First SSTC build - some questions

[zytra]

Duh me, well double duh.
1. Of course. With a full bridge each IGBT still sees half the bus voltage.
2. You're right, screenshot from a couple days ago shows the correct scaling ratio of 100:1, where as today, for some reason it defaulted out to 10:1. Had to reboot the scope the yesterday, maybe the settings didn't get saved properly. That makes more sense.

edit: Going off the idea of a full bridge and the 2-diode rectification, I drafted another one with full wave rectification. I'll double check tomorrow, but I think I need an extra trace for AC stuff, which will be more of a 4x2 layout. This still uses the doubler. Not 100% positive but it could also make sense to add a couple of MKP capacitors across the Vbus +/-. Even though the 2 large electrolytic capacitors are right there I think these would help, if I can fit them that is.

[davekni]

Each IGBT sees full bus voltage, same for both half and full bridges.  When one IGBT is on (roughly 0V), the other IGBT in that leg sees full bus voltage.

I still wonder about that 1.4MHz ring showing up at higher voltages but not at lower.  I have no idea if it is a problem or not.  Were you getting arcs during that test?

Not quite sure what schematic you are implementing with that 4-diode layout.  It looks like a voltage doubler with added diodes across the caps (which should never conduct, so not be of use).

Yes, some film caps are still a good idea.  Electrolytics tend to have higher impedance at high frequency.  I've recently started adding SMD ceramic caps across the gap between VBus+ and VBus- (630V C0G/NP0 dielectric 10nF, but will buy some 33nF ones shortly).  Their very-low inductance reduces the highest-frequency ringing where even film capacitor lead inductance is significant.

[zytra]

First let me thank you again for your continuous help with all this, I've learned a lot!

Let me clarify that my question regarding Half Vs Full bridge was about my case (using a doubler, with the SSTC2 as base schematics). In this schematics, a half bridge is used. So, when the high side IGBT's gate is open (the low side one is closed) the current flows from the 339V to the center tap of the doubler, which should be 170V or so. Is this incorrect?

By going to a full bridge, although each IGBT would also see the same 170V, the primary coil would actually see the full bus voltage (339V).

It could be that what I call "bus voltage" is not accurate. Or maybe I am still missing something.



I've attached a screenshot, with channel 1 on high side gate, channel 2 on low side gate, ground is on the low side emitter, and channel 3 is the (now corrected in scale) current probe on the primary.



As far for the schematics, I haven't double checked it yet, but it's based on the Quasar60 plans I had bought a while back, which uses a full bridge. Their power supply uses a full wave rectifier and a doubler. I'm attaching a screenshot. Those plans are for a DRSSTC but it doesn't matter I'm only looking at combining the power supply and the full bridge.



Regarding the high frequency ring, I don't know. Could that be relate to the ground clip picking up something? That seems to happen on every rising/falling edge of the gates. The fact it gets through the IGBT (since it's showing up on the current trace), suggests the IGBT is able to open and close that fast? Which is why I was thinking it might just be the ground clip picking up something. But to answer your questions, yes, all the screenshots were taken with the live coil releasing arcs.

[davekni]

The IGBTs see 339V for both half and full bridge.  The primary coil sees +-170V in the half bridge and +-339V in the full bridge.  Consider the half bridge.  There is 339V (Vbus) from low-side emitter to high-side collector.  When one IGBT is on (0V), the other has 339V.  Look at your IGBT voltage waveforms to see that.

The ring may have been related to connecting the scope "ground" to the high-side emitter.  I suggest retesting with the scope ground on the low-side emitter (VBus-), measuring low-side gate and collector voltages.  Another possibility is the changing arc load at higher voltage somehow excites a 1.4MHz resonance somewhere.

Yes, that circuit has two unused diodes in the bridge.  Was likely copied from a schematic that had a jumper between the bridge and caps.  That was common for supplies designed for both 120V and 240V options.  (Less common now that most supplies have PFC input stages.)  If you are always running on 120V into a doubler, there is no use for the other two diodes.

[zytra]

Thank you, that makes sense - I had no problem visualizing what the primary "sees" in half bridge vs. full bridge, and probably assumed the IGBT was seeing the same thing.

I should have mentioned that it would be good to eventually test it on split phase, which I suspect would need the extra 2 diodes. It's not for right now, and I would have to consider the specs of the components. I was actually wondering it (split phase) is not that frequently done, but maybe it's just a matter of transistor specifications.

I have finished another coil, slightly bigger, so I'll give it a shot and see if it changes anything on this ringing.

[zytra]

I have installed the bigger coil (quite a bit higher inductance, at 75 mH versus 47 mH with the original coil) as I really wanted to get an additional reference point.
The primary uses the same wire (12AWG) and 7 turns or so (same number of turns as before).

The system is substantially more power hungry. I am pulling as much (if not more) current as I was at a fraction of the bus voltage. For the first time I can also see the IGBT temperature go up.

Interesting observations
* On the waveform of the current, it takes quite a bit more time for the current to "ramp up"
* I couldn't understand why "nothing" was happening with my usual 1ms pulse
* but it makes as you can see it takes more than 1ms for the current to go up (and create discharges)

Considering the secondary as a lower resonant frequency and that I kept the same 4.7uF would it be possible that I have an unintentional DR?

I doubt it because after plugging numbers into JavaTC the primary is still much lower than that of the secondary.

Attached screenshots show:
* yellow: Low side gate (ground clip on low side emitter)
* purple: Low side collector
* cyan: primary current

[zytra]

I spaced the primary further away from the secondary and added more turns.
Current is a bit more manageable in a sense where I got to around 150V on the bus for current around 80A.

In terms of spark length I don't think it would be any better even if I was able to raise the bus to 330V. I might be able to as the current didn't seem to increase (much past a certain voltage, like before).


edit: by the way, I meant to ask this before and another reason why I wanted to test a second coil. Am I just "lucky" that it seems like switching it taking place at close to 0 current?

[davekni]

Looks like nice progress.  Yes, with arcs forming it is normal for current to not increase linearly with voltage as the larger arc lowers secondary Q.

The long initial delay is because the driver is oscillating at a higher frequency for a long time before locking in to the secondary frequency.  It may be that the feedback input HC14 chip has low hysteresis so that the 10k feedback resistor I'd suggest you add is causing high-frequency oscillation rather than at or below resonance as intended.  You could increase the 10k value, perhaps to 20k.  Or, you could add a small capacitor from HC14 input (pin 1) to ground to lower the initial oscillation frequency, perhaps 100pF or so.  (The long initial high-frequency oscillation may be causing most of your driver and IGBT power dissipation.)

[zytra]

That long delay at the beginning of each pulse (screenshots 38, 39), were resolved by playing with the primary. I tried lowering turns initially on a primary tightly wrapped around the secondary. That didn't help, actually, it made it quite a bit worse. Increasing the number of turns seemed to help although I could "hear" some action taking place somewhere between primary and secondary.

My solution was to space the primary a little bit and add some turns. I stopped there but I'm pretty sure I could add some more turns and still improve. Once that was done, arcs seem a little longer (maybe 13-14" up from 11-12") which. Current seemed a little bit higher but this is probably something to expect with a larger coil.

It sounds like this fits your theory, somehow. Perhaps the higher coupling resulted in a different feedback waveform causing that high frequency ringing to be more manageable?

I would really like to be able to address that high frequency ring, especially if it helps with efficiency.

Not sure if you saw my edit, is the switching taking place close to current zero crossing just pure luck? I guess the value of the DC blocking cap would be one way to play with phase if adjustments were needed?

[davekni]

As uspring pointed out, close to ZCS requires high coupling and high secondary Q.  The relatively-slow current increase after the frequency locks does suggest high secondary Q.  I'm guessing that your zoomed-in scope plots are late in the burst when current is close to maximum.  You may be less close to ZCS early in the burst, which could also be slowing the ramp a bit.

The initial high-frequency oscillation before frequency locks is (probably) unrelated to the 1.4MHz ring in some previous scope plots.  Don't see any of the 1.4MHz now, so am guessing it was caused b scope "ground" connected to half-bridge output.

The initial 1.3ms of high-frequency before current ramps up is dissipating power charging and discharging gates and IGBT output capacitance.  I can't tell what frequency that region is without a zoomed-in capture.  My favorite driver technique is to have it self-oscillate close to resonant frequency without feedback.  PLL drivers do that, but those are relatively uncommon.  A weakly self-oscillating circuit that is overdriven by feedback is simpler, and what the 10k is intended to do.  However, it looks like frequency is too high.  Raising to 20k will lower frequency way down, as it will be based on 10k and the 0.1uF capacitor between CT and diodes.  That may be OK, as one transition may be enough to start oscillation.  To adjust frequency, either add a small capacitor in series with the 1k resistor and change the 10k to 20k (or larger to tweak frequency down), or add a capacitor from HC14-1 to ground (and perhaps lower the 10k resistor value).  Either one should work.  (Or drop the 1k to 470 instead of increasing 10k to 20k.)

Most drivers w/o self-oscillation work fine.  However, the first bridge transition must make enough feedback signal to cause the next transition.  This requires the bridge's initial state to be different than the initial gate waveform drives it, which requires some bleed resistors to define initial state.  (Or luck in where leakage currents leave the initial state.)

[zytra]

I was re-reading the PLL SSTC thread from earlier this month with upspring's comments which is what reminded me to ask about ZCS. Sadly though, the quality factor is not something I can make a lot of sense of yet.

That being said you are correct, the zoomed in traces are taken after the initial burst, when the current maximum waveform was taking place (I was keeping a close eye on those).

I will implement one of your two suggestions tomorrow. But just to clarify the goal would be to address the high frequency ring showing up on all traces twice per period, right? I've highlighted them in the attached screenshot.

Also, I need to space the primary a bit more and add some turns. After looking in more details at the inductance and coupling values in JavaTC it looks like pushing the primary inductance without increasing the coupling (which I tend to keep around 0.30 or so) is what gave me the best results, so I'll try pushing it a bit further and see what's the impact on current and spark length.

[davekni]

My suggested change has nothing to do with those spikes.  They are caused by switching slightly after current zero-crossing.  The IGBT turning off is in diode-conduction.    When the opposing IGBT turns on, it forces the diode off.  The diode's reverse recovery current causes that ring.

Rather, my suggested change is to address the 1.3ms of oscillation before current starts to ramp up.  I can't tell from the zoomed-out scope plots just what frequency that is.  It looks to be well higher than resonant.

[zytra]

Ahhh. Sorry for the confusion!

I no longer have that long (over 1ms) period where the current was very small. This was fixed by spacing out the primary further and adding turns. I can't reproduce that without redoing the primary. I plan on working on the primary further so maybe I'll encounter that scenario again if so I'll make sure to take screenshots.


I did move my setup away from the desk and encountered some difficulties with the IR diodes again. I followed your advice and used a red LED instead. I first tested by holding by hand making sure the phototransistor was sensing the signal. It sure did. I didn't have any 5mm LED so I used a 3mm, and instead of drilling the LED, I drilled a 3mm hole in the body of the IR LED from IF. That works great, and was able to extend the length of the optical connection.

As far as your suggestion, I did add a 10K pot in series with the 10K that was there. I figured I would have a little bit more flexibility.

I will do two tests with the pot @ 0 (i.e. 10K total) and another one with the pot @ 10K (20K total), and scope the first few microsecond of a pulse.



edit: tests done
screenshots 52, 53, 54 are with 20K across pins 1 and 2 of the SN7414 inverter
screenshots 55, 56, 57 are with only 10K

[zytra]

Something rather strange happened after the previous set of tests, i.e. shortly I moved the coil away from the desk.

Since the beginning being short on differential probes, I had both the variac and the small 110-12VAC transformer on an isolation transformer. Although I didn't change anything to that part of the setup, shortly after finishing the tests reported in the previous post,  I blew a rectification diode (low side, which is where the oscilloscope ground clip was connected) in the middle of a test (70A or so on the primary). I replaced the diode and started investigating, as it turns out, the oscilloscope was the problem. The only thing that had changed was the outlet I was using for the isolation transformer. I tried putting the oscilloscope together with the rest of the stuff on the isolation transformer (I had never tried that), it didn't help. I tried putting the oscilloscope on another isolation transformer and same result.

Anyway, I removed the passive probes and used the one differential probe for the gate, I have another one the way.

screenshots 52 through 55, are the new larger primary (15mm of gap between it and the secondary) and 18 turns.
screenshots 56 through 59, only 16 turns.

The latter gave better sparks overall, and it starts "Cracking" at a lower voltage. Current draw is roughly the same (although the second set of screenshots have 2 turns less, the actual length of the wire is same as I didn't cut it, the extra 2 turns were left loose for now).

I'm not clear as to why the current trace more ringing than it did earlier today.


I'll keep playing the primary turns some more, I'm not sure I can do any better than the primary I had yesterday.

[davekni]

Sounds like either your isolation transformer has an internal fault or there's some connection to line ground in the bridge circuitry that I'm not aware of.  Normally running the bridge VBus supply from an isolation transformer allows grounding VBus- with a scope probe.  Wherever the issue is (isolation transformer or other ground path), that may explain some previous anomalies too.

The low-frequency current ring is likely at the resonant frequency of the 4.7uF cap and the primary inductance.  Adding a bleed resistor across the 4.7uF cap should help.  (The unwanted initial 1.3ms of high-frequency oscillation may have centered half-bridge voltage, so avoided that low-frequency ring.  Adding a bleed resistor across the 4.7uF cap is a better solution.  Although I do wonder what would cause that cap to charge between bursts.  Scoping the half-bridge output will show if it is going to one rail or the other after a burst.  Ideally it would remain at half of VBus.  That's what the bleed resistor should force, presuming it's only small leakage current pulling the output high or low.  It is also possible that one IGBT is partially damaged, causing high leakage current.  Perhaps the scope ground issue damaged one IGBT a little.

Since you happen to be switching slightly after zero-current point, fewer primary turns may move that switching earlier.  I think after-zero-current is unusual for SSTCs, but don't have much personal experience.  My SSTC is unusual (ferrite transformer instead of primary winding), so not useful as a reference point.

[zytra]

It's the same isolation transformer I have been using, so it should rule out it being defective. Though, as I read your message, I may accidentally inverted phase and neutral when I reinstalled the driver after adding the 10K pot this morning. The following two tests were low power where I wanted to see the impact of the added 10K. I then blew the diode (and the variac fuse as an immediate consequence, forgot to mentioned) when I raised the voltage as I was getting ready to play with primary turns.

I should be paying more attention to Phase/Neutral in general, and I think this is the perfect example as to why I should.
Edit: follow up question though, in the event of rectified split-phase (through a 220V isolation transformer), would you be able to use a scope to probe Vbus +/-?



Before I comment on the low-frequency ring, I meant to ask a general/safety question in regards to the testing environment. I have a steel cart, which I thought would be practical to put the coil on top, driver/bridge on the mid shelf, and oscilloscope on the bottom shelf. I would wire the steel cart to my RF ground of course. Is that a good idea? My idea behind this was that it would protect everything from possible discharges, although with the current spark length I am dealing with it's rather unlikely.



I am still not clear what you call low-frequency ring. Are you talking about the waves on the first hundred microseconds of each pulse (on those last screenshots, I was doing 10Hz, 1% duty, or 1ms pulses)? With that 6" primary and 15-20 turns, the primary resonance is between 9 and 12kHz. That seems to match the roughly 100 us period of those waves.

[davekni]

If the isolation transformer is working properly, the output is floating.  There is no difference between phase and neutral, so it makes no difference which way it is wired.  To me, seeing a difference with phase/neutral swapping is more evidence of a faulty isolation transformer.

It should make little difference in circuit operation even without an isolation transformer as long as no scope (or other) grounds are connected to the bridge circuitry.  It is convenient to have neutral at the doubler center-tap.  Neutral is roughly at ground potential.  Don't connect scope ground to neutral (unless isolated), but you can still scope bridge circuitry without connecting the ground clip.  With neutral on the doubler center-tap, you can connect the scope ground through a ~0.1uF capacitor to neutral for better high-frequency probing.

The cart has advantages and disadvantages.  The cart layers reduce possible magnetic or capacitive coupling between circuits.  If connected to line safety ground, it reduces issues of shock from touching the cart and some other grounded equipment.  It protects from discharges as you mentioned.  (Most DRSSTC designs have a not-quite-closed ground loop outside the primary as a strike rail.  Most SSTCs don't include that as arcs usually aren't long enough to pose a risk.)

The primary and secondary coils need to be spaced above the cart to allow a path for magnetic field at the coil bottoms.

However, when debugging live circuits, a large grounded structure (cart) can increase shock risk.  If you accidentally touch a live node such as VBus+ etc. while otherwise insulated (shoes on sitting on and sitting on a plastic chair), there's no shock.  If you were also grabbing the cart to steady it and/or you, then the cart completes the shock circuit.

Yes, it is exactly that 9-12kHz ring I'm referring to.  At 10Hz, it would require 4.7 mA to charge the 4.7uF cap to 100V between bursts.  That is high for leakage current.  It may be worth measuring half-bridge output voltage with the driver off (no gate signals).  If at VBus+, try pulling it down with a power resistor, say 10k-100k, to see if it goes close to 0V.  If at VBus-, try pulling it up with a power resistor.  Without gate drive, it should easily go to whichever rail you are pulling it.  If not, then one of the IGBTs or TVS diodes (if you have them) must be faulty.  (Or pull it with a resistor to doubler center-tap to measure what current is being drawn high or low.)

[zytra]

Thanks, I'll stick a wood top for now

I remember I failed to tell you that I forgot to add the bulk capacitor bleed resistors I originally had on the bridge when I rebuilt it a couple days ago. It's been a bit of pain (I hate keeping capacitors charged) to discharge them manually during all these tests today. I'll add them back tomorrow, but I doubt they have anything to do with that low frequency ring, I already had that with the old bridge. I'll definitely add a bleed resistor on the DC blocking cap as well, what value/power should I use for this one?

[davekni]

As you suspect, bulk-cap bleed resistors will have no effect on operational performance.

I was going to suggest a resistor value for across the 4.7uF cap, but realized that there must be excess (at least a few mA) leakage current to build up significant voltage between bursts.  It would take a lower value higher power resistor to overcome high leakage current.  And I don't know how high the leakage is.  So it is likely better to first figure out where leakage current is coming from.

On the other hand, one easy way to measure leakage current is to add a resistor across the 4.7uF cap and then measure voltage while not driven.  If leakage current is low, a 10k resistor would dissipate little power.  If leakage current is high, the resistor could see half bus voltage (170V) for ~3W power dissipation.  If you have a 3+W 10k resistor, that would work well.  Or, use a lower-power one and test quickly before it gets too hot.

The resistor value isn't critical.  100k would work fine if leakage current is low.  Lower than 10k is fine too.  Power needs to be high only to handle faults or high (faulty) leakage currents.

[zytra]

I have a 10W 20k that could work.
Would the (bleed) resistor alter the capacitor function of DC blocking?

For that measurement, the differential probe across the capacitor is all that is needed? One trace while the coil is in operation and some more shortly after? Or did you mean to simply use a voltmeter across? It's a rather small capacitor so it shouldn't take too long to discharge it. The much larger (almost a 1000 times) bulk capacitor take a minute or two to discharge with that same 20k resistor.