Patent ID: 8023294

Claim:
A method of reducing common-mode noise from unshielded coupling in a switch mode power supply; said switch mode power supply having a power input, a switch, a transformer, and a power output; said transformer having at least a primary winding on a primary side of said power supply and coupled to said power input via said switch, and a secondary winding of said transformer on a secondary side of said power supply and coupled to said power output; said common-mode noise from unshielded coupling comprising a noise voltage on a second of said primary and secondary windings coupled from a first of said primary and secondary windings, the method comprising: generating a noise suppression signal in antiphase to said noise from unshielded coupling using a voltage source comprising a winding of said transformer on a first power supply side that is one of said primary side and said secondary side of said power supply; and coupling said noise suppression signal into said transformer using a coupling structure capacitatively coupled to a winding of said transformer on a second power supply side that is the other one of said primary and secondary sides of said power supply, wherein the said winding on the second power supply side is the primary winding or the secondary winding; and wherein said coupling does not use a shield or screen between said primary and secondary windings of said transformer, and wherein said coupling structure is selected from a group consisting of: an electrostatic shield outside and adjacent said secondary winding, a flux band of said transformer, and an unearthed element of a core of said transformer.