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Limitations of the Marzzilli Flyback Driver

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Dr. Shark

Sun May 14 2006, 10:07AM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

I was just messing around for a couple of hours with this circuit, and I found no way of independently controlling output voltage and current. Basically I am looking for a way to maximize output voltage for a given amount of input power. Sadly it seems that due to the design of the circuit, using and input power supply with a different impedance (e.g. 50V 2A => 25Ohm instead of 24V 4A => 6Ohm), does not change the output impedance in the same way, i.e. I don't get twice the voltage at half the current. The only way to drive the resonant tank at a higher voltage is to pump in more current too, i.e. the driver has a constant impedance. Well, this is probably no surprise to anyone, but still I find it annoying, now I have to think about fitting an interruptor to my Marzzilli driver, or abandoning it alltogether. Too bad!

Marko

Sun May 14 2006, 11:05AM

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

I don't know if I understand you correctly, but for more current you can reduce supply choke, but if you go too much you will overvolt transistors.

Dr. Shark

Sun May 14 2006, 12:22PM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

Maybe I was a little unclear. What I want is to get more voltage, but at lower current. I dont think this is possible, as even if the HV output is open circuit, the driver burns a lot of power keeping resonance going in the primary and tank cap. Thus you cannot get the imporessive voltages the Marzzilli is capable of (50kV+), without also drawing insane currents.

Funny you should mention the choke though. As it convertes the input voltage source into a current source, it does probably influence the impedance of the circuit quite a bit. But I still think that in order do get more voltage with less current, one would have to reduce the losses in the resonant tank, which I don't have a clue how to do.

ragnar

Sun May 14 2006, 02:48PM

Registered Member #63 Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425

Joe, I'd have to say you haven't played with the circuit enough.

It's just a matter of finding the right combination of resonant capacitor and series choke. Work with high input voltages and a reasonably small resonant capacitor, a reasonably large choke, and you'll have nice long thin purple arcs...

The great thing about the ZVS is the purple arcs aren't squealy and annoying, they're wispy and flappy instead. :P

Just watch out, as with too much Vin you'll find that after starting an arc the output voltage will promptly double, sending a VERY squealy purple arc down the Ultor, HV cable, across/down the case to the HV return pin, or sometimes it punches through the plastic exit if you're not using proper HV cable (like me).

So, try lots of Vin, perhaps 10+10 turns, and a largeish (1.5" OD, 0.75" ID, 0.5" thick) powdered iron power supply choke, a small resonant capacitor, and you'll have beautiful stable long purple arcs.

Just try! =)

Marko

Sun May 14 2006, 03:10PM

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

powdered iron power supply choke,

You are going to fry a lot of power on lossy powdered iron.

ragnar

Sun May 14 2006, 04:02PM

Registered Member #63 Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425

Just how do you fry power in the series/positive choke? :)

Marko

Sun May 14 2006, 04:44PM

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

Just how do you fry power in the series/positive choke? :)

During transistents, you are going to pulse the choke quite a bit in reality...?
I wonder how it works good witout choke heating up, good if it does, altough I preffer ferrite more.

...

Sun May 14 2006, 05:47PM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

This was discussed on the old forum, it was decided that the ferrite might work, but would saturate too easily--which might or might not adversely affect the output. But powdered iron works fine, I have run at near kw powers at 100khz with a small powdered fe choke and it didn't even get warm

As to making long low current sparks... I haven't really experimented with it too much (I want high voltage AND high current as my home brew transformer can take it). I found that the circuit draws relatively little current until you hit cure saturation, where it begins to draw a few amps. Although the core is not what gets hot, it is the tank cap/primary.

Really the only way to reduce the losses in the tank circuit is to use a mmc of really low esr caps, and use BIG primary wire. At 50v in at 100 khz and a 1ufd tank cap my 4+4 turn 10awg primary gets hot enough to melt the insulation. I haven't accurately checked the current in the circuit, but by looking at the waveform the current in the tank circuit doubles as the core saturates. It goes from a nice sign wave to a distorted triangle wave that has a much higher amplitude.

Good Luck!

Marko

Sun May 14 2006, 05:56PM

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

I always used ferrite, and sometimes beads and never shown any problems.

I don't know how can you saturate an inductor, especially if ferrite bead is used like I do.

Steve Ward

Mon May 15 2006, 08:33AM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

I always used ferrite, and sometimes beads and never shown any problems.

I don't know how can you saturate an inductor, especially if ferrite bead is used like I do.

Powdered iron better for this, it has a higher max flux density, and its only really good use is for this exact kind of application.

Also, you can saturate any magnetic core. Im not sure what you meant by that second statement, since saturation of the inductor is highly possible, and would be a problem.

To further answer the original question, your voltage vs. current output is going to be strongly dependent on operating frequency. The secondary of the flyback doesnt "know" how many primary turns you have. All it "knows" is the frequency and flux intensity through the core. So, by changing your LC, you are changing up the frequency. Smaller C and bigger L means higher Z, so you need greater input voltage to drive it to the same power levels.

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