This Wikipedia entry specifically on the SMPS part, says:
Switched-mode power supplies are always regulated. To keep the output voltage constant, the power supply employs a feedback controller that monitors current drawn by the load.
However, I've found a range of mobile phone powersupplies, which claim to be switched-mode, but specify their output as being 4.7-6.5VDC at 900mA max, rated for 4W. Given that these were switch-mode and a range being specified, is what struck me as being contradictory. So, I am wondering if these are fakes ?
In fact, I went ahead and bought one of these, but at the moment, I am far from home and don't have my tools. It seems to be able to charge my Android phone, and is also able to power a RaspberryPi -- though I tried just the boot sequence and then switched-off, afraid of damaging the board. One thing in favor of their claim to being switched-mode, is that they are pretty light weight, and relatively small.
So, is the range, only because their manufacturing is not under tight quality control, and they actually operate at a fixed + regulated output voltage, but there could be variations within that range, from one piece to another ?
Edit: The power-supply is as such meant as generic / back-market replacement charger for Blackberry smartphones, that expect 5V supply.
My primary use for these supplies is to power a cache of RaspberryPi's (model-B), with minimal peripherals. The supplies available closer home, cost more than twice as much, so was wondering if this could be a good money saving bargain.
Edit2: (Feb 11, 2013) If the switch-mode power supply output is indeed unregulated, would a LDO as an intermediate step be a good way to regulate the final output to 5V ? Are there any inexpensive (<= $1) 5V LDO's which could do this job ?