  We discuss very low temperature experiments on superconducting micro-coolers
made of a double Normal metal - Insulator - Superconductor junction. We
investigate with a high resolution the differential conductance of the
micro-cooler as well as of additional probe junctions. There is an explicit
crossover between the single quasi-particle current and the phase-coherent
Andreev current. We establish a thermal model by considering the thermal
contribution due to the Andreev current. The related increase of the electron
temperature is discussed, including the influence of several parameters like
the phase-coherence length or the tunnel junction transparency.
