M.Ing. (Electrical & Electronic Engineering) / The technology of the use of electronic devices as power switches is still being developed. Because of the increasing demand for low mass and cost, the switching frequency must be as high as possible. The limiting factor for the frequency is the energy loss which is dissipated in the semiconductor crystal during each switching-transient. If tllis switching loss can be reduced, a higher frequency can be obtained. The switching loss is due to the non-instantaneous switching process of the semiconductor device. For the reduction of this switching loss, the device must be switched as fast as possible via its control-electrode. Further improvement can be obtained through the use of snubber networks. This thesis considers the bipolar high-voltage transistor as a power switch. The unfavourable switching loci of the transistor- switch is discussed in chapter 1. In chapter 2 the basedrive during transistor turn-off is investigated, in order to reduce the turn-off time. The following chapters de;al with the use of snubber circuits, which relieve the transistor during turn-on and turn-off. The advantages, as well as the disadvantages and limitations of snubbers are discussed. After the linear turn-off and turn-on snubbers have been treated, the non-linear turn-on snubber, which uses a saturable inductor, will be considered from an experimental point of view. The results show that this snubber is very effective in relieving the transistor during turn-on. In chapter 7 the non-linear turn-off snubber is treated on an experimental as well as on a mathematical bas e . Cri teria, which must be satisfied for realisation of a practical nonlinear capacitor, is also laid down. Because of the fact that a first order snubber is always loading the transistor during the complimentary switchin,]-transient, chapter 8 deals with a second order snubber, which consists of both a capacitor and an inductor. Experimentally it was found that the turn-on time is reduced with the use of a turn-on snubber, while the turn-off time is increased with the use of a turn-off snubber. In chapter 9 the physical behaviour of the switching processes is investigated, after which the influences of the snubbers on the switching times is explained qualitatively...
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:uj/uj:3983 |
| Date | 13 February 2014 |
| Creators | Steyn, Charl Gerhardus |
| Source Sets | South African National ETD Portal |
| Detected Language | English |
| Type | Thesis |
| Rights | University of Johannesburg |
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