A Spectre-based method is described for complex electrical-thermal simulations of smart power circuits. The packaged circuit of a system is divided into numerous small columns. They form a large electrical representation of the thermal network. The electrical circuit sends the dissipated power to the thermal network and receives the temperature. All transistors of the system have individual temperatures. An EKV transistor model with temperature input and power output is used. Transient simulations and measurements are compared.