Consequences of Relying on temporary hit points instead of armor for a homebrew class?

I am creating a homebrew tanking class for DND 5e, and I want to add some flavor rather than going with same formula as fighter, barbarian, and paladin.

I am thinking that instead of armor the class could rely on a reserve of temporary hit points to absorb damage. This reservoir of temporary hit points would be equal to half the hp maximum and would replenish fully every long rest, or when you regain hit points as part of a short rest, potion, or spell, you would regain half as many temporary hp.

What would the mechanical implications of such a class, and how would I prevent them from boosting their AC to make this combo OP?