Author Topic: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers  (Read 1617 times)

Offline AFreshLad

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Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
« Reply #20 on: September 19, 2021, 03:16:40 PM »
Quote
To me it logically makes sense that it would still work with biasing, and I confirmed that using a circuit simulator. The only thing I noticed was that the phase shift varied quite a bit depending on if the feedback frequency proportional voltage for the VCO was closer or further from the biasing voltage, as that would determine how much the biasing voltage would effect it the circuit, but still it works.
Ideal version would have phase comparator 2 output be + and - current sources rather than switches.  Then a DC current source would provide linear phase adjustment.  I did use a current source for biasing, even though 4046 phase comparator 2 output is not current source.

I've been giving it some though and I don't really understand why current sources would be ideal rather than voltage sources. I understand that the biasing works of principles of voltage dividers but don't know how a current version of that would be put together, especially when feeding the signal into the VCO which is voltage controlled. Could you send an example of a circuit that uses current sources so I could try to understand it. Thanks!

Offline davekni

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Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
« Reply #21 on: September 19, 2021, 08:19:01 PM »
Last time I used a 4046 was ~4 decades ago, so I don't have any circuits around to share.

Concerning why current is better for phase comparator 2:  Consider the nominal case when VCO is center-frequency (VCO input pin 9 at center between supply voltages).  Look at figure 5 of the Philips HEF4046B datasheet.  Phase comparator 2 output is mostly open (zero current).  At VCO output edges, it momentarily pulses high or low, causing a current of +-VDD/2/R3 to slightly charge or discharge low-pass filter capacitor C2.  The charge added to or subtracted from C2 is proportional to phase error (fixed current of +-VDD/2/R3 times varying pulse width based on phase error).

Instead of centered frequency, consider C2 at VDD/4.  Now phase comparator 2 output current is +3VDD/4/R3 or -VDD/4/R3.  PLL loop gain is now asymmetric, higher for increasing frequency and lower for decreasing frequency.  Switched current sources would fix this asymmetry, making PLL loop gain constant.
David Knierim

Offline AFreshLad

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Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
« Reply #22 on: September 20, 2021, 04:28:03 AM »
I think I get what your saying, but when you say current sources, do you mean constant current sources, because otherwise, it doesn't make sense to me. But assuming you do mean constant current sources, I'm guessing then the biasing would also work as a constant phase shift that wont change across the 2 F lock range as it would bias the capacitor the same amount across the whole range. But if I were to hypothetically build a current based system, wouldn't I need power rails beyond the logic levels the system works at, as otherwise, I'm guessing the current will taper off at limits of the logic levels (unless a theoretical system with 0 resistance is used somehow). Also, just out of curiosity, this phase shift that is created, is it a constant delay, like a constant time period that the output wave is shifted from the input, or is it a constant phase angle shift? My guess is that its an angle shift as adding a current bias to the comparator output means the VCO will have to compensate that by creating a phase shift. As phase comparator 2 when dealing with a phase shift and same frequency outputs only either positive or negative current sources, in this case the opposite polarity of the current biasing to cancel it out, it will create the perfect phase shift that creates an average current that cancels the bias current. But this same delay between the two comparator input signals, if replicated for a higher frequency means the current output will occur more frequently, and thus increase the average current above that of the biasing current. Thus to cancel out the effect of the bias, it would have to reduce the length of the current spikes/ reduce duty cycle, hence decreasing the delay between the two waves, and vise versa for lower frequencies. This to me isn't definitive proof of it shifting the phase angle a set amount, but does point more towards a phase angle shift rather than delay, as from my understanding the delay changes with frequency.

Offline AFreshLad

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Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
« Reply #23 on: September 20, 2021, 04:59:17 AM »
I have also been thinking about the XOR phase comparator in terms of a 'current' mode rather than voltage, and am I correct in saying that the only time a current mode will be useful is when biasing, as with voltage biasing it seems the phase shift varies faster from 0  - 180 degrees that without biasing, (due to asymmetric amounts of current from the biasing system at different frequencies) limiting the F lock range of the XOR gate below the VCO range, whilst a constant current biasing system would just introduce a constant bias that would just slightly clip the F lock range as the 0 - 180 phase shift range of the XOR gate has been slightly shifted, with a part of it outside the VCO range. But if no biasing was used, my guess is a current mode on an XOR comparator is useless, as, from what I know, an XOR comparator doesn't really have a gain, but rather, when two different frequencies are input, just outputs a sort of PWM signal where the duty cycle varies sort of sinusoidally, (which gets filtered into a sort of sine wave by the LC filter) and if this sine wave manages to clip the VCO voltage proportional to the frequency of the input signal, will lock onto it, no gain or anything like that. This concept was also how I was able to derive the equation of the F capture range of a type 1 PLL circuit.

Offline davekni

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Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
« Reply #24 on: September 20, 2021, 05:31:57 AM »
Yes, your reply #22 looks accurate.  A constant bias current would produce a constant phase offset.  This is true IF phase comparator 2 put out currents.  With a real 4046, phase comparator 2 currents depend on R3 and on VCO voltage, so phase shift will also depend on VCO voltage.

For me, it is easier to think of phase comparator 1 as putting out a voltage rather than current.  Unlike phase comparator 2, phase comparator 1 is never high-impedance.  Think of it as generating an average voltage using PWM.  Range is 0 to VDD for 0 to 180 degrees phase input.

Yes, a current source that handles 0 to VDD requires supply voltages beyond 0 and VDD.  You can get close with rail-to-rail opamps without additional supply voltages.  Look for app-notes and opamp datasheets with current-source examples.
David Knierim

Offline AFreshLad

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Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
« Reply #25 on: September 20, 2021, 06:03:57 AM »
Thanks for the input
I understand that a type 1 comparator never has a high impedance mode, but wouldn't the concept of a constant current source still apply, where the comparator will just output + and - current signals instead of positive and negative voltages, which are biased by a constant current. Further, even after the low pass filter, doesn't the whole concept of an XOR comparator depend on the voltage output having some oscillation. The F capture range of an XOR comparator on the datasheet is determined by the RC filter, where a 'stronger' RC filter with a lower cutoff frequency means the capture range is smaller, and thus if it were giving out a perfect, flat voltage, wouldn't it fail to lock onto a frequency from a state of not locked.

Offline AFreshLad

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Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
« Reply #26 on: September 20, 2021, 10:39:59 AM »
Hi
I have just finished attaching and testing the PLL circuit with a Schmitt trigger and it works perfectly. It was not at all finicky and the phase comparator 2 works perfectly. It keeps a lock on the signal through any situation, ground arcs etc, and even when shorting the secondary to ground recovers instantly after removing the rod. Further, by fine-tuning the phase shift using a circuit similar to Steve Wards linked here:
https://www.stevehv.4hv.org/CWcoil/PLLSchem.JPG
I was able to get 0 current switching to happen on my half-bridge, meaning I can finally attach the interrupt circuit without creating huge voltage spikes on my bridge. The 4046 IC was even run on a breadboard, with long looping wires connecting things at it still ran just fine (although the coil was running at very low power so less interference). The only problem I can see with phase comparator 2 is that I have set F min at ~200KHz whilst my coil resonates at ~296KHz which means the coil won't start by it self, instead I have to short the secondary and draw out an arc to start the coil. This problem might be solved at higher powers, but either way, it could also become a problem during interrupts if the secondary oscillations die out during the turn off period and doesn't restart oscillations again. This could also be solved by increasing F min.

Offline davekni

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Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
« Reply #27 on: September 21, 2021, 06:04:52 AM »
Quote
Further, by fine-tuning the phase shift using a circuit similar to Steve Wards linked here:
https://www.stevehv.4hv.org/CWcoil/PLLSchem.JPG
Yes, feedback delay is another way of adjusting phase lead.  Steve also has a switch to hold VCO frequency constant during interruption.

Was thinking more about phase comparitor 2 linearity.  Both phase comparitor 2 and phase adjustment can be made linear without using current sources, by using an opamp integrator instead of a plain R and C filter:



This keeps the integrator input voltage at VDD/2, so R3 and phase adjust R6 feed into a constant voltage.  Makes resistor currents constant without needing current sources.

Note that the opamp integrator is inverting.  To correct for this, the two phase comparitor input signals are swapped in the above schematic.

Of course, since you have something working now, no need to change anything.

Congratulations on success!
David Knierim

Offline AFreshLad

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Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
« Reply #28 on: September 22, 2021, 05:26:40 AM »
Congratulations on success!
Thanks!
I am not very familiar with Op amps and integrator circuits, but from the little I know, this looks similar to an inverting amplifier, but with a capacitor C2 rather than a resistor. If it were a resistor, I know the output will be just an amplified and inverted signal from the input, with the input acting as a virtual ground, in this case getting fixed at VDD/2. So, I guess it sort of makes sense that putting a capacitor there, which would only let through changes in voltage, aka dV/ dt, rather than the actual voltage means that to keep the virtual ground at VDD/2, the output must become the integral of Voltage over time. Is this correct? Also seeing that the capacitor differentiates, I'm guessing any information about DC offset is lost, so the output integral has an offset / 'C' value of 0. And this is where I think this actually is useful for adding linear phase shift, as by adding the phase adjust pot on the output of the capacitor which has removed DC offset, the effect of the offset pot is constant, as the same current will 'leak' into the pot at any frequency for the VCO which creates a constant shift on the output. I'm guessing the actual integration function of the circuit is useless.

And yes, the constant time period shift created using Steve Ward's circuit works just fine, so I won't be adding any of this Op amp stuff, I'm just curious as to how it works. Also, I didn't require the circuit that keeps the coil at resonance during interrupts, as when interrupted it has no problem 'pinging' the secondary to create enough feedback to start resonance. Further, in my brief look at the secondary feedback during interruptions, it doesn't really die down enough that the signal is lost between interrupts, so I don't think that safeguard was necessary for my coil at least.

Thanks a lot for all the help so far, I think now I'm going to try to put all this stuff on an actual circuit board rather than a bread board, try to reduce the parasitic inductances and finally, move to a full bridge design.

Offline davekni

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Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
« Reply #29 on: September 22, 2021, 07:07:48 AM »
Quote
aka dV/ dt,
I think you are confusing differentiation (dV/dt) with integration, which is the opposite.

Quote
means that to keep the virtual ground at VDD/2, the output must become the integral of Voltage over time.
Yes, this part is correct.

Quote
I'm guessing the actual integration function of the circuit is useless.
No, integration is the goal.  Current into a capacitor is also integrated.  Integrators are a type of low-pass filter with theoretically infinite DC gain and constant 90 degree phase shift.

For normal PLL use, a pure integrator would form an unstable loop.  It's 90 degree phase shift combined with 90 degrees of the VCO (phase is the integral of frequency) makes for 180 degrees total (0 degrees phase margin).  However, for Tesla coil use, a PLL is locking to a phase-shifted version of its own VCO output, not to an independent reference frequency.  That bypasses the 90 degrees of frequency-to-phase.
David Knierim

Offline AFreshLad

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Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
« Reply #30 on: September 22, 2021, 10:41:20 AM »
Doesn't a capacitor act as a differentiator. It only lets through changes in voltage, and the current through it is proportional to the rate of change of voltage, right? That's what I meant by the capacitor acting as a dV/dt differentiator of the output of the op amp, which if fed into the input like it is, means in order to keep the input at VDD/2 (virtual ground), the output of the opamp must be the integral of the signal, such that after the capacitor 'differentiates' this integral output, the original signal is present, hence keeping the input at VDD/2.

Offline davekni

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Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
« Reply #31 on: September 23, 2021, 05:48:40 AM »
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Doesn't a capacitor act as a differentiator.
A capacitor by itself has two equivalent properties.  Current is the derivative of voltage with time (times capacitance value), and voltage is the integral of current over time (divided by capacitance value).  Both are equally true, different ways to state the same fact.  This opamp circuit is an integrator.  Input voltage is converted to current by R3.  R3 current feeds C2.  C2 voltage (opamp output voltage) is the circuit output, the negative time integral of input voltage.  (This presumes input and output voltages are measured relative to VDD/2.)  If R3 and C2 were swapped, the circuit would become a differentiator (except fur opamp stability issues).
David Knierim

Offline AFreshLad

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Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
« Reply #32 on: September 23, 2021, 11:12:25 AM »
Thanks for the answers, but I'm still not fully getting it. I think I'll wait till I hopefully study this in uni before I burn myself out trying to understand it without basic knowledge.

Offline AFreshLad

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Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
« Reply #33 on: September 26, 2021, 10:32:14 AM »
Hi, I'm back with a new problem and I don't think this warrants a new thread so I'll put it here.
I have rebuilt my whole bridge and driver circuit as the old version was quite bad in terms of parasitic inductances etc. But now, with the new bridge, it runs perfectly, and I do get zero current switching, with no voltage spikes on the turn off of an interrupt cycle. But on the turn-on, there are huge voltage spikes. I probed the voltage across the primary and the feedback signal from the secondary and, while the coil is running, see a 90 ish degree phase shift between the feedback and the primary voltage, which makes sense. But during the off-time of the interrupt, the primary voltage induced by the coupling between the secondary and primary has a 180 ish degree phase shift from the feedback signal, changing instantly from the original 90-degree shift the moment the coil turns off. I'm not exactly sure why this occurs, but my guess is that it has something to do with the series capacitors with the primary that are present in steve ward's coil design (I don't have them) linked here:
https://www.stevehv.4hv.org/CWcoil/PLLSchem.JPG
These capacitors look like they have a similar job to the capacitor between the current transformer and the clipping diodes. I don't exactly know what exactly it does though, my guess is that it provides a consistent ish load for the CT or in this case for the primary coil rather than just loading it when the voltage reaches the rails where the clipping diodes/ body diodes of the mosfets start conducting. Also, due to the large currents that flow through the primary and high duty cycles, I'm guessing I'll need a very high-performance capacitor with low ESR, and I don't have any such capacitors on hand. Also, concerning these caps, what voltage should they be rated at, do they need to be rated at the voltage the bridge runs on or lower since I'm pretty sure that voltage will never fully fall across the capacitor.

Offline AFreshLad

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Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
« Reply #34 on: September 26, 2021, 02:26:25 PM »
I did some thinking and some research and my previous explanation doesn't really seem right. I read that adding those series capacitors cancels out the inductance of the bridge maybe? I'm now quite lost on what these capacitors do and if they will fix my problem at all, but as far as I can see this is the only fundamental difference between my design and Steve Ward's design, which implements interrupts. Here is a scope capture of what I was talking about in the previous post about voltage spikes.

Pink is the voltage across the drain and source of one of the Gate MOSFETs (I'm running a half-bridge)
Yellow is the gate to source voltage on the same MOSFET
Blue is the raw output from the CT on the secondary before any delays and filtering stuff is done.

You can see how after the yellow gate voltage shuts off, the voltage on the MOSFET gate to drain still oscillates due to, I'm guessing, it being back driven by the secondary that's oscillating by itself. But these oscillations are now suddenly shifted from where they were before with respect to the blue feedback from the secondary, which is why I think the voltage spikes at turn on are created as now the MOSFET switches at the wrong time and I'm guessing causing some sort of inductive discharge, but I'm sort of lost on this.
Any help will be appreciated, Thanks!

Offline davekni

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Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
« Reply #35 on: September 26, 2021, 07:51:40 PM »
Scope traces look roughly as would be expected.  I think most SSTCs are interrupted with longer off-times, so that ring-down is finished before the next enable edge.

For better understanding, I suggest learning an analog simulator (SPICE variant).  There are several free ones.  I use LTSpice.  There are simpler on-line simulators too.  No need to simulate your entire circuit.  Start with just a pulsed voltage source into a series L/C circuit.
David Knierim

Offline AFreshLad

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Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
« Reply #36 on: September 27, 2021, 05:26:36 AM »
Oh, that makes sense as I'm creating my interrupts with an arduino and the off-time is quite short. But I still don't understand why the phase of the voltage on the primary shifts so suddenly from the feedback when the gate drive signal is off. Is there any way to keep the on times high and the off times low as I would like the maximum output from this during music playback (as it is a SSTC).
Thanks for the help!

Offline AFreshLad

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Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
« Reply #37 on: September 28, 2021, 10:03:43 AM »
I've ran a simulation on my tesla coil circuit and I haven't seen any phase shift like present on the scope capture. I'm completely lost on what's causing it and my best guess is it has something to do with a smaller coil in the middle driving a larger coil one the outside (secondary driving primary) being different somehow to the large coil driving the smaller coil in the middle (primary driving secondary). Do you have any idea what's going on?
Thanks

Offline davekni

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Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
« Reply #38 on: September 29, 2021, 05:28:01 AM »
Please share your simulation schematic and result plots, and for reference which simulator you are using.  Then it will be easier to help.
David Knierim

Offline AFreshLad

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Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
« Reply #39 on: September 29, 2021, 06:36:30 AM »
I'm not too familiar with simulations so I just used a web-based one linked here:
https://www.falstad.com/circuit/
When I first used it, and fixed the primary gate drive signal at close to resonance (I simulated the MOSFETs as well), I got the primary and secondary to be in phase, and when the signal was disconnected it remained in phase, which is where I was stuck. But now, while tinkering with it more, I somehow got the primary to drive the secondary whilst being 180 degrees out of phase, and when the primary gate drive was turned off, cause the primary to flip 180 degrees when back driven by the secondary, just as in real life, but I have no idea why. I'm going to experiment more with the simulation to try to figure it out. I don't know what simulation schematics and result plots are, I only have a screenshot of where it did flip 180 degrees.
Thanks!

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Re: Help with SSTC feedback circuits with 4046 IC and Schmitt Triggers
« Reply #39 on: September 29, 2021, 06:36:30 AM »

 


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post Re: Starting on first coil, Medium DRSSTC
[Dual Resonant Solid State Tesla coils (DRSSTC)]
AeraCura_
October 20, 2021, 05:23:00 PM
post Re: Starting on first coil, Medium DRSSTC
[Dual Resonant Solid State Tesla coils (DRSSTC)]
Mads Barnkob
October 20, 2021, 01:22:22 PM
post Re: Battery powered DRSSTC
[Dual Resonant Solid State Tesla coils (DRSSTC)]
Mads Barnkob
October 20, 2021, 01:14:06 PM
post Re: QCW problem with ZVS and phase lead
[Dual Resonant Solid State Tesla coils (DRSSTC)]
Vtroxi
October 20, 2021, 09:18:37 AM
post Re: QCW problem with ZVS and phase lead
[Dual Resonant Solid State Tesla coils (DRSSTC)]
davekni
October 20, 2021, 06:48:43 AM
post Coin shrinking video
[Capacitor Banks]
MRMILSTAR
October 20, 2021, 06:01:40 AM
post Re: Where to buy DIY or home made capacitor book(s)?
[Capacitor Banks]
davekni
October 20, 2021, 05:45:27 AM
post Re: Where to buy DIY or home make capacitor book(s)?
[Capacitor Banks]
HVuser
October 20, 2021, 03:58:29 AM
post Re: CW Multiplier Killing ZVS Driver
[Voltage Multipliers]
Andrew321
October 20, 2021, 02:40:34 AM

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