A converter interface is a component of a plasmadyne relay. Depending on the design of the plasmadyne relay, a secondary phase inverter might be required in order to prevent the interface's failure due to excessive warp field stress.
In 2370, upon examining a plasmadyne relay aboard the USS Enterprise constructed by Geordi La Forge, Cadet Wesley Crusher sardonically mentioned that a secondary phase inverter would be required, despite La Forge's insistence that the relay could withstand over 500 cochranes of stress. (TNG: "Journey's End")