Abstract:
First, second and third capacitors ( 11, 12, 13 ) having predetermined capacitance values are connected in series in a commercial alternating current line. A rectifier bridge ( 14, 15, 16, 17 ) including two diodes ( 14, 15 ) having backward characteristics adjusted to a Zener voltage is connected in parallel with the capacitor ( 12 ) in the center, and a smoothing capacitor is connected between a first end and a second end of the rectifier bridge ( 14, 15, 16, 17 ). As a result, it is possible to divide alternating current (AC) from a power input terminal into a desired voltage while insulating it, and clip it with a predetermined voltage to shape it into a voltage waveform close to a rectangular wave and smooth it. Thereby, a stabilized DC voltage with less ripples can be obtained.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to power supply apparatus used in an electronic apparatus like a television receiver, for example. 
     2. Description of the Related Art 
     Among recent television receivers and other electronic devices, those a remote-controller for improving the convenience have been distributed widely. Such a remote-controller includes a microprocessor as its central element, and includes various operation keys, encoder, light emitting section, and so forth. It is powered by a battery, for example, to code signals responsive to operation of various operation keys, output infrared control signals to an electronic apparatus to be controlled. In receipt of an infrared control signal, the electronic apparatus decodes the received signal and makes up detected information. Then, based on the detected information, it outputs a control signal for controlling each section and with a predetermined operation effected. For this purpose, such an electronic apparatus has a stand-by power supply in addition to its main power supply so that it can always cope with control signals sent from the remote-controller by supplying power to its photodetector and controller even when the power supply from the main power supply is stopped. 
     FIG. 9 shows a power supply apparatus of an electronic apparatus matching with a conventional remote-controller. In FIG. 9, a commercial alternating current (AC) is supplied to power input terminals shown by  101   a  and  101   b . Numeral  108  denotes a rectifier bridge, and this rectifier bridge, smoothing capacitor and three-terminal regulator, which are not shown but connected to output terminals  109   a  and  109   b , constitute a main power supply section. 
     Between the power input terminal  101   b  and the rectifier bridge  108 , a main switch  103 , a choke coil  106   b  and a switch  122   a , forming a relay, are inserted in series. 
     Under the condition with the main switch  103  closed by manipulation of a user, when the switch  122   a  of the relay is turned ON, a rectified output is generated in the output terminals  109   a  and  109   b , and smoothed, stabilized and thereafter supplied to a main load circuit, not shown. 
     On the other hand, numeral  121  denotes the load circuit of the control system. By making a standby power supply section from a voltage converting and rectifying circuit made up of an insulating transformer  111 , rectifier bridge  112 , three-terminal regulator  114  and two capacitors  113 ,  115 , it is possible to supply a standby power supply to the control-system load circuit  121  and cope with a control signal sent from the remote-controller even when the relay switch  122   a  is turned OFF and the power supply from the main power supply is stopped. 
     When the main switch  103  is closed, a desired AC output is produced in the secondary coil  111   b  of the insulating transformer  111 , and this AC output is full-wave-rectified by the rectifier bridge  112 . Then, the full-wave-rectified output is smoothed by the capacitor  113 , and a direct current (DC) stabilized in 5V, for example, is generated at the output terminal of the three-terminal regulator  114 . This output voltage is supplied to LED  125 , photodetector  126  and microcomputer  127 , respectively, to activate the LED  125 , photodetector  126  and microprocessor  127 . 
     OBJECTS AND SUMMARY OF THE INVENTION 
     In the power supply apparatus of the conventional electronic device, as main power losses during standby periods, there are the excitation loss by the insulating transformer and the regulation loss of the three-terminal regulator. From the power-saving viewpoint, there is the demand for reducing these power losses in the standby periods. 
     According to the invention, there is provided a power supply apparatus connected to an alternating current to obtain a stabilized direct current voltage, comprising: 
     an AC voltage dividing means for dividing said AC voltage to a predetermined voltage including a first capacitor having one end connected to one of terminals of an alternating current and having a predetermined capacitance value, a second capacitor having one end connected to the other terminal of the alternating current and having a predetermined capacitance value, and a third capacitor having one end connected to the other end of the first capacitor and the other end connected to the other end of the second capacitor and having a predetermined capacitance value; 
     a rectifying and voltage-control means including a first rectifier element having a cathode connected to one end of the third capacitor and having a predetermined constant-voltage characteristic in a reverse voltage direction, a second rectifier element having a cathode connected to the other end of the third capacitor, an anode connected to the anode of said first rectifier element and a characteristic approximately equal to the first rectifier element in the backward voltage direction, a third rectifier element having an anode connected to one end of the third capacitor, and a fourth rectifier element having an anode connected to the other end of the third capacitor and a cathode connected to the cathode of the third rectifier element; and 
     a smoothing means for smoothing said DC voltage connected between a common connection point of the anodes of the first rectifier element and the second rectifier element and a common connection point of the cathodes of the third rectifier element and said fourth rectifier element. 
     In this embodiment, three capacitors having predetermined capacitance values are connected in series in an AC line, the rectifier bridge having two diodes whose reverse voltage characteristics are adjusted to a predetermined Zener voltage is connected in parallel with a capacitor located in the center, and a smoothing capacitor is connected to the rectifier bridge. As a result, it is possible to divide the AC voltage into a predetermined AC voltage and an isolate AC voltage at the same time. Additionally, it is possible to obtain a stabilized DC voltage with less ripples by clipping the AC voltage with the predetermined Zener voltage and smoothing it. 
     The above, and other, objects, features and advantage of the present invention will become readily apparent from the following detailed description thereof which is to be read in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit diagram showing a configuration of the first embodiment of the invention; 
     FIGS. 2A through 2E are waveform diagrams used for explanation of operations of the first embodiment of the invention; 
     FIG. 3 is a circuit diagram of a modified form of the first embodiment of the invention; 
     FIG. 4 is a circuit diagram showing a configuration of another modified form of the invention; 
     FIG. 5 is a circuit diagram of a modified form of the further embodiment of the invention; 
     FIG. 6 is a block diagram showing configuration of an electronic apparatus having the power supply apparatus according to any one of the embodiments shown in FIGS. 1 through 6; 
     FIG. 7 is a circuit diagram of a modified version of the embodiment of FIG. 1; 
     FIG. 8 is a perspective view of an AC adapter using the modified version shown in FIG. 7; and 
     FIG. 9 is a circuit diagram used for explaining a conventional power supply apparatus. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Explained below is an embodiment of the invention with reference to the drawings. FIG. 1 shows a configuration of this embodiment of the invention, and FIGS. 2A through 2E show voltage waveforms at various portions in the circuit. In FIG. 1, AC is supplied to power input terminals shown by  1   a  and  1   b . A line filter  4  is made up of a capacitor  5 , and a pair of choke coils  6   a  and  6   b  for removal of common mode noise. 
     Between one of power input terminals  1   a , and a rectifier bridge  8 , a fuse  2  and the choke coil  6   a  are inserted in series. Between the other input terminal  1   b  and the rectifier bridge  8 , a main switch  3 , choke coil  6   b  and a switch  32   a , forming a relay explained later, are inserted in series. 
     One end of the rectifier bridge  8  is connected to an output terminal  9   a , and the other end of the rectifier bridge  8  is connected to an output terminal  9   b . The rectifier bridge  8  forms a main power supply section together with a smoothing capacitor, regulator and other elements, not shown. 
     The main switch  3  is provided on an operation panel, not shown. While the main switch  3  is operated and closed by a user, when the relay switch  32   a  is turned ON, a rectified output is generated at the output terminals  9   a  and  9   b , and after being smoothed and stabilized, it is supplied to a main load circuit, not shown. 
     On the other hand, numeral  31  denotes a load circuit of a control system. The control system circuit  31  includes a relay made of a switch  32   a  and a coil  32   b , an NPN transistor  34  for switching, a LED section  35 , a photodetector  35  and a microprocessor  37 . 
     By constituting a standby power supply section from three capacitors  11 ,  12 ,  13  for dividing the AC voltage, two diodes  14 ,  15  which have functions for rectifying and stabilizing voltage, and rectifier diodes  16 ,  17  and smoothing capacitor  18 , it is configured to supply a standby power to the circuit  31  of control system load and cope with a control signal sent from a remote-controller even when the power supply from the main power supply section is stopped. 
     Between the connection point of the choke coil  6   a  and the rectifier bridge  8  and the connection point of the choke coil  6   b  and the switch  32   a , three capacitors  11 ,  12  and  13  are connected in series. The capacitors  11  and  13  are equal in capacitance, and the capacitor  12  has a capacitance of a predetermined value. 
     Connected to the point of the capacitors  11  and  12  are the cathode of the diode  15  and the anode of the rectifier diode  17 . Connected to the point of the capacitors  12  and  13  are the cathode of a diode  14  and the anode of a rectifier diode  16 . Forward voltage characteristics of the diodes  14  and  15  are the same as the rectifier diodes  16  and  17 , and their reverse voltage characteristics are different from the rectifier diodes  16  and  17  so as to have a predetermined Zener voltage and become conductive when a voltage of 6 volts, for example, is applied. And even when applied voltage is more than 6 volts, the anode of these diodes  14  and  15  are clipped and the voltage of them are kept at 6 volts. Anodes of these diodes  14  and  15  having these characteristics are connected together, and their connection point forms a secondary ground line. Further, cathodes of the rectifier diodes  16  and  17  are connected together, and a standby power is output from their connection point. The smoothing capacitor  18  is connected between the standby power output and the secondary ground line. 
     In the standby power supply section having the above-explained configuration, AC supplied voltage from the power input terminals  1   a  and  1   b  is divided into a predetermined voltage, insulated, rectified and smoothed so that a stabilized DC voltage with less ripples is output. This standby power supply section also functions to protect the circuit, and any noise generated at the input side and various kinds of noise generated at the output side are absorbed by the diodes  14  and  15 . 
     One end of the relay coil  32   b  is connected to the standby power output, the collector of the transistor  34  is connected to the other end of the relay coil  32   b , and the emitter of the transistor  34  is connected to the secondary ground line. Connected to the base of the transistor  34  is a control terminal of the microprocessor  37 , and a control signal is supplied from the microprocessor  37 . Connected at opposite ends of the coil  32   b  is a diode  33  for absorbing energy generated when the coil  32   b  is OFF. 
     Further connected to the standby power output are the LED section  35 , photodetector  36  and microprocessor  37 . 
     Operations of the embodiment having the above-explained configuration are explained below in greater detail. When the main switch  3  is turned ON, an AC voltage of 50 Hz, for example, is applied to opposite ends of the capacitors  11 ,  12 ,  13  as shown in FIG.  2 A. If the AC voltage is Vac, capacitance values of the capacitors  11 ,  12 ,  13  are C 11 , C 12  and C 13 , respectively, and the voltage across the opposite ends of the capacitor  12  is V 12 , then the relation of Equation (1) below is established between the voltage V 12  and the capacitance values of the capacitors, and the voltage V 12  divided into a predetermined voltage is obtained across the opposite ends of the capacitor  12 . FIG. 2B shows the voltage V 12  in an enlarged scale for easier view.                    V12   =     VacxC11   /     (       2      xC12     +   C11     )                   =         VacxC13   /     (       2      xC12     +   C13     )                       where                   C11     =   C13                   (   1   )                                
     The voltage divided to the opposite ends of the capacitor  12  is applied between the cathode of the diode  14  and the cathode of the diode  15 , and applied between the anode of the diode  16  and the anode of the diode  17 . Assuming that the Zener voltage of the diode  14  in the reverse voltage characteristic is D 14  and the Zener voltage of the diode  15  in the reverse voltage characteristic is D 15 , the peak voltage of a positive half cycle of the AC voltage shown by the broken line in FIG. 2C is clipped by the voltage of D 14 , and rectified by the line of the rectifier diode  16 , capacitor  18 , diode  15  and capacitor  12 . The peak voltage of the next negative half cycle is clipped by the voltage of D 15 , and rectified by the line of the rectifier diode  17 , capacitor  18 , diode  14  and capacitor  12 . As a result, the rectified waveform shown in FIG. 2D is obtained, and when it is smoothed by the capacitor  18 , a stabilized DC voltage with less ripples can be obtained. FIG. 2E shows the magnitudes of ripple voltages contained in the smoothed DC voltage for the comparison purpose, in which 1 denotes the ripple voltage not clipped whereas 2 denotes the ripple voltage clipped by a predetermined voltage. As the time between a peak to another peak becomes short and a voltage dropped by discharging becomes small, the ripples are improved largely. 
     When the Zener voltage D 14  of the diode  14  and the Zener voltage D 15  of the diode  15  are equal, and the forward voltages of the diodes  16  and  17  are Vfd 16  and Vfd 17 , respectively, output voltage Vc 18  between the standby power output line and the secondary ground line becomes the value shown by Equation (2). 
     
       
         Vc 18 =D 14 −Vfd 16 ≈D 15 −Vfd 17   (2) 
       
     
     Actually, the voltage V 12  generated at opposite ends of the capacitor  12  is affected by impedance of the load side as shown by Equation (3) below.                    V12   =     VacxZC11   /     (     Z2C12xZL   /     (     Z2C12   +   ZL     )       )                   =     VacxZC13   /     (     Z2C12xZL   /     (     Z2C12   +   ZL     )       )                     (   3   )                                
     where ZC 11  is the impedance of the capacitor  11 , ZC 13  is the impedance of the capacitor  13 , Z 2 C 12  is an impedance of a value that is twice that of capacitor  12 , and ZL is the impedance of the load side including the rectifier diodes  16  and  17 . Since the voltage drops due to the impedance of the load side in this manner, the condition shown by Equation (4) below is required for constant-voltage operations of the diodes  14  and  15 . 
     
       
         V 12 ≧D 14 , V 12 ≧D 15   (4) 
       
     
     Capacitance values of the capacitors  11 ,  12  and  13  are selected in optimum values by taking the condition required for constant-voltage operations, voltage fluctuations of the commercial AC voltage, and other factors into account. For example, for compliance with worldwide commercial ACs, it should be taken into consideration that the input voltage fluctuates from AC85V to AC280V, and capacitance values of the capacitors  11 ,  12  and  13  are selected to ensure that the voltage divided under the condition with the minimum voltage 85V and the maximum load be a voltage value not lower the Zener voltages of the diodes  14  and  15 . This range is capable of supplying currents of these standby power supplies is limited to the maximum allowable current of reverse voltage characteristics of the diodes  14  and  15 , and it is limited to 10 mA in the case of currently available ordinary Zener diodes, and is an optimum of the standby power supply of the remote-controller. 
     In this manner, the output voltage Vc 18  between the standby power line and the secondary ground line is smoothed by the smoothing capacitor  18 , and a DC voltage with less ripples is output. This standby power output voltage is supplied to the LED  35 , photodetector  36  and microprocessor  37 , respectively, to establish the standby mode. 
     LED section  35  is made of a limitative resistor and a LED element, and it is mounted on an operational panel, not shown, for example. The LED section  35  is lit by an output voltage from the voltage converting and rectifying circuit to show a user that the apparatus to be controlled is in a standby mode. 
     The photodetector  35  is made of a photo diode, decoder, and so on. In the operation mode where the output voltage from the voltage converting and rectifying circuit is supplied to the photodetector  35 , when it receives a control signal from the remote-controller, not shown, it decodes the signal to make detected information, and supplies this detected information to the microprocessor  37 . 
     The microprocessor  37  monitors the operation mode of the remote-controller on the basis of the detected information from the photodetector  36 . Therefore, when an instruction for operating the main power supply section is entered through the remote-controller, a control signal is output from the microprocessor  37 . In response to this control signal, the transistor  34  becomes conductive, a current flows in the relay coil  32   b , the switch  32   a  turns ON, and a full-wave rectified output from the rectifier bridge  8  is generated at the output terminals  9   a  and  9   b . The full-wave rectified output is smoothed and supplied to the main load circuit, and an operation of the main load side is started. In contrast, when an instruction for changing the main power supply section from the operative mode to the non-operative mode is entered through the remote-controller, the control signal becomes a LOW level, and the transistor  34  becomes non-conductive. Therefore, no current flows in the relay coil  32   b , the switch  32   a  turns OFF to interrupt AC, and therefore, the power supply from the main power supply section is stopped. 
     The above embodiment has been explained as using the diodes  14  and  15  having the same forward voltage characteristics as those of conventional rectifier diodes and having reverse voltage characteristics adjusted to have a predetermined Zener voltage. However, as shown in FIG. 3, rectifier diodes  21  and  22  having the same characteristics as the rectifier diodes  16  and  17  may be used in lieu of the diodes  14  and  15  to form a typical rectifier bridge, connecting the rectifier diodes  21  and  22  in parallel and connecting Zener diodes  19  and  20  for being conductive at the same reverse voltage as the diodes  14  and  15 . This constitution requires more circuit elements but ensures the same effect. 
     Further, the above embodiment has been explained as using the LED section  35  as means for displaying the standby mode. However, display may be provided by using another kind of display element or mechanical structure. Furthermore, although the above embodiment uses the photodetector  36  made of a photo diode, decoder and other elements to deal with control signals, if an ultrasonic wave or radio wave is used, a receiver section matching with the signal transmission mode of the remote-controlled transmitter is provided. 
     FIG. 4 shows configuration of another modified form of the invention. In FIG. 4, elements or parts equivalent to those of the foregoing embodiment shown in FIG. 1 are labeled with common reference numerals, and portions or sections having the same configuration are omitted from explanation. The modified form shown here additionally uses an isolating transformer  23  in the above-explained standby power supply section. 
     The primary coil  23   a  of the isolating transformer  23  is connected to opposite ends of the capacitor  12  which is the central one of three capacitors  11 ,  12  and  13  connected in series, adjusted to have predetermined capacitance values and connected in an AC line through a line filter  4 . 
     As to the secondary coil  23  of the isolating transformer  23 , the cathode of the diode  15  and the anode of the rectifier diode  17  are connected to one end, and the cathode of the diode  14  and the anode of the rectifier diode  16  are connected to the other end. 
     With this configuration, since the AC is isolated by the isolating transformer  23 , three capacitors  11 ,  12  and  13  connected in the power line can be selected by taking into consideration only their function of dividing AC into a predetermined AC voltage. Therefore, their capacitance values can be chosen more freely. 
     The above modified form has been explained as using the diodes  14  and  15  having the same forward voltage characteristics as those of conventional rectifier diodes and having reverse voltage characteristics adjusted to have a predetermined Zener voltage. However, as shown in FIG. 5, it is also possible to additionally connect the cathode of a Zener diode  25  to one end of the primary coil  23   a  of the isolating transformer, connect the cathode of a Zener diode  26  to the other end of the primary coil  23   a  of the isolating transformer, and connect the anodes of the Zener diodes  25  and  26  to one end of the rectifier bridge  8  nearer to the main load. In this constitution, although the number of circuit elements increases because of additional connection of the Zener diodes  25  and  26 , the same effect is obtained. 
     FIG. 6 shows configuration of an electronic apparatus  60  having the power supply apparatus according to the embodiments explained above. A power control signal sent from the remote-controller  52  is decoded by a control section of a control system load circuit  53 , and a power control signal is output to control a relay  51  connected in series to the main power supply section. As a result, AC input  50  is interrupted or supplied to control the supply of the main power. Also, the power control signal may be sent to the control section by using a contact switch, for example, instead of the remote-controller. 
     Some embodiments have been explained above as being composed of a main power supply section and a standby power supply section and being used in combination with a control system load circuit powered by the standby power supply section to cope with a control signal sent from the remote-controller even when the power from the main power supply section is stopped. However, by independently using the standby power supply section as the power supply apparatus, a DC voltage of a relatively small current capacitance can be supplied from AC. An embodiment of this concept is shown in FIG.  7 . In FIG. 7, elements or parts equivalent to those of FIG. 1 already explained are labeled with common reference numerals, and portions or sections having the same structures are omitted from explanation. In the power supply circuit having this configuration, AC voltage supplied from the power input terminals  1   a  and  1   b  is isolated and divided, then rectified and smoothed, and a stabilized DC with less ripples is output from the output terminals  19   a  and  19   b.    
     The AC adapter shown in FIG. 8 is one of applications of this embodiment. A container section  41  shown in FIG. 8 integrally contains the power circuit  20  surrounded by the outer broken line in FIG.  7  and mounted on a printed wiring board, a DC plug  43  attached to one end of a cable  42  to output a predetermined DC voltage and a plug  40  to be connected to an AC receptacle. Since the portion surrounded by the inner broken line  4  in FIG. 7 is not isolated from the AC voltage, an insulating material is used as the container section  41 . Since this AC adapter uses no isolating transformer, it can be reduced in weight and size. 
     Having described a specific preferred embodiment of the present 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 may be effected therein by one skilled in the art without departing from the scope or the spirit of the invention as defined in the appended claims.