Patent ID: 6842069

Claim:
An active EMI filter for reducing common mode noise current in a circuit comprising a rectifier coupled to an AC network, the rectifier supplying DC power to a DC bus, the DC bus feeding an inverter stage for providing AC power to a load, the load having a ground return line to a ground connection of the AC network, the active filter comprising: a transistor stage comprising two transistors coupled in series; a current sensor for sensing the common mode noise current in said circuit, said current sensor comprising a current transformer having a primary coupled to a branch of said circuit in which common mode noise occurs; the current sensor having an output coupled to the transistor stage, the transistor stage being driven by said current sensor in response to the common mode noise current; a capacitor coupling said transistor stage and said ground return line; said capacitor providing a cancellation current to said ground return line from said transistor stage to substantially cancel the common mode noise current in the ground return line; one of said two transistors of said transistor stage being coupled such that a respective main electrodes thereof is coupled to a respective legs of said circuit in which common mode noise occurs, another corresponding main electrode of said transistor being coupled by a respective secondary of said current transformer to a common node, and the control electrode of said transistors is coupled by another respective secondary of said current transformer to said common node; and the other of said two transistors being counted such that a respective main electrode thereof is coupled to said common node, another corresponding main electrode of said transistor being coupled by another corresponding secondary of said current transformer to another leg of said circuit in which common mode noise occurs, and the control electrode is coupled by another respective secondary to said other leg of said circuit.