IGBT tail current means it can't switch OFF quickly? - Page 1
Forum section: electronics / projects

author: ocset
author_rank: Super Contributor
timestamp: October 22, 2017, 04:39:55 pm
content: Hello,We have designed the attached Mains Transient imitator circuit, which exposes our LED Power supply to a synthesized “Mains Transient” voltage spike so as to see if it survives. Schem and LTspice simulation attached.We must use IGBTs (not MOSFETs) for the switchs, since they have far better overcurrent rating, and we may see overcurrents in the event that the overvoltage spike ‘kills’ the LED power supply.We need to be able to switch the IGBTs ON/OFF very quickly (within 1us). However, years ago, I learned that IGBTs have a large “tail current”, which means that they simply cannot be switched off quickly. We are using the STGW30NC120 IGBT (for all 4 IGBTs) , and its datasheet does not refer to this “tail current”…do you know why?...and do you believe there will be a significant “tail current”?The modus operandi of the circuit is that Switch 1 comes on first and gets the LED power supply up and running. ..Then, Switch 1 is suddenly turned OFF….and then 5us later Switch 2 is switched ON for just 100us and then suddenly switched OFF again…and then 5us later Switch 1 turns back ON again………………………………………….this is repeated continuously.-During the short period that Switch 2 is ON, the overvoltage spike is applied to the LED power supply under test. The Overvoltage is caused by the sudden voltage step input to the LC circuit.So what do you think about the “tail current “ problem?STGW30NC120 IGBT Datasheet:http://www.st.com/content/ccc/resource/technical/document/datasheet/23/1a/51/d9/f1/47/4a/0f/CD00077681.pdf/files/CD00077681.pdf/jcr:content/translations/en.CD00077681.pdf

author: ballanux
author_rank: Contributor
timestamp: October 22, 2017, 06:54:39 pm
content: Hi!The turn-off switching losses indicated in the datasheet should already include the effect of the tail current. Basically the tail current translates into power losses so the question would be if they are acceptable or not. It is possible to do quicker switching if you put negative voltage for the gate turn off, but it may not be necessary, you should calculate your conduction/switching losses first. IGBTs can work on the 20 to 100kHz range so it could work, but if you don't get the uC timings right you will certainly see some smokeMy advice if this is for a commercial design is to buy the test equipment that you need, making a fast transient generator that works properly and within the standard's specification may be more difficult than it seems (and it seems difficult). If you can't buy the equipment you could rent it or go to a pre-certification lab.

author: Simon
author_rank: Unknown
timestamp: October 22, 2017, 07:05:07 pm
content: In the professional test equipment that produces these spikes mercury wetted relays are used, there is a company called pickering relays in the UK that makes and supplies them, what you usually do is charge a capacitor and then use the relay to dump the accumulated energy into the DUT, because you spec the capacitor to hold a certain amount of energy at the given spike voltage you just close (which the mercury wetted contacts do very fast) and let the capacitor discharge, no need for the fast off again.

author: duak
author_rank: Super Contributor
timestamp: October 22, 2017, 09:32:30 pm
content: Here's a sort of answer to the tail current problem:http://www.ixys.com/Documents/AppNotes/IXYS_IGBT_Basic_I.pdfpages 8-9. It appears there's a stored charge inside the device that takes a while to clear out. In purely bipolar power switching circuits, a reverse voltage is often applied to the B-E junction to help sweep out the charge carriers faster. I don't know if IGBTs are available that have any internal nodes brought out.The IXYX note refers to PT devices (whatever they are) that have reduced tail current. I know nothing about themWhenever there is a parasitic current causing a problem in a circuit, provisions can be made to intercept or absorb it. Could a resistor be placed after the problematic switch to divert the current? Note that a few fast recovery diodes may also be needed to prevent backfeeding. If the resistor draws too much current during normal operation could another IGBT switch be added in series with the new resistor and only enabled when the tail current is to be absorbed?Cheers,

author: ahbushnell
author_rank: Frequent Contributor
timestamp: October 22, 2017, 10:32:17 pm
content: There are now SiC MOSFET's that can switch 100's of amps and you can get them at 1700 volts. They do not have tail currents.Look at Wolfspeed. You can buy them from Digikey.An example:https://www.digikey.com/products/en/discrete-semiconductor-products/transistors-fets-mosfets-arrays/289?k=&pkeyword=&pv608=3465&FV=fffc06a1%2C1f140000%2Cffe00121%2C978000e&mnonly=0&ColumnSort=0&page=1&quantity=0&ptm=0&fid=0&pageSize=100Andy

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 22, 2017, 11:22:00 pm
content: FYI, I don't see that that circuit is doing... much of anything. The traditional 8/20us surge generator uses a switch and pulse forming network to create the pulse, and LC coupling to superimpose it on the mains waveform. Different polarities are used, as well as phases synchronizing with the mains voltage.Hmm, I've solved for the equivalent circuit of the 8/20 network, but I don't seem to have it online.Ah, here we go.20uF is charged to the desired initial value (plus a little extra), and discharged through the network. RL is the load. When RL is large, the voltage is 1.5/50us; when small, the current is 8/20us. As mentioned above, the switch is normally a specialty relay; an SCR could also be used, if the right type is found, and triggered properly. IGBTs might be prohibitive because of the ratings required, but should also work.Typical output impedance is around 2 ohms, so for a 1.5kV line-to-line mains transient, 750A peak is required. You'd need a 2400V, 1000A (peak) IGBT or SCR to switch that.Tim

author: Simon
author_rank: Unknown
timestamp: October 23, 2017, 06:37:05 am
content: If you look up the relevant standard you will probasbly find a building blocks circuit. I once designed one but it never got made and the spec (mil spec at that) wanted a mercury wetted relay used and specified the charge resistor for the capacitor and the capacitor value.

author: ocset
author_rank: Super Contributor
timestamp: October 28, 2017, 06:40:10 pm
content: QuoteThanks, if you wish to run the LTspice simulation attached in the top post, you can see what our circuit does...it provides a high-ish voltage spike of some 600V or so into the DUT......and does it cheaply.We are cheapo types, i forgot to tell this to people. We are a startup and cant yet afford the big bucks super zapper type thingsWe already successfully use a reduced version of the circuit in the top post…however, it only has one IGBT “channel” and so can only give a transient overvoltage spike at power_ON time. The circuit of the top post has two IGBT “channels” and so we will be able to use it to give a transient voltage spike to the DUT whilst the DUT is up and running….ie, when its current clamp etc is biased up and ready.I believe that our circuit is the cheapest way to present an overvoltage spike to our DUT…admittedly it doesn’t do that 1.5us/50us voltage type transient , but the kit to do that is very expensive.The circuit of the top post works on the principle of just applying a step voltage to a LC circuit which then rings up to about 600V or so. This isn’t bad from a 230VAC mains supply. It’s a cheap way to give our product some overvoltage stress.

author: Simon
author_rank: Unknown
timestamp: October 28, 2017, 07:35:58 pm
content: I've already told you how this sort of circuit is made and that is going by what the standard you are testing to suggests, I'm sure a mercury wetted relay and a capacitor don't cost that much....... Test equipment is never cheap, but this can be made reasonably cheaply no matter how you do it.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 28, 2017, 08:06:05 pm
content: Your circuit underestimates the current available in a surge, by at least an order of magnitude. It also can't deliver transients of arbitrary polarity and phase, only connects to a single phase, and cannot test common mode surge.This is why I say it "doesn't do much of anything".Look. You say you're strapped for cash, but your actions seem to constantly repeat mistakes that cost precious time, and under-deliver in product quality. Starving yourself from the necessary knowledge and tools to bring a product to market is a classic case of spending dollars to save pennies.Tim

author: ocset
author_rank: Super Contributor
timestamp: October 29, 2017, 08:01:26 pm
content: Thanks , thats all quite right of course.Common mode surge is eg between live and earth....but of course, Neutral is connected to earth, so in some ways, we arent doing completely nothing relevant to that?

author: Simon
author_rank: Unknown
timestamp: October 29, 2017, 08:03:38 pm
content: If you are testing for a surge all you need to do is store a charge in a capacitor the value of which the spec may tell you or if it tells you the amount of joules you can work it out, and then discharge it into the DUT and then recharge with a resistor (constantly connected), it's really that simple.

author: ocset
author_rank: Super Contributor
timestamp: October 29, 2017, 08:10:39 pm
content: Quote…Thanks, you are absolutely correct there….if we pick the “wrong” mains peak then we don’t get any overvoltage spike……and we should have selected the one that’s 10ms later (or previous)….......so what we in fact do is get round this by giving a spike at a random peak….and then 30ms later, we give another spike….and since we are an odd number of multiples of 10ms later, then we know that at least one of the spikes that we give will be a high spike.Thanks Simon also, I will look into Mercury wetted relays..i hope they don’t bounce…..

author: Simon
author_rank: Unknown
timestamp: October 29, 2017, 08:16:50 pm
content: Mercury wetted relays are made for exactly this, the idea of the mercury is to stop the bounce and produce instantaneous low impedence contact with liquid metal rather than purely bouncing contacts which could rival the turn on time of semiconductors of similar price, I can't remember what the turn on times are but they are reed relays which I think is faster than the traditional sort.If you need to trigger at a particular point in the mains cycle why not use a comparator or other device, if you can detect the supply voltage rising or falling through a particular threshold you can trigger the relay taking into account the time required for the relay to react.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 29, 2017, 10:40:24 pm
content: Quote from: Simon on October 29, 2017, 08:03:38 pmClose. You can't use a capacitor directly. I gave the correct coupling network above. A switch is connected in series with the 20uF capacitor, and a charging network is attached to the capacitor as well.An isolation transformer may also be used, to allow positive and negative phase pulses to be coupled into a circuit.Tim

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 29, 2017, 10:42:14 pm
content: Quote from: treez on October 29, 2017, 08:10:39 pmWell, no, it gives the same sign as the mains, regardless.IEC 61000-4-5 surge is tested in all 90 degree phase positions (positive and negative, peak and zero crossing), on each line, 10 shots each.This is what I'm talking about.Tim

author: ocset
author_rank: Super Contributor
timestamp: November 05, 2017, 01:23:31 pm
content: ThanksQuote….Thanks, though what I meant was, if you were to view the attached LTspice sim of our spike generator, than you would see that it trys to trigger spikes at four times…..15ms, 25ms, 65ms and 95ms.….As you can see, despite triggering at all these four times, only the 25ms and the 65ms triggers actally give an overvoltage spike. This is why we have to give two triggers, an odd number multiple of 10ms apart, in order to guarantee getting a single overvoltage spike.We love overvoltage spikes, but must go to this extent to get them cheaply

author: Kleinstein
author_rank: Super Contributor
timestamp: November 05, 2017, 01:59:52 pm
content: That circuit is not going to work, and it would need a very special and large inductor. The main parts to the overvoltage would be the IGBT. There will also be quite some noise / transient injected back to the grid. So if you really want to test it, remove all sensitive gear.I don't think it would be cheaper than the correct circuit too.

author: ocset
author_rank: Super Contributor
timestamp: November 07, 2017, 08:31:40 pm
content: QuoteThe following (attached as an ltspice sim) , slightly simpler version of the circuit in the top post is already in use with us in the lab, and it works fine. As you can see, its mode of operation is just the same as the one in the above post, so we believe that will work too.We actually have it working on the bench.As you can see all it is is a switch switching a voltage into an LC circuit....surely there's not enough there to go wrong for it not to work.The circuit of the top post works fine in the sim..........and since the slightly simpler version of it works fine on the bench in the lab, then we dont see why it wont work. The simualtion shows it working......if someone could make the simulation "more real" , and show it not working, then we would be very grateful.