A Symmetry Correcting Pulsewidth Modulator for Power Conditioning Applications

A wide variety of power converters is of the transformer-coupled push-pull type and uses pulsewidth 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. Caused by small parameter dissymmetries of these transistors, asymmetrical switching 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 pulsewidth modulator circuit which can be advantageously utilized in the design of such converters, and which can effectively cope with the aforementioned symmetry problem. Symmetry correction is accomplished by monitoring the two primary currents and by providing proper feedback of the integrated difference between these two currents.