18.104.22.168.3 Primary Turns (NP)
The turn number on the primary side of the transformer (NP) is determined by two design considerations:
- The maximum flux density (BMAX) must be kept below the saturation limit (BSAT) of the magnetic core under the highest peak magnetizing current (IM+(MAX)) condition, a given cross-section area (AE) of the core geometry, and highest core temperature. When IFB = 0 A, such as VO soft-start or step-up load transient, the peak magnetizing current reaches IM+(MAX), since VCST = VCST(MAX) in those conditions. IM+(MAX) can be calculated based on the output power triggering an OPP fault (PO(OPP)) with VCST = VCST(OPP1) at VBULK(MIN). After NP is chosen, NS can be calculated through NPS.
- The AC flux density (ΔB) affects the core loss of a transformer. For a transition-mode active clamp flyback, the core loss at high line is usually highest, since the switching frequency is highest and duty cycle is smallest for a given load condition. The following equation is the ΔB calculation including the contribution of negative magnetizing current (IM-), used to put into the Steinmetz equation for more accurate core loss estimation. For VBULK ≥ NPS(VO+VF), IM- is calculated with VBULK divided by the characteristic impedance of LM and the lumped time-related switch-node capacitance (CSW). The expression of fSW is derived based on the triangular approximation of the magnetizing current, which also considers IM- effect over wide AC line condition.