Hi all. How can we have a converter with a floating output voltage but a fixed constant current. For e.g., we need an LED Driver with a constant current of 350 mA but can be used to drive 1 or 2 or some n number of LEDs in series. So, for 1 LED - 3.3 x 0.350 = 1 W; 2 LED - 2(3.3) x 0.350 = 2.3 W and so on.

An isolated current source converter.

I can think of a dozen or more different solutions, but I would suggest an isolated 2 switch forward converter with a properly sized output inductor to average the load current. Subtract the measured load current from your commanded current to derive your PWM command. The maximum number of LEDs will be limited by the primary voltage and the turns ratio of your transformer. For a "simple" forward converter the maximum output voltage would be Vin/(2 * TR).