The present study deals with the transient thermal management of electro-optical equipment using the phase-change materials (PCMs). These materials can absorb large amounts of heat without significant rise of their temperature during the melting process. This effect is attractive for using in the passive thermal management of portable electro-optical systems, particularly those where the device is intended to operate in the periodic regime, or where the relatively short stages of high power dissipation are followed by long stand-by periods without a considerable power release. In the present work, a so-called hybrid heat sink is developed. The heat sink is made of aluminum. The heat is dissipated on the heat sink base, and then is transferred by thermal conduction to the PCM and to a standard forced-convection air heat sink cooled by an attached fan. The whole system may be initially at some constant temperature which is below the PCM melting temperature. Then, power dissipation on the heat sink base is turned on. As heat propagates within the heat sink, some part of it is absorbed by the PCM causing a delay in the temperature growth at the heat sink base. Alternatively, the steady-state conditions may be such that the base temperature is below the PCM melting temperature, meaning that all the heat generated on the heat sink base is transferred to the cooling air. Then, the fan is turned off reducing the heat transfer to the ambient air, and the heat is absorbed into the PCM resulting in its melting. In both cases, the time that it will take the heat sink base to approach some specified maximum allowed temperature is expected to be longer than that without the PCM.