Inverter with Lots of Blown Mosfets - Page 1
Forum section: electronics / general

author: kolbep
author_rank: Frequent Contributor
timestamp: May 15, 2014, 06:19:41 am
content: Hi All.Well, this time I have a large inverter on my floor (I would say on the bench, but it is too big for that).Anyway, this Builder brought a few of these in from China. And 4 of the others worked fine.But this one does not work. It has lots of blown mosfets in the one bank (see pictures).Now, What could possibly cause this (is it maybe that the electrician who installed it - who is NOT me - shorted the output, or maybe swapped the input and output. [Of course he just says it did not work, so we have no way of knowing if it was his fault.]So, What can cause blown mosfets on this thing?How can I test this without blowing more mosfets (can I just use 1 new one on each bank, or will the transformer pull too much inrush)?And what basics can I test.It is a SCB-3KW 230vBatteries 48vThe mosfets are IRF3710and the Mosfet board is labelled FI3K Power V1.1I have googled, but coming from china, i doubt schematics are available.

author: amyk
author_rank: Super Contributor
timestamp: May 15, 2014, 01:14:50 pm
content: http://ludens.cl/Electron/chinverter/chinverter.htmlWhile yours looks a bit better built than that one, you may still want to look for any of the design flaws mentioned there.

author: Legit-Design
author_rank: Frequent Contributor
timestamp: May 15, 2014, 01:33:11 pm
content: is it just light playing tricks or is one of the winding darker than the other? Winding overheated, causing short, taking out the mosfets? Or the other way around, mostfets gone and dumping DC to windings causing them to overheat. That would explain why only one of them is black.Almost black coating on winding look really suspicious. Had one chinese/cheap crap alternator on car fail and it also had black windings, it wasn't just dirty because it was still reflecting the normal way. New one had more copper coloured windings.

author: DmitryL
author_rank: Regular Contributor
timestamp: May 15, 2014, 01:45:18 pm
content: The winding color looks quite familiar..I recently disassembled microwave oven transformer; its primary winding has the same nice "black-copperish" color - appeared to be aluminium

author: kolbep
author_rank: Frequent Contributor
timestamp: May 15, 2014, 03:28:59 pm
content: The laquer on the 2 windings is indeed a different colour. But the darkness is consistent, which makes me think that it was done on purpose like that. I cannot see any other signs on the windings of damage.I have pulled all the mosfets off (4 sets of 7 mosfets).Of all 28 Mosfets, only 3 test good still.7 gate resistors are high values (>900 ohms), the rest are all 20 ohms. There are 1 or 2 that are 18 ohms, and some are 23 ohms.How critical is the matching of the gate resistors?. Can I leave the ones that only have 2 ohms difference, and only change the ones that have an obvious problem, or must I change the others as well (to make all 20 ohms)?There are 4 banks of transistors on heatsinks. But I see that the banks are in pairs.Can I just put 1 good mosfet in each bank to test if I can just get it started (without pulling load), or will the transformers startup kick be too much for the mosfets (and turn them dead as well).What else *should* I test before I try firing it up?Thanks for all the help.P

author: johansen
author_rank: Super Contributor
timestamp: May 15, 2014, 03:48:10 pm
content: is it a pure sine wave inverter or msw?mosfets fail like that when high and low side were switched on at the same time.yes, its possible to blow the bridge when you short the output, but the state of the industry is such that the impedances of the transformer will protect the mosfets from failure. provided they turn off within thermal limits.however, when they turn off, you get this huge voltage spike, so if the manufacturer cheaps out on the bus capacitors, you can blow the mosfet due to avalanch failure, which will cascade down the chain. however, it won't blow all of them. so when the other side of the bridge turns on, you're turning on the H bridge into a short, which should, either cause the package of the blown mosfets to explode, opening the short, which then sends a voltage spike down the line and blows more, or, the Dc bus is robust enough to handle it, and you simply lose a few of them. but not all of them.so basically what you're looking at is an inverter with no appropriate nor safe failure modes. obviously there is no current sensor and it doesn't turn off when one of the inverter legs fails.it is possible those 900 ohm gate resistors (which might be blown due to 48 volts being sent down the gate)suppose they were a mistake.turn on won't matter, but if its a pwm sine wave inverter, you've got a few mosfets that are probably still turning off when the other leg is already turning on.so those mosfets would be fried, or run very hot.

author: NiHaoMike
author_rank: Super Contributor
timestamp: May 15, 2014, 06:04:25 pm
content: It appears to be a similar design to the rackmount APCs. Try disconnecting the low voltage winding of the transformer and applying mains (through a current limiter like a hair dryer) to the high voltage winding to rule out an internally shorted transformer. (If the transformer is bad, you have a door stop - it is not worth fixing.) After that, replace all the MOSFETs and find a way to power the control circuits without powering the DC bus in order to check the drive signals.

author: SeanB
author_rank: Super Contributor
timestamp: May 15, 2014, 06:42:51 pm
content: Peter, if you cooked all that silicon then it is likely as well the driver side is damaged as well. You can use a pair of mosfets per side, and use a series pair of 24V truck headlamps to limit current into the transformer ( place in series with power from the battery) when you test it. Very likely the battery was connected in reverse initially, which then blew up all the mosfets.

author: kolbep
author_rank: Frequent Contributor
timestamp: May 16, 2014, 02:13:23 pm
content: Thanks for the suggestions.I will give them a go just now.Any FYI, I did not do my solder sucking by mouth this time. I removed the Mosfets using a Desoldering pump.... :-)P

author: kolbep
author_rank: Frequent Contributor
timestamp: May 16, 2014, 02:46:13 pm
content: Until I get more of the Mosfets from A1 Radio.I was just wondering,The Current Mosfets are :IRF3710VDSS = 100VRDS(on) = 23m?ID = 57AI have some of these lying around :IRF2807VDSS = 75VRDS(on) = 13m?ID = 82ACan I perhaps use them for testing. I see the VDSS is lower, but isn't it ok because it runs off 48v batteries?and the RDS is lower, could that cause a problem?ThanksP?

author: krish2487
author_rank: Frequent Contributor
timestamp: May 16, 2014, 03:21:52 pm
content: Nope...The IRF2807s will blow out after a couple of mins of running. Faster on load.The nominal float voltage for tubular lead acid batteries is about 14.8 and about 14.4 for automotive batteries.The leads to atleast 57.6V when the batteries are fully charged. In the inverter mode of operation the switching of the mosfets and the primary inductance of the transformer leads to spikes on the FETs drains which might easily be of the order of several tens of volts.I see there are a couple of snubbers caps along with the (inadequate tank caps) but they are most likely designed for a 100V device and will not be of any use for a 75V FET.

author: johansen
author_rank: Super Contributor
timestamp: May 16, 2014, 06:11:59 pm
content: you can get away with the 75 volt mosfets provided you tighten up that DC bus quite a bit.it may not be worth the effort, given other failure modes.

author: kolbep
author_rank: Frequent Contributor
timestamp: May 18, 2014, 06:14:15 pm
content: Ok, So here is the latest.Hooked the secondary of the transformer to 230v, Just a nice gentle hum, with 26.5v coming out of the primary.With all the mosfets removed, I hooked up the Inverter board to the controller and batteries,Controller worked fine, was recognising the AC Voltage, Battery Voltage, etc.So I changed all the out-of-spec gate resistors, and put 3 Mosfets in (I am sure if it did work, it would have been a weird waveform entering the transformer), but it did not work. The 2 Mosfets that were on the same leg of the h bridge went kabluuias soon as the controller tried to start up the inverter.tOk, so I pulled out the mosfets again. Connected to the Controller again, hooked it up to the batteries (I really need to get an adjustable powersupply with Current limiting), pressed the power button, and a few seconds later a 7812 on the controller went up in smoke. Replaced it, tried again, and as soon as I hit the on button, the 7812 got hot very quickly (Short somewhere on the line)Managed to identify which pins on the 16 core cable (between the Inverter board and controller) are for the 0v and 48v supplies to the controller. Hooked those up direct to the battery (without the Inverter board connected at all), and the controller sprang to life, running fine off the batteries, recognising their voltage, etcAs the mains goes straight onto the controller board, it recognises the AC input voltage and frequency fine. But it is giving an error LED (obviously because of the missing inverter board).So at the moment there is a short on the inverter board (the section with the short gets powered by the 7812 regulator on the controller board. So now to go through the inverter board, fix the short, and try find out why the mosfets are not switching like they are supposed to.The Inverter board is nothing intelligent, it has:1x Quad 2 Input And Gate2x Hex Schmitt Triggers1 x Quad XOR GateA couple of transistors, and passives...Just thought you helpful people would like an update...

author: SeanB
author_rank: Super Contributor
timestamp: May 18, 2014, 07:42:19 pm
content: Dead driver transistors then. Just change the lot, as it is likely if they are not totally shorted then they have been made very upset by the companion ones that did go fully short. Change the CMOS chips there as well, they will have been sacrificed on the alter of fried silicon. Be suspicious of any small signal diodes as well. Check them and if there is any leakage or funny forward voltage change them.

author: NiHaoMike
author_rank: Super Contributor
timestamp: May 18, 2014, 08:54:53 pm
content: Quote from: kolbep on May 18, 2014, 06:14:15 pmSounds like someone installed the wrong transformer (24V or 36V input), most likely due to a mistake at the factory.

author: kolbep
author_rank: Frequent Contributor
timestamp: May 18, 2014, 08:59:09 pm
content: Might have been 46.5v . I will check tomorrow...Thanks for pointing that out

author: krish2487
author_rank: Frequent Contributor
timestamp: May 19, 2014, 03:18:01 am
content: What is the driver IC for the FETs??I am guessing IR2110/FAN7392. In such a case, then replace the same. Generally if the FETS blow out, then these drivers too blow out.And out of curiosity, what is the value of gate resistors that you replaced with??900 Ohms is too high. 22 ohms is more closer to the actual value.At 900 ohms turn on and turn off will be so slow that the FETs will be operating in linear mode.

author: kolbep
author_rank: Frequent Contributor
timestamp: May 19, 2014, 06:13:44 am
content: Quote from: NiHaoMike on May 18, 2014, 08:54:53 pmYou Hit something on the head.The Transformer is indeed giving 26v Out.Stamped on it is 0 0 26 230v.The Sticker on the transformer saysPrimary 3Kw/230vSecondary 3Kw/26vThe cabinet is marked as a 48v unit,and the Micro has 48v pencilled on it,So the Chinese must have messed up (howcome this was not caught in testing) Unless they don't test it.Could this have been the reason that the Mosfets Fried?, in which case the installing electrician is off the hook.ThanksPeter

author: krish2487
author_rank: Frequent Contributor
timestamp: May 19, 2014, 10:15:14 am
content: Not necessarily.What you are reporting is quite normal.You have a 4 battery model. (12 x 4 = 48V)For transformer ratings, no one uses the DC bus as the primary winding voltage. EVER.Generally, the primary windings are marked anywhere from 6 - 8 V per battery.What you are seeing is at the lower end of the range. about 6.5V per battery (6.5V x 4 = 26V), which is normal.What this essentially means is that1. The FETs will have a higher average current through them for a given load compared to, say, 32V transformer winding voltage.Given a 3KW rating (not 3KVA) at a battery low condition of 10.5 V per battery and running at full load, you ll draw 3000/42 = 72 A + Copper losses + General Efficiency losses of about a couple of amps. You should draw an average of 85 - 90 A through the TXR Primary.I would take the ratings of the TXR with a pinch of salt. They look grossly overrated.(Mind you, I have specifically mentioned TXR Primary, The FETS themselves will have spikes on the drains during the switching action. Owing to these spikes which cause a potential difference between drain-source during the turn on turn off, you ll also see short, perodic bursts of high current corresponding to ringing/spikes on the FETS. The lower(~6.5V) voltage of the txr has a direct bearing on the value of current spikes. Repetitively subjecting the FETS to this spikes will cause a breakdown. The FET totally sees a voltage of DC bus + Spike amplitude. The instantaneous current is a function of this net voltage across the FET)2. You will have excellent regulation on the AC Output voltage owing to the lower winding voltage at low battery conditions. This means that the Inverter will have a 230V from 10V to 14.8V DC range.I m guessing that replacing the FETS will not be fruitful. I m guessing that the transformer is likely to be operating at a higher flux density that recommended for the particular design. You will continue to have such issues. The best course of action is to simply replace the inverter with a better one.

author: G7PSK
author_rank: Super Contributor
timestamp: May 19, 2014, 03:02:32 pm
content: My guess would be it has no reverse polarity protection, I have seen inverters before where someone has reversed connected the batteries and blown the mosfets even did it myself once on a small one, but I had one old unit given to me that had 80 IRF 540's and all but one had been blown due to battery reversal.

author: kolbep
author_rank: Frequent Contributor
timestamp: June 27, 2014, 09:23:42 pm
content: Ok, Here is how far I have gotten with this thing now.I replaced almost all of the small transistors (2 have a code that does not seem to be findable)I replaced the 4 IC's and 1 x 7805vreg on the Inverter board.I found that there was a Zener that was shorting out the 12v rail (which is fed from the Control boards 7812 regulator). Replaced that.Now the UPS is trying to start up. It gets battery power fine, sees the mains present, and starts the fans.But it is still giving an error. I believe that may be because I have not put in any of the mosfets yet.I would like to know what is the best way to test that this thing will not be driving both sets of mosfets at the same time. I would like to do this to make sure that it will not blow any mosfets (cannot afford to use them to test with).I remember a previous comment about using truck headlights in series with the batteries, and I was thinking that this will limit the current through the Mosfets if they are both switched on. But then I thought that the batteries are connected to the bulk capacitors on the inverter board. Which means even with the lightbulbs, the batteries will charge up the caps (a bit slower), and then the caps will discharge into the mosfets (if there is still a problem), which could cause them to pop again.How about this :If I connect say 4 mosfets with jumper wires (instead of having them on the board), and then where the Drain of one Mosfet ties to the Source of the other mosfet, Do that tie in through the bulbs, So if there is both conducting (on one leg), the bulb will glow, but hopefully limit the current through the mosfets to under their 57A?Will this work? Just as a test before I start using the transformer...I do not have truck bulbs, but I do have several 100w 12v Bulbs. So If i connect 4 in series, it will handle the 48v, but will it be limiting the current enough to protect the mosfets?Thanks for the help.

author: jlmoon
author_rank: Unknown
timestamp: June 28, 2014, 08:06:58 pm
content: By chance on that inverter board, is there a 3524 or similar pwm ic that drives the outboard mostest? Shorted tantlum on that board heating up that 7812?

author: kolbep
author_rank: Frequent Contributor
timestamp: June 29, 2014, 10:01:27 am
content: Nope. on inverter board there was a few logic gates and schmidt triggers.The short on the 12v Line (output of a 7812), was a shorted zenere diode.Replacing that diode corrected that problem, now I am just wanting to confirm if how I want to connect lightbulbs (see above) will be able to protect against shoot-through if there is still a problem in the logic energising both the high and low mosfets.

author: tautech
author_rank: Super Contributor
timestamp: June 29, 2014, 11:10:58 am
content: Quote from: kolbep on June 29, 2014, 10:01:27 am100 W/~ 50 V = ~ 2 Amps. That shouldn't do any damage, but it might not tell you very much either. 2 meters needed, 1 at mains out, the other at DC in after the bulbs in series.The meters will tell you some of the story, the bulbs the rest.Try bulbs first to get an idea of how bright they are directly across the batteries, and if all is well the combination of all 3 indicators should dictate if you can go "live".If you are still suspicious although the bulbs don't indicate a dead short, you could try removing 1 bulb or more so to supply more current. (1 at a time)

author: kolbep
author_rank: Frequent Contributor
timestamp: July 01, 2014, 07:37:31 pm
content: Kaboom!More blown Mosfets, and a few other parts.I should have been more patient, and scoped the signal going to the gates of the mosfets (before I put them back in!)At least I only put in 4 mosfets.Have spent the evening pulling Everything off the board except resistors and capacitors,then testing the Diodes and Transistors 1 by one.Found 2 Schottky? Diodes that are reading short circuit.and 2 other transistors have died.Tomorrow I will get on to testing the Resistors and the capacitors...But, hopefully by the end of this week I will have the UPS working again.And don't even bother to ask the chinese manufacturer to give you a schematic, Their boss won't let them.

author: kolbep
author_rank: Frequent Contributor
timestamp: July 02, 2014, 05:55:39 pm
content: Another one of these UPS's has failed, after running a couple of months in a persons house.The Mosfets are also blown to bits.A few of the gate resistors have been so destroyed, it looks like they actually arced away.The board is full of dust, was wondering if it may have been some form of flashover.The schottkey diodes are reading 0 ohms either way around (with a normal DMM. I do not know much about schottkey diodes, what reading should I be getting)?The customer did have a spare Inverter board, which was brand new. I put that in the 1st UPS that I am trying to fix, and it is running fine, so I know on the 1st UPS, that the Transformer and Controller board are fine.So I guess I just have to carry on testing/changing parts on the 1st inverter board, until it is almost new, and then it should work.Then once that is out the way, it is on to the 2nd board (as a spare).P

author: SeanB
author_rank: Super Contributor
timestamp: July 02, 2014, 06:48:06 pm
content: Diodes reading zero both ways means dead. Typically forward will be between 100 to 300 mV, reverse can be anything above 1800, especially on bigger devices.

author: kolbep
author_rank: Frequent Contributor
timestamp: July 04, 2014, 07:14:13 pm
content: I am somewhat impressed with the chinese manufacturer of these units. Tanfon refuses to send me even a schematic, but once i told them another unit had died, they offered to ship a FREE replacement board. I just have to cover shipping.To get all the parts for one of these boards comes to at least 75 us dollars, so it should be a good deal, depending on the shipping charges.I still think it is strange that they would rather loose out, instead of just sending a diagram. Though.

author: SeanB
author_rank: Super Contributor
timestamp: July 04, 2014, 07:16:44 pm
content: They think you will make like any other Chinese manufacturer and use the diagram and clone it........

author: rob77
author_rank: Super Contributor
timestamp: July 04, 2014, 07:24:34 pm
content: Quote from: kolbep on July 04, 2014, 07:14:13 pm1. probably the design is copied from another manufacturer - sending you the schematic might reveal the truth2. probably the design is so crappy that they are ashamed to send it out3. the design is stolen - but they stole the gerber files , bill of materials and the testing procedure only and they simply don't have the schematic4. they think it's a intellectual property worth of not selling any more units (someone reading this thread will hardly buy their product)but anyways... the replacement board sounds like a good deal even if the shipping costs will be close to that $75.

author: kolbep
author_rank: Frequent Contributor
timestamp: July 06, 2014, 04:55:01 pm
content: Too Good to be true.The UPS Manufacturer responded that we will need to pay USD200 to cover the shipping cost,unless I have a DHL account, then they will ship on that account.To me that sounds very expensive. Works out to ZAR 2000.00!Will DHL really cost that much.P?

author: SeanB
author_rank: Super Contributor
timestamp: July 06, 2014, 05:20:28 pm
content: Unfortunately yes Peter. Living at the ass end of the planet does have drawbacks when it comes to airfreight. I use a courier account at work, and that bill can be frightening at the end of the month when it is reconciled and I have to tie up my usage with the bill.