Patent Document

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS 
   The present invention relates to Japan Patent Application 2006-117332, filed on Apr. 21, 2006, which is incorporated herein by reference in its entirety and which forms a basis for priority. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   Embodiments of the present invention relate to switching electrical power source devices and methods, and in particular to such devices and methods that reduce the hum that is produced in audio equipment. 
   2. Description of the Related Art 
   In recent years, there has been a significant progress in the computerization of control systems for switching electrical power source devices. They have become lighter and smaller, and are widely used as the electrical power sources for electronic and electrical devices. An important problem that has been studied involves designing circuits with switching electrical power sources in which the Electro-Magnetic Interference (EMI) noise is reduced. 
   For example, the Japanese Patent Application Publication (Kokai) Number 2004-236059 describes a noise filter circuit that is used with switching electrical power source devices.  FIG. 2  is a block diagram that shows the circuit configuration of a typical switching electrical power source device. The switching electrical power source device  10  comprises an input section  11 , a noise filter section  12 , a rectification and smoothing section  13 , and a converter section  14 . An alternating current voltage is input to the input section  11 . The external noise that has entered from the input side is eliminated by the noise filter section  12 , and then it is rectified and smoothed by the rectification and smoothing section  13 . It is then converted into a specified direct current voltage by the converter section  14 , and then it is outputted. 
   A commercial alternating current of, for example, 100 V is supplied to the input section  11  from a commercial alternating current power source and from an alternating current supply line comprising a hot conductor L and a neutral conductor N. A fuse F 11  for excess current protection is connected to the hot conductor L. 
   In the noise filter section  12 , a capacitor C 11  for the reduction of high frequency noise is connected between the hot conductor L and the neutral conductor N. In the stage following that, a line bypass capacitor C 13  for the reduction of high frequency noise is connected between the hot conductor L and a frame ground FG. Together with this, a line bypass capacitor C 14  for the reduction of high frequency noise is connected between the neutral conductor N and the frame ground FG. 
   In the following stage, the choke coil L 11  (for the elimination of the common mode noise) is connected between the hot conductor L and the neutral conductor N. In the stage following that, the choke coil L 11  and capacitor C 12  is connected between the line hot conductor L after the fuse F 11  and the neutral conductor N. In addition, in the following stage, a line bypass capacitor C 15  is connected between the hot conductor L and the frame ground FG. Together with this, a line bypass capacitor C 16  is connected between the neutral conductor N and the frame ground FG. 
   The line bypass capacitors C 13 , C 14 , C 15 , and C 16  in the noise filter section  12  eliminate the common mode noise that is generated by the current that flows through the parasitic capacitor between the collector or the source of the switching element in the DC-DC converter and the frame ground FG. 
   The alternating current that has passed through the noise filter section  12 , after being rectified by a diode bridge D 11  in the rectification and smoothing section  13 , is smoothed by a capacitor C 17  in the rectification and smoothing section  13 , and thus becomes an a stable direct current voltage. The a stable direct current voltage is stabilized by the DC-DC converter in the converter section  14 , and then becomes a specified direct current voltage for output. 
   In  FIG. 2 , in order to simplify the explanation, details of several circuits such as the oscillation start circuit, the oscillator circuit, and the protection circuit have been omitted. 
   SUMMARY OF THE DISCLOSURE 
   With the type of switching electrical power source device that was described above, it is possible to eliminate a constant line noise, but there is still a hum present that comes from the commercial alternating current power source. The hum can be mixed in with the sound that is output from the speakers when this electric power is supplied. In such audio equipments, the ground of the secondary circuit is connected to a metal panel. Because there is a minute shocking current that is lower than the leakage current limit value that is demanded by safety regulations, when the metal panel is touched, a weak shock can occur that imparts an uncomfortable tingling or prickling feeling. 
   According to certain embodiments of the invention described herein, both the hum and the weak shocks can be reduced. 
   One of the embodiments of the present disclosure includes a switching electrical power source device that includes: 1) a rectification element that rectifies a current that is supplied by a current supply line, 2) a switching element that switches the input voltage that has been rectified by the rectification element, 3) an oscillator circuit that oscillates at a specified frequency and drives the switching element by means of the oscillation, and 4) a transformer with which the voltage that has been switched by the switching element is input to the primary coil. The electrical power source output is obtained from the secondary coil. The switching electrical power source device includes a first capacitor, one end of which is connected to the hot conductor of the current supply line, and a second capacitor, one end of which is connected to the neutral conductor. The other end of the first capacitor and the other end of the second capacitor are connected to each other. Their connection point then is connected to the ground conductor on the secondary side that is connected to the secondary coil. 
   In a second embodiment, the capacitances of the first capacitor and the second capacitor of the switching electrical power supply are roughly the same. 
   Furthermore, in a third embodiment, the switching electrical power source devices is furnished with a third capacitor that is connected to the ground conductor of the primary side of the transformer and the ground conductor of the secondary side. 
   Here, the conductors that are at the operating reference potential in the circuit, are called the ground conductors. As such, they are to be differentiated from the grounding conductors that are grounded to the earth. 
   A first embodiment as described above, has an advantage that it can reduce or prevent the hum that is generated in audio equipment to which an electric power is supplied. Furthermore, the weak shock that are provided when the metal panel of the equipment is touched, can be further reduced. Because the ground on the secondary side is connected to the ground of the external equipment that is connected on the secondary side, the impedance of the external equipment is lowered due to the fact that the first and second capacitors are connected to the ground on the secondary side. Therefore, the first embodiment presented above not only can reduce the hum, but it can also reduce the leakage current due to the switching noise. 
   According to a second embodiment as described above, in addition to having the same advantages as was outlined by the first embodiment, it can also improve the reduction of common mode noise. By making the capacitance of the first capacitor and the second capacitor the same, a hypothetical midpoint potential is formed at the connection point of the first capacitor and the second capacitor. Because the connection point is connected to the ground conductor on the secondary side, the impedance of the ground on the secondary side can be reduced. 
   According to a third embodiment as described above, in addition to having the advantages outlined for the other embodiments there is the advantage that it is possible to reduce the switching noise that is generated on the primary side. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram that shows an electrical configuration of a switching electrical power source device according to an embodiment of the present invention; and 
       FIG. 2  is a block diagram that shows an example of a conventional switching electrical power source device. 
   

   DETAILED DESCRIPTION 
   Example embodiments of the present invention are described here in detail with reference to the attached drawings. 
     FIG. 1  is a block diagram that shows an electrical configuration of a switching electrical power source device  1 , according to a first embodiment of the present invention. As is shown in  FIG. 1 , the switching electrical power source device  1  includes an input section  2 , a noise filter section  3 , a rectification and smoothing section  4 , a switching section  5 , an oscillation start section  6 , an oscillator  7 , a transformer P 1 , and a secondary side rectification section  8 . 
   An alternating current voltage that is input to the input section  2 , an output on the line of the external noise that is entered from the input side, and a noise that is generated by the switching section  5 , are filtered by the noise filter section  3 . After being rectified and smoothed by the rectification and the smoothing section  4 , the voltage is switched by the switching section  5  and the voltage conversion is carried out on the secondary side by the transformer P 1 . The oscillation start section  6  is a circuit that supplies an electric power for oscillation to start by the oscillator section  7  when the power to the switching electrical power source device  1  is turned on. Once the oscillation starts, the oscillator section  7  drives the switching section  5 . 
   The switching section  5  switches the voltage that is supplied from the rectification and the smoothing section  4 , and a high frequency alternating current voltage is supplied to the primary side of the transformer P 1 . When an alternating current is supplied to the primary side of the transformer P 1 , an alternating current is produced on the secondary side. A voltage is also generated in the oscillator section  7 , and the oscillation continues. The voltage that has been produced on the secondary side of the transformer P 1 , is rectified and smoothed out, and is supplied to the audio equipment. 
   In the input section  2 , the commercial alternating current power of, for example, 100 V is input from the alternating current supply line that comprises the hot conductor L and the neutral conductor N. One end of each of the line bypass capacitors C 1  and C 2  is connected to the hot conductor L, and the neutral conductor N. The other ends of these capacitors are connected together, and their connection point is connected to the ground conductor of the secondary side circuit. 
   These capacitors reduce the high frequency noise and as a result, it is possible to reduce the generation of the hum from the frequency of the commercial electric power source (for example, 50 Hz or 60 Hz). In addition, by connecting these capacitors to the ground conductor of the secondary side circuit, it is possible to make the capacitance of the capacitor C 3  smaller. Accordingly, the weak shock can be reduced or prevented, in those cases where the panel of the equipment (that is connected to the ground conductor on the secondary side) is touched. 
   The fuse F 1  is connected in a stage following the line bypass capacitor C 1  and the line bypass capacitor C 2 . The capacitances of the capacitors C 1  and C 2  are made roughly of the same value in the range of 470 pF to 100 pF. 
   In the noise filter section  3 , a capacitor and a resistor for reducing high frequency noise are connected between the hot conductor L and the neutral conductor N. In the following stage, the choke coil CH 1  is connected and the noise of the common mode component is eliminated. A capacitor is also connected between the hot conductor L and the neutral conductor N in the stage following the choke coil CH 1 . It is then connected to the rectification and smoothing section  4 . 
   In the rectification and smoothing section  4 , a bridge is formed by four diodes. The positive pole that has been rectified is connected to the oscillation start section  6 . It is connected to one end of the primary side winding of the switching transformer P 1 , and the positive electrode of the electrolytic capacitor C 4 . The negative electrode of the electrolytic capacitor C 4  is connected to the ground conductor on the primary side. In addition, the negative pole of the bridge is connected to the ground conductor on the primary side through a thermistor. A thermistor is a device in which the value of the resistance changes (for example, becomes higher) as the temperature increases. It is affixed to the heat sink and the like that is attached to the switching element. Accordingly, in those cases where an abnormality due to a short circuit or a failure of a component and the like occurs, and the temperature of the switching element rises, the resistance value increases, and the flow of current is prevented. 
   The oscillation start section  6  is a circuit that supplies an electric power such that the oscillation is started by the oscillator at the time that the power source to the switching electrical power source device  1  is turned on. The oscillator section  7  may include an oscillator circuit that oscillates at a high frequency. The output of the oscillator is connected to the gate electrode of the field effect transistor Q 1 , which is a switching element, and drives the field effect transistor on and off. 
   The drain electrode of the field effect transistor Q 1  is connected to the other end of the primary side winding of the switching transformer P 1 . The source electrode of the field effect transistor Q 1  is connected to the ground conductor on the primary side. When the field effect transistor Q 1  is turned on and off, current flows in the primary side of the switching transformer P 1  and an alternating current having a specified voltage is produced on the secondary side of the transformer P 1 . Together with this, an electric power is supplied to the oscillator section  7 , and the stable oscillation continues in the oscillator section  7 . 
   The secondary side includes the diode D 2  that rectifies the alternating current having a specified produced voltage, and the capacitor C 5  that smoothes the current. An electric power having a specified voltage is supplied to the audio equipment that is connected to the secondary side. The capacitor C 6  is connected in parallel with the diode D 2  and reduces high frequency noise when carrying out the smoothening. With this switching electrical power source, the supply of electric power to various types of equipment such as an AC adapter is possible. A socket S 1  may be provided for connecting external equipment. 
   The capacitor C 3  is connected between the ground conductor on the primary side and the ground conductor on the secondary side. Since this capacitor C 3  discharges the noise that accompanies the switching operation, the capacitor is effective for reducing EMI. 
   As discussed above, in the embodiment involving the switching electrical power source device  1 , since one end of each of the line bypass capacitors C 1  and C 2  is connected to the hot conductor L and the neutral conductor N, the other ends are connected together, and the connection point is connected to the ground conductor of the secondary side circuit, the noise that accompanies the switching operation is reduced. As a result, it is possible to make the capacitance of the capacitor C 3  smaller. 
   As it was explained above, in the switching electrical power source device  1  in accordance with an embodiment of the present invention, one end of each of the line bypass capacitors C 1  and C 2  is connected to the hot conductor L and to the neutral conductor N (to which the commercial power source is supplied). The other ends are connected together, and the connection point is connected to the ground conductor of the secondary side circuit. As a result of this circuit configuration, the hum from the frequency of the power source, is reduced. It is also possible to reduce the shock by making the capacitance of the capacitor C 3  smaller for those cases where the panel (of the equipment to which the ground conductor on the secondary side is connected) is touched. 
   The explanation given above are samples of a few possible embodiments of the present invention. The present invention is not in any way limited to the embodiments that were discussed above. Various modifications and changes are possible without diverging from the scope of the present invention.

Technology Category: 5