In this work, the avalanche breakdown phenomenon in low-voltage trench power MOSFET is investigated through two-dimensional device simulations. The breakdown dependence from device geometrical characteristics is carefully characterized with particular attention to trench depth and drain region length and doping. An analytical model for breakdown calculation in a trench structure is presented. The analytical model is based on a well-known p + -n unilateral junction model, with a maximum electric field value correction to take into account the trench structure perturbation. It shows good results for a wide range of drift region characteristics and trench lengths.