It could also be a thermal cut-out operating. Sometimes restricted airflow will cause that if vent slots have become partially clogged or if a cooling fan isn't functioning properly.
Many of the latest designs use triacs on the control board to switch power to the u-wave power supply, heating elements etc.
The primary power circuits aren't usually too difficult to trace out -- Just a combination of thermal cutouts, microswitch interlocks and so on, although the exact arrangement can vary from model to model.
Many designs have three door switches, for example. Two are series interlocks, and the third is a "brute force" safety switch which puts a dead short on the supply to blow the fuse just in case the other two fail!
