Abstract:
When a relay ( 21 ) is controlled, an AC voltage ( 1 ) may be rectified in a full-wave rectification fashion. In a voltage-doubler rectification fashion, and a rectified output supplied to a main converter ( 6 ) or the AC voltage ( 1 ) may be rectified in a half-wave rectification fashion, and a rectified output supplied to a standby converter ( 10 ). A just detection circuit ( 23 ) detects a rectified voltage of the standby side, and a detection circuit ( 24 ) may detect a rectified voltage supplied to the main converter ( 6 ). The first and second voltage detection circuits ( 23 ) and ( 24 ) control operations of relay drive circuits ( 21 D and  22 D) based on both detection signals so that any one of two relay contacts ( 21   a  and  22   a ) may be turned on. Since a voltage of the rectified output supplied to the main converter ( 6 ) may be detected and its detection signal also may be used for control, a detection timing can be made fast so that, even when the voltage of the commercially-available voltage is fluctuated suddenly, the rectification systems can be switched immediately.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to a power supply apparatus for use with a television receiver, a monitor or the like, for example. More particularly, this invention relates to a power supply apparatus comprising a main power supply section for rectifying a voltage in either a full-wave rectification fashion or a voltage-doubler rectification fashion in response to a voltage supplied from a commercial alternating-current power supply and a standby power supply section for rectifying a voltage of a commercial alternating-current (hereinafter simply referred to as an AC) in which a full-wave rectification and a voltage-doubler rectification may be switched by detecting rectified outputted voltages from the standby power supply section and the main power supply section. 
     2. Description of the Related Art 
     FIG. 1 of the accompanying drawings shows an arrangement of a related-art power supply apparatus for use with a television receiver, a monitor or the like. A power supply apparatus  200 , generally depicted by reference numeral  200  in FIG. 1 can be used in the area in which a voltage of a AC power supply is 100V and in the area in which a voltage of a AC power supply is 220V. 
     As shown in FIG. 1, in the power supply apparatus  200 , one end of a AC power supply  1  may be connected to one input terminal of a bridge-type rectifier  2 . The other end of this AC power supply  1  may be connected to the other input terminal of the bridge-type rectifier  2  through a contact  3   a  of a relay  3 . The relay  3  may be adapted to supply or to interrupt the AC power supply  1 . A relay drive circuit  3 D which may drive the relay  3  may be driven under control of a power supply control signal (hereinafter simply referred to as “CPW”) supplied from a control section (not shown). When the signal CPW is held at high level, the relay  3  may be driven to close the contact  3 a thereby to supply the voltage of the AC power supply  1 . 
     When a contact  7   a  of a relay  7  is opened, the voltage of the AC power supply  1  may be rectified by the bridge-type rectifier  2  in a full-wave rectification fashion. Then, a DC voltage smoothed by capacitors  4  and  5  connected in series may be inputted to a main DC/DC converter 6. Also, when the contact  7   a  of the relay  7  is closed, a DC voltage (hereinafter simply referred to as a DC voltage) which was smoothed by the capacitor  4  after it had been rectified by one diode of the bridge-type rectifier  2  and a DC voltage smoothed by the capacitor  5  after it had been rectified by another one diode of the bridge-type rectifier  2  may be added and a voltage-doubler rectification can be performed. The main DC/DC converter  6  may convert the supplied DC voltage into DC voltages of a variety of voltages necessary for a television, a monitor or the like, for example. 
     The voltage from the AC power supply  1  may be rectified/smoothed by a diode  8  and a capacitor  9  and then inputted to a standby DC/DC converter  10 . This standby DC/DC converter  10  may supply a DC voltage necessary for maintaining functions of the control section, the relay drive, the voltage detection circuit or the like when the main DC/DC converter  6  is disabled. 
     A voltage detection circuit  11  may compare a voltage Vs rectified/smoothed by the diode  8  and the capacitor  9  with a predetermined threshold voltage. The voltage detection circuit  11  may output a detection output (hereinafter simply referred to as “VDE”) which goes to low level when the voltage Vs is higher than the predetermined threshold voltage and which goes to high level when the voltage Vs is lower than the predetermined threshold voltage. The voltage detection circuit  11  may have a hysteresis characteristic in which a high threshold voltage may be set to DC225V and a low threshold voltage may be set to DC190V, for example. The output VDE may be supplied to a relay drive circuit  7 D as a control signal. When the output VDE is held at high level, the relay drive circuit  7 D may drive the relay  7  to close: a relay contact  7   a , whereby a voltage-doubler rectification can be performed. 
     FIG. 2 shows an example of an arrangement of this voltage detection circuit  11  more concretely. As shown in FIG. 2, a voltage Vcc applied to a power supply terminal  51  may be supplied through a resistor R 7  to a shunt regulator IC 2  which may be used to generate a reference voltage. A series circuit of resistors R 8  and R 9  may be connected to the shunt regulator IC 2  in parallel. A voltage developed at a junction between the resistors R 8  and R 9  may be supplied to the shunt regulator IC 2 . The shunt regulator IC 2  may increase or decrease a current flowing thereto such that the above-mentioned voltage may become constant. Thus, a predetermined reference voltage Vrf may be developed at the junction between the resistors R 7  and R 8 . 
     As shown in FIG. 2, a voltage Vs may be supplied to a detection voltage input terminal  53  as a detection voltage. A voltage Va that was voltage-divided by resistors R 5  and R 6  may be supplied to a negative input terminal of a comparator IC 1 . The reference voltaege Vrf may be supplied through a resistor R 4  to a positive input terminal of the comparator IC 1 . The resistors R 3  and R 4  may be give a hysteresis characteristic to the comparator IC 1 , and a diode D 2  may be used in order to block a reverse current. 
     An output terminal of the comparator IC 1  may be connected to the base of a transistor Q 1 , and a bias may be applied to the base of the transistor Q 1  from the resistors R 1  and R 2 . The collector of the transistor Q 1  may be connected through a coil  7   b  of the relay  7  to the power supply terminal  51 . A diode D 1  which is connected in parallel to the coil  7   b  may be used to suppress the collector voltage of the transistor Q 1  from increasing due to a back electromotive force generated in the relay  7  when the transistor Q 1  is turned off. 
     When the voltage of the AC power supply  1  is low (e.g. 100V), accordingly, the detection voltage Vs is low, the voltage Va supplied to the negative input terminal of the comparator IC 1  may become lower than the voltage supplied to its positive input terminal so that the output VDE of the comparator IC 1  goes to high level. At that very moment, since the transistor Q 1  is turned on to drive the relay  7 , the contact  7   a  of the relay  7  may be closed, whereby the voltage-doubler rectification operation can be performed. 
     When on the other hand the voltage of the AC power supply  1  is high (e.g. 200V), accordingly, the detection voltage Vs is high, the voltage Va supplied to the negative input terminal of the comparator IC 1  becomes higher than the voltage supplied to its positive input terminal so that the output VDE of the comparator IC 1  goes to low level. At that very moment, since the transistor Q 1  is turned off and the relay  7  may not be driven, the full-wave rectification operation may be performed. 
     When the AC voltage is fluctuated suddenly, in particular, a voltage rises or when a power failure of a short period of time occurs or when a relay drive circuit is operated abnormally, it is unavoidable that the main DC/DC converter is considerably affected by a resultant stress. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a reliable power supply apparatus in which the aforementioned problem can be obviated. 
     According to the present invention, there is provided a power supply apparatus which is comprised of a main power supply section including a rectifying circuit supplied with a AC voltage through a first or second contact, the rectifying circuit for rectifying the AC voltage in a full-wave rectification fashion or in a voltage-doubler rectification fashion, a standby power supply section including a rectifying circuit for rectifying the AC voltage, a first voltage detection section for detecting whether the rectified output from the standby power supply section is higher than or lower than a predetermined threshold voltage, a second voltage detection section for detecting whether the rectified output from the main power supply section is higher than or lower than a predetermined threshold voltage, and a switch control section for closing any one of the first and second contacts based on a detection signal from the first voltage detection section and a detection signal from the second voltage detection section. 
     The main power supply section rectifies the AC power supply voltage supplied thereto through the first contact in a full-wave rectification fashion when the voltage of the AC power supply is high, and rectifies the AC power supply voltage supplied thereto through the second contact in a voltage-doubler rectification fashion when the voltage of the AC power supply is low. Control for opening and closing the first and second contacts may be executed by comparing the rectified voltage of the standby power supply and the rectified voltage of the main power supply with a predetermined voltage. Although the voltage change which was started on the rectified half-wave side can be detected by detecting the rectified voltage of the standby power supply, the voltage change which was started on the half-wave side which is not rectified cannot be detected by detecting the rectified voltage of the standby power supply. There is then the possibility that the detection will be delayed until the next half-wave is rectified. By detecting the rectified voltage of the standby power supply together with the rectified voltage of the main power supply, it is possible to provide a highly-reliable power supply apparatus in which the above-mentioned possibility can be obviated. 
     For example, the first and second voltage detection sections may comprise a comparator for comparing a first voltage corresponding to a threshold voltage and a second voltage corresponding to the rectified output and a charge and discharge circuit inserted into this comparator at its input side of the second voltage. A discharge time constant of the charge and discharge circuit may be made larger than a charge time constant thereof. Thus, under the state that the output level of the comparator selects the full-wave rectification, when a power failure of a short period of time occurs in the AC power supply, an output from the charge and discharge circuit is lowered slowly as compared with the case in which the second voltage is lowered. Hence, the output level of the comparator can be maintained in the state in which such output level selects the full-wave rectification. Thus, when the power failure is ended, the voltage-doubler rectification is not executed and the main DC/DC converter  6  can be protected. 
     Further, according to the present invention, there is provided a power supply apparatus which is comprised of a main power supply section including a rectifying circuit supplied with a AC voltage through a contact of a first relay or a contact of a second relay, the rectifying circuit for rectifying the AC voltage in a full-wave rectification fashion or a voltage-doubler rectification fashion, a standby power supply section for obtaining a standby power from the AC voltage, a voltage detection section for detecting whether a voltage of the AC voltage is higher than or lower than a predetermined threshold voltage, a first relay drive section for driving the first relay, a second relay drive section for driving the second relay, a switch control section for controlling operations of the first relay drive section and the second relay drive section based on the detected signal from the voltage detection section such that any one of the contact of the first relay and the contact of the second relay is turned on, a switch section for supplying a power source voltage obtained at the standby power supply section to the first relay drive section and the second relay drive section and a power supply on-off control section for controlling on-off of the switch section based on a power supply on-off control signal. 
     According to this invention, the main power supply section rectifies the AC power supply voltage supplied thereto through the contact of the first relay in a full-wave rectification fashion, and rectifies the AC power supply voltage supplied thereto through the contact of the second relay in a voltage-doubler rectification fashion. In this case, when the voltage of the AC power supply voltage is low, the contact of the second relay is closed such that the voltage-doubler rectification can be executed. When on the other hand the voltage of the AC power supply voltage is high, the contact of the first relay is closed so that the full-wave rectification can be executed. 
     Control operations for opening and closing the contacts of the first and second relays may be executed by controlling the first and second relay drive sections, respectively, based on the voltage detection signal of the standby power supply rectified output and the voltage detection signal of the main power supply rectified output. The power supply voltage may be supplied to the first and second relay drive sections from the standby power supply through the switch section. When the switch section is opened, the power supply voltage is not supplied to the first and second relay drive sections. Thus, both of the contacts of the first and second relays are turned off, thereby resulting AC voltage is not supplied to the main power supply. When on the other hand the switch section is closed, the power supply voltage is supplied to the first and second relay drive sections so that the contact of the first or second relay is closed based on the voltage detection signal, thereby resulting AC voltage is supplied to the main power supply. 
     As described above, since the contacts of the first and second relays may be served as also the power supply switch so that the voltage-doubler rectification operation, for example, is effected, only the second relay can be energized. Thus, a power consumption can be decreased. 
     For example, the power supply apparatus according to the present invention may further include an abnormal voltage detection section for detecting whether the rectified output from the main power supply section is higher than or lower than a predetermined threshold voltage. The power supply on-off control section may control on-off of the switch section based on a detection signal from the abnormal voltage detection section in addition to the power supply on-off control signal. Thus, when the contact of the second relay is closed due to an abnormal operation of the second relay drive section or the like so that an abnormally-high voltage is generated as a rectified output, the supply of power to the second relay is interrupted by de-energizing the switch section based on the detection signal from the abnormal voltage detection section. Thus, a rectified output voltage can be lowered, thereby resulting the main DC/DC converter  6  can be protected. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit diagram showing a power supply apparatus according to the related art; 
     FIG. 2 is a circuit diagram showing examples of a voltage detection circuit and a relay drive circuit according to the related art concretely; 
     FIG. 3 is a circuit diagram showing a power supply apparatus according to an embodiment of the present invention; 
     FIGS. 4A and 4B are diagrams to which reference will be made in explaining a voltage detection operation for each half-wave of a commercially-available power supply; 
     FIG. 5 is a flowchart to which reference will be made in explaining a concrete flow of a power supply control method; 
     FIG. 6 is a flowchart to which reference will be made in explaining a concrete flow of a power supply control method; 
     FIG. 7 is a circuit diagram showing an embodiment of a voltage detection circuit; 
     FIGS. 8A to  8 D are, respectively, diagrams to which reference will be made in explaining the manner in which the voltage detection circuit is operated when a momentary power failure occurs; and 
     FIG. 9 is a block diagram showing electronic equipment which incorporates therein a power supply apparatus according to the embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A power supply apparatus according to an embodiment of the present invention will hereinafter be described with reference to the drawings. 
     FIG. 3 shows a power supply apparatus  100  according to an embodiment of the present invention. In FIG. 3, elements and parts identical to those of FIG. 1 are marked with the identical reference numerals. This power supply apparatus  100  can be used in the area in which a voltage of a AC power supply may be 100V, 115V or 220V. 
     As shown in FIG. 3, the AC power supply  1  may be connected through a contact  21   a  of a relay  21  to the bridge-type rectifier  2 . The contact  21   a  of the relay  21  may be served as also a power supply switch so that, when the contact  21   a  of the relay  21  is closed, the full-wave rectification may be effected on voltage of the AC power supply  1 . This relay  21  may be driven by a relay drive circuit  21 D. 
     A voltage smoothed by the smoothing capacitors  4  and  5  after it had been rectified by the bridge-type rectifier  2  may be supplied to the main DC/DC converter  6 . The main DC/DC converter  6  may convert an inputted DC voltage into various kinds of DC voltages used in a television receiver, a monitor or the like, for example. 
     The other end of the AC power supply  1  may be connected to the capacitors  4  and  5  through a contact  22   a  of a relay  22 . The contact  22   a  of the relay  22  may be served as also a power supply switch so that, when the contact  22   a  of the relay  22  is closed, the voltage-doubler rectification may be effected on the voltage of the AC power supply  1 . This relay  22  may be driven by a relay drive circuit  22 D. 
     Since the relay contacts  21   a  and  22   a  may be operated as power supply switches as well as the full-wave rectification operation switch and the voltage-doubler rectification operation switch, respectively, a power consumption for driving the relays can be decreased as compared with the case in which the relays are provided independently. 
     The voltage of the AC power supply  1  may be rectified by the diode  8 , smoothed by the capacitor  9  and then supplied to the input of the standby DC/DC converter  10 . The standby DC/DC converter  10  may supply a DC voltage necessary for maintaining the functions of the control section, the relay drive circuit, the voltage detection circuit or the like when the main DC/DC converter  6  is disabled. 
     A junction P 1  between the diode  8  and the capacitor  9  may be connected to the input side of a voltage detection circuit  23 . This voltage detection circuit  23  may compare a voltage Vs developed at the junction P 1  with a predetermined threshold voltage, and may output an output VDE 1  of low level when the voltage Vs is higher than the predetermined threshold voltage and an output VDE  1  of high level when the voltage Vs is lower than the predetermined threshold voltage. A comparator in the voltage detection circuit  23  may have a hysteresis characteristic in which a high threshold voltage may be set to 225V and a low threshold voltage may be set to 190V, for example. 
     A voltage Vm developed at a junction P 2  between the bridge-type rectifier  2  and the capacitor  4  may be supplied to the input side of a voltage detection circuit  24 . This voltage detection circuit  24  may detect whether the voltage Vm is higher than or lower than a predetermined threshold voltage. The voltage detection circuit  24  may generate an output VDE 2  of low level when the voltage Vm is higher than the predetermined threshold voltage and the output VDE 2  of high level when the voltage Vm is lower than the predetermined threshold voltage. The comparator in the voltage detection circuit  24  has a hysteresis characteristic similarly. 
     The outputs VDE 1  and VDE 2  from the voltage detection circuits  23  and  24  may be supplied to an AND circuit  25 . The output from the AND circuit  25  may be supplied to the relay drive circuit  22 D as a control signal CS 1 . When both of the voltages Vs and Vm are lower than the predetermined threshold voltages, both of the outputs VDE 1  and VDE 2  go to high level and the control signal CS 1  also goes to high level, whereby the relay  22  may be driven to close the contact  22   a , thereby resulting in the voltage-doubler rectification being performed. Also, when any one of the voltages Vs and Vm is higher than the predetermined threshold voltage, one of the outputs VDE 1  and VDE 2  goes to low level, the control signal CS 1  goes to low level and the control signal CS 2  goes to high level, whereby the relay  21  may be driven to close the contact  21   a , thereby resulting in the full-wave rectification being performed. 
     The voltage Vm developed at the junction P 2  between the bridge-type rectifier  2  and the capacitor  4  may be supplied to a voltage detection circuit  27 . The voltage detection circuit  27  may detect an abnormal voltage by comparing the voltage Vm with a predetermined threshold voltage. When the voltage Vm is abnormally higher than the predetermined threshold voltage, the voltage detection circuit  27  may output an output VDE 3  of low level. The comparator in the voltage detection circuit  27  also has a hysteresis characteristic similarly, in which a high threshold voltage may be set to DC470V and a low threshold voltage may be set to DC300V, for example. 
     While the output VDE 3  outputted from the voltage detection circuit  27  may be supplied to one input of an AND circuit  28 , the signal CPW (power supply on-off signal) which goes to high level when the main power supply is turned on and which goes to low level with the power supply being turned off may be supplied to the other input terminal of the AND circuit  28  from a control section (not shown). When any one of the input is held at low level, a switch  29  may be turned off. Since the power supply voltage Vcc may be supplied through this switch  29  to the relay drive circuits  21 D and  22 D, when the power supply off signal or the abnormal voltage is detected, the supply of the power supply voltage Vcc to the relays  21  and  22  may be interrupted. Therefore, the switches  21   a  and  22   a  may open their contacts to de-energize the main DC/DC converter  6 . 
     When the contact  22   a  of the relay  22  is closed due to the abnormal operation of the relay drive circuit  22 D or the like so that the voltage-doubler rectification may be effected even though the voltage of the AC power supply  1  is held at high level and the control signal CS 1  supplied to the relay drive circuit  22 D is held at low level, the voltage Vm obtained at the junction P 2  may increase much more. As a consequence, the output VDE 3  outputted from the voltage detection circuit  27  goes to low level so that the switch  29  may be opened to interrupt the supply of the power supply voltage Vcc to the relay drive circuit  22 D. Accordingly, since the state in which the voltage Vm is abnormally high can be canceled, and the main DC/DC converter  6  can be protected. 
     As described above, the voltage Vm as well as the voltage Vs may be supplied as the detection voltages and the full-wave rectification and the voltage-doubler rectification can be switched. Since this voltage Vm is obtained from the full-wave of the AC power supply  1  shown in FIG. 4A, even when the voltage of the AC power supply  1  is fluctuated suddenly, the full-wave rectification and the voltage-doubler rectification may be switched immediately in response to such fluctuation. For example, even when the voltage of the AC power supply  1  increases suddenly under the state that the voltage-doubler rectification is performed, the voltage-doubler rectification may be immediately switched to the full-wave rectification, thereby resulting in the main DC/DC converter  6  can be protected. 
     While the voltage detection circuits  23 ,  24 ,  27 , and the AND circuits  25 ,  28  can be arranged in the form of the hardware as described above, the present invention is not limited thereto, and they can be arranged in the form of software as shown in flowcharts of FIGS. 5 and 6. 
     Referring to FIG.  5  and following the start of operation, at a step  1 , there may be compared a value corresponding to the detection voltage Vm and a predetermined threshold voltage. At a step  2 , there may be compared a value corresponding to the detection voltage Vs and a predetermined threshold voltage. At a step  3 , if it is determined based on these compared results that any one of the values corresponding to the detection voltage Vm and the detection voltage Vs is higher than the predetermined threshold voltage, then the switch  21   a  may be turned on by driving the relay  21  and the full-wave rectification may be performed. If both of the values corresponding to the detection voltage Vm and the detection voltage Vs are lower than the threshold voltages, then the switch  22   a  may be turned on by driving the relay  22  and the voltage-doubler rectification may be performed. 
     Referring to FIG.  6  and following the start of operation, at a step  1 , there may be compared a value corresponding to the detection voltage Vm and a predetermined threshold voltage. At a step  2 , it may be determined whether or not the power supply on-off signal CPW is held at low level. If it is determined at a step  3  based on these compared results that the value corresponding to the detection voltage Vm is higher than the predetermined threshold voltage or that the power supply on-off signal CPW is held at low level, then the switch  29  may be turned off to interrupt the supply of power to the relays  21  and  22 , thereby resulting in the switches  21   a  and  22   a  being turned off. If it is determined that the value corresponding to the detection voltage Vm is lower than the predetermined threshold voltage or that the power supply on-off signal CPW is held at high level, then the switch  29  may be held in the on-state to thereby continue the supply of power to the relays  21  and  22 . 
     FIG. 7 shows an embodiment of an arrangement of the voltage detection circuit  23 . In FIG. 7, elements and parts identical to those of FIG. 2 are marked with the identical reference numerals. 
     As shown in FIG. 7, the resistor R 7  and the reference voltage generation shunt regulator IC 2  may be connected between the power supply terminal  51  and the ground terminal  52  in series. A series circuit of the resistors R 8  and R 9  may be connected to the shunt regulator IC 2  in parallel. Then, a voltage developed at the junction between the resistors R 8  and R 9  may be supplied to the shunt regulator IC 2 . The shunt regulator IC 2  may increase or decrease a current flowing thereto in such a manner that this voltage may become constant. Thus, a predetermined reference voltage Vrf may be obtained at the junction between the resistors R 7  and R 8 . Incidentally, the power supply voltage Vcc may be supplied to the power supply terminal  51  from the standby DC/DC converter  10 . 
     The resistors R 5  and R 6  may be connected between the detection voltage input terminal  53  and the ground terminal  52  in series. The voltage Vs may be supplied to the detection voltage input terminal  53  as the detection voltage. A junction between the resistors R 5  and R 6  may be connected through a parallel circuit of the diode D 3  and the resistor R 10  and the capacitor C 1  in series to the ground terminal  52 . A junction between this parallel circuit and the capacitor C 1  may be connected to the negative input terminal of the comparator IC 1 . 
     A junction between the resistors R 7  and R 8  may be connected through the resistor R 4  to the positive input terminal of the comparator IC 1 . Then, the output terminal of this comparator IC 1  may be connected through a series circuit of the diode D 2  and the resistor R 3  to the positive input terminal of the comparator IC 1 . The resistors R 3  and R 4  may give a hysteresis characteristic to the comparator IC 1 . Also, the diode D 2  may function as a reverse-current blocking element. 
     Assuming that Va is a voltage developed at the junction between the resistors R 5  and R 6 , then when the voltage of the AC power supply  1  is low (e.g. 100V), since the voltage Va supplied to the negative input terminal of the comparator IC 1  may be set so as to become lower than the voltage supplied to the positive input terminal of the comparator IC 1 , the output VDE 1  of the comparator IC 1  goes to high level. When on the other hand the voltage of the AC power supply  1  is high (e.g. 220V), since the voltage Va supplied to the negative input terminal of the comparator IC 1  is set so as to become higher than the voltage supplied to the positive input terminal of the comparator IC 1 , the output VDE 1  from the comparator IC 1  goes to low level. 
     Also, under the state in which the voltage of the AC power supply  1  is high, accordingly, the contact  21   a  of the relay  21  is turned on so that the full-wave rectification is effected, when a power failure of a short period of time such as a time t occurs in the AC power supply  1  as shown in FIG. 8A, the detection voltage Vs progressively decreases and then increases after the power failure is ended as shown in FIG.  8 B. 
     At that very moment, the voltage supplied to the negative input terminal of the comparator IC 1  is changed as shown in FIG.  8 C. When a power failure of a short period of time occurs at a time t 10 , the detection voltage Vs progressively decreases. Thus, at a time t 11 , even when this voltage becomes a voltage at which the full-wave rectification is switched to the voltage-doubler rectification, the voltage supplied to the negative input terminal of the comparator IC 1  drops slowly as compared with the voltage Vs being lowered. 
     As a consequence, even at a time t 12  in which the power failure of the short period of time is ended, the voltage inputted to the negative input terminal of the comparator IC 1  is made higher than the voltage supplied to the positive input terminal. Then, the voltage supplied to the negative input terminal of this comparator IC 1  may increase in accordance with the increase of the voltage Vs after the time tl 2 . Accordingly, in the voltage detection circuit  23  thus arranged, even when the power failure of short period of time occurs in the AC power supply  1 , the output VDE 1  outputted from the comparator IC 1  can be prevented from going to high level, and hence the full-wave rectification can be continued. Thus, even when the voltage of the AC power supply  1  is high and the power failure occurs, the voltage-doubler rectification can be prevented from being effected, thereby resulting the main DC/DC converter  6  can be protected. 
     Incidentally, the reason that the diode D 3  is provided in parallel to the resistor R 10  is that the capacitor C 1  should be charged quickly when the detection voltage Vs is supplied. 
     Also, the voltage detection circuits  24 ,  27  are arranged similarly to the voltage detection circuit  11  shown in FIG.  2  and the charge and discharge circuit comprising the resistor R 10 , the capacitor C 1  and the diode D 3  shown in FIG. 7 may be omitted. The reason for this will be described below. When the voltage of the AC power supply  1  is high (e.g. 220V), the output VDE  1  from the voltage detection circuit  23  is held at low level. Therefore, regardless of the level of the output VDE 2  from the voltage detection circuit  24 , the output signal from the AND circuit  25  remains low level so that the voltage-doubler rectification can be prevented from being performed. Since the charge and discharge circuit is not provided in the voltage detection circuit  23  as described above, a manufacturing cost can be decreased. Instead of the voltage detection circuit  23 , the voltage detection circuit  24  may include the above-mentioned charge and discharge circuit. 
     FIG. 9 shows an example of electronic equipment  150  including the power supply apparatus  100  shown in FIG. 1 or the power supply apparatus control method shown in FIGS. 5 and 6. When an on-off operation of a power supply switch  120  is detected by a control section  110  and the power supply on-off signal CPW is outputted to the power supply section  100 , the main power supply may be switched from the off-state to the on-state or switched from the on-state to the off-state. Instead of the power switch  120 , it is possible that a remote control signal may be received by the electronic equipment  150 . 
     Having described a preferred embodiment of the invention with reference to the accompanying drawings, it is to be understood that the invention is not limited to that precise embodiment and that various changes and modifications could be effected therein by one skilled in the art without departing from the spirit or scope of the invention as defined in the appended claims.