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Steve Ward

Sat Apr 22 2006, 07:55PM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

Guys, any coil has a resonant frequency. For a flyback, the resonant frequency is controlled by the inductance of the coil and significantly by its self capacitance. With a half-bridge driver i usually find true resonance (you can use a scope to determine this, as the output is a sine wave with no other frequency component) around 35-65khZ depending on the transformer. And as expected, this produces the highest voltage from a voltage fed source (the half-bridge).

Dr. Shark

Tue May 09 2006, 10:09AM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

Steve Ward wrote ...

Guys, any coil has a resonant frequency. For a flyback, the resonant frequency is controlled by the inductance of the coil and significantly by its self capacitance. With a half-bridge driver i usually find true resonance (you can use a scope to determine this, as the output is a sine wave with no other frequency component) around 35-65khZ depending on the transformer. And as expected, this produces the highest voltage from a voltage fed source (the half-bridge).

I think this is an interesting point for discussion, and since the interest in the original topic has died down, I am hereby officially hijacking the thread. I claim:
1) For a flyback driven in "flyback mode", that is as an inductor with two windings, risetime and width of the pulses matter, so the frequency does have an influence on the output. This should definitely not be confused with resonance.
2) For a flyback driven in "transformer mode", with the airgap removed and a sine wave drive, the output simply follows the input multiplied by the turns ratio. The frequency does only come into play when when core parameters such as saturation and hysteresis losses are considered. again, no resonance.
3) Just like in a tesla coil, there is a possibility for resonance between the leakage inductance and the inter-winding capacitance, and I think this is what Steve is pointing at. I do not belive this to be significant in flyback drivers, since they are usually used to drive a load, so the Q of the circuit dies down and resonance would not have any significant effect. Under no load, it might, but for all practical purposes, it will not matter. To exploit resonance in a usefull fashion, one needs a topology like Steves Capacitor Charging Power Supply.

Now, convince me of the oposite! I am here to learn, you probably are too, so let's try and sort this out. I would like to _know_ why the frequency seems to have such a big influence I practive (yes, even I have noticed that

), and I am open for reasonable explanations.

...

Tue May 09 2006, 02:21PM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

is this legal?

In any case, you can definitely achieve resonance using it in flyback mode. I have experienced this with my zvs driver, the output voltage slowly ramps up to until about 70v in, where it dramatically increases for a range of ~2v, then it quickly drops off to where it was, and starts rising again.

Dr. Shark

Sat May 13 2006, 06:05PM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

Dang, noone interesting in jumping into the discussion? Trying to be provocative doesn't seem to help

Anyway, I'd _really_ like to know why flybacks are so damn frequency-dependent. It is so tempting to just up the frequency a bit, e.g. in the Marzzilli driver by using a resonant cap of only .22uF or so. This would allow to use less primary turns for a given voltage, and theoretically achive a bigger output. In practice this is not the case.
If I only knew why?

Marko

Sat May 13 2006, 06:34PM

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

You will eventually saturate the core by reducing number of turns on primary blowing the mazzili up.

Secondary of a flyback is resonant like any other coil, piece of wire, metal, etc.
Since it is actually a relatively big coil with lots of leakagge capacitance it forms LC at relatively low frequency. Q doesn't need to be pretty high for you to notice.

SSTC's may have Q of 10 and lower when we draw arcs and they seem to exploit resonance very well, so there is no reason why just a bit different shaped coil wouldn't.

Q may not be very high but you'l surely notice it.

For our ZVS/Mazzili/Andrineri flyback driver significant frequency splitting may occur because of high coupling trough the ferrite core, unlike the first case where we prefer high coupling since only secondary is resonant.

Steve Ward

Sat May 13 2006, 08:34PM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

Now, convince me of the oposite! I am here to learn, you probably are too, so let's try and sort this out. I would like to _know_ why the frequency seems to have such a big influence I practive (yes, even I have noticed that ), and I am open for reasonable explanations.

You already did explain it. The transformer will *always* have some leakage inductance and self capacitance that can resonate (even if there is no gap in the core). Removing the gap will reduce the leakage inductance, and simply increase the resonant frequency. The trick then, is to store energy in this L and C (by driving it at resonance) so that your open circuit voltage can build up much higher.

I suspect that half-wave rectified "DC" flybacks might make better use of resonance, since one half of the cycle is always "open circuit" thus allowing energy to be built up in that half-cycle, then discharged on the next. With an AC flyback you are constantly pulling energy out of the system if you have a load.

Hazmatt_(The Underdog)

Sat May 13 2006, 09:18PM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

Joe, I think you should plot the voltage versus frequency of a flyback.

Dr. Dark Current

Mon May 15 2006, 04:43PM

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

one silly question: Whats ok temperature for potted DC flybacks? Does the rule "if you can thold a hand on it, its ok" apply here? (maybe Joe can reply here with his bulged flyback

)

thanks, J.M.

Steve Ward

Mon May 15 2006, 04:48PM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

one silly question: Whats ok temperature for potted DC flybacks? Does the rule "if you can thold a hand on it, its ok" apply here? (maybe Joe can reply here with his bulged flyback )

Many transformers are made to run pretty darn hot. IF you can still hold it in your hand, you are probably fine. But, if you reach this condition after say 10 seconds or so, its likely that the transformer would really overheat if left to run continuously. If it takes a few minutes to warm, up, its probably alright. Just monitor the temp on longer runs and make sure it doesnt get hot enough to burn you.

Part Scavenger

Thu May 18 2006, 04:44PM

Registered Member #79 Joined: Thu Feb 09 2006, 11:35AM
Location: Arkansas
Posts: 673

I've run a flyback hot enough to not be able to hold, and it lived just fine. About 600W for about 45min. Yeah, I'm suprised I ran that long without trouble. Especially with those pissy transistors I was using. Probably about 60% on-time.

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