18.104.22.168.1 Primary-to-Secondary Turns Ratio (NPS)
NPS influences the design tradeoffs on the voltage rating between primary and secondary switches, and the balance between the magnetic core and winding loss of the transformer, which are explained in detail as follows:
- Maximum NPS (NPS(MAX)) is limited by the maximum derated drain-to-source voltage of QL (VDS_QL(MAX)). In the expression below, ∆VCLAMP is the voltage above the reflected output voltage. It can be either the ripple voltage of CCLAMP in AAM mode, or the voltage over-charge of CCLAMP by the leakage energy as QH is disabled in LPM mode. VO is the output voltage, and VF is the forward voltage drop of the secondary rectifier.
- Minimum NPS (NPS(MIN)) is limited by the maximum derated drain-to-source voltage of the secondary rectifier (VDS_SR(MAX)). In the expression for NPS(MIN), ∆VSPIKE should account for any additional voltage spike higher than VBULK(MAX)/NPS that occurs when QH is active and turns-off at non-zero current in AAM mode.
- Since the high-frequency transformer is usually a core-loss limited design instead of a saturation-limited design, the minimum duty cycle (DMIN) at VBULK(MAX) is more important. Lower DMIN increases core loss at VBULK(MAX), so this constraint creates another limitation on NPS(MIN).
- The winding loss distribution between the primary and secondary side of the transformer is the final consideration. As NPS increases, primary RMS current reduces, while secondary RMS current increases.