Converting appliance LEDs from constant current to constant voltage to allow for dimming
st2000's answer is on the right track, but does not go far enough. A constant voltage drive is likely to destroy the LED. The problem is that, for a given voltage, the current drawn will increase with increasing LED temperature. In the worst case, as the LED warms up, it will draw more current. Since power is voltage times current, the power dissipated by the LED goes up. This increases the LED temperature, which causes the current to increase, etc. It's called thermal runaway. Don't do it.

And yes, you get around this by adding a series resistor with a decent voltage drop across it, so that small changes do not have big results. And yes, it's terribly inefficient. The solution is to drive the LED with constant current. You can make a switching constant-current supply which is compact and efficient - which is why commercial LED drivers do it that way. 

Granted, this isn't the answer you're looking for, but reality can be cruel that way.