JPH10337023A - Switching power supply - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent generation of an abnormal DC output voltage due to operation only of a DC/DC converter while ensuring a required holding time. SOLUTION: When the input voltage to a power factor improving circuit 5 is interrupted, a voltage for continuing the operation is supplied from a smoothing capacitor 9 to a first control means 10. Consequently, a holding time for supplying a stabilized DC output voltage VOUT can be ensured under the operating state of a power factor improving circuit 8. When the smoothing capacitor 9 is discharged, operation of a second control circuit 15 is stopped together with a first control circuit 10 through a synchronous circuit 41. According to the arrangement, troubles due to operation only of a DC/DC converter 11 can be eliminated.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a so-called two-converter type switching power supply device provided with an active power factor correction circuit at a stage preceding a DC / DC converter.

[0002]

FIG. 3 shows a conventional configuration of a switching power supply device having a power factor correction circuit of this type and a DC / DC converter. In FIG. 1, reference numeral 101 denotes an AC power supply, and reference numerals 102 and 103 denote input terminals connected to the AC power supply 101. A choke coil 104 is connected between both ends of the input terminals 102 and 103 and the first switching element MO.
Connect a series circuit with S-type FET 105, MOS-type FET
A diode between both ends of 105, that is, between the drain and source
A power factor improving circuit 108 formed by connecting a series circuit of a smoothing capacitor 107 and a smoothing capacitor 107 is provided in a stage preceding the DC / DC converter 109. The power factor improving circuit 108 controls the power supply to cut off the MOS FET 105 by the drive signal from the first control means 110, thereby converting the input current waveform taken from the choke coil 104 into the input voltage waveform of the AC power supply 101. This is a sinusoidal wave which is substantially the same as. The smoothing capacitor 107 also functions as an input capacitor of the DC / DC converter 109.

On the other hand, in addition to the smoothing capacitor 107, the DC / DC converter 109 includes an insulating transformer 111 for insulating the primary side and the secondary side, and a second switching connected to the primary winding of the insulating transformer 111. MOS FET as an element
A rectifying and smoothing circuit 113 for rectifying and smoothing the voltage induced in the secondary winding of the insulating transformer 111;
And a second control means 114 for supplying a drive signal to the second control means 114. Then, the MOS is controlled by the second control means 114.
By controlling the cut-off of the FET 112, the smoothing capacitor
The DC input voltage VDC from the power factor correction circuit 108 generated between both ends of the inverter 107 is applied intermittently to the primary winding of the insulating transformer 111, and the stable DC output voltage VOUT is output from the rectifying and smoothing circuit 113 to the output terminal 115. , 116.

Next, the operation of the switching power supply device will be described with reference to the waveform diagrams of FIGS. In each of these graphs, the uppermost “AC”
Is the AC input voltage of the AC power supply 101. Hereinafter, the waveform corresponding to “VDC” is the DC input voltage VDC input to the DC / DC converter 109, and the waveform corresponding to “PFC operation” is The operation / non-operation state of the first control means 110 constituting the power factor improvement circuit 108, and the waveform corresponding to the “DC / DC operation” are the operation / non-operation states of the second control means 114 constituting the DC / DC converter 109. Non-operating state, "VOU
The waveform corresponding to "T" indicates the DC output voltage VOUT output from the DC / DC converter 109.

FIG. 4 shows waveforms when the first control means 110 and the second control means 114 operate / stop asynchronously. In this case, even if the power factor correction circuit 108 malfunctions or the power factor correction circuit 108 in which the AC input voltage from the AC power supply 101 is shut off is not operating, the DC / D
Since the C converter 109 continues to operate, the smoothing capacitor
107, the DC input voltage VDC drops, and the DC output voltage VOUT becomes unstable.
This causes a problem that the input current to the C / DC converter 109 becomes abnormal.

Further, when the input is restored after the instantaneous interruption of the AC input voltage, the following problem occurs as shown in FIG.
That is, when the AC input voltage from the power supply 101 is stopped and the power factor correction circuit 108 is not operated, the DC / DC converter 109 is operating, and the potential of the DC input voltage VDC discharges the smoothing capacitor 107. Even if the voltage gradually decreases, DC / DC is not applied until this potential reaches a certain level.
Converter 109 continues to supply a constant DC output voltage VOUT (T1: holding time). After that, as the DC input voltage VDC decreases, the DC output voltage VOUT of the DC / DC converter 109 also decreases. However, even if the AC power supply 101 returns, the power factor improving circuit 108 does not operate until the restart. Several hundred mSEC
It takes some time and does not return to operation immediately (T2: restart time). Accordingly, during this time, a capacitor input state occurs in which the AC input voltage from the AC power supply 101 is directly applied to the smoothing capacitor 107, and the DC input voltage VDC having a pulsating waveform is applied to the DC / DC converter 109, and the abnormal DC output voltage VOUT is output.

FIG. 5 shows waveforms when the first control means 110 and the second control means 114 operate / stop synchronously. Here, there are the following problems when the AC input voltage is stopped or cut off. That is, when the AC input voltage from the power supply 101 is stopped and the power factor correction circuit 108 is not operated, the operation of the DC / DC converter 109 is also stopped at the same time. Therefore, the DC output voltage VOUT of the DC / DC converter 109 cannot be maintained in the stable state up to that point, and immediately drops, and the holding time T1 as the switching power supply device is lost.

Therefore, the present invention has been made in view of the above problems, and
It is an object of the present invention to provide a switching power supply device capable of preventing generation of an abnormal DC output voltage due to operation of only a / DC converter and securing a required holding time.

[0009]

According to a first aspect of the present invention, there is provided a switching power supply device, wherein a first control means controls a cut-off of a first switching element to achieve the first object. Thus, the input current waveform is made sinusoidal, and a power factor improvement circuit that generates a DC input voltage between both ends of the smoothing capacitor, and the second switching element is turned on and off by the second control unit, so that A DC / DC converter for intermittently applying the DC input voltage to a primary winding of an insulating transformer and outputting a stable DC output voltage;
A synchronizing circuit for stopping the second control means in synchronization with the stop of the first control means, and when the second control means is operating, the operation from the smoothing capacitor to the first control means is performed. And an operation continuation circuit for supplying an operation voltage to be continued.

According to the first aspect of the present invention, even when the input voltage to the power factor improving circuit is cut off, the energy stored in the smoothing capacitor is utilized when the second control means is operating. , An operation voltage for continuing the operation is supplied to the first control means. Therefore, the DC / DC converter can be operated while the power factor correction circuit is operating, and the holding time for supplying a stable DC output voltage can be secured. Thereafter, when the smoothing capacitor discharges and the operating voltage supplied to the first control circuit decreases, the operation of the second control circuit together with the first control circuit is stopped by the synchronous circuit. Therefore, it is possible to solve the problem that only the DC / DC converter operates and the DC output voltage becomes unstable.
Further, even if the input voltage to the power factor correction circuit is restored, the power factor correction circuit and the DC / DC converter are both stopped synchronously.
It is possible to avoid a situation in which only the converter operates, and also in this case, it is possible to prevent occurrence of an abnormal DC output voltage.

According to a second aspect of the present invention, in addition to the configuration of the first aspect, the switching power supply supplies a voltage detection signal corresponding to an input voltage level of the power factor correction circuit to a control terminal of the switch element. When the input voltage level becomes equal to or higher than a predetermined value at the time of starting, the circuit further includes a starting circuit for turning on the switch element and supplying an operating voltage from the smoothing capacitor to the first control means. Comprises a switching element, and a series circuit of a diode and a resistor connected between a control terminal of the switching element and an operating voltage line of the second control means.

According to the second aspect of the present invention, even if the input voltage to the power factor correction circuit is cut off, the diode and the second control means are in operation when the operating voltage line is higher than a predetermined level. An ON signal is supplied to a control terminal of the switch element via a series circuit of resistors, and an operation voltage is supplied to the first control means from the smoothing capacitor. The switch element originally constitutes a start-up circuit, and by using this switch element also as the constitution of the operation continuation circuit, it is possible to use common components. And, in effect,
Since the operation continuation circuit can be configured only by the diode and the resistor excluding the switch element, the circuit configuration can be simplified.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the switching power supply according to the present invention will be described below with reference to the accompanying drawings. In FIG. 1 showing a circuit configuration, 1 is an AC power supply,
Reference numeral 3 denotes an input terminal connected to the AC power supply 1. An AC input voltage from the AC power supply 1 is rectified by a rectifier 4 such as a diode bridge. The power factor improving circuit 5 is composed of a boost type active smoothing filter circuit as in the conventional example, and a series circuit of a choke coil 6 and a MOS type FET 7 as a first switching element is connected between output terminals of the rectifier 4. Between both ends of the MOS FET 7, that is, the drain
A series circuit of a diode 8 and a smoothing capacitor 9 is connected between the sources. Also, while monitoring the DC input voltage VDC by the drive signal from the first control means 10 such as a control IC constituting the power factor correction circuit 5,
By controlling the cutoff of the S-type FET 7, the input current waveform taken in from the choke coil 6 is made to have a sine waveform substantially the same as the input voltage waveform of the AC power supply 1.

Reference numeral 11 denotes a DC / DC converter connected to the subsequent stage of the power factor correction circuit 5. This DC / DC converter 11 uses an insulating transformer 12 that shares the smoothing capacitor 9 as an input capacitor and insulates the primary side and the secondary side.
A MOS-type FET 13 as a second switching element connected to the primary winding 12A of the insulating transformer 12, a rectifying and smoothing circuit 14 for rectifying and smoothing a voltage induced in the secondary winding 12B of the insulating transformer 12, A second control means 15 such as a control IC for supplying a drive signal to the MOS FET 13 is provided. The second control means monitors the DC output voltage VOUT by using a DC output detection circuit (not shown).
The DC input voltage VDC from the power factor correction circuit 5 generated between both ends of the smoothing capacitor 9 is supplied to the primary winding 12A
And a stable DC output voltage VOUT is output from the rectifying / smoothing circuit 14 between the output terminals 16 and 17.

The first control means 10 has a first auxiliary power supply 21, and the second control means 15 also has its own second auxiliary power supply 22. Each of these supplies an operation voltage to the voltage supply terminal VCC of the first control means 10 or the second control means 15, and an operation voltage of a predetermined level or more is supplied from the first auxiliary power supply 21. When the first control means 10 and thus the power factor improving circuit 5 operate, the second control means 15 and therefore the DC / DC converter 11 Works.

The first auxiliary power supply 21 is wound around a common core with the choke coil 6 and has an auxiliary winding 23 having a center tap grounded, and a rectifier having an anode connected to each end of the auxiliary winding 23. Diodes 24 and 25 and diodes 24 and 25
Smoothing capacitor 26 connected between the cathode of the
It is composed of The second auxiliary power supply 22 is wound around a core common to the insulating transformer 12 and has the other end grounded, and a rectifying diode having an anode connected to one end of the auxiliary winding 12C. 27, and a smoothing capacitor 28 connected between the cathode of the diode 27 and the ground. The location from which the operating voltage is extracted is not particularly limited. Further, the circuit configuration of each of the auxiliary power supplies 21 and 22 is not limited to that of the present embodiment.

At the time of startup, the first control means 10
Is a start-up circuit for temporarily supplying an operating voltage to the voltage supply terminal VCC to operate the first control means 10. Also,
The second control means 15 also includes a resistor 32 as another starting circuit.
Is connected between one end of the smoothing capacitor 9 to which the cathode of the diode 8 is connected and the voltage supply terminal VCC of the second control means 15. The starting circuit 31 has resistors for voltage division connected between the output terminals of the rectifier 4, a gate connected to a connection point between the resistors 33 and 34, and a source connected to a voltage supply terminal of the first control means 10. MOS type F as a switch element connected to VCC
ET 35, a drain of the MOS FET 35 and a resistor 36 connected to one end of the smoothing capacitor 9. The gate of the MOSFET 35 is connected to the cathode of a Zener diode 37 whose anode is grounded. When the operating voltage is supplied from the first auxiliary power supply 21 after the start, the Zener diode 37 is turned on.
Is turned off to cut off the voltage supply from the starting circuit 31. Then, the starting circuit 31 sends a voltage detection signal corresponding to the input voltage level of the power factor improvement circuit 5 generated between the output terminals of the rectifier 4 from the connection point of the resistors 33 and 34 to the MOS type FE.
When the input voltage level of the power factor correction circuit 5 becomes higher than a certain value at the time of startup,
The OS-type FET 35 is turned on, the DC input voltage VDC generated between both ends of the smoothing capacitor 9 is dropped by the resistor 36 and supplied to the voltage supply terminal VCC of the first control means 10 as an operating voltage. doing.

In this embodiment, a synchronizing circuit 41 for stopping the second control means 15 in synchronization with the stop of the first control means 10 is provided. The synchronization circuit 41 is a synchronization terminal of the first control means 10.
When a non-operation signal of operation stop is output from the SYN, the potential of the voltage supply terminal VCC of the second control means 15 is reduced to a level at which the second control means 15 is in the non-operation state. . The circuit configuration is such that the gate is connected to the synchronization terminal SYN of the first control means 10 and the drain is connected to the second control means 15.
MOS FET 42 connected to the voltage supply terminal VCC
And a resistor 43 having one end connected to the gate of the MOSFET 42 and the other end grounded together with the source of the MOSFET 42.

Reference numeral 51 denotes a DC input voltage VDC generated across the smoothing capacitor 9 when the second control means 15 is in operation, and supplies the DC input voltage VDC to the voltage supply terminal VCC of the first control means 10 as an operating voltage. This is operation continuation means. The operation continuation means 51 includes a MOS type FET 35
And a series circuit of a diode 52 and a resistor 53 connected between the gate, which is the control terminal of the MOS FET 35, and the operating voltage line of the second control means 15. And
The diode 52 has an anode connected to an operating voltage line of the second control means 15, that is, a voltage supply terminal VCC, a cathode connected to one end of a resistor 53, and the other end of the resistor 53 connected to a gate of the MOSFET 35. Connected to.

Next, the operation of the above configuration will be described with reference to the waveform diagrams of FIGS. In FIG. 2, the waveform corresponding to “AC” at the top is the AC input voltage of the AC power supply 1. Hereinafter, the waveform corresponding to “VDC” is the DC input voltage input to the DC / DC converter 11. The waveforms corresponding to VDC and “PFC operation” are the operation / non-operation state of the first control means 10 constituting the power factor improvement circuit 5,
The waveform corresponding to "DC / DC operation" is the operation / non-operation state of the second control means 15 constituting the DC / DC converter 11, and the waveform corresponding to "VOUT" is output from the DC / DC converter 11. 5 shows a DC output voltage VOUT.

At the time of start-up when the AC power supply 1 is turned on, when the input voltage level from the rectifier 4 divided by the resistors 33 and 34 exceeds a predetermined value, the start-up circuit 31 starts to operate.
A potential difference is generated between the gate and the source of the 35
Turn on ET35. Then, the DC input voltage VDC immediately after startup generated between both ends of the smoothing capacitor 9 is supplied to the voltage supply terminal VCC of the first control means 10 via the resistor 36 and the MOS FET 35.

When the first control means 10 starts to start, it supplies a drive signal to the MOSFET 7 from its own drive terminal D1. Thereafter, the first control means constituting the power factor improvement circuit 5
10 is a DC input voltage VDC by a voltage detection circuit (not shown).
By controlling the pulse conduction width of the MOS FET 7 while monitoring the input current waveform, the input current waveform taken from the choke coil 6 is made substantially the same sine wave shape as the input voltage waveform of the AC power supply 1 to improve the power factor. . In addition, MOS type FET7
When a voltage is induced in the auxiliary winding 23 of the auxiliary power supply 21 by the switching of the
The current is rectified and smoothed by the capacitors 24 and 25 and supplied to the voltage supply terminal VCC of the first control means 10. Therefore,
After that, the first control means 10 operates by the voltage supply from the auxiliary power supply 21.

On the other hand, when the second control means 15 starts to be activated, a drive signal is transmitted from its own drive terminal D2 to the MOSFET.
Supply 13 Thereafter, the second control means 15 constituting the DC / DC converter 11 controls the pulse conduction width of the MOS FET 13 while monitoring the DC input voltage VDC. As a result, the DC input voltage V is applied to the primary winding 12A of the insulating transformer 12.
DC is applied intermittently, and a stable DC output voltage VOUT is output from the rectifying and smoothing circuit 14 between the output terminals 16 and 17. When a voltage is induced in the auxiliary winding 12C of the insulating transformer 11 by the switching of the MOS type FET 13, the induced voltage is rectified and smoothed by the diode 27 and the capacitor 28, and the voltage of the second control means 15 is reduced. Supply terminal V
Supplied to CC. Therefore, thereafter, the second control means 15 operates by the voltage supply from the auxiliary power supply 22.

Next, the operation when the AC input voltage of the AC power supply 1 is stopped or cut off for some reason will be described with reference to FIG. In this case, the current to the choke coil 6 constituting the power factor correction circuit 5 is cut off, and no voltage is induced from the auxiliary winding 23 of the auxiliary power supply 21.
The supply of the operating voltage from 21 to the first control means 10 is cut off. However, since a certain amount of energy is stored in the smoothing capacitor 9, the DC / DC converter 11
By switching the MOS FET 13, the DC output voltage V
Continue to stabilize OUT.

On the other hand, since the anode of the diode 52 constituting the operation continuation means 51 has the same potential as the voltage supply terminal VCC of the second control means 15, the diode 52 is in operation when the second control means 15 is operating. Becomes conductive, and the MOS-type FET 35
ON signal to the gate of. Thereby, the MOS type F
ET35 is turned on, and the DC input voltage VDC becomes the first through the resistor 36.
Is supplied to the voltage supply terminal VCC of the control means 10, the supply of the input voltage to the power factor correction circuit 5 is cut off, but the power factor correction circuit 5 itself continues to operate.

Eventually, when the smoothing capacitor 9 discharges and the DC input voltage VDC drops to a predetermined level VDC1, the first
The operating voltage supplied to the voltage supply terminal VCC of the control means 10 also becomes lower than the level necessary for operating the first control means 10, and the first control means 10 is in a stopped non-operation state. . At the same time, a non-operation signal is output from the synchronization terminal SYN of the first control means 10, and the MOS type FET 42 of the synchronization circuit 41
Turns on to forcibly ground the voltage supply terminal VCC of the second control means 15. In this way, by stopping the operation of the second control means 15 in synchronization with the first control means 10, the adverse effects caused by the operation of the DC / DC converter 11 when the rectifying / smoothing circuit 5 is not operating. Can be removed. As described above, even if the AC input voltage of the AC power supply 1 is stopped or cut off, the stable DC output voltage VOUT is maintained until the operations of the first control means 10 and the second control means 15 are stopped. The supply holding time T1 is secured. Even if the input voltage to the power factor improvement circuit 5 is restored after the operations of the first control means 10 and the second control means 15 are stopped, both the power factor improvement circuit 5 and the DC / DC converter 11 do not operate. In this state, it is possible to avoid a situation in which only the DC / DC converter 11 continues to operate and an abnormal voltage occurs as in the related art.

As described above, according to the above-described embodiment, even when the input voltage to the power factor correction circuit 5 is cut off, when the second control means 15 constituting the DC / DC converter 11 is operating, The first control means 10 constituting the power factor improvement circuit 5 by utilizing the energy stored in the smoothing capacitor 9.
, An operation voltage for continuing the operation is supplied. Therefore, the DC / DC converter 11 can be operated while the power factor improving circuit 5 is operating, and the holding time T1 for supplying a stable DC output voltage VOUT can be obtained.
Thereafter, the smoothing capacitor 9 discharges, and the first control circuit
When the operating voltage supplied to 10 decreases, the operation of the second control circuit 15 together with the first control circuit 10 is stopped by the synchronization circuit 41. Therefore, the conventional disadvantage that only the DC / DC converter 11 operates and the DC output voltage VOUT becomes unstable can be solved. Further, even when the input voltage to the power factor improvement circuit 5 is restored, both the power factor improvement circuit 5 and the DC / DC converter 11 are stopped in synchronization with each other. It is possible to avoid a situation in which only the DC / DC converter 11 operates, and also in this case, it is possible to prevent the occurrence of the abnormal DC output voltage VOUT as in the related art. In addition, the operation continuation circuit 51 uses the smoothing capacitor 9 constituting the power factor improvement circuit 5 instead of the dedicated power supply, and
Since the operating voltage is supplied to the components, there is also an advantage that components can be shared.

That is, the first control means 10 controls the cutoff of the MOS FET 7 serving as the first switching element, so that the input current waveform is sinusoidal, and the DC input voltage is applied across the smoothing capacitor 9. A power factor improving circuit 5 for generating VDC and a second control means 15 controlling the power supply through a MOS type FET 13 as a second switching element to thereby reduce the DC input voltage VDC to the primary winding of an insulating transformer 12. DC output voltage VOU applied intermittently to 12A
A DC / DC converter 11 for outputting T, a synchronizing circuit 41 for stopping the second control means 15 in synchronization with the stop of the first control means 10, and a smoothing circuit when the second control means 15 is operating. If a switching power supply is constituted by an operation continuation circuit 51 for supplying an operation voltage for continuing the operation from the capacitor 9 to the first control means 10, the abnormal DC output voltage VOUT due to the operation of only the DC / DC converter 11 is obtained. Can be prevented, and the necessary holding time can be secured. Further, the smoothing capacitor 9 is connected to the second control means 15.
It can be used to supply operating voltages to the components, and components can be shared.

Further, in this embodiment, in addition to the above configuration, a voltage detection signal corresponding to the input voltage level of the power factor improving circuit 5 is supplied to the control terminal, ie, the gate of the MOS FET 35 as a switch element. When the input voltage level becomes equal to or higher than a predetermined value at the time of start-up, a start-up circuit 31 for turning on the MOS FET 35 and supplying an operating voltage from the smoothing capacitor 9 to the first control means 10 is further provided. F
ET35 and a diode connected between the gate of the MOSFET 35 and the operating voltage line of the second control means 15.
It comprises a series circuit of 52 and a resistor 53.

With such a configuration, the power factor improving circuit 5
Even when the input voltage to the input is cut off, when the second control means 15 is operating and its operating voltage line is equal to or higher than a predetermined level, it passes through a series circuit of a diode 52 and a resistor 53,
An ON signal is supplied to the gate of the MOS FET 35, and an operating voltage is supplied from the smoothing capacitor 9 to the first control means 10. The MOS FET 35 originally constitutes the start-up circuit 31, and by using this as the configuration of the operation continuation circuit 51, the components can be shared. In addition, substantially, the diode 52 excluding the MOS FET 35
Since the operation continuation circuit 51 can be configured only by the resistor 53 and the resistor 53, the circuit configuration can be simplified.

The present invention is not limited to the above embodiment, and various modifications can be made within the scope of the present invention. For example, the configuration of the DC / DC converter is not limited to the forward type as in this embodiment, and various types can be applied. Further, the first and second switching elements are not limited to the MOS type FET. Further, an AC power supply may be directly connected without providing a rectifier in a stage preceding the power factor correction circuit. Further, the synchronization circuit shown in the embodiment is merely an example, and the configuration thereof can be changed as appropriate.

[0032]

In the switching power supply according to the first aspect of the present invention, the input current waveform is formed into a sine wave shape by the first control means to control the cut-off of the first switching element, thereby providing a smoothing capacitor. A power factor improving circuit that generates a DC input voltage between both ends, and a second control device that controls the switching of the second switching element so that the DC input voltage is intermittently applied to the primary winding of the insulating transformer. Apply
A DC / DC converter that outputs a stable DC output voltage, a synchronizing circuit that stops the second control unit in synchronization with the stop of the first control unit, and a synchronous circuit that stops operating the second control unit. And an operation continuation circuit for supplying an operation voltage from the smoothing capacitor to the first control means for continuing the operation, and generating an abnormal DC output voltage due to the operation of only the DC / DC converter. It is possible to provide a switching power supply device capable of preventing the switching and securing a required holding time.

According to a second aspect of the present invention, in addition to the configuration of the first aspect, the switching power supply supplies a voltage detection signal corresponding to an input voltage level of the power factor correction circuit to a control terminal of the switch element. When the input voltage level becomes equal to or higher than a predetermined value at the time of starting, the circuit further includes a starting circuit for turning on the switch element and supplying an operating voltage from the smoothing capacitor to the first control means. Comprises a switching element, a series circuit of a diode and a resistor connected between a control terminal of the switching element and an operating voltage line of the second control means, and a starting circuit. Can be shared by using the switch element constituting the same as the operation continuation circuit. Further, the operation continuity circuit can be easily constituted only by the diode and the resistor substantially excluding the switch element.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of a switching power supply device showing one embodiment of the present invention.

FIG. 2 is a waveform chart showing an operation state of the same.

FIG. 3 is a circuit diagram of a switching power supply device showing a conventional example.

FIG. 4 is a waveform diagram when the power factor improving circuit and the DC / DC converter are operated / stopped asynchronously.

FIG. 5 is a waveform diagram when the power factor improving circuit and the DC / DC converter are operated / stopped synchronously.

[Explanation of symbols]

 5 Power Factor Improvement Circuit 7 MOS FET (First Switching Element) 9 Smoothing Capacitor 10 First Control Means 11 DC / DC Converter 12 Isolation Transformer 13 MOS FET (Second Switching Element) 15 Second Control Means 31 Start-up circuit 35 MOS FET (switch element) 41 Synchronous circuit 51 Operation continuation circuit 52 Diode 53 Resistance

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁶ Identification code FI H02M 3/155 H02M 3/155 F 7/06 7/06 GA

Claims (2)

[Claims] 1. A power factor improving circuit for controlling a cut-off of a first switching element by a first control means to make an input current waveform sinusoidal and generate a DC input voltage across a smoothing capacitor. And a second control means for controlling the switching of the second switching element so that the DC input voltage is intermittently applied to the primary winding of the insulating transformer to output a stable DC output voltage. / DC converter and the second controller in synchronization with the stop of the first controller.
And an operation continuation circuit that supplies an operation voltage from the smoothing capacitor to the first control means when the second control means is in operation. A switching power supply device characterized by the above-mentioned. 2. A voltage detection signal corresponding to an input voltage level of the power factor correction circuit is supplied to a control terminal of a switch element.
When this input voltage level exceeds a predetermined value at startup,
A start-up circuit that turns on the switch element and supplies an operating voltage from the smoothing capacitor to the first control unit; the operation continuation circuit includes the switch element;
2. A circuit comprising a series circuit of a diode and a resistor connected between a control terminal of the switch element and an operating voltage line of the second control means.
A switching power supply as described.