Ah, well, if I knew what it actually does, I probably wouldn't be asking what it is in English
Well sometimes with technical translations, even if you know exactly what the object is, it can be hard to find the right technical term English.
Further input from other sources indicates that "platine" is short for "platine d'alimentation", and the platine refers to the fact that the unit in question includes a circuit board with the ballast required for such bulbs and usually also the integrated power supply/transformer. In the context, it would therefore seem appropriate to use "external ballast" so that the potential purchaser understands the installation constraints on the use of such an item.
I'm concerned that
platine is a more general term than "external ballast." An electrical ballast is a current limitation device... and the term in French is
un ballast. I believe the word
platine refers to a "plate" (i.e., a flat planar surface) on which the necessary components (e.g., ballast, power supply, etc.) are mounted. So the word gets used to talk about the whole circuit board assembly.
For the category "éclairage", the GDT defines
platine as
Élément rigide métallique ou isolant sur lequel sont fixés les accessoires nécessaires au fonctionnement des lampes à décharge.
...and translates it as
mounting plate. This sounds pretty good to me.
