Flyback speaker - Help adjusting scope waves and gate voltage

[TiagoBS]

Hello everyone!

After some time I came back with another project copied from the internet. As I said before, I have no background in electronics, I'm learning on my own and with the things I find here on this forum!

This time I copied the Flyback Speaker circuit from the Plasma Channel!

Original video Link:

/>
Well, just like in this video, I was successful and I can change the frequency and use a Bluetooth board to listen to music with plasma!

I made some changes to the original circuit and I have some doubts, mainly regarding the Gate voltage and also about the Scope Waves.

Changes: I added a TVS between DS and a pair of Zeners between GS. Also, between pin 5 of the NE555 and Ground a 10nF capacitor.
Another change was the use of an LM386 board to amplify the Bluetooth output and power pin 5 of the NE555.

- Main board, the LM386 amplifier and the Bluetooth board (with the small battery)


-All of this will be inside this small box mounted on top of a heat sink.

-


As you can see in the image, when I probe the Gate without the MOSFET connected, I have a perfect square wave, however, when I connect the MOSFET the wave becomes very triangular. In addition, the voltage seems to be very low, close to 8v.

- Without the MOSFET


- With the MOSFET


I tried using a push and pull configuration to try to increase the voltage at the Gate, but despite the circuit oscillating, the voltage remained the same.

My question is: With this low voltage at the Gate, would I be running the risk of not reaching the MOSFET saturation region and possibly generating more heat?

And how can I get around this?

I am currently regulating the voltage with a 7812, perhaps I could switch to a 7815?

[klugesmith]

Can you share a schematic so we don't need to sit through that Plasma Channel video?

What's the scope view if you probe the 555 power and ground pins (both right at the device) when MOSFET is connected?

[TiagoBS]

Can you share a schematic so we don't need to sit through that Plasma Channel video?

What's the scope view if you probe the 555 power and ground pins (both right at the device) when MOSFET is connected?

Here is the circuit  schematic before and after the changes I made and also the scope wave of pin 8 (Orange) and pin 6 (Blue) [Were you referring to these pins?]

-


-


-

[davekni]

As you can see in the image, when I probe the Gate without the MOSFET connected, I have a perfect square wave, however, when I connect the MOSFET the wave becomes very triangular. In addition, the voltage seems to be very low, close to 8v.

Scope trace shows Vgs reaching about 12V on each cycle just before rapid drop.  Vgs waveform likely looked worse before you added diode across 68ohm gate resistor.  Likely fine as is.  FET drain current is reverse-polarity at end of flyback pulse.  No need for FET to turn on rapidly.  You could make it faster by reducing value of 68ohm resistor if you'd like to.

[klugesmith]

Nice fabrication!   Thanks for posting schematic.  You're well on your way to learning electronic stuff.   
Here are some suggestions for polishing the schematic.

[pedantic] It's understood that I'm criticizing the sloppy schematic you screen-snipped from youtube.
Without context, it isn't clear what the red and blue and purple circles represent.  For the circuit to make any sense, the two blue circles must be on different nets.

There is no net identified as Ground, so we don't know the reference connection for your 'scope traces.
It's common but hardly universal for "ground" to be the most negative power supply point. In your schematic that appears to be a connection between one of the blue circles and one of the diodes.  Be easy to draw a ground symbol using your purple pen.

Reference designators are handy, so when discussing a resistor or diode it can be identified by number instead of type, value, or function (which often are not unique identifiers).

What's up with that arrow next to 100k resistor?  Ah, the original design has a 100k potentiometer in its BOM.  Was super sloppy to use that symbol without connecting the arrow -- it's worse than ambiguous about how one would actually wire the 3-terminal part.

There's a 4-way cross near lower left corner.   Do the vertical and horizontal wires connect at that point?  It's ambiguous without your purple edits, which exemplify a style of curved bridges when lines cross without a connection.   Better to add a dot at the cross, or reroute one of the lines, as in the original schematic near pin 2 of the IC. [edit] oh wait, original has a crossover bridge below the 100 nF capacitor.  We see that line, and the line from MOSFET source, not going to a straight extension of the horizontal line to the right of diode 5A.  Needless muddying of the picture, unless those line routings are supposed to offer useful guidance about physical placement and routing. That's not the job of a schematic diagram, except where clarified with plenty of text.

[\pedantic]

[TiagoBS]

Nice fabrication!   Thanks for posting schematic.  You're well on your way to learning electronic stuff.   
Here are some suggestions for polishing the schematic.

[pedantic] It's understood that I'm criticizing the sloppy schematic you screen-snipped from youtube.
Without context, it isn't clear what the red and blue and purple circles represent.  For the circuit to make any sense, the two blue circles must be on different nets.

There is no net identified as Ground, so we don't know the reference connection for your 'scope traces.
It's common but hardly universal for "ground" to be the most negative power supply point. In your schematic that appears to be a connection between one of the blue circles and one of the diodes.  Be easy to draw a ground symbol using your purple pen.

Reference designators are handy, so when discussing a resistor or diode it can be identified by number instead of type, value, or function (which often are not unique identifiers).

What's up with that arrow next to 100k resistor?  Ah, the original design has a 100k potentiometer in its BOM.  Was super sloppy to use that symbol without connecting the arrow -- it's worse than ambiguous about how one would actually wire the 3-terminal part.

There's a 4-way cross near lower left corner.   Do the vertical and horizontal wires connect at that point?  It's ambiguous without your purple edits, which exemplify a style of curved bridges when lines cross without a connection.   Better to add a dot at the cross, or reroute one of the lines, as in the original schematic near pin 2 of the IC. [edit] oh wait, original has a crossover bridge below the 100 nF capacitor.  We see that line, and the line from MOSFET source, not going to a straight extension of the horizontal line to the right of diode 5A.  Needless muddying of the picture, unless those line routings are supposed to offer useful guidance about physical placement and routing. That's not the job of a schematic diagram, except where clarified with plenty of text.

[\pedantic]

Thank you very much for the tips! I will definitely use them in my projects from now on!

I made some progress on the project that brought up a lot of questions.

-

-


As you can see in the photos and video, I made my own AC Flyback. It was my first one, so I didn't follow many standards. The important thing is that it worked.

However... Before, I was using a Flyback taken from an old TV. With it, the arcs weren't that big and the MOSFET didn't get too hot. I imagined that this had to do with the operating frequency.

To explain better: I tested this old Flyback by connecting my frequency generator to the primary coil and measuring the voltage in the secondary. In this particular Flyback, the highest voltage I got was when the frequency was around 20-21Khz.

In this new Flyback, when I performed the same test, the frequency at which I got the highest voltage was around 10Khz. I tried reducing the primary coil from 15 to 8 turns. But the difference in frequency was not as big as I expected.

I would like to operate this Flyback at higher frequencies to get rid of the noise generated at 10Khz.

Questions:

I can simply increase the frequency to 20Khz using the potentiometer adjustment. But wouldn't that take the Flyback out of resonance? (Assuming my frequency test was valid).

In the first circuit I used 150nF between DS. With this other Flyback I will need to find new values. Does this affect the resonance frequency?

Why did changing the number of turns in the primary coil not significantly impact the resonance frequency?

[davekni]

Best to check Vgs and Vds waveforms when making any adjustments.  There are two parameters that can be changed, off-time and on-time.  Your circuit has only one easily variable, on-time.  You could make the 11k resistor also variable to change off-time.

DC (ie. TV) flyback transformers typically have multiple internal diodes on HV side connected between each winding segment.  This isolates inter-winding capacitance between sections.  Makes self-resonant frequency of secondary high enough that it can be ignored in many applications.
AC flyback transformers as you've made have more secondary capacitance.  Secondary resonant frequency can be near enough to operating frequency to complicate behavior.  Becomes a dual-resonant system.

For DC flyback with minimal secondary capacitance, only significant resonance is primary coil with capacitor(s) in your circuit (drain to source or drain to V+).  That resonance is not your operating frequency.  It is the half-cycle you see during every off-time.  Off-time must be at least as long as that primary half-cycle time, but not too much longer.  On-time controls how far primary current ramps up, so how high voltage gets during off-time.  As you adjust on-time, you may be saturating transformer core or exceeding FET Vds limits.  That's one reason scoping is critical.  Other reason is to see effects of secondary resonance on primary waveform.

Hope above isn't too densely packed information.  Circuits are often more subtle and complex than they first seem, especially when tweaking away from standard mass-produced versions such as TV anode supplies.  If you are interested in exploring further, post some scope traces of Vgs and Vds.  Will be easier to discuss details based on actual data.

[TiagoBS]

Best to check Vgs and Vds waveforms when making any adjustments.  There are two parameters that can be changed, off-time and on-time.  Your circuit has only one easily variable, on-time.  You could make the 11k resistor also variable to change off-time.

DC (ie. TV) flyback transformers typically have multiple internal diodes on HV side connected between each winding segment.  This isolates inter-winding capacitance between sections.  Makes self-resonant frequency of secondary high enough that it can be ignored in many applications.
AC flyback transformers as you've made have more secondary capacitance.  Secondary resonant frequency can be near enough to operating frequency to complicate behavior.  Becomes a dual-resonant system.

For DC flyback with minimal secondary capacitance, only significant resonance is primary coil with capacitor(s) in your circuit (drain to source or drain to V+).  That resonance is not your operating frequency.  It is the half-cycle you see during every off-time.  Off-time must be at least as long as that primary half-cycle time, but not too much longer.  On-time controls how far primary current ramps up, so how high voltage gets during off-time.  As you adjust on-time, you may be saturating transformer core or exceeding FET Vds limits.  That's one reason scoping is critical.  Other reason is to see effects of secondary resonance on primary waveform.

Hope above isn't too densely packed information.  Circuits are often more subtle and complex than they first seem, especially when tweaking away from standard mass-produced versions such as TV anode supplies.  If you are interested in exploring further, post some scope traces of Vgs and Vds.  Will be easier to discuss details based on actual data.

Thank you very much for the explanations!

Well, with GPT help I was able to understand some things (correct me if I'm wrong):

There are two important parameters that I can control in my circuit (with the existing potentiometer and by changing the value of the 11k resistor). On Time and Off Time.

On-time: Controls the time that the MOSFET remains on and, therefore, how much the current increases in the transformer.

Off-time: Controls the time that the MOSFET remains off, allowing the energy stored in the transformer to be released to the load.

If the On-time is too long, this can cause the core to saturate.

-------

The operating frequency of the circuit is the frequency at which the MOSFET alternates between on and off, controlling the flow of energy to the transformer. This frequency is determined by the 555 circuit. [Scope pic 4]

The natural resonance frequency of the circuit is a frequency of natural oscillation between the primary inductance of the transformer and the capacitors connected to the circuit (drain and source of the MOSFET). This resonance occurs during the off-time of the MOSFET, that is, when it is turned off and the transformer is discharging the stored energy.

The secondary resonance frequency occurs in the secondary winding, formed by the secondary inductance and the parasitic capacitance between the turns.

Ideally, the primary and secondary resonance frequencies need to be far from the operating frequency to avoid interference. However, when the secondary resonance frequency is close to the operating frequency, interference can occur between the two sides of the transformer, causing a double resonance.

When the MOSFET turns off (off-time), the energy that was stored in the transformer core while it was on (on-time) is released to the secondary circuit. During this time, the voltage across the primary oscillates at a natural frequency due to resonance between the inductance and capacitance of the circuit. [Scope pic 1, 2, 3]

My frequency approximation: f=1/T 1/45×10^-6 = 22,22KHz

This resonance creates a half-wave (half-cycle) oscillation on the primary side. This oscillation occurs because the energy stored in the transformer is gradually released, and the circuit naturally oscillates until the energy is dissipated. [Scope pic 2]

The off-time of the circuit must be at least equal to the time of this half-cycle to ensure that all the energy in the transformer is discharged before the MOSFET turns on again. If the off-time is too short, the transformer will not be able to release all the energy, which can lead to current build-up and core saturation.


Scope pic 1


Scope pic 2


Scope pic 3


Scope pic 4


Circuit operating now (AC Flyback)


Circuit operating before (DC Flyback)



In summary.

The circuit as a whole has three "types of frequency", the oscillation generated by the 555 or operating frequency, the primary resonant frequency and the secondary resonant frequency.

Ideally, the primary's resonant frequency should not be close to the operating frequency. If this occurs, there may be double resonance that can cause voltage spikes and other problems.

Now, here's a part that I discovered through my own research and that is confusing me:

What is the ideal operating frequency?

According to the GPT example, if my primary's resonant frequency is 30kHz, it is recommended that the operating frequency (555) be close to 50-70kHz to avoid problems.

Well, I had imagined that for the circuit to work resonantly, the operating frequency should be close to the secondary resonance (like in Tesla Coils, where the secondary oscillation frequency is captured so that the circuit oscillates at the same frequency), but apparently, according to GPT:

"A resonant flyback is a variation of the traditional flyback circuit, designed to operate at the resonance frequency between the transformer's primary inductance and the circuit's capacitance. This type of configuration allows the circuit to operate more efficiently and with fewer switching losses, mainly by minimizing power dissipation in the switching transistors (like MOSFETs). [Zero Voltage Switching (ZVS)]"

[davekni]

In scope captures 1-3, it is not clear what node of what circuit you are measuring.  They look like typical ring-down resonance such as when a charged capacitor is connected across an inductor.  (That's a useful way to test inductors, by scoping waveform when a charged capacitor is connected to the inductor.)  In these images, period appears to be about 60us, for a frequency of about 17kHz.

In general, text or image label should define which signals are being measured, and which is CH1 and which is CH2.  At least in the later scope captures I can guess what is being measured based on waveform shape, gate and drain voltages of FET as I'd suggested posting.  (BTW, also best for most testing to set scope vertical channels to DC coupled.  Your CH1 is DC coupled, but CH2 is AC coupled, eliminating any information about actual 0V level of that signal.)

When the MOSFET turns off (off-time), the energy that was stored in the transformer core while it was on (on-time) is released to the secondary circuit.

Everything in your post above this looks accurate.  This statement is correct for non-resonant flyback circuits (boost converters) with no added drain capacitance.  With this resonant flyback circuit (what is normally called "flyback"), much of the stored energy transfers from primary winding to primary resonant capacitor (capacitor connected to FET drain) and then back to primary winding.  That's what resonance is, oscillatory transfer of energy from inductor to capacitor and then back to inductor.  Some energy does transfer to secondary of course.

During this time, the voltage across the primary oscillates at a natural frequency due to resonance between the inductance and capacitance of the circuit. [Scope pic 1, 2, 3]

Are these first three scope captures of FET drain with very long off-time?  That would make sense.
With normal flyback operation as in your image 6, only a half-cycle of primary frequency oscillation occurs.  That's intended operation of resonant flyback circuit.

This resonance creates a half-wave (half-cycle) oscillation on the primary side. This oscillation occurs because the energy stored in the transformer is gradually released, and the circuit naturally oscillates until the energy is dissipated. [Scope pic 2]

These first 3 pictures show many cycles of oscillation, as would occur with long FET off-time.  In normal operation FET isn't off long enough for more than first half-cycle, as in picture 6.

he off-time of the circuit must be at least equal to the time of this half-cycle

Yes, a bit longer than one half cycle, as in picture 6.  Too short causes FET to directly discharge drain capacitor and overheat.  Saturation is caused by too long on-time.

Ideally, the primary's resonant frequency should not be close to the operating frequency.

Yes.  As you can see in picture 6, period of 555 is about 7 times longer than the half-cycle of resonance.  Here primary resonant frequency is about 3.5x higher than operating frequency, a typical ratio.  Nothing directly to do with double-resonance.  That's when secondary resonant frequency gets close to primary frequency.

Scope picture 5 shows what is probably secondary resonant frequency causing two unequal cycles of oscillation during each FET off time.  That picture also appears to have some interference with scoping, perhaps unintended coupling from secondary to scope probes.  Would be worth reading up on scoping techniques, and perhaps share pictures of your physical circuit including scope connections etc.

I'd ignore the rest of GPT results at end of your post.  It is getting too many different circuits and modes of operation confused.

[TiagoBS]

In scope captures 1-3, it is not clear what node of what circuit you are measuring.  They look like typical ring-down resonance such as when a charged capacitor is connected across an inductor.  (That's a useful way to test inductors, by scoping waveform when a charged capacitor is connected to the inductor.)  In these images, period appears to be about 60us, for a frequency of about 17kHz.

During this time, the voltage across the primary oscillates at a natural frequency due to resonance between the inductance and capacitance of the circuit. [Scope pic 1, 2, 3]

Are these first three scope captures of FET drain with very long off-time?  That would make sense.
With normal flyback operation as in your image 6, only a half-cycle of primary frequency oscillation occurs.  That's intended operation of resonant flyback circuit.

Figures 1, 2, and 3 show the readings I took to measure the primary’s resonant frequency. With the circuit powered off, I briefly connected a 3.7V battery to the two terminals leading to the primary coil and scope DS.
I can assume that these approximate 17kHz are the primary's resonant frequency, right?
I made no changes to the off-time when I took this reading.

In general, text or image label should define which signals are being measured, and which is CH1 and which is CH2.  At least in the later scope captures I can guess what is being measured based on waveform shape, gate and drain voltages of FET as I'd suggested posting.  (BTW, also best for most testing to set scope vertical channels to DC coupled.  Your CH1 is DC coupled, but CH2 is AC coupled, eliminating any information about actual 0V level of that signal.)

I'll pay closer attention to these points. Without an explanation it's difficult for others to understand. Since I've been working on this circuit for a while, I tend to have a biased perspective

Ideally, the primary's resonant frequency should not be close to the operating frequency.

Yes.  As you can see in picture 6, period of 555 is about 7 times longer than the half-cycle of resonance.  Here primary resonant frequency is about 3.5x higher than operating frequency, a typical ratio.  Nothing directly to do with double-resonance.  That's when secondary resonant frequency gets close to primary frequency.

Alright, is there an ideal ratio, or does this 3.5-times value simply indicate that the circuit is operating at frequencies sufficiently apart to avoid double resonance?

The off-time of the circuit must be at least equal to the time of this half-cycle

Yes, a bit longer than one half cycle, as in picture 6.  Too short causes FET to directly discharge drain capacitor and overheat.  Saturation is caused by too long on-time.

In Figure 6, the off-time was not adjusted. However, this does not imply that adjustments to this value won't be necessary for the AC flyback transformer, correct?

Scope picture 5 shows what is probably secondary resonant frequency causing two unequal cycles of oscillation during each FET off time.  That picture also appears to have some interference with scoping, perhaps unintended coupling from secondary to scope probes.  Would be worth reading up on scoping techniques, and perhaps share pictures of your physical circuit including scope connections etc.

Okay, from what I understand, part of the solution involves finding an ideal capacitor value between D and S. But regarding the operating frequency, should I simply choose a value that’s far enough from 17 kHz (assuming this is the primary's resonant frequency)?

[davekni]

Figures 1, 2, and 3 show the readings I took to measure the primary’s resonant frequency. With the circuit powered off, I briefly connected a 3.7V battery to the two terminals leading to the primary coil and scope DS.
I can assume that these approximate 17kHz are the primary's resonant frequency, right?

So primary capacitor is still connected across primary, correct?
Unless battery impedance is quite high, battery in parallel changes circuit.  Battery connection is a great technique, but needs to be scoped as battery is disconnected.  Voltage typically rings much higher than battery voltage as battery current is removed.  Set scope trigger threshold higher than battery voltage, say 5V or 10V, to trigger on disconnect.  BTW, my best guess is that battery impedance is low enough to severely damp primary resonance.  You may be measuring secondary series-resonant frequency (frequency with transformer leakage inductance).
Even with disconnect technique, what you'll be measuring is a combination of primary and secondary resonances.  You could try that battery-disconnect test on secondary with primary disconnected from circuit and from primary capacitor.  That would give secondary parallel-resonant frequency.

Alright, is there an ideal ratio, or does this 3.5-times value simply indicate that the circuit is operating at frequencies sufficiently apart to avoid double resonance?

Again, this is not related to dual-resonance.  Resonant flyback operation needs operating frequency to be below primary resonant frequency.  (FET on time longer than the half-cycle resonant flyback pulse.)

In Figure 6, the off-time was not adjusted. However, this does not imply that adjustments to this value won't be necessary for the AC flyback transformer, correct?

Correct.  Or adjust capacitance to match fixed off-time if that works out better.

Okay, from what I understand, part of the solution involves finding an ideal capacitor value between D and S. But regarding the operating frequency, should I simply choose a value that’s far enough from 17 kHz (assuming this is the primary's resonant frequency)?

No.  Operating frequency is controlled by 555 circuit, not by D-S capacitor.  D-S capacitance change changes primary resonant frequency, not operating frequency.

[TiagoBS]

No.  Operating frequency is controlled by 555 circuit, not by D-S capacitor.  D-S capacitance change changes primary resonant frequency, not operating frequency.

What I meant is that I need to find a capacitance value across DS to adjust the primary resonant frequency, so it completes only half a cycle during the off-time.

I conducted some tests, setting the operating frequency to around 35kHz. Here are the probe readings I obtained:

150nF across DS and 15 turns on the primary. The off-time (11k resistor) remained unchanged from the previous tests.

Blue: GS
Orange: DS


-

-

-


The MOSFET is heating up a lot. Reducing the operating frequency would increase the ratio between the operation frequency period and the primary resonance period. Would this be a good solution to reduce the heating issue?

According to the images, does the off-time need to be adjusted?

[davekni]

I conducted some tests, setting the operating frequency to around 35kHz. Here are the probe readings I obtained:
150nF across DS and 15 turns on the primary. The off-time (11k resistor) remained unchanged from the previous tests.

Great scope captures with trace definitions, along with circuit values.  Is DC input supply 15V or so?

The MOSFET is heating up a lot.

FET current (same as primary current during on time) is quite high by end of on time, ~40A.  That's causing excess FET heating.  My first thought was ferrite core saturation.  But that doesn't fit unless your core is much smaller than it looks or is wrong type of ferrite material.  For future reference, what are core dimensions, especially diameter inside coils?  Do you know ferrite material type?  PC40 is common for parts from China.  What is the gap distance?  (Or distances if you have two gaps, one on each leg of core.)

Presuming not saturation, next guess is that the off-time half-cycle of resonance is upper pole frequency where impedance is lowest.  That could be cause of high current, though doesn't seem to quite fit plots either.  Hard to tell without knowing more circuit parameters, inductances  etc.  Presuming you don't have a meter to measure inductance, above core parameters would allow some guesstimating.  Do you know roughly what secondary turn count is?

One test that should show resonant frequencies:  Scope with very-long off-time, ie. with 10meg or as high a value as you have available instead of 11k.  This wouldn't be for final operation, just an experiment to see resonances after FET turn-off.

[TiagoBS]

Great scope captures with trace definitions, along with circuit values.  Is DC input supply 15V or so?

Thank you very much! I’m using a 17V DC power supply.

For future reference, what are core dimensions, especially diameter inside coils?  Do you know ferrite material type?  PC40 is common for parts from China.  What is the gap distance?  (Or distances if you have two gaps, one on each leg of core.)

Here are the core dimensions (I can take other measurements if needed). This ferrite was taken from an old AOC monitor flyback. Both legs of the core are in contact with those on the opposite side (although during some off-time adjustments, I could hear the core vibrating at times).

Should I leave a gap between the legs?

Presuming not saturation, next guess is that the off-time half-cycle of resonance is upper pole frequency where impedance is lowest.  That could be cause of high current, though doesn't seem to quite fit plots either.  Hard to tell without knowing more circuit parameters, inductances  etc.  Presuming you don't have a meter to measure inductance, above core parameters would allow some guesstimating.  Do you know roughly what secondary turn count is?

I have a low-cost inductance meter, so I’m not sure if the readings are accurate, especially for the primary coil inductance.

Here are the values I obtained:

Primary: 0.02 mH, 0.14 Ω
Secondary: 870 mH, 153 Ω

Well, I didn't measure the number of turns, but I believe it's between 1000 and 1500 turns.

One test that should show resonant frequencies:  Scope with very-long off-time, ie. with 10meg or as high a value as you have available instead of 11k.  This wouldn't be for final operation, just an experiment to see resonances after FET turn-off.

I'm not sure if I took the measurements correctly. I added 8 MΩ of resistance.

Blue: GS
Orange: DS


-


The oscillation was only visible when I turned off the power supply. Otherwise, the oscilloscope displayed the waveform as it would in normal operation, as if the off-time hadn't been adjusted (I expected to see the increase in off-time during normal operation), and the flyback worked normally, but at a lower frequency.

[davekni]

Here are the core dimensions (I can take other measurements if needed). This ferrite was taken from an old AOC monitor flyback. Both legs of the core are in contact with those on the opposite side (although during some off-time adjustments, I could hear the core vibrating at times).

Should I leave a gap between the legs?

When removing core from original flyback, did you find any spacers?  Core material would need to be something very unusual to operate as a flyback without gap(s).  I'd suggest reading up on gapped cores and flyback transformer design before proceeding.  This forum isn't the right place for a tutorial on transformer and core design (nor do I have time to write such a tutorial.)  Ungapped core is very likely saturating in most of your runs.  Makes for inefficient operation and complicates interpreting scope plots given extreme non-linearity in saturation.
For a very abbreviated summary, increased gap will: Increase resonant frequencies, increase current before core saturation, and decrease coupling from primary to secondary.

I have a low-cost inductance meter, so I’m not sure if the readings are accurate, especially for the primary coil inductance.

Yes, primary inductance looks low for an ungapped core.  Doesn't fit well with 17kHz resonant frequency with 150nF cap either.

The oscillation was only visible when I turned off the power supply.

I realized a couple issues with my suggestion.  First, 555 comparitor input current is too high for such high-value resistors.  Second is that increasing that initially-11k value increases on-time as well as off-time.  A very-long on-time will cause FET current to rise too high.
Not sure exactly why there's some operation as supply voltage drops, but it is likely enough.  Shows two frequencies, probably the lower and upper pole frequencies of this dual-resonant system.  Lower pole shows a half-cycle of roughly the same frequency you measured with battery experiment, ~17kHz.  Higher frequency of ~120kHz is presumably upper pole.  That 17kHz would require running ~3.5x lower, or about 5kHz.  Such low frequency is not surprising for an ungapped core.