Study and Application of Nonlinear Turn-Off Snubber for Power Electronic Switches

A nonlinear capacitive turn-off snubber is discussed. Customarily, only linear capacitors are used in high-frequency power electronic switches. However, ceramic ferro-electric materials, with ratios of initial to final capacitance of nearly 30, are found suitable and are investigated. The energy stored in these nonlinear capacitors, with an initial capacitance of 300 nF, amounts to only about 1.1 mJ at 300 V compared to 13.5 mJ in a 300-nF linear capacitor. As far as the power switch is concerned, the difference in switching behavior and switch turnoff loss between these two cases is negligible. This nonlinear snubber is analyzed approximately by assuming an exponential relation between capacitance and voltage, as well as with a more accurate analysis by means of a computer program. The latter uses an exact capacitance-voltage curve stored on a data file. Computed values correlated well with experimental measurements. As the dielectric losses in the ceramic material are very important, careful calorimetric measurements in situ revealed an energy loss in the capacitor of 135 μJ per turn-off cycle, taken at 500 Hz, 30 A, and 300 V.