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Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
I had very similar problems, the thing just doesn't work reliably off mains, and adding gate drivers just further complicates things. Really I think that for higher power levels you are better off dumping the self resonant design and try to get a PLL working, or better yet just go for a halfbridge.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
... wrote ...
I had very similar problems, the thing just doesn't work reliably off mains, and adding gate drivers just further complicates things. Really I think that for higher power levels you are better off dumping the self resonant design and try to get a PLL working, or better yet just go for a halfbridge.
PLL for what? I think we were talking about HV transformer driver, and I'm actually going away from half bridge and any hard switched designs, as the switching losses are too big and I exploded any halfbridge at the power level I want to run. So I'm investigating all and any soft-switching drivers that could be used to drive the ferrite transformer and tolerate both open load and shorted load conditions.
EDIT: what about running it from fixed frequency? (probably not)
Registered Member #95
Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308
Given the problems everyone seems to have with their royer oscillator deviations why not use a PWM chip to keep the frequency under control? You could still achieve ZVS by using the same output stage with a resonant capacitor, and just tune it manually or use PLL feedback like Peter suggested.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Uzzors wrote ...
Given the problems everyone seems to have with their royer oscillator deviations why not use a PWM chip to keep the frequency under control? You could still achieve ZVS by using the same output stage with a resonant capacitor, and just tune it manually or use PLL feedback like Peter suggested.
Yes but the problem is that the tank resonant frequency is diferent with open and shorted loads.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
The problem is that the 'zvs' inverter is 'supposed' to operate in near-sine mode, with some primary "Q" From memory the MINIMUM "Q" is theoretically 2.3, try to use Q>=5 for reasonable zvs operation, this means that the zvs inverter should never work into a short-circuit. In this case Q = (reactive power in resonant circuit)/(output power) If you don't maintain the Q then the invertor will not really be operating in zvs mode.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Today I tried "low voltage" (115VDC) version with 500V FETs and gate drivers. It starts to oscillate as I apply power but as soon as I make the first spark with the transformer, the circuit bursts into MHz oscillations and there is no output from the transformer.
So the circuit remains in 'Mhz oscillations' indefinitely after that?
I really have no clue what could be going on, not enough data, knowledge, intelligence on my side.
The problem is that the 'zvs' inverter is 'supposed' to operate in near-sine mode, with some primary "Q" From memory the MINIMUM "Q" is theoretically 2.3, try to use Q>=5 for reasonable zvs operation, this means that the zvs inverter should never work into a short-circuit. In this case Q = (reactive power in resonant circuit)/(output power) If you don't maintain the Q then the invertor will not really be operating in zvs mode.
I don't know what else to recommend...
Jan, what airgap do you have on your transformer? T think the core should be significantly gapped to give the circuit enough Q in worst case.
But, conner also seemed to have same thing happen on induction heater so I don't think that is the problem.
My biggest question here is, what could anywhere in the circuit cause it to oscillate in Mhz range? I really don't see any parasitic tank circuits that could do that.
Could the circuit just be jumping to Fres of transformer's secondary? But why it stays there?
I'm not sure why would faster gate drive provoke it either. I've made a mosfet converter of this type with active pullups and it worked well at low power levels.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Marko, I really believe that they are oscillations between the transistors' internal inductance and the tank caps (and their ESL). The fact that the oscillations are so fast that I can't really capture them on my 5MHz scope only supports that, the power supply for the gate drivers actually drops to ~1/2 voltage drawing about 0.5-1A.
This also explains why faster gate drive makes it worse, as the transitions are faster and provoke more ringing which eventually has enough amplitude to ring below the turn-on threshold of the gate drivers, and bad thing happens.
Registered Member #95
Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308
jmartis wrote ...
Yes but the problem is that the tank resonant frequency is diferent with open and shorted loads.
ZVS is most beneficial when switching some current, so tune it for ZVS with an arc. I'm not sure what happens without a load then, but I don't imagine the losses will be any worse than when running the flyback open circuit while hard switching. Is there really that much to gain from constant ZVS?
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
If you short-circuit the output of a Royer oscillator this short-circuit is reflected back to the primary side of the transformer and it kills off most of the inductance in the parallel resonant tank circuit. The only inductance remaining will be the "un-coupled" leakage inductance of the transformer referred to the primary side which will resonate with the primary cap, so I would expect the operating frequency to go right up.
If the transformer has really tight coupling then a short-circuit on the output might even decrease the Q of the circuit so much that is stops oscillating compeletely. See Sulaiman's post above. This is more likely to happen at high power levels than low, because a thicker high-current arc will present a lower load impedance to the output of the royer oscillator.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Marko, I really believe that they are oscillations between the transistors' internal inductance and the tank caps (and their ESL). The fact that the oscillations are so fast that I can't really capture them on my 5MHz scope only supports that, the power supply for the gate drivers actually drops to ~1/2 voltage drawing about 0.5-1A.
That inductance is absolutely tiny... I really don't see how could it have any significant Q on that much capacitance on it, and oscillate at 5Mhz. I don't see how is feedback loop necessary for continuous oscillation established.
Maybe you meant for gate circuit oscillation? Gate voltage going under threshold like that is very implausible unless your driver placement was totally crazy.
Can you clarify how exactly do you think this oscillation is forming?
Anyone else can give some insight?
ZVS is most beneficial when switching some current, so tune it for ZVS with an arc. I'm not sure what happens without a load then, but I don't imagine the losses will be any worse than when running the flyback open circuit while hard switching. Is there really that much to gain from constant ZVS?
Your resonating inductance will drop proportionally to load; you can think of it as, that with un-loaded transformer you have Lleakage series with Lmagnetizing, and with shorted transformer only Lleakage.
I'm not sure if this equation is correct:
XLp = wL1 - (w^3*M^2/Rload^2*w^2*L2^2),
M -mutual inductance, L1, L2 - magnetizing inductances, w ang. frequency;;
bleh,
If your core is not gapped enough your leakage inductance will be very small and circuit will probably crash if you short it's output or oscillate at very high frequency.
Maybe you could also use capacitive ballast in series with output like they do in those little royer CCFL inverters - I don't know what good does that do vs. simply gapping the core a bit more?
Geordieboy wrote ... If the transformer has really tight coupling then a short-circuit on the output might even decrease the Q of the circuit so much that is stops oscillating compeletely. See Sulaiman's post above. This is more likely to happen at high power levels than low, because a thicker high-current arc will present a lower load impedance to the output of the royer oscillator.
Hi Richie,
So you believe it's all nothing but simply a matter of Q getting too low?
Can any of those ''parasitic oscillations'' guys keep mentioning happen at all?
It is interesting that Steve reported same problem on a royer used as an induction heater, which had no core at all! I don't believe it would be too easy to get the Q too low... there, with so low coupling?
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"zvs" flyback driver on mains?
