This paper proposes an Active Thermal Control method conceived to increase the lifetime of power semiconductor modules in liquid-cooled converters. By acting on the cooling system, the coolant flow is varied and consequently the thermal resistance of the coldplate. This way, the amplitude of the thermal cycles can be reduced and, therefore, the lifetime of the converter increased. A traction converter in an urban electric bus application has been selected as a case study in order to analyse the impact of the proposed method. To do this, a mission profile-based reliability evaluation is performed under the typical start-run-stop driving profile, and the improvements in terms of lifetime consumption extracted.