Investigation of MOSFET power losses in emergency mode of buck converter

Operation modes of the compact SOT-23 package MOSFET in the high-frequency buck converter are considered. Transient analysis of a switching processes in the MOSFET based on a reliable SPICE model is obtained. Analysis were carried out in a wide variation of the supply voltage and load resistance in order to estimate energy losses in desired and emergency modes accompanied by a large amplitude of output voltage fluctuations of the converter. Converter operation in emergency modes with the increased transistor losses, which is related with an increase of the effective current value is shown. The possibility of high-frequency ripples occurrence on the transistor during the operation of the converter in the discontinuous-current mode is demonstrated. A circuit, consisting of the parasitic capacitance of the transistor and the inductance of the output filter which requires taking measures to compensate for these oscillations on practice in the discontinuous-current mode is shown. Investigated transistor demonstrates acceptable behavior in the circuit with the selected parameters, and the increase in energy losses during the transition to emergency modes is obtained. It requires special attention to design process for converters based on small-sized transistors in the SOT-23 package, since the latter have limited heat sink capabilities.