US Patent 7446641 Balance transformer

A small loop path 9A composed of a part of each of a first core 51 and a second core 52 is disposed with two windings 2A and 3A of a transformer portion 4A, and a small loop path 9B composed of another part of each of the first core 51 and the second core 52 is disposed with two windings 2B and 3B of a transformer portion 4B. A magnetic flux generated at the transformer portion 4A circulates along the small loop path 9A, and a magnetic flux generated at the transformer portion 4B circulates along the small loop path 9B with each flux circulating in a mutually reverse direction.