Wow. Nice little explanation. Why is it important if it is ac or dc?
The ballast is located outside the unit. Which is a plus in the case of having D/C LED's because, the light bar itself wont electrocute you if it would happen to land in the water (use extreme caution anyway LOL). and the cord going to the light unit is then D/C current. To me, it is just a safer and more ideal way of doing things, unlike some units that have the A/C going into the unit with the ballast inside and in turn having heat build up do to the ballast (heat is the enemy when it comes to LED's.)
Also, if the ballast is inside the unit, a larger unit is needed to accommodate the ballast.
and has to be able to transfer more heat (under a hood can always be a challenge, especially those of us that have used t5's and 8's and especially halides have posed the worst problems. Without the use of multiple fans and LOTS of additional noise). I have personally used adjustable snap switches (an on/off switch that controls the fans to turn on and off at a set tempature and off about 20F cooler) and was able to keep my hoods quite cool under high heat conditions with great success, but with the additional electric use and more moneys spent, not to mention the loss of the tranquil and peacefulness of a planted tank we loose the affect with fan noise.
Sorry went on a tangent LOL- Actual A/C LEDs have a few draw backs as with any A/C device there is a loss of power which is usually exchanged for heat. True A/C LEDs can also have a shorter lifespan do to the dual cathode/anode in one unit which creates more heat. I also believe A/C LEDs (not positive) are less energy efficient. Not to mention, if one side fails you have one color that "blinks very rappidly" but will not produce the color you wanted anymore and will be half of the color it was suppose to be (In my post I describe the green/red led to produce greenish yellow, if one side fails you have the remaining side still lit)
My electronics degree is quite old (1994 associates degree in electronic engineering) and a lot has changed since the days of my knowledge base, if I am mistaken in anyway please feel free to help clarify. Part of my education still involved "tube type" with "solid state", so I'm a bit outdated LOL.