The Mazilli ZVS flyback driver is well-known throughout the high voltage community for its simplicity and ability to deliver 20-50 kV at high currents for a flyback transformer.
About one and a half year ago, Marko from 4hv.org gave the circuit a comeback with it converted to a simple induction heater.
I build the circuit as a proof of concept model in order to show it to my father that would like to start doing black smith work on small knifes.
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The MOSFETs used need a voltage rating about 4 times higher than the supply voltage and a on-resistance below 150 mΩ. In ZVS operation the switches see a voltage that is π times input voltage, so 4 times rating of input voltage leaves some head room for playing it safe.
If supply voltage gets over 40 VDC, consider using resistors between 470R-800R for the gates. Supply voltage needs to be minimum 12 VDC, lower than 470R gate resistors can be used in that case, if supply voltage dips under 10 VDC, there is a risk of MOSFETs failing from overheating by working only in the linear region or short circuit if one of them stops switching.
Supply voltage should not exceed 60 VDC, as this is very close to 200 VDC across the MOSFET. The internal construction of MOSFETs with a higher voltage rating makes them unsuitable for use in a self oscillating circuit like this Royer oscillator.
A MMC is made from 27 capacitors to avoid excessive heating in a single capacitor. The capacitors will still heat as massive current flows between the tank and work coil. To get a good result, a large tank capacitance is needed, if a capacitance lower than 4 uF is used, results might be disappointing. It is strongly advised to use a capacitor with made from polypropylene (MKP) or similar that can handle large RMS currents, it might even be necessary to water cool the capacitor too. A MMC as the one I use here can only withstand short run times and will even then heat up.
The value of the inductors are advised to be between 45 to 200 uH and depending on core material the number of turns varies a lot, use a LCR meter to check the values.
Water cooling of the work coil is a must! Even at just small runs with moderate power input as the ones I have conducted, the work coil would take damage from heat.
|Voltage supply||35 VDC smoothed with 40000 uF|
|MMC||3 uF from 9 in parallel strings of 3x 2 275 VAC MKP X2 capacitors in series.|
|Power consumption||650 Watt.|
|Best result||Between red hot and white hot M10x20mm bolt|
17th January 2013
I succeeded in putting the entire setup together from parts I have salvaged from old equipment, only the MOSFETs was bought new and used before.
The transformer takes 230 VAC in for 32 VAC out, properly around 700 VA transformer estimated from the core size. It is rectified with a 25 A bridge rectifier smoothed with 40000 uF capacitance from four electrolytic capacitors 70 VDC / 10000 uF each.
The inductors are made from ferrite transformer cores from old power supplies. 14 turns of 1,5 mm^2 gave approximately 130 uH inductance.
Two IRFP250N MOSFETs mounted on each their fairly small heat sink, but big enough for the circuit to run for a couple of minutes and only get a little above hand warm. The heat sinks are glued together with a piece of acrylic plastic in-between to insure electrical isolation between the two heat sinks.
The work coil is made from 5 turns of 8 mm copper tubing, giving approximately 0,477 uH. The MMC consists of 9 parallel strings of 3 in series Rifa 1 uF / 275 VAC MKP X2 capacitors for 3 uF. This gives a resonant frequency calculated to about 133 kHz.
Measurements during a run of heating a M10x20 mm bolt at 33 VDC in, 260 VAC at 2.5 A input into transformer.
Resonant frequency is measured to 106 kHz. The measured frequency is different from the calculated as the work piece will influence on the coils electromagnetic properties.
In the following oscilloscope screenshot:
Yellow: Inverter current, here measured to 10 Ampere.
Blue: Inverter voltage, here measured to 100 Volt.
In the following oscilloscope screenshot:
Yellow: Tank current, here measured to 200 Ampere.
Blue: Tank voltage, here measured to 100 Volt.
Three pieces of metal heated to what is possible with input voltage of 35 VDC.
First picture shows a M4x10 mm screw heated red hot at 260 Watt input power, second and third picture shows a M10x20 mm bolt heated red hot at 650 Watt input power.
A good and reliable oscillator as long as supply voltage is kept within safe area of operation for the MOSFETs and only short run times are used unless there is used good components and water cooling on work coil, MOSFETs and capacitors.
Further improvements in use as a heater / melter would be a higher supply voltage.