The transformer inductance contains two components (L1A and L1B) on the primary side. Because the winding's inductance varies by the square of the turns, each half provides 25% of the total winding inductance:
L1A = L1/(N1A +N1B)2, or
L1/4 = L1A = L1B
where L1 is the total primary inductance, N1A is the number of turns on L1A, and N1B is the number of turns on L1B. The turns are on the same core. If L1A and L1B were on two separate cores, the inductance of the two windings added together would be two times that of each winding.
The secondary inductance is calculated by multiplying the primary turns by the turns ratio. For a 1:2 step-up transformer, the secondary has two times as many turns as the primary. Because they're on the same core, it boasts four times the inductance of the primary.
L2 = (2)2 × L1
where L2 is the total secondary winding inductance.
The primary inductance offers the greatest influence on the low-frequency roll-off. There has to be enough inductance to pass the low-frequency pulses, such as the link pulse. Meeting these requirements does demand an assurance that a sufficient amount of inductance will be left in the coil after a certain amount of bias current passes through the transformer.
Situated in tandem with the transformer, the common-mode choke provides high impedance to attenuate the EMI noise in the operating frequency range. That impedance is derived from the choke's inductance and core-loss resistance. The correct material must be chosen to achieve the core-loss characteristics and permeability needed to provide the right number of turns for the suppression.
The winding resistances (RCM1 and RCM2) have very little influence on the passband of the module's insertion loss. Again, these values represent the wire resistance of the winding as described for the transformer.
The choke's leakage inductances (LLC1 and LLC2) appear in series with the transformer leakage inductance. Their greatest effect can be seen in the deterioration of the return loss at the higher frequencies, as well as the slowing of the output waveform's rise time (Fig. 5). Leakage inductance should be kept to a minimum. However, the methods used to reduce it increase the coupling capacitance of the windings (CWC).
That coupling capacitance—the winding-to-winding capacitance of the common-mode choke—has its greatest impact on return loss. It appears in parallel with the distributed capacitance of the transformer. As it grows, the return loss at the higher frequencies improves to a point. Then, in the region of the resonant point, it starts to deteriorate again.
The distributed capacitances (CDC1 and CDC2) have very little effect on the waveform. Their greatest influence is seen in the stopband, where they tend to improve noise attenuation at higher frequencies.
In the core material, the parallel-equivalent core-loss resistances (RCA and RCB) represent eddy-current losses. The impedance produced by these losses peaks at a frequency determined by the number of turns on the core. The parallel-equivalent resistance and inductance both vary by the square of the turns used.
The distributed capacitance also has an effect on the impedance at higher frequencies. As a result, there is a point at which too many turns will eventually cause a problem at high frequencies, decreasing the amount of attenuation.
The primary inductances of the coil (LCM1 and LCM2) are determined by the equation:
L = permeability × N2 × core constant
where L is the open-circuit inductance, N is the number of turns, and the core constant is the core's cross-sectional area divided by its mean path length. For a ferrite material, permeability decreases with frequency, having its highest value at frequencies below 1 to 2 MHz. Above that point, permeability deteriorates with frequency until only the parasitic parameter of leakage inductance remains.
For the best return loss, the load resistances (RL1 and RL2) must match the source resistances based upon the turns ratio of the transformers. High return loss means no signal reflections back to the primary.
All of the components in this model have some direct impact on the desired waveform, as well as some indirect effects through their interactions. Naturally, some factors exert greater influence than others. Determining the component values that will achieve the best waveform is essentially a "balancing act," which encompasses not only transformer and/or choke design, but ultimately many aspects of system-level design.