Active common mode filter connected in a-c line

Abstract

An active filter for reducing common mode current in a pulse width modulating driving circuit for driving a load from a-c lines of an a-c source, wherein the driving circuit includes a rectifier connectable to the a-c lines and producing a rectified output voltage connected to a d-c bus, a PWM inverter having input terminals connected to the d-c bus and having a controlled a-c output for driving the load, the active filter comprising a current sensor for sensing the common mode current in the driving circuit, an active filter circuit comprising first and second transistors, each having first and second main electrodes and a control electrode, the transistors being coupled in a series circuit to feed an output node, the transistors being controlled such that they are alternately switched between a non-conducting state and a conducting state in a linear region by control of their control electrodes, their control electrodes being connected to an output from the current sensor, a output node between the transistors being coupled to an isolating capacitor, the isolating capacitor being coupled to a ground line coupled to the load, the two transistors providing alternate paths for current in the ground line to flow to respective ones of the a-c lines of the a-c source thereby to reduce the ground current flowing in the ground line by providing a circulating path for the current in the ground line, the active filter circuit being connected in the a-c lines coupled to the pulse width modulated driving circuit.