High voltage low current in avalanche mode in AC mosfets switch 55VDS IRFZ44N  - Page 1
Forum section: electronics / projects

author: eneuro
author_rank: Super Contributor
timestamp: October 04, 2015, 08:21:53 pm
content: Hello,I'd like to do some tricky thing like make zero current swithing of AC welder transformer secondary low voltage below 24VAC, high current up to 150A, so I could use a few AC mosfets swtitches in parallel eg. using IRFZ44N.However, tricky thing is that I'd like to connect HV a few kV flyback secondary limited by resistors in parallel to welder electrodes, so when AC mosfets switch on DC+ before DC+ electrode disconnects electrode from welder ground, a few kV HV limited low current <1mA from flyback secondary will create nice sparks between eelctrode and metal ground, but now this AC mosfets switch also is hit with a few kV voltage, so to some level maybe it will work like 55V zenner diode (IRFZ44N used), but is it capable to survive eg. 1mA shorted to 55V HV up to 10kV 100kHz hits when AC mosfets switch is turned off?Probably I will try this on flyback output limited to <1mA current and lookign for your opinions what will happenDoes this 55VDS AC mosfets switch has any chance to survive hits of a few kV low current when switched off in current zero crossing, so no welding current-I'd like to ignite arc using flyback HV, than turn on AC mosfets switch to provide high current and short flyback output to welder transformer secondary.....Looking into its datasheet:http://www.irf.com/product-info/datasheets/data/irfz44n.pdfWe have:QuoteMaybe this mosfet could survive 10kV 100uA shorted to 55VDS?If i didn't find any theory to calculate this effect with HV , than I will need to test this usingmethods, but maybe someone of you have some idea what might happend?Anyway I will need to immerse mosfet in solid electric insulation to avoid any sparks to ground via gate from mosfet drain (tap) and a few mm gap between drain and gate legs....

author: langwadt
author_rank: Super Contributor
timestamp: October 04, 2015, 08:57:30 pm
content: normally there is a capacitor on the output as a bypass for the hf start

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 04, 2015, 09:21:37 pm
content: Uh, that's not how you do HF start...Also, shorting mode commutation is required for welders. Try to ensure that you have adequate shoot-through time. Most gate driver ICs will not provide this, beware!Also, depending on the output capability of the welder itself, but I would guess based on most specs, that you should want 100 or even 150V devices. And put a huge MOV across them, as low a voltage as you can tolerate. MOSFETs do not appreciate repetitive avalanche, and you'll have a melty hunk of smoking goo if you violate that rating with *this* much current available.Tim

author: eneuro
author_rank: Super Contributor
timestamp: October 04, 2015, 10:14:29 pm
content: Quote from: T3sl4co1l on October 04, 2015, 09:21:37 pmDatasheet of IRFZ44N says as linked above, so probably we need to find how avalanche energy is calculated and see what does this 9.4mJ means for 55VDS mosfet... 0.0094 J/55V= 171uA, so maybe it could survive 10kV/20M~500uA ?QuoteI've found suchPower MOSFET Avalanche Design Guidelinesso maybe we'll be able to predict result of this HV avalanche in those 55VDS mosfets at low current 100uA.I think, I will try limit this HV current to 100uA and see what happends

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 04, 2015, 10:41:28 pm
content: Avalanche means it will act as a zener diode, clamping the voltage. If you turn on a high voltage supply across it, it will simply shunt all the available current, and the voltage won't rise above the lowest breakdown voltage of the parts (maybe 65-70V for a 55V rated device).The avalanche test waveforms should be in the datasheet too, I believe. Note that current is charged into an inductor, then released. The flyback is clamped by avalanche breakdown.Tim

author: eneuro
author_rank: Super Contributor
timestamp: October 05, 2015, 10:26:45 am
content: Quote from: T3sl4co1l on October 04, 2015, 10:41:28 pmWhen we calculate 55V*1.3~71V so this is what I expect, too, but I do not believe IRFZ44N will survive this testAnyway, I'll make two custom 10M resistors and put in series mosfet to flyback output via those resistors and we'll see if itor maybe surrviveHowever, maybe better will be add spark gap in series too, to start from lower voltages and see at which spark gap distance it fails?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 05, 2015, 11:21:55 am
content: What are you hoping to accomplish?An HF or HV start requires inducing high voltage across the output terminals, to initiate or maintain an arc at the electrode.With transistors in parallel with the electrode connections, the voltage cannot rise any higher than the maximum voltage drop of the transistors, which is the avalanche voltage.All the voltage will be dropped across the 10M resistor, and only a small fraction will appear on the output, perhaps not even the full avalanche voltage, because the current looks like leakage current.A spark gap may produce unexpected results, due to high frequency ESD effects. Indeed, this can be harnessed, using the traditional Tesla Coil style HF start circuit. The HF HV is added in series with the source, so that the output voltage can be higher than the voltage rating of the source or inverter.Tim

author: eneuro
author_rank: Super Contributor
timestamp: October 05, 2015, 09:16:30 pm
content: Quote from: T3sl4co1l on October 05, 2015, 11:21:55 amDrain-to-Source Leakage Current of IRFZ44N is 25uA @ 25*C 55VDS, but I will have a few of them in parallel, so it can be close to 100uA, but it might be fine since at 5kV with 10M+10M=20M series resistors it looks like 250uA HV output, so this resistance can be decresed if needed.Not sure if I will have any sparks at such low current, but I have now a few time to test itI can add output capacitor lets say 1nF to increase slightly spark current, but keep output energy still below 100mJ, but i will try first without any additional capacitance.... with 20M resistors in series with flyback output...

author: eneuro
author_rank: Super Contributor
timestamp: October 05, 2015, 11:45:38 pm
content: Soldered my custom 10M resistors (I need add thermal heatshrink to avoid any air discharges between connected 1M 1/4W resistors) and in series with my HV probe made of 3mm green+/yellow- leds in antiparallel, it looks like current is quite low level below 1mA, since green (positive HV DC+) has low (not bright) green light, but still a few mm spark is between led output and ground HV DC-, while HV DC+ connected to green ledIt looks like IRFZ44N mosfet maybe has a chance to survive in avalanche mode, since this current is quite small and very high frequency, since generated by flyback output oryginal secondary, but there is ZVS driver and a few split turns on its primary, so at least tens of kiloherts switching frequency, but ZVS driver 100nF capacitor is not getting hot as well as other my ZVS driver components 100uH 1A inductor and IRFZ44N mosfets in this custom ZVS driver shown in other threadsI haven't got time today setup IRFZ44N HV avalanche test mentioned above, but by adding additional capacitance (steel balls as spark gap) a few mm plasma speaks between those balls was easy created and much stronger air cracking noise is created, so it looks like it could work easy with those 2x 10M resistors as negative ions generator I'm working on in other project and probably those bulky custom 10M resistors (10cm long each) could be fine while space is not an issue in those projects.This experiment was to test if I still will be able create plasma air sparks with those two 10< series resistors on outputs without any additional capacitance and it is not possible make such long like without those current limiting resistors, but still it is a few mm and it looks like that adding those steel sphere balls helps create steady spark gap discharge at quite low HV current

author: eneuro
author_rank: Super Contributor
timestamp: October 07, 2015, 01:28:17 am
content: This was our hero - IRFZ44N with 100k and two 12Vz between gate and source and 10k gate resistor to test RDSON/OFFWhen connected flyback output with current limited by two 10M resistors and first HV DC+ to source, Hv DC- to drain there where no problems after a few seconds sparks between mosfet source-when tested with 9V battery applied to mosfet gate, IRFZ44N survived easy this HV VSD, despite at least 2mm spark was between HV DC+ and mosfet source pinHowever when connected HV DC+ to mosfet drain and created a few seconds sparks via HV DC- connected to mosfet source (avalanche mode) , than when disconnected everything to test mosfet resistance between drain and source I've got 5.3kI can still make low RDSON by applying 9VGS, but with 100k resistor between gate and source 5.3k resistance means that this IRFZ44N doesn't survived this avalanche test, so probably this continous avalache power was too high?Not sure, what this 5.3k mosfet resistance might suggest what kind of possible IRFZ44N mosfet damage after HV low current avalanche test?There is still 100k between gate and source and I can make RDSON low, but even with negative gate source voltage I can't make drain source resistance higher than 5.3k or so...That puzzles me what kind of internal damage happened during this avalanche HV test?Maybe neon (or floursecent lamp) with 80V forward voltage could be decent mosfet HV protection between drain and source or maybe this avalanche current was too high, so avalanche energy levels were exeeded ?Why it is not shorted, but has 5.3k drain to source resistance when IRFZ44N is switched off?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 07, 2015, 02:25:27 am
content: I don't know what the equivalent circuit of your HV output looks like (I'm guessing it's a lower impedance than a typical ESD source), but it sounds like you destroyed a small point in the MOSFET.Destruction due to local heating or vaporization causes a D-S (and usually G as well) short, which because it's small, and made of garbled silicon instead of metal, has fairly high resistance compared to the Rds(on) of the full device.Or another way to look at it, the MOSFET is composed of millions of small cells, all connected in parallel; each individual one has an Rds(on) of kohms, and shorting out just one or two gives that amount as a maximum ("Rds(off)" as it were) resistance.Tim

author: eneuro
author_rank: Super Contributor
timestamp: October 07, 2015, 07:13:41 am
content: There were at least 2mm spark gap between HV and mosfet source pin, but I've connected them together, too, to simulate final application, so we had short-circuit of this HV via mosfet avalanche.When we assume air breakdown voltage 3MV/1meter, than for 2mm spark gap we have ~6kV capability, so 6kV/20M resistor gives estimated HV short-circuit curent 300uA.If 55VDSmax mosfet working at avalanche mode had 55V*1.3=71.5VDS voltage drop, than disipated energy will be 300uA*71.5VDS~21mJ EARIRFZ44N datasheet linked above shows about half lower repetitive avalanche energy 9mJ EARmax and this is absolute maximum ratings, so it looks like those ratings were crossed, but HV DC is few tens kHz HV DC pulses, since it is flyback driven by ZVS driver on its primary.Anyway, in real AC mosfets switch I'll have at least three (total six mosfets like this) so this avalanche current should be three times lower I guess, so 21mJ/3 = 7mJ should be lower than 9mJ absolute rating (77% of absolute rating), so maybe by adding additional fluorescent lamp in parallel with this AC mosfets switch, will decrease this avalanche energy even more by providing additional HV current path bypassing mosfets breakdown VDS.If we had 9kV output than estimated 32mJ EAR we get, so above 9mJ per mosfet when three in parallel were used.Even now, since AC mosfet switch will turn on/off in curent zero crosiing 34Vmax voltage, while this partially failed mosfet has 5.3k resistance when turned off, isn't a problem since welder transormer primary SCRs will turn off/on welder output, so 34V/5.3k~7mA current, but while AC mosfets switch is only used to make this trick to disconnect welder transformer secondary winding from low current HV flyback rectified output, to create air breakdown when welder current/voltage is close to 0 amps, so to create low current plasma spark wheh AC waveform crosses zero, than those AC mosfets switch doesn't have to completelly cut off welder output circuit with high resistance.I think it is time to solder AC mosfets switch made of three in parallel, connect fluorescent lamp in parallel to this switch to try bypass HV mosfet avalanche, since at lower voltage at the range of 35Vmax-even smaller in other test with 11VAC (16Vmax)-hopefully will not be able break neon lamp.I didn't worked too much with ancient fluorescent lamps, so unsure if it there were HV gas discharges lets say with 50Hz AC mains at 100Hz zero switching frequency on welder transformer secondary low voltage , than neon ignited by HV short pulses will stop discharge at the rest of 100Hz cycle at low voltage 35Vmax/16Vmax ?I do not want any current limiting balast in series with flourescent lamp (neon) while I want bypass this AC mosfets switch to help IRFZ44N survive HV avalanche mode when welder electrode on secondary is disconnected from its windings at ZCS.Probably the best way will be to test this, since I haven't got highend HV measurement equipment stuff, but while one of those mosfets partially survived this HV scary sparks on its source pin, maybe AC mosfets switch made of three of them in parallel will have even more chances to survive and even if partially damaged still will be able to provide high current when turnen ON with very low RDSON like this one tested IRFZ44N

author: eneuro
author_rank: Super Contributor
timestamp: October 07, 2015, 11:52:48 am
content: I've made a few AC mosfets switches, where mosfets sources are reinforced (connected) with 2.5mm2 copper wire and drains taps directly to connectors, so no worries this PCB-mosfets should explode first nowI think, I will use two such AC mosfets switches in parallel with fluorescent (neon) lamp (18W 0.6m long) accross switch to try not to kill those two nice AC mosfets switches.Meantime tested previous hero switch and connected this current limited output HV DC+/- to its gate-source/source-gate to see if 12Vz zenners will protect mosfet gate and no worries those 2mm-3mm low current plasma sparks didn't killed mosfets gate

author: eneuro
author_rank: Super Contributor
timestamp: October 07, 2015, 02:53:52 pm
content: Now, with two AC mosfets switches in parallel and flourescent lamp in parallel to this switch IRFZ44N with switch turned off via 50k resistor (two100k in parallel) applying flyback secondary a few kV HV DC avalanche voltage including diode drop in AC mosfets witch was as predicted theoretically from IRFZ44N datascheet ~70V, and disconnecting HV source and testing resistance of turned off AC mosfets switch now everything is fine and we get resistance over its limit at 2M rangeI wouldn't like to disconnect neon lamp and test pure avalanche, since I'm not sure about HV value, so maybe I will try this when I make voltage divider to measure this flyback HV , since I could be able estimate avalanche currents to calculate avalanche energy and checkif it is below absolute rating-if it were more such AC mosfets witches in parallel, probably it shouldn't be any problem, but with only two such switches for the moment flourescent lamp, maybe also helps protect those IRFZ44N, since neon forward voltage might be at similar range like mosfet avalanche.Anyway, at 70V at very low current available and AC switch makeing in avalanche short-circuit , no visible sparks available at daylight conditions, so yep it was interesting experiment, trying to help mosfets to survive HV low current in AC mosfets switch, however I'd like on welder secondary regardless of welder transformer open circuit voltage nice 2mm-3mm plasma sparks like from flyback limited output by 20M resistance, so this is not a perfect way to provide HF HV low current to welding electrode to make nice plasma sparks for high current driving througth this conductive hot air

author: poorchava
author_rank: Super Contributor
timestamp: October 07, 2015, 04:07:00 pm
content: Looks at automotive qualified parts. Many of automotive mosfets are rated for repetitive avalanche breakdowns.Sent from my HTC One M8s using Tapatalk.

author: eneuro
author_rank: Super Contributor
timestamp: October 07, 2015, 05:30:15 pm
content: This single mosfet which failed HV avalanche test was from another assembly lot and I've choosen it intentionally, since only one left, so I woudn't like to use different lot mosfets when needed at least two for AC mosfets switch per PCBAdditionally it was not sourced from any reliable distributor like TME.eu etc, but prety random from local electronics shop where I have no idea from where it might be sourced.Another for which succesfully survived with neon lamp in parallel were from ebay like source and as I know Taiwan, but also not from known distributor.What is interesting both assmbly lots IRFZ44N has in place of YYWW part markings shown in datasheet something which do not fit into this pattern:P405D failed one and P4010D those four of double AC mosfets switch which survived, so I do not know probably datasheet I found have outdated part marking information or something else is going on there?Those IRF mosfets were not sourced from TME.eu I used to for small prototype series, but as I said from ebay like internet auctions, since there were very good shipment options and it was for quick prototype only-not production use-so I used them for testing-no cry if they fail...Update: Automotive version (AU* prefix)AUIRFZ44Nlooks like has slightly different those part markings YWWA.However, in absolute ratings we have the same 9.4mJ EAR , but probably difference is "Repetitive Avalanche Allowed up to Tjmax", which in the case of this test above where those mosfets had room temperature probably has nothing to do, but maybe closer looks will show other differencies too...