Bridgeless boost PFC circuits and systems with reduced common mode EMI

1. A bridgeless boost power factor correction (PFC) system comprising:

a first input for connection to a first line of an alternating current (AC) source;

a second input for connection to a second line of the AC source;

an output for delivering an output of the bridgeless boost PFC system;

a first boost choke coupled to the first input;

a second boost choke coupled to the second input;

a common mode choke coupled between the first and second inputs and the first and second boost chokes;

a first X capacitor coupled between the first input and the output; and

a second X capacitor coupled between the second input and the output.

2. The bridgeless boost PFC system of claim 1 wherein the first boost choke and the second boost choke are wound on a same magnetic core.

3. The bridgeless boost PFC system of claim 2 , wherein the first boost choke and the second boost choke are wound in opposite directions on the magnetic core.

4. The bridgeless boost PFC system of claim 2 , wherein the magnetic core is low permeability core.

5. The bridgeless boost PFC system of claim 1 , further comprising an EMI filter coupled to the first and second inputs.

6. The bridgeless boost PFC system of claim 5 wherein the EMI filter includes at least one of an additional X capacitor, a Y capacitor, an additional common mode choke and a differential mode choke.

7. The bridgeless boost PFC system of claim 5 , wherein said EMI filter is adapted to permit voltage boosting without disruption in the operation of said system.

8. The bridgeless boost PFC system of claim 1 , wherein said common mode choke comprises a low loss transformer ferrite.

9. The bridgeless boost PFC system of claim 1 , wherein said common mode choke is a high inductance choke.

10. The bridgeless boost PFC system of claim 1 , wherein the capacitance of each of the first and second X capacitors is at least 100 nF.

11. The bridgeless boost PFC system of claim 1 , wherein a number of turns on said common mode choke has no direct relationship to a number of turns on the first and second boost chokes.

12. The bridgeless boost PFC system of claim 1 , wherein an additional X capacitor is coupled across said common mode choke to bypass its leakage inductance.

13. The bridgeless boost PFC system of claim 1 , further comprising a Y capacitor coupled to ground.

14. The bridgeless boost PFC system of claim 1 , further comprising at least one switch and a PWM controller for controlling the switch, wherein said PWM controller is adapted to receive signals from an AC input polarity sensing circuit for an ‘always ON’ or ‘always OFF’ switching methodology.

15. A method for power factor correction in an AC power current

carrying system, said method comprising:

receiving and storing energy from an AC power supply in a first boost choke and a second boost choke;

presenting a relatively high input impedance to the boost chokes;

rectifying the incoming AC power using a combination of a pair of rectifier diodes and semiconductor switches;

controlling a duty cycle of the semiconductor switches; and

bypassing an input line of the AC power supply to an output terminal with a relatively low impedance so that a small high frequency potential difference exists between the input line of the AC power supply and said output terminal to reduce common mode conducted electromagnetic interference (EMI).

16. The method of claim 15 , further comprising filtering EMI from the AC power supply.

17. An AC power supply unit including a PFC system of claim 1 .