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Why does flux walking in a magnetic core occur?

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T3sl4co1l:

--- Quote from: Andy Chee on March 04, 2024, 12:35:21 pm ---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.

--- End quote ---

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:

--- Quote from: mercurial on March 04, 2024, 03:48:11 pm ---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?

--- End quote ---

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

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