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).
Equation 27. ![GUID-A90FA769-735A-4E58-BBF7-4D84EBA6E221-low.gif](/ods/images/JAJSJ67E/GUID-A90FA769-735A-4E58-BBF7-4D84EBA6E221-low.gif)
Equation 28. ![GUID-14CC4953-951F-4922-90DF-D4F6D6967BC8-low.gif](/ods/images/JAJSJ67E/GUID-14CC4953-951F-4922-90DF-D4F6D6967BC8-low.gif)
- The AC flux density (ΔB) affects the core loss of a transformer. For a transition-mode active clamp flyback, the core loss is usually highest at high line, 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.
Equation 29. ![GUID-1CB5FB68-48FC-44B0-B7FC-DD461A0E59EA-low.gif](/ods/images/JAJSJ67E/GUID-1CB5FB68-48FC-44B0-B7FC-DD461A0E59EA-low.gif)
Equation 30. ![GUID-3706B763-DB7A-43DE-959B-1E9EF9745B1F-low.gif](/ods/images/JAJSJ67E/GUID-3706B763-DB7A-43DE-959B-1E9EF9745B1F-low.gif)
Equation 31. ![GUID-7626E3E4-44F1-45A0-950A-7DDAD5AAE8E5-low.gif](/ods/images/JAJSJ67E/GUID-7626E3E4-44F1-45A0-950A-7DDAD5AAE8E5-low.gif)
Equation 32. ![GUID-D96D009F-CACD-4F47-AD6E-DB1D720FC62B-low.gif](/ods/images/JAJSJ67E/GUID-D96D009F-CACD-4F47-AD6E-DB1D720FC62B-low.gif)
Equation 33. ![GUID-DCA3E337-A18F-4805-9808-B87A4718ACFC-low.gif](/ods/images/JAJSJ67E/GUID-DCA3E337-A18F-4805-9808-B87A4718ACFC-low.gif)
Equation 34. ![GUID-B68FFB2D-E60D-4D9D-BD4B-03CEEEC3CF78-low.gif](/ods/images/JAJSJ67E/GUID-B68FFB2D-E60D-4D9D-BD4B-03CEEEC3CF78-low.gif)