# Effects of opposing fields in coupled inductor

This is a duplicate question that I already made in the electrical engineering part of stack exchange. As Im asking for the physics behind the effect I thought it might be worth to ask also here.

What happens to inductor di/dt when you have multiple MMF sources such as the schematic of a coupled buck converter below? Specifically when Duty cycle>0.5 so M1 and M2 are overlapping.

What I would expect is :

1. M1 turns on, increasing core B field in "positive" direction.
2. M2 turns on, creating an opposing H field
3. The opposing H field cancels the increase in B or dB/dt
4. No dB/dt means no back EMF at L1/L2 terminals, which I thought was the effect slowing down di/dt in the first place.
5. Assuming ideal components, di/dt goes to infinite.

Now obviously this does not happen, or converters such as these would not be effective. Hopefully somebody can correct my understanding.

A helpful comment was mentioning how higher duty cycles were dependent on leakage inductance, and attached a document explaining these issues. However the paper itself contradict this (page 20), stating that the higher coupling factor, the higher performance.

Borrowed figure 1.25 from the white paper.

Citation P.Wong:

"From Figure 1.25, the stronger the coupling, the better the performance of the multiphase coupled-inductor buck converter"

This suggests that even overlapping, the lesser di/dt effects would happen at perfect coupling, and zero leakage inductance. But I dont understand the electromagnetic physics of it at all.