Impact of switching sequence on zero current switching performance of power bipolar semiconductor devices

Bipolar devices significantly differ in their performance and behaviour in any given application. The knowledge of their behavior in order to design both devices and circuits that would allow less switching losses is essential. In this paper, a distinct and interesting phenomenon in the charge dynamics of BJTs and IGBTs under ZCS operation is reported. It is shown that the difference in charge dynamics results in contradicting switching sequence requirements between BJTs and IGBTs. It is shown, with the help of two-dimensional device-circuit simulations, that in IGBTs, a majority of excess carriers are flushed out during the no-current regime while in the case of the BJT, most of carriers are removed by recombination after the device is turned off. In order to use the semiconductor switch optimally, IGBT turn-off should be delayed as much as possible, while, in the case of a BJT, the switch needs to be turned off as soon as possible after the zero-current condition is established. A careful choice of switching sequence is shown to result in a reduction in switching power loss by a factor of 2 in the case of the BJT and a factor of 3 in the case of the IGBT.