A Technique for Increasing Power Transistor Switching Frequency

The use of bipolar transistors in high-frequency power conversion applications is often limited by device storage time. Storage time is related to saturation effects in the power transistor, and an example is shown of the application problem caused by excess storage time in a single-phase inverter operated at variable load. The general principle of reducing storage time by quasi-saturated operation has been known but not well documented. The operation of a quasi-saturated transistor switch is described, showing the improvements in storage time over operation in classical saturation. Further, the use of quasi-saturation techniques is extended to high-power Darlington transistors, where improvement in storage time is more significant than for the simple bipolar device. Results are given for both a 50-A bipolar transistor and a 50-A Darlington transistor. Practical guidelines are given for selecting suitable circuit components.