Abstract
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...