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ZVS Driver work Frequency

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MRacerxdl

Wed Feb 11 2009, 03:07PM

Registered Member #989 Joined: Sat Sept 08 2007, 02:15AM
Location: SÃ£o Paulo, Brazil
Posts: 476

I was trying here to made a ZVS based DRSSTC, and I tryed to tune the primary.

I was 5+5 Turns, and the secondary coil in, and my frequencymeter attached to one leg of the coil (not the center tap).
Independent of the size of the capacitor, the frequency stays at + 55kHz., but the ressoant frequency of the primary (Total L and C) was 240Khz. Why it doesnt ressoates at that frequency? The inductance of choke changes it?

Thanks!

101111

Wed Feb 11 2009, 03:10PM

Registered Member #575 Joined: Sun Mar 11 2007, 04:00AM
Location: Norway
Posts: 263

You will need to change the capacitor, there should be a formula to the Mazilli circuit somewhere here on the forum.

Maybe more turns would also help.

Dr. Dark Current

Wed Feb 11 2009, 03:12PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Probably the pri-sec coupling, but I'm not sure here.

Another possibility is that the gate resistors are not small enough to charge the gates at such high frequency. Remember it is designed for flybacks to work at ~<60kHz.

GeordieBoy

Wed Feb 11 2009, 04:39PM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

When you insert the secondary coil into the primary, the ZVS oscillator probably oscillates at one of the two new resonant modes of the system. These are also sometimes known as "split frequencies" and arise due to the magnetic coupling between the primary and secondary resonant networks.

Even though the primary or secondary might both resonate at 240kHz when each is tested in isolation, the oscillator will probably oscillate at some frequency above or below this when they are coupled together. In the coupled system there is no resonance at the original frequency of 240kHz.

-Richie,

Dr. Dark Current

Wed Feb 11 2009, 05:09PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Does changing supply voltage change the frequency noticably? If so, the feedback probably isn't working.

Try scoping the primary waveform, it should be a sine wave (or at least close to it).

Marko

Wed Feb 11 2009, 07:56PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Independent of the size of the capacitor, the frequency stays at + 55kHz., but the ressoant frequency of the primary (Total L and C) was 240Khz. Why it doesnt ressoates at that frequency? The inductance of choke changes it?

Did I understand you correctly, that your measured frequency jumps for + or - 55kHz around the original 240kHz frequency of the secondary coil? That sounds completely right for tesla-coil-like coupling.

It is not possible to tune such a system by measuring the frequency of the primary voltage alone and trying to make it equal to resonant frequency of the secondary alone, for reasons mentioned above by Richie.

You will have to tune while simply watching output of the coil, hence some way of tuning while power is up is preferable.
You should clearly notice the difference between two main modes as the system will jump into whichever is more energetically favorable at the moment, and should have some hysteresis in doing so.

With high Q coupled systems, I found out the greatest power transfer just at the brink of the in-phase (lower frequency) mode, before the system collapses into out-of phase mode. For a spark loaded tesla coil this might not matter as much due to low Q and high coupling.

Marko

Arcstarter

Wed Feb 11 2009, 10:32PM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

I have found that frequency is changed much more with the different primary inductance than with the different capacitor. I would just say mess with pri turns and coupling, perhaps you can get the freq to finally change.

Dr. Dark Current

Wed Feb 11 2009, 10:34PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

I have a feeling that it might even work without any capacitor for a SSTC if the couling is high. I could be wrong of course.

MRacerxdl

Thu Feb 12 2009, 10:54PM

Registered Member #989 Joined: Sat Sept 08 2007, 02:15AM
Location: SÃ£o Paulo, Brazil
Posts: 476

Marko:No, its stays at +55Khz Instead the 240Khz.
I tryed 47nF cap until 1uF cap, all ressoates at the same frequency (strange thing). Using flybacks, I get a variation, but less than 55Khz (some flybacks goes to 20Khz or even less when I pull arcs)

Dr. Dark Current

Fri Feb 13 2009, 06:04AM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

So there are two possibilities,
1) the circuit is locking to the series resonant frequency of the pri-sec system, and the capacitor just sits there connected and does nothing. If you remove it completely, it should still work at round the same frequency.

2) The gate pull-up resistors can't turn the gates on fast enough, which limits operating frequency. If this is the case, the transistors could run pretty hot.

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