This paper describes the design trade-offs presented in DC-DC power converters used in space applications. The major concern, in the design of power supplies for space applications, is to obtain a very high reliability level, which can be very difficult to quantify, considering the relatively low quantities production involved. Space converters have to deal with radiation effects, vacuum environment, shock and vibration and comply with EMI requirements. To cope with these stringent requirements several considerations have to be taken. Selection of the proper converter topology suitable to support radiation effects is of major importance. Component selection to resist radiation, mechanical dimensions together with thermal analysis to support vacuum conditions, are key parameters to achieve a high reliability level.