#SLLSEA03751 shows the ideal magnetizing curve for a push-pull converter with B as the magnetic flux density and H as the magnetic field strength. When Q₁ conducts the magnetic flux is pushed from A to A’, and when Q₂ conducts the flux is pulled back from A’ to A. The difference in flux and thus in flux density is proportional to the product of the primary voltage, V_P, and the time, t_ON, it is applied to the primary: B ≈ V_P × t_ON.

Figure 8-2 Core Magnetization and Self-Regulation Through Positive Temperature Coefficient of R_DS(on)

This volt-seconds (V-t) product is important as it determines the core magnetization during each switching cycle. If the V-t products of both phases are not identical, an imbalance in flux density swing results with an offset from the origin of the B-H curve. If balance is not restored, the offset increases with each following cycle and the transformer slowly creeps toward the saturation region.

Fortunately, due to the positive temperature coefficient of a MOSFET’s on-resistance, the output FETs of the SN6501 have a self-correcting effect on V-t imbalance. In the case of a slightly longer on-time, the prolonged current flow through a FET gradually heats the transistor which leads to an increase in R_DS-on. The higher resistance then causes the drain-source voltage, V_DS, to rise. Because the voltage at the primary is the difference between the constant input voltage, V_IN, and the voltage drop across the MOSFET, V_P = V_IN – V_DS, V_P is gradually reduced and V-t balance restored.