Active filter for reduction of common mode current

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, and
• a DC isolating capacitor, 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 bus respectively;
• and
• said DC isolating capacitor is connected between said common node and said ground wire;
• and
• said current sensor output is coupled to at least one of said control electrodes;
• and
• a reduced voltage filter bus coupled to said DC bus;
• and further wherein:
• said second electrode of said first transistor is connected to said reduced voltage filter bus;
• said reduced voltage filter bus has a reduced positive voltage bus and a reduced negative voltage bus;
• and
• one of said reduced positive voltage bus and said reduced negative voltage bus is connected in common with a corresponding one of said positive DC bus and said negative DC bus,
• further including first and second series connected capacitors connected across said positive and negative DC busses;
• and wherein said DC filter bus is connected to the node between said first and second capacitors.

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, and
• a DC isolating capacitor, 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 bus respectively;
• said DC isolating capacitor is connected between said common node and said ground wire;
• and
• said current sensor output is coupled to at least one of said control electrodes;
• and further comprising:
• a reduced voltage filter bus coupled to said DC bus, wherein:
• said second electrode of said first transistor is connected to said reduced voltage filter bus;
• said reduced voltage filter bus has a reduced positive voltage bus and a reduced negative voltage bus;
• and
• one of said reduced positive voltage bus and reduced negative voltage bus is connected in common with a corresponding one of said positive DC bus and negative DC bus,
• further comprising a low AC impedance path comprising a capacitance for common mode current disposed between the second electrode of at least one of said first and second transistors and one of the positive and negative DC busses,
• further including a series connected resistor and zener diode connected between said positive and negative DC busses, and wherein said positive DC filter bus is connected to the node between said resistor and said zener diode.

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, and
• a DC isolating capacitor, 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 bus respectively;
• said DC isolating capacitor is connected between said common node and said ground wire;
• and
• said current sensor output is coupled to at least one of said control electrodes;
• and further comprising:
• a reduced voltage filter bus coupled to said DC bus, wherein:
• said second electrode of said first transistor is connected to said reduced voltage filter bus;
• said reduced voltage filter bus has a reduced positive voltage bus and a reduced negative voltage bus;
• and
• one of said reduced positive voltage bus and reduced negative voltage bus is connected in common with a corresponding one of said positive DC bus and negative DC bus,
• further including first and second series connected capacitors connected across said positive and negative DC busses;
• and wherein said DC filter bus is connected to the node between said first and second capacitors.