Goleta-based Transphorm, which is developing Gallium Nitride (GaN) power conversion technology which is far more efficient than existing power transformer technology, has raised another $18M in funding, part of an ongoing $20M funding round for the company. The funding was disclosed in a regulatory filing earlier this week. Details on the funding, which came from four investors, have not yet been announced by the company. Transphorm has previously raised funding from Bright Capital, Foundation Capital, Google Ventures, Innovation Network Corporation of Japan. Kleiner Perkins Caufield and Byers, Lux Capital, and Nihon Inter Electronics Company. The new round will take Transphorm well beyond $100M in venture capital investment so far in its technology.