8.3.1 Push-Pull Converter

Push-pull converters require transformers with center-taps to transfer power from the primary to the secondary (see Figure 37).

Figure 37. 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 in order 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.

8.3.2 Core Magnetization

Figure 38 shows the ideal magnetizing curve for a push-pull converter with B as the magnetic flux density and H as the magnetic field strength. When Q₁ conducts the magnetic flux is pushed from A to A’, and when Q₂ conducts the flux is pulled back from A’ to A. The difference in flux and thus in flux density is proportional to the product of the primary voltage, V_P, and the time, t_ON, it is applied to the primary: B ≈ V_P × t_ON.

Figure 38. Core Magnetization and Self-Regulation Through Positive Temperature Coefficient of R_DS(on)

This volt-seconds (V-t) product is important as it determines the core magnetization during each switching cycle. If the V-t products of both phases are not identical, an imbalance in flux density swing results with an offset from the origin of the B-H curve. If balance is not restored, the offset increases with each following cycle and the transformer slowly creeps toward the saturation region.

Fortunately, due to the positive temperature coefficient of a MOSFET’s on-resistance, the output FETs of the SN6501 have a self-correcting effect on V-t imbalance. In the case of a slightly longer on-time, the prolonged current flow through a FET gradually heats the transistor which leads to an increase in R_DS-on. The higher resistance then causes the drain-source voltage, V_DS, to rise. Because the voltage at the primary is the difference between the constant input voltage, V_IN, and the voltage drop across the MOSFET, V_P = V_IN – V_DS, V_P is gradually reduced and V-t balance restored.

8.4.1 Start-Up Mode

When the supply voltage at Vcc ramps up to 2.4V typical, the internal oscillator starts operating at a start frequency of 300 kHz. The output stage begins switching but the amplitude of the drain signals at D1 and D2 has not reached its full maximum yet.

8.4.2 Operating Mode

When the device supply has reached its nominal value ±10% the oscillator is fully operating. However variations over supply voltage and operating temperature can vary the switching frequencies at D1 and D2 between 250 kHz and 495 kHz for V_CC = 3.3 V ±10%, and between 300 kHz and 620 kHz for V_CC = 5 V ±10%.

8.4.3 Off-Mode

The SN6501 is deactivated by reducing V_CC to 0 V. In this state both drain outputs, D1 and D2, are high-impedance.