yet, but there are some straightforward options to start with).It's no harder than doing just one channel, it's just more bother, repetition. Indeed you'll have a higher chance of success with the smaller channels, than with one big one.Tim

author: i_am_fubar
author_rank: Regular Contributor
timestamp: December 25, 2019, 07:24:52 am
content: Yea, the prototype doesn't have a choke or any filtering other than the caps (all elec, no ceramics so RF will be a pig). Will need to add some external filter to the actual switcher. Thankfully I'm not going for EMC tests, but how hard could they be....I've got 1 phase made up at the moment. Another I can make up, buy provision for 4. Using a PIC to generate PWM, won't be smooth as far as far as PSUs go, but it's driving a motor, so not too worried. Plus I can adjust motor speed with finer resolution from the drive PWM.Have got current and voltage monitoring on input and output. At the moment streams results to PUtty, but got a phone app that will give me a proper dash board in time.Going to try and attach a picture now.... Seems to be workingResistors give me 200ohm, so it's enough to mad a 10A bench supply out for now. Once I'm happy with control loops and voltage rating, I'll swap over to a (fused) battery.Have designed to take up to 36v input, so could knock it down to 50A input which is a much happier current.Also, I know this isn't an ideal way of doing it....bits just fun :pAlso, proof of concept... On verovoard... That blew up...https://youtu.be/SDAV5vs-Nlo

author: Zero999
author_rank: Super Contributor
timestamp: December 25, 2019, 09:00:47 am
content: Your best bet is to scrap the boost converter and use more batteries in series to give 180V. Failing that go with a lower battery voltage, but still higher than 12V, such as 48V or 60V, to keep the current low. It will be easier to construct an efficient boost converter to go from 60V to 180V, than 12V to 180V.

author: i_am_fubar
author_rank: Regular Contributor
timestamp: December 25, 2019, 09:09:38 am
content: I know it's not optimal and my efficiency will be lacking. But it's a good learning experience. First time dabbling above 1kW. And not the last by a long shot.Once I can tick this one off the list, I've got my eye on a 10kW BLDC driver I want to make.

author: MagicSmoker
author_rank: Super Contributor
timestamp: December 25, 2019, 11:17:30 am
content: Terrible idea for a number of reasons, with perhaps the worst being that a standard boost converter can't deliver an output voltage any lower than the battery voltage because there is only a diode in the path between source and load. This also means there isn't a way to limit, much less regulate, load current, and that is highly desirable both for protection and usability in a traction application. (NB - a synchronous boost can turn off the output, but can't regulate it below the source voltage except in the reverse direction, because then it becomes a synchronous buck)Third is that lead-acid batteries deliver less and less actual capacity as current draw approaches/exceeds the 1C mark; ie - a drawing 100A from a 100Ah battery might reduce usable capacity to 45-55Ah (see the Peukert Effect).Fourth is that treadmill motors are mostly rated on their peak power and if from a consumer grade device you can bet they won't withstand even running at 50% of their claimed power rating for more than a few minutes. The continuous power rating of a motor - much like with mains transformers - is rather inextricably linked to weight and size, after all.

author: iMo
author_rank: Super Contributor
timestamp: December 25, 2019, 11:18:28 am
content: Quote from: i_am_fubar on December 25, 2019, 09:09:38 amAs a child I spent some time with cherry pit spitting training. I did perhaps 5 meters. I knew it was not optimal and a waste of time. But it was a good learning experience too

author: i_am_fubar
author_rank: Regular Contributor
timestamp: December 25, 2019, 12:39:42 pm
content: Is this a forum forantsonly good ideas? I know this is very much not the way to do it. Ideally I would want a battery with a minimal full load voltage greater than the motor with an off the shelf controller.....BUT! It seemed like a project that has lots of oppertunities for experementing with funcky control loops and high power and some cool flat wire inductors I got.However, I have re-named the thread to something a bit more accurate (thanks for the inspiration Magic)Also, to address the reasons this is a terrible idea, and maybe make it sound like not-such-a-terrible-idea project......1) Minimum voltage outputYup. if i were running the motor directly off it... This would be bad. Buuuuuuuut I have an extra low side switch for fine speed control, right down to 0%.2) load current regulationI have current sensing on output and input. I can control my load through adjusting both the boost AND the motor drive FET.2.5) This is not a fixed output converterI have a PIC that measures the motors actual speed vs demand. If it starts to run on too low of a drive signal, it can reduce the boost output voltage. Negating a lot of the efficiency issues. this also means I can loose the boost in the future and just have a 180v supply.3) Flogging lead acidpB is not ideal. Would rather have Li-ion or Li-po, but on top of the cost, I want to minimise risk of bang bang when charging. If i could run this off a bench supply, at 100%, I would.... But I'm not allowed a 150A bench supply4) treadmill motorA few minutes of full power would make me very happyI'm not doing laps on this thing. If it goes ka-bloom then oh well. I get to buy brushlessthat does remind me though that I should put a temperature sensor on the motor.So, with all said, I'll try and get some results up at some pointor pictures of a charred PCB stack.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 25, 2019, 12:50:49 pm
content: Another advantage to the forward converter: it throttles to zero.You can do the same with a boost by making it SEPIC instead, but that would be terribly impractical at this level (even more so than boost is already). It would be better to make it flyback, but it's even better to use forward.Tim

author: Zero999
author_rank: Super Contributor
timestamp: December 25, 2019, 12:51:23 pm
content: Quote from: MagicSmoker on December 25, 2019, 11:17:30 amI don't see at as an issue. Use the boost converter to get a regulated 180V and add the standard motor + MOSFET and freewheeling diode to the output. Now the motor's speed can be PWM'd from 0 to full speed.QuoteYes those are far more important reasons why this is a bad idea. and the one about the lead acid reducing its capacity, at higher loads though is true irrespective of whether many small batteries are connected in series or a boost converter and larger battery is used.

author: i_am_fubar
author_rank: Regular Contributor
timestamp: January 02, 2020, 08:39:07 am
content: So far, I've hit a few snags, some real fails (like my floating current monitor IC back to front), some micro controller niggles (such as ADC cross coupling between inputs due to bad timings) and some legitimate gotchas (like noise on encoder screwing up speed readings. However, I've now hit....PROBLEM THAT I NEED HELP WITH #1My main boost switching element is a 80A, 250V MOSFET IXFH80N25X3. The gate is driven from a FAN3100TSX 3A gate driver running at 12v. Circuit as followsUnfortunately, I am getting some bad ringing as the gate voltage reduces past VGSon. Will get the scope traces when I get home.While the circuit mostly works, with the initial setup (gate resistor of 11 ohms) the drive chip was dying, ringing started and just didn't stop until failure, presumably through large negative voltage spikes or very high rate of on/off switching.Up rated the resistor to 100r and no more dying driver, but very slow on-off speed. Not what I want for a SMPS. Dropped down to 20r to hit speed spec, but ringing back.The layout is reasonably tight and as far as I can tell has limited inductance.Any thoughts or ideas on how to optimise this gate drive?Cheers All

author: mzzj
author_rank: Super Contributor
timestamp: January 02, 2020, 10:04:39 am
content: Quote from: MagicSmoker on December 25, 2019, 11:17:30 amNot problem really if you use 180v threadmill motor and 12v battery.Go-kart would probably barely if at all move with 12v.

author: MagicSmoker
author_rank: Super Contributor
timestamp: January 02, 2020, 11:14:03 am
content: Quote from: mzzj on January 02, 2020, 10:04:39 amSo letting the motor sit there stalled while the 12V battery continues to dump current into it through the boost diode isn't a problem?

author: fcb
author_rank: Super Contributor
timestamp: January 02, 2020, 01:49:50 pm
content: In the mid-90's I built small bost converters that went from 12V to 26V at 10A output for the film industry. We ended up with a custom planar magnetic inductor which was really expensive but dense. Going to 15x with a boost converter is pretty out-there, you'll need to be able to shut that FET off FAST, that's a heck of di/dt on the gate. Just doubling the number of lead-acids is going to halve your head-ache.And THE biggest problem (by far) was scoping out the gate drive - all the high currents and fast edges meant you chased ghosts all day long. Have you got access to an IsoVu?Good luck - you'll learn alot from the project. Look forward to the next installment.

author: i_am_fubar
author_rank: Regular Contributor
timestamp: January 02, 2020, 02:43:23 pm
content: Very fair point on the fast turn off. Hadn't really considered that turn on is much less critical.Inductor wise, I'm hoping these Wurth ones do the job, the spec is very tidy looking, but I'm also expecting more leakage than on the toroidal one I wound.I wish I had an Iso probe. Just measuring the ground, that was also the ground point I was grounded to, is giving me high levels of pickupAm going to build up another boost phase over the weekend and play around with different snubber arrangements, decoupling options, slew rate control.... etc. Drive it from Sig-gen as opposed to PIC so I can have a more consistent drive signal.Got my battery isolator, meter & fuse box on the go to make it a bit safer and be able to operate the whole thing floating.Oh, and I'm slowly relenting..... 10Ah 22.2v Lipo may be a better option for me :p

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 02, 2020, 03:06:26 pm
content: So what, ballpark 20nH there? Mostly from lead length. And Coss around 1nF? Resonates at 35MHz and impedance 4.5 ohms. So, if turnoff is faster than about 30ns, peak voltage will be noticeable.Note that the part of turn-off below Miller plateau is what's relevant: Vds has stopped moving (the diode has turned on) and current falls (until this moment, the transistor is still carrying all the inductor current). So this can still turn off fairly quickly even if your gate resistor is relatively large.The maximum overshoot is delta V = I_pk * Zo. So it's pretty significant that the impedance is quite high, compared to the switching impedance (which if it's say 100V and 50A peak or about 2 ohms). Preferably it should be the other way around (Zo < Zsw).No, there's no good way to bring it down -- you could load the drain with some capacitance, but for one it's not going to be any "closer" to the transistor itself (there's still the 10nH or so of the TO-247 package), and it's only going to increase losses (namely at turn-on). Increasing L or C only increases the reactive power and thus switching loss.Note, no source resistor, or current clamp on the inductor, is a disaster waiting to happen.Tim

author: i_am_fubar
author_rank: Regular Contributor
timestamp: January 02, 2020, 04:04:37 pm
content: Need to have a proper read through this reply when I've got the thing in-front of me.For reference here, what do you mean by source resistor or current clamp? (in this instance).Cheers.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 02, 2020, 04:12:22 pm
content: Anything to sense switch or inductor current. So, a source shunt resistor, or one referenced to the input (positive or ground, as, and if, applicable). Or a current sensor, like a Hall effect chip or clamp. Or etc.Tim

author: i_am_fubar
author_rank: Regular Contributor
timestamp: January 02, 2020, 05:00:18 pm
content: Ahhh, I've got a separate 100A hall sensor on another PCB that also generates the PWM. It's shared across phases but should be able to detect OC and failures to force shutdown.Ditto a 12A floating shunt on the output, shared across.

author: Siwastaja
author_rank: Super Contributor
timestamp: January 02, 2020, 05:12:39 pm
content: Quote from: MagicSmoker on January 02, 2020, 11:14:03 amWith a large mechanical switch or contactor, which likely needs to be there for safety shutdown anyway, maybe it's acceptable behavior if the builder is the only user? Off-state current and torque would be low. It sucks, though.

author: i_am_fubar
author_rank: Regular Contributor
timestamp: January 02, 2020, 05:45:33 pm
content: I did try a basic relay switch. 4x (rather cr4p condition) batteries gave an ok speed. 1 off would be a slow, but not insignificant providing the conductor losses aren't too low.

author: amyk
author_rank: Super Contributor
timestamp: January 02, 2020, 09:38:08 pm
content: For a relatively cheap and quick solution I would suggest getting an inverter (one with 120VAC output will be ~170VDC) and either putting a rectifier on the output or tapping into its internal HVDC bus.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 03, 2020, 12:57:41 am
content: Quote from: i_am_fubar on January 02, 2020, 05:00:18 pmYou need a per-channel sense. Think about current sharing, let alone compensation.Tim

author: i_am_fubar
author_rank: Regular Contributor
timestamp: January 03, 2020, 06:12:17 am
content: Assuming ultimate accuracy and voltage stabilty isn't my priority. I figured that hitting each channel with the same PWM duty for a few cycles before readjusting based on average current /voltage would be sufficient to keep the system in it's safe operating perimeters.Or do you think that the channel characteristics will vary that much?Re. Inverter. Considered it, but the goal is to get some experience designing high power switching systems. So not really applicable

author: james_s
author_rank: Super Contributor
timestamp: January 03, 2020, 06:35:55 am
content: Quote from: amyk on January 02, 2020, 09:38:08 pmYou can buy just the 12V->170VDC part from China, at least I've seen them available in the past.Personally I'd be looking at an array of RC LiPo packs to get >100V or a surplus EV battery module. Salvaged Tesla batteries are surprisingly affordable for what you get.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 03, 2020, 07:20:07 am
content: Consider what happens if there is a small say 1% duty mismatch between channels driven at the same PWM; consider further if they have negligible resistance (in the inductor, switch and diode) (which they do); and consider if there's say a 10% error in inductance. What does this do to current flow in the different channels? How can you predict this effect? How can you prevent transistors from exploding?Well, you can't, which is why we put current sensors on them to be sure.Tim

author: pepelevamp
author_rank: Regular Contributor
timestamp: January 03, 2020, 08:17:21 am
content: get one of those spinning wheels that one motor turns while a second one gets spun & generates a voltage.why bother with fast switching mosfets and silicon when you can add an extra wheel to your go-kart and use cogs to convert your power.

author: i_am_fubar
author_rank: Regular Contributor
timestamp: January 03, 2020, 08:44:14 am
content: Hmmmm, good point. There is definitely a risk here.I would argue that I can prevent the transistors from exploding. But it would be at a significant performance limit. Assuming that all inductors are actually at their -20% limit, I can configure my maximum on time to ensure their current can't rise above a safe value (say 50A). Yes, this means that if the inductor is actually at +20% tolerance, then I'm running nowhere near peak power throughput for the power components, but at the same time, it does mean nothing will go bang (in normal operation).The mismatch is a bigger worry and I may end up with 1 of the 4 phases doing 50% of the conversion, but again, providing my timing is such that I'm not exceeding SOA for each device, I don't see it as a magic smoke risk.Of cause, this is all my opinion as an engineer very new to high power, high speed switching.... So, please keep punching holesPlus, if I up my input voltage to 24v at half the current, then the performance degradation of playing it safe should be offset.Way to do efficient, low cost, design(sarcasm)On the bright side, I think I finally grasped the physics of gate ringing last night and how the parasitics come in to play, why snubbers work, and why slowing the gate reduces the effect. So, as a learning exercise, this is working

author: i_am_fubar
author_rank: Regular Contributor
timestamp: January 03, 2020, 08:46:48 am
content: Also, I have a bus bar design that I'm going to use for feeding the battery supply into each channel. I could integrate a current sensor into each branch of that to get independent monitoring.Onward an upward, more bits coming in today.

author: pepelevamp
author_rank: Regular Contributor
timestamp: January 03, 2020, 08:57:15 am
content: this sounds like a real cool project. i am looking forward to seeing footage of you sitting in a go kart moving along off a 12v battery. maybe race a friend riding a power drill.

author: jbb
author_rank: Super Contributor
timestamp: January 03, 2020, 06:29:59 pm
content: Quote from: T3sl4co1l on January 03, 2020, 12:57:41 amQuote from: i_am_fubar on January 03, 2020, 06:12:17 amYes, you almost always need it per-channel. This is because a small change in switch duty cycle translates to a small change in theDCoperating point of the inductor. Because your MOSFETs and inductors will be chosen to have low DC resistance, you will get a large current imbalance.However, there is one way to get around that. It will likely compromise the efficiency of your converter. Go to Discontinuous Conduction Mode (DCM).Advantages:MOSFETs turn on near zero current. No diode reverse recovery loss in synchronous rectifier diodes (which is good, because 200V MOSFETs have pretty crap body diodes)Inductor currents drop to zero each switching cycle, which reduces the system order and can make compensation design easierCurrent sharing will be largely - and stably! - determined by inductor toleranceInductorvalueswill be lowerDisadvantages are significant:Increased RMS currents in the converter mean higher conduction lossesIncreased current ripple will increase inductor core loss. Inductors may end up being quite large and difficultSomewhat more difficult to control synchronous rectifiersInductor currents drop to zero andwillring a bit (inductance + MOSFET capacitance = LC resonant tank at a few MHz). This could make a lot of radio noise, and may require snubbers to damp it away.Here's a question: do you really need a well controlled output voltage? I assume you will have some kind of motor speed controller after this stage (so that you can control the motor speed down to zero)?Getting a 1:10 step up out of a boost converter is a bit of an ask anyway, so why not use a fixed-ratio step-up converter (e.g. unregulated full bridge transformer)?

author: amyk
author_rank: Super Contributor
timestamp: January 04, 2020, 04:29:57 am
content: Quote from: i_am_fubar on January 03, 2020, 06:12:17 amYou can take the inverter apart and learn from its design too.