The ZVS flyback driver, made popular by Vladimiro Mazzilli, is one of the most efficient and powerful flyback drivers used by high voltage hobbyists from all over the world. The main advantages of this driver are simplicity of the circuit itself, very high efficiency and easily obtainable parts. By tuning it properly you are able to get insane power outputs; beefy flyback transformers will sustain a kilowatt for a short period.
The main workhorses of this circuit are the MOSFETs, input power supply and the resonant cap. Two fast, identical MOSFETs have to be used, with maximum drain-to-source voltage approx. 4 times the supply voltage, and drain-to-source resistance while on, below 100 milliohm. The higher this value is, the lower the efficiency you get, because more heat is generated between D-S. So, you’re safe to go with IRF540s (55mOhm Rds) if your supply voltage is no more than 30 volts, or IRFP250s up to 50 volts. My first version of this driver had two IRF540s and a voltage of circa 35V on load, and it worked flawlessly. To be on the safe side, don’t forget some heatsinking (and mica insulator mats if you decide to use a single heatsink, or else you are going to short both drains out).
The second main factor is the resonant capacitor. From my experience, use the physically biggest you got with a >250 volt rating. Do not try to use ceramics, or God forbid electrolytics; you will need some good “MKP” polypropylene (or “MKT” polyester) capacitors in the 100nF-2uF range. The reason why I have mentioned “physically biggest” is because you need to search for those with a high kVAr rating (kilovoltamperes reactive power), or else the capacitor will overheat, inflate and fail – sometimes with an accompanied “bang” 🙂
Use lower capacities, like 220n or 330n to get higher output voltages and lesser currents, and higher capacities like 1uF for some fat hot white arcs at 20-30 kilovolts out.
If you want to be perfectly sure, get at least 2 or 3 of these MKT or MKP caps and wire them up in parallel. This way, they are not going to heat up so much.
As for the input supply, get one which can give at least 24 volts DC and 10 to 16 A. Be sure to take the voltage drop at load into consideration. If your supply is working at mains frequency, you will need some hefty filtration caps; at least 10 to 15 mF is recommended to get rid of that mains hum. Also do not forget to heatsink the rectifier.
Now, this driver is really good when it comes to efficiency and output current in comparison with other topologies, however, it is quite difficult to obtain more than 40 kilovolts out of a single DST flyback without increasing input voltage, thus stressing the components more, or without an external high voltage multiplier.
So, my approach was to drive two identical DST flybacks with their secondaries in series, through an insulation “driver” transformer on the primary. However, phasing must be wired in correct polarity and the secondaries must be built exactly the same, or else it will fry the internal diodes out. This idea has been thus scrapped.
Nevertheless, I have managed to obtain circa 55 kilovolts from a single DST flyback, with 45 volts DC in, with a separate, “driver” transformer method. For this to work, I took some two ferrite cores out of a fried flyback, glued them together and wound 4+4 “primary” plus 4 “secondary” turns, in the same direction, primary was located directly beneath the secondary. With this setup, the 4+4 primary was connected to the driver itself, the 4 turn secondary to some 3, 4 or 5 turns which were wound on the flyback (check the photo or videos for more information, it is pretty self explanatory). This setup alters the resonant frequency; better than a directly wound “4+4” or “3+3” primary on the flyback itself would do. My approach results in lower current and lesser stress on the parts, too. However, you will have to experiment with the tank circuit, number of turns and with various flybacks for this to work the best for your application.
Just a note, this “driver transformer” idea was used in my third X-ray setup.
(at 45 volts in, 30kV out, 2x IRFP250s, 1.5uF resonant cap):
My first ZVS driver, with IRF 540s
Jacob’s ladder with the IRF540 ZVS driver
A second version with IRFP250s and a Soviet AC flyback
2nd version, with the ferrite “driver” transformer at the end
My very first working X-ray powered by the “driver” transformer method
The scariest voltage output I’ve ever got out of flyback transformers…
Here’s an insanity – a twin flyback ZVS driver – driving two DST television flybacks with secondaries in series, with the “driver transformer” method – more than 75 kV output, arcs *ignite* at 15-16 centimeters! 🙂