Abstract:
A power supply circuit including a first rectifying smoother for generating a DC output when connected to an AC power source. A standby power source that provides a plurality of DC outputs when supplied with the DC output from the first rectifying smoother. A second rectifying smoother converts the AC input into a DC output, which is converted by the main power source into a desired DC voltage. On the primary side of the main and standby power source, a relay controller, supplied with power by one of the DC outputs from the standby power source, responds to an actuation of a main switch. A relay switch connects the AC power source to the second rectifying smoother upon actuation of the relay controller. The standby power source also supplies power to the control means, which is disposed on the secondary side of the main and standby power source.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a power supply circuit for use in an electronic apparatus such as a video projector or a television receiver. 
     2. Description of the Prior Art 
     Recently in an electronic apparatus such as a video projector, a television receiver and so forth for projecting image, there is employed a system which, even when a main power source thereof is placed in its off-state, enables a user to close the main power source again by turning on a main power switch of the apparatus by means of a remote controller or the like. 
     The power supply circuit of this type known heretofore has such a circuit configuration as that shown in FIG.  3 . The power supply circuit shown in FIG. 3 has a power plug  1000 , a primary circuit  1001  and a secondary circuit  1002 . The power plug  1000  is capable of inserting into, e.g., an outlet of a commercial AC power source for supplying an AC input voltage. The power plug  1000  is connected to a relay switch  1003 , a rectifier diode bridge  1004  and so forth in the primary circuit  1001  of transformers  1020  and  1021 . The rectifier diode bridge  1004  is connected to a main switching power circuit  1005 . Power is supplied to a standby switching power circuit  1006  from a half-wave rectifier circuit consisting of a diode  1031  and a capacitor  1032 . 
     The secondary circuit  1002  of the transformers  1020  and  1021  is equipped with a relay controller  1007  and a microcomputer  1009  for turning on or off the relay switch  1003 . The relay switch  1003  is disposed in the primary circuit  1001  relative to the main switching power circuit  1005  and the standby switching power circuit  1006 . Meanwhile a relay control coil  1008  is disposed in the secondary circuit  1002  relative to the main switching power circuit  1005  and the standby switching power circuit  1006 . 
     In the known power supply circuit of FIG. 3 mentioned above, it is necessary to dispose the secondary coil, the rectifier and the regulator of the standby switching power circuit  1006  on the side of the secondary circuit  1002  side, and also to dispose the relay control coil  1008  and the power switch  1010  on the same side for turning on the main switching power circuit  1005 , and further to dispose the relay switch  1003  and the main switching power circuit  1005  in the primary circuit  1001 . When the power switch  1010  is turned on in a state where an on-control signal from the microcomputer  1009  is applied to the base of a transistor  1030 , then the transistor  1030  is switched on to cause a current flow in the relay control coil  1008 , thereby turning on the relay switch  1003 . Consequently, the main switching power circuit  1005  is driven to deliver a desired main power output  1011 . 
     Since the relay switch  1003  and the relay control coil  1008  are housed normally in one casing, the secondary circuit  1002  including the relay control coil  1008  is so positioned as to be proximate to the primary circuit  1001  with partial entry. 
     However, in view of conforming with required safety standards, a certain electrical insulation distance needs to be insured between the primary circuit  1001  and the secondary circuit  1002 . 
     For this reason, it is necessary to space the primary circuit structure and the secondary circuit structure apart from each other by increasing the area of a substrate where the primary circuit  1001  including the relay switch  1003  and the secondary circuit  1002  including the relay controller  1007  are formed. 
     Similarly in view of conforming with the required safety standards, the level of any spurious radiation noise generated in the electronic apparatus and leaking via its power cord needs to be reduced under a predetermined value. In preventing harmful influence of such spurious radiation noise generated from the secondary circuit  1002  and induced to the primary circuit  1001 , there exists a problem of necessitating increase of the substrate area to reduce the leakage level of the spurious radiation noise from the secondary circuit  1002  to the primary circuit  1001 . 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a power supply circuit which is adapted for curtailing the area of a circuit substrate and reducing any spurious radiation from a secondary circuit to a primary circuit while insuring a proper electrical insulation distance between the secondary and primary circuits. 
     According to one aspect of the present invention, there is provided a power supply circuit for converting an AC voltage of an input AC power into a desired DC output voltage. This power supply circuit comprises a first rectifying smoother for generating a DC voltage from the AC power supplied thereto; a standby power source supplied with the DC voltage from the first rectifying smoother and having a plurality of DC outputs; a second rectifying smoother for converting the AC voltage into a DC voltage; a main power source for converting the DC voltage of the second rectifying smoother into the desired DC output voltage; a main switch for turning on the main power source; a relay controller disposed on the primary side of the main power source and the standby power source and, in response to turn-on of the main switch, actuated by the first of the plural DC outputs obtained from the standby power source; and a relay switch disposed on the primary side of the main power source and the standby power source and, upon actuation of the relay controller, driven to connect the second rectifying smoother to the AC power source. 
     Since the relay controller and the relay switch are both disposed on the primary side of the main power source and the standby power source, the relay controller and the relay switch are positionally in mutual proximity, but the primary side of the main and standby power sources is not proximate to the secondary side of the main and standby power sources without the necessity of any particular portion for electromagnetic insulation, whereby the substrate area can be curtailed. Moreover, this structure is effective to reduce the leakage level of any spurious radiation noise from the relay controller to the relay switch. 
     The above power supply circuit further comprises a control means disposed on the secondary side of the main power source and the standby power source, and fed with an input signal which indicates an on-state or an off-state of the main switch, wherein the standby power source supplies the second of the plural DC outputs as a voltage for actuating the control means. 
     In this instance, the standby power source can supply the voltage for actuating to the control means. 
     The power supply circuit mentioned above further comprises a first optical switch for transmitting the signal, which indicates the on/off state of the main switch, to the control means, wherein the first optical switch consists of a light emitting element and a light receiving element to receive the light from the light emitting element, and the first optical switch is an electromagnetically insulating element disposed between the primary side of the main and standby power sources and the secondary side of the main and standby power sources. 
     Thus, electromagnetic insulation can be achieved between the primary side and the secondary side of the main and standby power sources. 
     In the power supply circuit mentioned above, the relay controller has a relay control coil, wherein the relay controller receives a control signal from the control means, and when the main switch is turned on, the relay controller operates the relay control coil thereof to turn on the relay switch. 
     Also in the power supply circuit mentioned above, the relay controller has a second optical switch turned on by the control signal from the control means, wherein the second optical switch consists of a light emitting element and a light receiving element to receive the light from the light emitting element; and the second optical switch is an electromagnetically insulating element disposed between the primary side of the main and standby power sources and the secondary side of the main and standby power sources. 
     Further in the power supply circuit mentioned above, the control means has an input terminal to detect the DC output voltage of the main power source, and has a function of discriminating between the presence and absence of the DC output voltage of the main power source, thereby making a decision as to whether the operation of the main power source is normal or not. 
     According to the present invention as described above, it becomes possible to curtail the area of the circuit substrate and also to reduce any spurious radiation from the secondary circuit to the primary circuit. 
     The above and other features and advantages of the present invention will become apparent from the following description which will be given with reference to the illustrative accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows an exemplary projector equipped with a preferred embodiment representing the power supply circuit of the present invention; 
     FIG. 2 is a circuit diagram of a preferred embodiment which represents the power supply circuit of the present invention; and 
     FIG. 3 is a circuit diagram of a known power supply circuit according to the related art. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter some preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings, wherein like component elements are denoted by like or similar numerals. 
     Since the following embodiments are merely preferred concrete examples of the present invention, some technically preferred restrictions are given thereto. However, it is to be understood that the scope of the present invention is not limited to such embodiments alone unless otherwise specified in the following description. 
     FIG. 1 shows a rear projector as an exemplary electronic apparatus equipped with a preferred embodiment of the power supply circuit of the present invention. This projector  10  has a mirror  14 , a projection unit  16  and so forth in its body  12 . A color image for example emitted from the projection unit  16  is magnified through a lens  18  and then arrives at the mirror  14 . The image is reflected by the mirror  14  and then is projected onto the inner plane of a screen  20  in the body  12 , so that a viewer  22  can watch the image projected onto the back side of the screen  20 . 
     FIG. 2 shows a preferred embodiment of the power supply circuit used in the projection unit  16  in FIG.  1 . This power supply circuit  30  is capable of supplying a main power output  99  to a light source of the projection unit  16  or a liquid crystal panel (light bulb). 
     Schematically, the power supply circuit  30  has a power plug  32 , a primary circuit  34 , a secondary circuit  36  and so forth. 
     The primary circuit  34  denotes the primary side relative to a transformer  102  in a main power source  40  and also a portion relative to a primary winding  64  and a secondary winding  66  of a transformer  101  in a standby power source  42 . The secondary circuit  36  denotes the secondary side relative to the transformer  102  in the main power source  40 , and a portion relative to a second secondary winding  82  of the transformer  101  in the standby power source  42 . 
     The power plug  32  is a type attachable to and detachable from a commercial AC power source  44 , and is adapted to obtain therefrom a commercial AC voltage of 100 V for example. 
     The primary circuit  34  has a relay switch  46 , a relay controller  94 , a rectifying smoother (rectifier)  48 , a main switching power circuit (main power circuit)  50 , a diode  111 , a capacitor  122 , a standby switching power circuit  52 , a power switch (main power switch)  54 , a light emitting diode  56 A of a photo coupler (first optical switch)  56 , a transistor  142 , a resistor  130 , a photo transistor  58 B of a photo coupler (second optical switch)  58 , a diode  112 , a relay control coil  60 , primary coils  62  and  64 , a secondary coil  66 , a diode  113  and a capacitor  123 . The primary circuit  34  is enclosed with a frame of one-dot chained line. 
     The secondary circuit  36  is enclosed with a frame of two-dot chained line. This circuit  36  has a photo transistor  56 B of the photo coupler  56 , a light emitting diode  58 A of the photo coupler  58 , a transistor  141 , a regulator  70 , a diode  116 , a capacitor  126 , resistors  131  and  132 , a microcomputer  72  serving as control means, diodes  114  and  115 , capacitors  124  and  125 , and secondary coils  80  and  82 . 
     In the primary circuit  34 , the rectifying smoother  48  comprises a rectifying diode bridge  48 A and a smoothing capacitor  121 . One terminal of the power plug  32  is connected to one input terminal of the diode bridge  48 A, while the other end of the power plug  32  is connected via the relay switch  46  to the other input terminal of the diode bridge  48 A. 
     One output terminal of the rectifying diode bridge  48 A and one terminal of the capacitor  121  are connected to an input terminal of the main switching power circuit  50 . Meanwhile the other output terminal of the diode bridge  48 A and the other terminal of the smoothing capacitor  121  are connected to a primary ground wire  90 . 
     The junction of the other terminal of the power plug  32  and the relay switch  46  is connected via the diode  111  and the primary coil  64  to the output terminal of the standby switching power circuit  52 . Further one terminal of the capacitor  122  is connected to the junction of the diode  111  and the primary coil  64 , while the other terminal of the capacitor  122  is connected to the primary ground wire  90 . 
     The relay switch  46  and the relay control coil  60  constitute a relay  92 . The relay controller  94  of the primary circuit  34  is a control circuit for operating the main switching power circuit  50  by actuating the relay control coil  60  to turn on the relay switch  46 . 
     The relay control coil  60  of the relay controller  94  is connected together with the diode  112  between the collector of the transistor  142  and the emitter of the photo transistor  58 B of the photo coupler  58 . The emitter of the transistor  142  is connected to one terminal of the secondary coil  66  via the power switch  54  and the diode  113 , and the other terminal of the secondary coil  66  is connected to the primary ground wire  90 . One terminal of the capacitor  123  is connected to the junction of the diode  113  and the power switch  54 , while the other terminal of the capacitor  123  is connected to the primary ground wire  90 . The anode of the light emitting diode  56 A of the photo coupler  56  is connected to the emitter of the transistor  142  via a resistor  133 , while the cathode of the light emitting diode  56 A is connected to the primary ground wire  90 . And a resistor  130  is connected between the emitter and base of the transistor  142 . 
     The base of the transistor  142  is connected to the collector of the photo transistor  58 B of the photo coupler  58 . 
     In the secondary circuit  36 , one terminal of the secondary coil  82  is connected to a secondary ground wire  98 , while the other terminal of the secondary coil  82  is connected to a capacitor  126  whose one terminal is connected to the secondary ground wire  98  via diode  116 . The junction of the diode  116  and the capacitor  126  is connected to the input terminal of the regulator  70 , and a source voltage Vcc is supplied from the output terminal of the regulator  70  to the microcomputer  72 . 
     A resistor  132  is connected between the control terminal  96  and the output terminal of the regulator  70 , and a resistor  131  is connected between an on/off detection signal input terminal  97  of the power switch  54  and the output terminal of the regulator  70 . 
     One terminal of the photo transistor  56 B of the photo coupler  56  is connected to the detection signal input terminal  97  of the microcomputer, while the other terminal of the photo transistor  56 B is connected to the secondary ground wire  98  of the photo transistor  56 B. 
     The junction of the diode  116  and the capacitor  126  is connected to one terminal of the light emitting diode  58 A of the photo coupler  58 , and the other terminal of the light emitting diode  58 A is connected to the collector of the transistor  141 . 
     The base of the transistor  141  is connected to the control terminal  96  of the microcomputer  72 , while the emitter of the transistor  141  is connected to the secondary ground wire  98 . 
     The secondary coil  80  consists of two windings, wherein one end of each winding is connected to the secondary ground wire  98 , while the other ends thereof are connected to the anodes of diodes  114  and  115 , respectively. 
     The cathodes of the diodes  114  and  115  are connected, respectively, to the other ends of smoothing capacitors  124  and  125  each connected at one end thereof to the secondary ground wire, and then a main power output  99  (DC output voltage) is delivered therefrom. 
     The cathode of the diode  114  is connected to an output voltage detection input terminal  95  of the microcomputer  72  via a signal line  100 . 
     The transformer  102  consists of a primary coil  62  and a secondary coil  80 , and the transformer  101  consists of primary coil  64  and secondary coils  66 ,  82 . 
     The feature of the power supply circuit  30  shown in FIG. 2 resides in that the relay switch  46  and the relay control coil  60  are both disposed in the primary circuit  34 . The photo couplers  56  and  58  are electromagnetically insulating elements which serve as optical switches having a function of achieving electromagnetic insulation between the primary circuit  34  and the secondary circuit  36 . 
     In response to the power switch on/off detection signal SD sent from the photo coupler  56  to the detection signal input terminal  97 , the microcomputer  72  is capable of detecting whether the power switch  54  is in its on-state or off-state. 
     Subsequently the microcomputer  72  outputs a control signal CS from the control terminal  96  to the transistor  141 . 
     In response to an information signal IS received at the output voltage detection terminal  95 , the microcomputer  72  is capable of detecting whether the main power output  99  is being properly delivered or not via the signal line  100 . 
     The power switch  54  is not connected directly to the power plug  32 . 
     When the power plug  32  is connected to the commercial AC power source  44 , the standby switching power circuit (standby power source)  52  and the microcomputer  72  are held in the normal operating state thereof. 
     Now an explanation will be given on an exemplary operation of the power supply circuit  30  in FIG.  2 . 
     When the power plug (AC plug)  32  is connected to an outlet of the commercial AC power source  44 , an AC power inputted via the power plug  32  is supplied to the first rectifying smoother consisting of the diode  111  and the capacitor  122 , and then the smoothed DC output therefrom is supplied to the standby switching power circuit  52  via the primary coil  64  of the transformer  101 . 
     Further the AC current generated in the secondary coil  82  of the transformer  101  is supplied to the regulator  70  via the rectifying smoother consisting of the diode  116  and the capacitor  126 , so that a source voltage Vcc of a predetermined value is supplied from the regulator  70  to the microcomputer  72 , which is thereby held in its normal operating state. 
     When the microcomputer  72  is thus placed in its operating state, the control terminal  96  of the microcomputer  72  is turned to a high level, which is then supplied to the base of the transistor  141  to thereby switch on the same. 
     Consequently, the light emitting diode  58 A of the photo coupler  58  sends light to the photo transistor  58 B, which is then switched on by the received light, so that the base of the transistor  142  is changed from a high level to a low level. 
     If the power switch  54  is turned on in this state, the power from the secondary coil  66  is supplied to cause conduction of the emitter-collector of the transistor  142 , whereby a current flow is induced in the relay control coil  60  to eventually turn on the relay switch  46 . 
     Then the commercial power obtained through the power plug  32  is supplied to the second rectifying smoother  48 . The DC output from the second rectifying smoother is supplied to the main switching circuit to thereby actuate the main switching power circuit  50 , so that the main power output  99  is delivered to the secondary circuit  36  via the transformer  102 . 
     When the power switch  54  is turned on, a current is caused to flow in the light emitting diode  56 A of the photo coupler  56  via the resistor  133 , so that the light from the light emitting diode  56 A reaches the photo transistor  56 B to switch on the same, whereby the power switch on/off detection terminal  97  of the microcomputer  72  is changed from a high level to a low level. 
     That is, the power switch on/off detection signal SD is changed from a high level to a low level. 
     Consequently, the microcomputer  72  can detect that the power switch  54  is in its on-state. 
     Meanwhile the control terminal  96  of the microcomputer  72  is held at a high level to thereby maintain the main switching power circuit  50  in its operating state. 
     Next, when the power switch  54  is turned off, no power is supplied to the relay control coil  60 , so that the relay switch  46  is turned off to thereby stop the operation of the main switching power circuit  50 . 
     Since no current is supplied to the light emitting diode  56 A of the photo coupler  56  either, the photo transistor  56 B is turned off, and then a high-level signal is inputted to the power switch on/off detection terminal  97  of the microcomputer  72 . 
     Thus, the microcomputer  72  can detect that the power switch  54  is in its off-state. 
     In case a standby command is outputted from a remote commander or the like when the power switch  54  is in its on-state and the main switching power circuit  50  is in operation, a low-level control signal CS is supplied from the control terminal  96  of the microcomputer  72  to the base of the transistor  141 . 
     Consequently, the transistor  141  is turned off to cause no more current flow in the light emitting diode  58 A of the photo coupler  58 , so that the light emitting diode  58 A is turned off. 
     As a result, the transistor  142  is turned off, and no more current is supplied to the relay control coil  60  to eventually turn off the relay switch  46 , thereby stopping the operation of the main switching power circuit  50 . 
     The feature of the present invention will now be described below. 
     In the power supply circuit  30  of the present invention shown in FIG. 2, both the relay switch  46  and the relay controller  94  including the relay control coil  60  are disposed in the primary circuit  34 , differently from the conventional circuit known heretofore. 
     Due to such a different configuration, there exists no necessity of caring the spurious radiation noise emitted from the secondary circuit  36 , and no portion of the secondary circuit  36  positionally enters the primary circuit  34 , hence realizing diminution of the spurious radiation noise and dimensional reduction of the substrate area. 
     The photo couplers  56  and  58  have a function of electromagnetically insulating the primary circuit  34  and the secondary circuit  36  from each other, thereby enhancing the electromagnetic insulation between the primary circuit  34  and the secondary circuit  36 . 
     The microcomputer  72  is capable of performing control from the secondary circuit  36  to the primary circuit  34  via the photo coupler  58  by the control signal CS obtained from the control terminal  96 . 
     The microcomputer  72  constitutes a power control system wherein the on/off state of the power switch  54  in the primary circuit  34  can be recognized in accordance with the power switch on/off detection signal SD outputted from the photo coupler  56 . 
     In the conventional example of FIG. 3, the clearance and creeping distance between the relay switch  1003  in the primary circuit  1001  and the relay control coil  1008  in the secondary circuit  1002  is required to be more than 6 mm according to the safety standards in Europe and some areas of Asia where the commercial AC supply voltage is 220 V, or more than 3 mm according to the safety standards in U.S.A. where the commercial AC supply voltage is 120 V, or more than 3 mm according to the safety standards in Japan where the commercial AC supply voltage is 100 V. 
     Otherwise it is necessary to employ, for the relay switch  1003  and the relay control coil  1008 , a requisite insulating structure (with regard to, e.g., material, thickness, dielectric strength, and non-flammability) prescribed in the safety standards. 
     In the present invention where the relay control coil  60  and the relay switch  46  are both disposed in the primary circuit, the predetermined clearance and creeping distance is not needed between the primary and secondary circuits as viewed from the safety standards, and the known insulating structure is not needed either, whereby the inter-terminal and inter-pattern distances can be shortened. 
     Consequently, the degree of freedom is raised in disposing the relay  92  and designing the patterns on the substrate, hence reducing the required area of the substrate. 
     In the conventional case where the relay controller is formed in the secondary circuit, it is necessary to ensure a sufficient distance between the patterns so as to diminish the level of the spurious radiation noise. Further, a certain insulation distance is needed since the secondary circuit partially enters the primary circuit. As a result, it has been difficult heretofore to attain effective use of the substrate area. 
     However, in the present invention where both of the relay switch  46  and the relay controller  94  can be disposed in the same primary circuit, it becomes possible to reduce the the substrate area. And the spurious radiation noise from the secondary circuit to the primary circuit can be sharply diminished. 
     According to the present invention, the microcomputer  72  is capable of detecting a power fault mode. That is, if the output of the standby switching power circuit  52  is existent, the microcomputer  72  can judge that the power plug  32  is in connection to the commercial AC power source  44 . 
     And if the output of the main switching power circuit  50  is not existent, i.e., if the information signal IS is not fed to the output voltage detection terminal  95  of the microcomputer  72  via the signal line  100  of the secondary circuit  36  even after the control signal CS from the control terminal  96  is turned to a high level during recognition of the on-state of the power switch  54 , then the microcomputer  72  can judge that the main switching power circuit  50  is abnormal. 
     Thus, it is possible in the microcomputer  72  to detect whether the main switching power circuit  50  is normal or abnormal. 
     It is further possible to reset the power supply circuit due to malfunction thereof. More specifically, when the main power circuit is kept at a halt by the operation of a protection circuit, the main switching power circuit  50  can be reset from its malfunction when restarted by changing the output level of the control signal CS, which is obtained from the control terminal  96  of the microcomputer  72 , as high/low/high. 
     In case the main switching power circuit  50  fails to be restarted, the microcomputer  72  judges that the main switching power circuit  50  is not restartable for its proper operation. In this case, such abnormal state can be indicated by blinking the light emitting diode (LED) under control of the microcomputer  72 . 
     As for a measure against lightning surge to the relay, there is no necessity of considering dielectric breakdown of the primary circuit  34  and the secondary circuit  36 . Because, in regard of the relay  92 , both the relay switch  46  and the relay control coil  60  are in the primary circuit, so that the known insulation structure required between the primary and secondary circuits in view of safety standards is not necessary. That is, a down-sized relay is usable to consequently curtail the cost. 
     Further, none of overvoltage is induced in the coil by any surge between the primary and secondary circuits. 
     The power switch (for the main power circuit) needs to meet merely the requirements for small signal, and any AC power supply standards is not necessary to eventually achieve down-sizing and cost reduction. 
     Since none of AC power switch is employed, there exists no extended layout of the AC power supply line due to a harness, so that the radiation noise or the like can be diminished with advantages of down-sizing the structure and curtailing the production cost. 
     It is to be understood that the present invention is not limited only to the above embodiments alone, and the power supply circuit of the invention may be used for some other kind of electronic apparatus than the projector mentioned.