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Registered Member #1694
Joined: Sat Sept 13 2008, 09:13AM
Location: Australia
Posts: 108
jonny5 wrote ...
With the IRFP450's, I wonder if the fets Rds might be causing a problem. The Vds of the fet that's ON is essentially I*Rds. An IRFP450 is spec'd at 0.33 ohms at 25C, about 0.5 ohms at 70C, and 0.66 ohms at 110C (approximations with 10Vgs). Also, the fet's threshold voltage begins at about 2V. What's the scenario? Let's say one of the fets is conducting 3A at 70C. With a 0.5 ohm rds, its drain voltage is 1.5V. The Mazzilli driver uses the cross-coupled diodes between the gate/drain to turn off the opposite fet, so the gate voltage of the fet that's off is 1.5V + diode drop (0.6V). That's 2.1V...barely threshold, but if it is then the fet that's off...actually isn't (just barely turned on). Problem is, this might be thermal feedback. (Off) Fet gets a little hotter (from barely turning on during off time) and, next 1/2 cycle when it's ON, its Rds is a little higher, increasing the opposite (Off-sorta ON)fet's Vgs...etc. Do you think this is even plausible? I've never seen it happen, just brain-stormin. Anyway, if you happen to have some lower Rds fets around, maybe they would be worth a try? Good luck!
That does sound like a very interesting theory!! I think you are right. With the lower voltage there was less heating which means Rds would of been lower and therefor the circuit would of been functioning normally for longer, which explains the high voltage on the caps I was charging. I've also noticed a definite relationship between mosfet temperature and the loudness/pitch of the squeel. Unfortunately I don't have any other mosfets at the moment and it'll be at least a week I get some, but I'll be sure to post back with results.
Also the springs in the battery pack annealed in their compressed state from over heating. *sigh* At least I've got something to do while I wait for more fets to arrive.
Registered Member #1819
Joined: Thu Nov 20 2008, 04:05PM
Location:
Posts: 137
Uzzors wrote ...
The problem here is most likely magnetizing current being too high. I never took note of the air gap size I used, but I'm sure it was smaller than 2mm. Try reducing the gap spacing to less than 1mm and see if that helps. IIRC I only used one or two of the spacers that come with the cores found in flybacks.
jonny5 wrote ...
Is the transformer core gapped? The Mazzilli utilizes a parallel LC tank circuit...your C is the resonant capacitor, and the L is the center tapped primary. The catch is if your transformer were to be perfectly coupled (no air gap, K=1) then your transformer couldn't store any energy. No energy storage, no resonance.
The only thing that I disagree with is inductors requiring an airgap to store energy. Some (or a lot) is indeed stored in a gap, but there's also energy stored in the magnetic field around the core. After all, an inductor's potential energy is 0.5*L*I^2. Resonance can still occur without an air gap.
You may be able to use an inductor without an air gap, but I would never recommend it. First of all, you won't get much leakage inductance, which is what stores the energy in the proper manner. Also, you could saturate the core easily if it is not driven right. Finally, the inductance that you can get is very hard to calculate; therefore, it is hard to predict the operating frequency. I've tried experimenting with inductors wound on ungapped ferrite cores, and the results were either catastrophic (explosions due to overcurrent) or unpredictable.
Registered Member #95
Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308
Forgive my bluntness, but that doesn't make much sense. Calculating inductance of an ungapped core is easier than a gapped core for one thing, and an ungapped core has greater inductance than a gapped core, so it's impedance is higher meaning less current. An air gap does improve the current capacity of an inductor though, so your saturation argument is true. You are talking about the main transformer though, right? Not the filter inductor in the circuit, which acts as a current source? Just so everything is on the clear, the main power transformer only works as a transformer in the Mazzilli ZVS circuit. There's no energy storage going on here like in a flyback driver. The reason an air gap is used is to tune the operating frequency without altering the primary turns, and help prevent saturation. When I said inductor above in I meant in general and not applied specifically to this problem, to clear up misconceptions about resonance.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
To be pedantic, the transformer in a ZVS does store energy. It couldn't resonate otherwise. The more inductive energy stored, the higher the loaded Q of the oscillator will be. This is important, because if the loaded Q gets too low, the ZVS will stop oscillating and blow itself up.
In practice this means that a ZVS works much better with an airgap in its transformer core.
Yellow and white toroids are iron powder, not ferrite. Great for the ZVS' DC link choke, lousy for its transformer core.
Registered Member #1819
Joined: Thu Nov 20 2008, 04:05PM
Location:
Posts: 137
Uzzors wrote ...
Calculating inductance of an ungapped core is easier than a gapped core for one thing, and an ungapped core has greater inductance than a gapped core, so it's impedance is higher meaning less current.
Calculating the effective permeability is what is very difficult for discrete air-gapped cores, since it includes things such as gap area, area correction factor, etc. Once the effective permeability is known, along with other characteristics such as B-H curves, the turns for a given inductance can be easily calculated.
What kind of "ungapped" cores are you talking about? Iron powder cores have their air gap distributed around the core, and inductance for a given number of turns is very easy to calculate. This is why I've never considered using a discrete-gapped ferrite core.
As for use of iron powder toroids in HF energy storage transformers, I'm assuming that the #26 material would probably cause too much core loss at standard ZVS frequencies, since it is not a good HF material. A good material for this application would be the #2 material, which has very low HF core loss, very flat B-H curves over a wide range of DC bias, and is also cheap and readily available.
Registered Member #95
Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308
Whoops, I've walked myself into a mess now, haven't I? Killah573, discrete-gapped ferrite cores were exactly what I was thinking about, in the context of the main transformer. I'm sorry if I caused any misconceptions myself trying to clear up anyone else's.
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
Okay, let me clear up the matter about inductance variation and gapping of ferrite core sets. I have designed many ferrite transformers for manufacture in the tens of thousands for commercial SMPS use, so have some practical experience in this area.
The production spread on magnetising inductance of a ferrite transformer without an air-gap is large. The absence of a deliberate air-gap means that the resulting inductance depends heavily on the permeability of the core material (which has a tolerance) and on the precise alignment of the core halves. In practice there can easily be grease or a few particles of dirt on the mating surfaces of the core halves which dramatically decreases the magnetising inductance. Generally the magnetising inductance will not be a specified parameter for an un-gapped ferrite transformer because of this production spread. When assembling an un-gapped transformer, keep the mating surfaces absolutely clean and free from dust. Also never glue together pieces of broken ferrite cores for use in an un-gapped transformer. For background information, un-gapped transformers are usually used in forward-type converters like the push-pull, half-bridge and full-bridge. In a well designed supply the magnetising current is insignificant compared to the load current so the actual magnetising inductance is of relatively little concern provided it is arbitrarily "high".
However, For the gapped core transformer the situation is quite different...
The production spread on magnetising inductance with a gapped core is much smaller. This is because the deliberately introduced air gap dominates the magnetic path. This makes the resulting inductance values far less sensitive to the permeability of the core and less dependent on the precise alignment of the core halves. Transformers with gapped ferrite cores are typically used in flyback converters where the magnetising inductance most definitely is specified and the value can be quite critical! The gapping of the core not only increases the energy storage of the component, but it acts to stabilise the inductance and reduce production spread. Fortunately assembly can be a little more casual without incuring too much inductance variation. Broken flyback core pieces can sometimes even be glued back together provided the amount of adhesive is kept to a minimum because there is already a reasonably large air gap in the magnetic path.
Finally, gapping a ferrite cored transformer generally does not alter the coupling significantly. As for the leakage inductance, this figure will typically fall initially as an air gap is introduced because of the overall drop in permeability. Introducing an air gap will cause the inductance of all windings to fall, and therefore the leakage inductance (being a percentage of total winding inductance) will also FALL. Any notion that introducing an air gap increases leakage inductance only holds true if the windings are far apart on different core halves, and the air gap is large between them! In general, coupling coefficient is mostly a function of the geometry of the two windings and their physical proximity to each other. Leakage inductance is a function of the total winding inductance and the coupling coefficient.
Registered Member #1819
Joined: Thu Nov 20 2008, 04:05PM
Location:
Posts: 137
GeordieBoy wrote ...
Finally, gapping a ferrite cored transformer generally does not alter the coupling significantly. As for the leakage inductance, this figure will typically fall initially as an air gap is introduced because of the overall drop in permeability. Introducing an air gap will cause the inductance of all windings to fall, and therefore the leakage inductance (being a percentage of total winding inductance) will also FALL. Any notion that introducing an air gap increases leakage inductance only holds true if the windings are far apart on different core halves, and the air gap is large between them! In general, coupling coefficient is mostly a function of the geometry of the two windings and their physical proximity to each other. Leakage inductance is a function of the total winding inductance and the coupling coefficient.
Lots of useful information here; in fact I didn't even know about cleaning the two core halves. I think that some of the confusion is resulting from semantics over the term "leakage inductance". I read in a transformer design guide that leakage inductance is inductance resulting from stray flux, meaning flux not coupled into the transformers' magnetic circuit. But when cores are gapped, the stray flux is an "intentional" part of the magnetic circuit. It seems that this is the definition that you are referring to. However, it also seems that there is another definition of leakage inductance, and I think I have seen it used on this forum. When used in this matter it is defined as any flux (meaning ANY flux) not directly in the core material. This would mean flux in an air gap would be counted as leakage inductance.
Sorry if this post is also confusing in itself. I hope this may help clear up the reasons behind any misconceptions formed.
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
Leakage inductance is the result of those flux lines originating from one winding that do not couple to the other winding. Due to the impedance transformation property of a transformer, the leakage inductance can be referred to either the primary winding or the secondary winding for the purpose of analysis.
If you take a practical real-life transformer, it's leakage inductance can be thought of as an additional discrete inductor in series with one of the windings of an otherwise perfect (ideal) transformer. Any decent textbook showing the transformer model equivalent circuit should have things like leakage inductance, magnetising inductance, coupling and mutual inductance shown.
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ZVS transformer core material
Killah573, discrete-gapped ferrite cores were exactly what I was thinking about, in the context of the main transformer. I'm sorry if I caused any misconceptions myself trying to clear up anyone else's.
