MOSFET get on fire with ZVS induction heater - Page 1
Forum section: electronics / projects

author: lamoule74
author_rank: Contributor
timestamp: September 05, 2022, 04:53:54 pm
content: Hey everyone,We are trying to buid a ZVS induction heater,so we inspire of this schematic below :https://sites.google.com/view/ez-weekend-projects/projets-%C3%A9lectroniques/zvs-module-de-chauffage-par-inductionWe checked the wiring several times, different personsbut each time we are turning the power on, one of the MOSFET (canal N) is destroyed.Last time it was on the source, check out the pictures attached.We obserded big sparks.Do you have any idea on what is going on ?Thank you in advance.Cheers.

author: boB
author_rank: Frequent Contributor
timestamp: September 05, 2022, 05:29:27 pm
content: Really need to look at the actual gate-source drive signal as well as drain-source voltage.That may give you a better indication of what is going on in the real circuit.What does the gate drive circuit consist of ?The simulation looks OK but real world signals may be quite different if the models are not correct.boB

author: boB
author_rank: Frequent Contributor
timestamp: September 05, 2022, 05:38:05 pm
content: Your schematic with the bottom FET gate being connected to the positive battery terminal looks like a big problem.It is shown as not switching. I might be missing something but after L2 saturates, you could have full current through the FET and gate voltage will possibly drop into the linear region ?

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: September 05, 2022, 06:09:52 pm
content: I've had good success with similar designs, but with a center tapped heating coil, avoiding the extra inductors, save one. Do an image search on diy induction heaters and you'll see several examples. Note that these designs will destroy most capacitors- you want polypropylene or heavy duty polystyrene, not Mylar or anything else. Needs extremely low losses.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 05, 2022, 06:41:54 pm
content: Try it at 5V with limited current first.My money's on bad layout. Keep connections short, wide, and close together. Multilayer PCB is ideal. Anything you can approximate like that from wires lashed together, will at least be an improvement.Tim

author: lamoule74
author_rank: Contributor
timestamp: September 06, 2022, 09:27:19 am
content: Hey guys,Thank you for all that suggestions!in order,@boB:OK il will take a look, but the default incoming so fast: I don't know if I will have the time to observe somethinkI will anyway check the gate signal with the scope.Note this is not a schematic from my design: it is pretty common on the web...It makes me think the schematic I used does not have any external gate resistor and could occasionate unwanted critical parameters and behaviours like:- switching time- power dissipation- power losses- voltage overshoot- gate ringingThis guy got a different design:https://www.schematix.co.nz/forum/how-to-s/1-4kw-induction-heaterhe also made a video:I think he's design is better...Note:I will try these IRFP4668PBF instead of 1N5943BG which seems to have bad publicity.@Conrad hoffman:oh, OK I'll take a look for the center-tap heating coil and maybe try it.I noted also for your suggestion about the capacitors' technology ; I suppose you talk about the "line" of capa before the heating coil?I will check what technology I'm using currently...@T3sl4co1l:yep, good idea.To be honnest I am working at a FabLab and this is a member's project;the current limit and the weird design was part of my first remarks...But I did not thought about the voltage limitation: thank you for that suggestion!I'll keep you informed guys.Cheers.

author: strawberry
author_rank: Super Contributor
timestamp: September 06, 2022, 04:05:25 pm
content: mine is betterhttps://www.eevblog.com/forum/projects/zvs-induction-heater-2000w/

author: lamoule74
author_rank: Contributor
timestamp: September 06, 2022, 06:31:00 pm
content: Quote from: strawberry on September 06, 2022, 04:05:25 pmjeeze, it seems to work so fast ! ...Ty, I'll take a look!

author: wraper
author_rank: Unknown
timestamp: September 06, 2022, 06:54:28 pm
content: Quote from: T3sl4co1l on September 05, 2022, 06:41:54 pmOr fake MOSFETs from China.

author: lamoule74
author_rank: Contributor
timestamp: September 08, 2022, 04:01:07 pm
content: Quote from: T3sl4co1l on September 05, 2022, 06:41:54 pmHey Tim, question:Does the powering up of the module have to be frank to avoid the simultaneous conduction of the MOS (powering up on the DC)?

author: strawberry
author_rank: Super Contributor
timestamp: September 08, 2022, 05:11:27 pm
content: yes, startup is both transistors conducting12V 200Wrms ATX powersupply must be enough for safe startup and not blowing transistors...

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 08, 2022, 08:52:13 pm
content: Quote from: lamoule74 on September 08, 2022, 04:01:07 pmNot really, I think? More voltage means faster rate of current rise -- accelerated resolution of that imbalance -- it starts up faster. If it can, of course.Note the powering down needs to be not instantaneous, otherwise gate drive disappears, drain voltage flies back, and avalanche breakdown ensues.Normally, hot-plugging these modules I think does not cause problems, but probably better to leave the supply connected and turn it on/off instead.Tim

author: strawberry
author_rank: Super Contributor
timestamp: September 08, 2022, 09:46:10 pm
content: at power off sequence circuit self power by resonant coil

author: Doctorandus_P
author_rank: Super Contributor
timestamp: September 09, 2022, 08:41:27 am
content: Quote from: T3sl4co1l on September 08, 2022, 08:52:13 pmNope.This simple circuit is notorious for destroying MOSfets.The trouble is that it is too simple, and does not have proper gate drivers, and when the fets are partially open, they have both voltage over and current through them, and they overheat extremely quickly.Therefore this circuit NEEDS to be switched on "instantly". So put an extra switch between the power supply output (and extra buffer elco's?) and the ZVS circuit itself and then turn on this switch last.Alternatively, you can use the MOS fet's themselves as a switch. First keep the gates shorted to the sources, and then remove these shorts when you want the circuit to oscillate.But for a proper design you need lots of extra's. and most of that stuff is protection to keep the MOSfets from blowing up. Some idea's:* Real gate drivers.* Undervoltage lockout.* Temperature monitoring and fan control.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 09, 2022, 10:42:56 am
content: I think there are some modules with protection, maybe even control(?!) onboard? So it depends, if that's the case. But if the module is as the OP's circuit, ya nah, turning it off suddenly will most likely brick it, among a variety of other things.Quote from: strawberry on September 08, 2022, 09:46:10 pmThis is true, after the inductor discharges, assuming it happens faster than the output takes to ring down. But therein lies the problem: as soon as power is removed, the inductor flies back, supply reverses (there's not even a clamp diode to catch this; the 470R resistors might just get popped in the process..?!), gate voltage goes to zero (well, slightly below -- clamp zeners doing their job), and avalanche ensues.Which, maybe the transistors can take a few hits of that, maybe millions of hits, maybe zero. Who knows.Using MOSFETs rated for avalanche current of at least as much as the maximum design input current, and using inductor(s) smaller than that used in the avalanche test, is necessary to survive at least one event. How many more you get after that... who knows.Quote from: Doctorandus_P on September 09, 2022, 08:41:27 amThis is tricky -- using them to turn on, is indeed okay. You can start in an open-circuit (non-oscillating) condition, enable gate bias, and it rings up.You cannot stop it by removing gate bias, lest the above happen!Just making this perfectly clear, in case any readers might make that inference -- being able to do the first step, absolutely does not mean the second is also allowable!How then? Disconnect supply to the inductor(s) first, leaving gate bias on. Only once energy has dissipated, remove gate bias.(This will then also work with less robust devices e.g. IGBTs.)If the input has a big enough capacitor on it, that the supply is not allowed to dip out due to inductor flyback -- then it can indeed be somewhat self-powered by the resonant tank, and energy will dissipate gradually. Self-power...doesn't really mean much, since such a capacitor will dominate total energy storage, anyway; but, in the sense of contributing to total energy, we can go with that.The capacitor needs to be larger than the oscillator's equivalent capacitance (the transformer/coil center tap (if present; note we can still imagine a virtual one even if using a 2-terminal coil and two supply inductors) presents an equivalent R || C impedance at frequencies much lower than Fosc), and needs to store several times more energy than the inductor(s) at maximum nominal supply current.Which should be the case when a PSU is attached: its output bulk capacitance is presented to the load.Actually, even that isn't sufficient help, because some PSUs use very little output capacitance indeed, even in quite high ratings; e.g. LLC and compact (high frequency) types can use surprisingly little. More capacitance might be needed to adequately dampen the oscillator in this way. (I really doubt you'll find a random -- cheap and available -- PSU that actually has this little output capacitance; this is more to say: it's still a technical possibility to need more external capacitance.)In any case, tossing in a few 1000 uF at the input to one of these circuits, is probably not a bad idea. Low ESR types should be good enough even for fairly powerful units.Note that, if you switch into said capacitor -- now you have its inrush to deal with as well. Now switching on is harder than switching off. So, again, probably better to let the PSU handle it, and switch its input instead.QuoteAlso great ideas; but maybe not so feasible? -- It's one of those problems where, obviously the full design is best, but if one doesn't know how to do it, maybe it really is better to just throw the dice with the simpler design and have a bucket of spare transistors handy.I fairly recently explained this elsewhere, actually, for those curious about more details why:https://electronics.stackexchange.com/questions/633217/why-zvs-driver-mazzilli-is-necessary-preferred/633239#633239Tim

author: strawberry
author_rank: Super Contributor
timestamp: September 09, 2022, 05:05:51 pm
content: low Q to get any overshootturning on/off gate supply is different

author: james_s
author_rank: Super Contributor
timestamp: September 09, 2022, 11:42:39 pm
content: That layout looks pretty bad to me, everything is so spread out, long thin wires, probably loads of parasitics. Those heatsinks are much larger than should be necessary, ZVS circuits typically are very efficient. You need heavy connections in the high current paths and as short as possible. You also need to make sure the supply voltage comes up quickly, if it's too slow you can have both transistors turn on at once.A better approach is to use gate drivers and have the circuit set up so that it starts oscillating at low power before the main high voltage power source is turned on.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 10, 2022, 12:57:53 am
content: Quote from: james_s on September 09, 2022, 11:42:39 pmWhat kind of drivers do you suggest?Tim

author: james_s
author_rank: Super Contributor
timestamp: September 11, 2022, 02:09:42 am
content: Quote from: T3sl4co1l on September 10, 2022, 12:57:53 amI used the MC33151This is what I based mine on:http://www.neon-john.com/Induction/Roy/Roy.htmer.I laid out my own PCB that was single sided, this was back before it became ridiculously cheap to have PCBs etched, I can dig up those files lat

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 11, 2022, 03:33:29 am
content: He's lucky it started up at all -- I made several attempts to digital-ize that circuit or relatives. This was the closest I got:https://www.seventransistorlabs.com/Images/1MHz_Ind_002.jpgassembled, tophttps://www.seventransistorlabs.com/Images/1MHz_Ind_Bottom.jpgPCB, bottom (hand etched, heh)https://www.seventransistorlabs.com/Images/1MHz_Ind_001.jpgoutput node (half bridge configuration)https://www.seventransistorlabs.com/Images/1MHz_Ind_Hot.jpgheating a block of what used to be #26 powdered iron...Oh, I don't think I ever released a schematic for this before... well, in captured form, not proper symbols unfortunately:https://www.seventransistorlabs.com/Images/1MHz_Ind.pngBut anyway, you can see it operates on a very similar dynamic, at least by intent, or compare witha more recent demo. It's designed to stay on for a bit, then turn off, fly back, then the opposite side is supposed to handle it, etc. Variable on-time being an implementation of frequency modulation, but local to the driver (clever, right?..).As I recall, it had weird modes where duty wouldn't balance, etc. So, some manner of symmetry-breaking going on.And therein lies the problem: the pulse widths aren't tied to anything so they can make runt pulses limited by propagation delays, or anything inbetween. (See my comments on the stack answer.)(Did anyone evenreadmy stack answer? I feel like it's now like triply relevant here. For posterity:https://electronics.stackexchange.com/questions/633217/why-zvs-driver-mazzilli-is-necessary-preferred/633239#633239Feel welcome to discuss any points made there.)I later (a few months by the looks of it) tried to alleviate some of these issues, by restricting pulse width, hence the name of this one:https://www.seventransistorlabs.com/Images/Pulse_Tightener.pngbut that also failed. Digital is simply too gross to handle the subtleties of a manifestly analog system.So the PLL+VCO is the only route I have seen, that is reliable enough to consider. Naive "this then that" pulse/timing based methods almost always fail -- you can get lucky with them sometimes, as John seems to have in that case, or under the particular conditions of the above scope shot and heating -- but I remember in particular that those conditions were quite narrow and hardly controllable.Even the  oscillator by itself, needn't be very reliable -- I'd tried a few multi-tuned networks, which I'm sure didn't have well-considered coupling factors, resulting in not only extra harmonics at the transistors, but resonance in an unintented mode, and subsequent heating of internal parts. An example use is, caps local to the transistors (required to prevent excess peak voltages), but also caps local to the work coil (to reduce required transformer capacity and overall losses), coupled through a transformer (which unavoidably has leakage) for matching and/or isolation purposes. Sometimes it starts at the coil resonance, sometimes it doesn't; sometimes, too much load on the coil quenches that resonance and it hops to a different mode.So, even in the analog case, these are special cases of special cases; the simple-enough base case happens to be somewhat useful, but any attempt at complexity will be thoroughly foiled by these unintended modes of operation, especially with digital control that doesn't filter it out (again, which is to say: PLL style).Tim