SSTC (Not) Working Correctly

[Laci]

Hello Everybody!
I recently upgraded my half bridge SSTC to a full bridge,from an IR2153 to 2xIR2109.The half bridge worked very well almost 10cm long arcs to not grounded objects at rectified mains.
With the full bridge i get pretty weird results;it lights up a CCFL lamp bright,at every frequency-around 100kHz-400kHz(andjusted by the pot at the 555),but very tiny arcs.The current drawn is also the half of the half bridge current drawn(1A-half bridge,+-0,4A-full bridge).
I tried to change the number of primary turns,changing polarity,adjusting frequency,everything I could,but the results are at least half or less as good as the half bridge.
I checked every frequency at the gates of the mosfets,with a frequency counter and they are the same.Sadly,I do not have an oscilloscope so I think that it is pretty hard to find out what is making the problem.
As I said,the driver circuit changed,so I'm sure that the problem is with that,but do not know what...The mosfets are also heating up simultaneusly,so they should work correctly...
If you have an idea,how can I improve this design to work correctly,please reply
Thank you very much!

[Mads Barnkob]

Are all MOSFETs heating up or maybe just 3 of them?

Is it polarity of the gate drive or polarity of the primary coil that you have changed? Your schematic is for sure wrong, you are driving both upper leg MOSFETs on at the same time and both lower leg MOSFETs on at the same time.

Since it oscillates, but draws very little power, it sounds like you are driving it out of phase, with wrong gate polarity or the supply to DC bus is not working and the energy transferred to the secondary circuit is only the capacitively coupled energy from the drive circuit.

[Laci]

I do not use gate drive transformer.In theory,I think that the 74HC14 shifts the NE555's output level,and the IR2109 chips will never turn both the high- or low side mosfets on,since the input signal is inverted.The setup is also weak to drive a TV flyback transformer.

[Mads Barnkob]

I completely missed the signal 1/2, so yes you have ofc taken care of the switching polarity.

Did you ground all the unused inputs on the 74HC14? If not, this could very well be the issue. There are other threads on the forum about that issue, like: https://highvoltageforum.net/index.php?topic=93.msg431#msg431

[Laci]

I will try to ground the other pins!Also I found out,that the high side output of the driver ic is not working correctly:the low side output goes high whenever the input signal is low/ground,but the high side just makes a pulse,and goes back to low,not remains high,when the input signal is high.

[Mads Barnkob]

Did it help to ground the unused 74HC14 input pins?

[CCX191]

Get a cheap DSO 138 oscilloscope

[Mads Barnkob]

Get a cheap DSO 138 oscilloscope

As long as the resonant frequency of the coil is not above 100 kHz, a DSO138 should be good enough, but it quickly begins to struggle above that.

I bought one and will write about it, once I get down through the pile of projects

[Laci]

Thank you guys for being so helpful!:)
I'm upgrading to a better coil recently.A coil of 0.3mm magnet wire is coming,the primary is made of 6mm copper wire- 15 turns.I will make the secondary 110mm dia,550mm long,and around 1800 turns,as calculated.

I made my uploaded schematic on a perf board,but I realized that it's not working with the transistors,as they should.As the load on the IRFP460 full bridge I put two LEDs in different polarity,decreased the frequency to 1Hz and(if I remember),one of the LEDs was blinking,and the other was turned on fully,and never turned off.
I could't find the problem,so I made the circuit on a breadboard,now it seems working there.
The other problem:all of the IRFP460s are dead,due to my stupidity(or just they are crappy fake mosfets and give up easily....)

An oscilloscope could solve a lot of my problems,but I want to buy good one.

[Laci]

So the magnet wire roll have arrive,0.3mm,636m.
My question is that 110mm diameter,550mm long,1850 turns or 160mm diameter,380mm long,1260 turns  secondary coil would work better for a SSTC or for a DRSSTC?

[Mads Barnkob]

So the magnet wire roll have arrive,0.3mm,636m.
My question is that 110mm diameter,550mm long,1850 turns or 160mm diameter,380mm long,1260 turns  secondary coil would work better for a SSTC or for a DRSSTC?

For a SSTC a smaller, not so tall, but lower diameter to height ratio is preferred. Something in the range of 1:2 to 1:3 ratio, since a SSTC needs a much higher coupling to perform than a DRSSTC, due to it operating with a much lower peak current(but high RMS current), it should give better performance with a smaller secondary coil, there is however also some increased spark length performance in longer secondary coils due to more secondary turns, but this increase is not that big and quickly diminishes when you exceed 1:4 or 1:5 ratios

A DRSSTC operates at very high peak currents and too high coupling will simply make it flash over between primary and secondary, but just the spark length that is achievable with a DRSSTC is another reason to make them longer, because if it is too short, all you get is limited spark length to ground

I would recommend going with:
a 1:4 ratio on the 110 mm diameter form, for 1330 turns, 440mm long winding (secondary circuit impedance around 47-49kOhm)
or
1:2.5 ratio on the 160 mm diameter form, for 1260 turns, 380mm long winding (secondary impedance around 61-70kOhm)

As a SSTC operates with long on-time, compared to a DRSSTC, I think the higher impedance secondary coil circuit would perform better. But I would someone else to comment on this, because I am not completely sure.

Regarding getting a good entry level oscilloscope, see the latest posts here: https://highvoltageforum.net/index.php?topic=107.msg883#msg883

[Uspring]

As a SSTC operates with long on-time, compared to a DRSSTC, I think the higher impedance secondary coil circuit would perform better. But I would someone else to comment on this, because I am not completely sure.

I don't have any practical experience with SSTCs, but from calculations I believe, that the primary inductance can look almost completely resistive if driven at the right frequency and that secondary Q is above a certain limit. That would probably be an advantage in order to not have too much reactive current in the primary. The Q needed is about 2/k^2, where k is the coupling. Secondary Q depends on arc loading. For a given arc load it increases with less secondary inductance. A large Q will also increase the resistance seen by the driver. There is a limit to increasing Q or equivalently reducing secondary inductance, i.e. less turns, since at some point the voltage won't be large enough to cause breakout at all.

Another issue is getting the frequency right. The feedback adjusts to a 0 degree phase all over the complete loop. That usually works but might not result in quite the optimal frequency. I wonder if anybody has tried to add a manual phase shifter for optimisation.

That being said, this is all theory.

[Laci]

Now i have the components for the secondary let's see what can I do
The coil,as calculated should be:125mm dia.,485mm long(1:3.88),1620 turns.

I hope,that if I can make it,it will work with a SSTC driver,then maybe upgrade to DRRSSTC...

[Mads Barnkob]

Now i have the components for the secondary let's see what can I do
The coil,as calculated should be:125mm dia.,485mm long(1:3.88),1620 turns.

I hope,that if I can make it,it will work with a SSTC driver,then maybe upgrade to DRRSSTC...

The secondary coil has a size that will make it useful for both SSTC and DRSSTC, but that primary coil might have too little coupling for a SSTC and way too high coupling for a DRSSTC. You can check the latter by plotting your coil dimensions into JavaTC and see the coupling vs. the suggest coupling.

Uspring, I will have read your reply a couple of more times before I can reply to it

[Uspring]

Mads, sorry if I've been to opaque in my reply. From my point of view, a SSTC suffers in comparison with a DRSSTC, that the load to the bridge circuit, i.e. the primary, can look mostly inductive. In a DRSSTC that is compensated by the MMC. A mostly inductive load can have much current in it, which is not causing any real power output out of the bridge. The primary should look as lossy as possible, implying, that there is much power transferred to the secondary.

That requires a large influence of the secondary on the primary. A high coupling helps and also high secondary currents. Large secondary currents appear, when secondary Q is big. For a given arc load, this requires a not too large secondary inductance. For Qsec > 2/k^2, there is a frequency, at which the primary inductance is mostly cancelled by effects of the secondary current. At this point, the load to the bridge appears similar to that of a DRSSTC, i.e. an approximately sine wave current in phase with the bridge voltage.

The cancellation of the primary inductance can lead to high primary currents, so care must be taken in order not to overload the bridge. It can be considerably larger than the estimate Vinp/(2*pi*f*Lpri).

Possibly your coils already run like this. Do you have scope shots of the primary current of your coils, wrt to bridge voltage?

[Laci]

The coil is made.I tried it with a slayer exciter circuit and I have a question.Why are the arcs large without topload at 3-5 turns primary,and weak with topload,but large again with topload and more(10+)primary turns?I thought it tunes automatically the primary frequency to the resonant frequency of the secondary,so why is it depending on the primary turns?

[Mads Barnkob]

Uspring: So I was wrong about saying high impedance would be better, as that is quite opposite of a larger gauge secondary wire which would give a higher Q, also be able to carry more current etc.

I think my SSTC2 is around Q~140 and that is much higher than a conservative estimate of k=0.2, so that 140 > 50. That could also blow the MOSFETs when pushing out almost 500mm sparks

I only had a old analog oscilloscope back then, so that would be a good experiment for the future, so get it back out and hook it up to DSO, CT and Diff. probe.

Laci: A topload will represent a much bigger load to the energy source on the primary circuit, I think you are simply loading down the slayer exciter. So when you add more primary turns with the topload on, it is not as high currents flowing and it has a chance to actually deliver power to the secondary circuit.

Can you measure what resonant frequency it was running at, in the different setups?

[Uspring]

Laci: You're right about the tuning not being dependent on the primary turns. I'd expect a larger primary current with with less primary turns, which might increase power throughput. The important term here is amp turns, which probably increases with less turns.
Adding a top load decreases secondary resonance frequency, which will also increase primary current, since primary impedance becomes lower with a lower running frequency.

The feedback will not always have the coil run at exactly the secondary resonance frequency. The condition for a working feedback is a 0 degree phase shift all over the feedback loop. I don't know, what your circuit looks like, but imagine, that the switching transistor is driven by an antenna. Then remove the antenna and replace it with a signal generator at exactly the same frequency, that the coil is running when using an antenna. The coil will behave as before and the antenna (now unconnected) will deliver the same frequency and phase as the signal generator. That shows, that the whole system phase shift between driver input and antenna output is 0 degrees.

The 0 degree condition is likely near the secondary resonance frequency, since the "whole system phase" varies strongly with frequency near resonance and the chance is high, that there is a frequency, that matches the 0 degree condition. That frequency might not be the best one for maximum power transfer between primary and secondary.

It's difficult to explain your observations without knowing the specs of your coil, e.g. feedback circuitry, inductances, capacitances coupling and such. You might try running a simulation.

Mads: Secondary Q depends mostly on arc loading, so it is probably much lower than that calculated from wire gauge and skin effects.

I don't quite understand your remark on loading down the slayer exciter. I'm not familiar with that circuit. How do you load it down aside from blowing up the transistor or overloading the power source?

[Mads Barnkob]

Mads: Secondary Q depends mostly on arc loading, so it is probably much lower than that calculated from wire gauge and skin effects.

I don't quite understand your remark on loading down the slayer exciter. I'm not familiar with that circuit. How do you load it down aside from blowing up the transistor or overloading the power source?

Yes I was using unloaded figures here

From what I understand, the slayer exciter circuit is quite simple and not a big power pusher, that from its feedback oscillating nature has a narrow performance and can not keep up with feeding a larger capacitance, but I could be wrong

I recall back when I build SSTCs that larger toploads always meant trouble, but it could be as you describe that the higher primary currents was the real culprit causing flashovers as I caught on video at 3:37:

[Laci]

So I made some measurements.The resonant frequency is 140kHz i think,but i'm not sure,because every frequency was measured with multimeter.The power supply is around 55 volts,large smoothing capacitor.
•Without topload:3 turns primary,current is 1A,and it outputs around 0.5cm arcs and they are pretty weak,can't pull a constant arc.With more primary turns,current drops,but the arcs gets more powerful,around 2cm.
•With topload <5 turns does not give any output,and by increasing the primary turns the current is dropping,arcs are increasing-the same thing as without topload,but the arcs are not as big as without topload,seems like it needs more primary turns with topload to work better.
The frequencies are pretty high(around 400kHz) in both cases,measured with multimeter so i think it's not accurate or it's not measuring what it should.

Also,the height of the coil is 50cm,a bit taller then I described,because I wound it until I ran out of magnet wire.