# Electronic – Center-tapped transformer and inductance calculation for transformer driver

inductanceswitch-mode-power-supplytransformer

Using a push-pull transformer driver, for example the LT3439 like below with a center-tapped transformer, more precisely this one here So we there is a center tapped transformer (with 1:1.3 ratio between two "full" windings, without the tap) and a half-bridge rectifier. At each half-cycle of the switching frequency, a current flows through half the primary and half the secondary at all time.

I wonder now:
The linked transformer datasheet says the primary inductor (the whole primary winding if I understand correctly) is 475uH, and the ratio is $$\a = V_p/V_s = 1/1.3\$$.

So I wonder what is the equivalent inductance at the secondary. Is it $$\L_p \cdot 0.5^2 \cdot (1/a^2) = 200uH\$$ or I'm I wrong.

Given $$\|Z_L| \propto L\omega\$$ and the general formula $$\|Z_{sec}| = Z_{Lprim}/a^2\$$ (load seen at the secondary), and the fact $$\L_{coil} \propto N^2\$$ (N=# of turns), I would think it's a correct assumption but I'm far from being sure. The 0.5^2 factor thus would be because current flows only in half the transformer coils at all instant.

Can anyone confirm? The reason I'd like to know the inductance at the secondary is to calculate some RLC filter with it (to match the output filtering to my desire), taking directly the L of the transformer into my calculations. I'm also interested at the answer for the sake of theory.

Thanks!