Hmm, how exactly do you expect variable voltage, without regulation? (Just that you don't NEED it, I guess?)
Is this a resonant design? The transformer is unspecified so I guess it could be planned as an LLC with integral leakage inductance. The secondary capacitance will be significant. Still, no output filtering at all is shown, if I'm not mistaken?
Do recommend full wave doubler rectifier, you only need a 1500V secondary and 3kV worth of diodes (so, probably 4 in series would still be a good idea).
Might as well get co-pack IGBTs, you're planning on putting diodes in with them anyway it seems (another indicator of resonant? -- forward doesn't technically need them, though diodes at all are likely still a good idea due to leakage, even if smaller ones).
Not sure why the gate clamp diodes (D2, etc.). If the gate blows up, the driver's toast either way; typically a transistor fails as a three-way short, thus delivering some fraction of its fault current to the gate pin. You need big TVSs and extra series resistance to actually have a driver survive. Add in a TVS to the 15V rail, and what would look like ESD protection between the logic source and the gate drivers (well, the optos in this case), if you want to treat the IGBTs and drivers as disposable and replace them after a fault occurs.
The opto is... rated for enormous isolation, and pricey! I don't see a need for this; typical digital isolators these days are rated for more than enough dV/dt and voltage to do the job. You probably have worse things to worry about if >5kV somehow appears across your isolation barrier...
Oh what the heck, it's slowed down a hell of a lot too, C5, C6, C11, C12 are way too big. The optos are 100/200ns and the drivers another 300/400ns (depending on rise/fall, I don't remember which is which), there's at least half a microsecond already between what the controller is doing, and what's actually happening. The RCs will add another almost a microsecond!
The discrete flip-flop seems... incongruous with the rest of the circuit? There's also no opto conveying its state to the controller, that's not great. RESET seems to be hanging.
Hm, all the temp indicator LEDs are wired hard in parallel. I suppose there's supposed to be diodes to FAULT, just a typo. FYI, you may want hysteresis to eliminate chatter, and/or latching to maintain which source caused the fault. For extra bits of flip-flops, consider a CD4043 or the like.
Speaking of CMOS, CD4011 or the like will be just fine for flip-flops. Or 4043s, or 4013s, etc.
Or don't run any logic on 15V, use 74HC on 5V, you have it handy already. Which is, I think just because the opto receivers need 5V?
BTW, beware of self-heating of the thermistors. 470R is kind of a strong pullup, even from +5V. I prefer >=10k thermistors. (Although I often run them from 12V, which is about the same situation, hah.) You can always set the threshold voltage lower (using higher pullups), too.
Oh and the input, no way in hell you need that much capacitance, about 2mF should be enough. One computer grade cap should do, or a handful of snap-in types. Preferably the latter, as they're quite low inductance -- probably fine to place them beside the inverter and skip out on the local (film?) bypasses. Or just a few uF local, may still be handy (DC Link MKP type are cheap and dense, well suited as the description suggests).
Oh, and I just noticed your main diagram suggests 20kV isolation, which, I don't have a clue where you'd even find an aux transformer rated for that. Another custom windup? You'll need to keep shopping for optos, that's only rated 16kV. Again, I don't get it, you'll have to put in real work to get anywhere close to that kind of offset. Like, intentionally charging the poor thing on top of a CRT's 2nd anode, or, stacking it on a live Tesla coil. Or an electrically-small antenna transmitting some ~kW. None of which makes any lick of sense for a transmitter power supply...
Likewise, RF immunity is easily provided by metal enclosure and heavily filtering all connections passing through it. Use a mains inlet filter. Create HV feedthrus for your output. Bypass grounds to chassis (typically with small Y1 rated caps), or ground to ground as the case may be. Actual RF inside should be ~volts, and most of that being electric field from the transformer and rectifier.
So, needless to say, this thing will be two dozen kinds of deadly in operation, but especially so if it's actually, intentionally, operationally, getting up to voltages like that. Like, I personally would not operate it without being on the far side of a lockout-gated metal fence, dangerous. (Power companies have test rooms like this, the safety procedures are quite ornate -- and with good reason, even if they're only doing a few mA in there for a given test.)
Also, again, if you're going to have that much voltage to isolate, then uh... how the hell are you gonna control the poor thing? All the controls are suggested as being at output level. It'd honestly be safer to wire everything on the primary side, where it's expecting only a measly 2.5kV (or 3-4 depending on CAT) transient above the paltry 240VAC mains...
So, show stoppers:
- I don't see ANY indication for AC current sense. Maybe you're putting a current transformer in the wiring diagram? Dunno. The control MUST be current mode, either for a square-wave forward converter (peak or average current mode), or for resonant (current limiting). (Voltage mode is a bad idea, best forgotten as a relic of primitive times gone by...)
- No feedback from the inverter. So the fault can latch and the controller just assumes everything is fine. And also has no way to reset it.
- You haven't shown the last sheet yet, but beware of short-circuit (transient) fault currents, especially with regards to the current sense, and anything else touching it (and also, for S&Gs, calculate the peak output current under such conditions
).
And the real worry point:
- You seem to have made enough mistakes that it suggests you're a novice when it comes to switching power supplies, or high power projects.
Please,
PLEASE, get some experience first.
You can get all the experience you need, just making piddly little, like, 12V 1A converters.
It all scales just fine.
Well, inverter parasitics don't scale, but that's part of the fun, make a few different ranges of voltages and currents, and make shitty layouts to exaggerate the strays.
Discover
why I'm so insistent about current-mode operation.
Then,
and only then, scale up to the low kW, and
maybe to the low kV, and build this project.
You can also discover why MCU controls are a bad idea -- unless you have much, much more experience. Mind -- I have no idea what you were planning, but I want to head this off too, just in case.
Would it surprise you to know, as many years experience as I have, I still have yet to design a software SMPS control? It's true. There are several reasons, some practical (who cares, who would pay for that when I can drop in a more-or-less proven IC that does the job already--), but some also to do with experience. Software
IS bugs. Only very, very good software is relatively free from bugs. And I don't know that I'd say any software project is truly free from bugs, aside from the most trivial cases. (There are "provably correct" software projects out there -- some even of quite surprising scale -- but this shifts the burden of proof onto the specifications themselves, which may in turn contain "bugs". Writing software compliant to a spec, is at least as hard as writing a truly comprehensive spec.) Let alone my software, which honestly is at or above average from what I've seen of other embedded projects, but is hardly, like, medical or aerospace grade.
No, an SMPS isn't likely to cause immediate bodily harm like a robotic surgeon or flight computer could, but... well, we're also talking about voltages that can reach out and touch you from whole centimeters away. So, do you really want to take the chance that you might have bugs in there?..
And so, that's the major reason why I don't, and prefer hardware which I'm both more confident in to begin with (but that's just me), and more confident in being able to produce a "correct" design, based on good design principles (like minimizing internal state).
Your design so far, by the way, does have this going for it, or the beginnings of it anyway -- if the driver fault signals work as planned, they should be able to turn off the IGBTs well within their maximum short-circuit time, and the circuit simply clicks off (the click will likely be audible!), rather than causing a cascade failure (typically, one transistor fails, then the other transistor switches into it, drawing short-circuit fault current; the driver, optos, maybe mains rectifier, maybe some output stuff, fail from there). Note that, as shown, this WILL NOT protect against transformer shorts to ground, as the high side transistors are not monitored -- for that, you may wish to consider additional circuitry, perhaps drivers with desat detection as well (or instead), etc. I've had excellent results with this, even just using a discrete desat circuit; thanks to this, plus a hardware fault latch, I'd just about be able to fit all the transistors I've ever blown, in the palm of one hand. (And the main reason I can't, is because I've blown a couple of industrial modules, that aren't quite palm-sized; if you allow stacking, then sure.) Other people seemingly almost brag about having blown "buckets" of transistors in their careers (and this, well before retirement age, I might add); I cannot say I agree with such an attitude.
Tim
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