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Registered Member #3429
Joined: Sun Nov 21 2010, 02:04AM
Location: Minnesota, USA
Posts: 288
I have some thoughts about how a flyback transformer behaves when driven by different waveforms, but I'd like to hear some comments from you transformer experts. Please tell me if my assumptions are correct:
1. When a flyback transformer is driven with a pure sinewave, the output voltage will be simply the input peak voltage multiplied by the turns ratio. 2. When driven by a sawtooth (i.e., linear ramp-up, followed by sudden voltage drop to zero), then the output voltage will be a huge spike, many times higher than the primary peak voltage multiplied by the turns ratio. 3. When driven by a rectangular wave or squarewave, then the output voltage will be a combination of #1 and #2. That is, the output voltage will have a complex waveform consisting of a sinewave intertwined with large spikes.
Are my assumptions correct on all three? I was not able to find anything on-line that explains this, so if anyone knows of a website that explains these properties of a flyback transformer, please provide the link. THANKS!
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716
Actually, no. The "flyback" component is just a transformer with a gapped core. That makes its magnetizing inductance relatively low, but it can take a relatively large magnetizing current without saturation. Its usual operation is very much like that of an engine-ignition coil.
You got case 1 right. With sinusoidal drive, the primary & secondary voltages and currents are all sinusoidal.
For case 2, beware of confusing the drive voltage with the drive current. In flyback converter operation, the driving element is actually a switch.
Starting with zero voltage and current: when the switch is closed, the primary voltage goes to V_in (beginning a rectangular pulse) and the primary current begins to ramp up. The secondary voltage begins a rectangular pulse at N times V_in, with no current (because typically blocked by a diode, or the voltage is insufficient to initiate a spark).
Opening the switch forces the primary -current- to zero very rapidly. The collapsing magnetic field can induce almost arbitrarily high voltages (of the other polarity) in both primary and secondary windings, still in a 1:N ratio. Typically, current begins to flow in the secondary circuit (because output diode is forward biased, or a spark is ignited). The load determines the secondary voltage, and the primary voltage follows it with 1/N ratio, until the energy stored in the core (I^2.L/2) has been dissipated in the secondary circuit.
The name flyback comes from the television application, since the 1940s if not earlier. One winding is connected to the horizontal deflection coils, and has a sawtooth current waveform (whose steeper side is the time when the e-beam direction rapidly returns to the left side of the screen). The associated voltage waveforms, proportional to di/dt, are bipolar rectangles: wide pulse with low voltage, and much narrower pulse with much higher voltage of the opposite polarity.
For your case 3, need to clarify whether you are driving voltage or current, whether there are diodes, etc.
Registered Member #3429
Joined: Sun Nov 21 2010, 02:04AM
Location: Minnesota, USA
Posts: 288
klugesmith wrote ...
Actually, no. The "flyback" component is just a transformer with a gapped core. That makes its magnetizing inductance relatively low, but it can take a relatively large magnetizing current without saturation. Its usual operation is very much like that of an engine-ignition coil.
You got case 1 right. With sinusoidal drive, the primary & secondary voltages and currents are all sinusoidal.
For case 2, beware of confusing the drive voltage with the drive current. In flyback converter operation, the driving element is actually a switch.
Starting with zero voltage and current: when the switch is closed, the primary voltage goes to V_in (beginning a rectangular pulse) and the primary current begins to ramp up. The secondary voltage begins a rectangular pulse at N times V_in, with no current (because typically blocked by a diode, or the voltage is insufficient to initiate a spark).
Opening the switch forces the primary -current- to zero very rapidly. The collapsing magnetic field can induce almost arbitrarily high voltages (of the other polarity) in both primary and secondary windings, still in a 1:N ratio. Typically, current begins to flow in the secondary circuit (because output diode is forward biased, or a spark is ignited). The load determines the secondary voltage, and the primary voltage follows it with 1/N ratio, until the energy stored in the core (I^2.L/2) has been dissipated in the secondary circuit.
The name flyback comes from the television application, since the 1940s if not earlier. One winding is connected to the horizontal deflection coils, and has a sawtooth current waveform (whose steeper side is the time when the e-beam direction rapidly returns to the left side of the screen). The associated voltage waveforms, proportional to di/dt, are bipolar rectangles: wide pulse with low voltage, and much narrower pulse with much higher voltage of the opposite polarity.
For your case 3, need to clarify whether you are driving voltage or current, whether there are diodes, etc.
Verstehen Sie?
I had to look up "Verstehen Sie". But, yes, I see your point. I recall from my early days of learning electronics that the voltage leads the current (I believe by 90 degrees, if memory serves me) in an inductor. Your explanation of how the transformer behaves under different conditions of driving current is very useful to me, and I thank you for that.
The particular flyback that I am experimenting with has no gap between the core halves. The faces of the core halves are in intimate contact with each other and secured with a clamp. I don't know for certain if it was designed that way, or if someone removed the fiber insert that you sometimes see between transformer core halves. I bought it used on Ebay. In this configuration (no gap), would it function better as a flyback (i.e., with the drive current being turned on/off with a fast "switch" (mosfet)) or as a normal transformer, being driven by a sinewave? If needed, I would modify the transformer in such a way so that it would be less likely to saturate, and at the same time to be efficient at transferring power between primary and secondary windings. Would that require a compromise in the physical design of the transformer?
I'm asking these questions because I am experimenting with using a flyback transformer in a Plasma Speaker circuit. I want to configure the flyback in such a way that it operates efficiently, and can handle nearly 100% amplitude modulation of the driving current without going into saturation, which would cause distortion of the audio.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
In a flyback transformer, the transformer stores engery while the switch is closed, when opened the engery stored is transfered to the coupled inductor (secondary) so this is a special type of a transformer. Remember in every other type of transformer energy storage is a bad thing, but in our flybacks its essential.
are you sure you arnt seeing this:
Is it possible the gap exists, but that you cant see it? can you post a pic of your transformer?
For a true "coupled inductor" the gap is pretty important...
Registered Member #3429
Joined: Sun Nov 21 2010, 02:04AM
Location: Minnesota, USA
Posts: 288
Patrick wrote ...
Is it possible the gap exists, but that you cant see it? can you post a pic of your transformer?
Here is one of the pictures from my Plasma Speaker project that I posted on 4hv:
It's a standard open-frame "AC" style flyback. I don't want to take it apart again unless I have to make changes, but I recall that it had no spacer between the cores when I did have it apart. What would the symptoms be if it was supposed to have a spacer in the gap between cores, but it fell out, and I reassembled it without the spacer? Would the transformer still work but overheat? It does get quite warm after about 5 minutes of a continuous arc, but it doesn't get so hot that it smokes or melts.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Xray wrote ...
Patrick wrote ...
Is it possible the gap exists, but that you cant see it? can you post a pic of your transformer?
Here is one of the pictures from my Plasma Speaker project that I posted on 4hv:
It's a standard open-frame "AC" style flyback. I don't want to take it apart again unless I have to make changes, but I recall that it had no spacer between the cores when I did have it apart. What would the symptoms be if it was supposed to have a spacer in the gap between cores, but it fell out, and I reassembled it without the spacer? Would the transformer still work but overheat? It does get quite warm after about 5 minutes of a continuous arc, but it doesn't get so hot that it smokes or melts.
Thats a great idea using a work light to contain the arc. anyway, if you dont have a gap then for sure you have a "normal" transformer functioning only by turns ratio. I dont see a problem so long as you expect lower kV.
On my flyback tank project, im putting in 77volts to the internal primary which collpases across the gap to about 200-250 volts on the primary, which then get shoved into the secondary turns.
Registered Member #3429
Joined: Sun Nov 21 2010, 02:04AM
Location: Minnesota, USA
Posts: 288
Patrick wrote ...
Thats a great idea using a work light to contain the arc. anyway, if you dont have a gap then for sure you have a "normal" transformer functioning only by turns ratio. I dont see a problem so long as you expect lower kV.
It's not actually a work light. It was intended as an outdoor halogen yard light. I removed all the halogen lamp connectors, brackets, and other stuff, and mounted my spark gap there. The reflector has somewhat of a focusing effect on the sound, which directs it outward and makes it louder than if the arc were in open air.
I believe that my flyback is operating like a normal transformer, and as a result, the arc is not extremely long, but it is thick and very "hot", which I believe is required in order to have good, clean, and loud sound from it.
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Flyback transformer "modes"
