To optimize the lifetime of switching power semiconductors, this paper presents a methodology to control power device junction temperature ${T_j}$ and its change during power cycles $\Delta {T_j}$ at thermal boundaries. This paper proposes a supervisory state machine to interrupt nominal system-level control only when temperature bounds are exceeded, and coordinates smooth transitions as ${T_j}(k)$ and $\Delta {T_j}(k)$ approach their respective boundaries. To ensure that thermal states are regulated via precise and independent modulation of conduction and switching loss elements, decoupling methods are proposed. Also proposed is a $\Delta {T_j}$ control law that closes a control loop on the rate of change state ${\bar{\dot{T}}_j}$ , and introduces active thermal capacitance and conductance into the closed-loop thermal system dynamics. Experimental evaluation of the proposed system illustrates well damped ${T_j}(k)$ and $\Delta {T_j}(k)$ responses, and gradual adjustment of the manipulated inputs switching frequency and duty ratio. Finally, comparison with a current limit-based ${T_j}$ regulation method illustrates how the proposed system allows power converters to push harder against their thermal limits.