LED street lighting

Hi, First of all please only people who has a lot of experience with LEDs. I am trying to put together a LED street light prototype and I need help with the circuit so the whole thing does last as long as possible, does not blow up in the long term because of some electrical moshumushu magical thing and is very power efficient (every % of the power consumption matters!). 1) I need to know how to wire up high power LEDs in multiple strings where each of them can be dimmed by PWM signal from the micro controller. What I understood so far is that this will require "constant voltage" power supply which can be a bit tricky with LEDs. For example I need to wire up 12 LEDs ([login to view URL]) into 2 to 4 strings, each dimmed with PWM and I am interested in how to do it in the most power efficient and safe way to do it so the LEDs don't suffer in the long term. 2) Secondly I would like to know how do I wire up for example 20 high power LEDs (the same ones) in the most power efficient and safe long term functioning way where all 20 LEDs can be controlled by a single PWM signal.