Questions on designing Fwd/Rev 12V DC motor controller for 80A typ (200A peak) - Page 1
Forum section: electronics / projects

author: Psi
author_rank: Super Contributor
timestamp: December 21, 2020, 02:49:49 am
content: I have been tasked with building a motor controller.12V DC (from SLA)Forward / ReverseMostly just 100% dutycycle, maybe a short PWM for a soft start80A typical but I have seen 200A when the motor is stalledIt will control two motors, but only one will be on at a time.My plan is an MCU controlling a mosfet gate driver IC connected to a few N-Channel mosfets in parallel, all forming 3 half bridges. With some Isense resistors and a current sense amplifier feeding back into the MCU.My question is around the best way to drive the mosfets.Obviously being N-channel I will need a voltage boost somewhere to switch on the high side fets.What mosfet drive IC would you recommend?

author: Psi
author_rank: Super Contributor
timestamp: December 21, 2020, 04:32:20 am
content: Currently looking at IRS2008(S,M)PBFhttps://www.infineon.com/dgdl/Infineon-IRS2008%28S%2CM%29-DataSheet-v01_00-EN.pdf?fileId=5546d46269e1c019016ae5475e943bd3&redirId=115112

author: Berni
author_rank: Super Contributor
timestamp: December 21, 2020, 07:40:15 am
content: Be careful with MOSFET driver chips that have a bootstrap capacitor, these need to switch constantly to keep that capacitor charged up. But one could still work around that by generating a extra supply rail above the main input using a boost switcher and feeding the upper side of the gate drive chips from that.But when it comes to driving large loads, i like integrated H bridge chips like the IFX007Thttps://www.infineon.com/cms/en/product/power/motor-control-ics/brushed-dc-motor-driver-ics/single-half-bridge-ics/ifx007t/#They make the job a lot easier since designing a H bridge with the same level of robustness is actually quite a lot of work. This is essentially a "smart mosfet" that protects itself from various hazards. So any shorts or overloads on the output don't kill it, any werid brownouts don't get the switches in the linear region.

author: Circlotron
author_rank: Super Contributor
timestamp: December 21, 2020, 07:41:22 am
content: About ten years ago I did a controller for an automatic sliding door using a 24V DC PM brush motor. Same basic idea but smaller. I used a pair of SPDT relays across the motor for polarity reversal and a single mosfet for PWM and on/off. After 100,000 opens and closes there was essentially zero wear on the relay contacts because they were arranged to operate or release under zero load. Much simpler than driving a H-bridge. Worth considering perhaps.

author: Psi
author_rank: Super Contributor
timestamp: December 21, 2020, 08:47:24 am
content: Quote from: Circlotron on December 21, 2020, 07:41:22 amYes, it is an option, but I would prefer a non-mechanical solution if I can get one to work

author: Psi
author_rank: Super Contributor
timestamp: December 21, 2020, 08:49:03 am
content: Quote from: Berni on December 21, 2020, 07:40:15 amYep, i saw that in the datasheet. figure I would just roll my own charge pump to supply the driver chip so it doesn't have to rely on output switching to generate the higher voltage.Yeah, i looked at a few smart H bridge chips and have used them in other projects but i've not seen any in the 200A output range. They all seem to stop around 50-80A

author: Berni
author_rank: Super Contributor
timestamp: December 21, 2020, 09:43:40 am
content: Quote from: Psi on December 21, 2020, 08:49:03 amYou could parallel up multiple of these to get the current handling and make heat dissipation easier.As for peak handling they actually internally limit the current by PWM cycling themselves. So even if a short is present they won't exceed there own current capability.

author: filssavi
author_rank: Frequent Contributor
timestamp: December 21, 2020, 09:55:27 am
content: I can give you few advices:1) Evaluate carefully what is the true maximum current the application needs, while machine designers love to specify both rated and peak torque (that translates to current), dont let them fool you, the drive must be rated for continuous operation at peak power, as the peak torque specified in the machine datasheet can be held for few seconds/minutes, and such long times are continuos for transitors2) Avoid bootstrapped gate drivers like the plague, if you can, use isolated gate drivers with a separate isolate supply (a 1W isolated dc/dc converter from traco power/ murata etc. is more than enough) while this is slightly more expensive it greatly simplifies the design3) Go with discrete devices(for example IRL40B212), or if you can find it a module with just the power transistors (something like the SEMiX201GD066HD could work, however this is large overkill as it is rated for 600V), as I doubt you will find anything with an integrated gate driver at such low voltages, high currents. The dv/dt are low enough that you should not have any significant signal integrity issue anyway4) As the voltage is low enough use sense resistors directly on the outputs with dedicated current sense amplifiers (high CMRR). While it will probably not be strictly necessary (again the voltage is low enough that I dont expect much noise) I would still try and do sampling in the middle of the PWM period, in order to avoid any commutation noise screwing with your current control5) if you just plan on doing direct connection with the battery (100% duty cycle), for most of the time, then I would also suggest to just use an appropriately rated contactor, it will be simpler, quicker to design, and much more efficient and cheaper too (probably even including replacing the contactor once or twice when the contact start to burn out)

author: max_torque
author_rank: Super Contributor
timestamp: December 21, 2020, 07:21:30 pm
content: Critical questions:1) How many are you making?2) Are there "cost" targets?3) Can you just buy one already?4) what is the actual load profile?Given that a 12V DC motor controller is not excatly cutting edge these days, i suspect unless you plan to sell the thing to make money as a product, you'll be better off just buying a COTS item.......

author: filssavi
author_rank: Frequent Contributor
timestamp: December 22, 2020, 12:11:38 am
content: Quote from: max_torque on December 21, 2020, 07:21:30 pmI can’t obviously answer for the OP to those questions, however I can say that COTS motor drives are extremely opinionated on how the machine “should” be run, and if the application is not completely standard you will start having a lot of trouble finding the perfect one.As an example a lot (maybe even most) of the general purpose commercial drives are only able to be run in constant speed mode, and not in constant torque, despite having a torque (current) control loop inside. It is just not possible to directly control its setpoint from the outside.While the vast majority of the times this is perfectly fine, there are several application where this isa deal breaker (multi machine arrangements like conveyor belts, dynamic braking, etc.)Now if OP is unlucky and manages to have few of these oddball requirements combined, he might not have a choice other than rolling its own

author: Psi
author_rank: Super Contributor
timestamp: December 22, 2020, 04:13:09 am
content: Quote from: Berni on December 21, 2020, 09:43:40 amI've not seen this mentioned in the datasheet of the driver chips I looked at, but i've not looked at all the brands/models.Are you aware of any that talk about paralleling chips for more current?Without a note from the manufacture showing how to do it correctly I would be worried strange things might occur, given the chips have smarts in them.I'd prefer to use smart driver chips if possible, it would make things easier and less likely to blow up.Quote from: filssavi on December 21, 2020, 09:55:27 amyeah.The peak current I saw on the clamp meter was 200A but that was under an almost motor stall.Most of the time it was around 80-130A.Quote from: filssavi on December 21, 2020, 09:55:27 amI considered 4x optoisolated fet driver chips instead of 2x half-bridge driver chips to make the H bridge.One of the reasons that put me off the opto idea was that i would need to implement my own hardware logic for dead time and preventing shoot through current. I could just do that in software on the MCU but that sounds like a bad idea. Should really be done in hardware.The Half-bridge chips do that all for me.Quote from: filssavi on December 21, 2020, 09:55:27 amMy current design has 3x PSMN2R2-40YSD in parallel.Quote from: filssavi on December 21, 2020, 09:55:27 amAgreed, yeah going to use proper Isense amp chip on the Isense resistor.Quote from: filssavi on December 21, 2020, 09:55:27 amWe really want to move away from mechanical contactors.That's what's used currently and is causing issues.I think it's mostly issues with contactor QC. But there's now a perception that contactors = bad and a solid state solution is needed.But yes, I agree that contactors is a simple solution when 100% duty is needed, maybe with a fet to avoid arcing.Quote from: max_torque on December 21, 2020, 07:21:30 pm1) ~400/year needed2) Can't give a price sorry3) For this application it needs to be customizable and interface to other systems as needed.4) It's similar to a hoist system, with a pretty short on-time, like a minute, and plenty of time to cool down before next use.

author: Circlotron
author_rank: Super Contributor
timestamp: December 22, 2020, 04:16:57 am
content: Quote from: Psi on December 22, 2020, 04:13:09 amWhat sort of issues are they having? Burnt contacts? Foreign material ingress? How often is it required to operate?

author: Psi
author_rank: Super Contributor
timestamp: December 22, 2020, 04:22:26 am
content: Quote from: Circlotron on December 22, 2020, 04:16:57 amPretty low number of operation cycles.Mostly burnt and dead/DOAIt does get some vibration and I suspect this is breaking the thin coil wires inside.I think it's mainly that the contactors being used are cheap shit.But it has created a perception that all contactors are shit and i'm not going to try fight that.

author: Circlotron
author_rank: Super Contributor
timestamp: December 22, 2020, 04:28:11 am
content: Okay. Just for interest's sake, this is how I did mine anyway. Someone here might find it useful.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 22, 2020, 05:27:05 am
content: Drive speed is probably not something you will need, or want. That's a lot of current to switch at a time, especially with parallel transistors.I recommend doing a basic layout, inspecting it for loop inductance (and consistency between transistors), optimizing on that a bit, and then choosing gate resistors (likely it'll be one per transistor) and PWM frequency accordingly. If possible, PWM at a low frequency, or even restrict switching to a certain number of cycles in some time frame (to reduce peak heating).Current limiting would be nice (and would obviate the need for 200A peak capacity), but I suppose if you're going the brute-force way, yes, it is possible to handle LRA or even battery short-circuit current, at the cost of a few additional transistors.There may be the possibility of feature creep. I don't know anything about the underlying application -- just spitballing here. If someone happens to note that, "hey, we have a PWM driver here, let's add speed control and all that!", well, now you need to do PWM, but you've got an inverter that's optimized against it (high switching losses, no current sense) -- commence wailing and gnashing of teeth. As long as that's not a danger, yeah, a slow and lumbering inverter is fine here.EMI -- how long are the battery and motor cables, can shielding be provided, should it be filtered to some extent?Nice part about slow switching, like over a whole microsecond, both radiated EMI, and filtering needed to address it, are relaxed.Mind, this may still require special design consideration -- modern switching transistors have very steep capacitance curves, so that it's very easy to get fast switching edges, whether you intended to or not. A typical fix is more feedback, such as source degeneration (stray inductance or ferrite bead, say -- controls dI/dt), and shunt feedback (R+C from drain to gate, perhaps -- controls dV/dt). Nothing crazy, just something to provide on the layout.Edit: speaking of shorting out battery -- you may want to provide an independent, monitored supply for control power. That is, maybe you take raw 12V through a diode, into a relatively large reservoir cap (like some 1000s uF). Then into the control circuit (3.3/5V MCU, >10V gate drivers, charge pump, whatever), possibly using a switching regulator so that the higher voltages can stay in the safe zone for longer. Anyway, a UVLO circuit detects if the reservoir voltage drops too far, and sends a stop signal to the MCU -- disabling the outputs (or even better, do this in hardware with some logic gates as well). That way the transistors are turned off sharply, without spending any time burning up in an indeterminate range. (The alternative is to just wire everything together, getting gate drive voltage from raw battery. But then when it dips below, oh, 6V or so, now the transistors aren't turned so "on" anymore, current drops and power dissipation skyrockets... This could very easily happen with a dying battery or short-circuited motor -- it's worth consideration.)Tim

author: Psi
author_rank: Super Contributor
timestamp: December 22, 2020, 08:28:23 am
content: Thanks.Yes will definitely need hardware cutouts for- UVLO.- Over temp- And some sort of overcurrent shutdown that can protect against a shorted output. Which is a bit more tricky when 200A can occur normally.For this application PWM wont be needed and wont be requested in the future.Only thing I might do is a very short burst for a soft start. but even then it's probably out of scope. So i'm not worried about doing PWM at all.Here's the alpha test PCB.Note: I know this has issues, It's a very early alpha test prototype, to see just how hot things get.The HALL ACS770 current sensor I have here is not what I'm using in the actual product. It's just a quick and dirty sensor so i can do some datalogging. I'm aware its too slow to do much else. Real design with have resistor + Isense amp.The 5x5 grid holes are for 4x large copper SMT M8 ring terminal blocks. I'm testing using the thick gauge wire terminals as the heatsink.

author: Berni
author_rank: Super Contributor
timestamp: December 22, 2020, 08:48:09 am
content: Id avoid using a PCB if possible, dealing with currents this high on one is not easy.Use transistors in those big screw terminal packages to handle the high current switching. Get to know a small time laser cutting supplier to cut you some custom bus bars out of a copper sheet, use those to carry the high currents around between the output terminals, transistors and shunt.Then put your electronics on a small board off to the side, wired into all this with wires (Or if you want to be fancy for high volume you could use a flat flex PCB that spiders out to all the required connections).If you do want to use SMD transistors id recommend splitting the design into two boards. Put the transistors on the bottom board and make that a single sided aluminum core PCB with really thick copper such as 8oz. Bolt that board to something to help cool it such as a die cast aluminum housing. Then put the fancy electronics on the 2nd board being a regular classic 2 sided PCB where all the actual electronics live.Just to put things into perspective a 50mm2 (0 AWG) copper wire is required to pass 200A. This is a cable that is already the size of a small garden hose.

author: Psi
author_rank: Super Contributor
timestamp: December 22, 2020, 08:56:54 am
content: Quote from: Berni on December 22, 2020, 08:48:09 amYeah, it definitely may come to that.One of the reasons i'm making this PCB above is to get a real-world gauge of just what i'm dealing with at 200A.The amount of current the boards able to handle safely will tell me a lot, be that 50A 100A etc..And how long it lasts before melting at 200A too haha.I just to happen to have a 230v to 12V @ 115A PSU to do some tests withand i can probably get 2nd one.Also have a box of around 20x 0.1R 100W chasse mount resistors to make a dummy load.Got to love having lots of junk around, never know when it will be useful

author: CJay
author_rank: Super Contributor
timestamp: December 22, 2020, 09:10:39 am
content: www.4qd.co.ukused to run a forum on their site at the height of Robot Wars in the UK, lots of useful info on there about designing and the rationale behind design choices for some simple to complex DC motor controllers.

author: xavier60
author_rank: Super Contributor
timestamp: December 22, 2020, 09:13:33 am
content: Most MOSFET driver ICs are susceptible to voltage undershoot. It will be difficult to design a satisfactory layout on the first attempt if it's going to be a one off project.http://www.irf.com/technical-info/designtp/dt97-3.pdfFor my current project, I intend using Galvanically isolated drivers.https://www.st.com/en/power-management/stgap2d.htmlIm expecting that they will allow the layout to be much less critical.One problem is that this part will not be available until March.Added: Found some here,https://www.futureelectronics.com/p/semiconductors--analog--drivers--motor-drivers/stgap2dmtr-stmicroelectronics-5113851

author: Berni
author_rank: Super Contributor
timestamp: December 22, 2020, 10:02:10 am
content: Quote from: Psi on December 22, 2020, 08:56:54 amI can tell you that at 20A things get toasty on regular jellybean 1oz copper PCB. Going up from there things just get exponentially worse due to P=I^2*R. So once you go higher the expensive > 3oz becomes pretty much a must.Also the transistors datasheets always specify specs with maximum possible cooling to get the best numbers. The cooling provided by a regular 2 layer board is next to none, so the transistors can get very hot very fast at only 10% of the rated "maximum current". So running things at these sort of levels you need to keep in mind some way of actually cooling your transistors. The very large motor drives >300A often have water cooling.I have a 500A lab PSU here and dealing with anything above 100A becomes very difficult without a hefty mass of copper.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 22, 2020, 12:13:23 pm
content: Yeah, 3, sometimes 4oz is reasonably affordable, and obviously triples the ampacity (or more). It's still usually near the realm of the custom stackup -- but a hundred bucks here or there is just a couple hours labor elsewhere, it's hardly a big deal.Alternately, get 2oz x 4 layers, that's 8oz total copper if you stitch them all in parallel, or 4oz + 4oz when used in pairs, that can also be interleaved for better heat transfer and stray inductance, etc.You can increase ampacity just fine, as long as you have adequate cooling -- clamp thermal pads and heatsinks onto it for example. Does nothing for voltage drop of course. Can also leave the copper bare (remove soldermask and silkscreen) and solder or tack copper sheet on top. Helps for heat dissipation too.The Hall effect sensors are more than fast enough for motor control purposes, I wouldn't discount them by any means. Don't know exactly which you were looking at, but I recall >20kHz bandwidth being available. Maybe not quite enough time to react to a short, but alright to control motor current.Don't worry too much about wire size. Ampacity tables are usually listed by code, for use in conduit, in insulated walls, etc. (Sometimes you'll see chassis ratings, which are more optimistic, and closer to what is likely the case here.) Several steps thinner is still acceptable for continuous duty, in short lengths, and when heat dissipation is adequate. Presumably the 200A is only momentary overload? I'd be perfectly happy with 80A (nominal) through 6AWG, maybe even 8. And you can always bolt on bus bar where needed.BTW, is that a TVS looped around the outside? That's not going to do very much -- that's a good 50, 100nH there. 200A/1us * 0.1uH = 20V already, and accidentally switching any faster (up to a fully powered gate drive, that would crank that off in 50ns or even less) very quickly uses up any overhead you had.Put it up close and personal, right beside the MOSFETs. One per set (of FETs) I suppose is fine, nothing wrong with using extras, especially if you can't afford to place them right across the supply [because of the extreme priority to current handling].Alternately -- and preferably -- use a lot of electrolytics, probably at a minimum, 8 x 1000uF, >= 25V, < 0.1 ohm ESR each. That'll keep the peak voltage rise down to a few volts [due to inductance between battery and switch], and may allow for faster switching. (I mean, you can still have a TVS across the supply, it's not a bad idea still; it just won't be needed on ~every switching edge.)And, if you aren't using the bottom side copper over most of the switching area -- fill it with the other supply, and interleave each capacitor with each pair of transistors (in its respective half-bridge). This keeps loop inductance to a minimum; downside, it leaves little room to route gate drive signals, which might actually be worth wiring via jumpers, or using axial resistors to the same effect, or going to 4+ layers.Or heh, I can't say I've ever done this -- nor needed to do this -- but you could easily leave some SMT pads on top copper, to solder down cut-off chits of copper-clad -- yep -- doing Manhattan style breadboarding,on top of a printed damn board. Why not, it's only a one-off? You can do whatever you want!Tim

author: Wolfram
author_rank: Frequent Contributor
timestamp: December 22, 2020, 12:18:23 pm
content: I've done PCB designs carrying up to around 100 A steady state, and I found the cheapest option to be multi-layer boards with moderate copper thickness, usually four to six layers of 70 um copper depending on how constrained the planes are by the component placement. If your 200 A peak requirement is for a shorter time period than the thermal time constant of the PCB, you mainly have to worry about the steady-state current. Board-wire interconnections are also a challenge, but there are a lot of nice press-fit connectors for this available these days.

author: filssavi
author_rank: Frequent Contributor
timestamp: December 22, 2020, 12:40:52 pm
content: You are on the right track, and I don’t see why you should not succeed in your endeavourAs for deadtime there is no need to roll your own you just need to chose the right MCU, the ones with advanced timers (meant for motor control like NXP kinetis V) will add deadtime for you automatically. Provided you use the right pins for the right transistors (basically the Timer has coupled outputs, you should use one for the top switch and the other for bottom one in a single leg)Avoid software deadtime like the plague^2 on a MCU, as it is a recipe for dead power devicesThe high current at startup is probably only due to the fact that at standstill you have no back-emf to limit it and you are not Ramping up the Voltage gradually. However it might also be that your load has very high static friction and needs a bunch of torque to get it going, but that should be fairly clear depending on the type of load

author: Berni
author_rank: Super Contributor
timestamp: December 22, 2020, 01:43:50 pm
content: Oh yes forgot to mention, if you do PWM ramp up the motor speed you will indeed not get that starting spike. The current will simply ramp up along with it up to the normal running current without any overshoot (provided you ramp it up slowly enugh).The motor itself will also be happier since it reduces sparking on the brushes and won't have that typical DC motor tendency to violently jump on startup since they produce maximum torque output when at 0 rpm.But you do get extra heat on the transistors from the switching losses during the PWM soft start. Also makes monitoring the motor current more important because PWMing a DC motor actually creates a buck DC/DC converter with the motor as the inductor. So you can for example apply 25% duty PWM on the motor, draw ~25A from the battery while there is ~100A flowing trough the motor. This is the reason why PWM control of DC motors is so nice, it can create more output torque from less supply current. But you do need to make sure your transistors can handle it.

author: Siwastaja
author_rank: Super Contributor
timestamp: December 22, 2020, 02:00:57 pm
content: 80A is well doable on a PCB. Tight layout, including minimizing the distance from the MOSFETs to the connectors is the key. If you understand the concept of ohms/square, you'll immediately see that making links short solve the current handling issue, for example a 10mm x 10mm trace is just fine for 80A, but if you need 100mm distance then you'd need 100mm track width to get equal resistance!Stall current is irrelevant because having an actual controller, you'll never see the stall current. Actually, if you have measured 200A stall current from 12V supply, I suspect actual peak current you'd ever like to run at would be less than 80A, maybe some 50A, unless the motor is a very inefficient one in which case the usable peak current is closer to the stall current. Excess current just saturates the motor iron and gives diminishing returns on torque so limit it to something sensible, typically that would be like 2-3 times the nominal "running" current, or maybe one quarter of stall current.

author: max_torque
author_rank: Super Contributor
timestamp: December 22, 2020, 08:43:22 pm
content: For "only" 400 units, you may find that using a mosfet "Brick" means you don't have to worry about carrying the high current on the pcb?

author: Psi
author_rank: Super Contributor
timestamp: December 22, 2020, 11:36:13 pm
content: Quote from: T3sl4co1l on December 22, 2020, 12:13:23 pmYep, that is exactly what my board above is, 2oz x 4 layers all in parallel.Quote from: T3sl4co1l on December 22, 2020, 12:13:23 pmInteresting

author: Psi
author_rank: Super Contributor
timestamp: December 22, 2020, 11:41:36 pm
content: Quote from: max_torque on December 22, 2020, 08:43:22 pmThanks, will investigate the cost of this, my initial look seemed to show them horrendously expensive.But digikey prices aren't always reflective of reality for specialized items.

author: Berni
author_rank: Super Contributor
timestamp: December 23, 2020, 06:25:27 am
content: Quote from: Psi on December 22, 2020, 11:41:36 pmYeah the modules do seam to be ridiculously expensive, but the "screw terminal brick" package individual mosfets are not that bad, here is one example for 23 bucks in 100qty:https://www.digikey.com/en/products/detail/ixys/IXTN660N04T4/6053919Only difference between a module and these beefy single mosfets is that you need to wire them up yourself. But as i said you can simply pay a laser cutting supplier to cut you custom fit busbars for your motor drive for the price of the raw copper stock + a few bucks for cutting it out. If you need the busbars to cross over each other without touching you also get copper spacers or get the people that laser cut it also give them a few quick bends on the press (its just sheet metal anyway). For prototyping you can just buy a straight copper busbar of the correct thickness, cut it to length and drill the hole for the screw. You will find this kind of busbar construction in most high current power supplies >200AOne significant benefit of these type of transistors is that they already have an insulated tab. The silicon die inside is already bonded to the mounting tab via a heat conductive ceramic insulator. So the tab can be directly screwed down onto the heatsink without causing shorts between the various transistors. Power losses are also recuded due to the low resistance of bus bars connecting things, so the bus bars also serve as cooling (Rather than heating up themselves from the current and making the transistors run even hotter)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 23, 2020, 08:04:43 am
content: Mind that you don't save much on inductance with those -- some of the integrated modules are good, some are literally as lazy as possible (~40nH!). Shop carefully.The SOT-227 mini-modules are around 10nH each, not bad considering they're about twice the size of a TO-247 for example -- but it still adds up to enough that you need to be careful. (And thanks to the Kelvin gate terminals, you can indeed drive them altogether too fast.) They're best suited to high voltages, where the inductance is less of an issue (say 200V+), and the sheer amount of available power is crazy (for some types, say, 10kW from a pair). Which, also because of that -- if you don't mind the sticker price -- since you can get so much more MOSFET inside these beasts, you can easily use 100V types for example, and just ride through the transients, who cares.Oh also, there's this thread ongoing if you haven't seen it,https://www.eevblog.com/forum/manufacture/high-current-pcb-options/Tim

author: nuno
author_rank: Frequent Contributor
timestamp: December 23, 2020, 08:38:49 am
content: If the application is kind of critical, you may still want a contactor at the power input, since FETs tend to fail shorted.You may also have to consider slow rail power capacitor charge.And protect the uC section from rail voltage transients.

author: Berni
author_rank: Super Contributor
timestamp: December 23, 2020, 12:08:19 pm
content: Oh and regarding thick copper you can get huge copper thickness on PCBs (but don't ask about the price of it)

author: S. Petrukhin
author_rank: Super Contributor
timestamp: December 23, 2020, 08:32:25 pm
content: Quote from: Berni on December 23, 2020, 12:08:19 pmWeird... I've seen copper build-up boards a long time ago, more than 10 years ago. Has the technology not evolved and become cheaper?

author: xavier60
author_rank: Super Contributor
timestamp: December 23, 2020, 09:43:24 pm
content: I ordered 2oz for my project. The prices go crazy for higher thickness. It's cheaper to increase the number of layers.https://www.pcbway.com/orderonline.aspx

author: wraper
author_rank: Unknown
timestamp: December 23, 2020, 10:30:40 pm
content: Quote from: Berni on December 23, 2020, 12:08:19 pmIt makes more sense to use multilayer PCB with traces on multiple layers connected in parallel before using that stuff.

author: S. Petrukhin
author_rank: Super Contributor
timestamp: December 23, 2020, 11:16:44 pm
content: Quote from: wraper on December 23, 2020, 10:30:40 pmThe inner layers are 2 times thinner than 1oz and they don't have convection contact with the air, isn't it?4 inner layers is less than 1 layer of 2oz, and the cost of such a board is off the scale.I'm currently developing a power unit, I'm thinking of trying to laser cut out ~ 4oz copper foil pads on the tracks that will be soldered with paste. I don't know what will happen, but I'll try. I don't know yet how to make the laser machine understand Gerber...

author: wraper
author_rank: Unknown
timestamp: December 24, 2020, 12:15:39 am
content: Quote from: S. Petrukhin on December 23, 2020, 11:16:44 pmYou can use wider traces on inner layers. Or free outer layers for wide traces and place majority of small traces inside. Example on the picture makes no sense as it uses exorbitantly thick traces which are very narrow. 1oz internal copper is nothing special, 2oz is possible too. Wider traces mean more heat dissipation, therefore you can use less copper for the same current. FWIW achieving copper thickness by adding internal layers in general is cheaper than using very thick outer layers.

author: S. Petrukhin
author_rank: Super Contributor
timestamp: December 24, 2020, 01:54:11 am
content: Quote from: wraper on December 24, 2020, 12:15:39 amAmong others, I still have an idea to try to make a power board on an aluminum base and screw it to the radiator. I do not know why copper buildup is so expensive, this method was used even in the USSR, I studied it at the Institute. On some boards, copper was not etched, but deposited. This is a longer process, but technically it is not difficult at all.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 24, 2020, 07:06:03 am
content: Metal core boards are plenty available, but like everything we're talking about here, they're a custom order, except for some very specific cases.Your typical protos optimize for 1-1.6mm (sometimes 0.8 and 2.4mm as well) overall thickness, two layers, 0.5-2oz foil, and multilayer in 4, sometimes 6 layers, from the same foil thicknesses give or take. (Often, one, sometimes two stackups are offered in each kind. So whatever the house uses, if it's 0.5oz inner, 0.5oz outer plated to 1oz; or 1 and 1-plated-to-2, etc.)So it may be that some fabs offer a 2oz-all-layers fab, or it's going to be custom. (Personally, I've ordered such -- with more evenly spaced layers too so it was definitely a custom stack, and Chinese fabs do those for low $100s in proto quantities.)Now -- some do offer metal-core PCBs as a standard proto, which is nice. What's maybe not so nice, they're usually single layer -- not very useful here, mostly intended for LEDs and such.AFAIK, multilayer metal core can be done two ways: building up the top layer (basically making a metal-backed flat flex / semirigid; no connections to the core), or both layers and drilling and insulating vias somehow (which may or may not support connections to the core). Not sure which kinds are most available.As for copper buildup in general -- simply, time. First off you're paying a premium for a custom stackup. Your bill starts at say $200 and grows from there. Every hour your panel sits in the plating bath is an hour that production panels (dozens?) can't use it (or use its slot, if they cycle piecemeal rather than by complete batches).There's also a fab in Australia (the only one left in .au I think?) that does vacuum deposited copper film on the core (a few um), then resist in deep wells, so that the plating is molded perfectly to size, while essentially the full thickness is deposited -- meaning, they completely avoid the issue of undercutting or overplating, so can offer very close width/space even on heavy copper (high aspect ratios). But as you might've already guessed, yeah, it's not the cheapest process!Have also seen one that looked like it might be machined (or waterjet or whatever) copper plate, embedded in prepreg/resin/whatever, and plated and laminated on top of that -- a fantastic way to get massive thermal and electrical capacity, without waiting for electrodeposition. But at the expense of something probably much worse. I forget now who offered that, and it was only ever a "contact sales for more information!" thing... if you have to ask [the price], and all that...So anyway, yeah, overall, 2-ish oz (all layers) multilayer is probably the best value: it's adjacent to standard proto processes; for N layers it's basically the cost of N/2 2-layer protos, plus custom fab overhead; and the density is good, you can get >10% copper in the total thickness of the board (also great for planar transformers). You're welcome to check out other options (ceramic PCB too!), just don't expect stupid cheap prices for any of this.Tim

author: Berni
author_rank: Super Contributor
timestamp: December 24, 2020, 07:46:58 am
content: Quote from: wraper on December 24, 2020, 12:15:39 amYeah that particular board makes no sense. I assume it was a Chinese PCB manufacturer trying to show off how thick of a copper layer they can make by just running some random board design trough the process and cranking up the plating thickness as far as it will go. Could be that they used this design because it appears to have very large spacing between tracks (These thick copper processes really don't like tight clearances because the traces grow outwards)Quote from: S. Petrukhin on December 24, 2020, 01:54:11 amYeah this is one of the technologies that tends to be used more for military application where the cost of it is not an issue. These thick boards are usually done by simply chemicaly depositing lots and lots of copper onto the traces. This slows down production a lot hence the price, then the price makes it unattractive for mass production, so it become a niche technology that is even more expensive as a result.The aluminum core PCBs however are dirt cheep for single sided types. Cheaper than 4 layers. The amount of cooling they provide is also pretty impressive. They make it pretty much impossible to use a soldering iron even if you crank up the heat. I ended up using hot air preheating and some aluminium foil to keep the board hot while hand soldering, while for re flow i had to really crank up the oven to 11 to actually get it to solder properly. The cooling effect works on copper traces just as well as it does to the stuff soldered to it, so you can put much more current trough the tracks.Some people making big 800A BLDC motor drives also used the aluminum core boards to cool there power stage and they had too much trouble getting it to solder in the reflow oven so they resorted to just placing them on a hotplate for reflow.

author: Psi
author_rank: Super Contributor
timestamp: December 24, 2020, 11:19:54 am
content: Quote from: nuno on December 23, 2020, 08:38:49 amThanks, this is a very good point. Might need an extra mosfet to disconnect the input if current is detected when no mosfet is being told to turn on.That, or perhaps intentionally trigger both the High and Low mosfets to cause the fuse to pop. Na, that's a bad idea.

author: Psi
author_rank: Super Contributor
timestamp: December 24, 2020, 11:31:08 am
content: I'm finding lots of mosfets in tiny packages, like DFN5*6, which are rated for massive currents over 100A.I'm aware that the max current on a mosfet is under ideal (unrealistic) conditions and with the chip running within an inch of its life temperature wise, but still. How are you supposed to use a mosfet in DFN5*6 package when it's going to dissipate like 5 watts?Are you expected to have a thermal pad and heatsink clamped to the top of it somehow?

author: S. Petrukhin
author_rank: Super Contributor
timestamp: December 24, 2020, 11:39:27 am
content: Quote from: Psi on December 24, 2020, 11:31:08 amIs it supposed to dissipate 5W?100A is peak currents that shouldn't be frequent, it's the endurance of the transistor, the ability to survive in critical conditions. The operating DC is almost an order of magnitude less. Isn't?

author: Psi
author_rank: Super Contributor
timestamp: December 24, 2020, 11:41:59 am
content: Prices of mosfets look much better on LCSC, so I might over-design this thing a bit with many more mosfets to get the total Rds and total dissipation really low.What issues do you run into when paralleling lots of mosfets? I'm thinking like 10x 1.5mR fets to get 0.15mR totalWhich is only 1.5W spread across all 10 mosfets with 100A flowing.That sort of thing. I assume there's a downside to paralleling too many fets?

author: Psi
author_rank: Super Contributor
timestamp: December 24, 2020, 11:47:27 am
content: Quote from: S. Petrukhin on December 24, 2020, 11:39:27 amHere's an example of the sort of thing i'm talking about.https://www.digikey.com/en/products/detail/on-semiconductor/FDMT80040DC/7356044420A continuous 2644A peakPower dissipation 156WBut it's in a tiny 8x8mm package.It gives some pcb layout pad size info in the DS but still, seems a little crazy.I suspect it's intended for like 1U or 2U servers with crazy amounts of airflow.

author: S. Petrukhin
author_rank: Super Contributor
timestamp: December 24, 2020, 12:24:59 pm
content: Quote from: Psi on December 24, 2020, 11:47:27 amAmazing! It's fantastic. We connect the 400A cabinet with a cable of 270 square millimeters.Let's say it is "Current-Continuous Drain (Id) @25°C 420A (Tc)". But how to connect to it?And I'm worried about @25°C - with a high current, it will probably quickly run away from 25°C and On Semiconductor will say "you did not provide cooling with the creosystem - so it did not survive".I think a hint about its capabilities is here: Rds On (Max) @Id, Vgs: 0.56mOhm @64A, 10V.

author: Siwastaja
author_rank: Super Contributor
timestamp: December 24, 2020, 01:04:20 pm
content: 5x6mm DFN is some Rth = 40-50 degC/W with mediocre heatsinking approaches and maybe down to some 10-15 degC/W with 2oz board, a lot of thermal vias, then larger fill on the backside, which is coupled to metal heatsink through thin and better-than-the-cheapest-2W/mK thermal pad material. So 5W is possible, but not completely trivial. I'd aim to stay below some 3-4W with such packages and that is with coupling the board to actual metal heatsink. Maybe <1.5W with PCB only heatsinking.With metal-core board, 10W dissipation from such small package would be quite easy.In motor controls, it's quite easy to keep switching losses down because the massive amount of inductance which is there want it or not allows use of low frequencies (10kHz-ish) compared to SMPS (where it's desirable to go for f_sw > 100kHz). That leaves Rds(on) loss dominating which is also easier to calculate. Just remember to multiply the datasheet figure by 1.5 for operation around Tj=90-100degC.

author: nuno
author_rank: Frequent Contributor
timestamp: December 24, 2020, 01:40:20 pm
content: Quote from: S. Petrukhin on December 24, 2020, 12:24:59 pmThose figures are only useful to compare between devices. And many times they state the die current capability, not even the package current capability. They test the dies with some kind of cryogenic cooling, that's right, to keep them at 25ºC. IRF has an app note talking about that.

author: Siwastaja
author_rank: Super Contributor
timestamp: December 24, 2020, 01:44:45 pm
content: Reading datasheet takes practice as some numbers are there "just for fun" or to trick young players, some others areactuallyused. I_d specifically is almost always useless. Sometimes I_d has alternative specification at Tj = 80 degC, device mounted on some typical heatsink, it will be a lot smaller number and actually useful. Most datasheets don't bother.The relevant numbers are:Rds(on) (for conduction loss)Qg (for switching loss)Rth_j-cMaximum current needs to be calculated from maximum power dissipation, which is calculated from the sum of Rth_j-c and Rth_c-a the latter of which you need to model from your heatsinking (for SMD device, typically just the board layout).Some datasheets give Rth_j-a with some "typical" layout, for example a 25.4 x 25.4 mm area of 2oz copper on 1.6mm thick FR4 acting as a heatsink. With such arrangements, you may have 50-60 degC/W on such tiny package. Obviously, you can do a lot better, but still, with such SMD parts, your Rth_c-a will dominate over the very small Rth_j-c.

author: Berni
author_rank: Super Contributor
timestamp: December 24, 2020, 06:48:43 pm
content: Quote from: Psi on December 24, 2020, 11:47:27 amThat is very impressive indeed, but be careful with the numbers you find on the first page of the datasheet.While it might be possible to put 400A continuously trough it doing so according to the resistance of say 0.8 mOhm (Add a bit for drop due to channel saturation, temp rise etc) you end up dissipating about 130W of heat so if you cool it from the bottom to 25°C it experiences a drop of 0.8 °C/W so the die ends up at about 130°C. It is really a best case possible laboratory scenario. To achieve this they likely soldered the mosfet directly to a fist sized block of copper that is used for both cooling and current transfer. If we assume this copper to be about 1cm around the transistor this gives an additional 0.26 °C/W. So due to the thermal resistance of the copper and extra resistive losses in the connection, this adds another 40°C temp rise, so to get the actual transistor to 25°C the outside of the block needs to be cooled to -15°C. They likely did this by forcefully pushing boiling refrigerant through cooling channels in that copper block.Safe to say you will not be able to recreate anywhere near such conditions in the actual product. However, the engineers do end up including real world achievable thermal resistance numbers and sneaking it past the marketing department into the datasheet. The actual real world numbers for thicker copper 2oz PCB with lots of airflow is 26°C/W or 38°C/W with no forced airflow. So punching in the numbers for a reasonable 75°C ambient you get a power dissipation of 2W before the transistor blows up and this lets it pass about 50A max. However, this is still assuming your copper traces are an electrical superconductor, so on an actual 2oz PCB the traces will add additional heating and make it blow up even sooner.If you add heatsinking you can bring the thermal resistance down to 9°C so this gets you 8W of thermal dissipation and so a max current of about 100A. If you have a good aluminium core board you might be able to push it up to 15W of dissipation so about 140A. So even the best case real-world scenario is not even close to 400, but is still very impressive for such a little mosfet.You can parallel mosfets when in switching applications reasonably safely, just make sure they see the same resistance both thermally and electrically so that they share nicely.

author: Psi
author_rank: Super Contributor
timestamp: September 02, 2021, 03:11:40 am
content: This product is almost done but I just wanted to post this scope image and confirm my understanding is correctThis is the voltage across the worst mosfet (furthest away from TVS/CAP).It's the high side mosfet at switch-off with the maximum loading I should ever see.It looks to me like the TVS is clamping the back EMF nicely at 33V. (The TVS used (LNBTVS4-222S) Vclamp is 33.2V max (10/1000 µs).There's also a flat area around 22V which matches the breakdown voltage of the TVS, so I assume it corresponds to that.The mosfet's are 40V so there's 7V before that happens at worst case.So my question is, does this waveform all make sense for a TVS correctly preventing back EMF that might damage the mosfet.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 02, 2021, 04:07:49 am
content: What was that then, 30A at the transistor? 60?If 60, that implies an average dI/dt of 170A/us, and an overshoot of 10V implies L = (10V)/(170A/us) = 60nH between the transistor and TVS.Seems kind of high? I wonder if dI/dt is falling by that point.During the clamping period, it's what 8V above baseline, for about 90ns, or 0.72uVs... but this depends on the load, supply and TVS inductances, and how much is left after the transistor's switched off, so it's not apparent what current or inductance is in play here.Tim

author: moffy
author_rank: Super Contributor
timestamp: September 02, 2021, 04:20:49 am
content: I couldn't find on the data sheet a continuous power rating for the device. I would be concerned with exceeding the continuous power dissipation, but you don't know what that is for the device. I personally feel a little uncomfortable with 40v MOSFETs and a repetitive 33v clamp, not a lot of headroom, I would go for higher voltage rated MOSFETs just for peace of mind, but that's my paranoia speaking.

author: Psi
author_rank: Super Contributor
timestamp: September 03, 2021, 12:23:01 pm
content: Quote from: T3sl4co1l on September 02, 2021, 04:07:49 amShould be 20-30A through the mosfet.

author: Psi
author_rank: Super Contributor
timestamp: September 03, 2021, 12:25:09 pm
content: Quote from: moffy on September 02, 2021, 04:20:49 amIn this application the situation which causes the spike is reoccurring in the human timescale of seconds to hours, even weeks. There is plenty of time for cooldown so the pulse power rating is more relevant than continuous.I could try some 18V TVS diodes instead of 22V and see what I get then, that would drop it a little lower.

author: moffy
author_rank: Super Contributor
timestamp: September 03, 2021, 03:28:24 pm
content: If it is non repetitive, what is causing the spike? Perhaps an RC snubber across the MOSFET might help absorb some energy and reduce the peak.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 03, 2021, 04:31:54 pm
content: Take note of the power level here: it will have to be quite a large capacitor, and small resistor, to be able to do that. Switching loss will be greatly impacted at lighter load.And more importantly, the capacitors will be so (physically) large that it's even less practical to interleave them with the transistors!Yes, I have no problem with the spike, if that's worst-case already then who cares, done and done; if current can be a good 30% or so higher then that should be enough to push it into avalanche, but even that isn't fatal due to the low repeat rate. But do mind it is a wear mechanism, it won't last forever that way.Tim

author: Psi
author_rank: Super Contributor
timestamp: September 04, 2021, 01:54:20 am
content: Quote from: T3sl4co1l on September 03, 2021, 04:31:54 pmYeah.Perhaps under a output dead short scenario. But given the product application this is unlikely in the products entire life.The circuit does have hardware overcurrent trip using the comparator in the MCU which has hardware comparator output pin driving fets which short the gate optocoupler LEDs to GND.

author: moffy
author_rank: Super Contributor
timestamp: September 04, 2021, 04:19:39 am
content: Did a quick sim in LTSpice and 0.5R/220nf was pulling about 9A for a similar edge. Thing about RC's, if the ratings are not exceeded they last.

author: Psi
author_rank: Super Contributor
timestamp: September 21, 2021, 08:59:30 am
content: What is the usual technique to measure the peak current through a TVS diode?Obviously you cannot add anything in series or the reading wont be correct.Some sort of hall effect current probe?The current TVS i'm using (LNBTVS4-222S) is 3 kW (10/1000 µs) 22.5 kW (8/20 µs).It's probably way overkill, but I don't know how to figure out the ideal TVS.(Other than trying smaller and smaller TVS diodes until one fails then add a good amount of margin on top of that one)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 21, 2021, 02:33:04 pm
content: If you can get a loop around it, a Rogowski coil will do.With a pulse under 200ns width and up to 200A at 30V = 6kW, you're delivering well under the 22.5kW rated for a 100 times longer pulse.This one doesn't, but others show a power vs. time curve, which will be helpful in selecting minimal parts. I'd just as well stick with the SMC as it's a wider body, so potentially lower inductance.Tim