How to drive high amp DC mosfets from MCU? - Page 1
Forum section: electronics / beginners

author: superUnknown
author_rank: Regular Contributor
timestamp: February 19, 2014, 02:34:19 am
content: I'm trying to figure a dead nuts simple way to drive mosfets cabable of 200 amps 70VDC. Because.I want to take an MSP430 or arduino and produce alternating polarity DC pulses (AC square wave) say 50-100Hz. All the big mosfets, igbts need a driver, bootstrap blah blah blah. Is there a drop in to trigger on a 3-5v 20ma signal? Maybe just drop in a motor drive board to drive the big mosfets?Thanks!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 19, 2014, 02:47:06 am
content: Welder polarity switcher..?There's no "zero component" method, and you wouldn't want to anyway. It takes all of 0.5% of that load current sneaking into the MCU to burn it to ashes. Not only do you need the drivers (or bootstraps or whatever) to get isolation (keep the MCU out of harm's way), you need the amplification (enough voltage and current to switch MOSFETs that size) and voltage withstanding to handle a load like that.If it is a welder, or something to do with motors, or anything with the least bit of inductance, you will also need much higher voltage ratings (200V transistors would be fine -- you'd probably prefer this range anyway, because there's so damn many to choose from, and their performance is still good), and some consideration of handling peak voltages and currents during switching. If all transistors turn off at the same time, before the reverse side has time to switch on, the open circuit will cause a huge voltage spike, which at the very least will have to be clamped with an MOV or something like that.Tim

author: Psi
author_rank: Super Contributor
timestamp: February 19, 2014, 04:45:34 am
content: It sounds to me like he's trying to add AC mode to an old welder, or replace the dead driver logic in one.This project really does need to be done properly and not with plug-n-play arduino parts.However if you're really set on trying it an H bridge motor driver board is an ok starting point to drive large fets.Just be sure the drive output cannot ever get above the max gate voltage of the mosfets. Usually +/- 20V.I would add some 17V TVS diodes and drive it with 15V.

author: Smokey
author_rank: Super Contributor
timestamp: February 19, 2014, 04:54:33 am
content: When you get down to it, the heart of a power device gate driver is just another smaller half bridge usually on the other side of an opto. Check out these Avago gate drivers. They have everything from simple single opto drivers, to fully integrated protection right in the driver. They also have a lot of good app notes about generating the high side drive voltage if you need to do that.http://www.avagotech.com/pages/en/optocouplers_plastic/plastic_integrated_gate_drive_optocoupler/gate_driver/

author: superUnknown
author_rank: Regular Contributor
timestamp: February 19, 2014, 05:19:06 am
content: Quote from: Psi on February 19, 2014, 04:45:34 amguilty.

author: poorchava
author_rank: Super Contributor
timestamp: February 19, 2014, 09:44:22 am
content: IXYS has a good portfolio of gate drivers for high power MOSFETS and IGBT.And as Psi already said: this is not something that you can slap together with plug-and-play modules. At those power levels the thing has to designed carefully or you end up with an explosion.No teletubbies-level electronics here. [appropriate wallpaper attached]

author: superUnknown
author_rank: Regular Contributor
timestamp: February 19, 2014, 05:17:22 pm
content: Quote from: GEuser on February 19, 2014, 05:56:07 amJa, thanks I'll look that up! I've got a DC tig welder that I want to use for AC tig aluminum... I'm not sooper smart on the electronics side, so I hear "bootstrap" and tune out...

author: superUnknown
author_rank: Regular Contributor
timestamp: February 19, 2014, 05:20:05 pm
content: Quote from: poorchava on February 19, 2014, 09:44:22 amGotcha', well this teletubby is giving her the old college try. I'll be sure and post the ensuing blue smoke, explosions and house fires. Thanks for the heads up guys!

author: superUnknown
author_rank: Regular Contributor
timestamp: February 19, 2014, 05:22:34 pm
content: love the traces on that pic #6. I can smell the vintage from here!Thanks for your good advice guys! Really appreciate getting a nudge in the right direction. Cheers!

author: poorchava
author_rank: Super Contributor
timestamp: February 19, 2014, 07:39:20 pm
content: I didn't want to be offensive or something. I just wanted to point out that this is not a trivial task.Let's say you look for a transistor on Digikey. Sorting above 130V and above 200A, then ordering by Rds(on) in increasing order you can find IXFX360N15T2. 150V, 260A, 4mOhm@Vgs=10V and Id=60A. Now what? Look at the graph, at 200A the R(ds) is a bit higher, at junction temperature of 25*C. But wait, tj will not be 25*C. Thermal resistance junction to case is 0.09 *C/W. Assuming Rds of 4.1mOhm, at 200A you will dissipate 160W. So assuming zero thermal resistance between case and heatsink and between heatsink and ambient (not gonna happen in reality) and ambient temperature of 25*C your junction will be at 14.4*C higher temperature. Say your heatsink in reality will make for another 0.09 *C/W. You get junction temp of about 55*C. So Rds(on) gets a bit higher and so does power loss. This can be solved using some differential equations. And you need to dissipate continuous power of over 160W. This is one thing.Now the gate drive: look at the Cg=f(Vgs) graph. You have about 700nC of gate charge at 10V. This comes down to 70nF. And you need to charge/discharge that 70nF FAST. I'd shoot at ~10A peak current gate driver. And that's considering you buy a $21 transistor (which you may quite possibly blow up) from Digikey (which may mean exorbitant shipping fee depending on your location).At that's the tip of the iceberg. You need to consider gate snubber, account for voltage and current spikes, blablabla. You can consider parallel mosfets, but then again: gate charge will most likely be larger and physical layout may not necessarily allow for driving them with single driver. Multiple mosfets and multiple drivers can be more expensive as well.Signal source is sctually the easiest part, as most of the drivers will accept some common signal level like 5V or 3.3V.

author: superUnknown
author_rank: Regular Contributor
timestamp: February 19, 2014, 08:21:01 pm
content: Quote from: poorchava on February 19, 2014, 07:39:20 pmNo offence taken (or implied). I had no idea we'd hit 10A peak gate current.I've spent the past hour messing with a driving a 10A load with a 90A IGBT. Using a 500mA reed relay, coil snubber, and 17V zener on the gate. Thinking, WTF? this seems pretty easy. 450 million cycles out of a reed, 100Hz, 1200 hr to mean failure. What's the big deal?No big deal until the reed welds together driving 10A hahahaha. Oops.

author: superUnknown
author_rank: Regular Contributor
timestamp: February 19, 2014, 08:33:57 pm
content: Quote from: Smokey on February 19, 2014, 04:54:33 amThank you, this helps!

author: cellularmitosis
author_rank: Unknown
timestamp: February 19, 2014, 08:51:07 pm
content: subscribed.(owner of a DC-only TIG unit).

author: Smokey
author_rank: Super Contributor
timestamp: February 19, 2014, 10:18:07 pm
content: Mechanical gate drive?Another thing to look at is integrated power modules. They have all the power devices and gate drive circuity in one package. Just hook up your micro. They are more expensive than discrete solutions, but they also are tested working and have rated specs so you don't have to guess.Powerx is just an example. A bunch of people make these now.http://www.pwrx.com/

author: superUnknown
author_rank: Regular Contributor
timestamp: February 20, 2014, 02:08:41 am
content: Quote from: Smokey on February 19, 2014, 10:18:07 pmI tried hydraulic gate drive, couldn't get the flow rates right.Reed relays work great. Good isolation and as bonus we get a nice little 100Hz hum! (this is in the Beginner forum right?)That IPM link is perfect! Plug in and easy, just like I wanted. 200 bucks beats a week of dicking around. Thank you!!

author: granz
author_rank: Regular Contributor
timestamp: February 20, 2014, 03:23:17 pm
content: Rather than any dicking around, it's most certainly easier to just pick up a cheap aluminum TIG welder:http://www.ebay.com/itm/AC-DC-200-TIG-MMA-PULSE-Welder-welding-aluminum-WSE200P-/130451031165?pt=BI_Welders&hash=item1e5f7cc47dI once thought of trying to modify a cheap TIG welder, but then I asked myself a few questions:1) How much money will I save?2) How much time will I waste?3) Is this something I would commercialize (in which case the time isn't wasted)?4) Do I value the learning potential / blog credits enough to make it worth it?And answered them like this:1) Maybe a hundred bucks after considering boards and wasted/changed parts.2) A lot, realistically meaning I would be paying myself something like $2/hour.3) No, I would not try to sell it.4) No, I'd rather spend my time on something more innovative.Maybe you have different answers to these questions in which case it might be worth it. It's definitely not a trivial exercise of spinning a quick board.There are some youtube reviews of those Chinese welders, and quality seems to vary, but I believe that one above is OK.Good luck !

author: superUnknown
author_rank: Regular Contributor
timestamp: February 20, 2014, 04:15:35 pm
content: Quote from: granz on February 20, 2014, 03:23:17 pmAgree with you 100%. However my thinking is, once I get a handle on high current switching, it's not so much of a leap to use any cheapo DC welder as AC tig, High frequency induction heater, EDM source...And, even if I can't get it to work, I'll already learned a lot more about electronics than I knew yesterday...

author: granz
author_rank: Regular Contributor
timestamp: February 20, 2014, 04:47:40 pm
content: Quote from: superUnknown on February 20, 2014, 04:15:35 pmTotally true that it could be very valuable as a learning experience, I just thought that wasn't your main goal since you were considering going with the module solution. You should definitely post your work as you go along if you're going to take the leap

author: superUnknown
author_rank: Regular Contributor
timestamp: February 20, 2014, 05:35:33 pm
content: Most definitely. Samples ordered. Let the smoke show begin!!!