Burning FET's and driver troubles

Things at the limits.

Re: Burning FET's and driver troubles

Postby JonathanH13 » Sat Nov 20, 2010 12:25 pm

>You're going to need more drive, or less fet.

I have replaced the SPW47N60C3 fets with IXFH52N30P (52A, 300V). If/when they burn, I will get some IRFP450.

>those rises and falls are just too slow.

I have 6 ohm resistors at the gates and 470nF ultralow tantalums as bootstrap caps. When correctly tuned, this arrangement with the new fets seems to give a reasonable rise time. See pic 'LO2 & Output of same' (I measure the duration of the on pulse=13.8uS, rise time=1.6uS, fall time=0.8uS. So risetime is around 11% of on duration and 6% of total time. One complete cycle = 28uS = 35.7KHz)

The fets are staying cold at low or no load, but still at low voltage...

>if I could see one of the bridge outputs into the load.

See same pic as above - blue trace is gate, red trace is output of fet board at transformer primary.

>but you need bulk filtering.

I have connected two 4700uF @ 400VDC caps in parallel across the rails, so it's all smooth DC now, I'm also driving the chip with a large 12V battery.

I have added a snubber between the two output points (1000pf and 100ohms, 50w resistor) and it does not seem change much (tried different caps and resistor values). This is still with the main rail voltage at 10VDC, so perhaps when I turn things up it will make a difference?

>A key diagnostic is what does the output do during that dead time.

I identified the deadtime (see pic 'deadtime between the gates'), and then looked at this same time at the output. See pic 'Output during deadtime' - blue trace gate, red trace output. It looks to me like the output flies to the other polarity, pretty hard.

I have added a pic of the chip output without the fets connected, red trace is low gate signal, blue trace is high gate signal. What's with the little 'trough' before the rise on the high gate signal? And the little notch knocked out of the pulse just before the fall? I understand this is the deadtime, but do you know why it effects the high pulse like this, and not the low one? This little 'disturbance' is what is generating the more serious glitches and ringing in the fets... I have posted a pic of the ringing on the gates due this deadtime 'trough'.

I have tuned for the lowest no-load current: 310mA @ 10VDC which is around 42.37KHz (this is where the glitches disappear, just the same as in your video) but cannot get rid of this nasty ringing effect). I am going to connect it to the stack and see what happens at higher voltages...
Attachments
gate ringing.jpg
High gates ringing
driver with no FETs connected.jpg
Driver with no FETs connected
Output during deadtime.jpg
Output during deadtime
Deadtime at the gates.jpg
Deadtime between the gates
LO2 & output of same.jpg
LO2 & Output of same
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Re: Burning FET's and driver troubles

Postby JonathanH13 » Sat Nov 20, 2010 3:04 pm

I just connected this to the stack, and the fusor. It worked brilliantly! That is, until I let in too much gas, the pressure jumped up and the stack's 50K resistor arced (that's never happened before). The backlash took out the little driver chip, judging from the smell (I cannot test it until the caps discharge). But all in all, very succesful - a massive improvement. With 20VAC on the variac I get -24KV, with 35VAC on the variac, I get -40KV (no load). The variac goes up to 270VAC, so it might be time to sink the stack into oil. I think that the maximum current being drawn on the fets was just under 5 amps at one point, and they are only slightly warm, running with heatsinks, but no fan. The high frequency component on the rising edge is still a problem, as this obviously grows as a spike as the voltage is increased. Curiously, I now get three electron streams, where before (with a 50Hz MOT supply) I only got one. That image is at around a few hundred microns I think. The pink image is at a lower pressure, somewhere between 10^-3 and 10^-4 Torr, with a touch of deuterium added.
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fusor deuterium.jpg
Deuterium added
3 jets.jpg
Three electron streams
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Re: Burning FET's and driver troubles

Postby Doug Coulter » Sat Nov 20, 2010 3:59 pm

True beauty, Jon! I'd been reading your prior post with all the juicy data I can get my teeth into so as to make a good response, but other things have slowed me down a little; new members and so on, coming from Physics World, IOP, and other "real" places.

Yes, I've fried more chips than fets, to tell the truth -- once I got them to be either all the way on or all the way off -- the fets live and don't need to be much overrated. A fast 15v zener from G-S on the fets (or better, right at the chip so the gate R can protect the zener some too) might help with that one. Resistor arcing has been an issue here too, till we got those huge 13" long 225 watt ones we use now, or the ones we've made up of series parallel strings of bulk ceramics CliffS reccomends (Mouser electronics has those fairly cheap -- I used about 30 10k ones to make a 33k ballast about a foot long in a glass pipe, that works fine too). Of course if you're going to higher voltages, the arc over problem with a ballast becomes worse along with the rest, so keep that in mind. I have voltages up to 53k handled quite well, the next step up is going to have all those challenges all over again, but much worse. Above about 50kv, streamer formation with high field concentration at the streamer tip starts to happen, and at that point it's like lightning -- length of the resulting spark has no obvious limits. I had a 125kv one go 6 feet one way, make a u turn in mid air (right near a ground, too) and come back near where it started once.

Yes, I think oil or other not-air thing might not be a bad idea. I am planning for little to no air exposure for my new HV supply for the reasons above. One scary 12 foot spark in the lab right next to me gave me a little religion. I am thinking of boring out some inch or bigger teflon rod to use as HV wire insulation, and adding a ground mesh over that. (note, boring HDPE is really hard to do for deep holes, but teflon is easy and better anyway) I'm running my ballast R screwed/mounted right to the FT, and it's inside a 2" PVC pipe with forced air blown into it to keep it cooler. The whole mess is inside a grounded screen -- no exposed HV anywhere even at a "mere" 53 kv.

Arcs are murder on this stuff -- no limit on the risetime of that, so even small capacitors like those in the fets, and various strays can move big amounts of power backwards through things.
At least you have a full-wave CW stack, without that, you can have really horrible issues with the full stack voltage trying to get through the transformer backwards, which can be the same multiple of the fet rail voltage as the stack multiplication factor, very very ugly. Even with full wave, as you just found out, there are issues...for one thing, it's never perfect balance.

For everyone else -- Jon is setting an excellent example of good posting here -- a good set of questions, the data we need to answer them when requested, all very clean (Thanks, Jon).
This sort of thing makes this more useful to all, including the lurkers, and brings the better lurkers out of hiding as well, to everyone's advantage over the longer term. Good show :D

Jon, you might want to look over what I'm doing now for gas control, it's truly a breakthrough over the old ways of doing this....if you can get to where it works. That problem is just gone here now. I have a little of it under hands free fusor runs. I'll gladly write more detail on that if anyone wants (or maybe no matter what, it's good stuff).

I'm of course you'll be adding to that forum soon with some results -- you should be getting them pretty soon now. And nice focus for a first go, you'll find as you improve that, so will your net output.
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Re: Burning FET's and driver troubles

Postby Doug Coulter » Sun Nov 21, 2010 10:03 am

Ok, responding to your post above. Easy stuff first.
On the snubber, it's intended to get rid of some ringing and slow down the output edge foolery during deadtime. Since very fast output edges wind up coupling back into the gates, you are trying to kind of slow the thing down -- rise and fall times can otherwise be very fast when driving an inductive load. In other words, tuned below resonance a little, which is where all mine work best. As I noted elsewhere, I'll often put at least some of the primary turns on the other leg of the core to get a little more leakage L, makes things much easier to drive.

Of course, this depends heavily on the magnetics you've got, and sometimes the easiest thing to do is play with or rewind the primary winding, it's usually easy and not many turns.
If you're seeing no effect with those numbers, there can be a couple of reasons -- and you should see it even at low inputs. One possibility is that your resistor itself is really inductive, so the fast edge coupled in via the 1k pf cap simply doesn't see a load. Some WW resistors are really bad that way, and it's getting hard to find big carbon ones these days, which are much better. Of course, you can make a big one out of some smaller ones. You'd like to see a few watts in the resistor (at full voltage), and if it's not getting warm -- it's either really inductive or it needs to get smaller, and perhaps the cap a little larger. You probably don't want to tune the RC below or too close to the 3rd harmonic of the base frequency, or it will simply eat too many watts.

I want you to try something for me here. Short your probe, and hook the scope to some big square wave (such as a bridge output) and look at the resulting trace -- I want to see if your scope can really be floated on a big SQ wave, because if you really can, heck, I want one, I've never seen one you could really do that with without a lot of spurious stuff making it into the trace when you should see nothing (note, by doing this you make a tiny one turn inductive pickup, so don't do it near a wire carrying a lot of current). If that's happening, then some of the other strange things I see in your pictures are explainable that way.

Some of the gate drive waveforms look very-very strange -- I am trying to understand why there is stuff there during the on time for example. Nothing in the chip is happening, nor should there be anything happening on the output during that time, so where's all the junk (rather than a flat top) coming from? See above scope test -- you gotta be where you can trust your tools pretty absolutely.

Your "high gates ringing" might be almost entirely due to a scope issue here" It's actually sort of hard to make that happen unless the output is ringing hard and coupling back there via the fet capacities, so I find that slightly suspicious -- a possible measurement artifact that isn't really happening. It's actually pretty safe in this circuit to assume the top drives look pretty much like the bottom ones once your bootstrap cap issues are taken care of, and they should be at this point. Your extra diodes may be causing a little of this by allowing more outside the rails stuff.
I know the book says don't use the fet body diodes, but I do, and they are not all that bad these days with the more modern parts we have. They may cause a slight bit more of fet heating, but if you've got thermal headroom, it's not a big deal. The fact that they are a little bit slower than the best diodes you can get isn't all bad -- helps control the output waveform some, though the snubber is supposed to be doing that job. But it's not necessarily bad to share that job around (spread the pain/heat).

Though perhaps not needed in either of our projects, this trick is often used in cases where the self resonance of the transformer is just too low.
HVTrick.gif
Tuning trick

Here, L1 is used to raise the effective resonant frequency of the magnetics. You can only take this so far without running into big losses, because it's the secondary doing the resonating and running it above that frequency means resistive losses in that winding as it tries to drive its own capacitive load, but it might get you "enough" without being too bad. Obviously, the inductor has to not saturate at full drive for this to be good, and it has to be not very lossy itself, so it's a big chunk of extra expense. The series L-C is tuned to resonate at the drive frequency (which is about the same as the new net output resonant frequency). This does two things for you. One is you get the capacitor to keep from having the magnetic walk in the main core if there's a bit of DC on the bridge, with the series L2 canceling out the capacitor's reactance, and the series L2 also acts like a higher impedance back to the bridge for both the fast edge on your desired signal, and any blow back from arcing on the output. You don't want this to be very high Q, else the voltage at the junction of L2 and C1 will go wild, and may even need to make L2 delibearly lossy (or put some R across it, but that's going to eat some power). Wouldn't be surprising to accidentally make many times the normal signal voltage at that point, so you have to be a little careful choosing the L2/C1 values from the infinite number of combinations that create a particular resonance. Perhaps Cliff will chime in and mention some good rough rules of thumb here, as I don't normally do this and don't have a lot of experience with doing it this way.



There are several ways to help with burning fets and chips during arcing situations (but it's best to not arc of course). One is the zeners I mentioned above. MOV's generally aren't "hard" enough, you have to go way overvoltage on them to get them to do very much, and you probably don't want to choose them so low voltage as to draw current when things are normal.
Spark gaps, though, are fast and can take a certain amount of pounding, so if you can find some that don't "go" at your full rails voltage, then hooking them between each output and each rail is good (eg you need 4 total). You could try zeners there too, but I've not had great luck with them, and it's hard to find ones that will take really big peak currents, no point in putting anything there that will fry (and fail shorted!) itself. You can count some on the big filter caps and a simpler catch diode circuit to keep the outputs inside the rails, and you just might find some fast ones that turn on before or at the same time as the fet body diodes without needing the one in series with the fet drain. Most big electrolytics do have some series impedance, though, which is why you often see the use of a bunch of smaller ones in parallel -- this results in less ESR and other stuff, not to mention better cooling. You'll find your filter caps getting warm under some tuning conditions due to the ESR and the catch diodes shoving quick peak current pulses back into them.

I would be trying to run the chip as close to the 15v as you can. I like using a 3 terminal regulator (7815) for that job, so I don't have to depend on heating the zener in the chip, or a resistor outside it -- the 7815 is a lot easier to shove on a heatsink anyway. The internal zener is a little above this, so you should be fine, but in my board I made provision for a 50-100 ohm series R from the 7815 to the chip just in case. You get a free current limit this way too, a nice thing to have anyway.

Even if you don't want the ability to have a constant current output (I really like having that, by the way, which is why I'm going to try and go down the path suggested by JohnF, with a current fed topology and a current limiter in the input switcher) you do need some kind of fast acting way to shut this down under arc/short conditions.
I'd suggest a current sensing resistor in one of the main supply rails, a series R to an optocoupler (else a big spike will just fry that) and using the output of the optocoupler output to drive the chip shutdown pin, for probably the simplest way of doing that. Yes, it's annoying to have to power cycle to reset the thing -- but it's more annoying to have to change out parts. On my fusors, it takes awhile of "conditioning" before little sparks and arcs stop happening of the sharp edges of the grid, and the "arc warning" light on my Spellman goes constantly for awhile. I had to disable its shutdown for that or I'd never get through that phase which happens pretty much every time the system has been up to atmosphere, and especially whenever I put in a new grid.

Especially until you get really good gas control (and that will take some doing, as I've found) a fusor can easily get into a kind of avalanche mode, where the more current that goes in, the more gas gets ionized, so it draws more current, and eventually winds up as a near short circuit. A larger value ballast helps there, but you can't make it so large as to completely solve that problem unless you're getting it so big it's going to need a fan and so on -- and when not needed, it wastes a lot of power when it's big valued.

There is a link to a very nifty reactance/resonance slide rule here. I use this all the time, it's a big time-saver when calculating snubbers, resonance problems, an RC lowpass filters, to name a few. It's worth the hassle to make one from the images there.
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Re: Burning FET's and driver troubles

Postby Doug Coulter » Sun Nov 21, 2010 11:19 am

I want to note here that getting insulation for HV above the 50kv range is really hard, and we should probably start a thread for that. But what I want to say here, is that oil has a much higher dielectric constant than air, so putting your stack into it will cause you to need a major retuning of your driver, most likely -- the extra C that causes will resonate your transformer at some lower frequency. There ae other complex issues in this too -- like C coupling out to the world is increased, which can result in some DC showing up in odd places until you "charge" the resulting capacitor -- we ran into this in a very nasty way with some sparkplug wires that used a high D insulation. We had to put braid around them and ground that to make them useful, or you'd still get shocks, dust would fly around and so on.
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Re: Burning FET's and driver troubles

Postby JonathanH13 » Mon Nov 22, 2010 6:51 pm

Thanks for the help and encouragement - I couldn't do it without you and the other suggestions that have been made
(I know because I have been struggling with this for months).

On the snubber


I have checked the resistors I was using and you are spot on - they are wire wound (that just would never had occurred to me, even though they look just like fat little inductors staring me in the face!) :shock:

I just ordered a bunch of non-inductive 5watt thick film resistors - they should do the trick, and I can add or subtract them in units of 22ohms, until I find a workable value.

Your points about streamer formation and insulation for HV above 50KV are a little scary - I think I'm going have to start taking this all a lot more seriously. I used to launch home-made wire-towing rockets into African thunderstorms, to generate targeted strikes. But now I'm just chicken - or maybe I'm slowly getting smarter 8-)

short the probe,


I'm sorry, but you are going to have to spell this out to me: when you mean short the probe, you mean connect the probe tip to the (conductive clip) ground lead? And then connect that junction to the source of the square wave?

Or do you mean drop the 'one turn inductive pickup' loop over a lead that is carrying a square wave?

and hook the scope to some big square wave.


How big? a couple hundred volts?

I have tried to do this using my frequency generator @ 100KHz, but it is only 20VDC and the scope appears to just drop to ground...

I have spent some time incorporating what I have learnt on this thread so far into a new circuit for the driver chip (see image below). I have tried to keep the output tracks wide and have used some 45 degrees to reduce parasitics. I have included space for some zeners, and a little bypass cap right up next to the power rails of the chip. This will all be shielded in an RF box, which will hopefully clean up the waveform somewhat. I will also dig out a 15 Volt transformer and add a regulator to the circuit. Will work out the current sensing resistor and optocoupler once the first version is built.

gates ringing
your extra diodes may be causing a little of this


I can check this easily enough by removing them.

Thanks for the details on the tuning trick - very sneaky. I'm hoping I wont need to resort to something like this, and I'm also hoping that I can get this tuned without touching the transformer windings.

you might want to look over what I'm doing now for gas control


I have replicated your high-speed solenoid bypass arrangement on my diffusion pump, but my needle valve is till way too coarse - could you give me the exact spec of one which works? (I have a whole bunch and they are all rubbish!)
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kv driver.jpg
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Re: Burning FET's and driver troubles

Postby Doug Coulter » Mon Nov 22, 2010 8:03 pm

Not all WW resistors are bad, just nearly all of them are. In you should know that you want to over rate your R's there by at least factor 2, eg use a 10w resistor if it's going to see 5 watts. The ratings are on the optimistic side, and also assume nothing else nearby is making heat. Else they die young, and do bad things to PCBs.

Yes, what I meant is short the probe -- ground clip to the center input, then hook that junction to something really noisy, such as the output of a bridge half and see if you still see no signal.
I'll be real surprised if you don't see that something somehow gets capacitively coupled into the scope innards and shows up, even though the signal is shorted out at the probe. I've seen it many times here, it's the norm.

One good thing to do with real HV is put it into thickwall pipes, and have a continuous grounded screen around that. If something arcs, it can't get you that way. Teflon is real good, but you can't find thickwall pipe of it too easily -- I'm going to just make some out of rod, and perhaps in sections where one piece plugs into another to a good depth of overlap. You could also try to find pipes that telescope one inside the other to kind of create a thick wall pipe. I do that here sometimes. Wire screen mesh used to be easier to find than it is now, but "hardware cloth" or "chicken wire" will work. Window screens are now all plastic, darnit. And yeah -- I'm not particularly easy to frighten, but that 12 foot spark within a couple feet of me did the job fine.

I found it particularly important to avoid anything like inductive coupling and ground loops between the chip ground and the timing set parts ground. In my case I had the fets and those big currents on the same board, you might have less trouble (hope so) with that.

Since you're here, I'll put this here for now, but we should move this to some thread under vacuum tech....keep the power supply stuff clean and single topic.

I am not sure how well the throttling trick works on diff pumps myself -- JohnF said it works a little on one of his setups, so maybe you can find that on the diff pump thread or ask him.
I'd be afraid that in the case of the pump "stalling" that it would backstream oil too much, but I'm not an authority on those, just heard some hearsay. For the one diff pump system we have here, we used a big valve between it and the tank to accomplish that function and let the diff pump do it's best or most of it's best (we had a lamp dimmer in the heater power, but there's a limit to how well you can throttle that down and still have it pump at all). I would suggest a normal gate valve you open for fast pumping, and a tiny valve and long small pipe "around" it for the slow speed...long skinny pipes are a good throttle.

I strongly suspect you didn't get a glow at e-3 or -4 millbar (which is about the same as torr, a torr is 1/760 atm, where a millibar is 1/1000), as I can't get one here with 50kv at those pressures. Paschen's law curve is very steep there.
In fact, I now adjust my fusor pressure by the voltage on a secondary grid while it's in current limit....it's much more accurate than my fancy gage! Or I should say, much higher resolution.
There are some notes on that in the fusor section on run data. Since your tank has some long paths in it, it may go to lower pressure than mine, but at about 8e-3 mbar, all my grids go out at 53kv input.

No needle valve I've run across is really good enough, even the ones I have that are 1/32" needle -- it's just way too big, which is why I came up with the other thing, where I just use the valve as a "switch" -- let a fixed (tiny) volume fill to the regulator pressure, and then let that leak slowly into the tank. If it's not enough, do it again. The slow leak is key so you don't need to have super repeatable timing on the valve open time. I do that by getting small (.005" id) SS capillary tubing and using a couple inches of it. It's 1/16" OD so it solders into small Cu tubing nicely with the right flux (there are special ones that make SS able to be soft soldered). I get all that stuff at McMaster, which I hear doesn't ship to your side of the pond. I do, however, so if there's anything you see at McMaster you want, drop me a line.
Hopefully you can find a local source, it'd be cheaper, though -- shipping is pretty high these days. I have to electropolish the SS cap tubing after cutting it to get the hole open again, it's not hard, but you'll never do it with a file, it smears over the hole again. And of course, you have to keep crud out of it, it clogs easily. Which is why you can't braze the stuff, it oxidizes inside. Epoxy works fine, too, but is easier to crack by accident.

The closest I've come to a usable needle valve is when one of them twisted off just above the O ring stem seal, and I then had the choice of pushing on the needle with the screw, or not, with the needle always full in. Touchy, but barely usable. I am working on a new design here for that, so I'll put up posts when I have something worth posting on it. Speed might be able to substitute for tiny if it's fast enough, ala, automotive fuel injector type design and a short pulsed drive, we shall see -- I'm going to try that one here.

I've found that with these "inexpensive" valves you want to run them over atmospheric so any stem leaks are "out" by the way. That way at least you don't contaminate your gas into the fusor.
And we've found it matters, a lot.
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Re: Burning FET's and driver troubles

Postby JonathanH13 » Wed Nov 24, 2010 3:37 am

I'm going to have to build the new driver circuit before I can do the probe test properly. I included a rectifier and regulator and have finalised the layout:
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Re: Burning FET's and driver troubles

Postby Doug Coulter » Wed Nov 24, 2010 11:05 am

Nice looking layout. Not obvious to me where the gate R's are -- are they here or at the fet end of things? Probably doesn't matter too much, but beware coupling from the high current paths back into the gate via wiring issues. Seems everyone knows about stray C coupling (which shielding fixes easy), but with these and some pretty hefty peak currents during switch time, you can actually make a transformer with two wires running parallel that give you no end of mysterious issues...
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Re: Burning FET's and driver troubles

Postby JonathanH13 » Sat Nov 27, 2010 7:43 pm

Doug,

I am now running the chip at 15v. I have added a transformer, rectifier, regulator, some smoothing caps and a 50ohm 5 watt resistor to the circuit. I'll post the updated schematic when its complete, and add details as to where all the components go and their values. I have included an image of the one of the outputs, measured when the stack was at -25kV. The circuit was very well behaved and the waveshape did not change all the way up to -40kV (the limit of my high voltage probe). The performance of the circuit and the waveshape is much improved :D A+

Looks like the 'mexican wave' on the top of my waveform is at least partly due to some characteristic in my transformer. When I designed it I included an extra feature which has come back to bite me: it has two primary coils, which can be connected in series and driven at 500VAC or connected in parallel and driven with 250VAC. Connected in parallel caused some pretty serious distortion (I could find no frequency at which the little dead-time glitches fell away). This was improved considerably by connecting the primaries in series(compare the images). Not sure if it can be flattened out further?

The probe test: Once I had the circuit running, I shorted out the probe and connected it to the output of the bridge at about 20VDC 40KHz. I do see some spikes, but only when I zoom in to 200mV per division. At 5V per division I see nothing but a flat ground line.

I have the new snubber in place - I used five 22ohm non-inductive resistors to give me 110ohms at 25watts. They do not get warm and do not seem to change the waveform - looks like I need to up that cap value (it is 1000pF at the moment). All these measurements were done with the large schottky diodes in place next to the FETs, I still need to remove them and see how that changes things.

So the driver seems to be well tuned and working perfectly (the FETs don't even get warm, shifting at least 3 amps). However, the next weakest point has made itself apparent - the high voltage feed on the inside of the tank. The high voltage is shorting past the insulating alumina tube and arcing to the nearby mesh screen. I was reading your thread on this the other day, about the quartz and/or pyrex arrangement - looks like I need something similar...
Attachments
25KV.jpg
Between one of the outputs and ground, at -25kV on the stack.
probe test.jpg
Probe test
snubber.jpg
The new snubber
primaries in series.jpg
Xfmer primaries in series
primaries in parallel.jpg
Xfmer primaries in parallel
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