A symmetry correcting pulse-width modulator for power conditioning applications

A wide variety of power converters of the transformer-coupled push-pull type uses pulse-width modulation for output regulation. In designing and operating such converters, serious complications usually arise as a result of unsymmetrical switching of the two power transistors. Asymmetrical switching, caused by small parameter dissymmetries of these transistors, normally drives the transformer into saturation. Resulting excessive saturation currents reduce efficiency and put a severe strain on the power transistors, which can eventually cause circuit failure. This paper presents a new pulse-width modulator circuit which can be advantageously utilized in the design of such converters, and which can effectively cope with the above mentioned symmetry problem. Syrnmetry correction is accomplished by monitoring the two primary currents and by providing proper feedback of the integrated difference between these two currents.