GB2349517A

GB2349517A - Power factor correction circuit

Info

Publication number: GB2349517A
Application number: GB9909759A
Authority: GB
Inventors: Jim H Liang
Original assignee: Skynet Electronic Co Ltd
Current assignee: Skynet Electronic Co Ltd
Priority date: 1999-04-29
Filing date: 1999-04-29
Publication date: 2000-11-01
Anticipated expiration: 2019-04-29
Also published as: GB9909759D0; GB2349517B

Abstract

A power factor correction circuit 10 comprises an series connection of a bridge rectifier, DB<SB>1</SB> a first winding L<SB>a</SB> , a diode D<SB>1</SB> and an electrolytic capacitor C<SB>1</SB>; a series connection of the electrolytic capacitor C<SB>1</SB>, a second winding L<SB>b</SB> and a DC/DC converter 2; and a common core T<SB>r1</SB> wound round by the first winding L<SB>a</SB> and the second winding L<SB>b</SB>. The diode D<SB>1</SB> is able to switch between a reverse bias and a forward bias by controlling the polarities of the windings such that an input current always flows to the electrolytic capacitor C<SB>1</SB> during each sinusoidal period of an ac voltage. Also, a dc ripple voltage of the electrolytic capacitor C<SB>1</SB> will not rise due to a drop in load when the DC/DC convertor 2 is operating in a continuous current mode. Furthermore an output of the DC/DC converter 2 will not be adversely affected by 120 Hz ac voltage input. The DC/DC converter may be a fly back convertor (figure 9).

Description

POWER FACTOR CORRECTION CIRCUIT The present invention relates to a power factor correction circuit for improving a power factor of an off-line switching power supply in order to comply with the requirements of Class A or Class D stipulated in harmonic current rules I EC-1000-3-2.

A typical off-line switching power supply is shown in FIG. 1. The supply comprises an AC/DC rectifier 1, and a DC/DC converter 2 in which an electrolytic capacitor C, is connected as a filter for the bridge rectifier DB,. As such, the electrolytic capacitor C, begins to charge only when the bridge rectifier DB, is conducted if the input ac voltage Vsz is higher than the voltage of the electrolytic capacitor C, Note that the input current pc is a pulsating current as shown in the graph of FIG. 2. The power factor of the input current of the conventional off-line switching power supply is significantly decreased (e. g., approximately 50%), and the total harmonics distortion (hereinafter referred as"THD") is even higher than 100% after the rectification performed by the AC/DC rectifier 1. As a result, the total harmonics is seriously distorted, the quality is poor, and, to the worse, the precious energy is wasted.

Thus, many countries have promulgated a number of harmonic current rules (e. g., IEC-1000-3-2) which specify the current waveshape of the power supply for manufacturers to obey in order to improve the efficiency and quality of the power source being supplie.

As such, various designs of power factor correction circuits have been proposed by researchers in order to improve power factor of the conventional off-line switching power supply. These designs have been located in a search as follows ; 1. Inductor type power factor correction circuit: As shown in FIG. 3, the prior art discloses a design in which a low frequency large winding L, is in series between a bridge rectifier DB, and a electrolytic capacitor C,. The winding L, and the capacitor C, form a low pass filter to rectify the input current of a DC/DC converter 2. Such design is similar to the ballast for correcting the power factor of a fluorescent lamp in functionality. However, the winding L, has the drawbacks for being relatively large, limited power factor improvement, and abnormal high temperature developed.

2. Active type power factor correction circuit: As shown in FlG. 4, the prior art discloses a design in which the ACIDC rectifier is redesigned to form a two-stage circuit with the DC/DC converter 2.

Further, a complex control circuit 11 and a large switch element Q, are added therein to improve the power factor. However, it is relatively complex in circuit design and will cause high manufacturing cost.

3. Dither type power factor correction circuit of single-stage single-switch: As shown in FIG. 5, the prior art is simple in circuit design. However, the whole circuit is redesigned, and a number of deficiencies have been found in use as follows : a) The ripple voltage Vdc will rise to approximately 100% to 200% if the load suddenly drops significantly when the DC/DC converter 2 is operating in a continuous current mode. As such, a high-voltage electrolytic capacitor is required. b) The alternating current component of the ac source Vs1 will be brought into the DC/DC converter 2 when the switch element Q, is conducted. As a result, the output of the DC/DC converter 2 will be adversely affected by the 120 Hz ac voltage input, resulting in the rise of the ripple voltage. c) The large winding L, hardly improves the power factor when the DC/DC converter 2 is operating in the continuous current mode.

4. U. S. Pat. No. 5, 301,095 to S. Teramoto is disclosed in FIG. 6. Teramoto's patent replaces the diode D2 of the dither type circuit shown in FIG. 5 with a small capacity capacitor C3 in order to improve the power factor. However, the deficiencies of b) and c) as stated above are not effectively eliminated.

5. U. S. Pat. No. 5,600,546 to Fu-Sheng Tsai is disclosed in FIG. 7. Tsai's patent adds another winding L3, which is in series with the diode D2, into the dither type circuit shown in FIG. 5. Such design will resolve the problem of the rise of ripple voltage Vdc of the electrolytic capacitor C,, when the DC/DC converter 2 is operating in continuous current mode, by lowering the induction ratio Lp/L1 of the primary winding Lp of the DC/DC converter 2 to the winding L1, or increasing the induction ratio L3/Lp of the winding L3 to the winding Lp. However, the deficiencies of a) and c) as stated above are not effectively eliminated.

Thus, it is desirable to provide a power factor correction circuit in order to overcome the above drawbacks of prior I It is an object of the present invention to provide a power factor correction circuit comprising a series connection of a bridge rectifier, a first winding, a diode, and an electrolytic capacitor; a series connection of the electrolytic capacitor, a second winding, and a DC/DC converter; and a common core wound round by the first winding and the second winding. The diode is able to switch between a reverse bias and a forward bias by controlling the polarities of the windings such that an input current always flows to the electrolytic capacitor during each sinusoidal period of an ac voltage. Further, the power factor of the off-line switching power supply is increased to above 0.9 by appropriately adjusting the induction ratio of the first winding to the second winding in order to comply with the requirements of Class A or Class D stipulated in harmonic current rules IEC 1000-3-2. Furthermore, the ripple voltage of the electrolytic capacitor will not rise if the load suddenly drops significantly when the DC/DC converter is operating in a continuous current mode. As such, the output of the DC/DC converter will not be adversely affected by the 120 Hz ac voltage input.

The invention will now be described further by way of example with reference to the accompanying drawings in which: FIG. 1 is a circuit diagram of a prior art off-line switching power supply ; FIG. 2 is a graph showing the wave shapes of the input voltage versus the input current of FIG. 1; FIG. 3 is a circuit diagram of an inductor type power factor correction circuit; FIG. 4 is a circuit diagram of an active type power factor correction circuit; FIG. 5 is a circuit diagram of a dither type power factor correction circuit of single-stage single-switch ; FIG. 6 is a circuit diagram of U. S. Pat. No. 5,301,095; FIG. 7 is a circuit diagram of U. S. Pat. No. 5,600,546; FIG. 8 is a circuit diagram of a preferred embodiment of the present invention; FIG. 9 is a modified circuit diagram of FIG. 8 in which the DC/DC converter is a fly-back converter; FIG. 10 is a graph showing the wave shapes of the voltage Vds of the switch element versus the current !, a, Lb of windings La, Lb of FIG. 9; FIG. 11 is a graph showing the wave shapes of the input voltage V, n versus the input current), of FIG. 9; FIG. 12 is a graph showing the voltage VdCof the electrolytic capacitor C, ouf FIG. 9 versus varying load ; FIG. 13 is a graph showing the wave shapes of the ripple voltages in the secondary electrolytic capacitors of the present invention shown in Fig. 9 and the prior art shown in FIG. 7; FIG. 14 is a graph showing the dc ripple voltage Vdcof FIG. 9 versus varying load current; FIG. 15 is a circuit diagram of a second embodiment of the present invention; and FIG. 16 is a circuit diagram of a third embodiment of the present invention.

TABLE 1 is the experiment data of the harmonics of the input current lin of the embodiment shown in Fig. 9 comparing with the requirements of Class A stipulated in harmonic current rules IEC-1000-3-2 ; and TABLE 1 is the experiment data of the harmonics of the input current), of the embodiment shown in Fig. 9 comparing with the requirements of Class D stipulated in harmonic current rules IEC-1000-3-2.

Referring to FIG. 8, there is shown a preferred embodiment of a power factor correction circuit 10 constructed in accordance with the present invention.

The power factor correction circuit 10 comprises a series connection of a bridge rectifier DB,, a first winding La, a diode D,, and an electrolytic capacitor Cl ; a series connection of the electrolytic capacitor C,, a second winding Lb, and a DC/DC converter 2; and a common core Trt wound round by the first winding La and the second winding Lb.

In FIG. 9, the DC/DC converter 2 can be a fly-back converter comprising a transformer Ta, a switch element Q"a a control circuit 21, a diode D2 on a secondary side of the transformer T, and a capacitor C3 on the secondary side of the transformer Tr2, The diode D2 and the capacitor C3 form an output filter. As to the transformer Tr2, ratio of the primary winding Lp to the secondary winding Ls with respect to the number of turns is Np/Ns. A series connection consists of the primary winding Lp, the switch element Q,, the electrolytic capacitor C,, and the winding Lb. Another series connection consists of the secondary winding Ls, the diode D2, and the capacitor C3. An output voltage Vu is measured across the output terminas X3 and X4 by the control circuit 21 so as to generate a control signal for adjusting the conduction time of the switch element Q,. As a result, the voltage V, is kept at a predetermined value.

Again referring to Fig. 9, wherein Vs1 is the ac input; Vin is an input voltage of the bridge rectifier DB, measured across the input terminals X, and X2 ; 1in is an input current thereof; Vjn (r, < is output output voltage of the bridge rectifier DB, ; ILa and l, are currents of the first winding La and the second winding Lb respectively ; Vy is a voltage measured between the first winding La and the diode D, ; and Vdc is an input dc voltage of the electrolytic capacitor Cl.

In the above embodiment, the current ILb having a slope VdC/(Lb+Lp) is generated on the secondary winding Lp when the switch element Q, is conducted. Then, the current ILb flows from the anode of the electrolytic capacitor C, through the windings Lb, Lp and the switch eiement Q, to the cathode of the electrolytic capacitor C1. Since the windings Lb, Lp wind round the same core Troll the polarity between the windings Lb, Lp will then let the diode D, be reverse biased and act as an open circuit to block the current ILa of the winding La, i. e., the current ILa is approximately zero. The magnetic lines in the core T,, will expand because the current ILb is still flowing through the winding Lb.

The magnetic lines in the core T, will diminish when the switch element Q, is cut off. As a result, the polarity is reversed. Then the diode D, is forward biased and acts as a closed circuit to conduct the current ILa of the winding La having a slope of (Vin(rec)-Vdc)/La which flows from the bridge rectifier DB1 to the electrolytic capacitor C,.

In FIG. 10, a graph shows the wave shapes of the voltage \les of the switch element Q, versus the current IL, of the winding La and the current ILb of the winding La. In FIG. 11, a graph shows the wave shapes of the input voltage V,, versus the input current 1in. In view of these two graphs, it is found that the input current), generated in the present embodiment complies with the requirements of Class A or Class D stipulated in harmonic current rules IEC-1000-3-2. The above feature will be more apparent from table I and table 11 by comparing the experiment data of the harmonics of the input current li, of the embodiment shown in Fig. 9 with the requirements of Class D stipulated in harmonic current rules IEC-1000-3-2.

In view of the above, the power factor correction circuit 10 according to the present invention will make sure the input current lin always flows to the electrolytic capacitor C, during each sinusoidal period (i. e., the period of the switch element Q, switching from conduction to cutoff and vice versa) of an ac voltage V,,,. The power factor is then increased to above 0.9 by appropriately adjusting the induction of the windings Las Lb and the induction of the secondary winding Lp of the transformer T, of the DC/DC converter 2. Moreover, the THD is dropped to below 15%.

As shown in FIG. 12, the voltage of the electrolytic capacitor C, is kept at a stable range when load is varied. Further, as shown in FIG. 13, the ripple output voltage of the DC/DC converter 2 of the present embodiment is not affected by the 120 Hz ac voltage input, while that of the prior art power supply shown in FIG.

7 is adversely affected. Furthermore, as shown in FIG. 14, the dc ripple voltage Vdc of the electrolytic capacitor C, will not rise if the load suddenly drops significantly when the DC/DC converter 2 is operating in the continuous current mode.

FIG. 15 illustrates a second embodiment of the present invention in which a high frequency filter capacitor C2 is added to be in a parallel connection with the power factor correction circuit 10 for filtering out the high frequency component of the circuit 10. As such, the output current of the bridge rectifier DB1 is smoothed by the action of the capacitor C2.

FIG. 16 illustrates a third embodiment of the present invention in which a high frequency filter capacitor C2 is added to be in a parallel connection with the bridge rectifier DB, for filtering out the high frequency component of the circuit 10.

As such, the input current lit ouf the ac source Vs, is smoothed by the action of the capacitor C2.

Note that the fly-back converter as implemented in the above embodiments being readily substituted with a forward converter, a half-bridge converter, a fullbridge converter, a pus-pull converter, or a boost converter should also be deemed as falling within the scope of the present invention.

While the invention herein disclosed has been described by means of specific embodiments, numerous modifications and variations could be made thereto by those skilled in the art without departing from the scope of the invention set forth in the claims.

Claims

CLAIMS 1. A power factor correction circuit comprising: a first series connection of a bridge rectifier, a first winding, a diode, and an electrolytic capacitor; a second series connection of the electrolytic capacitor, a second winding, and a DC/DC converter ; and a common core wound round by the first winding and the second winding; wherein the diode is able to switch between a reverse bias and a forward bias by controlling poiarities of the windings such that an input current of an alternating current voltage always flows to the electrolytic capacitor during each sinusoidal period of the alternating current voltage.
2. The power factor correction circuit of claim 1, wherein the DC/DC converter further comprising: a transformer in which a first series connection consists of a primary winding of the transformer, a switch element on a primary side of the transformer, the electrolytic capacitor on the primary side of the transformer, and the second winding, and a second series connection consists of a secondary winding of the transformer, a diode on a secondary side of the transformer, and a capacitor on the secondary side of the transformer; and a control circuit connected between the switch element and an output of the DC/DC converter for measuring an output voltage of the converter so as to generate a control signal for adjusting a conduction time of the switch element for keeping the output voltage at a predetermined value.
3. The power factor correction circuit of claim 1 or 2, wherein a first high frequency filter capacitor is connected in parallel with the power factor correction circuit for filtering out a high frequency component of the power factor correction circuit for smoothing an output current of the bridge rectifier.
4. The power factor correction circuit of claim 1 or 2, wherein a second high frequency filter capacitor is connected in parallel with the bridge rectifier for filtering out a high frequency component of the power factor correction circuit for smoothing the input current of the alternating current voltage.
5. A power factor correction circuit substantially as herein described with reference to and as illustrated in Figures 8,9,15 and 16 of the accompanying drawings.