cheapest way to get 4x isolated +-15V supplies? - Page 1
Forum section: electronics / projects

author: SArepairman
author_rank: Frequent Contributor
timestamp: October 03, 2014, 07:01:19 pm
content: So, I need four +-15V supplies for a IGBT bridge. I would prefer to go with isolation rather then gate drive transformers.I think I need 0.4 amps per +-15V pair.Isolated DC/DC converter modules that can do this cost about 20-30$ each, meaning 80-120$ total.Is it possibly cheaper to get a switching transformer that has 12 taps on it? (I don't think the iron core is the way to go here).

author: David Hess
author_rank: Super Contributor
timestamp: October 03, 2014, 07:29:48 pm
content: Quote from: SArepairman on October 03, 2014, 07:01:19 pm0.4 amps? Is that the expected continuous current draw by the isolated gate driver circuits?QuoteOr use 4 separate isolation transformers with the primaries connected in parallel. This also has the advantage of increasing the secondary to secondary isolation. I would drive them as simple inverters without any sort of secondary to primary side feedback.It is worth noting that transformers intended for isolated gate drive also have the characteristics needed for isolated power supplies.

author: NiHaoMike
author_rank: Super Contributor
timestamp: October 03, 2014, 07:59:29 pm
content: If it's a common H bridge, you only need 3 supplies.

author: SArepairman
author_rank: Frequent Contributor
timestamp: October 03, 2014, 08:07:29 pm
content: I think 0.4 amps is the average current draw, check my other thread for the method of my calculations if you know the details.https://www.eevblog.com/forum/projects/estimating-gate-driver-requirements/I spent some time looking at switching transformers on digikey and I got absolutely no where.Should I be designing a switcher around a transformer? Any transformer recommendations that would be good for paralleling up? Digikey has miserable search parameters.

author: moffy
author_rank: Super Contributor
timestamp: October 04, 2014, 12:42:48 am
content: You might want to look at:http://www.ti.com/lit/ug/tidu355/tidu355.pdfNormally you design and build your own transformers. I never seem to be able to find one already available.

author: SArepairman
author_rank: Frequent Contributor
timestamp: October 04, 2014, 01:56:08 am
content: what kind of price do you usually come to winding your own transformer that can do a few watts? I am getting the sense that this is a massive amount of hoopla to jump through for not that much savings.

author: ovnr
author_rank: Frequent Contributor
timestamp: October 04, 2014, 02:09:21 am
content: Just look at the prices for transformer cores - the copper wire itself is practically free in comparison, at least for small qty work.I'm a bit skeptical of your 400mA needs - ordinarily, gate drive doesn't eat that much. Anyway: There are plenty of smaller transformers available on Digikey; I'd just pick up however many you need and parallel up the primaries, then feed it with a simple oscillator + MOSFET. Drop some regulators on the output if you can't get it to sit in a comfortable range.Or, if this is a one-off, just buy off-the-shelf modules. It'll take you way more time and effort to engineer something that's significantly cheaper while still reliable, and it's probably not worth it. I'd be pretty shocked if you couldn't get away with some $5-6 2W modules.Also, regardless of what you do: Consider using a +15V/-5V scheme instead, you probably don't need to drive the gatequitethat low.

author: mzzj
author_rank: Super Contributor
timestamp: October 04, 2014, 06:56:53 am
content: Quote from: SArepairman on October 04, 2014, 01:56:08 amyou can get ferrite toroids for something like 0.5-1 usd per piece and rest of the parts will cost buck or two. Look for something like 15-20 mm outer diameter cores, reasonable size to handle and you dont need that many turns as with tiny cores. Core material is not critical as long its ferrite and not iron powder. even emi interference filter toroid cores work pretty well in my experience and they are easiest to source. Run unregulated half or full bridge, 100 khz is a good all-around compromiše for switching frequency.

author: SArepairman
author_rank: Frequent Contributor
timestamp: October 04, 2014, 07:37:05 am
content: so, mosfet switching into the core line, 2x diodes per transformer, have it run at like 17V and use a LDOs to bring it down and stabilize it., the dissipation could probably be handled by the PCB.2x diode, 2x filter cap, 2x 7815 (50 cents each to220) per transformer output, no feedback. 4x hand wound torroid transformer, 1x switching transistor, clock from 555are there any caveats to this?are there any particularly evil failure modes compared to using 4x switching module? it looks like 4-5$ of parts per +-15V rail doing it this way,what demons does this kind of regulator have? I got so used to seeing fancy switch mode IC's that I never considered this kind of solution.I don't care about efficiency as much as reliability in this case.160mA is a very reasonable number for me. cheapest acceptable module is 20$ each on digikey. they do have SCP and OVP though. I'm not sure what this solution has.so, 80$ vs 25$ of parts.40 vs 25 if I use ebay modules though, but its not repeatable for others then*i did run the numbers on 400mA igbt drive. Maybe when I get 3 phase power lol

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 04, 2014, 09:12:57 am
content: Considering you're working with $100+ transistors each, I don't see why "cheapest" should matter for anything less than, say, $50/channel. Or, considering we're talking about the converter alone, not the whole driver and fault/feedback channel, $20 is still quite reasonable.You can't go wrong with off-the-shelf converters (that are rated for this duty, e.g. Recom's reinforced series of DC-DC converters).For a similar purpose, I once designed an eight channel converter, 100W total capacity -- it still ended up around a buck per watt, as many things seem to.Tim

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 04, 2014, 09:15:05 am
content: Quote from: SArepairman on October 04, 2014, 07:37:05 amUmmm...yes?Short ProtectionUndervoltageOvervoltageStartup Transients (will it startup at all?)Flux BalancingLoad Matchingetc.Tim

author: David Hess
author_rank: Super Contributor
timestamp: October 04, 2014, 11:48:35 am
content: Quote from: SArepairman on October 03, 2014, 08:07:29 pmI came up with about 60 milliamps per driver.QuoteCoilcraft makes appropriate ones and like I said, gate drive transformers can also be used:http://www.coilcraft.com/prod_isolation.cfmhttp://www.coilcraft.com/prod_gatedrive.cfmQuoteI would just use an inverter instead of a switching regulator. Regulation will still be good and each output only needs a rectifier and filter capacitor.

author: SArepairman
author_rank: Frequent Contributor
timestamp: October 04, 2014, 04:39:08 pm
content: oh yea, the calculation in that thread was just a sample. with the specs I have it will be 150mA or so. 400mA would be utterly beastly.thats quite a scary list of problems therei can see teslacoil getting upset if he saw this near a 150$ IGBT.I think I will save him the heart ache.but for the sake of stirring a pot of shitShort Protection - fuseUndervoltage - handled by driverOvervoltage - handled by 7815 (unless it really spikes?) (mov?)Startup Transients (will it startup at all?) - not sureFlux Balancing - not sureLoad Matching - not sureetc. - i'm interested if you have the caffeine

author: Dago
author_rank: Frequent Contributor
timestamp: October 04, 2014, 04:42:52 pm
content: Why don't you just use a high-side driver?I'm using an ADuM7234 (http://www.analog.com/static/imported-files/data_sheets/ADuM7234.pdf) in the same application, just need one ~15V supply for the whole bridge.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 04, 2014, 07:05:06 pm
content: Quote from: SArepairman on October 04, 2014, 04:39:08 pmMeh, last time I was working with them, they were $600 each. But they were quads (dual half bridge), so about the same idea either way. 1200V 300A range stuff.QuoteSo when it shorts, you *want* the circuit to fail irreparably?QuoteGood, but, if nothing else knows that the driver stopped driving, you're missing pulses. Likely followed closely by somewhat larger quantities of magic smoke.QuoteYou shouldn't need a post regulator on a switcher, that's absurd -- went out of styledecadesago! Certainly not a non-LDO kind.MOVs don't buy you anything -- breakdown voltage is already 20-30% above nominal operating voltage, and under pulsed conditions, it's more than 100%. In other words, a "15V" MOV will clamp no less than 30V under applicable surge conditions, meaning your circuit needs to handle 40V, from that supply alone.IIRC, gate drivers of the IGBT module class are good for 30V swing, so either 40 or 60V supply ratings. But that's marginal, and you'd want 80V. Iffy.Not that MOVs are even relevant at low voltages -- you'd use TVSs of course. They're better (peak more like 50% of nominal), but you still need that overhead if you want the circuit to survive.On the upside, a gate driver is not going to be exposed to 8/20 or 10/1000 off-line surges. On the downside, it'll be exposed to 200V+ on the gate when the IGBT fails (all-way shorted, typically -- at least until the arc flash burns everything off).I've tried to protect drivers from this abuse before, but it's tough. Even with bigass schottky diodes on the output, you still get partial failures of the driver chip (excessive supply consumption, impaired voltage swing, etc.).Drivers are cheap, anyway, so it's a good plan to build them as modules (under $50 like I said elsewhere), so they can be disposable with the transistors. Or, even just buy ready-made modules and interface those. When design time and low-quantity assembly is included, they start looking really good.QuoteCaffeine? I was thinking ethanol...Tim

author: ovnr
author_rank: Frequent Contributor
timestamp: October 04, 2014, 10:51:17 pm
content: As an aside, this is how you drive large-ass IGBTs:This sat on top of a 1200V 600A dual IGBT brick (the kind that costs $600 and weighs 1.5 kilos). A second duplicate was above it forming a somewhat ridiculous H-bridge, mounted on a 6 kg heatsink. Yay power electronics.

author: SArepairman
author_rank: Frequent Contributor
timestamp: October 04, 2014, 11:49:15 pm
content: Do you maybe have a schematic of that circuit?i see switcher/isolated gate drive module, a push/pull, multiple resistors for the push pull dissipation,but what is that 8 pin chip connected to nnc2x? is that a discrete Desaturation detection? (this is what I am curious about)It's pretty much what I had in mind of building.

author: ovnr
author_rank: Frequent Contributor
timestamp: October 05, 2014, 01:45:05 am
content: No schematic, sorry. This is just a rather large power unit I pulled apart.More detail:I'm pretty sure it's a desat detector, yes. Chip is a LM393 comparator, and the upstream opto (white chip,HCPL-316J) has a desat detector and feedback function. It looks like they're kind of bodging it on, but I'm sure it works. The trace coming in from the bottom is from the collector.As an aside, here's a couple pictures of the whole unit:(The large round object with fins in the lower right corner is aridiculous inductor- 100uH, 50A. Almost 3 kilos.)

author: SArepairman
author_rank: Frequent Contributor
timestamp: October 05, 2014, 02:02:35 am
content: oh, what are the push pull mosfets used? Can you get a close up of them?they must be able to do like 70A iPeak if they can do 400A igbts.why do they have a seperate desat if there is one on the opto?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 05, 2014, 04:06:49 am
content: Well, no more than 12A peak, pull down. I suppose they could be MOSFETs, but more likely I'm thinking they are DPAK BJTs, complementary. The 1218? resistors look to be a total 2.5 ohms pull-down, 4 ohms pull-up. The white chip must have a high voltage driver on it. Also, a feedback channel -- the desat stuff goes back thataway. Guessing the black SOIC just beside the isolator is a logic interface chip or something like that...Also, momentous waste of a four layer board.Tim

author: SeanB
author_rank: Super Contributor
timestamp: October 05, 2014, 06:06:57 am
content: If you were already doing a 4 layer board for the rest then it probably was cheaper to use the same larger panel for the power devices as well. One less setup charge, and a single test for all the boards during PCB manufacture. no need to double the board setup and test costs if you are only doing small runs, as each batch will incur this again.

author: ovnr
author_rank: Frequent Contributor
timestamp: October 05, 2014, 09:11:57 am
content: Quote from: SArepairman on October 05, 2014, 02:02:35 amDrivers: Sadly, they're completely boring BJTs. Surprisingly boring -4H11G and 5H11G. 8A units, 16A Ipk.The IGBT bricks areFF600R12KF4's. Total allowable powerdissipation: 3.9 kW.Desat: The opto/driver is a 2.5A integrated unit. They probably figured it wasn't up to the task of driving these bricks, and added a discrete stage instead, and for some reason decided to coerce the built-in desat circuit to still work. Personally I'd just have added a second opto...Quote from: T3sl4co1l on October 05, 2014, 04:06:49 amHa, this machine had some totally ridiculous boards, but this one takes the cake:39x33cm 2-layer 3.2mm (!) board with amassivecutout - all to add some buttons around a screen. And since it's a 3.2mm board, the cutout area is not reused anywhere. Practically everything else was on 4-layer boards if it had more than 10 nets.(sorry for the shitty cellphone image.)

author: SArepairman
author_rank: Frequent Contributor
timestamp: October 05, 2014, 10:45:04 pm
content: I figured that you would need a bigger Ipeak to turn on such massive IGBT, although I don't see a rating for columbs on the driver.I never heard of a mWs unit. (under turn on energy)I am building my own driver, I can get 500mA +-15V isolated modules for 10$ each, so I thought I could build the driver to be completely beastly with a better iPeak then that.The most powerful driver I found was the 30A/30nS from Ixys (many machines... new machines...)But, what if I wanted something discrete more powerful then that? What transistors would I turn to?I suppose I cascade these behind some medium-power BJThttp://www.onsemi.com/pub_link/Collateral/2N5684-D.PDFBut its alot slower then the 30A Ixys units. buuuuuut the ixys units don't have negative voltage.

author: SArepairman
author_rank: Frequent Contributor
timestamp: October 05, 2014, 11:40:09 pm
content: or these guys?http://www.onsemi.com/pub_link/Collateral/MJ11028-D.PDFI could drive some house brick with that. Any objections?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 06, 2014, 12:10:20 am
content: I've seen it done with anywhere from IRF540/9540s to BJTs (NOT freaking TO-3 monsters, actual switching transistors) to integrated chips (e.g.IXDD430CI).Tim

author: SArepairman
author_rank: Frequent Contributor
timestamp: October 06, 2014, 12:18:28 am
content: do you have any recommended MOS complimentary transistors so I can do a negative swing? Is it ever done that way?'How do you recommend decoupling these? I was thinking a few 100nF ceramics right up close to the transistor.

author: ovnr
author_rank: Frequent Contributor
timestamp: October 06, 2014, 01:03:39 am
content: As for the Ixys drivers: Can't you just tie their GND pin to a -5V rail, and the IGBT emitter to the real 0V ground? You'd have to levelshift the inputs, but that's entirely doable.Here's how the circuit would essentially work:(Components chosen at random.)Edit re: levelshifting - just stick the ground/emitter of your optocoupler or whatever on the -5V rail and all should be right with the world - the Ixys driver linked to earlier takes any input voltage up to Vcc, so just drop a pullup resistor to the +15V rail. Easy.

author: ovnr
author_rank: Frequent Contributor
timestamp: October 06, 2014, 01:14:38 am
content: Quote from: SArepairman on October 06, 2014, 12:18:28 amYou don'treallyneed complimentary MOSFETs as long as their delay times are reasonably similar - it's not like you're using them in the linear region. You will also want a MOSFET driver to drive your MOSFET driver transistors that are driving your IGBT. And by the end of the day you've cascaded enough stages that your control loop gets all out of whack... But I digress.Decoupling: Well, how would you ordinarily decouple a power rail that's seeing 30+A spikes with rise and fall edges of <100 ns? (First answer that comes to mind: By leaving the design on someone else's desk and running far, far away...)Go look at the impedance graphs for ceramics, and find something that's as low as possible near the frequency in question (100ns edge? 10MHz. Etc.). Say it's 10nF. Then stack on a couple of those, then a couple 100nF caps, and I'd probably just jump straight to 10µF ceramics. Then some largeish electrolytics (polymer?) for good measure. It's rather annoying to deal with.

author: SArepairman
author_rank: Frequent Contributor
timestamp: October 06, 2014, 03:50:20 am
content: If the transistor is capable of 16A @ 15V, why is it going through a 4.7 ohm resistor (12.1 x3 in parallel) in the circuit that you linked, that limits the peak to 3A. The converter is capable of 2.5 by itself. They went through all that trouble for 500mA?It should be more like 1 ohm?then again, I guess driving that thing at 2.5 amps is probably not the brightest idea. I'm suprized they can drive those bricks with 3.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 06, 2014, 06:29:26 am
content: The pull-down resistors are larger, and the swing is not 15V (-7.5 to +7.5?), but 30V. I stated this earlier.You don't even need 100mA peak to drive one, all that matters is how fast you need. With MOSFETs, there are very good reasonsnotto drive as fast as absolutely possible, and with IGBTs, it's pointless to drive any faster than they can do internally to begin with.Tim

author: mzzj
author_rank: Super Contributor
timestamp: October 06, 2014, 09:00:03 am
content: Quote from: SArepairman on October 05, 2014, 10:45:04 pmBased on your questions are you sure that ziggawatt inverter with "big-ass" igbt bricks is a good idea or starting point to power electronics?It looks like you have really STEEP learning curve ahead. Plus plenty of expensive blown up igbt's, nasty igbt goo flying around your lab and a shrapnel in your (remaining) eye.

author: Dago
author_rank: Frequent Contributor
timestamp: October 06, 2014, 11:20:27 am
content: Also one thing to take in to account that you actually do not necessarily want to turn on the IGBT "as fast as you can". Even though it will result in lower losses it will increase the dv/dt over the IGBT and make it possible to exceed the voltage rating of the device turning in into a grenade. Also increases EMI. It is usually a trade off between losses and dv/dt.

author: SArepairman
author_rank: Frequent Contributor
timestamp: October 06, 2014, 04:31:16 pm
content: Quote from: mzzj on October 06, 2014, 09:00:03 amI don't know why you assume that my lack of knowledge on electrical theory means that I would do some stupid ass dangerous shit that puts me in harms way? Yea, I might hear a bang and blow out a 30$ 2nd hand eBay IGBT. Electronics is pretty safe, you can decide exactly when it has the possibility of blowing up (un like some other things)What do you think, I'm going to be hunched over a water block connected to a IGBT connected to a bread board running at 370V poking it with some low voltage oscilloscope probes when my face is 6 inches away? I appreciate your help but come on, thats just condescending.I would have this thing far away in a non fragmenting enclosure with remote probes that I hooked up while its turned off and any caps discharged.

author: David Hess
author_rank: Super Contributor
timestamp: October 06, 2014, 06:26:12 pm
content: Quote from: SArepairman on October 06, 2014, 04:31:16 pmThat is not a bad idea. I have had pieces of plastic power packages stick in the walls, the ceiling, and me when the power transistor failed by disappearance and they were small compared to the ones being discussed here.

author: SeanB
author_rank: Super Contributor
timestamp: October 06, 2014, 07:13:10 pm
content: I have had steel packages do that. Wet tantalum capacitors can do an impressive act, easily going through a hollow core door. I only ever soldered one in backwards once.......

author: SArepairman
author_rank: Frequent Contributor
timestamp: October 06, 2014, 08:05:15 pm
content: then again, I look at my dad and I guess the fear mongering in this thread is warranted. he would set up a ladder at a 75 degree angle if he could

author: SArepairman
author_rank: Frequent Contributor
timestamp: October 06, 2014, 09:06:45 pm
content: Any ideas why they chose to go with the additional de-saturation detection circuitry?I was going to use the ISO5500, which is almost the same chip. I am worried that the DESAT might have some kind of problem when using external drivers? I can't imagine what that could be though.Maybe they wanted a different threshold? Here is the relevant stuff about desatI'm worried there is some "bug" (like the lt3080 problems dave jones was having) or is this scheme just poor for some reason?