Why does flux walking in a magnetic core occur?

[MrAl]

1. Does the flux accumulate over time primarily due to "hysteresis" property of the core or are there any other factors contributing to it?

The hysteresis property is not really a factor, but the permeability property is a major one.

2. Could a simple drive circuit to a coil wound on the core cause ratcheting of flux.

Potentially yes.

3. Does that mean that it is mandatory to always drive a transformer with a bipolar drive?

No. 

Single-switch flyback, two-switch flyback, single-switch forward, and two-switch forward converters, are all unipolar.  They are all limited to 50% duty cycle maximum, in order to allow the core to reset during the other 50%.  Or if driven with low duty cycle 10%, the remaining 90% is plenty of time to allow the core to reset.  If you go beyond 50% duty cycle, the core will not properly reset, resulting in eventual core saturation.

Bipolar drive permits high duty cycle beyond 50% (and commensurately higher power conversion), because the core is reset via polarity reversal.  But unbalanced drive waveforms will eventually lead to core saturation as well, for example 90% in one direction and 91% in the other.

Hi,

I am not sure if you could say that bipolar drive permits duty cycles above 50 percent when the core is reset via polarity reversal.  That's because in that case we would need a 50 percent one polarity and 50 percent the other polarity, or at least each half cycle would have to have the same pulse width.

It's hard to pin this down in simple terms though because even that isn't really the end of the story.  For example, we could have one pulse that is 25 percent V+ and 75 percent V-, which is asymmetrical, but then the next pulse that is 75 percent V+ and 25 percent V-.  The first and second pulses are asymmetrical, but together they balance out the core.  This is as long as the initial inductance is high enough to accept that without saturating after the first pulse.  This is the case with pure sine synthesized converters where there is a long train of varying pulse widths of the positive polarity followed by a long train of pulses of the negative polarity (or similar).  After the second train of pulses the core is back to the original state.

[mercurial]

Here are some plots that show how the core flux can increase over time with multiple pulses.

What software was used to create those plots, is it a magnetic simulator of some sort?

[MrAl]

Bonjour, bravo for the question.

As a power electronics designer since 70s, I have not hear this term "flux walk" , Iit is commonly called  core saturation.

Any ferrous material  has a non linear  a B-H (flux vs exitation) relation.    near linear at low flux  and  a flux  limit at very high exitation.

Core losses due to heysterysis etc are also changng nonlinear with H.

For a perfect AC exitation, zero DC omponent, the cycle by cycle peak B is not changinbg.

In case a DC bias exists in exitation, eg now quite 50% DC in puch pull/bridge, the DC then causes a sucessive creeping cycle by cycle of the B in one dirsction.

As the B increases cycel by cleyc the magnetizing current increases as incermental L decreaes. See the B-H curves by core manufactureses, Arnold, Mag Mtls , Thomas for lams and tapewound iron, TDK, EPCOS, Siemens, etcv for ferrite.

Suggest you read a classic text on static electromagneti devices like Hunt and Stein, to lear about the reasons for ferrous material nonlinearities.

We never had these ussus: in 1970s various  DC cancelling FB were used to even the V-S area plus/minus applies.

We avoid push pull, centertapped transformers.

If some DC is present use an airgap in the core.

Most modern topologies use a series  cap to remove change of DB bias.

Bon chance,

Jon

Hi,

Very surprised to hear you have never heard of the term "flux walk" maybe it is just a US thing.
All the engineers I have ever met that worked with power converters knew of this term, but that was in the US mostly.

As you probably know, the air gap allows some DC mainly because it lowers the permeability which then requires more wire turns.  The net effect is it takes more offset to saturate the core.
It's also interesting that the air gap permeability is so low compared to the base core metal permeability that it can be very short and still affect the entire construction significantly.

[MrAl]

Here are some plots that show how the core flux can increase over time with multiple pulses.

What software was used to create those plots, is it a magnetic simulator of some sort?

Hi,

I created those plots with a circuit simulator like the LT Spice simulator which you can download for free.
I wanted to show the essence of what happens while staying away from the more complicated operation, but I have other plots that show a more complete picture which I will try to find now.

The second picture shows the BH curve when the flux is imbalanced and saturating on the positive part while not quite on the negative part.

[mercurial]

Hi MrAl

For someone who worked in the power electronics industry for so many years it would be interesting to hear how did you really measure core saturation did you only use current as your eyes into the magnetics.
Did you use any special tools to measure the magnetic field while the transformer was operating?

[T3sl4co1l]

Single-switch flyback, two-switch flyback, single-switch forward, and two-switch forward converters, are all unipolar.  They are all limited to 50% duty cycle maximum, in order to allow the core to reset during the other 50%.  Or if driven with low duty cycle 10%, the remaining 90% is plenty of time to allow the core to reset.  If you go beyond 50% duty cycle, the core will not properly reset, resulting in eventual core saturation.

Bipolar drive permits high duty cycle beyond 50% (and commensurately higher power conversion), because the core is reset via polarity reversal.  But unbalanced drive waveforms will eventually lead to core saturation as well, for example 90% in one direction and 91% in the other.

Single-switch flyback, and forward if using RCD clamp snubber for reset, can go higher than 50%.  Or conversely, conditions exist where unwanted DC imbalance occurs below 50%.  Forward, as long as magnetizing inductance DCM or BCM is maintained, full reset is achieved (e.g. using a RCD clamp snubber for reset).  Flyback can simply be operated in CCM with no consequence, as long as the control is suitable (it often isn't, e.g. peak current mode control requires high ripple fraction).  In that case, the limit is not 50% unilaterally, but depends on the voltage ratio in CCM, D = Vo / (Vi + Vo).

Similarly, the subharmonic oscillation threshold that is often erroneously repeated in appnotes, is precisely the same threshold: it's simply to say current is continuous, regardless the voltage ratio.  You do most of your design calculations at 50% duty, so it's not far in practice, but it is strictly wrong to claim a fixed 50% for all cases as appnotes inevitably do.

Incidentally, this means the intentional opposite of "flux walking" is simply CCM.  That is, precisely the same phenomena, but actually intended.  It's just total unbalanced DC bias current in the core.

Ah, yes-- not to mention that's another reason I object to the term -- it implies some ignorance of the system, some agency of it, unconstrained by the designer, that it can just wander off on its own for reasons inexplicable.  As a designer, I find this wholly antithetical.

Tim

[MrAl]

Hi MrAl

For someone who worked in the power electronics industry for so many years it would be interesting to hear how did you really measure core saturation did you only use current as your eyes into the magnetics.
Did you use any special tools to measure the magnetic field while the transformer was operating?

Hi,

I did design work and also some troubleshooting when needed so I had theoretical as well as hands on experience in that field.  What I liked best was trying to come up with new ways to make power converters, especially the synthesized sine type converters.

The main way to find out if the core is going to saturate is to bring the power to the converter up slowly, which meant bringing the DC buss voltage up little by little often with the control circuitry powered with a separate power source so it would operate normally even with a low DC buss voltage.
As the voltage pulses to the transformer get higher and higher, if the core starts to enter into saturation, the current goes from a regular up and down sawtooth to a sawtooth where near the end of the pulse it rises up sharply.  That tells you right there, and if you raise the voltage higher the current at the end rises up more and more so you see the spike go up even higher and sharper.  Going too high at that time will blow the bridge transistors.
In some designs we used current sensors to sense transistor currents so we could provide feedback to prevent them from blowing out.  That came later though.

The field of the transformer core is almost all confined within the core so the only measurements I ever did was with a hall effect device inside the gap.  This ends up being like a hall effect current probe used with oscilloscopes and DC current clamp on meters.  I don't remember much about the measurements though because this was more of a curiosity than a regular test technique.

Now that you mention it, the best way to test a core like in an inductor made for say a buck converter is to test it in a buck converter.  The actual buck converter provides the normal operating conditions so it's like a full test.  Testing it any other way is much harder to do because you have to provide all the operating conditions the buck converter would see anyway, such as the average DC current which is an important factor in a buck converter.

[jonpaul]

Mercurial:

>>1. Why center tapped transformers to be avoided?
Wdg is More expensive and takes more space for taps. Single P and single S always preffered in mfg, prod.

 >>Don't center tapped also reverse the flux in the core?
V-S  depends on the drive symmetry, switch saturation, switch recovery, and drive pulse symmetytr.

2. >>What is wrong with push-pull why that needs to be avoided?
see above.

3. >>What is the V-S area. (VOLT*Seconds) Voltage applied integrated over the time = Area integrated .

Enjoy,

Jon

[jonpaul]

>> how did you really measure core saturation 
Did you use any special tools to measure the magnetic field while the transformer was operating?

1. Change in magnetizing current with applied exitation voltage.

2. Wdg a few turns search coil on core leg, use   X-Y scope to see B-H curve.

https://phys.libretexts.org/Bookshelves/Electricity_and_Magnetism/Book%3A_Applications_of_Maxwells_Equations_(Cochran_and_Heinrich)/06%3A_Ferromagnetism/6.02%3A_B-H_Curves

Jon