Managing soft-switching constraints

One of the fundamental conclusions to be drawn from the study of a switching cell consisting of two switches, two sources and no reactive elements is that the operating point of the controlled switch must pass through a zone in the plane (v _K , i _K ), passing through the point (V, I) corresponding to a very high instantaneous power for the duration of the switching operation (see [D 3 076] , § 2). In order to limit the losses corresponding to this path in the dissipative zone, known as switching losses, it is important to carry out this switching as quickly as possible. Switching speed depends in particular on the characteristics of the semiconductors and the way they are controlled, and improving this characteristic is one of the main objectives of semiconductor technology development. It should be noted, however, that increases in dV/ dt and dI/ dt lead respectively to an increase in the currents induced in parasitic capacitances and the voltages induced in the meshes of surrounding circuits, thus creating a number of generally undesirable interactions grouped under the generic term of electro-magnetic compatibility (EMC). Consequently, while increasing switching speeds is one way of reducing switching losses, it would be dangerous to go too far in this direction. This is all the more true as fast switching accentuates phenomena resulting from component imperfections (diode crossover current in particular) that can lead to additional losses and excite fast oscillatory modes. Other ways of reducing switching losses in semiconductors have therefore been considered.

• Switching assistance circuits don't actually reduce switching losses, but transfer them to auxiliary elements. These circuits were practically indispensable when power semiconductors had mediocre switching performance, but today, given the meteoric progress of modern semiconductors (IGBTs in particular), their use is virtually reserved for very high-power applications (> 1 MW) using IGCT-type semiconductors.

• Switching aid circuits have also made it possible to devise the concept of soft switching, which consists in slightly modifying the switching cell to reduce switching losses in the switch, but without transferring them to auxiliary elements.

• Finally, there's another way of reducing the constraints of the traditional switching...