This paper studies the control of dc-dc converters subjected to constant-power loads using a passivity-based technique. This technique allows for global analysis and avoids the limitations present in small-signal-based techniques. Passivity-based control in this application leads to a systematic derivatton of a linear proportional-differential controller for a buck converter and a nonlinear proportional differential controller for boost and buck-boost converters. The latter are emphasized here. It is discussed how for boost and buck-boost converters an equivalent linear proportional differential controller achieves stable operation with a constant power load. It is also suggested that for all types of converters an integral control term achieves line and load regulation. Experimental work is included to verify the analysis.