GaN-transistors are a new kind of power transistors, which can be used for example on power supplies. GaN-transistors are tinier compared to traditional power transistors and their switching frequency can be over 1 MHz. There are only a few surveys done about the GaN-transistors at this moment, so there was a need for a power supply prototype to research GaN-transistors. This bachelor's thesis explains about realizing a PCB and a heatsink for this power supply. The main issues were reducing capacitive and inductive coupling and inventing a way to cool switching losses down. The heatsink was tested by measurements and it was proven that the heatsink can cool down the switching losses below 112 watts to keep the GaN-transistors below their highest operating temperature 150 degrees celsius.