Study and application of saturating, voltage dependent, non-linear turn-off snubbers for power electronic switches

• Main Author: Steyn, Charl Gerhardus
• Published: 2014
• Subjects: Power electronics; Semiconductor switches
• Online Access: http://hdl.handle.net/10210/8964

D.Ing. (Electrical & Electronic Engineering) === 'Ihis thesis considers the application of non-linear, voltage dependent, saturating capacitors as turn-off snubbers for power electronic switches. The concept of using a non-linear turn-off snuJ::ber for the relieving of semiconductor switching devices is shown theoretically and experimentally. As proven by the results, the most outstanding advantage of non-linear snubbers is the much smaller quantity of energy which is being stored, during and after snubbing, in such a non-linear snubber device, compared to normal linear snubber elements. Depending on the saturation level of such a non-linear capacitor, the stored energy can (for existing ceramic capacitors) be an order of magnitude lower than the energy in a comparable linear capacitor. After the profitability of using non-linear saturating capacitors has been demonstrated, the non-linear capacitive snubber is analyzed by means of a computer simulation which uses the exact capacitancevoltage curve, as stored on data file. The circuit is, however, also analyzed analytically by approximating the non-linear capacitor characteristics in two different ways: (L) by an exponential approximation, and (ii) by a two-step piecewise linearization. Most of the results are within about 20 %of those obtained by the exact analysis, and the approximate analyses can therefore be regarded as very useful. Using the analysis of the non-linear snubber, an optimization in terms of minimum energy losses is carried out on a general turn-off snubber circuit which consists of a non-linear saturable capacitor in a regenerative snubber configuration. The equations and results are awlicable to most snubber cases. From the results it is evident that the same performance as obtained with complex linear regenerative snubber circuits can be obtained by a simple non-linear dissipative snubber...