let's visualize a transformer. Transformers consist of 2 coils, primary and secondary coils and both these coils possess some parasitic inductance. When we try to do some impedance matching, we look at the effective resistance seen by the voltage source to which primary coil is connected. And we must take primary coil's parasitic inductance into account for better results but we dont do the same for parasitic inductance of secondary coil. I dont understand why. Can you explain?
The parasitic inductance of the primary coil can be said to be formed by those turns that don't effectively couple their magnetic field to the secondary therefore those few turns form what is called a leakage inductance and is regarded as equivalent to an external inductor in series with the "ideal" primary.
However, that leakage inductance (as a single component) represents both the primary turns and the secondary turns that don't couple magnetically. In other words, the primary parasitic inductance value that you might read in a data sheet represents (in one component) the leakages of both primary and secondary.
Some data sheets will specify both leakages but they can be grouped mathematically (and quite easily) into one component at the primary.