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Before discussing the calculation of magnetic components for switching power supplies, let me just quickly go over the basic concepts and definitions. A changing current in the primary winding creates an alternating magnetic field in the core. The core multiplies this field and couples most of the flux through the secondary windings.
Power transformer in SMPS is used to change amplitude of high-frequency pulses by the turns ratio and to provide isolation between circuits. Note that a transformer can’t transfer a DC component of a pulse: in a steady state mode net volt-seconds across any winding should be zero, otherwise the core will soon saturate. DC output voltage can be obtained only by using rectifiers. Nevertheless, an average voltage across a real coil’s terminals can be non-zero due to non-zero wire resistance.
This DC offset can be used for lossless sensing of an average current across an inductor or a transformer winding with unidirectional current: if you add an RC network parallel to the coil, the voltage across the capacitor will be proportional to the coil’s average current. For better thermal stability the wire can be made of low TCR material, such as a copper alloy. MAGNETICS DESIGNING normally involves trade-offs between size, cost and power losses. BMAX should not approach the core material’s saturation flux value BSAT.
Note that in higher frequencies, core loss rather than saturation can become the main limiting factor for BMAX. The flux change is a function of the applied volt-seconds and the core geometry. Excessive volt-seconds applied to a coil cause core saturation. When it happens, the windings are effectively shorted out.
Ac product, applied voltage and frequency for common voltage waveforms. Contrary to popular misconception, BMAX does not depend on the magnetic material properties or air gaps.
It does not depend on the transferred power neither. That’s why in theory the core size does not depend on the wattage. However, for efficiency and thermal reasons, we have to limit ohmic losses in the wires.