Abstract:
A discharge circuit unit for minimizing standby power occurring in a standby mode and an image forming apparatus having the same are provided. The discharge circuit unit is connected to an input line of alternating current (AC) power and discharges a capacitive element for reducing noises. The discharge circuit unit includes a discharge circuit including first and second resistance units connected in series to discharge the capacitive element in response to a discharge control signal generated when an input of the AC power is interrupted, and a detection circuit that detects whether the input of the AC power is interrupted, and includes third and fourth resistance units connected in series so as to generate the discharge control signal when it is detected that the input of the AC power is interrupted. Each of the first to fourth resistance units includes at least one of a resistor and a switch.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is related to, and claims the priority benefit of, Korean Patent Application No. 10-2013-0085336, filed on Jul. 19, 2013, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference. 
     BACKGROUND 
     1. Field 
     Embodiments relate to a discharge circuit unit and an image forming apparatus having the same, and more particularly, to a discharge circuit unit for reducing standby power and an image forming apparatus having the same. 
     2. Description of the Related Art 
     From the viewpoint of environmental protection, standby power regulations for electric products are strictly enforced for energy saving, for example, in the Americas (e.g., EPA1.2) and Europe (e.g., ErP step 2). To meet the standby power regulations, a variety of efforts are made to reduce standby power in the electric products. 
     An electromagnetic interference (EMI) filter for removing noise may be installed on an input terminal of a power supply (e.g., switching mode power supply (SMPS)). When electric charges charged in an X-capacitor (hereinafter referred to as “X-cap”) provided for the EMI filter flow to metal terminals of a plug in the event of plug-off, this may give rise to a problem with safety. To attempt to address this problem, a discharge resistor for discharging the electric charges charged in the X-cap may be used. However, due to the discharge resistor, power loss may occur in a standby mode. This power loss counters efforts to reduce the standby power in the electric products. A solution to such a problem is desired. 
     SUMMARY 
     It is an aspect of an exemplary embodiment to provide a discharge circuit unit for minimizing standby power occurring in a standby mode of an electric product. 
     Additional aspects of embodiments are set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention. 
     In accordance with an aspect of an embodiment, a discharge circuit unit is connected to an input line of alternating current (AC) power and discharges a capacitive element for reducing noises. The discharge circuit unit includes a discharge circuit that includes first and second resistance units connected in series so as to discharge the capacitive element in response to a discharge control signal generated when an input of the AC power is interrupted, and a detection circuit that detects whether the input of the AC power is interrupted, and includes third and fourth resistance units connected in series so as to generate the discharge control signal when it is detected that the input of the AC power is interrupted. Each of the first to fourth resistance units includes at least one of a resistor and a switch. 
     The third and fourth resistance units of the detection circuit may have a relatively greater resistance value than the first and second resistance units of the discharge circuit. 
     The first resistance unit of the discharge circuit may include first and second resistors, and the second resistance unit of the discharge circuit may include first and second switches. 
     The first resistor, the first switch, the second switch, and the second resistor may be connected in series between opposite ends of the input line of the AC power, and the first and second switches may be turned on or off by the discharge control signal. 
     The first and second switches of the discharge circuit may be N-channel enhancement type metal oxide semiconductor field effect transistors (MOSFETs). 
     Sources of the first and second switches may be interconnected to form a first node. Drains of the first and second switches may be connected to the first and second resistors, respectively. The discharge control signal may be input into gates of the first and second switches. 
     The first and second resistors of the discharge circuit may have the same resistance value. 
     The third resistance unit of the detection circuit may include third and fourth resistors, and the fourth resistance unit of the detection circuit may include third and fourth switches. 
     The third resistor, the third switch, the fourth switch, and the fourth resistor may be connected in series between opposite ends of the input line of the AC power. 
     The detection circuit may be configured so that the third and fourth switches are turned on and off by the input and interruption of the AC power, and the discharge control signal is generated by turning on the third and fourth switches. 
     The third and fourth switches of the detection circuit may be PNP type bipolar transistors. 
     The detection circuit may be configured so that collectors of the third and fourth switches are interconnected and connected to the first node, emitters of the third and fourth switches are connected to the third and fourth resistors respectively, and the discharge control signal is output from the emitters of the third and fourth switches. 
     The detection circuit may be configured so that a fifth resistor is connected between the emitter and a base of the third switch, a second capacitor is connected between the base of the third switch and the first node, a sixth resistor is connected between the emitter and a base of the fourth switch, a third capacitor is connected between the base of the fourth switch and the first node, a seventh resistor is connected in parallel to the second capacitor, and an eighth resistor is connected in parallel to the third capacitor. 
     The third and fourth resistors of the detection circuit may have the same resistance value. 
     In accordance with an aspect of an embodiment, an image forming apparatus has a discharge circuit unit that is connected to an input line of alternating current (AC) power and discharges a capacitive element for reducing noises, in which the discharge circuit unit includes a discharge circuit that includes first and second resistance units connected in series so as to discharge the capacitive element in response to a discharge control signal generated when an input of the AC power is interrupted, and a detection circuit that detects whether the input of the AC power is interrupted, and includes third and fourth resistance units connected in series so as to generate the discharge control signal when it is detected that the input of the AC power is interrupted. Each of the first to fourth resistance units includes at least one of a resistor and a switch. 
     The third and fourth resistance units of the detection circuit may have a relatively greater resistance value than the first and second resistance units of the discharge circuit. 
     The first resistance unit of the discharge circuit may include first and second resistors, and the second resistance unit of the discharge circuit may include first and second switches. 
     The first and second resistors of the discharge circuit may have the same resistance value. 
     The third resistance unit of the detection circuit may include third and fourth resistors, and the fourth resistance unit of the detection circuit may include third and fourth switches. 
     The third and fourth switches may be turned on and off by the input and interruption of the AC power, and the discharge control signal may be generated by turning on the third and fourth switches. 
     The third and fourth resistors of the detection circuit may have the same resistance value. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and/or other aspects of the invention will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which: 
         FIG. 1  illustrates a laser printer that is an image forming apparatus according to an embodiment; 
         FIG. 2  illustrates a control system of the image forming apparatus illustrated in  FIG. 1 ; 
         FIG. 3  illustrates an embodiment of a power supply; 
         FIG. 4  illustrates an exemplary circuit configuration of a discharge circuit unit; 
         FIGS. 5A and 5B  illustrate an operation of the discharge circuit unit for a positive (+) half period of AC power supplied to an image forming apparatus according to an embodiment; 
         FIGS. 6A and 6B  illustrate an operation of the discharge circuit unit for a negative (−) half period of AC power supplied to an image forming apparatus according to an embodiment; 
         FIGS. 7A and 7B  illustrate positive (+) discharge of the X-cap in the discharge circuit unit in the event of plug-off of an image forming apparatus according to an embodiment; and 
         FIGS. 8A and 8B  illustrate negative (−) discharge of the X-cap in the discharge circuit unit in an event of plug-off of the image forming apparatus according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to the embodiments, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. 
       FIG. 1  illustrates a laser printer that is an image forming apparatus according to an embodiment. An image forming apparatus  102  may be provided with a power cable  104  for receiving power. A plug  106  may be provided at one end of the power cable  104 . The plug  106  may be inserted into a socket  108  of a plug receptacle or a power strip. Thereby, commercial alternating current (AC) power supplied from an external power source can be supplied to the image forming apparatus  102 . The image forming apparatus  102  may be supplied with power through the plug  106  inserted into the socket  108  and the power cable  104 , and performs an operation associated with image processing. Inserting the plug  106  into the socket  108  may be referred to as plug-on, and separating the plug  106  inserted into the socket  108  from the socket  108  may be referred to as plug-off. 
       FIG. 2  illustrates an exemplary control system of a image forming apparatus, for example, as illustrated in  FIG. 1 . As illustrated in  FIG. 2 , a control unit  240  for controlling overall operations of the image forming apparatus  102  may be electrically connected, for example, to a paper feed unit  230 , a paper eject unit  220 , an image transfer unit  250 , a fixing unit  270 , a display  206 , and a speaker  208  so as to be able to conduct communication. A power supply (e.g., switching mode power supply (SMPS))  202  generates system direct current (DC) powers of 5 V and 24 V by AC-DC conversion, and supplies the generated power to the control unit  240 , the image transfer unit  250 , for example. The DC power of 5 V may be supplied to the control unit  240  made up of a microprocessor, circuit elements, etc. The DC power of 24 V may be supplied to the fixing unit  270 . The system DC powers of 5 V and 24 V output from the power supply  202  may be selectively supplied to other components of the image forming apparatus  102 . The power supply  202  may supply the input commercial power to varied components (e.g., a fixing heater of the fixing unit  270 ). The paper feed unit  230  feeds a printing medium (e.g., paper) stacked in a paper feed cassette to the image transfer unit  250 . The image transfer unit  250  forms a predetermined image in response to an image signal, and transfers the formed image to an image plane of the printing medium. The fixing unit  270  fixes, e.g., semi-permanently fixes the image transferred to the printing medium. The paper eject unit  220  ejects the printing medium to which the image is fixed by the fixing unit  270  to the outside. The control unit  240  controls the operation, e.g., overall operations of the image forming apparatus  102  and may be electrically connected to a plurality of sensors for detecting conditions of the components of the image forming apparatus  102  so as to be able to conduct communication. The display  206  displays guide messages for informing a user of information about operations and/or conditions of the image forming apparatus  102 . The speaker  208  outputs a guide sound and/or a warning sound generated during the operation of the image forming apparatus  102 . 
       FIG. 3  illustrates an embodiment of a power supply  202 , for example, as illustrated in  FIG. 2 . The power supply  202  illustrated in  FIG. 3  includes a discharge circuit unit  302 , an electromagnetic interference (EMI) filter  304 , a rectifier  306 , and a transformer  308 . The discharge circuit unit  302  discharges an X-cap (first capacitor) of the EMI filter  304 . The EMI filter  304  may be a line filter made up of a coil and a capacitor in order to remove various noises included in the power, e.g., commercial power (AC power) supplied through the power cable  104 . The rectifier  306  converts AC power to DC power, or converts a phase of the AC power to another desired phase. The transformer  308  lowers a voltage of the DC power rectified by the rectifier  306  to a desired level of DC voltage. 
     As illustrated in  FIG. 3 , the X-cap of the EMI filter  304  may be charged in a plug-on state by commercial power input to the power supply  202 , and discharged by an action of the discharge circuit unit  302  when the plug-on state is changed to a plug-off state. If the X-Cap is not discharged after being charged, the charged voltage may be applied to opposite metal terminals of the plug  106 . As such, it may be necessary to rapidly discharge the X-Cap for safety. 
       FIG. 4  illustrates a configuration of a discharge circuit unit  302 , for example, as illustrated in  FIG. 3 . As illustrated in  FIG. 4 , the discharge circuit unit  302  according to the embodiment includes a first discharge circuit unit  312  powered to detect power, for example, for a positive (+) half period of the AC power, and a second discharge circuit unit  322  powered to detect power, for example, for a negative (−) half period of the AC power. The first and second discharge circuit units  312  and  322  may be connected in series between opposite ends Live and Neutral of an AC power input line, and may be disposed so as to have a symmetrical structure with respect to a node (first node) N 1 . A discharge resistor R 11  and an N-channel enhancement type metal oxide semiconductor field effect transistor (MOSFET) Q 11  and another N-channel enhancement type MOSFET Q 21  and another discharge resistor R 21  may be connected in series with the node N 1  centered therebetween. Sources of the two MOSFETs Q 11  and Q 21  may be connected to the node N 1 . The two discharge resistors R 11  and R 21  discharge the X-cap in the event of plug-off. A current-limiting resistor R 12  and a PNP type bipolar transistor Q 12  and another PNP type bipolar transistor Q 22  and another current-limiting resistor R 22  may be connected in series between the opposite ends Live and Neutral of the AC power input line, and are connected in parallel with the discharge resistors R 11  and R 21 . An emitter of the PNP type bipolar transistor Q 12  may be connected to a gate of the MOSFET Q 11 , and an emitter of the other PNP type bipolar transistor Q 22  may be connected to a gate of the other MOSFET Q 21 . Collectors of the two bipolar transistors Q 12  and Q 22  may be connected to the node N 1 . A resistor R 13  may be connected between the emitter and a base of the bipolar transistor Q 12 , and a resistor R 14  may be connected between the base and the collector of the bipolar transistor Q 12 . A resistor R 23  may be connected between the emitter and a base of the other bipolar transistor Q 22 , and a resistor R 24  may be connected between the base and the collector of the other bipolar transistor Q 22 . An AC coupling capacitor C 11  may be connected to opposite ends of the resistor R 14 . An AC coupling capacitor C 21  may be connected to opposite ends of the resistor R 24 . 
     According to an embodiment, the first discharge circuit unit  312  and the second discharge circuit unit  322  are symmetrically connected in series between the opposite ends Live and Neutral of the AC power input line with the node N 1  centered therebetween. Due to this symmetrical serial connection structure, no current flows through the MOSFETs Q 11  and Q 21  or the bipolar transistors Q 12  and Q 22  for the positive and negative half periods of the AC power in the plug-on state, and only a very small amount of standby current flows through the resistors R 12  and R 22  having a relatively very great resistance value. In the discharge circuit unit  302  of  FIG. 4 , the resistance value of the resistor R 12  may be relatively greater than that of the resistor R 11  (e.g., about five times). The resistance value of the resistor R 22  may be relatively greater than that of the resistor R 21  (e.g., about five times). When the plug  106  is in the plug-on state, the current flows through the resistor R 12  or R 22  having the relatively greater resistance value. Thereby, power consumption caused by the standby current when the plug  106  is in the plug-on state is minimized. 
     In the event of the plug-off, the voltage charged in the X-cap should be reduced, for example, to a level harmless to a human body within a very short time (e.g., within one second). To do so, the X-cap should be able to be rapidly discharged. In the event of the plug-off, the electric charges of the X-cap should be discharged within as short a time as possible (e.g., within one second). In consideration of this, values of the resistor R 14  and the capacitor C 11  and values of the resistor R 24  and the capacitor C 21  are determined. Assuming, for example, that turn-on voltage of the bipolar transistor Q 22  is 0.7 V and that input voltage is rectified DC voltage, magnitudes of the resistor R 24  and the capacitor C 11  are set so that 
             Vt   =     Vt   ×   0.9   ×     (     1   -     ⅇ       -   1       R   ⁢           ⁢   24   ⁢           ⁢   C   ⁢           ⁢   11           )             
and a time constant R 24 C 11  is less than 8.45, and thereby the X-cap has only to be set to be discharged for a shortest time while the standby power is minimized. Since a discharge time of the X-cap relates to the electric charges charged by the unrectified AC power, the resistance values of the resistors R 11  and R 21  which meets the time constant RC&lt;1 (where R is R 11  or R 21 , and C is X-Cap) have only to be set.
 
     The discharge circuit unit  302  illustrated in  FIG. 4  may be divided into a discharge circuit and a detection circuit, in addition a division into the first discharge circuit  312  and the second discharge circuit  322 . In the discharge circuit unit  302  of  FIG. 4 , the discharge circuit may be a circuit including the resistors R 11  and R 21  and the MOSFETs Q 11  and Q 21 , and the detection circuit may be a circuit including the resistors R 12 , R 13 , R 14 , R 22 , R 23 , and R 24  and the capacitors C 11  and C 21 , and the bipolar transistors Q 12  and Q 22 . 
     In the configuration of the discharge circuit unit  302  of  FIG. 4 , the other components excluding the resistors R 11  and R 21  for the discharge and the resistors R 12  and R 22  for limiting the current may be packaged into one semiconductor chip. Alternatively, in the configuration of the discharge circuit unit  302  of  FIG. 4 , the other components (including the resistors R 12  and R 22 ) excluding the resistors R 11  and R 21  for the discharge may be packaged into one semiconductor chip. Due to this packaged configuration, the discharge circuit unit can be simply configured by designing only the magnitudes of the resistors R 11  and R 21  or only the magnitudes of the resistors R 11 , R 21 , R 12 , and R 22 . 
       FIGS. 5A and 5B  illustrate an operation of the discharge circuit unit for the positive (+) half period of the AC power supplied to the image forming apparatus according to an embodiment. Before the AC power is supplied (prior to the plug-on), no power is supplied, and thus the two MOSFETs Q 11  and Q 21  and the two bipolar transistors Q 12  and Q 22  are in a turn-off state. In this state, when the AC power begins to be supplied by the plug-on, only a very small amount of current flows through the resistor R 12  whose resistance value is relatively greater than that of the resistor R 11 . A flow of the current is equal to a path indicated by an arrow of  FIG. 5A . As illustrated in  FIG. 5A , a small amount of standby current (detection current) flowing through the resistor R 12  flows through: the resistor R 13 , the resistor R 14 , the MOSFET Q 21  having a diode function, and the resistor R 21 ; the resistors R 13 , R 14 , R 24 , R 23 , and R 22 ; or the capacitors C 11  and C 21  and the resistors R 23  and R 22 . An amount of the current flowing along this path may be sufficiently restricted by the resistor R 12  having a great resistance value, and thus is very small. The resistance values of the resistors R 12  and R 22  may be determined in consideration of standby power regulations. As illustrated in  FIG. 5B , since the capacitors C 11  and C 21  are virtually electrically short-circuited when the AC is input, a current of the base of the bipolar transistor Q 12  flows to the end Neutral through the resistor R 14  and the capacitor C 11 . Thus, PNP type bipolar transistor Q 12  is turned on. When the bipolar transistor Q 12  is turned on, a voltage of the emitter of the bipolar transistor Q 12 , i.e. the gate of the MOSFET Q 11 , electrically becomes a low level, and the N-channel enhancement type MOSFET Q 11  is continuously maintained in a turn-off state. When the plug  106  is in the plug-on state, the current is caused to flow the resistors R 12  and R 22  having the relatively greater resistance value, and thereby the standby power of the image forming apparatus  102  is greatly reduced. 
       FIGS. 6A and 6B  illustrate an operation of the discharge circuit unit for the negative (−) half period of the AC power supplied to the image forming apparatus according to an embodiment. When the AC power is converted from the positive (+) half period to the negative (−) half period, only a very small amount of current flows through the resistor R 22  whose resistance value is relatively greater than that of the resistor R 21 . In this case, a flow of the current is equal to a path indicated by an arrow of  FIG. 6A . As illustrated in  FIG. 6A , a small amount of standby current (detection current) flowing through the resistor R 22  flows through: the resistor R 23 , the resistor R 24 , the MOSFET Q 11  having a diode function, and the resistor R 11 ; the resistors R 23 , R 24 , R 14 , R 13 , and R 12 ; or the capacitors C 21  and C 11  and the resistors R 22  and R 23 . An amount of the current flowing along this path is sufficiently restricted by the resistor R 22  having a great resistance value, and thus is very small. The resistance values of the resistors R 12  and R 22  may be preferably determined in consideration of the standby power regulations. As illustrated in  FIG. 6B , since the capacitors C 21  and C 11  are virtually electrically short-circuited when the AC is input, a current of the base of the bipolar transistor Q 22  flows to the end Live through the resistor R 24  and the capacitor C 21 . For this reason, the PNP type bipolar transistor Q 22  is turned on. When the bipolar transistor Q 22  is turned on, a voltage of the emitter of the bipolar transistor Q 22 , i.e. the gate of the MOSFET Q 21 , electrically becomes a low level, and thus the N-channel enhancement type MOSFET Q 21  is continuously maintained in a turn-off state. In this way, when the plug  106  is in the plug-on state, the current is caused to flow the resistors R 22  and R 12  having the relatively greater resistance value, and thereby the standby power of the image forming apparatus  102  is greatly reduced. 
     As illustrated in  FIGS. 5A, 5B, 6A, and 6B , the X-cap is charged while the AC power is supplied. When the X-cap is charged, as illustrated in  FIGS. 7A and 7B , the side of the end Live may be positive (+) polarity, and the side of the end Neutral may be negative (−) polarity. As illustrated in  FIGS. 8A and 8B , the side of the end Live may be negative (−) polarity, and the side of the end Neutral may be positive (+) polarity. In this way, in the state in which the side of the end Live is the positive (+) polarity, and the side of the end Neutral is the negative (−) polarity, the plug  106  undergoes plug-off, and the X-cap is discharged, which is defined as positive (+) discharge. In the state in which the side of the end Live is the negative (−) polarity, and the side of the end Neutral is the positive (+) polarity, the plug  106  undergoes plug-off, and the X-cap is discharged, which may be defined as negative (−) discharge. 
       FIGS. 7A and 7B  illustrate positive (+) discharge of the X-cap in the discharge circuit unit in an event of the plug-off of an image forming apparatus according to an embodiment. As illustrated in  FIG. 7A , in the state in which the side of the end Live of the X-cap is the positive (+) polarity, and the side of the end Neutral of the X-cap is the negative (−) polarity, when the plug  106  undergoes plug-off, the electric charges charged in the X-cap begin to undergo positive (+) discharge. The MOSFET Q 11  is turned off, and the bipolar transistor Q 12  is turned on. As such, the X-cap is discharged through the resistor R 12 , the turned-on bipolar transistor Q 12 , the MOSFET Q 22  having a diode function, and the resistor R 21 . The X-cap is discharged through the resistors R 12 , R 13 , R 14 , R 24 , R 23 , and R 22 . The capacitor C 11  is charged by discharge current of the X-cap. When the capacitor C 11  is charged, the flow of the current of the base of the bipolar transistor Q 12  is interrupted. Thus, as illustrated in  FIG. 7B , the bipolar transistor Q 12  is turned off, and the voltage of the emitter of the bipolar transistor Q 12  is electrically converted to a high level. Accordingly, the voltage of the gate of the N-channel enhancement type MOSFET Q 11  is also converted to a high level, and the MOSFET Q 11  is turned on. When the MOSFET Q 11  is turned on, the X-cap is discharged through the resistor R 11  having a relatively small resistance value, the turned-on MOSFET Q 11 , the MOSFET Q 21  having a diode function, and the resistor R 21 . Due to R 11 &lt;R 12 , and R 21 &lt;R 22 , the X-cap can be rapidly discharged through the resistors R 11  and R 21  having a relatively small resistance value. 
       FIGS. 8A and 8B  illustrate negative (−) discharge of the X-cap in the discharge circuit unit in the event of the plug-off of the image forming apparatus according to the embodiment. As illustrated in  FIG. 8A , in the state in which the side of the end Live is the negative (−) polarity, and the side of the end Neutral is the positive (+) polarity, when the plug  106  undergoes plug-off, the electric charges charged in the X-cap begin to undergo negative (−) discharge. The MOSFET Q 21  is turned off, and the bipolar transistor Q 22  is turned on. As such, the X-cap is discharged through the resistor R 22 , the turned-on bipolar transistor Q 22 , the MOSFET Q 12  having a diode function, and the resistor R 21 . The X-cap is discharged through the resistors R 22 , R 23 , R 24 , R 14 , R 13 , and R 12 . The capacitor C 21  is charged by discharge current of the X-cap. When the capacitor C 21  is charged, the flow of the current of the base of the bipolar transistor Q 22  is interrupted. Thus, as illustrated in  FIG. 8B , the bipolar transistor Q 22  is turned off, and the voltage of the emitter of the bipolar transistor Q 22  is electrically converted to a high level. Accordingly, the voltage of the gate of the N-channel enhancement type MOSFET Q 21  is also converted to a high level, and the MOSFET Q 21  is turned on. When the MOSFET Q 21  is turned on, the X-cap is discharged through the resistor R 21  having a relatively small resistance value, the turned-on MOSFET Q 21 , the MOSFET Q 11  having a diode function, and the resistor R 21 . Due to R 11 &lt;R 12 , and R 21 &lt;R 22 , the X-cap can be rapidly discharged through the resistors R 11  and R 21  having a relatively small resistance value. 
     An exemplary embodiment can be applied to various electric appliances operated by the power supplied through the plug inserted into the socket and the power cable connected to the plug. For example, an exemplary appliance can be applied to various industrial apparatuses, office automation apparatuses, household electric appliances, etc. using the power as an energy source. 
     The discharge circuit unit and the image forming apparatus having the same in accordance with an embodiment can minimize standby power occurring in a standby mode of an electric product. The discharge circuit unit may be configured to be connected in series between opposite ends of an AC power input line. Thereby, it is possible to reduce the number of elements, to be easily commercialized, and to reduce consumption of standby power (detection power) while AC power is supplied. AC blocking is realized using MOSFET having a diode function, and thereby a separate blocking diode is not required. 
     Although a few embodiments have been shown and described, it would be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.