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"zvs" flyback driver on mains?

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Dr. Dark Current

Tue Feb 26 2008, 08:08PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Marko wrote ...

Can you clarify how exactly do you think this oscillation is forming?

yeah, at least I can try to.. BTW the 5MHz figure is the "rating" of my scope, I believe the frequency was way higher.

I think it goes like this: The drain voltage of one FET starts rising and once it crosses the other gate driver's threshold, it's supposed to keep the other FET on. However there is a very high frequency ringdown of unknown cause, superimposed on the rising drain voltage. If the amplitude of this ringing noise is bigger than the input hysteresis of the gate drivers (in my case probably below 100mV because of the PNP transistor amplifying the signal), it causes the other gate driver and FET to turn off again, initiating the very high frequency oscillation.

One thing to note is that in the case of the VHF oscillation, the power circuit does not draw any "fault" current (just slightly higher than with open transformer) and there is no output from the xfmr; the FETs survive it most of the time.

Sulaiman

Tue Feb 26 2008, 11:12PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

If you make a zvs inverter with separate supplies for gates and power, like this

and connect just V2, the system will oscillate !
(this is why I say you should connect the gate supply before the main supply)
Not at full power - but it oscillates

So not only do we have the parasitic oscillations when Drain1-source1-source2-drain2 inductance resonates with the capacitor, there is a second more subtle 'parasitic' oscillation mode.

Dr. Dark Current

Sat Mar 08 2008, 12:17PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

So has anyone any idea how to get rid of the parasitic oscillation? Right now I've made one additional winding on the core and I'm using it for voltage feedback, to an opamp (switch at zero cross). I think the additional-winding-voltage-feedback is the way to go - but the only problem is that I can't get it to oscillate well.
Any ideas?

Marko

Sat Mar 08 2008, 07:18PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

and connect just V2, the system will oscillate !
(this is why I say you should connect the gate supply before the main supply)
Not at full power - but it oscillates

So not only do we have the parasitic oscillations when Drain1-source1-source2-drain2 inductance resonates with the capacitor, there is a second more subtle 'parasitic' oscillation mode.

Yes, that is normal, and circuit works same as it does usually except it is powered only through gate resistors.
Oscillation happens at frequency of the LC tank, it will only jump in amplitude once drain power is applied.

According to Richie, the parasitic oscillation forms when circuit is overloaded and primary inductance drops too low, making mosfets effectively parallel.

Also, too low inductance may simply reduce Q of the circuit too low for it to oscillate.

A proper solution seems to be capacitive ballast on the output, as it is done with CCFL inverters.

Low value HV cap (few tens to few hundred pF, depending on frequency -> wanted impedance) should be used in series with output, and relying on leakage inductance to the transformer to limit the current is apparently a bad way -

One royer CCFL inverter I had here kept blowing up because it's ballast capacitor was shorted, despite it lit the tube well even in that state.

Marko

Proud Mary

Fri Mar 28 2008, 12:51PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Plasmaddict wrote ...

So has anyone any idea how to get rid of the parasitic oscillation? Right now I've made one additional winding on the core and I'm using it for voltage feedback, to an opamp (switch at zero cross). I think the additional-winding-voltage-feedback is the way to go - but the only problem is that I can't get it to oscillate well.
Any ideas?

You could try putting some anti-parasitic chokes on the gates - chokes which have low Z at the desired frequency, and a very much higher Z in the parasitic range. Make sure the two chokes are of slightly different inductances/self-resonance so they do not set up an oscillatory circuit all of their own!

Steve Conner

Fri Mar 28 2008, 01:01PM

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

I can't see any reason to use anything other than a SLR converter at this high power level. We already know the SLR works- Steve Ward has got some great arcs from SLR-driven ferrite transformers- and just needs a fixed frequency oscillator with 50% duty. A fullbridge of TO-247 IGBTs should handle >3kW in the SLR, even those crappy BUP314Ds that kick around Eastern Europe

I believe the parasitics happen in a single-ended mode. The resonant circuit is between gate and source, and it's powered by feedback through the Miller capacitance. (The same feedback mechanism as Sulaiman observed, just a different resonant mode.)

The transconductance of vertical MOSFETs is extremely high to start with, and increases even more with drain current, so as you crank your variac up, you get more gain, more feedback, until the loop gain exceeds 1 and it bursts into song.

I'd bet the cure would involve ferrite beads on gate leads, gate stopper resistors, RC snubbers from drain to ground, etc. IGBTs would also suffer less than MOSFETs because they're slower. The original Mazzilli oscillator did in fact use BUP314s (they are 1200V) running off rectified 220V mains, and powered a SSTC with good results.

But that doesn't solve the fundamental problem of running a ZVS off mains: that the choice of 1200V devices is poor, and they're slower and more expensive than the 600V ones which are driven by development for consumer SMPS. It was much easier for me to go to a PLL driven halfbridge for my induction heater than try to debug the ZVS.

Dr. Dark Current

Fri Mar 28 2008, 01:37PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

I actually got this circuit going right, the problem was in the feedback path. Now i use a voltage sense winding on the transformer and use this to switch the transistors on zero crossings. It works well and no "parasitic" oscillations.

The only thing I'd want to improve is that the delay in the fb path causes that the transistors switch a bit too late. Maybe PLL wil help here? I have no experience with it so I have no idea how would I set it up.

As to the 1200V devices - well you can use a current fed halfbridge (that operates with the same principle) and 600V devices can be used at mains 325V voltage.

jpsmith123

Sat Mar 29 2008, 02:03AM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
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Posts: 843

Plasmaddict, I'm curious as to how you built your transformer? Did you wind the coil yourself? Did you encapsulate it with anything?

(BTW somewhere I saw a schematic from "Information Unlimited" where they drove what is supposedly a 10 kv 60 ma "flyback" transformer from the AC mains, selectable for either 120 or 240 vac, with a half-bridge using IRFP450s and an IR2153 driver).

Dr. Dark Current

Sat Mar 29 2008, 07:39AM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

jpsmith123 wrote ...

Plasmaddict, I'm curious as to how you built your transformer? Did you wind the coil yourself? Did you encapsulate it with anything?

(BTW somewhere I saw a schematic from "Information Unlimited" where they drove what is supposedly a 10 kv 60 ma "flyback" transformer from the AC mains, selectable for either 120 or 240 vac, with a half-bridge using IRFP450s and an IR2153 driver).

I just used two quite large cores from old flyback transformers and two identical HV coils which I had no idea where they come from, in series. I was pushing around 600Watts of real power, and the little IRF740's got just warm.

However I noticed one odd thing: If I use larger resonant cap, the arcs are thick but shorter and with smaller caps they are longer and thinner! How can this be? The primary voltage is still the same so how can higher frequency draw bigger arcs?? I don't get it...

Arcstarter

Sat Mar 29 2008, 06:02PM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

I experience the same thing with my 555 driving irfp460.I get 2 inch sparks at audible frequencies but 1 inch powerful arcs at above audible range. I haven't as much experience as you do but i figure it is because there are so many sparks at a high frequency that they fuse together. And i base this upon the theory of absolutely nothin'.:)

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