A “ZVS driver” is a very simple circuit that can oscillate a large amount of power with about 90% efficiency. To the right exists a simplified version of the oscillator, known as the “Baxandall converter”.
When power is applied at +V current starts to flow through both sides of the primary and on to the mosfets’ drains. Simultaneously that voltage appears on both of the mosfets’ gates and starts to turn them on. Because no two components are exactly alike one mosfet turns on a little faster than the other one and more current can then flow through that fet. The extra current flowing in that side of the primary robs the gate current from the other fet and starts to turn it off. A condenser forms an LC tank with the primary and the voltage proceeds to rise and fall sinusoidally. If it were not for that capacitor, the current would continue to increase until the transformer’s core saturated and the mosfets exploded.
Imagine that Q1 was the first to turn on. The voltage at point Y will be at near ground while the voltage at Z rises to a peak and falls back down as the LC tank goes through one half cycle. As the voltage at Z passes through zero the gate current to Q1 is removed and the mosfet turns off. The voltage at point Y is now allowed to start rising and Q2 turns on. That mosfet clamps the voltage at Z to ground; something that makes sure Q1 stays off. This same process repeats for Q2 completing the other half cycle, and the oscillator continues cycling. In order to prevent the oscillator from drawing huge peak currents and exploding, L1 is added in series with +V as a choke. The LC impedance is what limits the actual current (the choke just mitigates current spikes).
A keen eye will notice that this oscillator is zero-voltage switching (ZVS), meaning that the mosfets switch when they have zero volts across them. This is good because it allows the mosfets to switch when they are carrying the least power; something that for the most part eliminates the switching losses which generate huge amounts of heat. This means only small heat sinks are needed, even when oscillating 1000 watts!
Being a resonant oscillator the frequency that the mazilli will run at is determined by the inductance of the transformer’s primary coil and the capacitor. You can use the following formula to figure this out:
f = 1 / ( 2π * √[L * C] )
f is the frequency in Hertz
L is the inductance of the primary in Henries
C is the capacitance of the capacitor in Farads
Now in reality mosfets are rather fragile components and if the gates are +/- more than 30V from the source the mosfets will be destroyed, or at least degraded significantly. In order to prevent this scenario from occurring we’ll need gate protection; something easily added with a few extra components. See the schematic to the right.
• The 470 ohm resistors limit the current that charges the gates as too much gate current can cause damage.
• The 10K resistors pull the gates down to ground to prevent latchup; a process in which the mosfet gets stuck on.
• The Zener diodes prevent the gate voltage from exceeding either 12, 15 or 18V depending on the zeners you use.
• The UF4007 diodes pull the gates down to ground when the voltage on the opposite leg of the tank is at ground.
One may notice that instead of charging the gates with the LC tank we are instead using +V to charge them up and we are using the LC tank to discharge them via the ultrafast diodes. This improves the overall performance of the circuit.
The following schematic was made very easy to understand, I hope you like it.
Due to a bit of black magic known as resonant rise the voltage in the LC tank will be about pi*vcc, so you’ll need to make sure your mosfets can withstand this tension. A good rule of thumb is to use mosfets that are rated at 4x the voltage you plan on feeding the oscillator and the IRFP250 or the better IRFP260 is a good mosfet for the task. You’ll need some heatsinks for the mosfets, but they do not need to be large. They must not be put on the same heatsink unless insulating pads are used since the metal back of the mosfet is not electrically insulated (it is connected to the drain). Also be sure to use thermal goop when you attach a heatsink else the thermal transfer will be crap.
The capacitor must be a good one, an MKP, mica or Mylar cap is a good option. Do not use an electrolytic cap, it will without a doubt explode. The two primary windings must also be wound in the same direction or else the oscillator will not function. The oscillator will also fail to function if there is no air gap in the transformer’s core, so always make sure that one exists.
Below is a youtube video of the oscillator powering a flyback transformer at 12, then 24, then 36V. Skip to 0:47 for the 36V if you are impatient.
Problems with the Circuit
The oscillator has one fatal flaw: it likes to explode above 70V. 60V, does well, 70 is meh… 80 KABLOOEY. The problem is above 70V the powers tend to be so high that the diodes responsible for turning off the gates fail to fully do so, and the oscillation stops with one mosfet left on. That’s essentially a short circuit so the mosfet responds with suicide. To anyone who is reading this article, I propose to you a challenge: fix this problem. First one to do so will receive a present. I don’t know what but it’ll be something. Neon John attempted a fix, but it’s still pretty unreliable…
UPDATE: I partially solved the problem by placing a 0.5 ohm wirewound resistor in series with the filter inductor. Now things don’t explode if the load inductance plummets. Still asplodes when VCC>70V though. ∎