Cyclic photophosphorylation is part of Photosystem I (PS I). It is perhaps the earliest way cells formed ATP.

In modern plants, it occurs when so much NADPH is made that no NADP+ is available to pick up the energized electron & H+ (NADP+ is the final electron acceptor). This potentially could shut down PS I. Instead, the excited electrons are shunted through the cyclic electron transport chain allowing it to keep using the excited electrons while creating ATP.