Driving IGBTs. Opto isolated or not? - Page 1
Forum section: electronics / beginners

author: TheBrick
author_rank: Regular Contributor
timestamp: March 30, 2014, 08:51:32 pm
content: Hi,I'm looking into all sorts information regarding driving IGBTS, reading all sorts of application notes on IGBTs and their drivers. The main aim seems to be reducing ground loops, gate inductance, and being careful with your overall design when it comes to switching speed and switching losses due to parasitic capacitance between collector and gate. All of which I am slowly working out.My question is one of optical isolation. Some items I have read suggest using an optical isolated gate driver. Looking on farnels something simple like thishttp://uk.farnell.com/fairchild-semiconductor/fod3180tv/optocoupler-dip-8-2a-mosfet/dp/2083102but if I look directly under IGBT drivers they are more like thishttp://uk.farnell.com/international-rectifier/irs4427pbf/ic-igbt-mosfet-50-ns-dip-8/dp/2066734which are not optically isolated.I would be wanting to drive a IGBT like thishttp://uk.farnell.com/stmicroelectronics/stge200nb60s/igbt-sot-227/dp/1293645the application would be the inversion and current control stage of a AC tig (constant current) welder, so the load is considered to be inductive.To me it seem sensible that there is optical isolation, however the fact that all the IC listed as IGBT drivers are not make me think I must be missing something

author: fcb
author_rank: Super Contributor
timestamp: March 30, 2014, 09:33:07 pm
content: I've just prototyped a big 3 phase bridge design and I used FOD3182's and little DC/DC blocks:http://uk.farnell.com/jsp/search/productdetail.jsp?CMP=i-ddd7-00001003&sku=2083114http://uk.farnell.com/jsp/search/productdetail.jsp?CMP=i-ddd7-00001003&sku=2079683Each IGBT had it's own D/CDC (which in the next unit will probably be a tap off a little SMPS transformer).I used the dual output DC/DC to create a +/-12 v swing on the gate - worked well.Having done a few low-volume projects with IGBT's, the cost of isolation is worth it.

author: diyaudio
author_rank: Frequent Contributor
timestamp: March 30, 2014, 09:54:00 pm
content: Quote from: TheBrick on March 30, 2014, 08:51:32 pmThe problem with *any* optocoupler is this.* They are meant for medium switching applications.* They have a limited slew rate (hence point 1, regarding medium switching).* Suffer from pulse width distortion.* Suffer from false triggering which will destroy a power supplys switching stage. if common-mode dV/dt is exceeded.* Larger turn-on and turn-off typical propagation delays.They useful, depends on the intent of the design.

author: TheBrick
author_rank: Regular Contributor
timestamp: March 30, 2014, 10:00:15 pm
content: Perfect link to the DC/DC power supplies fcb! Thanks. That solve another problem!The price difference of the optically isolated driver does not seem anything I was just perturbed by the fact that items listed a IGBT drivers were not opto isolated but the one not listed as IGBT drivers, which seemed to be up to the job where. Good to know you've used the FOD3182 successfully.Any further tips on driver design? I was planning on having a zener from gate to emitter back to back with a zener to allow any high gate voltage to drop to the emitter but stop any high emitter spike killing the driver.

author: TheBrick
author_rank: Regular Contributor
timestamp: March 30, 2014, 10:05:35 pm
content: Quote from: diyaudio on March 30, 2014, 09:54:00 pmExcellent info thanks, thing I did not know. The inverter I expect to run at 50 - 400 hz, which to me sounds medium so for the h bridge I don't see a problem, but I'm inexperienced in these things. I'm also going to have a fifth IGBT to do some current control which will probably need to be run faster. I need to do some more research on this and full understand more all of the switching losses and energy dissipation. In my head I'm thinking around the 1khz figure but I'm not sure.

author: fcb
author_rank: Super Contributor
timestamp: March 30, 2014, 11:01:05 pm
content: Quote from: TheBrick on March 30, 2014, 10:00:15 pmI didn't bother, my circuit was very simple: the FOD3182, a 10R series resistor for the gate, a couple of 100nF caps across the the DC/DC and FOD3182. Read the d/s on the FOD3182, you'll see it is pretty straightforward to use.Post your schematic when you've done it, we can then take a peek.On another note, don't get too hung up on the posting about the deficiencies of opto's, they aren't perfect - but nothing is.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 30, 2014, 11:59:55 pm
content: Oddly enough, there are different kinds of gate drivers! Big and small; fast and slow; isolated, HVIC, or neither.There are completely standalone drivers, for example, which use a photovoltaic stack inside, powered by the control LED. Even driving small transistor gates, they develop all of 4-6V, after several milliseconds! These are the basis of AC/DC solid state relays, by the way -- that's why they're so slow, have a resistive "on" characteristic, and switch AC or DC (as opposed to the traditional AC-only SSR, which is a big TRIAC driven by a photo-TRIAC, optionally with a zero-crossing detector or whatever).The FOD3180 is an isolated type, which requires an isolated power supply (it's not completely standalone). You'll do very well with something like that (the price isn't awful, either). You'll still need additional circuitry if you need protection (like desat detection) or what have you. Beware that its rated dissipation is very low (seems typical of most opto devices), so you can't drive much load at very high of a frequency, which limits certain uses (it probably wouldn't be so great for a 10kW power converter).The IRS4427 is a second-source buffer/driver, also made by Micrel (MIC- prefix), Microchip (TC-) and others. Think of it as a fucking massive CD4000 buffer gate (except usually with a different input voltage threshold, and better speed and switching anyway). You provide the logic-level signal, and it does the heavy lifting. So, you still need to provide isolation and power. This is the sort of chip you might use with an optocoupler to drive big loads that a chip like FOD3180 can't do.There are combined high-low drivers, I think originally starting with the IR2110 series (but everyone makes them now). They are not isolated. You provide logic signals and power, it does the signal coupling (coupling, not isolation) and driving. Usually, the high side is bootstrap powered, meaning, when the low side turns on, the high side is pulled down near GND, so a diode can charge up its capacitor from low-side VCC. The capacitor will only run the high side driver itself for so long, so this type requires that you keep the inverter always switching. You can't have it stuck high forever unless you provide isolated DC or whatever. (Exceptions include the HIP4081, which has an integrated charge pump: not as powerful as an isolated supply, but enough to keep things running.)These drivers tolerate a relatively high voltage on the high side, but positive only. Some may stop working, malfunction or get destroyed if the high side suddenly ends up negative for any reason (overshoot being the most common concern). The highest (lowest?) rating I've seen is -6V to 600V.What you cannot do is use a high-low driver to control transistors on the AC line side, while your controller is on the DC load side. You need isolation for that. Besides the principle of galvanic isolation, and the circuit being common to the low side anyway, because the voltage rating (of the high side at least) is one-sided, you simply can't use them on AC.As for isolation, take your pick:Capacitive (usually via modulation)Inductive:- Gate drive transformer (no external power at all!)- Signal transformer- Radio tricks (usually via modulation)- Monolithic magnetic logic couplersOptical:- Dumb phototransistors and stuff (e.g., 4N35)- Proper "high speed" isolators (SFH6345 = a 6N136 with actual shielding -- an order of magnitude better dV/dt!)- Integrated amp / logic output- Fiber optics (ranging from simple on/off LED-fiber-detector pairs to whatever modulation, mixing and splitting network you care to build!)An example of the first (capacitive) would be one of those huge-$$$ ISOxxx chips from BB or ADI. At least a few of them I think, used a capacitively coupled isolation barrier, coupling the proportional analog signal via frequency modulation. A similar method (using FSK or PSK, for example) could be used for digital isolation, but... yeah, you'll have a hard time with that, and ultimately, the performance (especially dV/dt) won't be all that great. Not hard to make something that works alone, but hard to do well for an application like this.Transformers are nice, especially at higher frequencies and lower power levels. Not terribly expensive, and can be custom wound for not terribly more. But they also go up steeply from there, especially if you need small quantities, or short lead times (if non-stock), or agency rated isolation (anywhere you see, like, UL rated 'Reinforced' 5kV+ isolation, automatically add $5 to the price).Transformers are good at driving small loads, but it's very difficult to make one beefy enough to handle bigass gates. You can use a signal transformer followed by a conventional driver, but now you could just as well use any other logic isolator.Although the transformer itself is AC only, you can use simple pulse encoding, followed by a latch, to achieve DC coupled behavior. But I intensely dislike the idea of leaving a gate driver hanging on its own memory; what if it happens to miss a pulse?Monolithic couplers are very interesting. They are actual copper wire transformers, printed on kapton, printed over the die. Air cored. The electrical performance of this structure is nine degrees of terrible, but it's enough not only to couple a logic signal, but is in fact capable of delivering fractional watt power levels at practical efficiencies (I think 30-70%) -- enough for, I don't know, an isolated RS-485 receiver let's say. ADI's iCoupler series I think is the original. One distinct advantage: the dV/dt rating is far in excess of any opto I've seen, 25kV/us and up. And, being monolithic, it's no problem for them to stuff two or four channels into one SOIC package, so you can transmit *and receive* multiple channels -- put a MCU up there to monitor the transistor switching performance if you want! They are fairly priced (comparable to most logic-output optos), and offer higher bandwidth as well (10Mbps and up). FYI, I wouldn't trust other name brands, especially the very cheap ones: they probably don't have as good isolation (the ADI parts qualify as Reinforced) or upset resistance (typically, they only send pulses when the input changes... same problem as DC coupling with a transformer -- difference is, the ADI parts pulse constantly, and they have a fail-safe state as well).Finally, fiber optics of course offer the greatest possible isolation of any method. (Well, you could make a full-on radio transmitter and receiver just as well -- as long as it's above the switching noise from the rest of the circuit of course... but that's equivalent, just doing the same thing at a lower frequency!) This is commonly used in massive converters, from motor drives and induction heaters to HVDC power stations. Hardware starts in the $10-20 range, so you only bother if you need it.Tim

author: TheBrick
author_rank: Regular Contributor
timestamp: March 31, 2014, 01:09:11 am
content: Thanks for writing all that out Tim that is is a massive post and will take some time for me to digest.I did wonder if you could use a FOD3180 to boot strap to the IRS4427 on on the the gate of the IGBT. I think I will be back to you on this subject. I may end up starting a thread on the project when I have worked out a few more details. I understand most of the basic ideas but the subtle points of implementation is where I'm struggling so this is all good info thanks.

author: TheBrick
author_rank: Regular Contributor
timestamp: March 31, 2014, 01:26:13 am
content: One thing I can't figure out it how much current I need to drive a IGBT like thishttp://www.farnell.com/datasheets/1690099.pdfVG(th) is 5 v and because the gate is a mosfet its voltage driven so both the FOD3182 or IRS4427 can reach a high enough voltage to drive it. The IRS4427 still requires the isolation as you state but the FOD3182 looks good enough on its own to me.

author: senso
author_rank: Frequent Contributor
timestamp: March 31, 2014, 01:41:35 am
content: Sorry for not adding anything extra, but really love to see all that much information in a single post.

author: fcb
author_rank: Super Contributor
timestamp: March 31, 2014, 08:44:03 am
content: Post of the day, made me smile:Quote from: T3sl4co1l on March 30, 2014, 11:59:55 pmTheBrick, yes you could use a FOD3180 to drive another driver chip - however if your going to do that, you might as well just spend the money on a faster opto (teslacoil mentioned a faster 6N136).One think to watch for on those single chip HVIC drivers is the bootstrapping - BE WARNED - you need to understand throughly the drive waveform you are feeding the chip, bootstraps only work well for certain dutycycle limits. I tend to design for static situations, so use isolated supplies in-favour of bootstrapping.Another thing teslacoil pointed out is transformers. Not much experience using these to drive gates - plenty using them to make isolated supplies. Probably now considered 'old-fashioned' for gate drives.I have seen fibre-optics (POF) drivers on very high power IGBT supplies - never used then yet for that purpose, might be fun though.

author: Dago
author_rank: Frequent Contributor
timestamp: March 31, 2014, 12:25:04 pm
content: Quote from: TheBrick on March 31, 2014, 01:26:13 amYou need to calculate the current needed from the gate charge and the required rise-time. Deciding on a rise-time can be complicated. In terms of power dissipation you want it to be as small as possible but that increases switching voltage peaks over the device due to higher di/dt.Here is a good document:http://www.ti.com/lit/ml/slup169/slup169.pdf

author: TheBrick
author_rank: Regular Contributor
timestamp: April 01, 2014, 01:16:05 am
content: Cheers Dago. I'll take a look at that one. For others interested in this subject I've also found these to be useful.IR DT04-4 Using monolithic high voltage gate driversAdvance power technology app node APT0201APT9302IR AN-1045I'm taking a bit fro each one and slowly getting the idea.

author: TheBrick
author_rank: Regular Contributor
timestamp: April 01, 2014, 02:05:21 am
content: The penny has dropped! Its about supplying the required gate charge for the required switch on time!@Slaps head for being so stupid@