JPH08149816A - Switching power supply circuit - Google Patents

Abstract

PURPOSE: To improve power factor and power conversion efficiency by connecting a first capacitor to the output line of a bridge rectification circuit, connecting two second capacitors to the positive and negative input terminals of the bridge rectification circuit, and by supplying the switching output at a primary coil winding to the bridge rectification circuit. CONSTITUTION: A low-pass filter in normal mode consisting of a filter choke coil LN and a filter capacitor CN is provided at the rectification current path of a bridge rectification circuit D1 . Then, a first capacitor C1 A whose total electrostatic capacity is nearly equal to the electrostatic capacity corresponding to a resonance capacitor which is connected to a primary coil winding N1 for forming a series resonance circuit and two second capacitors C1 B, are provided. The first capacitor C1 A is connected to the output line of the bridge rectification circuit D1 and two second capacitors C1 B, are connected to the positive and negative input terminals of the bridge rectification circuit D1 . Then, the switching output at the primary coil winding N1 is supplied.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a switching power supply circuit for improving power factor, for example.

[0002]

2. Description of the Related Art In recent years, by developing a switching element capable of withstanding a relatively large current and voltage of high frequency, most of the power supplies of a switching system are used as a power supply device for rectifying a commercial power supply to obtain a desired DC voltage. It is a device. The switching power supply is used as a power supply for various electronic devices as a high-power DC-DC converter, while increasing the switching frequency to downsize transformers and other devices.

By the way, in general, when a commercial power supply is rectified, the current flowing through the smoothing circuit has a distorted waveform, which causes a problem that the power factor indicating the utilization efficiency of the power supply is impaired. In addition, there is a need for measures to suppress harmonics generated by the distorted current waveform.

Therefore, as one of the switching power supply circuits with improved power factor, a switching power supply circuit as shown in the circuit diagram of FIG. 6 has been previously proposed by the present applicant. This power supply circuit is a half-bridge type self-exciting current resonance type converter.

In this figure, AC indicates a commercial AC power source. Further, D ₁ is a bridge rectifier circuit composed of four low-speed recovery type diodes, and performs full-wave rectification on the input AC power supply AC. The filter choke coil L _N is connected to the line between the positive output terminal of the bridge rectifier circuit D _{1 and} the positive electrode of the smoothing capacitor Ci.
A fast recovery type diode D ₂ and a choke coil CH are provided in series as shown in the figure. The normal mode filter capacitor C _N has been inserted between the positive pole of the filter choke coil L _N and high-speed connection point of recovery diode D ₂ and the smoothing capacitor Ci, a filter capacitor C _N and filter choke coil L _N L
It forms a C low-pass filter.

This LC low pass filter is designed to prevent high frequency noise of the switching frequency from flowing into the AC line. The fast recovery type diode D ₂ is provided in response to the high-frequency current of the switching cycle described later flowing through the full-wave rectified output line.

C ₂ is a parallel resonance capacitor,
As shown in the figure, it is connected in parallel with the choke coil CH to form a parallel resonance circuit together with the choke coil CH. The resonance frequency of the parallel resonance circuit is set to be substantially the same as the resonance frequency of the switching power supply. The operation will be described later.

Q ₁ and Q ₂ are switching elements forming a half-bridge type switching converter circuit, and as shown in the figure, a collector and an emitter are respectively connected between the connection point on the positive electrode side of the smoothing capacitor Ci and the ground. Connected. This switching element Q ₁ , Q ₂
Resistor R inserted between each collector and base of
_S and R _S are starting resistors, and switching elements Q ₁ and Q
D _D and D _D inserted between the bases and emitters of ₂ respectively represent damper diodes. Also, the resistors R _B and R
_B indicates resistors for adjusting the base current (drive current) of the switching elements Q ₁ and Q ₂ , respectively. And
C _B and C _B are capacitors for resonance, and together with drive windings N _B and N _{B of} the drive transformer PRT described below,
A series resonance circuit for self-excited oscillation is formed.

PRT represents a drive transformer for variably controlling the switching frequencies of the switching elements Q ₁ and Q ₂ , and in the case of this figure, the drive windings N _B and N _B and the resonance current detection winding N _D are wound. Further, a control winding N _C is wound in a direction orthogonal to each of these windings to form an orthogonal saturable reactor. This drive transformer PR
_One end of the drive winding N _B on the switching element Q ₁ side of T is connected to the resistor R _B , and the other end is connected to the emitter of the switching element Q ₁ . Also, the drive winding N on the switching element Q ₂ side
One end of _B is grounded and the other end is connected to a resistor R _B so that a voltage having a polarity opposite to that of the drive winding N _B is output. Further, one end of the current detection winding N _D is connected to the contacts of the emitter of the switching element Q _{1 and} the collector of the switching element Q ₂ , and the other end is connected to the primary winding N ₁ of the insulating transformer PIT.

PIT is an insulating transformer for transmitting the switching outputs of the switching elements Q ₁ and Q _{2 to} the secondary side. One end of the primary winding N ₁ of this insulating transformer PIT is the end of the current detection winding N _D. The other end is connected to the connection point between the fast recovery diode D ₂ and the choke coil CH via the series resonance capacitor C ₁ . Then, by the inductance component of the insulating transformer PIT including the series resonance capacitor C ₁ and the primary winding N ₁ ,
A resonance circuit is formed to make the switching power supply circuit a current resonance type. In the case of this switching power supply circuit,
On the secondary side of the insulation transformer PIT, the induced voltage induced in the secondary winding N ₂ by the primary winding N ₁ is converted into a DC voltage by the bridge rectifier circuit D ₃ and the smoothing capacitor C ₃ and the output voltage E ₀ is obtained. To be done.

The control circuit 1 compares the DC voltage output E _O on the secondary side with the reference voltage and supplies a DC current corresponding to the error to the control winding N _C of the drive transformer PRT as the control current I _C. Error amplifier.

In the switching operation of the switching power supply having the above-mentioned structure, when the commercial AC power supply is first turned on, the switching element Q is turned on, for example, via the starting resistors R _S and R _{S.
Although the} base current is supplied to the bases of ₁ and Q ₂ , if the switching element Q ₁ is turned on _first , the switching element Q ₂ is controlled to be turned off. Then, as the output of the switching element Q ₁ , the current detection winding N _D → the primary winding N ₁ → the series resonance capacitor C
_Although a resonance current flows in ₁ , the switching element Q ₂ is controlled to be turned on and the switching element Q ₁ is controlled to be turned off in the vicinity where the resonance current level becomes 0. Then, a resonance current in the opposite direction to the above flows through the switching element Q ₂ . After that, the self-excited switching operation in which the switching elements Q ₁ and Q ₂ are alternately turned on is started. In this way, the switching elements Q ₁ and Q ₂ are alternately opened and closed by using the terminal voltage of the smoothing capacitor C _i as the operating power supply, thereby supplying the drive current close to the resonance current waveform to the primary winding N ₁ of the insulation transformer. Then, an alternating output is obtained from the secondary winding N ₂ .

Further, when the DC output voltage (E _O ) on the secondary side decreases, the control circuit 2 controls the current flowing through the control winding N _C so that the switching frequency becomes low (close to the resonance frequency. Controlled by the primary winding N ₁
The drive voltage is controlled so that the drive current is increased to achieve a constant voltage.

The power factor improving operation is as follows. In this circuit, the primary winding N ₁ of the isolation transformer PIT
The switching output corresponding to the resonance current flowing through
The choke coil CH is superposed on the rectified voltage of the commercial AC power source flowing through the self-inductance Li of the winding Ni. As a result, the smoothing capacitor Ci is charged in a state where the switching voltage is superimposed on the full-wave rectified voltage, and the terminal voltage of the smoothing capacitor Ci is reduced in the switching cycle by the superimposed amount of the switching voltage. Then, the charging current starts flowing while the terminal voltage of the capacitor Ci is lower than the rectified voltage level of the bridge rectifier circuit, and the average AC input current approaches the AC voltage waveform to improve the power factor. .

In the power supply circuit for improving the power factor by such a method, the drive current of the insulating transformer PIT becomes small when the load is light, so that the switching current flowing in the full-wave rectified output line is also made small by this drive current. Become. Therefore, since the level of the charging current becomes small when the load is light and the charging current becomes large when the load is heavy, the phenomenon that the terminal voltage of the smoothing capacitor Ci abnormally rises is solved especially when the load is light, and it is difficult for the normal MS method. The regulation can be improved. Therefore, for example, the fluctuation of the rectified and smoothed voltage Vi is suppressed even if the fluctuation of the _AC input voltage V _AC ± 20%, and it is not necessary to consider the improvement of the breakdown voltage of the switching element or the smoothing capacitor.

Further, the resonance capacitor C ₂ connected to the self-inductance Li of the choke coil suppresses the switching voltage fed back to the rectifying and smoothing line when the load of the switching power supply becomes light. As a result, the increase of the terminal voltage Ei of the smoothing capacitor Ci at the time of light load is suppressed.

That is, in the power supply circuit having the configuration shown in FIG. 6, the switching frequency is controlled to increase as the power supply load decreases. At this time, the self-inductance L
The impedance of the resonance circuit composed of i and the parallel resonance capacitor C ₂ suppresses the switching voltage returned to the charging circuit side and prevents the terminal voltage from rising. Further, when the power supply load increases, the switching frequency lowers, approaches the resonance frequency of the resonance circuit of the self-inductance Li and the parallel resonance capacitor C ₂ , and acts to increase the feedback switching voltage. Therefore, in this power supply circuit, the voltage fluctuation rate at which the terminal voltage of the smoothing capacitor fluctuates due to the power supply load is reduced, which makes it easy to make the DC output E _O a constant voltage.

The filter choke coil L _N and choke coil CH actually used in the circuit of FIG.
Are shown in FIGS. 7 (a) and 7 (b), respectively. The filter choke coil L _N is constructed by winding a single wire such as a polyurethane copper wire directly around a drum-shaped ferrite core D without a bobbin as shown in FIG. 7A.

As shown in FIG. 7 (b), the choke coil CH forms an EE type core in which E type ferrite cores C _R are combined as shown in the figure. G is provided so that the leakage magnetic flux does not leak to the outside of the choke coil. And
As the winding Ni, for example, a winding bobbin (not shown) has a diameter of 6
A litz wire made of 0 μ polyurethane polyurethane wire is wound so that the self-inductance Li is not saturated at the maximum load.

[0020]

In view of the size and cost of equipment, the switching power supply circuit can be made small and lightweight by reducing the number of parts as much as possible and using small and inexpensive parts. And it is preferable to achieve low cost. For example, in the case of the switching power supply circuit having the configuration shown in FIG. 6 described above, the outer shape of the choke coil CH (see FIG. 7B) becomes large as the load power increases, and thus the size is small. It is disadvantageous from the viewpoint of weight reduction, and since the litz wire is wound, it is expensive and disadvantageous in cost. In addition, it is preferable to improve the characteristics such as power conversion efficiency and guarantee of power factor improvement with respect to the range of AC input voltage. For example, in the choke coil CH formed by winding a litz wire around a ferrite material, This is a component having a relatively large power loss due to factors such as copper loss, iron loss, and eddy current loss, which is disadvantageous in improving power conversion efficiency.

[0021]

SUMMARY OF THE INVENTION Therefore, the present invention provides a bridge rectifier circuit for rectifying a commercial power source, a smoothing capacitor for smoothing the output of the bridge rectifier circuit, and a switching converter for interrupting the voltage supplied from the smoothing capacitor. And a switching transformer intermittently switched by the switching converter to supply the primary winding with an insulating transformer, and a switching power supply circuit configured to obtain a DC output from the secondary side of the insulating transformer. A normal mode low-pass filter consisting of a choke coil and a filter capacitor is provided in the rectification current path of the bridge rectification circuit, and the capacitance corresponding to the resonance capacitor that forms a series resonance circuit is connected to the primary winding and its total static capacitance. One first capacitor and two capacitors with almost the same capacitance The second capacitor is provided, the first capacitor connected to the output lines of the bridge rectifier circuit, 2
The two second capacitors are respectively connected to the positive / negative input terminals of the bridge rectifier circuit, and are configured to supply the switching output obtained from the primary winding to the bridge rectifier circuit. And the resonance capacitor, the first
Of the first capacitor and the second capacitor after setting the respective capacitances so that A = (B + 2C), where A, B, and C are the capacitances of the second capacitor and the second capacitor, respectively. It was decided that the power factor could be set arbitrarily by changing the capacitance ratio, and a high-speed recovery type was used as the rectifying element forming the bridge rectifying circuit.

As the constant voltage control, the switching frequency of the switching converter is varied based on the DC output voltage obtained on the secondary side of the insulation transformer, or based on the DC output voltage obtained on the secondary side of the insulation transformer. To perform constant voltage control by changing the magnetic flux of the insulation transformer, or if the switching converter is of the separately excited type, change the switching drive signal based on the DC output voltage obtained on the secondary side of the insulation transformer. It was decided to configure so that a constant voltage could be obtained.

[0023]

According to the above configuration, in the switching power supply circuits of various types of current resonance type, the normal mode low pass filter provided in the AC line and the bridge rectification circuit using the high speed recovery type diode are provided, and the series resonance capacitor is provided. It is divided and connected to the bridge rectifier circuit side, and the power output is obtained by supplying the switching output obtained in the primary winding of the insulation transformer to the rectifier circuit side by capacitive coupling of these divided capacitors. Although improvements will be made, this will reduce the components such as the choke coil, the parallel resonance capacitor, and the fast recovery type diode provided in the rectification line. Further, with the above configuration, a high power factor characteristic can be obtained from a heavy load to a light load.

[0024]

FIG. 1 shows an embodiment of a switching power supply circuit according to the present invention. The same parts as those in FIG.
The description of the switching operation and constant voltage control will be omitted. In the circuit of this embodiment, the normal mode low-pass filter formed by the filter choke coil L _N and the filter capacitor C _N is provided in the line of the AC power supply AC as shown in the figure. The filter choke coil L _{N according to} the present embodiment is, for example, as shown in FIG.
It suffices if the inductor is configured by the open magnetic circuit of the drum choke coil similar to that shown in FIG. Further, a fast recovery type is used for each of the four diodes forming the bridge rectifying circuit D ₁ in this case, and these fast recovery type diodes D _F1 , D _F2 , D _F3 , and D _{F4 are used.}
Will be bridge-connected as shown in the figure.

Further, in this embodiment, the capacitance of the series resonant capacitor C ₁ forming the series resonant circuit described with reference to FIG. 6 is divided into the capacitor C _1A and the two capacitors C _1B and C _1B. And C ₁ = (C _1A +
2C _1B ), the two capacitors C _1B and C _1B preferably have the same capacitance. Then, one pole of the capacitor C _1A is connected to the primary winding N ₁ of the insulation transformer PIT, and the other pole is connected to the cathode and D of the fast recovery diode D _F1.
Connection point with cathode of _F2 , that is, bridge rectifier circuit D ₁
Is connected to the positive electrode output line. Further, one pole of the two capacitors C _1B is commonly connected to the primary winding N _1, and the other poles thereof are respectively fast recovery type diodes D 1.
It is connected to the connection point between the cathode of _F4 and the anode of D _F1 and the connection point between the cathode of the fast recovery diode D _F3 and the anode of D _F2 . That is, the bridge rectifier circuit D ₁ is connected to the positive / negative AC input terminals. In this way, the capacitors C _1A and C _1B ,
By providing C _1B , the switching elements Q ₁ , Q ₂
The switching output obtained by the switching operation of (1) is fed back from the primary winding N ₁ to the full-wave rectification output line via the capacitor C _1A , and the capacitors C _1B and C _1B form a bridge rectification circuit D ₁ for high-speed recovery. It is possible to obtain an operation in which the rectified current flowing in the diode is switched according to the switching cycle. That is, in this embodiment, when the switching output is superimposed on the current path of the primary side rectifying / smoothing circuit, the capacitors are coupled by the electrostatic capacitances of the capacitor C _1A and the capacitors C _1B and C _1B .

The operation of the switching power supply circuit of this embodiment having the configuration shown in FIG. 1 will now be described with reference to the waveform chart of FIG. For example, when the AC input voltage VAC is supplied to the _AC power supply AC as shown in FIG.
The AC power supply in the normal mode low-pass filter provided in AC line filter capacitor C _N, a capacitor C _1A, that flows sinusoidal high frequency switching cycle supplied from the primary winding N ₁ through C _1B However, the current I _1A flowing through this filter capacitor C _N is
As shown in FIG. 2B, a low level high frequency signal waveform is obtained.

Then, during the period of the positive half cycle of the _AC input voltage V _AC shown in FIG.
_{At 1} , the rectified currents I ₁ and I ₃ flow into the fast recovery type diodes D _F1 and D _F3 , respectively. This rectified current I
_{As I} and I _3, as shown in the periods 0 to π and 2π to 3π in FIG. 2C, a substantially convex waveform with a high frequency (sine wave) of the switching cycle is obtained. That is, the current is made to flow during a period other than the τ period when the AC input voltage is lower than the rectified and smoothed voltage Ei of the smoothing capacitor Ci. On the other hand, in the period when the AC input voltage V _AC is negative,
Rectification currents I ₂ and I ₄ flow through the fast recovery diodes D _F2 and D _F4 of the bridge rectification circuit D ₁ . Similarly, the rectified currents I ₂ and I ₄ also have substantially convex waveforms due to the high frequency of the switching period as shown in the period π to 2π in FIG. 2D. Such a waveform as shown in FIG. 2 (c) or (d) is obtained because of the switching output supplied through the primary winding N ₁ of the two capacitors C.
_{This is because the} currents I ₆ and I ₇ (FIG. 2E) due to the switching cycle via _1B and C _1B are supplied to the positive / negative input terminals of the bridge rectifier circuit D ₁ , respectively.

As described above, the switching output is the bridge rectifier circuit D from the primary winding N ₁ through the capacitor C _1A.
The current I ₅ flowing through the capacitor C _1A has a sinusoidal high frequency waveform as shown in FIG. 2F. The charging current I ₈ flowing through the smoothing capacitor Ci is the current I ₅ (see FIG. 2).
(F)) and the rectification currents I _{1 to} I _{4 of the} bridge rectification circuit D _1.
2 (c) and (d), a high-frequency waveform as shown in FIG. 2 (g) is obtained. The waveform of the _AC input current I _AC flowing through the AC power supply AC is, for example, as shown in FIG.
The rectified current shown in (d) has the waveform shown in FIG. 2 (h) in which I _{1 to} I ₄ are averaged, and the power factor is improved.

FIG. 3A shows the AC input power Pin for the AC input voltage V _AC of the switching power supply circuit of this embodiment.
7 is a diagram showing the characteristics of the above in comparison with the switching power supply circuit shown in FIG. 6. In this case, the load power P _O is used as a parameter, the characteristics of the circuit of this embodiment are indicated by straight lines, and the characteristics of the circuit shown in FIG. 6 are indicated by broken lines. In the switching power supply circuit of this embodiment, L _N = 100 μH, C _N = 1 μF, C _1A =
8200pF, C _1B = 4700pF, D _{F1 to} D _F4 = 3A
/ 200V is selected, and in the circuit shown in FIG. 6, L _N
= 100 μH, C _N = 1 μF, Li = 22 μH, C ₁ =
It is assumed that 0.081 μF and D ₂ = 3 A / 200 V are selected.

According to this figure, when the load power P _{O is} 120 W and the load is heavy, the switching power supply circuit of this embodiment has an AC input voltage V _{AC of about} 80 V to 140 V and is about 1 W more than the circuit shown in FIG. The AC input power Pin is reduced, and even when the load power P _{O is} zero, the AC input voltage V _{AC is in} the range of 80 V to 140 V, and the circuit of the present embodiment is 3 more than the circuit of FIG. It is reduced by about 5W. Further, when the load power P _{O is} 12 W and the load is light, the characteristic of the circuit of this embodiment is a straight line on the upper right as shown in the figure, but the AC input voltage V _AC is in the vicinity of 80 V to 130 V as shown in FIG. A power factor characteristic higher than that of the circuit is obtained, and for example, when the AC input voltage V _AC = 100V, the AC input power Pin is reduced by about 1.5 W as compared with the circuit of FIG.

As described above, the reason why the power loss is reduced in the switching power supply circuit of this embodiment is that the choke coil CH provided in the switching power supply circuit of FIG. This is because the power loss due to copper loss and iron loss has disappeared.
Further, since the high speed recovery type diode D ₂ is provided in the circuit of FIG. 6, the _two diodes of the bridge rectifying circuit D ₁ and the high speed recovery type diode D ₂ are provided for each positive / negative half cycle of the AC input current. _In the present embodiment, the fast recovery type diode D ₂ is omitted and the diodes provided in the respective paths of the positive / negative AC input current are This is because the two bridge rectifier circuits D ₁ are provided and the loss due to the forward voltage drop of the diode element is eliminated accordingly.

Next, FIG. 3B shows the power factor characteristic of the switching power supply circuit shown in FIG. 1 with respect to the AC input voltage V _AC .
It is a figure compared and shown with the switching power supply circuit shown in FIG. 6, and load electric power P _O is made into a parameter. In addition,
Even in this case, the circuit shown in FIG. 1 and FIG.
The characteristics described under (a) are the characteristics under the selected conditions. As can be seen from this figure, in the circuit shown in FIG. 7, a high power factor can be obtained only under a heavy load of load power P _O = 120 W and in the range of _AC input voltage V _AC = 100 V or less. In the circuit, the load power P _O = 120W
A high power factor can be obtained not only when the load is heavy, but also when the load power P _{O is} 12 W and a light load, and the AC input voltage V _AC = 80
A high power factor is stably obtained over a range of V to 140V.

In this embodiment, the capacitance of the divided capacitors C _1A and C _1B is C ₁ = C _1A + 2C _1B
The power factor can be arbitrarily set by changing the ratio while maintaining the relationship of. For example, capacitor C _1A ,
If the electrostatic capacitance of C _1B is C _1A <C _1B , the power factor can be improved.

Further, comparing the present embodiment with the switching power supply circuit shown in FIG. 6 from the viewpoint of components of the power supply circuit, as described above, in this embodiment, the fast recovery type diode D ₂ and the parallel resonance capacitor C are provided. ₂ and choke coil C
H will be reduced. As a result, the circuit board can be made smaller and lighter than the circuit shown in FIG. In particular, the choke coil CH is a large-sized component and increases in size in response to a heavy load. Therefore, the effect of omitting it is great. Incidentally, in the present embodiment, the series resonance capacitor C ₁ is a capacitor C _1A, C _1B, but is provided is divided into three C _1B, the capacitance of equal to _{C 1 = C 1A + 2C 1B} , The costs are similar and there is no particular problem.

Next, the circuit diagram of FIG. 4 shows the configuration of a switching power supply circuit which is another embodiment of the present invention. The same parts as those of FIG. In this switching power supply circuit, the drive transformer is the control winding N
CDT (Converter Drive Transf) without _C
ormer), and therefore the switching frequency is fixed. In this case, the PR as the orthogonal saturable reactor in which the control winding N _C is provided orthogonal to the primary and secondary windings N ₁ and N ₂ of the insulating transformer PIT.
It is called T (Power Regulating Transformer). In this case, the control circuit 1 changes the control current flowing through the control winding N _C on the basis of the DC output voltage E _O to isolate the insulating transformer P.
A so-called series resonance frequency control method is employed in which the saturation characteristic of IT is changed to control the leakage magnetic flux to perform constant voltage control.

In this case, by the same operation as described with reference to FIG. 1, a high power factor can be obtained during heavy load and light load, power loss can be reduced, and the capacitance of the capacitors C _1A and C _1B can be reduced. The power factor can be set arbitrarily by changing the ratio of.

Next, the circuit diagram of FIG. 5 shows the configuration of a switching power supply circuit of still another embodiment. The same parts as those of FIGS. 1 and 2 are designated by the same reference numerals and the description thereof will be omitted. The current resonance type converter in the embodiment of this figure is of the separately excited type by half bridge connection using, for example, MOS-FETs as the switching elements Q ₁ and Q ₂ . In this case, the control circuit 1 controls the oscillation drive circuit 2 based on the DC output voltage E _O , and changes the switching drive voltage supplied from the oscillation drive circuit 2 to the gates of the switching elements Q ₁ and Q _2. Then, constant voltage control is performed.
D _D and D _D connected to the drain and source of each switching element Q ₁ and Q _{2 in} the direction shown in the figure form a current path that is fed back when the switching elements Q ₁ and Q _{2 are} off. It is used as a clamp diode. Further, the starting circuit 3 is provided to detect the voltage or current obtained in the rectifying and smoothing line at the time of starting the power source to start the oscillation drive circuit 2, and the starting circuit 3 is provided to the insulating transformer PIT. The low-voltage DC voltage supplied by the tertiary winding N ₃ and the rectifying diode D ₄ is supplied. Further, in this embodiment, the capacitor C _1A is connected to the ground-side output terminal of the bridge rectifier circuit, that is, to the connection point of the anodes of the fast recovery type diodes D _F3 and D _F4 , and the capacitor C _1A shown in FIGS. The polarity is opposite to that of the case so that the switching output is superimposed on the full-wave rectification line. In this embodiment, the configuration of the power factor correction circuit is the same as that shown in FIG. 1, and as described above, the power factor is improved, the power loss is reduced, and the power factor can be arbitrarily set. Made possible.

The power factor improving method of the present invention described so far in each of the above embodiments is the self-excited oscillation type / excited oscillation type as the switching power supply circuit of the current resonance type.
Depending on the combination pattern of switching frequency control method (drive transformer is orthogonal PRT) / series resonance frequency control method (insulation transformer is orthogonal PRT), switching element half bridge coupling type / full bridge coupling type, etc. It is needless to say that the present invention can be applied to the configured power supply circuit and is not limited to the combination patterns shown as the embodiments in the above respective drawings.

[0039]

As described above, the switching power supply circuit of the present invention is provided with the normal mode low-pass filter provided in the AC line and the rectification circuit by the high speed recovery type diode, and the capacitance of the series resonance capacitor is divided. Then, the power factor is improved by supplying the switching output obtained to the primary side winding of the insulating transformer to the rectifier circuit through the electrostatic capacitance of the divided capacitors. As a result, the components such as the choke coil, the parallel resonance capacitor, and the high-speed recovery type diode provided in the rectification line are reduced, so that the circuit board can be further reduced in size and weight. In addition, since the power loss caused by the choke coil and the high speed recovery type diode is eliminated, the power conversion efficiency is improved accordingly. Further, it has an effect that a high power factor can be obtained from a heavy load to a light load.

[Brief description of drawings]

FIG. 1 is a circuit diagram of a switching power supply circuit as an embodiment of the present invention.

FIG. 2 is a waveform diagram showing the operation of the switching power supply circuit of the embodiment.

FIG. 3 is a diagram showing an AC input power characteristic and a power factor characteristic with respect to an AC input voltage in the example.

FIG. 4 is a circuit diagram showing a switching power supply circuit as another embodiment.

FIG. 5 is a circuit diagram showing a switching power supply circuit as still another embodiment.

FIG. 6 is a circuit diagram showing a switching power supply circuit as a prior art.

FIG. 7 is a perspective view showing a structure of a filter choke coil and a choke coil.

[Explanation of symbols]

1 Control circuit 2 Oscillation drive circuit 3 Start circuit L _N Filter choke coil C _N filter capacitor D ₁ Bridge rectifier circuit DF _{1 to} DF ₄ Fast recovery type diode CH Choke coil PIT Isolation transformer PRT Control transformer CDT Drive transformer Q ₁ , Q ₂ Switching element Ci Smoothing capacitor C _1A , C _1B capacitor N ₁ Primary winding

Claims (6)

[Claims] 1. A bridge rectifier circuit for rectifying a commercial power source, a smoothing capacitor for smoothing an output of the bridge rectifier circuit, a switching means for connecting and disconnecting a voltage supplied from the smoothing capacitor, and a connection for connecting by the switching means. In a switching power supply circuit including an insulating transformer adapted to supply a switching output to a primary winding, and a DC power output obtained from a secondary side of the insulating transformer, a normal mode including a filter choke coil and a filter capacitor. Is provided in the rectification current path of the bridge rectification circuit, and the capacitance corresponding to the resonance capacitor that is connected to the primary winding to form a series resonance circuit and the total capacitance are substantially equal to each other. One capacitor and two second capacitors are provided, and the first capacitor Capacitors is connected to the output line of the bridge rectifier circuit, each of the two second capacitor connected to the positive / negative input terminal of the bridge rectifier circuit,
A switching power supply circuit configured to supply a switching output obtained from the primary winding to the bridge rectification circuit. 2. The capacitances of the resonance capacitor, the first capacitor, and the second capacitor are A, respectively.
B and C are set so that A = (B + 2C), and the power factor can be set arbitrarily by changing the capacitance ratio of the first capacitor and the second capacitor. The switching power supply circuit according to claim 1. 3. The switching power supply circuit according to claim 1, wherein the rectifying element forming the bridge rectifying circuit is a high speed recovery type. 4. A constant voltage control is performed by varying a switching frequency of the switching means based on a DC output voltage obtained on the secondary side of the insulation transformer. The switching power supply circuit according to claim 1, claim 2, or claim 3. 5. The insulation transformer is of a quadrature type and is configured to perform constant voltage control by varying the magnetic characteristics of the quadrature type insulation transformer based on a DC output voltage obtained on the secondary side. Claim 1 or claim 2 characterized in that
Alternatively, the switching power supply circuit according to claim 3. 6. The switching means is a separately excited type,
3. The constant voltage control is performed by changing the switching drive signal based on the DC output voltage obtained on the secondary side of the insulation transformer. Item 3. The switching power supply circuit according to Item 3.