Push-pull converters require transformers with center-taps to transfer power from the primary to the secondary (see Figure 7-3).

Figure 7-3 Switching Cycles of a Push-Pull Converter

When Q₁ conducts, V_IN drives a current through the lower half of the primary to ground, thus creating a negative voltage potential at the lower primary end with regards to the V_IN potential at the center-tap.

At the same time the voltage across the upper half of the primary is such that the upper primary end is positive with regards to the center-tap to maintain the previously established current flow through Q₂, which now has turned high-impedance. The two voltage sources, each of which equaling V_IN, appear in series and cause a voltage potential at the open end of the primary of 2×V_IN with regards to ground.

Per dot convention the same voltage polarities that occur at the primary also occur at the secondary. The positive potential of the upper secondary end therefore forward biases diode CR₁. The secondary current starting from the upper secondary end flows through CR₁, charges capacitor C, and returns through the load impedance R_L back to the center-tap.

When Q₂ conducts, Q₁ goes high-impedance and the voltage polarities at the primary and secondary reverse. Now the lower end of the primary presents the open end with a 2×V_IN potential against ground. In this case CR₂ is forward biased while CR₁ is reverse biased and current flows from the lower secondary end through CR₂, charging the capacitor and returning through the load to the center-tap.