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Weird intermittent problem with a DRSSTC driver (edit4: fixed?)

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Goodchild

Thu Aug 29 2013, 06:23PM

Registered Member #2292 Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795

Well the UD2 works on a slightly different set of parameters, so it doesn't surprise me that you can just drop in a UD1.3 and have it work. What works for the UD1.3 won't necessarily work for the UD2. The UD2 is a bit for finicky about its feedback input and proper tuning than the UD1.3 is. Simple things like this can affect the system drastically.

All Iâ€™m saying is that you may want to examine the rest of the system in more detail before you pin the problem on the controller. I'm still very skeptical of you feedback, it doesn't seem like you have checked this in much detail. I would really put your scope in two places, across the phase lead network and on the input of the comparator and look at what you are feeding into the system.

As a wise man once said "crap in = crap out"

Kizmo

Thu Aug 29 2013, 06:36PM

Registered Member #599 Joined: Thu Mar 22 2007, 07:40PM
Location: Northern Finland, Rovaniemi
Posts: 624

I would have scoped that long time ago but everything around the feedback comparator is eeextremely sensitive to scoping, even my differential probe is enough to throw it from kind of working to not working at all.

And funny thing is, Mads Barnkob has exactly same problem.. :l

Mads Barnkob

Thu Aug 29 2013, 07:01PM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

Here is a scope shot of IN- on the comparator, I am a little vary about that the ringing being a problem as the voltage phase shift only happens on the the peak primary current. But the ringing also seems to seize after the primary current starts to ring down. Perhaps it is just interference from the primary?

Steve Conner

Thu Aug 29 2013, 09:30PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Goodchild wrote ...
As a wise man once said "crap in = crap out"

A Tesla coil is an extremely noisy system, so there is always going to be "crap in". Switching spikes from the bridge are bad enough, never mind the high frequency hash from ground strikes.

The challenge is to clean this noisy signal up and produce a nice drive waveform for the bridge even while the system is basically being struck by lightning 200 times per second.

It strikes me that differentiating it with a series inductor maybe isn't the best way to start, as it could amplify the high-frequency noise considerably.

Mads did a test that disproved my flip-flop hypothesis, so here's another one that might fit the new evidence better: Current spikes from the CT get exaggerated by the phase lead inductor and drive the comparator outside of its common-mode range, causing it to latch up.

Another thought: What happens when you remove the phase lead network from the UD2.x (and maybe fit it to the UD1.x?)

Kizmo

Fri Aug 30 2013, 02:24AM

Registered Member #599 Joined: Thu Mar 22 2007, 07:40PM
Location: Northern Finland, Rovaniemi
Posts: 624

i dont have phase lead inductor in place.

Mads Barnkob

Fri Aug 30 2013, 06:02AM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

Steve Conner wrote ...

Goodchild wrote ...
As a wise man once said "crap in = crap out"

I thought about that, because the elevated signal from the feedback CT is approximately up to 2,5V, input range for the comparator is -0,2V to 3,5V, so the spikes in the above scope shot at 3,68V might be it. But the problem also exists at very low duty cycles, where the noise is not yet passing the 3,5V mark.

I tried removing the inductor in the phase lead network and also tried with another type of inductor, it did not change anything.

Yellow: CT input, Blue: Comparator input.

Yellow: Comparator output Q, Blue: Comparator input

Yellow: Comparator output !Q, Blue: Comparator input

Yellow: Comparator output Q, Blue flip-flop Q

Yellow: Pin3 AND gate out, Blue: Pin6 AND gate out (phasing)

Yellow: Pin3 AND gate tout, Blue: Inverter voltage

Yellow: IC9 MOSFET driver, OUT A, Blue OUT B

Yellow, IC9 driven GDT primary, Blue: IC8 driven GDT primary

That last very short peak seen on the GDT primaries, I could only track it back to the AND gate outputs for the phasing, this could be what makes the double inverter voltage?

EDIT 1539CET:
No effect: I tried adding ferrite beads to the supply rails and a 10uF tantalum in parallel with the 0,1uF for the 74HC08 (AND gate)

No effect: Change 74HC08 for another manufacturer, checked soldering and path fot pin6, cleaned out between traces.

Kizmo

Fri Aug 30 2013, 12:41PM

Registered Member #599 Joined: Thu Mar 22 2007, 07:40PM
Location: Northern Finland, Rovaniemi
Posts: 624

Because scope shots are nice.

Red: feedback CT input
Yellow: feedback comparator input

These were captured with conventional probes and full 100MHz bw.

Here its kind of working. It sounds like drsstc but burst beginning is still messed up.

Same up close.

And here its glitching heavily. From constant bzzzz to brpbrpbprbprpbprpbrbrbrr.

And closer

At the same time my burst starts look like this:

yellow trace: H-bridge output measured with high voltage differential probe
red trace: primary current measured with pearson current transformer

Steve Conner

Fri Aug 30 2013, 09:14PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Mads, the big series of scope shots you just posted, are these with the circuit operating properly, or is it glitching? They all look fine... :|

Mads Barnkob

Fri Aug 30 2013, 09:51PM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

Steve Conner wrote ...

Mads, the big series of scope shots you just posted, are these with the circuit operating properly, or is it glitching? They all look fine... :|

This is with the circuit working without jittering, but still with the weird inverter voltage phase shift. If I had turned the duty cycle pot meter just a little it would jitter all over the screen and give the brpbrpbrpbrpbrpbrp sound

Steve Conner

Sat Aug 31 2013, 11:42AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Well, it would be easier to debug the problem if the scope results actually showed it.

What we are trying to establish is a chain of causation. We have the symptoms, we are trying to figure out the root cause. The problem is that the whole system is one big feedback loop, so the chain of causation could be circular.

I would be interested to see what happens when the feedback input is driven from a signal generator as opposed to the CT. This breaks the feedback loop so you can rule out any circular arguments.

I would also investigate the possibility of the switching spikes from the bridge getting back into the comparator. They can be pretty violent especially near the beginning of the burst when there's not enough current to achieve smooth commutation. And, now the comparator has hysteresis, a switching spike could permanently flip it instead of just temporarily glitching the output. (Did the UD1.x have hysteresis?)

The amount of switching spikes present in your CT signal is a system level issue, mainly down to your shielding and grounding schemes. But the driver's sensitivity to them is another matter. My PLL driver uses the type-1 phase detector which works on the average value, so it ignores the spikes completely.

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