What to look out for in a wide output DC - DC converter?

[tmadness]

My switching power supplies fundamentals are a bit rusty . if I wanted to design a dc dc converter with constant input voltage but variable output voltage, what should i be looking out for in a converter/controller?
a few parameters that come to mind:
Min switch on time, determines how low the output voltage can go.
what about switching characteristics? PFM vs PSM vs PWM? this should not be an issue... I think.
feedback mechanism? again should not matter.
what else? inductor choice?  angle of the sun and moon?

[moffy]

If going for PWM(I am talking about a transformer with secondary and rectifier) to get the variable output voltage vs duty cycle the output choke has to be conducting continuously. This means either large choke or a large static load, otherwise for low conduction of the output choke you are going to get a voltage leaning towards the peak pulse voltage. Flyback can be a little easier as the energy transfered is LI*I/2 for each pulse. You can adjust the frequency of the pulses to match the load/voltage, but ripple can sometimes be an issue.