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Registered Member #567
Joined: Tue Mar 06 2007, 10:55AM
Location: Singapore
Posts: 147
I've built a couple of these for fun, and much fun have I had with them. But there's been something nagging at me, and that's that I cannot figure out how it works. Here's what I think I know:
-The capacitor and primary windings form a tank circuit. -The FETs conduct when the opposite... erm, side of the capacitor is high and switches the as the tank circuit approaches 0v. They are turned off by the fast diodes grounding them as the part of the tank circuit they're connected to goes low.
That's what I've figured out. The inductor between the flyback primary and the power supply is where I'm a bit puzzled, though. My woefully uneducated guess is that when the appropiate FET turns on, current is conducted through the inductor, through the primary winding, and then through the FET. When the FET turns off, the inductor tries to keep the current flowing and so 'forces' more current through the next winding.
How close am I?
EDIT: Forgot something. I hear you can configure one of these as an induction heater, for which I'm assuming the usual LCLR arrangement. But how? I mean, it seems to already have the first L (the big inductor), the C (obvious), second L and R. Do I just expand the primary and chuck my load in there?
Registered Member #540
Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969
One thing that the inductor does it filter some of the ripple and I think it makes it more efficient because some energy gets stored in the core for the next cycle. I'm not entirely sure what it exactly is meant to do though.
The ZVS flyback driver is a weird circuit though because it doesn't seem to work in any simulations (because there is no thermal noise to get the oscillations started).
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
The LC primary circuit keeps going at its resonant frequency and is just "poked" with the transistors with the switching transitions at the right times (primary voltage waveform zero crossings).
The choke is a "current buffer" supplying more-or-less constant current through every cycle. Without it the transistors could not switch a sine wave oscillator from a voltage supply.
Registered Member #142
Joined: Sat Feb 11 2006, 01:19PM
Location:
Posts: 102
Yes, you just use make a center-tapped work coil and drive it with the Mazilli circuit. I lashed up a Mazilli induction heater as an experiment. I just wound a dozen turns of zip cord around a screwdriver and cross-connected the ends to make the electrical equivalent of a center-tapped coil. Simple as that. You don't wind a secondary on an induction heater. The conductive workpiece acts as a shorted single-turn secondary. You can look into the theory of induction heaters at Richie Burnett's highly informative web site. I used the component values recommended in the Mazilli diagram. 0.68 uF cap, 100 uH inductor. Bgmicro.com has 100 uH 13.2 amp inductors for something like $1.50. I read a post from somebody a while back that said the cap actually resonates with the choke, not the work coil. I can believe that the choke determines the frequency, rather than the work coil, because I changed the number of turns on my work coil and it didn't change the frequency, only the current draw. However, the circuit ran at a far higher frequency than a .68 uF and 100 uH tank circuit theoretically would. So YMMV, mine certainly did.
I used the circuit with the 470 ohm gate resistors and I also ran it with totem poles to drive the gates.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
kell wrote ...
I read a post from somebody a while back that said the cap actually resonates with the choke, not the work coil.
That's not true. The series choke should ideally have infinite inductance to supply constant current through every cycle. If it's too small, it can distort the primary waveform and in turn change the resonant frequency, but that's not desirable.
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Mazzilli flyback driver explanation?
