Patent ID: 7385438

Claim:
An active filter for reducing a common mode current in a PWM switched power supply, said power supply comprising an AC source, a rectifier connected to said AC source which produces a rectified output voltage between a positive DC bus and a negative DC bus, a PWM inverter having input terminals coupled to said positive DC bus and negative DC bus and having a controlled AC output adapted to be connected to a load device, a ground wire adapted for connection to the load device, and a current sensor which measures a common mode current and produces an output current related to the common mode current, said active filter comprising: first and second transistors, each having first and second electrodes providing a controlled current path and a control electrode, a DC isolating capacitor, and said first electrodes of said first and second transistors are connected to a common node; said second electrodes of said first and second transistors are connected to said positive DC bus and said negative DC respectively; said DC isolating capacitor is connected between said common node and said ground wire; and said current sensor output is coupled to said control electrodes for producing said output current related to said common mode current, and a headroom voltage control circuit coupled to said first and second transistors for maintaining sufficient voltage between the first and second electrodes of each transistor to permit linear amplification operation of each transistor over substantially the full rectified cycle of said AC source, wherein said current sensor comprises a differential current transformer having first and second windings connected in series with said positive and negative DC busses respectively and an output winding coupled to said control electrodes to provide said output current related to said common mode current.