High Frequency IGBT H-Bridge snubber network question!  Reusing MEDAR spotwelder - Page 1
Forum section: electronics / projects

author: applicanon
author_rank: Contributor
timestamp: November 30, 2019, 12:59:58 pm
content: Hello! Well, i happened to find a medar MFDC inverter used in a spot welder. Complete with the juicy 1200 amp IGBT's that all the tesla coil freaks and induction heater nerds tend to be using for their projects. It's got 4 of em connected in an H-Bridge configuration. Well.. They were.. I dismantled the system a couple years ago because I had intended on using the parts individually for some smaller projects, but I've decided now that I actually want to use the IGBT's in the H-bridge configuration they were in already, and I'd like to try to make use of the gate driving circuitry that was already being used in the MEDAR welder system. It saves me from having to do all the engineering to design a gate drive circuit as well as an appropriate snubbber network to protect the IGBT's, because I already have it all! One kind of minor issue.. after dismantling the thing, I am left with some power resistors mounted to the same heatsink as the IGBT's which I can't recall where they were connected to.I'm not sure if the resistors were part of the snubber network, and if so, I'm not sure how to connect them again.. I'm also thinking they may have played some roll in the DC link portion of the system. The original Medar spotweld circuit had a large bank of electrolytic capacitors (8 of them, 3300uF @ 420V) and I'm thinking its possible the resistors I'm talking about may have been connected to the caps instead of to the snubber network? Perhaps they were connected in a third use that I haven't thought of?I'm hoping somebody can shed some light on my plight! Here's a ton of photos that should give you an idea of what I'm working with!

author: applicanon
author_rank: Contributor
timestamp: November 30, 2019, 01:01:33 pm
content: more photos

author: applicanon
author_rank: Contributor
timestamp: November 30, 2019, 01:02:15 pm
content: morer

author: capt bullshot
author_rank: Super Contributor
timestamp: November 30, 2019, 01:11:42 pm
content: Can you look up the DC link capacitor circuit somewhere?Resistors like the ones you shown (though usually at higher resistance value than 5k) are often used as bleeders and for even voltage distribution along series connected capacitors. 8 large electrolytics could have been arranged as 4 groups of 2 series connected ones, matching the four groups of 2 series connected resistors.For your snubbers, two resistors should be sufficient from the look of the board.

author: applicanon
author_rank: Contributor
timestamp: November 30, 2019, 01:42:01 pm
content: Thanks for the quick response! Very much appreciated.Unfortunately I can't find the schematics anywhere as this type of welder is rare in the first place as its an industrial grade piece of equipment, and it's also no longer being manufactured, and has been out of manufacture for some time now I think. I can't even find pictures of the specific inverter model I've got the pieces of, the only pictures online I'vev found are ones that I posted online before taking it apart. unfortantely I didn't take pictures of some key spots which could have answered some questions for me.Anyhow.. It would make sense to have the configuration you mentioned of 4 sets of 2 caps in series.. 4P2S, as I have the bus bars still which connected the caps together, and there are 4 of these bars.. each bar has 2 holes and are only long enough to connected 2 caps together if placed side by side. So, this would make 4 sets of caps with 840v ratings, and 8 terminals with nothing connected to them. I'm guessing that 6 of these terminals would get the red ring lugs connected to them from that resistor network? the remaining 2 capacitor terminals would connect to ground and to the positive DC rail? is this sounding about right?

author: applicanon
author_rank: Contributor
timestamp: November 30, 2019, 02:33:26 pm
content: I drew this up real quick... so do you think this is basically how the DC link would look with the resistor network connected with the capacitors in this 'bleeder' style configuration?? It makes sense I guess... Because at 5k ohms and at 420v, each resistor will be consuming about 35.28 watts if my math's correct.. Which matches up pretty well perfect to the 50 watt rating of the resistors!Can you confirm please? Just give me the good old thumbs up buttercup and I'll be on my way to customizing the gate firing board so that its triggered with an arduino instead of by PLC which it originally operated on... way back when PLC's were all the balls..

author: capt bullshot
author_rank: Super Contributor
timestamp: November 30, 2019, 03:51:26 pm
content: Yes, that's the configuration I figured. Looks very plausible to me. 5k is on the low side (quite high power dissipation), for standard bleeders the resistance value would be higher, maybe they had a reason for this value.Driving the IGBTs should be simple, you've got the booster board mounted on the modules, so all you need is an isolated +15V / -5V (typical value, up to +20V / -10V might be used) supply per IGBT and some modern IGBT driver optocouplers to drive the booster input. Your isolated supply must withstand the switching edges and supply a few Watts (depending on gate charge and switching frequency), you should find useful stuff on the old driver board.

author: applicanon
author_rank: Contributor
timestamp: November 30, 2019, 09:44:04 pm
content: thanks for the help! Yea I've looked over the gate driver board and think I've been able to figure out where the gate firing signal is getting sent for each IGBT.. so I'll attempt to simply replace the existing signal source on the board with an arduino firing signal. The gate driver board already has all the necessary power supplies at the optimum voltages and all that, so I'd be silly to scrap it all and try to rebuild the whole thing again myself. Basically, I'm just replacing what is providing the signalling, and the board will be doing all the rest as far as providing the correct voltage source / sinks etc. etc.! I'm very happy with this as I've been hanging onto these parts for a few years now, and it seems I'll be able to implement them into a project pretty soon now!Thanks again my friend!Oh and... what do you suggest... do I build a solid state tesla coil, or an induction heater... or ... something else can you suggest anything??

author: capt bullshot
author_rank: Super Contributor
timestamp: December 01, 2019, 10:41:05 am
content: Quote from: applicanon on November 30, 2019, 09:44:04 pmA SSTC has IMO the best coolness factor

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 01, 2019, 12:10:02 pm
content: Nice! Leave it as-is, bolt it back up -- you're not going to improve on the parasitics and snubber over the OEM.Might even be able to use the controller, which...I would hazard the guesses that:- The Xilinx part (FPGA) can still be found in their tool, so you just need a programmer dongle to connect to it.- The FPGA runs all the low level stuff, gate drives and whatnot. Timing critical, system operation and control, glue logic. It may connect to many of the I/Os as well.- May be possible to disassemble the config (presumably stored in the adjacent Flash ROM), but that's really advanced and you're probably better off tracing pins and figuring out your own basic system. Upside, I doubt it's encrypted.- The other QFP is an application processor for running all the other piddly high level stuff. Check the datasheet and see what it can do. Probably has an external memory bus, as well as a bunch of onboard stuff.- I/Os consist of some combination of serial (including RS-232 or 422 or 485?), digital I/O (contact closures?), and some ADCs (the LT parts??) and their signal conditioning parts.It's... fairly daunting to RE, just because of the quantity of things, and the PCB layers frustrating visual inspection -- but you could get started by, say, writing a bare bones pin reader for the FPGA and just reading out which pins do what. (Maybe even doing this directly in JTAG boundary scan?) Poke at them with a pull-up/down resistor and see which traces go from/to what pins and functions. Build up a toolkit and start poking at peripherals.Be on the lookout for signals intended for fault detection or latching (comparators in the gate drive or ADC section, e-stop from the I/Os?), and, don't neglect them -- while you can get away with just blindly pulsing transistors to run TCs or whatnot, you stand a much better chance of not exploding your only set of them by respecting the hardware they likely already put in there for you.And the MCU, is that a MC68k something or other? Bet some interesting things could be done with that. Also all the headers around there, that's not PC104 is it, was that an x86-only thing? Some kind of stacker in any case, and willing to bet it's got the full bus on there, or at least a bunch of GPIOs. Lots of stuff you could connect...Tim

author: Marco
author_rank: Super Contributor
timestamp: December 01, 2019, 05:59:19 pm
content: H bridges don't generally have snubbers, (body) diodes clamp everything to begin with. What you drew would be more aptly named balancing resistor network.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 02, 2019, 05:52:03 am
content: The stud diodes on the connector board are connected to parallel capacitors, apparently serving some snubber function (Vpk clamp?). Would expect a few resistors serve the snubbers.The diodes aren't particularly low inductance, but I doubt the IGBTs are either, or that they're very fast (maybe sub-1us switching?). I would guess, okay for pulsed operation below 100kHz, or continuous operation below 20kHz.Tim

author: krish2487
author_rank: Frequent Contributor
timestamp: December 03, 2019, 09:27:17 am
content: Not directly related to your question,I do believe those are 600A IGBTS not 1200. The 1200 figure is only for a pulse power delivery application. Its nominal rating is only 600A.Those drivers look like M57962L hybrids. Easy enough to come by.. and looks like a standard supporting circuitry.In fact both the IGBts and the drivers are from powerex. You might have some luck in the application notes section of powerex or Mitsubishi website.Something like a reference design or so..

author: krish2487
author_rank: Frequent Contributor
timestamp: December 03, 2019, 09:32:58 am
content: The hybrid drivers for sure have a desaturation detection output. and it looks (there is a small 4 ppin SMD opto I presume) detecting it.and the CTs would definitely have a fast OC trigger as well, apart from the average current sensing.Id imagine it is not impossible to reverse engineer, certainly doable if you know what the broad topology is like... The FPGA does certainly complicate things.Quote from: T3sl4co1l on December 01, 2019, 12:10:02 pm

author: applicanon
author_rank: Contributor
timestamp: December 05, 2019, 12:31:06 am
content: Wow thanks for all the replies and interest guys! A lot of what you guys are talking about is going way over my head though! Here's a link to a youtube video I took of the system before dismantling it.. It also shows the PLC system with the medar card which provided the signalling output.The IGBT's are in fact cm600ha-24h which are 600A, 1200V. You guys are correct. I had the volts and amps reversed.They have the following timing specs:turn on delay - 300nsrise time -700nsturn off delay - 450nsfall time - 350nsdiode reverse recovery time - 250nsI've taken the time to draw out the schematic for the IGBT h-bridge with the snubber network as is from the original equipment manufacturer. Let me know your thoughts on it, and whether or not I drew it correctly!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 05, 2019, 04:51:44 am
content: Looks right. I'd have drawn the diodes the other way around but the connection is right.Tim

author: applicanon
author_rank: Contributor
timestamp: December 06, 2019, 07:13:08 am
content: Tim, that's the way the diodes are connected. If they were reversed, then current would be able to flow from Vcc across the first resistor, through one of the diodes and across the load, bypassing the IGBT h-bridge, which would not be a good thing.Maybe you meant you would simply draw them differently? I don't follow that logic though.. How else can they be drawn?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 06, 2019, 09:00:17 am
content: I just mean by layout. Still needs more moved around for readability but this is the basic direction:Tim

author: applicanon
author_rank: Contributor
timestamp: December 06, 2019, 09:45:51 am
content: Ah I follow your logic now. But now you have the capacitor connections crossing through the drawing making a mess of things visually.. I like how my layout looks just finethanks for the help and input i really do appreciate it!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 06, 2019, 02:58:42 pm
content: Yeah the crossed supplies are a bit annoying, takes some space to draw everything cleanly. Anyway, that's how it's laid out physically -- easy to have all the inside connections where they are, then the supply rails beyond that. That's really what matters, the physical length of the loops thus formed -- length gives inductance, and the snubbers need a minimum of it!Tim

author: ow19m
author_rank: Contributor
timestamp: January 11, 2024, 02:05:10 pm
content: Any Update on the welding inverter designed ? what was the response for the calculated snubbers used by Medar ? Thanks