[ZVS] What's dirting my sine wave ? / General Science and Electronics / Forums

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[ZVS] What's dirting my sine wave ?

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LAN_403

Noxxx

Fri Aug 28 2009, 01:29PM

Registered Member #2314 Joined: Sun Aug 23 2009, 08:01PM
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Posts: 1

Hey there,
just a quick question... take a look at this oscillogram

The sine wave is coming out from a ZVS inverter (the Mazzilli's one... here's the schematic in case someone doesnt know/remember it), and the signal is picked up on the mosftes' drains while the circuit is "idling" at 5Vdc.

What's that noise (<= here's isolated) on my beautiful sine wave!?!?

Is that the soft-switching noise or something else?

Here u can see a few more shots

GeordieBoy

Fri Aug 28 2009, 02:48PM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

It's either stray wiring inductance ringing with the MOSFET's output capacitance when the device turn's off...

_OR_

It may be the start of HF oscillation (instability) in the circuit if the MOSFETs are lingering too long in the linear region where HF gain is high.

There's quite a lot of "un-clamped" inductance between your parallel resonant tank circuit and the actual MOSFETs. This is likely the cause of the problem. You could try shortening the length of these leads, or alternatievely moving one of the tank capacitors so that it is connected directly between the MOSFET drain terminals right at the MOSFETs! I wouldn't worry about the ringing too much, but that is the first action I would take if I wanted to eliminate it. Also make sure you have good DC supply rail decoupling on the PCB!

Edit: You've actually answered the question yourself with the collection of scope traces you posted. The voltage measured across the tank capacitors shows no ringing, but the voltage at the MOSFET drain shows ringing! You even showed that the voltage across this connecting wire only displays the ringing, so it is obviously due to the stray inductance of this wiring resonating with the MOSFET drain capacitance.

-Richie,

kell

Wed Sept 02 2009, 03:39AM

Registered Member #142 Joined: Sat Feb 11 2006, 01:19PM
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Posts: 102

Mazilli switches at the zero crossing of the floating ac signal from the tank (floating if your choke is big enough, that is). For perfectly clean switching you would want the mosfets, in theory, to switch exactly at the zero crossing -- in practice, just a few degrees before the zero crossing, but by its nature the Mazilli can't do that. The switching lags the zero crossing just a bit, for obvious reasons. Switching after the zero crossing causes the glitch you see. If you wanted to eliminate it completely you would need some kind of phase angle detection or something, and you would end up with a much more complicated circuit. Just don't worry about it, and minimize the effects as much as possible using good circuit layout.

Steve Conner

Wed Sept 02 2009, 09:25AM

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

Like this?

As I increased the supply voltage, that ringing got less and less damped, then turned into full-blown oscillation, and ultimately transistor shrapnel.

Everyone posting here is right in one way or another: it's ringing caused by non-zero current switching, and the stray inductance of the wiring and MOSFET input capacitance. But the ringing circuit is also hooked up to an amplifier: the MOSFET when it's passing through its linear region, with feedback through the Miller capacitance and so on. The FET's transconductance increases with the drain current, and so the loop gain of this unwanted oscillator also increases as you turn up the wick, until it gets over unity and bursts into song. (and flames)

This is the main problem with the Mazilli circuit. Some cleverly placed RC snubbers, as seen in push-pull MOSFET RF amps, would probably cure it. But I've never figured out just where to put them myself.

Myke

Wed Sept 02 2009, 10:06PM

Registered Member #540 Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969

I have heard somewhere that ferrite beads on the gate should help.
I'm wondering if the forward voltage drop of the diodes between the drain and the gate of the other MOSFET determines when it should switch. If it does, wouldn't reducing the drop make it switch closer to zero crossing? You could try to find some 200V schotty diodes and put two in series. I haven't seen any around but I read that they are the highest voltage they are made in. If it doesn't exist, I guess you could use 4 100V ones or a UF4004 (the UF4007 is slower and has more of a forward voltage drop).

teravolt

Thu Sept 03 2009, 02:58AM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

maby a greater resistance in the gate would help

kell

Thu Sept 03 2009, 08:24PM

Registered Member #142 Joined: Sat Feb 11 2006, 01:19PM
Location:
Posts: 102

There's a consensus that the amount of time spent in the linear region is a problem.

Your induction heater probably runs in the 100 kHz ballpark. It could be significantly more.

The drive for the gates is just power resistors. You can see where I'm going. The gate drive is very weak for that kind of frequency, just way too weak.

The time I built an induction heater using the mazilli, I recognized that the drive was a big problem. I did away with the crummy resistor drive and used a bipolar totem pole. Standard totem pole with a npn on top and pnp below, bases connected, a resistor from Vcc to drive the upper transistor. Just connect that resistor/base node through a diode to the drain of the opposing mosfet, same as the connection for the power resistor in the classic Mazilli.
You still won't get a perfect square wave, but if you scope it, you will find that the totem pole drive makes a world of difference in the gate drive waveform.

Steve Conner

Fri Sept 04 2009, 10:06AM

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

Well, yes. If you read on in that thread, I changed to a halfbridge driven by a PLL and gate driver ICs, and went from warming nuts a little, to melting them.

But this thread is specifically about how to improve the ZVS, not what to replace it with.

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