According to Ptolemy, the Sun resided between Venus and Mars and was sorted among the planets. The card depicts the sun god Helios (Roman Sol), driving his chariot across the skies. As a little extra the artist had added the unlucky Phaeton falling from the chariot - at least this is assumed. Legend had it that Phaeton, Helios' son, drove the chariot by himself, failed to control it and almost set the world on fire. To prevent disaster, Zeus struck him dead with a lightning bolt. The legend failed to explain how exactly that saved the earth and who exactly took control of the leaderless chariot after Phaeton was toast, but I digress...
Note the difference of the Sun in this card to the 'Genius of the Sun' in the card XXXI Iliaco. Iliaco presented the energy generated by the Sun, the life force emanating from it - Sol is the Sun itself. The trump card of the Sun in traditional Tarot can likely be seen as a combination of both.
Further reading: Sun (Planets)