We propose a minimal model of a Coulomb-coupled fermionic quantum dot thermal diode that can act as an efficient thermal switch and exhibit complete rectification behavior, even in the presence of a small temperature gradient. Using two well-defined dimensionless system parameters, universal characteristics of the optimal heat current conditions are identified. It is shown to be independent of any system parameter and is obtained only at the mean transitions point "-0.5", associated with the equilibrium distribution of the two fermionic reservoirs, tacitly referred to as "universal magic mean".
Keywords: coulomb blockade; master equation; quantum dots; quantum thermodynamics; thermal diode.