Totem-pole power factor corrector and current-sampling unit thereof

Abstract

A totem-pole PFC and a current-sampling unit of the totem-pole PFC are provided. The totem-pole PFC is electrically connected to an AC power source and a DC-to-DC converter, and is electrically connected to a load through the DC-to-DC converter. The current-sampling unit has a first sampling switch and a second sampling switch. The first sampling switch and the second sampling switch are controlled to be turned on and turned off so that a magnetizing current flows through the magnetizing inductor when a magnetizing inductor is magnetized and a demagnetizing current does not flow through the sampling resistor when the magnetizing inductor is demagnetized, thereby increasing the demagnetization efficiency and overcoming superimposed operations to improve current detection and increase conversion efficiency of the power conversion.

Claims

1. A current-sampling unit of a totem-pole power factor corrector that is electrically connected to an alternating-current (AC) power source and a load through a DC-to-DC converter, the current-sampling unit configured to detect a current flowing through a switch unit of the totem-pole power factor corrector, and comprising: a primary-side winding connected in series to the switch unit; a full-bridge rectifying unit having a first input end, a second input end, a first output end, a second output end, a first sampling switch, and a second sampling switch, and further having a first current sampling path and a second current sampling path, wherein the first sampling switch is on the first current sampling path and the second sampling switch is on the second current sampling path; a secondary-side winding coupled to the primary-side winding; a magnetizing inductor; a demagnetizing component connected in parallel to the secondary-side winding and the magnetizing inductor between the first input end and the second input end of the full-bridge rectifying unit; a sampling resistor electrically connected between the first output end and the second output end of the full-bridge rectifying unit; wherein when the AC power source is in a positive half cycle, the first sampling switch is turned on, the second sampling switch is turned off, the magnetizing inductor is magnetized, and through the first current sampling path of the full-bridge rectifying unit, the secondary-side winding and the sampling resistor form a first loop, and when the AC power source is in the positive half cycle, the first sampling switch is turned on, the second sampling switch is turned off, the magnetizing inductor is demagnetized, and the magnetizing inductor and the sampling resistor are disconnected from each other; wherein when the AC power source is in a negative half cycle, the second sampling switch is turned on, the first sampling switch is turned off, the magnetizing inductor is magnetized, and through the second current sampling path of the full-bridge rectifying unit, the secondary-side winding and the sampling resistor form a second loop, and when the AC power source is in the negative half cycle, the second sampling switch is turned on, the first sampling switch is turned off, the magnetizing inductor is demagnetized, and the magnetizing inductor and the sampling resistor are disconnected from each other.

2. The current-sampling unit of the totem-pole power factor corrector as claimed in claim 1, wherein the full-bridge rectifying unit further comprises: a first diode; a second diode, a cathode of the second diode electrically connected to an anode of the first diode; a third diode; a fourth diode, a cathode of the fourth diode electrically connected to an anode of the third diode; wherein a connection node at which the first diode and the second diode are connected is electrically connected to the first input end of the full-bridge rectifying unit, and a connection node at which the third diode and the fourth diode are connected is electrically connected to the second input end of the full-bridge rectifying unit; wherein a connection node at which a cathode of the first diode and a cathode of the third diode are connected is electrically connected to the first output end of the full-bridge rectifying unit, and a connection node at which an anode of the second diode and an anode of the fourth diode are connected is electrically connected to the second output end of the full-bridge rectifying unit; wherein the first current sampling path is formed by the first input end of the full-bridge rectifying unit, the first diode, the first output end, the second output end, the fourth diode, and the second input end of the full-bridge rectifying unit; wherein the second current sampling path is formed by the second input end of the full-bridge rectifying unit, the third diode, the first output end, the second output end, the second diode, and the first input end of the full-bridge rectifying unit.

3. The current-sampling unit of the totem-pole power factor corrector as claimed in claim 2, wherein the first sampling switch of the full-bridge rectifying unit is located between the cathode of the first diode and the first output end of the full-bridge rectifying unit, and the second sampling switch is located between the cathode of the third diode and the second output end of the full-bridge rectifying unit.

4. The current-sampling unit of the totem-pole power factor corrector as claimed in claim 3, wherein the first sampling switch and the second sampling switch are each respectively a silicon controlled rectifier (SCR); or the first sampling switch and the second sampling switch are each respectively a metal-oxide-semiconductor field-effect transistor (MOSFET).

5. The current-sampling unit of the totem-pole power factor corrector as claimed in claim 3, wherein the demagnetizing component is a demagnetizing resistor or two Zener diodes connected in back-to-back series.

6. The current-sampling unit of the totem-pole power factor corrector as claimed in claim 2, wherein the first sampling switch of the full-bridge rectifying unit is located between the cathode of the fourth diode and the second input end of the full-bridge rectifying unit, and the second sampling switch of the full-bridge rectifying unit is located between the cathode of the second diode and the first input end of the full-bridge rectifying unit.

7. The current-sampling unit of the totem-pole power factor corrector as claimed in claim 6, wherein the demagnetizing component is a demagnetizing resistor or two Zener diodes connected in back-to-back series.

8. The current-sampling unit of the totem-pole power factor corrector as claimed in claim 6, wherein the first sampling switch and the second sampling switch are each respectively a silicon controlled rectifier (SCR); or the first sampling switch and the second sampling switch are each respectively a metal-oxide-semiconductor field-effect transistor (MOSFET).

9. The current-sampling unit of the totem-pole power factor corrector as claimed in claim 2, wherein the demagnetizing component is a demagnetizing resistor or two Zener diodes connected in back-to-back series.

10. The current-sampling unit of the totem-pole power factor corrector as claimed in claim 2, wherein the first sampling switch and the second sampling switch are each respectively a silicon controlled rectifier (SCR); or the first sampling switch and the second sampling switch are each respectively a metal-oxide-semiconductor field-effect transistor (MOSFET).

11. The current-sampling unit of the totem-pole power factor corrector as claimed in claim 1, wherein the full-bridge rectifying unit comprises: a first rectifying switch; a second rectifying switch; a second diode, a cathode of the second diode is electrically connected to the first output end sequentially through the first rectifying switch and the first sampling switch; and a fourth diode, a cathode of the fourth diode is electrically connected to the first output end sequentially through the second rectifying switch and the second sampling switch; wherein a connection node at which the first rectifying switch and the cathode of the second diode are connected is electrically connected to the first input end of the full-bridge rectifying unit, and a connection node at which the second rectifying switch and the cathode of the fourth diode are connected is electrically connected to the second input end of the full-bridge rectifying unit.

12. The current-sampling unit of the totem-pole power factor corrector as claimed in claim 11, wherein the demagnetizing component is a demagnetizing resistor or two Zener diodes connected in back-to-back series.

13. The current-sampling unit of the totem-pole power factor corrector as claimed in claim 11, wherein the first sampling switch and the second sampling switch are each respectively a silicon controlled rectifier (SCR); or the first sampling switch and the second sampling switch are each respectively a metal-oxide-semiconductor field-effect transistor (MOSFET).

14. The current-sampling unit of the totem-pole power factor corrector as claimed in claim 1, wherein the full-bridge rectifying unit comprises: a first rectifying switch; a second rectifying switch; a second diode, a cathode of the second diode is electrically connected to the first output end sequentially through the second sampling switch and the first rectifying switch; and a fourth diode, a cathode of the fourth diode is electrically connected to the first output end sequentially through the first sampling switch and the second rectifying switch; wherein a connection node at which the second sampling switch and the first rectifying switch are connected is electrically connected to the first input end of the full-bridge rectifying unit, and a connection node at which the first sampling switch and the second rectifying switch are connected is electrically connected to the second input end of the full-bridge rectifying unit.

15. The current-sampling unit of the totem-pole power factor corrector as claimed in claim 14, wherein the demagnetizing component is a demagnetizing resistor or two Zener diodes connected in back-to-back series.

16. The current-sampling unit of the totem-pole power factor corrector as claimed in claim 14, wherein the first sampling switch and the second sampling switch are each respectively a silicon controlled rectifier (SCR); or the first sampling switch and the second sampling switch are each respectively a metal-oxide-semiconductor field-effect transistor (MOSFET).

17. The current-sampling unit of the totem-pole power factor corrector as claimed in claim 1, wherein the demagnetizing component is a demagnetizing resistor or two Zener diodes connected in back-to-back series.

18. The current-sampling unit of the totem-pole power factor corrector as claimed in claim 1, wherein the first sampling switch and the second sampling switch are each respectively a silicon controlled rectifier (SCR); or the first sampling switch and the second sampling switch are each respectively a metal-oxide-semiconductor field-effect transistor (MOSFET).

19. A totem-pole power factor corrector electrically connected to an alternating-current (AC) power source and a DC-to-DC converter, and further electrically connected to a load through the DC-to-DC converter, the totem-pole power factor corrector comprising: a power factor correction inductor; a first bridge arm comprising: a first switch unit; a second switch unit connected to the first switch unit at a first node; a second bridge arm comprising: a third switch unit; a fourth switch unit; a first current-sampling unit and a second current-sampling unit; wherein the first current-sampling unit is configured to detect a current flowing through the third switch unit, and the second current-sampling unit is configured to detect a current flowing through the fourth switch unit; wherein the fourth switch unit is connected to the third switch unit through the second current-sampling unit, and the third switch unit is connected to the second current-sampling unit at a second node; wherein the AC power source and the power factor correction inductor connected in series to the AC power source are connected between the first node of the first bridge arm and the second node of the second bridge arm; a positive output end; a grounding end; wherein the positive output end and the grounding end are electrically connected to the DC-to-DC converter; and a capacitor; wherein the first bridge arm, the second bridge arm, and the capacitor are connected in parallel between the positive output end and the grounding end; the first switch unit of the first bridge arm is electrically connected to the positive output end, the third switch unit of the second bridge arm is electrically connected to the positive output end through the first current-sampling unit, and the second switch unit of the first bridge arm and the fourth switch unit of the second bridge arm are electrically connected to the grounding end; wherein each of the first current-sampling unit and the second current-sampling unit comprises: a primary-side winding connected in series to the third switch unit or the fourth switch unit, a full-bridge rectifying unit having a first input end, a second input end, a first output end, a second output end, a first sampling switch, and a second sampling switch, and further having a first current sampling path and a second current sampling path, wherein the first sampling switch is on the first current sampling path and the second sampling switch is on the second current sampling path; a secondary-side winding coupled to the primary-side winding; a magnetizing inductor; a demagnetizing component connected in parallel to the secondary-side winding and the magnetizing inductor between the first input end and the second input end of the full-bridge rectifying unit; a sampling resistor electrically connected between the first output end and the second output end of the full-bridge rectifying unit; wherein when the AC power source is in a positive half cycle, the first sampling switch is turned on, the second sampling switch is turned off, the magnetizing inductor is magnetized, and through the first current sampling path of the full-bridge rectifying unit, the secondary-side winding and the sampling resistor form a first loop, and when the AC power source is in the positive half cycle, the first sampling switch is turned on, the second sampling switch is turned off, the magnetizing inductor is demagnetized, and the magnetizing inductor and the sampling resistor are disconnected from each other; wherein when the AC power source is in a negative half cycle, the second sampling switch is turned on, the first sampling switch is turned off, the magnetizing inductor is magnetized, and through the second current sampling path of the full-bridge rectifying unit, the secondary-side winding and the sampling resistor form a second loop, and when the AC power source is in the negative half cycle, the second sampling switch is turned on, the first sampling switch is turned off, the magnetizing inductor is demagnetized, and the magnetizing inductor and the sampling resistor are disconnected from each other.

20. The totem-pole power factor corrector as claimed in claim 19, wherein when the AC power source is in the positive half cycle, the first sampling switch of the first current-sampling unit is turned on, the second sampling switch of the first current-sampling unit is turned off, the first sampling switch of the second current-sampling unit is turned on, and the second sampling switch of the second current-sampling unit is turned off; when the AC power source is in the negative half cycle, the second sampling switch of the first current-sampling unit is turned on, the first sampling switch of the first current-sampling unit is turned off, the second sampling switch of the second current-sampling unit is turned on, and the first sampling switch of the second current-sampling unit is turned off.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) The features of the present disclosure believed to be novel are set forth with particularity in the appended claims. The present disclosure itself, however, may be best understood by reference to the following detailed description of the present disclosure, which describes an exemplary embodiment of the present disclosure, taken in conjunction with the accompanying drawings, in which:

(2) FIG. 1 is a schematic circuit diagram of a totem-pole power factor corrector (PFC) according to a first preferred embodiment of the present disclosure;

(3) FIG. 2 is a schematic circuit diagram of a current-sampling unit according to the first preferred embodiment of the present disclosure;

(4) FIG. 3 to FIG. 6 are schematic circuit diagrams of operations of the current-sampling unit according to the present disclosure;

(5) FIG. 7 is a schematic curve diagram of a sampling current according to the first preferred embodiment of the present disclosure;

(6) FIG. 8 is a schematic circuit diagram of the current-sampling unit according to a second preferred embodiment of the present disclosure;

(7) FIG. 9 is a schematic circuit diagram of the current-sampling unit according to a third preferred embodiment of the present disclosure;

(8) FIG. 10 is a schematic circuit diagram of the current-sampling unit according to a fourth preferred embodiment of the present disclosure;

(9) FIG. 11 is a schematic circuit diagram of the current-sampling unit according to a fifth preferred embodiment of the present disclosure;

(10) FIG. 12 is a schematic circuit diagram of the current-sampling unit according to a sixth preferred embodiment of the present disclosure;

(11) FIG. 13 is a schematic circuit diagram of the current-sampling unit according to a seventh preferred embodiment of the present disclosure;

(12) FIG. 14 is a schematic circuit diagram of a conventional totem-pole PFC;

(13) FIG. 15 to FIG. 18 are schematic circuit diagrams of operations of the conventional totem-pole PFC;

(14) FIG. 19 is a schematic curve diagram of a current provided by an AC power source, a current detected by a first current-sampling unit, and a current detected by a second current-sampling unit;

(15) FIG. 20 is a schematic circuit diagram of a conventional current-sampling unit; and

(16) FIG. 21 to FIG. 24 are schematic circuit diagrams of operations of the conventional current-sampling unit.

DETAILED DESCRIPTION

(17) Reference will now be made to the drawing figures to describe the present disclosure in detail.

(18) Referring to FIGS. 1 to 7, a first preferred embodiment of a current-sampling unit 100 of a totem-pole power factor corrector (PFC) 10 is disclosed in the present disclosure. With reference to FIG. 1, the totem-pole PFC 10 is electrically connected between an alternating-current (AC) power source V.sub.AC and a DC-to-DC converter 20, and the totem-pole PFC 10 is further electrically connected to a load R.sub.L through the DC-to-DC converter 20. The current-sampling unit 100 is provided to detect a current flowing through a switch unit Q of the totem-pole PFC 10.

(19) In one embodiment, the totem-pole PFC 10 includes a power factor correction (PFC) inductor L, a first bridge arm 11, a second bridge arm 12, a capacitor C, a positive output end V.sub.CC, and a grounding end GND.

(20) The first bridge arm 11 has a first switch unit Q1 and a second switch unit Q2. The second switch unit Q2 is connected to the first switch unit Q1 at a first node n1.

(21) The second bridge arm 12 has a third switch unit Q3, a fourth switch unit Q4, and two current-sampling units 100 including a first current-sampling unit 101 and a second current-sampling unit 102. The first current-sampling unit 101 is provided to detect a current flowing through the third switch unit Q3. The second current-sampling unit 102 is provided to detect a current flowing through the fourth switch unit Q4.

(22) The fourth switch unit Q4 is connected to the third switch unit Q3 through the second current-sampling unit 102. The third switch unit Q3 is connected to the second current-sampling unit 102 at a second node n2.

(23) The AC power source V.sub.AC and the PFC inductor L connected in series to the AC power source V.sub.AC are connected between the first node n1 of the first bridge arm 11 and the second node n2 of the second bridge arm 12. The positive output end V.sub.CC and the grounding end GND are electrically connected to the DC-to-DC converter 20.

(24) The first bridge arm 11, the second bridge arm 12, and the capacitor C are connected in parallel between the positive output end V.sub.CC and the grounding end GND. The first switch unit Q1 of the first bridge arm 11 is electrically connected to the positive output end V.sub.CC. The third switch unit Q3 of the second bridge arm 12 is electrically connected to the positive output end V.sub.CC through the first current-sampling unit 101. The second switch unit Q2 of the first bridge arm 11 and the fourth switch unit Q4 of the second bridge arm 12 are electrically connected to the grounding end GND.

(25) Since the circuit structure and operations of the totem-pole PFC 10 are disclosed in the prior art, the detail description of the totem-pole PFC 10 is omitted here for conciseness.

(26) Referring to FIG. 2, the current-sampling unit 100 of the totem-pole PFC 10 includes a primary-side winding W.sub.1, a full-bridge rectifying unit 111, a secondary-side winding W.sub.2, a magnetizing inductor L.sub.m, a demagnetizing component 112, and a sampling resistor R.sub.S.

(27) The primary-side winding W.sub.1 is connected in series to the switch unit Q. The full-bridge rectifying unit 111 has a first input end I/P1, a second input end I/P2, a first output end O/P1, a second output end O/P2, a first sampling switch S.sub.1, and a second sampling switch S.sub.2. The full-bridge rectifying unit 111 further has a first current sampling path and a second current sampling path, wherein the first sampling switch S.sub.1 is on the first current sampling path and the second sampling switch S.sub.2 is on the second current sampling path. The secondary-side winding W.sub.2 is coupled to the primary-side winding W.sub.1.

(28) The demagnetizing component 112, the secondary-side winding W.sub.2, and the magnetizing inductor L.sub.m are connected in parallel between the first input end I/P1 and the second input end I/P2 of the full-bridge rectifying unit 111. In this embodiment, the demagnetizing component 112 may be a demagnetizing resistor R.sub.C.

(29) The sampling resistor R.sub.S is electrically connected between the first output end O/P1 and the second output end O/P2 of the full-bridge rectifying unit 111.

(30) When the AC power source V.sub.AC is in a positive half cycle, the first sampling switch S.sub.1 is turned on, the second sampling switch S.sub.2 is turned off, the magnetizing inductor L.sub.m is magnetized, and through the first current sampling path of the full-bridge rectifying unit 111, the secondary-side winding W.sub.2 and the sampling resistor R.sub.S form a first loop. When the AC power source V.sub.AC is in the positive half cycle, the first sampling switch S.sub.1 is turned on, the second sampling switch S.sub.2 is turned off, the magnetizing inductor L.sub.m is demagnetized, and the magnetizing inductor L.sub.m and the sampling resistor R.sub.S are disconnected from each other

(31) When the AC power source V.sub.AC is in a negative half cycle, the second sampling switch S.sub.2 is turned on, the first sampling switch S.sub.1 is turned off, the magnetizing inductor L.sub.m is magnetized, and through the second current sampling path of the full-bridge rectifying unit 111, the secondary-side winding W.sub.2 and the sampling resistor R.sub.S form a second loop. When the AC power source V.sub.AC is in the negative half cycle, the second sampling switch S.sub.2 is turned on, the first sampling switch S.sub.1 is turned off, the magnetizing inductor L.sub.m is demagnetized, and the magnetizing inductor L.sub.m and the sampling resistor R.sub.S are disconnected from each other.

(32) The first sampling switch S.sub.1 located on the first current sampling path of the full-bridge rectifying unit 111 is turned on by the current-sampling unit 100 when the AC power source V.sub.AC is in the positive half cycle so that an induction current generated at the secondary-side winding W.sub.2 flows through the sampling resistor R.sub.S through the first current sampling path and the current, i.e., a current flowing through the switch unit Q is sampled by the sampling resistor R.sub.S. Simultaneously, the second sampling switch S.sub.2 located on the second current sampling path of the full-bridge rectifying unit 111 is turned off by the current-sampling unit 100 so that the demagnetizing component 112 is demagnetized and the magnetizing inductor L.sub.m and the sampling resistor R.sub.S are disconnected from each other, thereby increasing the demagnetization efficiency.

(33) The second sampling switch S.sub.2 located on the second current sampling path of the full-bridge rectifying unit 111 is turned on by the current-sampling unit 100 when the AC power source V.sub.AC is in the negative half cycle so that the induction current generated at the secondary-side winding W.sub.2 flows through the sampling resistor R.sub.S through the second current sampling path and the current, i.e., a current flowing through the switch unit Q is sampled by the sampling resistor R.sub.S. Simultaneously, the first sampling switch S.sub.1 located on the first current sampling path of the full-bridge rectifying unit 111 is turned off by the current-sampling unit 100 so that the demagnetizing component 112 is demagnetized, and the magnetizing inductor L.sub.m and the sampling resistor R.sub.S are disconnected from each other, thereby increasing the demagnetization efficiency.

(34) Accordingly, the current-sampling unit 100 is provided to increase the demagnetization efficiency and overcome superimposed operations to improve current detection and effectively provide the function of power factor correction to the totem-pole PFC 10.

(35) In one embodiment, the full-bridge rectifying unit 111 further has a first diode D1, a second diode D2, a third diode D3, and a fourth diode D4. A cathode of the second diode D2 is electrically connected to an anode of the first diode D1. A cathode of the fourth diode D4 is electrically connected to an anode of the third diode D3. A connection node at which the first diode D1 and the second diode D2 are connected is electrically connected to the first input end I/P1 of the full-bridge rectifying unit 111, and a connection node at which the third diode D3 and the fourth diode D4 are connected is electrically connected to the second input end I/P2 of the full-bridge rectifying unit 111. A connection node at which a cathode of the first diode D1 and a cathode of the third diode D3 are connected is electrically connected to the first output end O/P1 of the full-bridge rectifying unit 111, and a connection node at which an anode of the second diode D2 and an anode of the fourth diode D4 are connected is electrically connected to the second output end O/P2 of the full-bridge rectifying unit 111.

(36) In this preferred embodiment, the first current sampling path is formed by the first input end I/P1 of the full-bridge rectifying unit 111, the first diode D1, the first output end O/P1, the second output end O/P2, the fourth diode D4, and the second input end I/P2 of the full-bridge rectifying unit 111. The second current sampling path is formed by the second input end I/P2 of the full-bridge rectifying unit 111, the third diode D3, the first output end O/P1, the second output end O/P2, the second diode D2, and the first input end I/P1 of the full-bridge rectifying unit 111.

(37) In one embodiment, the first sampling switch S.sub.1 is located between the cathode of the first diode D1 and the first output end I/O1 of the full-bridge rectifying unit 111, and the second sampling switch S.sub.2 is located between the cathode of the third diode D3 and the first output end I/O1 of the full-bridge rectifying unit 111.

(38) Referring to FIG. 3 to FIG. 6, the detailed operations of the current-sampling unit 100 are disclosed, and the second current-sampling unit 102 is exemplified for demonstration as follows. As mentioned above, the switches of the totem-pole PFC are controlled to magnetize and demagnetize the PFC inductor L so as to implement the function of power factor correction.

(39) As mentioned in FIG. 15, when the AC power source V.sub.AC is in the positive half cycle, the first sampling switch S.sub.1 is turned on, the second sampling switch S.sub.2 is turned off, the PFC inductor L is magnetized, the fourth switch Q4 is turned on, and a current flows through the PFC inductor L. Similarly shown in FIG. 3, a current flows through the primary-side winding W.sub.1 of the current-sampling unit 100 since the fourth switch Q4 connected in series to the primary-side winding W.sub.1 is turned on. An induction current generated from the secondary-side winding W.sub.2 flows through the magnetizing inductor L.sub.m and the sampling resistor R.sub.S via the first current sampling path of the full-bridge rectifying unit 111 for sampling current.

(40) As mentioned in FIG. 16, when the AC power source V.sub.AC is in the positive half cycle, the first sampling switch S.sub.1 is turned on, the second sampling switch S.sub.2 is turned off, the PFC inductor L is demagnetized, the fourth switch Q4 is turned off, and no current flows through the PFC inductor L. Similarly shown in FIG. 4, no current flows through the primary-side winding W.sub.1 of the current-sampling unit 100 since the fourth switch Q4 connected in series to the primary-side winding W.sub.1 is turned off so that no current is induced from the secondary-side winding W.sub.2. At this time, the magnetizing inductor L.sub.m is demagnetized. The demagnetizing current of the magnetizing inductor L.sub.m flows through the demagnetizing component 112, i.e., the demagnetizing resistor R.sub.C since there the magnetizing inductor L.sub.m and the sampling resistor R.sub.S are disconnected from each other, thereby increasing the demagnetization efficiency of the magnetizing inductor L.sub.m by the demagnetizing resistor R.sub.C.

(41) As mentioned in FIG. 17, when the AC power source V.sub.AC is in the negative half cycle, the second sampling switch S.sub.2 is turned on, the first sampling switch S.sub.1 is turned off, the PFC inductor L is magnetized, the fourth switch Q4 is turned off, and no current flows through the PFC inductor L. Similarly shown in FIG. 6, no current flows through the primary-side winding W.sub.1 of the current-sampling unit 100 since the fourth switch Q4 connected in series to the primary-side winding W.sub.1 is turned off so that no current is induced from the secondary-side winding W.sub.2. At this time, the magnetizing inductor L.sub.m is demagnetized. The demagnetizing current of the magnetizing inductor L.sub.m flows through the demagnetizing component 112, i.e., the demagnetizing resistor R.sub.C, since the magnetizing inductor L.sub.m and the sampling resistor R.sub.S are disconnected from each other, thereby increasing the demagnetization efficiency of the magnetizing inductor L.sub.m by the demagnetizing resistor R.sub.C.

(42) As mentioned in FIG. 18, when the AC power source V.sub.AC is in the negative half cycle, the second sampling switch S.sub.2 is turned on, the first sampling switch S.sub.1 is turned off, the PFC inductor L is demagnetized, the fourth switch Q4 is turned on, and a current flows through the primary-side winding W.sub.1 of the current-sampling unit 100 in an opposite direction to the current shown in FIG. 3. Similarly shown in FIG. 5, a current flows through the primary-side winding W.sub.1 of the current-sampling unit 100 since the fourth switch Q4 connected in series to the primary-side winding W.sub.1 is turned on. An induction current generated from the secondary-side winding W.sub.2 flows through the magnetizing inductor L.sub.m and the sampling resistor R.sub.S via the second current sampling path of the full-bridge rectifying unit 111 for sampling current.

(43) Similar to the first current-sampling unit 101, when the AC power source V.sub.AC is in the positive half cycle, the second sampling switch S.sub.2 is turned on, the first sampling switch S.sub.1 is turned off, the PFC inductor L is magnetized, the third switch Q3 is turned off, and no current flows through the PFC inductor L so that the magnetizing inductor L.sub.m is demagnetized. The demagnetizing current of the magnetizing inductor L.sub.m flows through the demagnetizing component 112, i.e., the demagnetizing resistor R.sub.C, since the magnetizing inductor L.sub.m and the sampling resistor R.sub.S are disconnected from each other, thereby increasing the demagnetization efficiency of the magnetizing inductor L.sub.m by the demagnetizing resistor R.sub.C.

(44) Similar to the first current-sampling unit 101, when the AC power source V.sub.AC is in the positive half cycle, the second sampling switch S.sub.2 is turned on, the first sampling switch S.sub.1 is turned off, the PFC inductor L is demagnetized, the third switch Q3 is turned on, and a current flows through the PFC inductor L. An induction current generated from the secondary-side winding W.sub.2 flows through the magnetizing inductor L.sub.m and the sampling resistor R.sub.S via the second current sampling path of the full-bridge rectifying unit 111 for sampling current.

(45) Similar to the first current-sampling unit 101, when the AC power source V.sub.AC is in the negative half cycle, the first sampling switch S.sub.1 is turned on, the second sampling switch S.sub.2 is turned off, the PFC inductor L is magnetized, the third switch Q3 is turned on, and a current flows through the PFC inductor L. An induction current generated from the secondary-side winding W.sub.2 flows through the magnetizing inductor L.sub.m and the sampling resistor R.sub.S via the first current sampling path of the full-bridge rectifying unit 111 for sampling current.

(46) Similar to the first current-sampling unit 101, when the AC power source V.sub.AC is in the negative half cycle, the first sampling switch S.sub.1 is turned on, the second sampling switch S.sub.2 is turned off, the PFC inductor L is demagnetized, the third switch Q3 is turned off, and no current flows through the PFC inductor L so that the magnetizing inductor L.sub.m is demagnetized. The demagnetizing current of the magnetizing inductor L.sub.m flows through the demagnetizing component 112, i.e., the demagnetizing resistor R.sub.C, since the magnetizing inductor L.sub.m and the sampling resistor R.sub.S are disconnected from each other, thereby increasing the demagnetization efficiency of the magnetizing inductor L.sub.m by the demagnetizing resistor R.sub.C.

(47) FIG. 7 shows a curve of the sampling current of the sampling resistor R.sub.S. Since the sampling resistor R.sub.S can sample a current flowing through the sampling resistor R.sub.S only when the fourth switch Q4 is turned on, a curve of a sampling current of the sampling resistor R.sub.S is shown in FIG. 7 when the AC power source V.sub.AC is in the positive half cycle. Since the sampled current is zero when the magnetizing inductor L.sub.m is fully demagnetized, no superimposed operation exists.

(48) The first sampling switch S.sub.1 and the second sampling switch S.sub.2 are switched according to different half cycles of the AC power source V.sub.AC. In other words, a switching frequency of the first sampling switch S.sub.1 and the second sampling switch S.sub.2 is identical to a line frequency of the AC power source V.sub.AC. When the AC power source V.sub.AC is changed from the positive half cycle to the negative half cycle, the on/off conditions of the first sampling switch S.sub.1 and the second sampling switch S.sub.2 are changed, thereby reducing switching losses of the first sampling switch S.sub.1 and the second sampling switch S.sub.2 by reducing switching times of the first sampling switch S.sub.1 and the second sampling switch S.sub.2.

(49) Since a direction of the current flowing through the fourth switch Q4 when the AC power source V.sub.AC is in the positive half cycle is just opposite a direction of the current flowing through the fourth switch Q4 when the AC power source V.sub.AC is in the negative half cycle, the current-sampling unit 100 may be used for sampling current in a bidirectional manner, thereby increasing the applicability of the present disclosure.

(50) In a second preferred embodiment shown in FIG. 8, since the first sampling switch S.sub.1 of the full-bridge rectifying unit 111 is located on the first current sampling path and the second sampling switch S.sub.2 of the full-bridge rectifying unit 111 is located on the second current sampling path, the first sampling switch S.sub.1 may be located between the cathode of the fourth diode D4 and the second input end I/P2, and the second sampling switch S.sub.2 may be located between the cathode of the second diode D2 and the first input end I/P1 of the full-bridge rectifying unit 111.

(51) In a third embodiment shown in FIG. 9, the demagnetizing component 112 is composed of two Zener diodes connected in back-to-back series.

(52) In a fourth embodiment shown in FIG. 10, the first sampling switch S.sub.1 and the second sampling switch S.sub.2 are a silicon controlled rectifier (SCR), respectively.

(53) In a fifth embodiment shown in FIG. 11, the first sampling switch S.sub.1 and the second sampling switch S.sub.2 are each respectively a metal-oxide-semiconductor field-effect transistor (MOSFET).

(54) In a sixth embodiment shown in FIG. 12, the full-bridge rectifying unit 111 has a first rectifying switch Q11, a second rectifying switch Q21, a second diode D2, and a fourth diode D4.

(55) The cathode of the second diode D2 is electrically connected to the first output end O/P1 sequentially through the first rectifying switch Q11 and the first sampling switch S.sub.1. The cathode of the fourth diode D4 is electrically connected to the first output end O/P1 sequentially through the second rectifying switch Q21 and the first sampling switch S.sub.2. A connection node at which the first rectifying switch Q11 and the cathode of the second diode D2 are connected is electrically connected to the first input end I/P1 of the full-bridge rectifying unit 111. A connection node at which the second rectifying switch Q21 and the cathode of the fourth diode D4 are connected is electrically connected to the second input end I/P2 of the full-bridge rectifying unit 111.

(56) As shown in FIG. 13, the full-bridge rectifying unit 111 includes a first rectifying switch Q11, a second rectifying switch Q21, a second diode D2, and a fourth diode D4.

(57) The cathode of the second diode D2 is electrically connected to the first output end O/P1 sequentially through the second sampling switch S.sub.2 and the first rectifying switch Q11. The cathode of the fourth diode D4 is electrically connected to the first output end O/P1 sequentially through the first sampling switch S.sub.1 and the second rectifying switch Q21. A connection node at which the second sampling switch S.sub.2 and the first rectifying switch Q11 are connected is electrically connected to the first input end I/P1 of the full-bridge rectifying unit 111. A connection node at which the first sampling switch S.sub.1 and the second rectifying switch Q21 are connected is electrically connected to the second input end I/P2 of the full-bridge rectifying unit 111.

(58) Although the present disclosure has been described with reference to the preferred embodiment thereof, it will be understood that the present disclosure is not limited to the details thereof. Various substitutions and modifications have been suggested in the foregoing description, and others will occur to those of ordinary skill in the art. Therefore, all such substitutions and modifications are intended to be embraced within the scope of the present disclosure as defined in the appended claims.