Abstract:
In a direct-current power supply device that includes a smoothing capacitor C 1 , which performs a DC/DC converter operation, a transformer T 1 , a switching element Q 1 , a diode D 2 , a smoothing capacitor C 2 , a reactor L 1 , which performs a PFC operation, a fast recovery diode D 1  and a switching element Q 1 , when compared with the case of a rated load, the voltage of the smoothing capacitor C 1  of a PFC circuit rises at a time when a load is light. Therefore, the following has been required: a capacitor having a sufficient withstanding voltage rating, or an operation of connecting a plurality of capacitors in series or any other operation to secure a voltage-withstanding capability. 
     A direct-current power supply device  1 , in which a switching element Q 1  used by a PFC circuit is shared as a switching element Q 1  by a DC/DC converter, includes voltage suppression means (switching elements Q 2  and Q 3  and resistance R 2 ) for supplying electric charge accumulated in a smoothing capacitor C 1  to a power supply Vcc of a control circuit CTL 1  that controls the switching element Q 1  at a time when a load is light in order to suppress a rise in voltage in the smoothing capacitor C 1.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a direct-current power supply device and particularly to a technique of reducing a rise in voltage of a smoothing capacitor of a PFC (Power Factor Correction: Power factor improvement) circuit at a time when a load is light in a direct-current power supply device designed to control a DC/DC converter and the PFC circuit by means of one switching element. 
     2. Description of the Related Art 
     A direct-current power supply device, which converts a commercial alternating-current power supply to a direct-current power supply using a rectification smoothing circuit and then converts the direct-current power supply to a desired direct-current voltage using a DC/DC converter to output the direct-current voltage, has been used. When the direct-current power supply is obtained by the rectification smoothing circuit from the commercial alternating-current power supply, current flows through a smoothing capacitor only around a peak of sine-wave alternating-current voltage. Accordingly, a power factor becomes worse; a higher harmonic wave is generated, which badly affects surrounding areas. To solve the above problem, a PFC circuit may be provided in the rectification smoothing circuit. In this case, a switch used by the PFC circuit can be shared as a switching element by the DC/DC converter; the sharing of the switch is effective in making the direct-current power supply device smaller and reducing costs. Those sharing the switching element between the PFC circuit and the DC/DC converter include, for example, the one disclosed in Jpn. Pat. Appln. Laid-Open Publication No. 2002-247843 (Patent Document 1) or U.S. Pat. No. 5,991,172 (Patent Document 2). 
     CITATION LIST 
     Patent Document 
     
         
         [Patent Document 1] Jpn. Pat. Appln. Laid-Open Publication No. 2002-247843 
         [Patent Document 2] U.S. Pat. No. 5,991,172 
       
    
     The above conventional techniques are effective in making devices smaller and reducing costs because the switching elements and controllers used in the PFC circuit and DC/DC converter are put into one. However, it is only output voltage of the DC/DC converter that can be controlled in a stable manner. Therefore, the problem is that when a load of the DC/DC converter is light, terminal voltage of the smoothing capacitor of the PFC circuit rises.  FIG. 6  shows a characteristic, with the horizontal axis representing the output voltage (the state of the load) of the DC/DC converter and the vertical axis representing the terminal voltage of the smoothing capacitor of the PFC circuit. It is clear that as the output voltage of the DC/DC converter falls (or turns into a light-load state), the terminal voltage of the smoothing capacitor of the PFC circuit rises. 
     In general, an electrolytic capacitor is used for the smoothing capacitor with a unique absolute rated voltage; there is a limit to the voltage that can be applied to the smoothing capacitor. The reason why the terminal voltage of the smoothing capacitor of the PFC circuit rises at a time when the load of the DC/DC converter is light has been unclear. Therefore, the following has been required for the smoothing capacitor: a capacitor having a sufficient withstanding voltage rating, or an operation of connecting two or more capacitors in series or any other operation to secure a voltage-withstanding capability. Or alternatively, an overvoltage protection circuit has been provided to protect the capacitor against overvoltage. The measures described above, however, lead to an increase in costs and become a snag in terms of implementation when the device is made smaller. 
     The inventors of the present invention have found as a result of careful examination that the inefficiency in a process of transferring the energy released from a reactor of the PFC circuit to a secondary side of a transformer at a time when the load is light is a cause of the above problem. That is, when the load is light, the ON pulse width, which is used to switch the switching element ON/OFF, becomes narrower. Therefore, stray capacitance that exists on a primary winding of the transformer, or a capacitor of a snubber circuit that is connected to the primary winding of the transformer to absorb a surge voltage, is not fully charged. As a result, the voltage of the transformer is lowered. Thus, the energy released form the reactor of the PFC circuit cannot be transferred to the secondary side of the transformer in an efficient manner. The phenomenon will be described with reference to  FIGS. 7 and 8 . 
       FIG. 7  shows the circuit configuration of a direct-current power supply device  100  of a conventional technique, which is so formed as to control a DC/DC converter and a PFC circuit with one switching element.  FIG. 8  shows the operational waveform of each portion to explain an operation of the direct-current power supply device  100  when a load is light. Since the load is light, the direct-current power supply device  100  oscillates intermittently. What is shown is the rising terminal voltage of a smoothing capacitor C 1  of the PFC circuit. 
     As shown in  FIG. 7 , the direct-current power supply device  100  includes the smoothing capacitor C 1 , which performs a DC/DC converter operation; a transformer T 1 ; a switching element Q 1 ; a diode D 2 ; a smoothing capacitor C 2 ; a reactor L 1 , which performs a PFC operation; a fast recovery diode D 1 , which serves as a backflow preventing diode; and a switching element Q 1 . In this case, the switching element Q 1  is shared by a DC/DC converter section and a PFC section. The DC/DC converter section works as a flyback converter. The voltage polarity of the transformer T 1  is set as indicated by ● in the diagram so as to work as a flyback converter. 
     As the switching element Q 1  is turned ON/OFF, a change in voltage of a high frequency wave occurs at a tap section where two windings N 1   a  (first primary winding) and N 1   b  (second primary winding) of the primary winding N 1  of the transformer T 1  are connected in response to the ON/OFF operation of the switching element Q 1 . As the voltage changes, high frequency current flows through the reactor L 1 . The amplitude of the current varies according to the voltage amplitude of a commercial alternating-current power supply Vs. Therefore, the PFC operation with improved power factors is achieved. 
     The circuit configuration of the direct-current power supply device  100  will be described with reference to  FIG. 7 . To a rectification circuit RC 1  where diodes are so connected to form a bridge, a commercial alternating-current power supply Vs is connected. Between a positive electrode-side output terminal and negative electrode-side output terminal of the rectification circuit RC 1 , a bypass capacitor C 3 , whose capacitance is smaller than that of the smoothing capacitor C 1 , is connected. To a connection point where the positive electrode-side output terminal of the rectification circuit RC 1  and one terminal of the bypass capacitor C 3  are connected, one terminal of the reactor L 1  is connected. To the other terminal of the reactor L 1 , an anode terminal of the fast recovery diode D 1  is connected. A cathode terminal of the fast recovery diode D 1  is connected to the tap section of the primary winding N 1  of the transformer T 1 . The primary winding N 1  of the transformer T 1  is made up of two windings N 1   a  (first primary winding) and N 1   b  (second primary winding). A connection point where the other terminal of the first primary winding N 1   a  and one terminal (at the side indicated by ● in the diagram) of the second primary winding N 1   b  are connected together is the tap section described above. One terminal (at the side indicated by ● in the diagram) of the first primary winding N 1   a  is connected to one terminal (at the positive electrode side) of the smoothing capacitor C 1 . The other terminal (at the negative electrode side) of the smoothing capacitor C 1  is connected to a connection point where the negative electrode-side output terminal of the rectification circuit RC 1  and the other terminal of the bypass capacitor C 3  are connected. The other terminal of the second primary winding N 1   b  is connected to a drain terminal of the switching element Q 1 . A source terminal of the switching element Q 1  is connected to a connection point where the negative electrode output terminal of the rectification circuit RC 1 , the other terminal of the bypass capacitor C 3  and the other terminal of the smoothing capacitor C 1  are connected together. 
     The other terminal of the secondary winding N 2  of the transformer T 1  is connected to an anode terminal of the diode D 2 . A cathode terminal of the diode D 2  is connected to one terminal (at the positive electrode side) of the smoothing capacitor C 2 . One terminal (at the side indicated by ● in the diagram) of the secondary winding N 2  of the transformer T 1  is connected to the other terminal (at the negative electrode side) of the smoothing capacitor C 2 . One terminal and the other terminal of the smoothing capacitor C 2  serve as a positive electrode-side output terminal A and negative electrode-side output terminal B of the direct-current power supply device  100 , respectively. The voltage between the positive electrode-side output terminal A and negative electrode-side output terminal B of the direct-current power supply device  100  is input to a control circuit CTL 5 , which outputs a pulse signal to a gate terminal of the switching element Q 1  to turn the switching element Q 1  ON/OFF so that a target voltage is obtained. 
     The waveforms shown in  FIG. 8  represent, from top to bottom, positive electrode-side output voltage Vin (=voltage Vc 3  of the bypass capacitor C 3 ) of the rectification circuit RC 1 , drain-to-source voltage Vds of the switching element Q 1 , voltage Vc 1  of the smoothing capacitor C 1 , current IL 1  of the reactor L 1 , current IC 1  of the smoothing capacitor C 1 , drain current IQ 1  of the switching element Q 1 , current ID 2  of the diode D 2 , and output voltage Vo of the direct-current power supply device  100  (=voltage VC 2  of the smoothing capacitor C 2 ), with t 1  to t 18  at the bottom representing time. 
     (Until t 0 ) 
     The positive electrode-side output voltage Vin (=voltage Vc 3  of the bypass capacitor C 3 ) of the rectification circuit RC 1  is substantially at a constant level, as the DC/DC converter consumes less power because the load is light. At time t 0 , the output voltage Vo goes down to a switching operation restart voltage, which is lower than the rated voltage. The control circuit CTL  5  outputs a gate signal to the switching element Q 1  to prompt ON/OFF control. 
     (t 0  to t 1 ) 
     After the switching element Q 1  is turned on at time t 0 , the voltage waveform of the drain-to-source voltage Vds of the switching element Q 1  becomes substantially 0V as shown in  FIG. 8 , and the discharging of electricity of the smoothing capacitor C 1  takes place through the first and second primary windings N 1   a  and N 1   b  of the transformer T 1  (The discharge current is IC 1 ). Accordingly, the voltage VC 1  of the smoothing capacitor C 1  drops over time t 0  to t 1 . From t 0  to t 1 , the current IL 1  of the reactor L 1  flows through the second primary winding N 1   b  of the transformer T 1  and the switching element Q 1 , rising from 0 A. At this time, the drain current IQ 1  of the switching element Q 1  is a flow of current that is the sum of the discharge current IC 1  of the smoothing capacitor C 1  and the current IL 1  from the reactor L 1 . 
     (t 1  to t 2 ) 
     From time t 1  to t 2 , after the switching element Q 1  is turned off by OFF signal, the magnetic energy accumulated in the reactor L 1  charges the smoothing capacitor C 1  via the first primary winding N 1   a  of the transformer T 1 . At this time, because of the voltage applied to the first primary winding N 1   a , the voltage of the secondary winding N 2  occurs in proportion to the turns ratio, but does not go above the output voltage Vo (=the voltage VC 2  of the smoothing capacitor C 2 ). Therefore, the diode D 2  remains off. Thus, the secondary-side smoothing capacitor C 2  is not charged with the voltage of the secondary winding N 2 . 
     (t 2  to t 6 ) 
     Then, a similar operation takes place from time t 2  to t 6 . At this time, the charging and discharging of the voltage of the smoothing capacitor C 1  is repeatedly performed, and the voltage of the smoothing capacitor C 1  gradually rises as shown in  FIG. 8 . The charging and discharging of the stray capacitance between the windings of the transformer T 1  (or a capacitor C 5  of the snubber circuit (the stray capacitance is not shown in the diagram; the snubber circuit is shown briefly with the capacitor C 5  and resistance R 1 )) is also repeatedly performed, and the voltage thereof also gradually rises. However, the stray capacitance (or the capacitor C 5  of the snubber circuit) between the windings of the transformer T 1  discharges more easily than the smoothing capacitor C 1  because the stray capacitance and the windings of the transformer T 1  are connected in parallel. Therefore, the voltage of the stray capacitance (or the capacitor C 5  of the snubber circuit) between the windings of the transformer T 1  rises more slowly than the voltage of the smoothing capacitor C 1 . As the load becomes lighter, an ON period of the switching element Q 1  and a charging period of the stray capacitance (or the capacitor C 5  of the snubber circuit) between the windings of the transformer T 1  become shorter. Thus, when the load is light, it is difficult to charge the stray capacitance (or the capacitor C 5  of the snubber circuit) between the windings of the transformer T 1 . However, the stray capacitance discharges easily. As the load becomes lighter, the above slowdown trends to intensify because the pulse width becomes narrower during the charging process. As a result, it takes more time for the secondary voltage of the transformer T 1  to rise to the output voltage Vo after an ON pulse has been supplied to the gate of the switching element Q 1  from the control circuit CTL 5 . Meanwhile, the voltage of the smoothing capacitor C 1  goes higher. 
     (t 7  to t 9 ) 
     At time t 8 , a middle point between time T 7  and t 9 , current starts to flow through the diode D 2 , meaning that at time t 8 , the voltage of the stray capacitance (or the capacitor C 5  of the snubber circuit) between the windings of the transformer T 1  is being charged, and that the voltage that occurs at the secondary winding N 2  has risen to voltage VC 2  where the smoothing capacitor  2  can be charged. With the voltage that occurs at the secondary winding N 2  of the transformer T 1 , the smoothing capacitor C 2  is being charged via the diode D 2  during the period of time t 8  to t 9 . Incidentally, from time t 0  to t 8 , the smoothing capacitor C 2  is not charged by the secondary winding N 2  of the transformer T 1 , and the output voltage Vo (voltage VC 2  of the smoothing capacitor C 2 ) continues to fall. 
     (t 9  to t 10 ) 
     After the switching element Q 1  is turned on at time t 9 , the voltage waveform of the drain-to-source voltage Vds of the switching element Q 1  becomes substantially 0V as shown in  FIG. 8 . The voltage VC 1  of the smoothing capacitor C 1  discharges via the first and second primary windings N 1   a  and N 1   b  of the transformer T 1  (the discharge current is IC 1 ), and therefore falls over time t 9  to t 10 . From time t 9  to t 10 , the current IL 1  of the reactor L 1  flows through the second primary winding N 1   b  of the transformer T 1  and the switching element Q 1 , rising from 0 A. At this time, the drain current IQ 1  of the switching element Q 1  is a flow of current that is the sum of the discharge current IC 1  of the smoothing capacitor C 1  and the current IL 1  from the reactor L 1 . 
     (t 10  to t 11 ) 
     At time t 10 , after the switching element Q 1  is turned off by OFF signal, the magnetic energy accumulated in the reactor L 1  charges the smoothing capacitor C 1  via the first primary winding N 1   a  of the transformer T 1 . At this time, because of the voltage applied to the first primary winding N 1   a , the voltage of the secondary winding N 2  occurs in proportion to the turns ratio. At this time, the voltage of the secondary winding N 2  has already risen to voltage VC 2 . Therefore, the diode D 2  is turned on and, from time t 10  to t 11 , the smoothing capacitor C 2  is charged with the voltage of the secondary winding N 2 . 
     (t 11  to t 15 ) 
     Then, a similar operation takes place from time t 11  to t 15 . At this time, the voltage of the smoothing capacitor C 1  changes as the charging and discharging of the smoothing capacitor C 1  is repeatedly performed. However, when compared with the situation between time t 0  and t 8 , the amount of charge becomes smaller, and the amount of discharge larger, because energy has been transferred to the secondary side of the transformer T 1 . Therefore, the voltage of the smoothing capacitor C 1  gradually decreases as shown in  FIG. 8 . The voltage VC 2  (=output voltage Vo) of the smoothing capacitor C 2  gradually rises as the smoothing capacitor C 2  is charged with the secondary voltage of the transformer T 1 . During the above period of time t 8  to t 15 , the magnetic energy released from the reactor of the PFC circuit is transferred to the secondary side of the transformer T 1 . 
     (t 15  to t 16 ) 
     After the output voltage Vo rises to a target rated voltage at time t 15 , the control circuit CTL 5  detects the output voltage Vo reaching the target rated voltage and outputs an OFF signal to the switching element Q 1 . In response, the switching element Q 1  remains in a halting state until time t 16 . At this time, the voltage of the smoothing capacitor C 1  has risen above the voltage of time t 0 . Since there is no circuit for discharging electric charge from the smoothing capacitor C 1 , the voltage VC 1  is substantially kept constant. Meanwhile, the electric charge of the stray capacitance (or the capacitor C 5  of the snubber circuit) between the windings of the transformer T 1  is discharged through the windings of the transformer T 1 . Therefore, like the above-described situation between time t 0  to t 8 , even if the switching of the switching element Q 1  restarts, the voltage of the secondary winding N 2  of the transformer T 1  does not rise immediately. 
     (t 16  to t 17 ) 
     Then, at time t 16 , the output voltage Vo drops to a switching operation restart voltage, which is detected by the control circuit CTL 5 . The control circuit CTL  5  outputs a gate signal, as in the case of time t 0 , to the switching element Q 1  to prompt ON/OFF control. As a result, the switching operation starts. However, from time t 15  to t 16 , the electric charge of the stray capacitance (or the capacitor C 5  of the snubber circuit) between the windings of the transformer T 1  has been discharged. Therefore, until t 17 , for the same reason as the above situation between time t 0  and t 8 , the smoothing capacitor C 2  is not charged with the voltage that occurs at the secondary winding N 2  of the transformer T 1 . If the period of time t 16  to t 17  is almost equal to the period of time t 0  to t 8 , the voltage of the smoothing capacitor C 1  rises from a starting point at time t 16  by an amount substantially equivalent to the rise in voltage between time t 0  to t 8 . Therefore, the voltage of the smoothing capacitor C 1  goes higher than that at time t 8 . 
     (t 17  to t 18 ) 
     At time t 17 , current starts to flow through the diode D 2 , meaning that as in the case of time t 8 , the voltage of the stray capacitance (or the capacitor C 5  of the snubber circuit) between the windings of the transformer T 1  is being charged, and that the voltage that occurs at the secondary winding N 2  has risen to voltage VC 2  where the smoothing capacitor  2  can be charged. Therefore, during the period of time t 17  to t 18 , a similar operation to that during the above period of t 8  to t 15  takes place. When the switching operation stops at time t 18 , the voltage of the smoothing capacitor C 1  goes higher than that at time t 16  as in the case where the voltage of the smoothing capacitor C 1  goes higher at time t 15  than that at time to. 
     (t 18  and Thereafter) 
     After time t 18 , a similar operation is repeatedly performed, and the voltage of the smoothing capacitor C 1  rises. However, after the magnetic energy released from the reactor L 1  and the energy output from the secondary side of the transformer T 1  rise and are equally matched, the magnetic energy from the reactor L 1  and the energy from the secondary side of the transformer T 1  become balanced, and the voltage of the smoothing capacitor C 1  stops rising. In this manner, the output voltage Vo is controlled by the control circuit CTL 5  and kept at the rated voltage. However, the voltage of the smoothing capacitor C 1  is not controlled by the control circuit CTL 5  and goes higher than that at the start of the switching operation. 
     As described above, the voltage of the smoothing capacitor C 1  is unstable, and there is a fear that the voltage of the smoothing capacitor C 1  could rise above the withstanding voltage of the capacitor at a time when the load is light. Therefore, according to a conventional technique, the following has been required: a capacitor having a sufficient withstanding voltage rating, or an operation of connecting two or more capacitors in series or any other operation to secure a voltage-withstanding capability. Or alternatively, an overvoltage protection circuit has been provided to protect the capacitor against overvoltage. The measures described above, however, lead to an increase in costs and become a snag in terms of implementation when the device is made smaller. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to solve the problems of the conventional techniques in view of the above problems and to provide a direct-current power supply device, in which a switch used by a PFC circuit is shared as a switching element by a DC/DC converter and which prevents terminal voltage of a smoothing capacitor of the PFC circuit from rising at a time when the DC/DC converter is a light load. 
     As for a direct-current device of the present invention, a direct-current power supply device, which converts energy obtained from an alternating-current power supply into direct-current energy, includes: a rectifier that converts alternating-current voltage of the alternating-current power supply into direct-current voltage; a transformer that includes a primary winding, which includes a tap at a connection point where a first primary winding and a second primary winding are connected, and a secondary winding; a primary-side smoothing capacitor whose positive electrode-side terminal is connected to a terminal at a side opposite to the tap of the first primary winding and whose negative electrode-side terminal is connected to a negative electrode-side output terminal of the rectifier; a first switching element whose drain and source terminals are connected between the negative electrode-side output terminal of the rectifier and a terminal at a side opposite to the tap of the second primary winding; a reactor and backflow preventing diode that are connected in series between a positive electrode-side output terminal of the rectifier and the tap of the transformer; a direct-current smoothing circuit that includes a rectifying diode, which is connected to the secondary winding of the transformer, and a secondary-side smoothing capacitor; and discharging means for detecting a light-load state of an output and discharging electric charge of the primary-side smoothing capacitor in a way that suppresses an increase in voltage of the primary-side smoothing capacitor. 
     Moreover, in the direct-current power supply device of the present invention, the discharging means is so formed that the electric charge of the primary-side smoothing capacitor is supplied to power supply of a control circuit that performs ON/OFF control of the first switching element. 
     Moreover, in the direct-current power supply device of the present invention, the discharging means includes: a second switching element whose drain and source terminals are connected between a power supply terminal of the control circuit and a connection point where the first primary winding and the positive electrode-side terminal of the primary-side smoothing capacitor are connected; resistance that is connected between drain and gate terminals of the second switching element; a third switching element whose drain and source terminals are connected between the gate terminal of the second switching element and the negative electrode-side output terminal of the rectifier; and a control circuit that outputs an ON/OFF signal to a gate terminal of the third switching element, wherein the control circuit is so formed as to detect a decrease in power supply voltage of the control circuit and output, when the decrease in power supply voltage is detected, an OFF signal to the third switching element. 
     Moreover, as for a direct-current power supply device of the present invention, the direct-current power supply device, which converts energy obtained from an alternating-current power supply into direct-current energy, includes: a rectifier that converts alternating-current voltage of the alternating-current power supply into direct-current voltage; a transformer that includes a primary winding, which includes a tap at a connection point where a first primary winding and a second primary winding are connected, and a secondary winding; a primary-side smoothing capacitor whose positive electrode-side terminal is connected to a terminal at a side opposite to the tap of the first primary winding and whose negative electrode-side terminal is connected to a negative electrode-side output terminal of the rectifier; a first switching element whose drain and source terminals are connected between the negative electrode-side output terminal of the rectifier and a terminal at a side opposite to the tap of the second primary winding; a reactor and backflow preventing diode that are connected in series between a positive electrode-side output terminal of the rectifier and the tap of the transformer; a direct-current smoothing circuit that includes a rectifying diode, which is connected to the secondary winding of the transformer, and a secondary-side smoothing capacitor; and electromagnetic energy supplying means for detecting a light-load state of an output and supplying part of electromagnetic energy of the reactor to a power supply of a control circuit, which performs ON/OFF control of the first switching element, via the second primary winding. 
     Moreover, in the direct-current power supply device of the present invention, the electromagnetic energy supplying means includes: a second switching element whose drain and source terminals are connected between a power supply terminal of the control circuit and a connection point where the second primary winding and the drain terminal of the first switching element are connected; resistance that is connected between drain and gate terminals of the second switching element; a third switching element whose drain and source terminals are connected between the gate terminal of the second switching element and the negative electrode-side output terminal of the rectifier; and a control circuit that outputs an ON/OFF signal to a gate terminal of the third switching element, wherein the control circuit is so formed as to detect a decrease in power supply voltage of the control circuit and output, when the decrease in power supply voltage is detected, an OFF signal to the third switching element. 
     Moreover, as for a direct-current power supply device of the present invention, the direct-current power supply device, which converts energy obtained from an alternating-current power supply into direct-current energy, includes: a rectifier that converts alternating-current voltage of the alternating-current power supply into direct-current voltage; a transformer that includes a primary winding, which includes a tap at a connection point where a first primary winding and a second primary winding are connected, and a secondary winding; a primary-side smoothing capacitor whose positive electrode-side terminal is connected to a terminal at a side opposite to the tap of the first primary winding and whose negative electrode-side terminal is connected to a negative electrode-side output terminal of the rectifier; a first switching element whose drain and source terminals are connected between the negative electrode-side output terminal of the rectifier and a terminal at a side opposite to the tap of the second primary winding; a reactor and backflow preventing diode that are connected in series between a positive electrode-side output terminal of the rectifier and the tap of the transformer; a direct-current smoothing circuit that includes a rectifying diode, which is connected to the secondary winding of the transformer, and a secondary-side smoothing capacitor; and electromagnetic energy supplying means for supplying part of electromagnetic energy of the reactor to a power supply of a control circuit, which performs ON/OFF control of the first switching element, via a diode using an auxiliary winding provided in the reactor. 
     Moreover, in the direct-current power supply device of the present invention, the electromagnetic energy supplying means is so formed that: the auxiliary winding of the reactor and the diode are connected in series between the negative electrode-side output terminal of the rectifier and the power supply of the control circuit; and current flows from the auxiliary winding to the power supply of the control circuit via the diode as the power supply voltage of the control circuit decreases. 
     Moreover, as for a direct-current power supply device of the present invention, the direct-current power supply device, which converts energy obtained from an alternating-current power supply into direct-current energy, includes: a rectifier that converts alternating-current voltage of the alternating-current power supply into direct-current voltage; a transformer that includes a primary winding, which includes a tap at a connection point where a first primary winding and a second primary winding are connected, and a secondary winding; a primary-side smoothing capacitor whose positive electrode-side terminal is connected to a terminal at a side opposite to the tap of the first primary winding and whose negative electrode-side terminal is connected to a negative electrode-side output terminal of the rectifier; a first switching element whose drain and source terminals are connected between the negative electrode-side output terminal of the rectifier and a terminal at a side opposite to the tap of the second primary winding; a reactor and backflow preventing diode that are connected in series between a positive electrode-side output terminal of the rectifier and the tap of the transformer; a direct-current smoothing circuit that includes a rectifying diode, which is connected to the secondary winding of the transformer, and a secondary-side smoothing capacitor; a control circuit that outputs an ON/OFF signal to the first switching element to control output voltage so that the output voltage becomes predetermined voltage; and an output voltage detection circuit that increases detection voltage relative to the same output voltage and outputs a feedback signal to the control circuit at a time when a load is light, wherein an operation takes place in a way that lowers the output voltage when the load is light. 
     Moreover, as for a direct-current power supply device of the present invention, the direct-current power supply device, in which a switch used by a PFC circuit is shared as a switching element by a DC/DC converter, includes voltage suppression means for suppressing a rise in voltage of a primary-side smoothing capacitor of the PFC circuit at a time when a load is light. 
     Moreover, as for a direct-current power supply device of the present invention, the direct-current power supply device, in which a switch used by a PFC circuit is shared as a switching element by a DC/DC converter, includes voltage suppression means for supplying electric charge accumulated in a primary-side smoothing capacitor to a power supply of a control circuit that controls the switching element at a time when a load is light in order to suppress a rise in voltage in the primary-side smoothing capacitor. 
     Moreover, as for a direct-current power supply device of the present invention, the direct-current power supply device, in which a switch used by a PFC circuit is shared as a switching element by a DC/DC converter, includes voltage suppression means for also supplying magnetic energy released from a reactor of the PFC circuit to a power supply of a control circuit that controls the switching element via a second winding of a primary winding of a transformer at a time when a load is light in order to suppress an amount of charge of a primary-side smoothing capacitor. 
     Moreover, as for a direct-current power supply device of the present invention, the direct-current power supply device, in which a switch used by a PFC circuit is shared as a switching element by a DC/DC converter, includes voltage suppression means for supplying, with a main winding and auxiliary winding provided in a reactor of the PFC circuit, magnetic energy of the reactor to a power supply of a control circuit via a diode from the auxiliary winding after power supply voltage of the control circuit that controls the switching element at a time when a load is light falls. 
     Moreover, as for a direct-current power supply device of the present invention, the direct-current power supply device, in which a switch used by a PFC circuit is shared as a switching element by a DC/DC converter, includes voltage suppression means for controlling the DC/DC converter in a way that lowers output voltage at a time when a load is light in order to suppress an amount of charge of a primary-side smoothing capacitor. 
     According to the present invention, the direct-current power supply device, in which the switch used by the PFC circuit is shared as a switching element by the DC/DC converter, can prevent the terminal voltage of the smoothing capacitor of the PFC circuit from rising at a time when the DC/DC converter is a light load. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram showing the circuit configuration of a direct-current power supply device  1  according to a first embodiment of the present invention; 
         FIG. 2  is a diagram showing a voltage characteristic of a smoothing capacitor of the direct-current power supply device  1  according to first to fourth embodiments of the present invention; 
         FIG. 3  is a diagram showing the circuit configuration of a direct-current power supply device  2  according to the second embodiment of the present invention; 
         FIG. 4  is a diagram showing the circuit configuration of a direct-current power supply device  3  according to the third embodiment of the present invention; 
         FIG. 5  is a diagram showing the circuit configuration of a direct-current power supply device  4  according to the fourth embodiment of the present invention; 
         FIG. 6  is a diagram showing a voltage characteristic of a smoothing capacitor of a direct-current power supply device  100  of a conventional technique; 
         FIG. 7  is a diagram showing the circuit configuration of the direct-current power supply device  100  of a conventional technique; and 
         FIG. 8  is a diagram showing the operational waveform of each section of the direct-current power supply device  100  of a conventional technique. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following describes embodiments of the present invention in a concrete way with reference to the accompanying drawings. A direct-current power supply device illustrated in an embodiment of the present invention is a direct-current power supply device in which a switch used by a PFC circuit is shared as a switching element by a DC/DC converter, including voltage suppression means for suppressing an increase in voltage of a smoothing capacitor of the PFC circuit at a time when a load is light. 
     A direct-current power supply device  1  of a first embodiment shown in  FIG. 1  is one that, as voltage suppression means, suppresses a rise in voltage of the smoothing capacitor C 1  by supplying electric charge accumulated in the smoothing capacitor C 1  to power supply of the control circuit CTL 1  at a time when a load is light. 
     A direct-current power supply device  2  of a second embodiment shown in  FIG. 3  is one that, as voltage suppression means, suppresses an amount of charge of a smoothing capacitor C 1  by also supplying magnetic energy released from a reactor L 1  of a PFC circuit to power supply of a control circuit CTL 2  via the other winding of a primary winding of a transformer T 1  at a time when a load is light. 
     A direct-current power supply device  3  of a third embodiment shown in  FIG. 4  is one in which a main winding P and an auxiliary winding S are provided, as voltage suppression means, in a reactor L 2  of a PFC circuit. The main winding P is used in the same way as a reactor L 1 ; as the power supply voltage of a control circuit CTL 3  falls at a time when a load is light, magnetic energy of the reactor L 2  is supplied to power supply of the control circuit CTL 3  from the auxiliary winding S via a diode D 4 . 
     A direct-current power supply device  4  of a fourth embodiment shown in  FIG. 5  is one that controls a DC/DC converter in such a way that output voltage decreases at a time when a load is light, thereby shortening the period of time t 0  to t 8  (the period of time t 16  to t 17 ) shown in  FIG. 8  and curbing an amount of charge of a smoothing capacitor C 1 . 
     (First Embodiment) 
       FIG. 1  shows the circuit configuration of a direct-current power supply device  1  of the first embodiment of the present invention. The direct-current power supply device  1  is different from the direct-current power supply device  100  of the conventional technique shown in  FIG. 7  in that a circuit (switching elements Q 2  and Q 3 , and resistance R 2 ) for supplying electric charge of a smoothing capacitor C 1  to power supply Vcc of a control circuit CTL 1  is provided, with the switching elements Q 2  and Q 3  controlled by an ON/OFF signal from the control circuit CTL 1  at a time when a load is light. The switching elements Q 2  and Q 3  and the resistance R 2  make up a circuit that also serves as a start-up circuit of a control circuit. Incidentally, an auxiliary winding N 3  of a transformer T 1 , a diode D 3 , a smoothing capacitor C 4  and the like are not shown in  FIG. 7  but are shown in  FIG. 1 . The bypass capacitor C 3 , which is shown in  FIG. 7  but not in  FIG. 1 , a high frequency component removing capacitor and a filter provided between a commercial alternating-current power supply Vs and a rectification circuit RC 1 , which are disclosed in Patent Document 1, or the like may be provided when needed. 
     The circuit configuration of the direct-current power supply device  1  will be described with reference to  FIG. 1 . A rectification circuit RC 1  where diodes are so connected as to form a bridge is connected to a commercial alternating-current power supply Vs. To a positive electrode-side output terminal (voltage Vin) of the rectification circuit RC 1 , one terminal of a reactor L 1  is connected. To the other terminal of the reactor L 1 , an anode terminal of a fast recovery diode D 1  is connected. A cathode terminal of the fast recovery diode D 1  is connected to a tap section of a primary winding N 1  of the transformer T 1 . The primary winding N 1  of the transformer T 1  is made up of two windings N 1   a  (first primary winding) and Nib (second primary winding). A connection point for the other terminal of the first primary winding N 1   a  and one terminal (at the side indicated by ● in the diagram) of the second primary winding N 1   b  is the tap section described above. One terminal (at the side indicated by ● in the diagram) of the first primary winding N 1   a  is connected to one terminal (at the positive electrode side) of the smoothing capacitor C 1 . The other terminal (at the negative electrode side) of the smoothing capacitor C 1  is connected to a negative electrode-side output terminal (GND) of the rectification circuit RC 1 . The other terminal of the second primary winding N 1   b  is connected to a drain terminal of the switching element Q 1 . A source terminal of the switching element Q 1  is connected to a connection point where the negative electrode-side output terminal of the rectification circuit RC 1  and the other end of the smoothing capacitor C 1  are connected. The voltage polarity of the primary winding of the transformer T 1  is set as indicated by ● in the diagram. 
     The other end of a secondary winding N 2  of the transformer T 1  is connected to an anode terminal of the diode D 2 . A cathode terminal of the diode D 2  is connected to one terminal (at the positive electrode side) of a smoothing capacitor C 2 . One terminal (at the side indicated by ● in the diagram) of the secondary winding N 2  of the transformer T 1  is connected to the other terminal (at the negative electrode side) of the smoothing capacitor C 2 . The voltage polarity of the transformer T 1  is set as indicated by ● in the diagram so as to work as a flyback converter. One terminal and the other terminal of the smoothing capacitor C 2  work as a positive electrode-side output terminal A and negative electrode-side output terminal B of the direct-current power supply device  1 , respectively. The voltage between the positive electrode-side output terminal A and negative electrode-side output terminal B of the direct-current power supply device  1  is detected by an output voltage detection circuit DTC, which is connected between the positive electrode-side output terminal A and the negative electrode-side output terminal B. A resultant detection signal is input, as a feedback signal FB, to the control circuit CTL 1 . The control circuit CTL 1  makes a comparison between a preset chopping-wave voltage and the feedback signal FB; a pulse signal for turning the switching element ON/OFF is output from an OUT terminal of the control circuit CTL 1  to a gate terminal of the switching element Q 1  so that output voltage Vo (=voltage VC 2  of the smoothing capacitor C 2 ) comes to a desired target voltage. Adjustments to the output voltage Vo can be made by changing the size of the feedback signal FB with the use of an output voltage control signal vc, which is input into the output voltage detection circuit DTC. 
     In the transformer T 1 , an auxiliary winding N 3  is provided for power supply of the control circuit CTL 1 . The voltage of the auxiliary winding N 3  is rectified and smoothed by a diode D 3  and a smoothing capacitor C 4  before being supplied as power supply Vcc of the control circuit CTL 1 . The voltage polarity of the primary winding N 1  of the transformer T 1  and of the auxiliary winding N 3  is set as indicated by ● in the diagram. 
     What is provided for the smoothing capacitor C 1  is a circuit (switching elements Q 2  and Q 3 , and resistance R 2 ) that also serves as a start-up circuit of the control circuit CTL 1  and supplies electric charge of the smoothing capacitor C 1  to the power supply Vcc of the control circuit CTL 1  at a time when the load is light: the smoothing capacitor C 1  is connected in parallel to the circuit. That is, to one terminal (at the positive electrode side) of the smoothing capacitor C 1 , a drain terminal of the switching element Q 2  (FET, for example) is connected. A source terminal of the switching element Q 2  is connected to a terminal of power supply Vcc of the control circuit CTL 1 . To a gate terminal of the switching element Q 2 , a drain terminal of the switching element Q 3  is connected. A source terminal of the switching element Q 3  is connected to GND. Resistance R 2  is connected between the gate and drain of the switching element Q 2 . A gate terminal of the switching element Q 3  is connected to an ON/OFF signal output terminal of the control circuit CTL 1 . 
     As the voltage of the power supply Vcc of the control circuit CTL 1  decreases, the decrease is detected by the control circuit CTL 1 , which then outputs a signal for turning the switching element Q 3  off from an ON/OFF signal output terminal of the control circuit CTL 1 . As a result, the switching element Q 3  is turned off. Therefore, the switching element Q 2  is turned on, supplying power from the smoothing capacitor C 1  to the power supply Vcc of the control circuit CTL 1 . That is, when the output of the direct-current power supply device  1  turns into a light-load state or when the output voltage is lowered by the output voltage control signal vc, a period during which the switching element Q 1  is ON is shortened. Accordingly, the average value of the voltage waveform that appears at the auxiliary winding N 3  of the transformer T 1  falls, resulting in a decrease in voltage of the power supply Vcc. After the decrease in voltage of the power supply Vcc is detected by the control circuit CTL 1 , a signal for turning the switching element Q 3  off is output from the ON/OFF signal output terminal of the control circuit CTL 1 . As a result, the switching element Q 3  is turned off. Therefore, the switching element Q 2  is turned on, and the electric charge of the smoothing capacitor C 1  is discharged. Thus, it is possible to lower the voltage of the smoothing capacitor C 1 . 
       FIG. 2  shows a characteristic, with the horizontal axis representing the output voltage (the state of the load) of the DC/DC converter and the vertical axis representing the terminal voltage of the smoothing capacitor of the PFC circuit. Among characteristic curves, the solid-line characteristic curve represents a characteristic of the direct-current power supply device  1  of the first embodiment of the present invention. The dotted-line characteristic curve represents a characteristic of the direct-current power supply device  100  made up of conventional circuits. It is clear that when compared with the conventional circuits, the terminal voltage of the smoothing capacitor C 1  of the PFC circuit of the present invention is kept lower at a time when the load is light. 
     According to the present first embodiment, with the circuit that also serves as a start-up circuit, it is possible to curb an increase in voltage of the smoothing capacitor C 1  at a time when the load is light. Therefore, the advantage is that the circuit configuration is simplified. Moreover, according to the present embodiment, part of the magnetic energy released from the reactor L 1  can be used as power of the power supply of the control circuit CTL 1 . Therefore, compared with the one in which the energy generated by a rise in voltage of the smoothing capacitor C 1  is simply consumed by resistance, it is possible to improve the efficiency of the direct-current power supply device. Since the voltage of the smoothing capacitor C 1  falls, a capacitor with low voltage-withstanding capability can be used. Thus, it is possible to achieve a reduction in costs of smoothing capacitors and an improvement in reliability. If the power supply Vcc of the control circuit CTL 1  is obtained by rectifying the voltage of the auxiliary winding N 3  of the transformer T 1 , the power supply Vcc decreases when the output of the direct-current power supply device  1  turns into a light-load state or when the output voltage is lowered by the output voltage control signal vc. However, according to the present first embodiment, power is supplied from the smoothing capacitor C 1 . Therefore, it is possible to keep the power supply Vcc of the control circuit CTL 1  from decreasing. 
     (Second Embodiment) 
       FIG. 3  shows the circuit configuration of the direct-current power supply device  2  of the second embodiment of the present invention. The direct-current power supply device  2  is different from the direct-current power supply device  1  of the first embodiment shown in  FIG. 1 : while the drain terminal of the switching element Q 2  is connected to one terminal (positive-electrode terminal) of the smoothing capacitor C 1  in the direct-current power supply device  1 , the drain terminal of the switching element Q 2  is connected to a connection point where the other terminal of the second primary winding N 1   b  of the transformer T 1  and the drain terminal of the switching element Q 1  are connected together in the direct-current power supply device  2 . The configuration of the other parts is the same as that of the first embodiment and therefore will not be described in detail. 
     According to the present second embodiment, unlike the first embodiment, the energy accumulated in the smoothing capacitor C 1  is not supplied to the power supply Vcc of the control circuit CTL 1 ; part of the electromagnetic energy released from the reactor L 1  is supplied to the power supply Vcc of a control circuit CTL 2  via the second primary winding N 1   b  of the transformer T 1 . As in the case of the first embodiment, even in the present second embodiment, when the voltage of the power supply Vcc of the control circuit CTL 2  decreases, the decrease is detected by the control circuit CTL 2 . A signal for turning the switching element Q 3  off is output from the ON/OFF signal output terminal of the control circuit CTL 2 , and the switching element Q 3  is turned off. As a result, the switching element Q 2  is turned on, supplying power from the smoothing capacitor C 1  to the power supply Vcc of the control circuit CTL 2 . 
     That is, when the output of the direct-current power supply device  2  turns into a light-load state or when the output voltage is lowered by the output voltage control signal vc, a period during which the switching element Q 1  is ON is shortened. Accordingly, the average value of the voltage waveform that appears at the auxiliary winding N 3  of the transformer T 1  falls, resulting in a decrease in voltage of the power supply Vcc. After the decrease in voltage of the power supply Vcc is detected by the control circuit CTL 2 , a signal for turning the switching element Q 3  off is output from the ON/OFF signal output terminal of the control circuit CTL 2 . As a result, the switching element Q 3  is turned off. Therefore, the switching element Q 2  is turned on, and part of the magnetic energy released from the reactor L 1  is consumed as power supply of the control circuit CTL 2 . Thus, it is possible to lower the voltage of the smoothing capacitor C 1 . Even in the present second embodiment, the circuit also serves as a start-up circuit. Moreover, it is possible to curb an increase in voltage of the smoothing capacitor C 1  at a time when the load is light, and the advantage is that the circuit configuration is simplified. Moreover, even in the present second embodiment, part of the magnetic energy released from the reactor L 1  can be used as power of the power supply of the control circuit CTL 2 . Therefore, compared with the one in which the energy generated by a rise in voltage of the smoothing capacitor C 1  is simply consumed by resistance, it is possible to improve the efficiency of the direct-current power supply device  2 . Moreover, since the voltage of the smoothing capacitor C 1  falls, a capacitor with low voltage-withstanding capability can be used. Thus, it is possible to achieve a reduction in costs of smoothing capacitors and an improvement in reliability. If the power supply Vcc of the control circuit CTL 2  is obtained by rectifying the voltage of the auxiliary winding N 3  of the transformer T 1 , the power supply Vcc decreases when the output of the direct-current power supply device  2  turns into a light-load state or when the output voltage is lowered by the output voltage control signal vc. However, according to the present second embodiment, power is supplied from the reactor L 1 . Therefore, it is possible to keep the power supply Vcc of the control circuit CTL 2  from decreasing. 
     (Third Embodiment) 
       FIG. 4  shows the circuit configuration of the direct-current power supply device  3  of the third embodiment of the present invention. In the direct-current power supply device  3 , the switching elements Q 2  and Q 3  and resistance R 2 , which the direct-current power supply devices of the first and second embodiments include, are removed. Instead of the reactor L 1 , a reactor L 2  including a main winding P and an auxiliary winding S is provided. The main winding P is used in the same way as the reactor L 1  of the first or second embodiment. Part of the magnetic energy of the reactor L 2  is supplied to the power supply Vcc of a control circuit CTL 3  from the auxiliary winding S via a diode D 4  at a time when the load is light. The configuration of the other parts is the same as that of the first or second embodiment and therefore will not be described in detail. 
     When the load is heavy, as in the case of a circuit of a conventional technique, the power supply Vcc of the control circuit CTL 3  is supplied from the auxiliary winding N 3  of the transformer T 1 . However, when the output turns into a light-load state or when the output voltage Vo is lowered by the output voltage control signal vc, a period during which the switching element Q 1  is ON is shortened. Accordingly, the average value of the voltage waveform that appears at the auxiliary winding N 3  of the transformer T 1  falls, and part of the magnetic energy of the reactor L 2  is supplied to the power supply Vcc of the control circuit CTL 3  from the auxiliary winding S via the diode D 4 . Therefore, part of the magnetic energy, which is accumulated in the reactor L 2  when the switching element Q 1  is turned on, is supplied to the power supply of the control circuit CTL 3 ; the electric charge that is supplied to the smoothing capacitor C 1  decreases. Thus, it is possible to lower the voltage of the smoothing capacitor C 1 . According to the present third embodiment, the switching elements Q 2  and Q 3  are unnecessary; a portion of the control circuit for the switching elements Q 2  and Q 3  is also unnecessary. Thus, the advantage is that the circuit configuration is simplified. Moreover, since the voltage of the smoothing capacitor C 1  falls, a capacitor with low voltage-withstanding capability can be used. Thus, it is possible to achieve a reduction in costs of smoothing capacitors and an improvement in reliability. If the power supply Vcc of the control circuit CTL 3  is obtained by rectifying the voltage of the auxiliary winding N 3  of the transformer T 1 , the power supply Vcc decreases when the output of the direct-current power supply device  3  turns into a light-load state or when the output voltage is lowered by the output voltage control signal vc. However, according to the present third embodiment, power is supplied from the auxiliary winding S of the reactor L 2 . Therefore, it is possible to keep the power supply Vcc of the control circuit CTL 3  from decreasing. 
     (Fourth Embodiment) 
       FIG. 5  shows the circuit configuration of the direct-current power supply device  4  of the fourth embodiment of the present invention. The direct-current power supply device  4  is substantially the same as the direct-current power supply device  100  of the conventional technique shown in  FIG. 7 . However, the direct-current power supply device  4  is different from the direct-current power supply device  100  in that the output voltage Vo is controlled so as to be lowered by the output voltage control signal vc at a time when the load is light. In this case, the output voltage can be decreased when the output voltage detection circuit DTC increases the detection voltage relative to the same output voltage and outputs a feedback signal FB to a control circuit CTL 4 . When the target output voltage of the output voltage Vo is lowered, the voltage of the stray capacitance (or the capacitor C 5  of the snubber circuit) between the windings of the transformer T 1  is being charged after the start of a switching operation of the switching element Q 1 . A period of time (the period of time t 0  to t 8  shown in  FIG. 8 , or the period of time t 16  to t 17 ) required for the voltage occurring at the secondary winding N 2  to rise to voltage VC 2  where the smoothing capacitor C 2  can be charged is shortened, thereby curbing an increase in voltage of the smoothing capacitor. According to the present fourth embodiment, the switching elements Q 2  and Q 3  are unnecessary; a portion of the control circuit for the switching elements Q 2  and Q 3  is also unnecessary. Thus, the advantage is that the circuit configuration is simplified. Moreover, since the voltage of the smoothing capacitor C 1  falls, a capacitor with low voltage-withstanding capability can be used. Thus, it is possible to achieve a reduction in costs of smoothing capacitors and an improvement in reliability. 
     The above has described the present invention through specific examples. However, the above description is given for illustrative purposes only. Needless to say, the present invention may be modified and embodied without departing from the scope of the present invention. For example, according to the present embodiment, to the direct-current power supply device shown in  FIG. 1  of the specification of Patent Document 1, the present invention is applied. However, the present invention is not limited to the above. The present invention may be applied in a way that curbs an increase in voltage of a smoothing capacitor Cdc (equivalent to the smoothing capacitor C 1  of the embodiments of the present invention) shown in  FIGS. 5 to 11  of the specification of Patent Document 1. Moreover, in the examples described above, a MOSFET is used for the switching element Q 1 . However, a bipolar transistor, FET, IGBT or any other transistor can also be used. Moreover, in the examples described above, FETs are used for the switching elements Q 2  and Q 3 . However, bipolar transistors or MOSFETs can also be used.