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Power module heat dissipation with spatial inhomogeneity and induced nonuniform temperature distribution presents a challenging concern for system reliability due to thermo-mechanical stresses. These reliability challenges are especially important for nonplanar designs using 3-D packaging principles. Here, we develop a heat spreader thermal switch capable of actively reducing temperature gradients between silicon carbide (SiC) devices in a three-level T-type power converter, which can change depending on electronic operating conditions. The heat spreader thermal switch consists of a stainless-steel (SS) heat spreader and a copper sliding switch embedded within the spreader. Heat transfer from the SiC devices and within the spreader can be controlled by moving the slider between positions within the spreader. To understand heat transfer mechanisms and design the heat spreader thermal switch, we conducted 3-D finite element method simulations to calculate the minimum attainable temperature difference between SiC devices. We used the finite element simulation to quantify the reduction in junction temperature swing during dynamic operation and coupled the results to the Coffin–Manson reliability model to quantify lifetime to failure. We integrated the heat spreader thermal switch with one phase of a three-phase T-type converter and demonstrated isothermalization at different working conditions. At 2.4-kW converter power, each hot SiC device dissipated 4.7 W of heat, resulting in a device case temperature of 43 °C, with each cold SiC device dissipating 1.7 W at 38 °C. The device-to-device temperature difference was decreased from 5 °C to 0 °C by moving the switch a distance of 20 mm. Finite volume method (FVM) simulations of the conjugate heat transfer problem validate the experimental results and support analysis of key performance parameters. This work demonstrates successful isothermalization of a power converter with an active heat spreader thermal switch and develops simulation and design guidelines for successful electro-thermal codesign and implementation of thermal switches for other electronics applications for which isothermalization and enhanced device reliability is a key issue. [ABSTRACT FROM AUTHOR]