Abstract:
A power supply circuit employed in various types of electronic appliances, telecommunications equipment, and the like, is provided for generating a voltage pulse by resonance effect of a primary side of a transformer, and outputting the voltage pulse from a secondary side after raising a voltage thereof. The power supply suppresses fluctuations of electric current, and prevents noise from being propagated through a device powered by the supply. A control circuit actuates a second switching element into a non-conducting state while a first switching element is in a conducting state. The power supply switches the second switching element into a conducting state when voltage of a primary coil is greater than voltage of a driving power supply after an electric current flows in a first diode while the first switching element is in a non-conducting state.

Description:
THIS APPLICATION IS A U.S. NATIONAL PHASE APPLICATION OF PCT INTERNATIONAL APPLICATION PCT/JP00/00548. 
     FIELD OF THE INVENTION 
     The present invention relates to a power supply circuit employed in a various kind of electronic appliances, telecommunications equipment, and the like, for generating a voltage pulse by resonance effect of a primary side of transformer, and outputting the voltage pulse from a secondary side after raising a potential thereof. 
     BACKGROUND OF THE INVENTION 
     With reference to accompanying figures, a power supply circuit of the prior art will be described hereafter. 
     FIG. 16 represents a circuit diagram illustrating a power supply circuit of the prior art, and FIG. 17 is a drawing of waveforms showing changes in voltage, current, and switch pulse in the power supply circuit with time. 
     The power supply circuit of the prior art shown in FIG. 16 is intended to stabilize an output voltage of high potential applied by a transformer to a display. Its composition has been such that it comprises a driving power supply  403  connected to one side of terminals of a primary coil  402  of the transformer  401 , and a switching element  404 , a capacitor  405  and a diode  406  connected to the other side of the terminals of the primary coil  402 . 
     The switching element  404  is comprised of a MOS type field-effect transistor (MOS FET) that has an internal diode. This MOS type field-effect transistor is disposed in a manner that a drain is connected to the other side terminal of the primary coil  402 , a source is connected to a ground side, and a gate is connected to a PWM control circuit  407 , which generates a pulse wave for controlling the switching element  404 . It contains the internal diode with its anode connected to the ground side, and a cathode to the other side terminal of the primary coil  402 . The capacitor  405  has its one end connected to the other side terminal of the primary coil  402 , and the other end connected to the ground side. The diode  406  has its cathode connected to the other side terminal of the primary coil  402 , and anode connected to the ground side. In addition, the cathode of the diode  406  and the one end of the capacitor  405  are connected to a point where the drain of the switching element  404  and the primary coil  402  make connection. 
     A display  409  (CRT) having high horizontal and vertical scanning frequency is connected to a secondary coil  408  of the transformer  401 . 
     Furthermore, waveforms of voltage, current and switch pulse in this power supply circuit, as they change with a lapse of time are shown in FIG.  17 . 
     In FIG. 17, a reference letter (a) represents a waveform illustrating a time series of change in value of voltage induced in the primary coil  402  of the transformer  401  taken at a point “O” in the power supply circuit; a letter (b) a waveform illustrating a time series of change in amount of current flowing at the point “O” in the power supply circuit; and a letter (c) a waveform illustrating a time series of change in shape of an output wave of the PWM control circuit fed to the switching element  404 . 
     During a period of A to B in FIG. 17, when a pulse wave (the output wave) of a predetermined duration shown by the waveform (c) is input from the PWM control circuit  407  to the switching element  404 , making the switching element  404  into an ON state, amount of electric current in the point “O” increases with time in proportion to a duration of the ON state of the switching element  404  as shown by the waveform (b), and thereby energy is charged into the primary coil  402 . 
     During a period of B to C, when input of the pulse wave from the PWM control circuit  407  to the switching element  404  is ceased, as shown by the waveform (c), to turn the switching element  404  into an OFF state, the energy charged in the primary coil  402  begins to be charged into the capacitor  405 , and amount of the current in the point “O” decreases with time, as shown by the waveform (b). The voltage of the primary coil  402  reaches its peak value as shown by the waveform (a), upon completion of the charge. 
     During a period of C to D, after completion of the charge into the capacitor  405 , the energy charged in the capacitor  405  begins to be recharged into the primary coil  402  again, and amount of the current in the point “O” decreases with time, as shown by the waveform (b). The voltage of the primary coil  402  becomes zero as shown by the waveform (a), when the charge is completed. 
     During a period of D to E, when the charge to the primary coil  402  is completed, the energy charged in the primary coil  402  is about to start being recharged into the capacitor  405  again, and this recharge of the capacitor  405  is to begin through the ground side due to an effect of a positive-negative relation in polarity of the voltage across the primary coil  402 . However, the capacitor  405  is not charged, but a current flows through the diode  406  having a low impedance, since the diode  406  is placed between the other side terminal of the primary coil  402  and the ground with the anode connected to the ground side. Therefore, although amount of the current flowing in the point “O” increases with time as shown by the waveform (b), the voltage of the primary coil  402  remains zero as shown by the waveform (a), since no energy is charged into the capacitor  405 . 
     During a period of E to F, since the energy charged in the primary coil  402  has been discharged by the flow of current through the diode  406 , amount of the current shown by the waveform (b) in the point “O” shall now remain theoretically zero, unless the switching element  404  is turned into an ON state with the waveform (c). In reality, however, amount of the current through the point “O” increases for a certain period of time as shown by the waveform (b). 
     A certain amount of energy is therefore charged in the primary coil  402  due to the increase of current through the point “O”. 
     Subsequently, during a period of F to G, the energy charged into the primary coil  402  begins to be charged to the capacitor  405  in the same manner as above, after the charge to the primary coil  402  is completed. Thus, amount of the current in the point “O” decreases with time, as shown by the waveform (b), and voltage of the primary coil  402  reaches its peak value as shown by the waveform (a), when the charge is completed. 
     During a period of G to H, after completion of the charge into the capacitor  405 , the energy charged in the capacitor  405  begins to be recharged into the primary coil  402  again, and amount of the current in the point “O” decreases with time, as shown by the waveform (b). The voltage of the primary coil  402  becomes zero as shown by the waveform (a), when the charge is completed. 
     During a period of H to I, when the charge into the primary coil  402  is completed, the energy charged in the primary coil  402  is about to start being recharged into the capacitor  405  again. While amount of the current flowing in the point “O” increases with time as shown by the waveform (b), the voltage of the primary coil  402  remains zero as shown by the waveform (a), since no energy is charged into the capacitor  405 . 
     During a period of I to J, amount of the current in the point “O” again increases for a certain period of time in the same manner as described above, as shown by the (b), and energy is hence charged in the primary coil  402 . 
     During a period of J to K (A), after the charge to the primary coil  402  is completed, the energy charged in the primary coil  402  begins to be charged into the capacitor  405 . Amount of the current in the point “O” decreases with time, as shown by the waveform (b), and the voltage of the primary coil  402  reaches its peak value as shown by the waveform (a), when the charge is completed. Since the switching element  404  is turned into an ON state during this period, as shown by the waveform (c), this becomes a new starting point and the same steps as above are repeated over again. 
     In the above composition, an output voltage of the secondary coil  408  changes depending on value of the voltage of the primary coil  402 . The voltage of the primary coil  402  varies depending on a duration of time in the ON state of the switching element  404 , and the longer the ON state, the greater the voltage. 
     As the pulse wave of the predetermined duration is input to the switching element  404  from the PWM control circuit  407  during this step, the voltage of the primary coil  402  becomes zero instantly at a timing the pulse wave goes on. Due to this sudden change in value of the voltage, amount of the current in the point “O” increases while producing undulation (W) shown by the waveform (b) in FIG. 17 during the ON state of the switching element  404 . It has a substantial effect, especially if the voltage of the primary coil  402  changes to zero from a value greater than a voltage of the driving power supply in the timing the pulse wave turns on. 
     As has been described, there has been a problem with the above composition in that noises are generated on an display screen due to an influence of the undulation (W) in the current. 
     DISCLOSURE OF THE INVENTION 
     An object of the present invention is to provide a power supply circuit that suppresses undulation in current, and prevents screen noises from being generated on a display, for instance, when it is used as a load. 
     In order to achieve this object, the present invention provides a composition comprising; 
     a driving power supply connected to one side of terminals of a primary coil of transformer, 
     a first switching element, 
     a capacitor, and 
     a first diode, all connected to the other side of the terminals of the primary coil. 
     The first switching element is comprised of a first MOS type field-effect transistor (MOS FET) having a drain connected to the other side terminal of the primary coil, a source connected to a ground side, and a gate connected to a control circuit. 
     The capacitor has its one end connected to the other side terminal of the primary coil, and the other end connected to the ground side. 
     The first diode has its cathode connected to the other side terminal of the primary coil, and an anode connected to the ground side. 
     In addition, there is provided a noise suppression means between the transformer and the control circuit to restrict generation of noises. 
     With the composition as described above, generation of noises can be restricted. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit diagram illustrating a power supply circuit of a first exemplary embodiment of the present invention; 
     FIG. 2 is a drawing of waveforms showing time series of changes in voltage, current, and switch pulse in the same power supply circuit; 
     FIG. 3 is a drawing of waveforms showing time series of changes in voltage, current, and switch pulse in a power supply circuit of a second exemplary embodiment of this invention; 
     FIG. 4 is a circuit diagram illustrating a power supply circuit of a third exemplary embodiment of this invention; 
     FIG. 5 is a drawing of waveforms showing time series of changes in voltage, current, and switch pulse in the same power supply circuit; 
     FIG. 6 is a circuit diagram illustrating a power supply circuit of a fourth exemplary embodiment of this invention; 
     FIG. 7 is a drawing of waveforms showing time series of changes in voltage, current, and switch pulse in the same power supply circuit; 
     FIG. 8 is a circuit diagram illustrating a power supply circuit of a fifth exemplary embodiment of this invention; 
     FIG. 9 is a drawing of waveforms showing time series of changes in voltage, current, and switch pulse in the same power supply circuit; 
     FIG. 10 is a circuit diagram illustrating a power supply circuit of a sixth exemplary embodiment of this invention; 
     FIG. 11 is a drawing of a waveform showing a high voltage wave output by a transformer in the same power supply circuit; 
     FIG. 12 is another drawing of a waveform showing a high voltage wave output by a transformer of poor dynamic characteristic; 
     FIG. 13 is still another drawing of a waveform showing a high voltage wave output by a transformer of poor rising characteristic; 
     FIG. 14 is a circuit diagram of a power supply circuit of a seventh exemplary embodiment of this invention; 
     FIG. 15 is a drawing of waveforms showing time series of changes in voltage, current, and switch pulse in the same power supply circuit; 
     FIG. 16 is a circuit diagram of a power supply circuit of the prior art; and 
     FIG. 17 is a drawing of waveforms showing time series of changes in voltage, current, and switch pulse in the same power supply circuit. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Exemplary Embodiment 
     With reference to accompanying figures, a power supply circuit of a first exemplary embodiment of this invention will be described hereinafter. 
     In FIG. 1, the power supply circuit of the first exemplary embodiment of this invention comprises a driving power supply  23  connected to one end of terminals of a primary coil  22  of a transformer  21 , a first switching element  24 , a capacitor  25 , and a first diode  26 , all connected to the other end of the terminals of the primary coil  22 . 
     Furthermore, the first switching element  24  is comprised of a first MOS type field-effect transistor (MOS FET) that has an internal diode. This first MOS type field-effect transistor is disposed in a manner that a drain is connected to the other side terminal of the primary coil  22 , a source is connected to a ground side, and a gate is connected to a PWM type control circuit  27 , which generates a pulse wave to control the first switching element  24 . It contains the internal diode with an anode connected to the ground side, and a cathode to the other side terminal of the primary coil  22 . The capacitor  25  has its one end connected to the other side terminal of the primary coil  22 , and the other end connected to the ground side. The first diode  26  has its cathode connected to the other side terminal of the primary coil  22 , and an anode connected to the ground side. In addition, the cathode of the first diode  26  and the one end of the capacitor  25  are connected to a point where the drain of the first MOS type field-effect transistor and the primary coil  22  make connection. 
     There is also provided with an auxiliary coil  30 , which is mutually inductive with the primary coil  22 , with one of terminals connected to the ground side. In addition, a second diode  31  and a second switching element  32  are disposed in connection to the other side of the terminals of the auxiliary coil  30 . The second diode  31  and the second switching element  32  compose a noise suppression means. The second switching element  32  is comprised of a second MOS type field-effect transistor having a drain connected to the other side terminal of the auxiliary coil  30  through the second diode  31 , a source connected to the ground side, and a gate connected to the PWM type control circuit  27 . The second diode  31  has an anode connected to the other side terminal of the auxiliary coil  30 , and a cathode connected to the drain side of the second MOS type field-effect transistor. The control circuit  27  is so designed as to switch the second switching element  32  into an OFF state while the first switching element  24  is in its ON state, and into an ON state when voltage of the primary coil  22  is greater in value than voltage of the driving power supply after an electric current flows in the first diode  26  while the first switching element  24  is in its OFF state. 
     A display  29  (CRT) having high horizontal and vertical scanning frequency, or the like, is connected to an output side of a secondary coil  28 . 
     In this power supply circuit, waveforms of voltage, current and switch pulses, as they change with a lapse of time, are shown in FIG.  2 . 
     In FIG. 2, a reference letter (a) represents a waveform illustrating a time series of change in value of voltage induced in the primary coil  22  of the transformer  21  taken at a point “O” in the power supply circuit of FIG. 1; a letter (b) a waveform illustrating a time series of change in amount of current flowing in the point “O” in the power supply circuit ; a letter (c) a waveform illustrating a time series of change in shape of an output wave of the control circuit  27  fed to the first switching element  24 , a letter (d) a waveform illustrating a time series of change in shape of an output wave of the control circuit  27  fed to the second switching element  32 ; and a letter (e) a waveform illustrating a time series of change in amount of current flowing through a point “P” in the power supply circuit of FIG.  1 . 
     During a period of A to B in FIG. 2, when a pulse wave (the output wave) of a predetermined duration shown by the waveform (c) is input from the control circuit  27  to the first switching element  24 , making the first switching element  24  into an ON state, amount of electric current in the point “O” in FIG. 1 increases with time in proportion to a duration of the ON state of the first switching element  24  as shown by the waveform (b), and thereby energy is charged into the primary coil  22 . 
     During a period of B to C, when an input of the pulse wave from the control circuit  27  to the first switching element  24  is ceased as shown by the waveform (c), causing the first switching element  24  into an OFF state, the energy charged in the primary coil  22  begins to be charged into the capacitor  25 , and amount of the current in the point “O” decreases with time, as shown by the waveform (b). The voltage of the primary coil  22  reaches its peak value as shown by the waveform (a), when the charge is completed. 
     During a period of C to D, after completion of the charge into the capacitor  25 , the energy charged in the capacitor  25  begins to be recharged into the primary coil  22  again, and amount of the current in the point “O” decreases with time, as shown by the waveform (b). The voltage of the primary coil  22  becomes zero as shown by the waveform (a), when the charge is completed. 
     During a period of D to E, when the charge to the primary coil  22  is completed, the energy charged in the primary coil  22  is about to start being recharged into the capacitor  25  again, and this recharge of the capacitor  25  begins through the ground side due to an effect of a positive-negative relation in polarity of the voltage across the primary coil  22 . However, a current flows through the first diode  26  having a low impedance, instead of charging the capacitor  25 , since the first diode  26  is disposed between the other side terminal of the primary coil  22  and the ground, with the anode connected to the ground side. Therefore, although amount of the current flowing in the point “O” increases with time as shown by the waveform (b), the voltage of the primary coil  22  remains zero as shown by the waveform (a), since no energy is charged into the capacitor  25 . 
     During a period of E to F, since the energy charged in the primary coil  22  has been discharged by the flow of current through the first diode  26 , amount of the current shown by the waveform (b) in the point “O” shall theoretically remain zero, unless the switching element  24  is turned into an ON state with the waveform (c). In reality, however, amount of the current through the point “O” increases for a certain period of time as shown by the waveform (b). 
     A certain amount of energy is therefore charged in the primary coil  22  due to the increase in amount of the current through the point “O”. 
     During a period of F to G, the energy charged in the primary coil  22  begins to be charged into the capacitor  25  in the same manner as above, after the charge to the primary coil  22  is completed. Thus, amount of the current in the point “O” decreases with time, as shown by the waveform (b), and the voltage of the primary coil  22  reaches its peak. value as shown by the waveform (a), when the charge is completed. 
     During a period of G to H, after completion of the charge into the capacitor  25 , the energy charged in the capacitor  25  begins to be recharged into the primary coil  22  again, and amount of the current in the point “O” decreases with time, as shown by the waveform (b). The voltage of the primary coil  22  becomes zero as shown by the waveform (a), when the charge is completed. 
     During a period of H to I, when the charge to the primary coil  22  is completed, the energy charged in the primary coil  22  is about to start being recharged into the capacitor  25  again. While amount of the current in the point “O” increases with time as shown by the waveform (b), the voltage of the primary coil  22  remains zero as shown by the waveform (a), since no energy is charged into the capacitor  25 . 
     During a period of I to J, amount of the current in the point “O” again increases for a certain period of time in the same manner as described above, as shown by the waveform (b), and energy is hence charged in the primary coil  22 . 
     During a period of J to K (A), after the charge to the primary coil  22  is completed, the energy charged in the primary coil  22  begins to be charged into the capacitor  25 . Amount of the current through the point “O” decreases with time, as shown by the waveform (b), and the voltage of the primary coil  22  reaches its peak value as shown by the waveform (a), when the charge is completed. Since the first switching element  24  is turned into an ON state during this period, as shown by the waveform (c), this becomes a new starting point again. 
     In addition, the second switching element  32  is so operated that it turns into an ON state when voltage of the primary coil  22  is greater than the voltage level of the driving power supply after an electric current flows in the first diode  26  while the first switching element  24  is in its OFF state (at points X and (X)), and it turns into an OFF state while the first switching element  24  is in its ON state (at points Y and (Y)). 
     At a time when the second switching element  32  goes into an ON state as shown by the waveform (d), an electric current flows through the point “P” simultaneously, as shown by the waveform (e). 
     In a period of (A) through (D), same operations as described above are repeated. 
     Waveforms of (c) and (d) are adjusted in advance of their timings according to a cyclic period of the waveform (a). 
     With the above-described composition, voltage of the primary coil  22  decreases to the voltage level of the driving power supply at the moment the second switching element  32  is turned on, when the voltage of the primary coil  22  is greater than the voltage of the driving power supply, and the voltage of the primary coil  22  then becomes zero at the moment the first switching element  24  is turned into an ON state, since the second switching element  32  is switched into its ON state when the voltage of the primary coil  22  is greater than the voltage level of the driving power supply after electric current flows in the first diode  26  while the first switching element  24  is in its OFF state (at the points X and (X)). 
     The voltage of the primary coil  22  decreases gradually to the voltage level of the driving power supply during this moment, because the energy built up in the transformer  21  is gradually discharged, while the auxiliary coil  30  is also short-circuited. 
     In other words, there is no sudden change in voltage, and undulation liable to occur in the current wave can be restricted, since the voltage always changes from a value equal to or below the voltage of the driving power supply to zero at the moment the first switching element  24  is turned on. 
     As described above, the first exemplary embodiment of this invention eliminates a sudden change in voltage, and restricts the undulation likely to occur in the current wave, because it always changes the voltage from a value equal to or below the voltage of the driving power supply to zero at the moment the first switching element  24  is turned on. 
     Second Exemplary Embodiment 
     Referring now to FIG.  1  and FIG. 3, a power supply circuit of a second exemplary embodiment of this invention will be described hereinafter. 
     A power supply circuit of the second exemplary embodiment of this invention employs a change in control function of the control circuit  27  in the power supply circuit, shown in FIG. 1, in the first exemplary embodiment of this invention. 
     A control circuit  27  of the power supply circuit in the second exemplary embodiment of this invention generates a pulse wave to control a first switching element  24  and a second switching element  32  and thereby it switches the second switching element  32  into an OFF state while the first switching element  24  is in an ON state, and into an ON state while an electric current is flowing through a first diode  26 . 
     In this power supply circuit, waveforms of voltage, current and switch pulses, as they change with a lapse of time, are shown in FIG.  3 . 
     In FIG. 3, a reference letter (a) represents a waveform illustrating a time series of change in value of voltage induced in a primary coil  22  of a transformer  21  taken at a point “O” in the power supply circuit; a letter (b) a waveform illustrating a time series of change in amount of current in the point “O” in the power supply circuit; a letter (c) a waveform illustrating a time series of change in shape of an output wave of the control circuit  27  fed to the first switching element  24 , a letter (d) a waveform illustrating a time series of change in shape of an output wave of the control circuit  27  fed to the second switching element  32 ; and letter (e) a waveform illustrating a time series of change in amount of current in a point “P” in the power supply circuit of FIG.  1 . 
     During a period of A to C in FIG. 3, when a pulse wave (the output wave) of a predetermined duration shown by the waveform (c) is input from the control circuit  27  to the first switching element  24  to turn the first switching element  24  into an ON state, amount of electric current through the point “O” increases with time in proportion to a duration of the ON state of the first switching element  24  as shown by the waveform (b), and thereby energy is charged into the primary coil  22 . An amount of current in the point “P” remains zero while there is an electric current flowing in the point “O”, since no current flows in the point “P”. 
     The pulse wave (output wave) of predetermined duration input to the second switching element  32  from the control circuit  27  is cut off at any given timing of a point “B” during the period of A to C, as shown by the waveform (d), to keep the second switching element  32  in its OFF state. 
     During a period of C to D, when the first switching element  24  is turned into an OFF state by terminating the input of pulse wave of the control circuit  27  to the first switching element  24 , as shown by the waveform (c), the energy charged into the primary coil  22  begins to be charged into the capacitor  25 . This causes the amount of current in the point “O” to decrease with time as shown by the waveform (b), and the voltage of the primary coil  22  reaches its peak value as shown by the waveform (a), when the charge is completed. 
     During a period of D to E, after completion of the charge into the capacitor  25 , the energy charged in the capacitor  25  begins to be recharged into the primary coil  22  again, and amount of the current in the point “O” decreases with time, as shown by the waveform (b). The voltage of the primary coil  22  becomes zero as shown by the waveform (a), when the charge is completed. 
     During a period of E to G, when the charge to the primary coil  22  is completed, the energy charged in the primary coil  22  is about to start being recharged into the capacitor  25  again, and this recharge of the capacitor  25  begins through the ground side due to an effect of a positive-negative relation in polarity of the voltage across the primary coil  22 . However, a current flows through the first diode  26  having a low impedance, instead of charging the capacitor  25 , since there is the first diode  26  between the other side terminal of the primary coil  22  and the ground, with the anode connected to the ground side. Therefore, although amount of the current in the point “O” increases with time as shown by the waveform (b), the voltage of the primary coil  22  remains zero as shown by the waveform (a), since no energy is charged into the capacitor  25 . 
     The pulse wave (output wave) of a predetermined duration from the control circuit  27  is input to the second switching element  32  at any given timing of a point “F” during the period of E to G, as shown by the waveform (d), to keep the second switching element  32  in its ON state. 
     During a period of G to H, since the energy charged in the primary coil  22  has been discharged by the flow of current through the first diode  26 , amount of the current shown by the waveform (b) in the point “O” shall theoretically remain zero, unless the switching element  24  is turned into an ON state with the waveform (c). In reality, however, amount of the current in the point “O” increases for a certain period of time as shown by the (b). 
     Although a certain amount of energy is due to be charged in the primary coil  22  with the increase in amount of the current through the point “O”, no energy is charged in the primary coil  22 , because the second switching element  32  is held in its ON state at the point “F” of given timing during the period of E to G, which turns the auxiliary coil  30  into a short-circuited condition. 
     This is due generally to the phenomenon involving two coils that are mutually inductive, in which energy is not charged into one of the coils even when electric current flows in it, if the other coil is in a short-circuited condition. 
     During a period of H to I, no energy is charged in the capacitor  25 , since energy is not charged in the primary coil  22 . Thus, amount of the current through the point “O” decreases with time to zero, as shown by the waveform (b), and the voltage of the primary coil  22  is kept at same value as the voltage of the driving power supply  23 , as shown by the waveform (a). 
     During a period of I to J(A), when the amount of current becomes zero as shown by the waveform (b), a current begins to flow in the point “P” as shown by the waveform (e), to make the voltage of the primary coil  22  kept at the voltage of the driving power supply, and this continues until there turns into an ON state in the waveform (c). 
     Subsequently, the same steps are repeated throughout a period of (A) to (G). 
     With the above-described composition, an electric current, which is a cause of charging energy in the primary coil  22 , flows into the primary coil  22 , after the first switching element  24  turns into its OFF state from the ON state by the pulse wave generated in the control circuit  27 , and the voltage becomes zero as the current flows for the first time in the first diode  26 . However, the primary coil  22  can be prevented from being charged with energy, even when the current flows through the primary coil  22 . 
     In an ordinary case, the voltage waveform is liable to be subjected under an L-C resonance with reference to the voltage of the driving power supply  23 , after the first current flows through the first diode  26 . In the above composition, however, the voltage is maintained constant at the same voltage as the driving power supply  23 , which is greater than zero. 
     Therefore, this composition can reliably produce the voltage across the primary coil  22  of the transformer  21  even for a display  29  having high resolution with high horizontal and vertical scanning frequency, or the like, since it does not allow electric current to flow through the first diode  26  at the second time and thereafter, thereby preventing the phenomenon in that no current flows in the first switching element  24  when the pulse wave is turned into ON state. 
     The control circuit  27 , in particular, is so devised that it switches the second switching element  32  into its ON state while there is an electric current flowing through the first diode  26 . Therefore, the primary coil  22  is not actually charged with energy, even if the electric current, a cause of charging energy to the primary coil  22 , flows in the primary coil  22 , after the voltage becomes zero as the current flows through the first diode  26  for the first time. 
     Accordingly, since no energy is charged in the primary coil  22  to cause an L-C resonance with reference to the voltage of the driving power supply  23 , it prevents the resonance from taking place, thereby maintaining the voltage waveform at the same voltage as the driving power supply  23  after the first electric current flows through the first diode  26 . 
     According to the second exemplary embodiment, as described above, the control circuit  27  switches the second switching element  32  into its ON state while electric current flows through the first diode  26 . Therefor, no energy is actually charged in the primary coil  22 , even if the electric current, a cause of charging energy to the primary coil  22 , flows in the primary coil  22 , after the voltage becomes zero due to the first current flowing through the first diode  26 . As the result, it prevents a resonance from taking place, thereby maintaining the voltage waveform at the same voltage as the driving power supply  23  after the first electric current flows through the first diode  26 , since no energy is charged in the primary coil  22  to cause the L-C resonance with reference to the voltage of the driving power supply  23 . 
     Accordingly, it can prevent the phenomenon in that no current flows in the first switching element  24  when the first switching element  24  is switched into its ON state, and produce the voltage reliably across the primary coil  22  of the transformer, even for the display  29  having high resolution with high horizontal and vertical scanning frequency, or the like. 
     Third Exemplary Embodiment 
     A power supply circuit of a third exemplary embodiment of this invention will be described hereinafter with reference to the accompanying figures. 
     In FIG.  4  and FIG. 5, the power supply circuit for a transformer in the third exemplary embodiment of the present invention is a modified version of the transformer power supply circuit of the first exemplary embodiment of this invention. Therefore, the same structural components are assigned with the same reference numerals. 
     The power supply circuit of the third exemplary embodiment of this invention has a composition that comprises a third diode  33  provided between a drain of a MOS type field-effect transistor constituting a first switching element  24  and a cathode of a first diode  26 , wherein a cathode of the third diode  33  is connected to a drain of the MOS type field-effect transistor serving as the first switching element  24 , and an anode of the third diode  33  is connected to the cathode of the first diode  26 . 
     In this power supply circuit, waveforms of voltage, current and switch pulses, as they change with a lapse of time, are shown in FIG.  5 . 
     In FIG. 5, a reference letter (a) represents a waveform illustrating a time series of change in value of voltage induced in a primary coil  22  of a transformer  21  taken at a point “O” in the power supply circuit; a letter (b) a waveform illustrating a time series of change in amount of current in the point “O” of the power supply circuit; a letter (c) a waveform illustrating a time series of change in shape of an output wave of a control circuit  27  fed to the first switching element  24 ; a letter (d) a waveform illustrating a time series of change in shape of another output wave of the control circuit  27  fed to a second switching element  32 ; and a letter (e) a waveform illustrating a time series of change in amount of current at a point “P” of the power supply circuit of FIG.  4 . Illustration of the waveforms in the third exemplary embodiment is identical to the corresponding illustration in the first exemplary embodiment, except for the waveforms (a) and (b) in a period between E and K(A). 
     During the period of E through K(A), a voltage shown by the waveform (a) does not become smaller than zero, even though the generated voltage indicates an L-C resonance around a driving power supply voltage, since an addition of the third diode  33  reduces an amount of current produced, as shown by the waveform (b). 
     Therefore, once an electric current flows through the first diode  26  for the first time, this composition does not allow subsequent current to flow through it at the second time and thereafter, thereby preventing the phenomenon that no current flows in the first switching element  24  when the first switching element  24  is turned into an ON state, as shown by the waveform (c). Hence, it can produce a voltage reliably across the primary coil  22  of the transformer, even for a display  29  having high resolution with high horizontal and vertical scanning frequency, or the like. 
     Additionally, in the same manner as the power supply circuit of the first exemplary embodiment, the second switching element  32  is so operated that it turns into an ON state when voltage of the primary coil  22  is greater than a voltage level of the driving power supply after an electric current flows in the first diode  26  while the first switching element  24  is in its OFF state (at points X and (X)), and it turns into an OFF state while the first switching element  24  is in its ON state (at points Y and (Y)). Therefore, voltage of the primary coil  22  decreases to the voltage level of the driving power supply at the moment the second switching element  32  is turned on, when the voltage of the primary coil  22  is greater than the voltage of the driving power supply, and that the voltage of the primary coil  22  becomes zero at the moment the first switching element  24  is turned on. 
     In other words, there is no sudden change in voltage, and undulation liable to occur in the current wave can be restricted, since the voltage always changes from a level equal to or below the voltage of the driving power supply to zero at the moment the first switching element  24  is turned on. 
     According to the third exemplary embodiment as described, it can prevent the phenomenon in that no current flows in the first switching element  24 , and thereby it can produce a voltage reliably across the primary coil  22  of the transformer even for the display  29  having high resolution with high horizontal and vertical scanning frequency, or the like. In addition, since the voltage always changes from a value equal to or below the voltage of the driving power supply to zero at the moment the first switching element  24  is turned on, no sudden change occurs in voltage, and thereby restricting undulation that is liable to occur in the current wave. 
     Fourth Exemplary Embodiment 
     A power supply circuit of a fourth exemplary embodiment of this invention will be described hereinafter with reference to the accompanying figures. 
     In FIG. 6, a power supply circuit of the fourth exemplary embodiment of the present invention is an improved version of the transformer power supply circuit of the first exemplary embodiment of this invention. 
     The power supply circuit for transformer in this fourth exemplary embodiment of the invention is provided with a driving power supply  123  connected to one side of terminals of a primary coil  122  of a transformer  121 , a first switching element  124 , a capacitor  125 , and a first diode  126 , all connected to the other side of the terminals of the primary coil  122 . 
     Furthermore, the first switching element  124  is comprised of a first MOS type field-effect transistor (MOS FET) that has an internal diode. This first MOS type field-effect transistor is disposed in a manner that a drain is connected to the other side terminal of the primary coil  122 , a source is connected to a ground side, and a gate is connected to a PWM type control circuit  127 , which generates a pulse wave to control the first switching element  124 . It contains the internal diode with an anode connected to the ground side, and a cathode to the other side terminal of the primary coil  122 . The capacitor  125  has its one end connected to the other side terminal of the primary coil  122 , and the other end connected to the ground side. The first diode  126  has its cathode connected to the other side terminal of the primary coil  122 , and an anode connected to the ground side. The cathode of the first diode  126  and the one end of the capacitor  125  are connected to a point between the drain of the first MOS type field-effect transistor and the primary coil  122 . 
     In addition, there are provided an L-C resonance circuit  135  connected to the primary coil  122 , a second diode  131  connected to the L-C resonance circuit  135 , and a second switching element  132  connected to the second diode  131 . 
     Moreover, the second switching element  132  is comprised of a second MOS type field-effect transistor, and this first MOS type field-effect transistor has a drain connected to a cathode side of the second diode  131 , a source connected to the ground side, and a gate connected to the control circuit  127 . The second diode  131  has its anode connected to one side of the L-C resonance circuit  135 . The L-C resonance circuit  135  has one end connected to the other side terminal of the primary coil  122 , and the other end connected to the anode side of the second diode  131 . 
     The control circuit  127  generates a pulse wave to control the first switching element  124  and the second switching element  132 , thereby the second switching element  132  is switched into an ON state when voltage of the primary coil  122  is greater than the voltage level of the driving power supply after an electric current flows in the first diode  126  while the first switching element  124  is in its OFF state (at points X and (X) in FIG.  7 ), and into an OFF state while the first switching element  124  is in its ON state (at points Y and (Y) in FIG.  7 ). 
     In addition, the secondary coil  128  is connected with a display  129  (CRT) having high horizontal and vertical scanning frequency, or the like. 
     In this power supply circuit, waveforms representing voltage, current and switch pulses, as they change with a lapse of time, are shown in FIG.  7 . 
     In FIG. 7, a reference letter (a) represents a waveform illustrating a time series of change in value of voltage induced in the primary coil  122  of the transformer  121  taken at a point “O” in the power supply circuit of FIG. 6; a letter (b) a waveform illustrating a time series of change in amount of current flowing in the point “O” in the power supply circuit; a letter (c) a waveform illustrating a time series of change in shape of an output wave of a control circuit fed to the first switching element  124 ; a letter (d) a waveform illustrating a time series of change in shape of another output wave of the control circuit fed to the second switching element  132 ; and a letter (e) a waveform illustrating a time series of change in amount of current flowing in a point “P” in the power supply circuit of FIG.  6 . 
     During a period of A to B, when a pulse wave (the output wave) of a predetermined duration shown by the waveform (c) is input from the control circuit  127  to the first switching element  124 , turning the first switching element  124  into its ON state, amount of electric current in the point “O” increases with time in proportion to a duration of the ON state of the first switching element  124 , as shown by the waveform (b), and thereby energy is charged into the primary coil  122 . 
     During a period of B to C, when an input of the pulse wave from the control circuit  127  to the first switching element  124  is ceased as shown by the waveform (c), causing the first switching element  124  to turn into an OFF state, the energy charged in the primary coil  122  begins to be charged into the capacitor  125 , and amount of the current in the point “O” decreases with time, as shown by the waveform (b). The voltage of the primary coil  122  reaches its peak value as shown by the waveform (a), when the charge is completed. 
     During a period of C to D, after completion of the charge into the capacitor  125 , the energy charged in the capacitor  125  begins to be recharged into the primary coil  122  again, and amount of the current in the point “O” decreases with time, as shown by the waveform (b). The voltage of the primary coil  122  becomes zero as shown by the waveform (a), when the charge is completed. 
     During a period of D to E, when the charge to the primary coil  122  is completed, the energy charged in the primary coil  122  is about to start being recharged into the capacitor  125  again, and this recharge of the capacitor  125  begins through the ground side due to an effect of a positive-negative relation in polarity of the voltage across the primary coil  122 . However, a current flows through the first diode  126  having a low impedance, instead of charging the capacitor  125 , since the first diode  126  is disposed between the other side terminal of the primary coil  122  and the ground, with the anode connected to the ground side. Therefore, although amount of the current in the point “O” increases with time as shown by the waveform (b), the voltage of the primary coil  122  remains zero as shown by the waveform (a), since no energy is charged in the capacitor  125 . 
     During a period of E to F, since the energy charged in the primary coil  122  has been discharged due to a flow of the current through the first diode  126 , amount of the current shown by the waveform (b) in the point “O” shall theoretically remain zero, unless the switching element  124  is turned into an ON state with the waveform (c). In reality, however, amount of the current in the point “O” increases for a certain period of time as shown by the waveform (b). 
     A certain amount of energy is therefore charged in the primary coil  122  due to the increase in amount of the current through the point “O”. 
     Subsequently, during a period of F to G, the energy charged in the primary coil  122  begins to be charged into the capacitor  125  in the same manner as above, after the charge to the primary coil  122  is completed. Thus, amount of the current in the point “O” decreases with time, as shown by the waveform (b), and the voltage of the primary coil  122  reaches its peak value as shown by the waveform (a), when the charge is completed. 
     During a period of G to H, after completion of the charge into the capacitor  125 , the energy charged in the capacitor  125  begins to be recharged into the primary coil  122  again, and amount of the current in the point “O” decreases with time, as shown by the waveform (b). The voltage of the primary coil  122  becomes zero as shown by the waveform (a), when the charge is completed. 
     During a period of H to I, when the charge to the primary coil  122  is completed, the energy charged in the primary coil  122  is about to start being recharged into the capacitor  125  again. While amount of the current in the point “O” increases with time as shown by the waveform (b), the voltage of the primary coil  122  remains zero as shown by the waveform (a), since no energy is charged into the capacitor  125 . 
     During a period of I to J, amount of the current in the point “O” again increases for a certain period of time in the same manner as described above, as shown by the waveform (b), and energy is hence charged in the primary coil  122 . 
     During a period of J to K (A), after the charge to the primary coil  122  is completed, the energy charged in the primary coil  122  begins to be charged into the capacitor  125 . Amount of the current in the point “O” decreases with time, as shown by the waveform (b), and the voltage of the primary coil  122  reaches its peak value as shown by the waveform (a), when the charge is completed. Since the first switching element  124  is turned into an ON state during this period, as shown by the waveform (c), this becomes a new starting point again. 
     In addition, the second switching element  132  is so operated that it turns into an ON state when voltage of the primary coil  122  is greater than the voltage level of the driving power supply  123  after an electric current flows in the first diode  126  while the first switching element  124  is in its OFF state (at points X and (X)), and it turns into an OFF state while the first switching element  124  is in its ON state (at points Y and (Y)). 
     At a time when the second switching element  132  goes into an ON state as shown by the waveform (d), an electric current flows through the point “P” simultaneously, as shown by the waveform (e), and thereby the voltage output to the primary coil  122  becomes equal to or smaller than the voltage level of the driving power supply  123 , as shown by the waveform (a), due to an effect of the L-C resonance circuit  135 . 
     In this embodiment, the L-C resonance circuit  135  is used as a waveform shaping circuit for reducing the-output voltage to the primary coil  122  to a value equal to or smaller than the voltage level of the driving power supply  123 , and it has a resonance time constant in synchronization with undulation of the current waveform generated in the primary coil  122 , as it outputs a shaping wave that cancels the undulation of the current generated in the primary coil  122 . 
     In respect of the resonance time constant, values of an inductance (L) and a capacitance (C) employed in the L-C resonance circuit  135  are determined to satisfy a condition of (a period of fundamental frequency of the undulation)=1/(resonance frequency of the L-C resonance circuit  135 ). 
     Subsequently, in a period of (A) to (D), same operations as described above are repeated. 
     With the above-described composition, voltage of the primary coil  122  decreases to nearly zero at the moment the second switching element  132  is turned on, when the voltage of the primary coil  122  is greater than the voltage of the driving power supply, and the voltage of the primary coil  122  then becomes zero at the moment the first switching element  124  is turned on, since the second switching element  132  is switched into its ON state when voltage of the primary coil  122  is greater than the voltage level of the driving power supply after an electric current flows in the first diode  126  while the first switching element  124  is in its OFF state (at the points X and (X)). 
     In other words, there is no sudden change in voltage, and undulation liable to occur in the current wave can be restricted, since the voltage always changes from a value equal to or below the voltage of the driving power supply to zero at the moment the first switching element  124  is turned on. 
     However, since the first switching element  124  is not turned into its ON state during a state wherein voltage of the primary coil  122  has already been zero, there occurs undulation to a certain extent due to this difference in voltage. 
     This undulation can be suppressed, however, since the L-C resonance circuit  135  has the resonant time constant in synchronization with the undulation of the current wave generated in the primary coil  122 , and outputs the shaping wave in a manner to cancel the undulation. 
     According to the fourth exemplary embodiment as described, there occurs no sudden change in voltage, and undulation liable to occur in the current wave can be suppressed, since the voltage always changes from a value equal to or below the voltage of the driving power supply  123  to zero at the moment the first switching element  124  is turned on. 
     Even if undulation occurs to a certain extent because the voltage of the primary coil  122  is not zero, although it is equal to or less than the voltage of the driving power supply  123 , at the moment the first switching element  124  is turned on, the undulation can be suppressed to a great extent, since the L-C resonance circuit  135  has the resonance time constant synchronized with the undulation in the current waveform generated in the primary coil  122 , and it outputs the shaping wave that cancel the undulation. 
     Fifth Exemplary Embodiment 
     Referring now to the accompanying figures, a power supply circuit of a fifth exemplary embodiment of this invention will be described hereinafter. 
     In FIG.  8  and FIG. 9, the power supply circuit for a transformer in the fifth exemplary embodiment of the present invention is a modified version of the transformer power supply circuit of the fourth exemplary embodiment of this invention. Therefore, the same structural components are assigned with the same reference numerals. 
     The power supply circuit of the fifth exemplary embodiment of this invention has a composition comprising a third diode  133  disposed between a drain of a MOS type field-effect transistor constituting a first switching element  124  and a cathode of a first diode  126  in the transformer power supply circuit of the fourth exemplary embodiment of this invention, wherein a cathode of the third diode  133  is connected to a drain of the MOS type field-effect transistor serving as the first switching element  124 , and an anode of the third diode  133  is connected to the cathode of the first diode  126 . 
     In this power supply circuit, waveforms of voltage, current and switch pulses, as they change with a lapse of time, are shown in FIG.  9 . 
     In FIG. 9, a reference letter (a) represents a waveform illustrating a time series of change in value of voltage induced in a primary coil  122  of a transformer  121  taken at a point “O” in the power supply circuit of FIG. 8; a letter (b) a waveform illustrating a time series of change in amount of current flowing in the point “O” in the power supply circuit; a letter (c) a waveform illustrating a time series of change in shape of an output wave of a control circuit  127  fed to the first switching element  124 ; a letter (d) a waveform illustrating a time series of change in shape of an output wave of the control circuit  127  fed to a second switching element  132 ; and a letter (e) a waveform illustrating a time series of change in amount of current flowing in a point “P” in the power supply circuit of FIG.  8 . Illustration of the waveforms in the fifth exemplary embodiment is identical to the corresponding illustration in the third exemplary embodiment, except for the waveforms (a) and (b) in a period between E and K(A). 
     During the period of E through K(A), a voltage shown by the waveform (a) does not become smaller than zero, even though the generated voltage indicates an L-C resonance around a driving power supply voltage, since an addition of the third diode  133  reduces an amount of current produced, as shown by the waveform (b). 
     Therefore, once an electric current flows through the first diode  126  for the first time, this composition does not allow subsequent current to flow through it at the second time and thereafter, thereby preventing the phenomenon that no current flows in the first switching element  124  when the first switching element  124  is turned into an ON state, as shown by the waveform (c). Hence, it can produce a voltage reliably across the primary coil  122  of the transformer  121 , even for a display  129  having high resolution with high horizontal and vertical scanning frequency, or the like. 
     Additionally, in the same manner as the power supply circuit of the third exemplary embodiment, the second switching element  132  is so operated that it turns into an ON state when voltage of the primary coil  122  is greater than a voltage level of the driving power supply after an electric current flows in the first diode  126  while the first switching element  124  is in its OFF state (at points X and (X) in FIG.  9 ), and it turns into an OFF state while the first switching element  124  is in its ON state (at points Y and (Y) in FIG.  9 ). Therefore, voltage of the primary coil  122  decreases to the voltage level of the driving power supply  123  at the moment the second switching element  132  is turned on, when the voltage of the primary coil  122  is greater than the voltage of the driving power supply  123 , and that the voltage of the primary coil  122  becomes zero at the moment the first switching element  124  is turned on. 
     In other words, there occurs no sudden change in voltage, and undulation liable to occur in the current wave can be restricted, since the voltage always changes from a level below the voltage of the driving power supply  123  to zero at the moment the first switching element  124  is turned on. 
     According to the fifth exemplary embodiment as described, it can prevent the phenomenon that no current flows in the first switching element  124 , and thereby producing a voltage reliably across the primary coil  122  of the transformer  121  even for the display  129  having high resolution with high horizontal and vertical scanning frequency, or the like, in addition to the advantages attained in the fourth exemplary embodiment. Moreover, since the voltage always changes from a level equal to or below the voltage of the driving power supply  123  to zero at the moment the first switching element  124  is turned on, no sudden change occurs in the voltage, and thereby restricting undulation that is liable to occur in the current wave. 
     In the above exemplary embodiment of this invention, although the L-C resonance circuit  135  has been employed as a waveform shaping circuit, it may be replaced by a resistance circuit, which outputs such a shaping wave that absorbs undulation of the current wave generated by the primary coil  122 , to yield a similar effect. 
     Sixth Exemplary Embodiment 
     Referring now to the accompanying figures, a power supply circuit of a sixth exemplary embodiment of the present invention will be described hereinafter. 
     In FIG. 10, the power supply circuit of the sixth exemplary embodiment of this invention is a modified version of the transformer power supply circuit of the first exemplary embodiment of this invention shown in FIG.  1 . 
     The power supply circuit of the sixth exemplary embodiment of this invention comprises a flyback transformer represented by a transformer  234  having a primary coil  231  and a secondary coil  232  for supplying anode voltage to a display  233  such as a display device, a PWM type control circuit  235  for controlling low output voltage input to the primary coil  231 , and a high-voltage detection circuit  236  for detecting high output voltage output from the secondary coil  232  as a detected voltage. 
     The control circuit  235  controls the low output voltage based on the detected voltage. The high-voltage detection circuit  236  is disposed between an intermediate point  237  connecting the secondary coil  232  and the display  233  and the control circuit  235 . The high-voltage detection circuit  236  is so composed that a group of serially connected capacitors  240  comprising a first detection capacitor  238  and a second detection capacitor  239  and a group of serially connected resistors  243  comprising a first resistor  241  and a second resistor  242  are connected in parallel between the intermediate point  237  and a ground, a first midpoint  244  between the first detection capacitor  238  and the second detection capacitor  239  and a second midpoint  245  between the first resistor  241  and the second resistor  242  are connected with a third detection capacitor  251 , and the second midpoint  245  is connected to the control circuit  235 . 
     Furthermore, the primary coil  231  of the transformer  234  is connected with a first switching element  246 , a damper diode  247 , a resonance capacitor  248 , a diode  249  for canceling a reverse recovery time of the first switching element  246 , and a driving power supply  250 . 
     A capacitance (C 1 ) of the first detection capacitor  238 , a capacitance (C 2 ) of the second detection capacitor  239 , a resistance (R 1 ) of the first resistor  241 , and a resistance (R 2 ) of the second resistor  242  are arranged in such values that there is a relation of (C 1 )×(R 1 )=(C 2 )×(R 2 ). With a contribution of a capacitance (C 3 ) of the third detection capacitor  251 , the capacitance (C 2 ) of the second detection capacitor  239  may increase or decrease ,so as to change the relation of (C 1 )×(R 1 )=(C 2 )×(R 2 ) to either of (C 1 )×(R 1 )&gt;(C 2 )×(R 2 ) and (C 1 )×(R 1 )&lt;(C 2 )×(R 2 ). 
     A schematic illustration representing a waveform of the high output voltage of the transformer  234  is shown in FIG.  11 . In this waveform illustration, a range “A” represents a start-up characteristic of the power supply circuit at power-on, and a range “B” represents a dynamic characteristic of the same. 
     Because of the above-described composition, in which the first midpoint  244  between the first detection capacitor  238  and the second detection capacitor  239  and the second midpoint  245  between the first resistor  241  and the second resistor  242  are connected by the third detection capacitor  251 , the capacitance (C 2 ) of the second detection capacitor  239  comes to be a value derived by adding the capacitance (C 3 ) of the third detection capacitor  251  to it, at the moment the power supply circuit is turned on, since all of the first detection capacitor  238 , the second detection capacitor  239 , and the third detection capacitor  251  do not carry electric charge, and therefore values of the respective capacitances (C 1 ), (C 2 ), and (C 3 ) are zero at the moment of power-on. Hence, their relation with the resistance (R 1 ) of the first resistor  241  and the resistance (R 2 ) of the second resistor  242  becomes (C 1 )×(R 1 )&lt;(C 2 )×(R 2 ). 
     Furthermore, since the capacitance (C 1 ) of the first detection capacitor  238 , the capacitance (C 2 ) of the second detection capacitor  239 , and the capacitance (C 3 ) of the third detection capacitor  251  carry electric charge in a steady state after the power is turned on, a value of the capacitance (C 2 ) of the second detection capacitor  239  decreases as a composite capacity, and they become (C 1 )×(RI)&gt;(C 2 )×(R 2 ). 
     It can therefore improve both of the start-up characteristic and the dynamic characteristic simultaneously, as shown in FIG. 11, without holding on to a poor dynamic characteristic as shown in FIG. 12, and a poor start-up characteristic as shown in FIG.  13 . 
     As has been described, since the first midpoint  244  between the first detection capacitor  238  and the second detection capacitor  239  and the second midpoint  245  between the first resistor  241  and the second resistor  242  are connected by the third detection capacitor  251 , the sixth exemplary embodiment can improve the start-up characteristic as well as the dynamic characteristic at the same time. 
     Seventh Exemplary Embodiment 
     A power supply circuit of a seventh exemplary embodiment of this invention will be described hereinafter with reference to the accompanying figures. 
     In FIG. 14, a power supply circuit of the seventh exemplary embodiment of the present invention is an improved version of the transformer power supply circuit shown in FIG. 1 of the first exemplary embodiment. 
     The power supply circuit in the seventh exemplary embodiment of this invention is provided with a driving power supply  323  connected to one side of terminals of a primary coil  322  of a transformer  321 , a first switching element  324 , a capacitor  325 , and a first diode  326 , all connected to the other side of the terminals of the primary coil  322 . 
     Furthermore, the first switching element  324  is comprised of a first MOS type field-effect transistor (MOS FET) that has an internal diode. This first MOS type field-effect transistor is disposed in a manner that a drain is connected to the other side terminal of the primary coil  322 , a source is connected to a ground side, and a gate is connected to a control circuit  327 , which generates a pulse wave to control the first switching element  324 . It contains the internal diode with its anode connected to the ground side, and a cathode to the other side terminal of the primary coil  322 . The capacitor  325  has its one end connected to the other side terminal of the primary coil  322 , and the other end connected to the ground side. The first diode  326  has its cathode connected to the other side terminal of the primary coil  322 , and an anode connected to the ground side. The cathode of the first diode  326  and one end of the capacitor  325  are connected to a point between the drain of the transistor and the primary coil. 
     In addition, a third diode  330  is provided between the cathode of the first diode  326  and the drain of the transistor. The third diode  330  is disposed with its cathode connected to the drain side of the transistor, and its anode connected to the cathode side of the first diode  326 . 
     Furthermore, one end of the capacitor  325  is connected to a point between the cathode of the first diode  326  and the primary coil  322 . 
     The first diode  326  and the third diode  330  have their current reverse-recovery time shorter than that of the internal diode of the transistor. The secondary coil  328  is connected with a display  329  (CRT) having high horizontal and vertical scanning frequency, or the like. 
     In this power supply circuit, waveforms of voltage, current and switch pulses, as they change with a lapse of time, are shown in FIG.  15 . 
     In FIG. 15, a reference letter (a) represents a waveform illustrating a time series of change in value of voltage induced in the primary coil  322  of the transformer  321  taken at a point “O” in the power supply circuit of FIG. 14; a letter (b) a waveform illustrating a time series of change in amount of current in the point “O” of the power supply circuit; and a letter (c) a waveform illustrating a time series of change in shape of an output wave of the control circuit  327  fed to the first switching element  324 . 
     During a period of A to B, when a pulse wave (the output wave) of a predetermined duration shown by the waveform (c) is input from the control circuit  327  to the first switching element  324 , making the first switching element  324  into an ON state, amount of electric current in the point “O” increases with time in proportion to a duration of the ON state of the first switching element  324  as shown by the waveform (b), and thereby energy is charged into the primary coil  322 . 
     During a period of B to C, when an input of the pulse wave from the control circuit  327  to the first switching element  324  ceases as shown by the waveform (c), causing the first switching element  324  into an OFF state, the energy charged in the primary coil  322  begins to be charged into the capacitor  325 , and amount of the current in the point “O” decreases with time, as shown by the waveform (b). The voltage of the primary coil  322  reaches its peak value as shown by the waveform (a), when the charge is completed. 
     During a period of C to D, after a completion of the charge into the capacitor  325 , the energy charged in the capacitor  325  begins to be recharged into the primary coil  322  again, and amount of the current in the point “O” decreases with time, as shown by the waveform (b). The voltage of the primary coil  322  becomes zero as shown by the waveform (a), when the charge is completed. 
     During a period of D to E, when the charge to the primary coil  322  is completed, the energy charged in the primary coil  322  is about to start being recharged into the capacitor  325  again, and this recharge of the capacitor  325  begins through the ground side due to an effect of a positive-negative relation in polarity of the voltage across the primary coil  322 . However, a current flows through the first diode  326  having a low impedance, instead of charging the capacitor  325 , since the first diode  326  is disposed between the other side terminal of the primary coil  322  and the ground, with the anode connected to the ground side. Therefore, although amount of the current in the point “O” increases with time as shown by the waveform (b), the voltage of the primary coil  322  remains zero as shown by the waveform (a), since no energy is charged in the capacitor  325 . 
     During a period of E to F since the energy charged in the primary coil  322  has been discharged due to the current flowed through the first diode  326 , amount of the current shown by the waveform (b) in the point “O” shall theoretically remain zero, unless the switching element  324  is switched into an ON state with the waveform (c). In reality, however, amount of the current in the point “O” increases for a certain period of time as shown by the waveform (b). 
     A certain amount of energy is therefore charged in the primary coil  322  due to the increase in amount of current in the point “O”. 
     During a period of F to G, the energy charged into the primary coil  322  begins to be charged into the capacitor  325  in the same manner as above, after the charge to the primary coil  322  is completed. Thus, amount of the current in the point “O” decreases with time, as shown by the waveform (b), and the voltage of the primary coil  322  reaches its peak value as shown by the waveform (a), when the charge is completed. 
     During a period of G to H, after completion of the charge into the capacitor  325 , the energy charged in the capacitor  325  begins to be recharged into the primary coil  322  again, and amount of the current in the point “O” decreases with time, as shown by the waveform (b). The voltage of the primary coil  322  becomes equal to the voltage of the driving power supply  323  as shown by the waveform (a), when the charge is completed. 
     During a period of H to I, when the charge to the primary coil  322  is completed, the energy charged in the primary coil  322  begins to be recharged into the capacitor  325  again, and amount of the current in the point “O” increases with time, as shown by the waveform (b). The voltage of the primary coil  322  becomes smaller than the voltage of the driving power supply  323 , but greater than zero, as shown by the waveform (a), when the charge is completed. 
     During a period of I to J, after completion of the charge into the capacitor  325 , the primary coil  322  begins to be recharged once again. As amount of the current in the point “O” increases with time, as shown by the waveform (b), and the voltage of the primary coil  322  comes to be equal to the voltage of the driving power supply  323  as shown by the waveform (a), when the charge is completed. 
     During a period of J to K, after the charge into the primary coil  322  is completed, the energy charged in the primary coil  322  begins to be charged into the capacitor  325 . Amount of the current in the point “O” decreases with time, as shown by the waveform (b), and the voltage of the primary coil  322  reaches its peak value as shown by the waveform (a), when the charge is completed. 
     During a period of K to L, after completion of the charge into the capacitor  325 , the energy charged in the capacitor  325  begins to be recharged into the primary coil  322 . Amount of the current in the point “O” decreases with time, as shown by the waveform (b), and the voltage of the primary coil  322  becomes equal to the voltage of the driving power supply  323  as shown by the waveform (a), when the charge is completed. 
     During a period of L to M(A), when the charge to the primary coil  322  is completed, the energy charged in the primary coil  322  begins to be recharged into the capacitor  325  again, and amount of the current in the point “O” increases with time, as shown by the waveform (b). While the voltage of the primary coil  322  becomes smaller than the voltage of the driving power supply  323 , but greater than zero, as shown by the waveform (a), upon completion of the charge, the first switching element  324  is turned into its ON state during this period as shown by the waveform (c). Hence, the same steps as described above are repeated again at this time, making it a new starting point. 
     The composition, as described above, comprises the third diode  330  disposed between the cathode of the first diode  326  and the drain of the transistor, i.e. the first switching element  324 , with its cathode connected to the drain side of the transistor and the anode connected to the cathode side of the first diode  326 , and the capacitor  325 , of which one end is connected to a point between the cathode of the first diode  326  and the primary coil  322 . In addition, the first diode  326  and the third diode  330  have their current reverse-recovery time shorter than the internal diode of the transistor. According to the above composition, although an electric current, which is a cause of charging energy into the primary coil  322 , flows into the primary coil  322 , after the switching element  324  turns into its OFF state from the ON state with the pulse wave generated by the control circuit  327 , and the voltage becomes zero as current flows for the first time through the first diode  326 , amount of this electric current can be reduced substantially so as to reduce the energy charged in the primary coil  322 . 
     Accordingly, although an L-C resonance occurs in the voltage waveform with reference to the voltage of the driving power supply  323 , after the first current flows through the first diode  26 , an amplitude of the resonance can be kept small, and the voltage can be maintained greater than zero. 
     Therefore, this composition can reliably produce a voltage across the primary coil  322  of the transformer  321  even for a display  329  having high resolution with high horizontal and vertical scanning frequency, or the like, since it does not allow electric current to flow through the first diode  326  at the second time and thereafter, and to prevent the phenomenon in that no current flows in the first switching element  324  when the pulse wave is turned into an ON state. 
     In respect of the internal diode, in particular, which is indispensable for the MOS type field-effect transistor, since the current reverse-recovery time of the first diode  326  and the third diode  330  is shorter than that of the internal diode of the transistor, i.e. the first switching element  324 , a dampening time (the time in which a flow of current stops, i.e. the current reverse-recovery time) of the current that flows when the voltage impressed on two ends of the first diode  326  and the third diode  330  is discharged becomes shorter than that of the internal diode of the transistor, or, the first switching element  324 . 
     As a result, a delaying current (the current that flows after measures are taken to dampen the current, until the time the current flow actually stops) due to the first diode  326  and the third diode  330  is suppressed within a shorter time than a delay current that originates in the internal diode of the transistor, and thereby the delay current originating in the internal diode of the transistor is suppressed by the third diode  330  when the electric current flows from the anode to the cathode of the first diode  326 . Moreover, since the delay current that originates in the first diode  326  and the third diode  330  can be suppressed in a shorter time than the delay current originating in the internal diode of the transistor, it prevents the transformer and the capacitor from being charged with extra energy. 
     According to the seventh exemplary embodiment as described, the third diode  330  is connected between the cathode of the first diode  326  and the drain of the transistor, or the first switching element  324 , with its cathode connected to the drain side of the transistor and the anode connected to the cathode side of the first diode  326 , and one end of the capacitor  325  is connected to the point between the cathode of the first diode  326  and the primary coil  322 , and further that the first diode  326  and the third diode  330  have their current reverse-recovery time shorter than the internal diode of the transistor, or the first switching element  324 . Therefore, it can positively generate a voltage across the primary coil  322  of the transformer  321 , even for the display  329  that has a high resolution with high horizontal and vertical scanning frequency, or the like. 
     In the seventh exemplary embodiment, although one end of the capacitor  325  is connected to the point between the cathode of the first diode  326  and the primary coil  322 , the same effect can also be attained even if it is connected to a point between the cathode of the first diode  326  and the anode of the third diode  330 . 
     Industrial Applicability 
     As described above, the present invention can provide a power supply circuit that suppresses undulation liable to occur in current wave, and prevents noise from being generated on an display screen, since it employs a noise suppression means between a transformer and a control circuit.