Overview

In Chapter 1, the basic forward converter was covered extensively. The topology treated is a single-transistor version of the forward family. As such, the power switch shoulders all the burden of providing load power and power consumed locally. The configuration therefore is limited to low-power applications, for instance, less than 100 W, due to concerns for transistor ratings and thermal management. In addition, the transformer used in the single transistor version utilizes only the first quadrant of the core B-H plane. It not only carries a DC current and underutilizes the magnetic core, but also requires external core reset. In light of these deficiencies, solutions must be found. This is where the push-pull configuration steps in.

The conventional push-pull topology employing a single-loop voltage-mode control comes in several forms. Figure 2.1 shows the two most popular implementations. Both configurations enjoy the benefits of dual transistors, four-quadrant core B-H utilization, and natural magnetic reset. However, they all suffer potential flux imbalance and, consequently, core saturation. Numerous techniques were conceived to prevent the threat. Most result in additional circuits but with only limited success. It turns out that peak-current current-mode control easily alleviates the problem by, in effect, placing a current limit on the power switches. With this technique, albeit with minute volt-second imbalance, the power switch is never subjected to destructive overcurrent conditions. Given the advantage, this chapter gives the complete coverage of a center-tap push-pull converter with peak-current current-mode control, slope compensation, and the ability to operate...