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Odd problem with ZVS

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Saz43

Thu Sept 10 2009, 10:19PM

Registered Member #1525 Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 294

Good Afternoon All,

I just finished construction of Uzzor's ZVS capacitor charger:

Everything on my circuit is to-spec, and my inductor is labeled as 185uH. Diodes are ultra fast, res. caps are MKP and the inductor is a big toroidal-type, so I believe all of my components are suitable for this application. Input power is ~13V from a high-drain battery. The comparator chip is not plugged in to remove that variable, for now.

The problem is, when my capacitor bank gets to exactly 300V, my driver, which has been dead silent up to this point, starts emitting this squealing noise, which changes pitch in steps higher and higher until I shut it off out of fear. Charging slows or ceases as this occurs and MOSFETs get hot.

Here are some crappy pictures:

Any idea what could be wrong? Many thanks!

Arcstarter

Fri Sept 11 2009, 01:24AM

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

As the cap's voltage gets higher, the load is smaller. The LC tank circuit (primary and capacitor) is probably just oscillating at lower frequency.

My ZVS driver did this when i drove a LOPT and i had drawn the arcs out as far as they would go. It squealed very loud, the spark was not as hot, and it flailed alot more.

Just use a lower capacitance in your LC circuit. What capacitance are you using now? Try something like 1uf with a LOPT core.

Saz43

Fri Sept 11 2009, 01:49AM

Registered Member #1525 Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 294

I had used a capacitance of 4.2uF, as specified in the diagram. I also tried 4.4 and 2.2, with no noticeable changes. I just tried 1uF as you had suggested, it actually went very slowly, charging my bank at about 2 or 3 volts/second. The problem must be somewhere else... I'm not sure.

I am currently using a flyback core with 3.5 + 3.5 primary and ~240 secondary.

jonny5

Fri Sept 11 2009, 05:02AM

Registered Member #1807 Joined: Tue Nov 11 2008, 07:36AM
Location: San Luis Obispo, California
Posts: 19

Unless I am mistaken, it looks like you have hot glued the secondary to the ferrite core? With only 300V, it's probably ok, but ferrite is conductive (to a degree). At least the mag wire is insulated...however, I would suggest more insulation if you decide to push for higher voltages.

GeordieBoy

Fri Sept 11 2009, 12:55PM

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

You did put some heatsinks on the power transistors before testing, right?

Dr. Dark Current

Fri Sept 11 2009, 03:45PM

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

The circuit does not like when the Q of the resonant circuit gets too low (high power in output resistive load) and starts acting erratic. This is normal and I don't think the circuit suffers much during this condition.

You can also see this when drawing arcs, if the arc gets too long, it starts emitting loud screaming noises...

uzzors2k

Fri Sept 11 2009, 07:14PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

How big is your test bank, or to the point, what is the output power? My transformer squeals away too, comparable to nails scratching chalkboard, it comes from the frequency dropping as the load decreases. It's not really a problem, but can cause saturation or heating of the core. If it bothers you try adding some more turns and using a smaller capacitor, like the 2,2ÂµF one. Heatsinks are recommended.

Also the comparator needs to be in place to charge. If you can charge without it the 2N3904 and diodes aren't working properly.

Saz43

Sat Sept 12 2009, 05:08AM

Registered Member #1525 Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 294

Thank you all very much for your responses. My capacitor bank is 15.6mF.

The output power is very low (I would estimate 5W), and the MOSFETs begin to heat instantly. My battery gets sucked dry very fast, which is unusual since it could charge my bank several times with the boost converter before needing a recharge.

I am running it w/o the comparator, and it does work (to the limited extent described above). What might be wrong with my diodes and transistor? I checked my circuit 3+ times and it seems right.

uzzors2k

Sun Sept 13 2009, 02:09PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

The comparator is required to hold the base of the 2N3904 low, otherwise the pull-up will turn it on, causing the diodes to turn off the Mazzilli driver. You seem to have enough problems that I would suggest just using the Mazzilli driver alone first (remove diodes), and optimizing it for more power. You should be able to get at least 10x as much. The instant mosfet heating is also unusual. Once that's done troubleshoot the comparator section.

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