A cooling unit for cooling in particular power semiconductors contains a distributor for guiding liquid across a surface to be cooled. The distributor comprises an inlet manifold (8) and outlet manifold (9), whereby the inlet and outlet manifolds are connected through a flow cell, which has a main flow channel (50). The main channel is formed as a meandering sequence of channel segments (61,62,63,64). It has been found, that the transfer of heat by the liquid in the main flow channel can be improved by introducing a bypass flow channel (71,72,73) which allows the flow of liquid from the cell inlet to the cell outlet, wherein the bypass flow channel interconnects the channel segments of the main flow channel.