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2nd May 2017, 12:13 #1
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Flyback source an magnetic air gap.
I was studying the dcdc converters because I want to do a flayback voltage source.
That's a circuit which ON Semiconductor gives and does all the calculus but there is something missing and it's the ferrite core.
First of all what I found it that in the flayback's switching source the analize of the transformer is more like a couple inductance. There is a difference between a transformer with a couple inductors? I think there is not. But the ferrite transformer's operation is like this:
1When the transistor is on there is a current flow througth the primary coil and there is an energy store in the magnetic flux.
2When the trensistor is off the energy stored in the magnetic flux applies in the secundary, generating a voltage.
I think that differs from the operation of a silicon steel transformer at 60Hz Which do not have an energy store. That's energy storage it's because a magnetic air gap?
For example if we applies the voltage red signal to a simple transformer (steel transformer at 60Hz),
The flux will be:
and the secundary's voltage will be
As we can see there is not difference of phase so there is not energy storage but in the ferrite cores does; if we takes a look here:
https://www.maximintegrated.com/en/a...dex.mvp/id/848
And for example, if we see what texas instrument gives. The next graph are the signal for a flayback
from here: http://www.ti.com/lit/an/slva559/slva559.pdf
I do not understand why there is an energy storage, which displays for a difference of phase between the primary and secundary voltages and in a simple transformer at 60Hz do not.Last edited by julian403; 2nd May 2017 at 12:35.

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2nd May 2017, 14:37 #2
Re: Flayback source an magnetic air gap.
The difference between a transformer and a coupled inductor, is that the transformer does not store energy.
**actually, there is a small amount of unwanted energy storage due to fringe fields in a realworld transformer**.
And you are correct, the magnetic energy is stored in the air gap. This is a concept that many times causes confusion to people studying magnetic theory.My batteries are recharged by "Helpful Post" ratings.
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2nd May 2017, 16:03 #3
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Re: Flayback source and magnetic air gap.
I thought that the air gap is because there must be an smaller mangetic permeability to can have more ampereturn. So the energy store is a secundary effect? or is something wanted, too?
Because as a flayback have a dc average value, so the curve is:
So, whit an air gap there is the red curve:
Which at the point where there is a saturation in the black curve there is not in the red. And what everyone wants is have a transformer which the max B is the saturation (almost saturation as a top) and therefore more oppositive fem in the coil.

2nd May 2017, 16:54 #4
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Re: Flayback source an magnetic air gap.
The B/H curves don't make sense, particularly not the B offset.
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2nd May 2017, 19:38 #5
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2nd May 2017, 20:11 #6
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Re: Flyback source an magnetic air gap.
I see nothing wrong with Texas Instruments waveforms and there is no energy storage. The net increment of the flux is 0 as you can see in the current waveforms. Charges up with energy in the first half cycle, and then discharges the energy in the second half cycle (discharges the energy in the secondary load).
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2nd May 2017, 20:45 #7
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Re: Flyback source an magnetic air gap.
Charges up with energy in the first half cycle, and then discharges the energy in the second half cycle (discharges the energy in the secondary load).

2nd May 2017, 22:30 #8
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Re: Flyback source an magnetic air gap.
Yes, I know. Where is the problem ? The energy stored during the first half cycle is wasted during the second half cycle. The transformer can not store energy because its core would saturate in a short amount of time... that is the reason why always square waves applied to the transformer swing between a positive value and a negative value and not between positive and 0.
   Updated   
Regarding your calculations of the flux (with sinusoidal voltage), as you can see in the following plot (assuming initial flux=0), the flux rises up and then decreases in order to have a net flux = 0 at the beginning of the next cycle.
I took N1=N2 = 1 turn
   Updated   
Aaaaaaahhhhhhhhhhh I think I get you know. Just to make sure, please mark in red the phase shift in voltage waveforms you are talking about in post #1 and I will tell you where you got confused. Because seems like you see a voltage phase shift which I do not see
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3rd May 2017, 01:20 #9
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Re: Flyback source an magnetic air gap.
The red arrow is the primary current and the blue arrow is the secundary current.
Yes, I know. Where is the problem ? The energy stored during the first half cycle is wasted during the second half cycle. The transformer can not store energy because its core would saturate in a short amount of time... that is the reason why always square waves applied to the transformer swing between a positive value and a negative value and not between positive and 0.
I did the calculus with sin signal but if there is a square or pulse signal in primary there will be the same without a delay in the secundary because by fourrier, the armonic (represented by sin signals) do not have a difference of phase. And the signals will have just a difference of amplitude.
So,The energy stored during the first half cycle is wasted during the second half cycle
I propouse that if there is an air gap, and as the coils (primary and secundary) are in the ferrite material. The flux in the first cycle goes all to the air and not througth the ferrite. And when the primary's ampereturn goes down the air deliver the energy to ferrite.
So the ferrite's flux would be something like this. where is the magnetic flux in ferrite and is the magnetic flux in the air gap. Therefore there will be a delay time in the secondary or a 90º phase difference between the signal in the primary with the secundary.
and so and (where k is a constan which deppends of the permeabilities )because if there is not a derivate in somewhere there can't be energy storage.
Look! I made a simulation with a coupled inductors
There is not delay timeLast edited by julian403; 3rd May 2017 at 01:36.

3rd May 2017, 12:08 #10
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Re: Flyback source an magnetic air gap.
Look what I found today (something like that there must be), there is a derivative (in this case is an integral)
https://en.wikipedia.org/wiki/Magnetic_capacitivity
https://en.wikipedia.org/wiki/Magnetic_inductance
and take a look
from here http://icosymnt.cvut.cz/courseodl/node44.htmlLast edited by julian403; 3rd May 2017 at 12:35.

3rd May 2017, 14:41 #11
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Re: Flyback source an magnetic air gap.
As you can see there is not delay time in the secundary's signal.
Exactly what I thought... You are talking about voltage phase shift but you are showing a current waveform. I knew this was your misunderstanding..just wanted to make sure. Did you have a course in Electrical Machines ?
Look! I made a simulation with a coupled inductors
[..]
There is not delay time
Can you find some voltage waveforms for the flyback transformer and see by yourself that there is no delay in voltage ?

3rd May 2017, 14:47 #12

3rd May 2017, 17:09 #13
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Re: Flyback source an magnetic air gap.
Yes, no delay in voltage waveform but you are talking about the current waveform in the flyback transformer.. you are mixing up current waveform with voltage waveform...

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3rd May 2017, 18:59 #14
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Re: Flyback source an magnetic air gap.
In a flyback transformer (coupled inductor), energy is stored in the transformer inductance as the primary current increases when the primary switch is on.
The switch can be left on until the core nears saturation and no more energy can be stored.
When the primary switch turns off, this inductive current with its energy, is blocked from flowing in the primary path so it flows in the secondary path, transferring the energy to the secondary circuit.
Thus the primary and secondary currents are out of phase.
That is unlike a standard transformer which just transfers the energy from primary to secondary without storing it, thus the primary and secondary waveforms are (must be) in phase.
The transformer air gap is to increase the magnetic flux saturation point of the core material.
The transformer inductance will store energy without the gap but will store more energy with the gap.
This is because the inductive energy is proportional to the current squared but proportional to the inductance^1 (E =½ LI²).
Thus, suppose the air gap reduced the inductance by ½ but allowed an increase in current by a factor of 2 before core saturation.
The net increase in stored energy is then a factor of 2.
   Updated   
That's because your simulation is using the transformer as a normal transformer, not as a flyback converter.
A flyback applies a voltage to the primary through a switch and then opens that switch, generating an open circuit on the primary, at which point the energy is transferred to the secondary.
Applying a squarewave to a transformer is not the same thing since the source is never opencircuit but is always a low impedance (zero in the case of your simulation).
So in a flyback converter the output current is always out of phase with the input current for a resistive (or any kind) of load.Zapper
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3rd May 2017, 21:20 #15
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Re: Flyback source an magnetic air gap.
Ok, let's see if I got it.
The ampere turn in the core is:
to find we must look at the primary coil circuit which is a RL's circuit where is the primary coil's inductance and the transistor chanel's resistance but as the switching conmutation is more faster than the time constant we can assume:
the same for the secondary circuit:
So:
D is the cycle dutty
and in DT the transistor turn off but there is still an magnetomotive force Hmax
so
and as
The complete equation for the secundary circuit is:
so:
That would be the solution, where the current I2 takes places from the time DT And as every power electronic circuit such as a simple DCDC converter there is a succesion of transistors until reaching a premanent state.
What do you think?Last edited by julian403; 3rd May 2017 at 21:36.

3rd May 2017, 22:15 #16
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Re: Flyback source an magnetic air gap.
Sorry, but I seldom get involved in looking at math equations unless I have to.
This doesn't qualify as one of those times.Zapper
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4th May 2017, 06:22 #17
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Re: Flyback source an magnetic air gap.
Sorry, but I seldom get involved in looking at math equations unless I have to.
This doesn't qualify as one of those times.
I see that the original post is asking about the relation of forward (direct inductive coupling) and flyback (inductive coupling with intermediate energy storage in main inductance) principle in DC/DC converters.
My general answer is that all transformers can be modeled by a combination of main and leakage inductance, the difference between transformers dedicated for forward and flyback operation is the ratio of both parameters. Other parameters like windings resistance, core losses and nonlinear core characteristic can be considered in the analysis, but you can ignore it at first.
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