Charakterisierung und Einfluss parasitärer elektromagnetischer Effekte in leistungselektronischen Systemen

Abstract

Structure-caused parasitic electromagnetic effects have a great importance in power electronic modules and systems, since they affect both their function mode and the security of the system. This work deals with the extraction of parasitic elements from the circuit layout based on field theory, clarifies its different influences on the switching process of power semiconductors and points constructional proposals for solution out for the avoidance or reduction of parasitic effects.Using a PEEC ("Partial element Equivalent Circuit") approach it is shown, how partial elements with consideration of skin- and proximity-effect can be determined by suitable computer simulations from the power electronic circuit layout. For typical module dimensions partial self and mutual inductances of the conductors, bond wires and busbars are indicated, whereby also the influence of the substrate thickness and the dependence on the frequency are examined.It is shown by simulations that layout-dependent inductive positive and negative feedbacks between power and control circuit can affect the switching behaviour of power transistors considerably. It is illustrated, as this inductive coupling can be used consciously, in order to adjust switching speeds and to realize a symmetrical switching behaviour in the parallel connection of power devices.On the basis of a half bridge module on DCB substrate it is shown by simulation and measurement that eddy currents in the backside metallization of the substrate have a great influence on the symmetrical load of IGBTs in a parallel connection.During the turn-off phase of bipolar power devices paralleled in power modules very disturbing high frequency oscillations in the range between 70 MHz and 700 MHz have sometimes been observed. The excitation mechanism of these spontaneously appearing oscillations in the tail current phase can be explained by a transit-time diode effect which works in analogy to a BARITT-type diode. The new PETT-effect ("Plasma Extraction Transit Time") is based on a periodic extraction of holes from the electron-hole plasma in combination with a suitable transit-time of the charge carriers passing the space-charge region. The LC resonant circuit is formed by the junction capacitances of the power devices itself and the parasitic inductances between the chips arranged in parallel. The experiences from measurements are confirmed by suitable device simulations. The observed oscillations react very sensitively to changes in the operating parameters (temperature, DC link voltage and tail current level) and the natural frequency of the resonant circuit.In order to absorb PETT oscillations in the power module considerably or prevent their emergence, three new constructional measures based on the integration of resistive elements are presented. The effectiveness of these measures is confirmed by electromagnetic field simulations.During dynamic current measurements with magneto-resistive sensors in power modules measuring errors were observed because of parasitic proximity effects. It is shown by electromagnetic field simulation that the measuring errors can be substantially suppressed by adding a ferrite core or an eddy current plate brought in the position suitably.