Electrification plays a significant role in combating climate change, and the electrification of aviation has been a challenge, particularly because of the strict power density requirements of the power and propulsion systems in electric aircraft. Recently, hydrogen-fuel celled planes have been a promising solution to this problem, but two of the major obstacles with this new technology are various voltages needed on an airplane and the cryogenic temperatures involved with liquid hydrogen. One critical voltage level needed is a low voltage 28VDC bus that is used to power all the avionics on an aircraft and will be the objective of this two part thesis. The first part of this thesis details the construction and design decisions of a double pulse test testbench to observe and analyze the hard switching power loss and the dynamic resistance characteristics at various drain source voltages of cryogenically cooled Gallium Nitride transistors. The data and information gathered from the test is analyzed and used for the second part of the thesis to build a prototype converter that is capable of converting the output from a typical aircraft-sized hydrogen fuel cell (250VDC) to the aircraft standard low voltage (28VDC) that has features optimized for cryogenic temperatures. The novel contributions of this thesis involve investigating the dynamic resistance behavior of pure enhancement-mode GaN transistors at cryogenic temperatures and the construction of a cryogenic power converter based on pure enhancement-mode GaN transistors.