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ZVS Circuit Theory

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Xray

Fri Dec 17 2010, 07:58PM

Registered Member #3429 Joined: Sun Nov 21 2010, 02:04AM
Location: Minnesota, USA
Posts: 288

ZVS appears to be a popular circuit for driving Flyback transformers, but can someone please explain how ZVS is any different than a standard multivibrator circuit? I searched on the well known Mazilli website as well as on the HV Wiki, which explains very briefly that ZVS (Zero Voltage Switching) works in such a way that the transistors switch ON during the time when there is zero voltage across them. Why is that important? I understand why ZVS is important in circuits that drive a Triac (for example) because it's best for the Triac to turn ON and OFF when the AC sinewave is crossing zero volts. That is important especially when switching an inductive load. But how does ZVS play a role in driving a Flyback transformer?

Thanks!

Wolfram

Sat Dec 18 2010, 01:01AM

Registered Member #33 Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971

Zero voltage switching is advantageous in that you switch the transistors when the voltage across them is zero. That way you avoid the losses that would otherwise occur when the energy stored in the parasitic drain-source capacitance would be discharged through the transistor. It can also reduce EMI. An other advantage is that the gate of the transistor will be easier to switch, as the miller effect is eliminated, but that's not really significant here.

The advantages of the "Mazzilli" style flyback driver is that it can push a lot of power efficiently. It uses a resonant primary circuit, so the transistors don't have to deal with the reactive power, and the losses are therefore minimized.

Xray

Sat Dec 18 2010, 03:01AM

Registered Member #3429 Joined: Sun Nov 21 2010, 02:04AM
Location: Minnesota, USA
Posts: 288

Anders M. wrote ...

Zero voltage switching is advantageous in that you switch the transistors when the voltage across them is zero. That way you avoid the losses that would otherwise occur when the energy stored in the parasitic drain-source capacitance would be discharged through the transistor. It can also reduce EMI. An other advantage is that the gate of the transistor will be easier to switch, as the miller effect is eliminated, but that's not really significant here.

The advantages of the "Mazzilli" style flyback driver is that it can push a lot of power efficiently. It uses a resonant primary circuit, so the transistors don't have to deal with the reactive power, and the losses are therefore minimized.

So, if I understand you correctly, you are describing two different properties of the ZVS driver. One is that the circuit is more efficient than a standard multivibrator circuit, and the other, which probably can also be done with a standard multivibrator, is the resonant primary circuit of the flyback transformer. Together, they form a design that was developed by Mazilli.

Thanks Anders!

Wolfram

Sat Dec 18 2010, 11:28AM

Registered Member #33 Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971

Yeah, I think that is correct. I'm a bit unsure about this myself, but this is how I understand it to be. Technically, this sort of circuit is called a self-oscillating current fed parallel resonant push pull converter, and it has a few other advantages in addition to ZVS. The fact that it is current fed means that it is inherently current limited, and therefore well suited to driving capacitive loads like voltage multipliers.

There are many different ways of accomplishing these goals, but the Mazzilli driver is by far the simplest way to do it.

Steve Conner

Sat Dec 18 2010, 07:42PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

The ZVS is a "multivibrator" if you want to call it that. The similarity with the old two-transistor flip-flop circuit from your Radio Shack 1001 Electronic Projects box is fairly obvious.

The only differences are:

Power MOSFETs for extreme current output. The multivibrator is a small signal circuit.

Operating frequency is determined by the resonant tank circuit. It's not a relaxation oscillator like the astable multivibrator.

The circuit is fed through an inductor, so the current stays constant in the short term, giving it different properties to what you intuitively expect.

ZVS and Mazzilli driver are names coined by us, the 4hv community. They're not generally recognised. If you showed the ZVS to a random EE, he would say that it was a version of the Royer oscillator, which was invented in the 1950s, as a simple power inverter using the new germanium power transistors.

Vladimiro Mazzilli, the ZVS inventor, referred to his own circuit as a Royer oscillator. His "Royer" was quite an impressive one made with 1200V IGBTs and driving an air-cored Tesla transformer.

The original Royer was voltage-fed and non-resonant, the frequency was set by magnetic saturation of the transformer core. But resonant current-fed versions were also well-known in the literature.

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