NPS is a ratio of primary winding turns
to secondary winding turns and although each winding must have a whole number of
turns, the ratio of the two is not required to be a whole number. The choice of
NPS influences the design tradeoffs on the voltage ratings between
primary and secondary switches, and the balance between the magnetic core and
winding loss of the transformer, which are explained in more 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
total voltage deviation above VBULK when the primary TVS or RCD clamp
absorbs the leakage inductance energy of the trasnformer when QL
turns off. VO is the output voltage, and VF is the
forward voltage drop of the secondary rectifier.
Equation 21. ![GUID-3BD338D1-6DB2-4402-A79F-88391503C2E8-low.gif](/ods/images/SLUSEI6/GUID-3BD338D1-6DB2-4402-A79F-88391503C2E8-low.gif)
- 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 QSR is
active and turns-off at non-zero current in AAM and ABM modes.
Equation 22. ![GUID-6319E0D8-E5D7-41E3-AF2E-E135C7F42F43-low.gif](/ods/images/SLUSEI6/GUID-6319E0D8-E5D7-41E3-AF2E-E135C7F42F43-low.gif)
- 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. Conversely, as NPS decreases, primary RMS current increases, while secondary RMS current reduces.