Flux walking is an issue in full-bridge, half-bridge and push-pull transformers….Flux walking (flux stair casing) is an accumulation of flux in the core due to a non-zero average of the applied voltage over each period of time. This causes the flux to “staircase” toward one end of the cores flux density ability which inevitably leads to core saturation.
The issue is more common in high frequency switching transformers due to the ordinarily small number of PRI turns and hence low PRI DCR….
In higher PRI DCR transformers as the flux “walks” toward one end there is an increasing dc component of the magnetizing current that causes an opposing IR drop in the winding. This usually and eventually cancels the dc voltage component of the drive waveform so saturation can be avoided…when the PRI DCR is low this cancellation does not usually occur and so core saturation occurs instead.
If the PRI DCR is insufficient one can try gapping the core to increase reluctance. Gapping the core decreases PRI inductance and so increases magnetizing current. This increased current results in a larger voltage drop which, in some cases, can counteract the low PRI DCR.
Below is a pictorial view of what occurs during flux walking….
Q1, Q2, Q3 & Q4 are transistor states….Q1 & Q4 on times are shown exaggerated so one can see as more positive volt-sec’s are applied than negative ones the flux and magnetizing current both “walk”…at some point the core can handle no more flux at which time it saturates which leads to a core permeability of nearly zero….this drops PRI inductance considerably and so the magnetizing current increases dramatically spiking toward something far too large…
The options of increasing PRI DCR and gapping the core are the simplest means to try and control flux walking….
One can also use current mode control which senses the current allowing the transistors to turn off at the same peak current levels each period…. this increases and decreases ON times accordingly which counters a flux buildup and hence negates flux walking