Active filter for reduction of common mode current
View Patent ↗An active filter for reducing the common mode current in a pulse width modulated drive circuit driving a load, said drive circuit comprising an a-c source, a rectifier connected to said a-c source and producing a rectified output voltage connected to a positive d-c bus and a negative d-c bus, a PWM inverter having input terminals coupled to said positive d-c bus and negative d-c bus and having a controlled a-c output, a load driven by said a-c output of said PWM inverter, a ground wire extending from said load, and a current sensor for measuring the common mode current in said drive circuit in said ground wire, said current sensor producing an output current related to said common mode current, said active filter comprising a first and second MOSFET transistor, each having first and second main electrodes and a control electrode, and an amplifier driving a respective one of the transistors; said first electrode of said first and second transistor coupled to a common node, said second electrodes of said first and second transistors being coupled to said positive d-c bus and said negative d-c respectively; each of said amplifiers having an input coupled to said output of said current sensor and each having an output connected to a respective one of said control electrodes; and a d-c isolating capacitor connecting said common node of said first electrode of said first and second transistors to said ground wire.
1. 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 buffer amplifier;
a DC isolating capacitor, 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 a full rectified cycle of said AC source, wherein:
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; and said buffer amplifier has a signal input provided by the output of said current sensor and an output connected to at least one of said control electrodes; and said DC isolating capacitor is connected between said common node and said ground wire.
2. The device of claim 1 , wherein said current sensor is a differential current transformer having first and second windings connected in series with said positive and negative DC busses respectively and an output winding connected as the signal input to the buffer amplifier.
3. The device of claim 1 , wherein said current sensor is a differential current transformer having first and second windings connected in respective ones of lines of said AC source connected to said rectifier and an output winding connected as the signal input to the buffer amplifier.
4. 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 buss 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
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 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.
5. The device of claim 4 , wherein said isolating capacitor is indirectly connected to said common node of said first electrode of said first and second transistors.