Abstract:
A power supply circuit includes a power converter and a control circuit. The power converter converts AC power to DC power, and the control circuit controls operation of the power converter. The control circuit repeatedly enables the conversion operation of the power converter for a first predetermined period of time and disables the conversion operation for a second predetermined period of time, greater than said first predetermined period of time, in a power saving mode of operation.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a power supply, in particular to a switched mode power supply (SMPS) and a method of controlling the same. 
     2. Description of the Background Art 
     In a conventional switched mode power supply (SMPS), when the electrical loads connected with an output terminal of the switched mode power supply do not draw electric power, the switched mode power supply assumes a standby state. Only is when the electric loads require power, the SMPS provides power to the electric loads, by operating in a normal mode. 
     FIG. 1 illustrates a circuit of a switched mode power supply which is composed of a main power unit, an auxiliary power unit and a controller. 
     The main power unit includes an input filter unit  101  removing noise from AC power supplied from an AC power source, a rectification/smoothing unit  102  rectifying the AC power to DC by a full wave rectification and smoothing the DC voltage to a certain level, a main transformer  104  to a primary winding of which is repeatedly supplied and disconnected DC power supplied from the rectification/smoothing unit  102  in accordance with an ON/OFF operation of a switching device Q 1 , a snubber  103  removing noise from power supplied to the primary winding of transformer  104 , the noise generated when the switching device Q 1  is operated, a rectification/smoothing unit  105  rectifying AC voltages induced in the secondary windings of the main transformer  104  and smoothing the resulting DC voltage to a certain level, a rectification/smoothing unit  105 - 1  generating a certain direct current voltage from the AC voltage induced in an auxiliary winding of the main transformer  104 , and a driving controller  106  receiving an output voltage of the rectification/smoothing unit  102  and the direct current voltage from the rectification/smoothing unit  105 - 1  and controlling an ON/OFF operation of the switching device Q 1  in accordance with a control signal from a signal feedback unit  107 , which will be explained later. 
     The auxiliary power unit includes a driving controller  111  receiving an output voltage of the rectification/smoothing unit  102  and controlling an ON/OFF operation of a switching device Q 2 , and an auxiliary transformer  108  to a primary winding of which is repeatedly supplied and disconnected the output voltage of the rectification/smoothing unit  102  in accordance with the ON/OFF operation of the switching device Q 2  thereby inducing an AC voltage in a secondary winding of the auxiliary transformer  108  which is connected to a rectification/smoothing unit  109  and a rectification/smoothing unit  109 - 1 . The rectification/smoothing unit  109 - 1  rectifies the AC voltage induced in the secondary winding of auxiliary transformer  108  and outputs a certain direct current voltage to the driving controller  111  in accordance with the switching operation of the switching device Q 2 . The rectification/smoothing unit  109  rectifies the voltage induced in the secondary winding of the auxiliary transformer  108  in accordance with a switching operation of the switching device Q 2  and smoothes the thusly rectified a direct current voltage. The auxiliary power unit also includes a snubber  110  removing noise which is generated in the auxiliary transformer  108  when the switching device Q 2  is operated. 
     The control unit includes a microcomputer  114  receiving as its supply power in a normal operating mode an output voltage among the output voltages outputted from the rectification/smoothing unit  105 , and also receiving an output voltage of the rectification unit  109  as a supply voltage in a power saving mode. The microcomputer  114  outputs a control signal for controlling an ON/OFF operation of the switching device Q 1  in the normal mode. The control unit also includes a signal feed back unit  107  detecting information on the state of the power consumption of loads connected to the output terminals of the rectification smoothing unit  105  receiving a control signal from the microcomputer  114   10  and transferring the information and the control signal to the driving controller  106 , a timer  112  generating a timing signal and supplying the thusly generated timing signal to the microcomputer  114 , and a backup unit  113  receiving a direct current voltage from the rectification/smoothing unit  105  and also from the rectification/smoothing unit  109  and storing backup power for operating of the microcomputer  114  in the standby mode. 
     As shown in FIG. 2 illustrating in detail the circuit of FIG. 1, the rectification/smoothing units  105 ,  105 - 1 ,  109  and  109 - 1  are circuits each formed of a diode D 1 -DM, DD 5 -DD 6  and a capacitor C 1 -CM, CC 5 -CC 6 . The rectification/smoothing unit  109  further includes a regulator  109 - 2  and a diode DD 7 . 
     The operation of the thusly constituted conventional switched mode power supply will be explained. 
     When AC mains power is applied into the input filter unit  101 , the input filter unit  101  removes noise from the AC mains power and outputs it to the rectification/smoothing unit  102 . The rectification/smoothing unit  102  rectifies the noise-removed AC power by a full wave rectification, smoothes it, and then outputs a certain level direct current voltage to the primary windings of the transformers  104  and  108 . 
     At this time, as the driving controller  106  in the main power unit turns on/off the switching device Q 1  on/off, AC voltages are induced in the secondary windings of the main transformer  104  and are applied to the rectification/smoothing unit  105 . As the driving controller  111  in the auxiliary power unit turns the switching device Q 2  on/off, an AC voltage is induced in the secondary winding of the auxiliary transformer  108  and is applied to the rectification/smoothing unit  109 . 
     The rectification/smoothing unit  105  outputs the DC voltages which are smoothed after a half wave rectification to the loads (not shown), and the rectification/smoothing unit  109  outputs a regulated voltage, which is smoothed after a half wave rectification through the regulator circuit  109 - 2  and the diode DD 7 . At this time, the regulated voltage outputted from the rectification/smoothing unit  109  is supplied to the back-up unit  113  and the microcomputer  114  via a line connected with diode DD 7 . 
     In the normal mode in which the loads normally consume power, the rectification/smoothing unit  105  outputs a plurality of output voltages, and if one of the output voltages is applied as a supply voltage of the microcomputer  114 , a positive voltage is applied at the output terminal of the diode DD 7  connected with the output terminal of the rectification/smoothing unit  109 , so that the diode DD 7  does not forwardly conduct. Therefore, the regulated voltage outputted from the rectification/smoothing unit  109  is not applied to the microcomputer  114 . The microcomputer  114  receives a supply voltage from the rectification/smoothing unit  105  and controls the entire operations of the system based on a timing signal generated by the timer  112  for thereby controlling the power state of the loads. 
     When the microcomputer  114  checks the power consumption state of the loads, if it is judged that the loads do not consume much power, the microcomputer  114  controls the system and changes the normal mode into a power saving mode. 
     In the power saving mode, the microcomputer  114  outputs an ON/OFF control signal which controls the switching device Q 1  at a low frequency by controlling the driving controller  106  via the signal feedback unit  107  so that power corresponding to the lower power consumption of the loads is outputted. Namely, when the microcomputer  114  increases the ON/OFF operation time of the switching device Q 1 , the levels of the voltages induced in the secondary windings of the main transformer  104  are decreased. Therefore, the rectification smoothing unit  105  which receives the lower voltage levels supplies DC voltages corresponding to the power consumed by the loads. 
     If the microcomputer  114  judges that the loads do not consume power, the operation mode of the switched mode power supply is changed to a standby mode. In the standby mode, the microcomputer  114  controls the driving controller  106  via the signal feed back unit  107 , so that the switching device Ql is not operated, and the inductive voltages are not generated in the secondary windings of the main transformer  104 . Therefore, the rectification/smoothing unit  105  does not output any voltages. In the standby mode, only the auxiliary power supply circuit continues to operates. When the power supply assumes a standby mode in accordance with the control of the microcomputer  114 , the regulated voltage outputted from the rectification/smoothing unit  109  of the auxiliary power supply circuit is applied as a supply voltage to the microcomputer  114  via the diode DD 7 . Here, when the power supply operates in the normal mode or the power saving mode, the backup unit  113  receives an output voltage from the rectification smoothing unit  105  or  109  and charges to a certain voltage. Even in the standby mode, if the power is OFF, the charged voltage is supplied to the microcomputer  114 , so that the microcomputer  114  continuously operates. 
     In the conventional switched mode power supply, there are provided both the main power supply circuit which supplies electric power to the loads in the normal mode, and the auxiliary power supply circuit which supplies a minimum power when the system operates in the standby mode in which the loads do not consume power. Therefore, the system becomes bulky and complicated. In the normal mode in which the loads normally consume power, since the main power supply circuit and the auxiliary power supply circuit are both operated at the same time, excessive noise is generated at the switching devices Q 1  and Q 2 . In addition, since the switching devices Q 1  and Q 2  are continuously operated, as time elapses, the reliability of the switching devices Q 1  and Q 2  is decreased, so that the power consumption of the switched mode power supply is increased. Therefore, the switched mode power supply in accordance with the conventional art is not widely used for an electric appliance. Namely, the above-described switched mode power supply is used only for an expensive apparatus. 
     As another example of a conventional switched mode power supply, there is known a power supply in which the power outputted from the secondary winding of a transformer is controlled in the standby mode. 
     However, the above-described example also becomes complicated, since the system must include a higher number of circuit parts. Additionally the system is bulky, the reliability of the system is decreased, and also the fabrication cost is increased. 
     In a switched mode power supply for turning off the unnecessary power supply circuit in a standby mode, there is a limit on decreasing the power consumption since the switched mode power supply which uses power outputted from the auxiliary power supply circuit in the standby mode must necessarily include a certain number of parts. Therefore, the power supply becomes complicated, and the reliability of the circuit is also decreased. 
     A power supply capable of disconnecting from the AC power source by using a mechanical switch, in the view of decreasing the power consumption, offers advantages. However, since the AC power source turn on/off switch must be operated mechanically whenever the loads require power, the power supply is very inconvenient to use. In addition, when the AC power source on/off switch is turned off, for example, an apparatus such as a VCR can not record a reserved TV program or can not perform other functions. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a switched mode power supply which makes it possible to decrease a power consumption by supplying electric power for a certain time during a recurring period in a standby mode. 
     It is another object of the present invention to provide a switched mode power supply which is capable of quickly supplying electric power in accordance with a power turn-on instruction signal which is inputted by a user in a standby mode. 
     It is a further object of the present invention to provide a controlling method for a switched mode power supply which is capable of decreasing power consumption by supplying power for a certain time during a certain period in a standby mode. 
     In order to achieve the above objects, there is provided a switched mode power supply according to a first embodiment of the present invention which comprises a driving controller for controlling a driving operation of the power supply circuit, a signal feedback device for repeatedly stopping the operation of the driving controller for a certain time tOFF during a certain period tON+tOFF in the standby mode, and a controller for controlling the operation of the driving controller via the signal feedback device so that the operation of the power supply is repeatedly stopped for the certain time tOFF during the recurring period tON+tOFF when the standby mode is set, whereby the power consumption in the standby mode is minimized. 
     In a second embodiment of the present invention, the signal feedback device comprises a first signal feedback unit receiving a level of the output voltage in a secondary winding of a transformer of the power supply circuit and controlling the driving controller in accordance therewith, whereby out voltages of the power supply circuit are maintained to be a constant voltage level when loads connected with output windings of the power supply circuit consume power in an active state of the SMPS, and a second signal feedback unit for stopping the operation of the driving controller in order to stop the power generation by the power supply circuit for the certain time tOFF in accordance with a control of the controller in the standby mode. 
     In a third embodiment of the present invention, the switched mode power supply further comprises a switching unit connected with output windings of the power supply circuit for disconnecting loads connected with output windings of the power supply circuit from the power supply circuit in accordance with a control of the controller. 
     In a fourth embodiment of the present invention, the signal feedback device comprises, a first switching unit for controlling the operation of the driving controller, a second switching unit for transferring a control signal from the controller to the first switching unit, a first signal feedback unit for receiving a level of the output voltages in a secondary winding of a transformer of the power supply circuit and controlling the driving controller in accordance therewith, whereby out voltages of the power supply circuit are maintained to be a constant voltage level, and a second signal feedback unit for transferring a control signal to the second switching unit in accordance with a control of the controller. 
     In a fifth embodiment of the present invention, the signal feedback device comprises a third switching unit and a fourth switching unit for controlling the operation of the first switching unit, a first signal feedback unit for receiving a level of the output voltages in a secondary winding of a transformer of the power supply circuit and controlling the driving controller in accordance therewith, whereby out voltages of the power supply circuit are maintained to be a constant voltage level, and a second signal feedback unit for transferring a control signal to the third switching unit in accordance with a control of the controller. 
     In a sixth embodiment of the present invention, there provided a power supply which generates a plurality of DC supply voltages from AC mains power, the power consumption decreasing apparatus for the power supply comprises a controller for controlling an operation of the power supply and generating a control signal for repeatedly disconnecting the power supply from AC mains power for a certain time tOFF during a certain period tON+tOFF in a standby mode. A signal feed back device for controls the supply of AC mains power to the power supply and the disconnects of AC mains power therefrom in accordance with the control signal from the controller. 
     In a seventh embodiment of the present invention, the signal feed back unit comprises a first signal feed back unit for receiving a level of the output voltages in a secondary winding of a main transformer of the power supply circuit and controlling the driving controller in accordance therewith, whereby out voltages of the power supply circuit are maintained to be a constant voltage level, and for activating the driving controller when an output voltage of the power supply circuit is an active state of SMPS, a second signal feed back unit for generating an output signal for controlling the supplying of AC mains power to the power supply to stop a power generation of the power supply for the certain time tOFF in accordance with the control signal from the controller in the standby mode, and a switching unit for supplying AC mains power to the power supply and disconnecting the AC mains power therefrom in accordance with the output signal of the second signal feedback unit. 
     In a eighth embodiment of the present invention, a method for decreasing a power consumption for a switched mode power supply comprises a first step for judging whether a standby mode is set and a second step for periodically outputting power to the windings of the power supply only during a certain interval tON of a certain period interval tON+tOFF when the standby mode is judged set as a result of the judgement of the first step. 
     Additional advantages, objects and features of the invention will become more apparent from the description which follows. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, wherein like reference numerals designate like components or elements to eliminate redundant description, and wherein: 
     FIG. 1 is a schematic block diagram illustrating a switched mode power supply according to the conventional art; 
     FIG. 2 is a detailed schematic circuit diagram of the switched mode power supply of FIG. 1; 
     FIG. 3 is a schematic block diagram illustrating a switched mode power supply implementing a reduced power consumption method according to a first embodiment of the present invention; 
     FIG. 4 is a detailed schematic circuit diagram of the switched mode power supply of FIG. 3; 
     FIGS. 5A through 5C are wave form diagrams illustrating respective control signals which control the switched mode power supply according to the present invention in a standby mode; 
     FIG. 6 is a schematic circuit diagram illustrating a switched mode power supply implementing a reduced power consumption method according to a second embodiment of the present invention; 
     FIG. 7 is a schematic circuit diagram illustrating a switched mode power supply implementing a reduced power consumption method according to a third embodiment of the present invention; 
     FIG. 8 is a schematic circuit diagram illustrating a switched mode power supply implementing a reduced power consumption method according to a fourth embodiment of the present invention; and 
     FIG. 9 is a flow chart illustrating a power consumption decreasing method for a switched mode power supply according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The embodiments of a switched mode power supply implementing a reduced power consumption according to the present invention will be explained with reference to the accompanying drawings. 
     FIG. 3 illustrates a first embodiment of a switched mode power supply implementing a reduced power consumption method according to the present invention which is composed of a main power unit and a controller. 
     The main power unit includes an input filter unit  201  removing noise from mains power supplied from an AC power source, a rectification/smoothing unit  202  receiving noise-removed AC power from the input filter unit  201 , rectifying it by a full wave rectification and smoothing the same and outputting a DC voltage, a main transformer  204  to a primary winding of which is supplied the DC voltage from the rectification/smoothing unit  202  in accordance with an ON/OFF operation of a switching device Q 11  controlling the flow of current through the primary winding of the transformer  204 , a rectification/smoothing unit  205  rectifying AC voltages induced in secondary windings of the main transformer  204  and outputting a plurality of DC voltages, a rectification/smoothing unit  205 - 1  rectifying an AC voltage induced in an auxiliary winding of the main transformer  204  in accordance with an ON/OFF operation of the switching device Q 11  and outputting a DC voltage, and a driving controller  206  receiving output voltages outputted from the rectification/smoothing unit  202  and the rectification/smoothing unit  205 - 1  and controlling the switching device Q 11  in accordance with control signals outputted from signal feedback units  207  and  208 , which will be described later. 
     The control unit includes a microcomputer  210  receiving as its supply voltage one output voltage among the plurality of output voltages outputted from the rectification/smoothing unit  205  and controlling the system, a backup unit  211  charging based on a voltage outputted from the rectification/smoothing unit  205  and supplying power to the microcomputer  210  in a standby mode, a timer  209  generating a timing signal for the microcomputer  210 , a signal feedback unit  207  sensing the level of the output voltage received from the rectification/smoothing unit  205  and transferring the voltage to the driving controller  206  so that the driving controller  206  controls the switching device Q 11  to operate the transformer  204 , and a signal feedback unit  208  receiving a control signal from the microcomputer  210  and transferring it to the driving controller  206  in the standby mode. 
     FIG. 4 is a detailed schematic circuit diagram of the switched mode power supply of FIG.  3 . As shown, the power supply which further includes a switching unit  212 , which receives the plurality of output voltages from the rectification/smoothing unit  205  and outputs respective voltages Vol, Vo 2 , . . . , VoN in accordance with a control of the microcomputer  210 . The signal feedback unit  208  includes a photo-coupler PC 1  the receiver of which is supplied with DC power from the rectification/smoothing unit  202  the output of which is connected to ground via a resistor R 14 , and which outputs a certain DC voltage in accordance with a control signal from the microcomputer  210  applied at an emitter thereof. The signal feedback unit  208  also includes and a switching device Q 12  outputting a control signal for activating or deactivating the driving controller  206  in the standby mode in accordance with the DC voltage outputted from the photo-coupler PC 1  via a resistor R 13 . In addition, the backup unit  211  includes a diode D 15  and a capacitor C 16 , which charges based on an output voltage from the rectification/smoothing unit  205 . 
     As further shown in FIG. 4, a resistor R 11  connects the rectifier  202  to the driving controller  206 , a capacitor C 15  connects the output of the signal feedback unit  208  to ground, and the primary winding and each of the secondary windings of the rectification/smoothing unit  205  are connected to a diode DD 11 -DDN and a capacitor CC 11 -CCN. 
     The operation of the first embodiment of the switched mode power supply according to the present invention will be explained as follows. 
     In the normal mode in which loads (not shown) normally consume power, the input filter unit  201  receives the mains AC voltage from the AC power source, removes noise from power supplied from the AC power source and outputs the noise-removed AC voltage to the rectification/smoothing unit  202 . The rectification/smoothing unit  202  rectifies the noise-removed AC power by a full wave rectification and smoothes and outputs a DC voltage to the primary winding of the main transformer  204  and the driving controller  206 . The driving controller  206  turns the switching device Q 11  on/off, whereby AC voltages are induced in the secondary windings of the main transformer  204 . The thusly induced AC voltages are applied to the rectification/smoothing unit  205 . The rectification/smoothing unit  205  rectifies the induced AC voltages by a half wave rectification and smoothes and outputs the resultant DC voltages to the switching unit  212 . The switching unit  212 , which is controlled by the microcomputer  210 , outputs the plurality of DC voltages Vo 1 , Vo 2 , . . . , VoN to the loads(not shown) connected with its output terminals. 
     At this time, the backup unit  211  charges based on an output voltage outputted from the rectification/smoothing unit  205 . 
     In the normal mode in which the loads consume power normally, when a user actuates a power saving mode key and thereby a signal as shown in FIG. 5A is generated to the microcomputer  210  or when the loads do not consume power for a certain time, the microcomputer  210  outputs a control signal via the resistor R 12  so that the photo-coupler PC 1  of the signal feedback unit  208  operates. Namely, as shown in FIG. 5B, when the light emitting diode PD 1  emits light for the time tOFF, for example 19.5 s, in accordance with the control signal outputted from the microcomputer  210  and the photo-transistor PT 1  is thereby turned on, a voltage developed across the resistor R 11  and the resistor R 14  is applied to the base of the switching device Q 12  via the resistor R 13 , so that an input port of the driving controller  206  assumes a ground level (or low level) and deactivates the operation of the switching device Q 11 . Therefore, no current flows through the primary winding of the main transformer  204 . When the switching operation of the DC from rectification/smoothing unit  202  to the main transformer  204  is stopped, the backup unit  211  supplies power, previously charged in the backup capacitor C 16  during the normal mode, to the microcomputer  210 . Therefore, the microcomputer  210  maintains a standby mode. The backup capacitor C 16  discharges continuously as time elapses, and after the time tOFF, as shown in FIG. 5B, is elapsed, the microcomputer  210  outputs a low level control signal to the feedback unit  208  for a set time tON, for example 0.5 s, so that the photo-coupler PC 1  of the signal feed back unit  208  is not operated. Therefore, the switching device Q 12  which operates in accordance with the operation of the photo-coupler PC 1  is turned off. Because the input port of the driving controller  206  is no longer held at a low (ground) level when the switching device Q 12  is turned off, the driving controller  206  is reactivated. As shown in FIG. 5C, when a train of high level pulse signals is outputted from the driving controller  206  to the switching device Q 11 , the switching device Q 11  is repetitively turned on and off. When the switching device Q 11  is repeatedly turned on and off, AC voltages are induced in the secondary windings of the main transformer  204 . The rectification/smoothing unit  205 , which receives the induced AC voltages outputs a plurality of DC output voltages. The backup capacitor C 16  of the backup unit  211  receives and charges based on one of the output voltages from among the DC voltages outputted from the rectification/smoothing unit  205 . 
     Next, after the certain charging time tON has elapsed, the microcomputer  210  again outputs the high level control signal to the signal feedback unit  208  for thereby operating the photo-coupler PC 1 , so that the switching device Q 12  is turned on and a ground level is maintained at the input port of the driving controller  206 . Thus no high level pulses are applied to the switching device Q 11 , and the switching device Q 11  does not operate. 
     Next, after the set time tOFF has elapsed, the microcomputer  210  brings the control signal low and thereby stops the operation of the photo-coupler PC 1  of the signal feedback unit  208 , so that driving controller  206  once again operates the switching device Q 11  and power is supplied so that the backup capacitor C 16  of the backup unit  211  is again charged. 
     In the standby mode, the microcomputer  210  controls the signal feed back unit  208  and controls the ON/OFF operation of the driving controller  206  to thereby control the operation of the switching device Q 11 . The AC voltages induced in the secondary windings of the main transformer  204  are outputted to the rectification/smoothing unit  205 , and a charging and discharging operation occurs at the backup capacitor C 16  of the backup unit  211 . FIG. 5C illustrates the turning ON/OFF periods of the switching device Q 11 , that is, of the driving controller  206 , which correspond to the periods tON/tOFF in FIG.  5 B. 
     In the standby mode, when a user actuates a power turn-on key so that a power on key signal is sent to the microcomputer  210 , the microcomputer  210  controls the power supply to escape from the standby mode and once again supply power to the loads. 
     In the case of inputting the power turn-on key signal: The microcomputer  210  outputs the control signal to the signal feedback unit  208  which turns off the photo-coupler PC 1 . The switching device Q 11  is changed from the turned off state to the turned on state, so that the current is switched through the transformer  204  whereby the rectification/smoothing unit  205  supplies power to the loads. 
     In the standby mode, when the power turn on key signal is inputted, the microcomputer  210  operates the timer  209  and judges whether the inputted key signal is a signal set to supply the power to the loads. When the inputted key signal corresponds to the power turn on key signal, the switching device Q 11  is operated for supplying power to the loads, and otherwise the system is maintained in the standby mode. 
     In the above description, even though the operation of the SMPS in accordance with the present invention is exemplified by the intervals of tOFF and tON as 19.5 ms and 0.5 ms in the standby mode respectively, the duty rate (tON/(tOFF+tON)) can be less. 
     FIG. 6 illustrates a second embodiment of the present invention which includes a main power unit and a control unit. 
     The main power unit includes a rectification/smoothing unit  202 , a transformer  204  transferring power from the primary winding to the secondary winding in accordance with a switching operation of a switching device Qll, a rectification/smoothing unit  205 - 1  receiving an AC voltage outputted from the auxiliary winding of the transformer  204  and outputting a certain DC voltage, a rectification/smoothing unit  205  receiving AC voltages induced in the secondary windings of the transformer  204  and outputting a plurality of DC voltages, and a driving controller  206  receiving a control signal from a controller which will be described later and controlling the switching device Q 11 . 
     The control unit includes a microcomputer  210  controlling the system, a backup unit  211  supplying power to the microcomputer  210  in the standby mode, a timer  209  generating a timing signal to the microcomputer  210 , signal feedback units  207  and  215  operated in accordance with a control of the microcomputer  210 , a switching device Q 22  turning the driving controller  206  on/off, and a switching device Q 21  repeatedly changing the electrical potential of a control electrode of the switching device Q 22  (which is connected between the rectifier  202  and the driving controller  206 ) as between the high and low levels in accordance with a control signal from the signal feedback unit  215  in the standby state. A resistor R 22  is connected between the base and emitter of switching device Q 22 , and a resistor R 23  connects the switching devices Q 22  and Q 21 . 
     The operation of the second embodiment according to the present invention will be explained with reference to the accompanying drawings. 
     In the normal mode, when the signal feedback unit  215 , which is operated in accordance with a control of the microcomputer  210 , turns on the switching device Q 21 , the electrical potential of the base of the switching device Q 22  becomes a low state (i.e., ground level), and the switching device Q 22  is thereby turned on. Therefore, the output voltage of the rectification/smoothing unit  202  is applied to the driving controller  206  via the switching device Q 22 . As the driving controller  206  repeatedly turns the switching device Q 11  on/off, AC voltages are induced in the secondary windings of the main transformer  204 , so that the rectification/smoothing i 15  unit  205  outputs a plurality of DC voltages each having a certain level. 
     One voltage among a plurality of DC voltages outputted from the rectification/smoothing unit  205  is provided as a supply voltage of the microcomputer  210 . The signal feedback unit  207  which also receives the above-described supply voltage maintains the operation of the driving controller  206  in an active state in the case that the supply voltage is a certain level, and the backup unit  211  charges based on an electric charge into a capacitor using the supply voltage. 
     If there is an external signal which corresponds to a standby mode setting key actuated by a user or if the loads do not consume power, the microcomputer  210  outputs a control signal to the signal feedback unit  215  so that the driving voltage supplied from the rectification/smoothing unit  202  to the driving controller  206  is repeatedly supplied and disconnected as switching devices Q 21 , Q 22  are turned on/off. When the switching device Q 21  maintains a turned-off state for a certain time in accordance with a control signal from the signal feedback unit  215 , since the electric potential of the control electrode of the switching device Q 22  maintains a floating state, the switching device Q 22  assumes a turned-off state (i.e., Q 22  is biased OFF). Therefore, when the supply voltage to the driving controller  206  is disconnected, the driving controller  206  does not turn the switching device Q 11  on/off. Therefore, since the AC voltages are not induced in the secondary windings of the main transformer  204 , the rectification/smoothing unit  205  does not output any voltages. 
     The signal feedback unit  207  senses the level of the output voltage rectified and smoothed from one secondary winding of the main transformer  204  and applies the voltage to the driving controller  206  so that the driving controller  206  controls the switching operation of switching device Q 11 . When no output voltages are outputted from the rectification/smoothing unit  205 , the microcomputer  210  receives a supply voltage from the backup unit  211 . When the backup unit  211  supplies power to the microcomputer  210  for a certain time which is the discharging time of the backup capacitor, the microcomputer  210  outputs a control signal to the signal feedback unit  215  so that the switching device Q 21  maintains a turned-on state for a certain time. 
     When the switching device Q 21  is turned on, a low level signal is applied the base of the transistor composing switching device Q 22 , and a DC voltage is inputted from the rectification/smoothing unit  202  to the driving controller  206  via the switching device Q 22 . The driving controller  206  repetitively turns on and off the switching device Q 11  and induces voltages in the secondary windings of the transformer  204 . The rectification/smoothing unit  205 , which receives the induced voltages outputs a plurality of DC voltages, each having a certain level. The signal feedback unit  207  which receives a DC voltage from the rectification/smoothing unit  205  changes the operation state of the driving controller  206  to an active state in accordance with a control of the microcomputer  210 , which receives the DC voltage. The backup unit  211  receives the DC voltage and charges the capacitor based thereon. 
     In the standby mode, the microcomputer  210  controls the signal feed back unit  215  and repeatedly turns the switching devices Q 21  on/off to Q 22  and thereby activate/deactivate the driving controller  206  and repeatedly turn the switching device Q 11  on/off accordingly power is supplied only for a period of time needed to charge the backup capacitor of the backup unit  211 , thereby minimizing the power consumption of the switched mode power supply. 
     FIG. 7 illustrates a third embodiment of the present invention which is composed of a main power unit and a control unit. 
     The main power unit includes a rectification/smoothing unit  202 , a transformer  204  performing a transforming operation of the DC voltage from rectification/smoothing unit  202  based on the operation of a switching device Q 11 , and a rectification/smoothing unit  205  receiving AC voltages induced at the secondary side of the transformer  204  and generating a plurality of DC voltages. 
     The control unit includes a microcomputer  210  controlling the system and receiving a DC supply voltage from the rectification smoothing unit  205 , a backup unit  211  receiving the DC supply voltage and supplying backup power to the microcomputer  210  in the standby mode, a timer  209  generating a timing signal to the microcomputer  210 , a photo-coupler  216  receiving a DC voltage from the rectification/smoothing unit  205  and accordingly outputting a high level voltage, a switching device Q 31  which is turned on by an output voltage of the photo-coupler  216  and is turned off when the current flow direction is changed at the transformer  204 , thereby turning ON/OFF the switching device Q 1 , a photo-coupler  217  supplying and disconnecting the DC voltage from the rectification/smoothing unit  202  for a certain time in response to a control signal from the microcomputer  210  in the standby mode, and a switching device Q 32  turning on the switching device when the photo-coupler  217  is operated for a certain time in the standby mode, and turning off when the photo-coupler  217  is not operated for a certain time. 
     As shown in FIG. 7, a pair of resistors R 31  and R 32  are connected between the output of the rectifier  202  and a common node. The base of the switching device Q 11  is connected to the common node, the switching devices Q 31  and Q 32  are connected between the common node and ground, and the series connection of a resistor R 33 , a diode DD 11 , a resistor R 36  and a diode D 31  are connected between the photo-coupler  216  and the common node. A capacitor CC 11  and a resistor R 35  are connected between the resistor R 33  and the photo-coupler  216 . An auxiliary primary winding of the transformer  204  is connected to the resistors R 35  and R 36 . 
     As further shown, a resistor R 34  connects the switching device Q 11  to ground, a capacitor C 32  is connected between the resistor R 34  and the common node, and a diode D 32  is connected between the resistor R 34  and the base of the switching device Q 31 . A capacitor C 31  is connected between ground and the base of the switching device Q 31 , and the base of the switching device  31  is also connected to the photo-coupler  216 . Furthermore, resistors R 37  and R 38  connect the base of the switching device Q 32  to ground, and the connection between resistors R 37  and R 38  is connected to the photo-coupler  217 . The photo-coupler  217  is also connected to the connection between resistors R 31  and R 32 . Both photo-coupler  216  and  217  are connected to the microcomputer  210  via resistors R 40  and R 39 , respectively. 
     The operation of the third embodiment of the present invention will be explained. 
     In the normal mode, when the microcomputer  210  outputs a low level control signal to the photo-coupler  217 , the photo-coupler  217  is not operated, and the switching device Q 32  maintains a turned-off state. As a result so that the switching device  031  assumes an ON/OFF state in accordance with the control signal from the feedback unit  216 . 
     At this time, a certain level DC voltage outputted from the rectification/smoothing unit  202  is applied to the base terminal of the switching device Q 11  via the resistors R 31  and R 32 , and the switching device Q 11  is turned on. 
     At this time, voltages are induced in the secondary windings based on the current flowing in the primary winding of the transformer  204 . 
     The rectification/smoothing unit  205  rectifies the voltages induced in the secondary windings of the transformer  204  and outputs a plurality of DC voltages each having a certain level, and the photo-coupler  216  receives the supply voltage applied to the microcomputer  210  and outputs a certain level voltage to the base terminal of the switching device Q 31 . In this state, since the switching device Q 31  is turned on, the electric potential of the base terminal of the switching device Q 11  becomes a ground electrical potential, so that the direction of the current flowing in the primary winding of the transformer  204  is reversed. Therefore, the direction induced voltages generated in the secondary windings of the transformer  204  are reserved. 
     Since the electrical potential of the base terminal of the switching device Q 31  becomes connected to ground voltage by an output voltage outputted from the auxiliary winding of the transformer  204  via resistor R 36 , the electrical potential at the base terminal of the switching device Q 11  becomes a high level. As the turning ON/OFF operation of the switching device Q 31  is repeatedly performed, the switching device Q 11  is repeatedly turned ON/OFF, so that voltages are induced in the secondary windings of the transformer  204 . The thusly induced AC voltage are rectified and smoothed into a plurality of DC voltages each having a certain level and respectively outputted to each load. 
     If there is no external input signal for a certain time in a state that the user operates a setting key for the standby mode (power consumption mode) or the loads are not normally operated, the microcomputer  210  outputs a control signal to the photo-coupler  217 , and the switching device Q 32  is turned on, and the electrical potential of the base terminal of the switching device Q 11  becomes a ground electric potential. When the base terminal of the switching device Q 11  is connected with the ground, no current is switched through the primary winding of the transformer  204 . In this state, the microcomputer  210  receives power from the backup unit  211 . 
     After a certain time, which is the discharging time of the backup capacitor, has elapsed, the microcomputer  210  outputs a low level ON control signal, so that the photo-coupler  217  is turned off. If there is no output from the photo-coupler  217 , the switching device Q 11  is turned on, and current flows through the primary winding of the transformer  204 , and AC voltages are induced in the secondary windings. 
     The rectification/smoothing unit  205  rectifies and smoothes the AC voltages induced in the secondary windings of the transformer  204  and outputs a plurality of DC voltages. 
     When there is an output voltage at the rectification/smoothing unit  205 , the photo-coupler  216  applies a certain level voltage to the base terminal of the switching device Q 31 . The switching device Q 31  is turned on, and the base terminal of the switching device Q 11  becomes a ground electrical potential. The flowing direction of the current in the primary winding of the transformer  204  is reversed, and the flowing direction of the currents in the secondary windings is reversed, so that AC voltages are induced. 
     The electrical potential of the base terminal of the switching device Q 31  is lowered to ground level via the auxiliary winding of the transformer  204 , so that the electrical potential of the base terminal of the switching device Q 11  becomes a high level. As the switching device Q 31  is repeatedly turned ON/OFF, the switching device Q 11  is repeatedly turned ON/OFF, so that AC voltages are induced in the secondary windings of the transformer  204 , and the thusly induced AC voltages are rectified and smoothed into a plurality of DC voltages each having a certain level. One DC voltage among the plurality of DC voltages is supplied as a supply voltage to the microcomputer  210 , and the backup capacitor of the backup unit  211  is charged thereby. 
     As time elapses, the backup capacitor of backup unit  211  is discharged, and the microcomputer  210  outputs a high level control signal and operates the photo-coupler  217 , so that a high level voltage is applied to the base terminal of the switching device Q 32 . 
     At this time, as the switching device Q 32  is turned on, the electrical potential at the base terminal of the switching device Q 11  becomes a ground electrical potential, so that the switching device Q 11  maintains a turned off state, so that the switching operation of current through the transformer  204  is stopped. 
     Therefore, the operation in which the photo-coupler  217  is operated for a certain time(discharging time of the backup capacitor) and then is stopped, with This operation being repeatedly performed so that the charging and discharging operation is repeatedly performed for the backup capacitor in the backup unit  211 . In the standby mode, the power which is used by the power supply is minimized. 
     FIG. 8 illustrates a fourth embodiment of the present invention which is formed of a main power unit and a controller. 
     The main power unit includes an input filter unit  201  receiving AC mains power, a rectification/smoothing unit  202 , a transformer  204  to a primary winding of which is supplied a DC voltage from rectification/smoothing unit  202  in accordance with a switching operation of a switching device Qll, a rectification/smoothing unit  205 - 1  generating a certain DC voltage from the AC voltage induced in an auxiliary winding of transformer  204 , a rectification/smoothing unit  205  generating plural DC voltages from the AC voltages induced in multiple secondary windings of the transformer  204 , and a driving controller  206  receiving the output voltages of the rectification/smoothing unit  202  and the rectification/smoothing unit  205 - 1  and receiving an output signal of a signal feedback unit  207 , which will be explained later, for thereby controlling the switching device Q 11 . 
     The controller includes a switch unit  213  connected with the AC mains power source, and an input terminal of the input filter unit  201  and, via a resistor R 15 , the output of the rectifier  202 . This switch unit  213  repeatedly performs an AC power supplying and disconnecting operation. The computer further includes a microcomputer  210  controlling the system and judging the standby mode, a backup unit  211  supplying backup power to the microcomputer  210  in the standby mode, a timer  209  generating a timing signal to the microcomputer  210 , a signal feedback unit  207  controlling the driving controller  206  in accordance with a control of the microcomputer  210 , and a signal feedback unit  214  repeatedly performing a turning ON/OFF operation of the switch unit  213  for a certain time in accordance with a control of the microcomputer  210 . Here, the switch unit  213  checks the output voltage of the rectification/smoothing unit  202 . If there is an error, the switch unit  213  is turned off. 
     The operation of the fourth embodiment according to the present invention will be explained. 
     In a normal state where the loads normally consume power, in the case that the power supply is not being normally operated, the microcomputer  210  controls the signal feedback unit  214  and maintains the turned-on state of the switch unit  213 . Therefore, when the switch unit  213  is in the turned-on state, noise is removed from the AC mains power by the input filter unit  201  and the thusly noise-removed AC voltage is changed to a certain level DC voltage by the rectification/smoothing unit  202  and is applied to the transformer  204  and the driving controller  206 . 
     As the driving controller  206  turns the switching device Q 11  on/off AC voltages are induced in the secondary windings of the transformer  204 . The thusly induced voltages are rectified and smoothed by the rectification smoothing unit  205  and are outputted as a plurality of DC voltages each having a certain level. 
     One DC voltage among the plurality of DC voltages outputted from the rectification/smoothing unit  205  is supplied as a supply voltage of the microcomputer  210 , and the signal feedback unit  207 , which checks the supply voltage, maintains the operation of the driving controller  206  in an active state in the case that the supply voltage is a certain level voltage. The backup unit  211  receives the voltage supplied to the microcomputer  210  and charges the backup capacitor of the backup unit  211 . 
     In the normal state that the switched mode power supply normally operates, if there is no external input signal for a certain time in a state that the user inputs an external input signal for setting the standby mode or the loads are not normally operated, the microcomputer  210  outputs an ON/OFF control signal to the signal feedback unit  214 . This causes the switch unit  213  connected, with one input terminal of the input filter unit  201 , to repeatedly turn ON/OFF for a certain time. Therefore, the DC voltage is not outputted from the rectification/smoothing unit  202  in the case that the switch unit  213  is turned off for a certain time in accordance with the control of the microcomputer  210 , so that the driving controller  206  does not operate the switching device Q 11 . If the switching device Q 11  is not operated, since no AC voltages are induced in the secondary windings of the transformer  204 , the microcomputer  210  receives a supply voltage from the backup unit  211 . In addition, the signal feedback unit  207  renders the driving controller  206  to an inactive state since a voltage is not supplied from the rectification/smoothing unit  205  to the microcomputer  210 . 
     As the time elapses, if the voltage charged in the backup unit  211  is discharged to below a certain level, the microcomputer  210  controls the signal feedback unit  214 , so that the switch unit  213  maintains a turned on state for a certain time. The DC voltage is applied to the driving controller  206  via the input filter unit  201  and the rectification/smoothing unit  202 , and then the driving controller  206  operates the switching device Q 11  so that AC voltages are induced in the secondary windings of the transformer  204 . The rectification/smoothing unit  205 , which receives the thusly induced AC voltages, outputs a plurality of DC voltages each having a certain level. 
     At this time, the signal feedback unit  207  which senses the DC voltage supplied from the rectification/smoothing unit  205  to the microcomputer  210  maintains the operation of the driving controller  206  in an active state since the DC voltage is maintained as a supply voltage for the microcomputer  210 . In addition, the backup unit  211  charges the supply voltage into the capacitor of the backup unit  211 . 
     Therefore, in the standby mode, the microcomputer  210  controls the signal feedback unit  214  so that the switch unit  213  is repeatedly turned ON/OFF for a certain time for thereby minimizing the power consumption of the powered apparatus. 
     FIG. 9 is a flow chart illustrating the power consumption decreasing method for a switched mode power supply according to the present invention. 
     When AC mains power is inputted into the switched mode power supply in Step ST 1 , the microcomputer judges whether the loads normally consume power in Step ST 2 . If the microcomputer determines that the loads do not normally consume power, it re-determines that continuously until a certain time elapses in Step ST 3 . If the microcomputer determines that the loads normally do not consume power after the certain time has elapsed in Step ST 3 , it controls the system to stop supplying power in Step ST 5 . Then the microcomputer continuously does not supply power to the loads for a certain time (tOFF) in Step ST 6  unless it is judged that a power turn on key has been inputted in the subroutine in Step St  7 . If it is judged that the power turn on key is inputted in Step ST 7 , flow returns again to Step ST 2 . 
     If the certain time which is the discharging time (tOFF) of the back-up capacitor is elapsed in Step ST 6 , the microcomputer causes the system to supply power to the loads in Step ST 8  and the backup capacitor is charged for a certain time (tON) in Step ST 9 . If the certain time (tON) which is the backup capacitor charging time is judged to have elapsed in Step ST 9 , the microcomputer controls the system to stop supplying power the loads by returning flow to Step ST 5 . Thereafter, Steps ST 5 ,  6 ,  7 ,  8  and  9  are repeatedly performed, so that the power saving mode is implemented. 
     However, if the microcomputer judges that the loads normally consume power in Step ST 2  and the user inputs any other key signal except for the power saving mode key signals in Step ST 4 , the switched mode power supply supplies power to the loads or does not respond to the key signal. If the power saving key signal is judged to have been inputted by the user in Step ST 4 , the control unit causes the power not to be supplied to the loads in ST 5 . 
     As described above, since the switched mode power supplies according to the present invention do not require the conventional auxiliary power supply, their sizes may become small and their fabrication cost may be decreased. In another embodiment of the present invention, in the standby mode, it is possible to supply a minimum power for driving the microcomputer, so that the power supply operates based on a minimum power. In addition, it is possible to decrease the switching noises by a switching operation of the supply of the AC mains power for a certain time tOFF during a certain period tON+tOFF, so that the reliability of the device is increased, and the present invention may be adapted to all kinds of electric apparatuses which use the switching mode power supply. The advantageous reduction in standby mode power supply consumption is achieved by making the interval of tOFF greater, preferably much greater, than the interval of tON. 
     Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as recited in the accompanying claims.