PATENT ABSTRACT
A resonant switching power source apparatus has a first switching element and second switching element being connected in series between output terminals of a DC power source and are alternately turned on and off; a series resonant circuit having a primary winding of a transformer and a current resonance capacitor and connected in parallel with the second switching element; a controller to control the first and second switching elements; a rectifying/smoothing circuit to rectify and smooth a voltage, which is generated on a secondary winding of the transformer during an ON period of the second switching element, and output the rectified-smoothed voltage; a current detector to detect a current passing through the series resonant circuit; and an overcurrent protector to turn off the first and second switching elements for a predetermined period so that excitation energy of the transformer is reset, if the current detector detects a predetermined current value.

PATENT DESCRIPTION
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a resonant switching power source apparatus having an overcurrent protection function, and particularly, to a technique of activating the overcurrent protection function even in a case where an output voltage is below a working voltage. 
   2. Description of the Related Art 
     FIG. 1  is a circuit diagram showing a resonant switching power source apparatus according to a related art. The power source apparatus includes a diode bridge circuit DB to rectify an output from an AC power source AC and a smoothing capacitor C to smooth the rectified output and provide DC source power. Between terminals of the capacitor C that supplies the DC source power, there are a first switching element Q 1  and second switching element Q 2  that are connected in series. The first and second switching elements Q 1  and Q 2  are, for example, MOSFETs. 
   Between the source and drain of the second switching element Q 2 , there are connected a voltage resonant capacitor Crv, a resonant reactor Lri, and a series resonant circuit including a primary winding Np of a transformer T 1  and a current resonant capacitor Cri. The resonant reactor Lri may be a leakage inductance of the transformer T 1 . Between terminals of a second winding Ns of the transformer T 1 , there is connected a rectifying/smoothing circuit including a diode D 1  and a capacitor C 1  that are connected in series. The rectifying/smoothing circuit rectifies and smoothes a voltage generated on the secondary winding Ns of the transformer T 1 . A voltage (terminal voltage of the capacitor C 1 ) provided by the rectifying/smoothing circuit is an output voltage supplied to a load L. 
   Between terminals of the capacitor C 1 , there is connected an output voltage detector including a resistor R 2 , a resistor R 3 , a shunt regulator SR, and a photocoupler emitter PC 1 - 1 . In the output voltage detector, the resistors R 2  and R 3  divide the output voltage into a divided voltage that is sent to the shunt regulator SR. The shunt regulator SR internally compares the divided voltage with a reference voltage and provides a current representative of a differential voltage (hereinafter referred to as “error voltage”) to the photocoupler emitter PC 1 - 1 . 
   A photocoupler receiver PC 1 - 2  has a first end connected through a resistor R 1  to a reference power source Vref (not shown) that is incorporated in a controller  110 . A second end of the photocoupler receiver PC 1 - 2  is grounded. The photocoupler receiver PC 1 - 2  generates a voltage Vpc in response to the error voltage detected by and transmitted from the output voltage detector. The voltage Vpc is supplied to a non-inverting input terminal (+) of a comparator Comp 1 . An inverting input terminal (−) of the comparator Comp 1  is connected to a first end of a capacitor Ct. A second end of the capacitor Ct is grounded. The first end of the capacitor Ct is also connected to a first end of a resistor Rt. A second end of the resistor Rt is connected to an output terminal N of a flip-flop FF. An output terminal of the comparator Comp 1  is connected to a first input terminal of an AND gate AND 1 . 
   The resistor Rt is connected in parallel with a diode D 2 . An anode of the diode D 2  is connected to the first end of the resistor Rt and a cathode of the diode D 2  is connected to the second end of the resistor Rt. The output terminal N of the flip-flop FF is connected to a high-side driver  11 . An inverting output terminal C of the flip-flop FF is connected to the second switching element Q 2 . The inverting output terminal C of the flip-flop FF is also connected to an input terminal of a pulse generator  12 . An output terminal of the pulse generator  12  is connected to a set terminal S of the flip-flop FF. 
   Operation of the resonant switching power source apparatus with the above-mentioned configuration will be explained. If the output terminal N of the flip-flop FF is at high level, the capacitor Ct is charged through the resistor Rt and the first switching element Q 1  is driven through the high-side driver  11 . Namely, the first switching element Q 1  turns on. At this time, the inverting output terminal C of the flip-flop FF is at low level to turn off the second switching element Q 2 . The capacitor Ct is continuously charged, and when a voltage Vct of the capacitor Ct reaches the voltage Vpc of the photocoupler receiver PC 1 - 2 , the comparator Comp 1  provides a low-level output. 
   The output of the comparator Comp 1  is supplied to the first input terminal of the AND gate AND 1 . A second input terminal of the AND gate AND 1  is connected to an overcurrent detector to be explained later. In a normal state, the second input terminal of the AND gate AND 1  receives a high-level signal and the output of the comparator Comp 1  is supplied to a reset terminal R (negative logic input) of the flip-flop FF, to reset the flip-flop FF. As a result, the output terminal N of the flip-flop FF becomes low and the inverting output terminal C thereof becomes high. 
   Switching operation of the first and second switching elements Q 1  and Q 2  is controlled to have a dead time in which the first and second switching elements Q 1  and Q 2  are both OFF by a circuit (not shown) that delays ON timing of the elements Q 1  and Q 2 . 
   When the output terminal N of the flip-flop FF becomes low, the first switching element Q 1  turns off and the capacitor Ct is rapidly discharged through the diode D 2 . As a result, the output of the comparator Comp 1  again becomes high. Then, the inverting output terminal C of the flip-flop FF becomes high to turn on the second switching element Q 2  and provide the pulse generator  12  with a high-level signal. 
   In response to the high-level signal, the pulse generator  12  sends, after a predetermined period, a low-level signal to the set terminal S (negative logic input) of the flip-flop FF. As a result, the output terminal N of the flip-flop FF becomes high to again turn on the first switching element Q 1  and start charging the capacitor Ct through the resistor Rt. At this time, the inverting output terminal C of the flip-flop FF becomes low to turn off the second switching element Q 2 . 
   The above-mentioned operation is repeated with an ON period of the first switching element Q 1  being determined according to an error voltage detected by the output voltage detector. The predetermined period generated by the pulse generator  12 , i.e., a period starting when the pulse generator  12  receives a high-level signal and ending when the pulse generator  12  sets the flip-flop FF determines an ON period of the second switching element Q 2 . With these ON periods, the first and second switching elements Q 1  and Q 2  are alternately turned on and off. 
   The overcurrent detector includes a capacitor Coc, a resistor Roc, a comparator Comp 2 , and a reference power source OCP 1 . An overcurrent protector is served by the AND gate AND 1  that resets the flip-flop FF in response to an output from the overcurrent detector, thereby turning off the first switching element Q 1 . 
   The capacitor Coc and resistor Roc in the overcurrent detector form a series circuit that is connected in parallel with the current resonant capacitor Cri. A non-inverting input terminal (+) of the comparator Comp 2  is connected to the reference power source OCP 1  that supplies a reference voltage Vref 1 . An inverting input terminal (−) of the comparator Comp 2  is connected to a connection point between the capacitor Coc and the resistor Roc. An output terminal of the comparator Comp 2  is connected to a second input terminal of the AND gate AND 1 . 
   A current passes through a path extending along the series resonant circuit having the resonant reactor Lri, the primary winding Np of the transformer T 1 , and the current resonant capacitor Cri, in which a part of the current passes through the capacitor Coc in a manner of a capacitance ratio of the current resonant capacitor Cri and the capacitor Coc. Accordingly, the resistor Roc generates a voltage corresponding to the current passing through the series resonant circuit. 
   The voltage generated by the resistor Roc is supplied to the inverting input terminal (−) of the comparator Comp 2  and is compared with the reference voltage Vref 1  of the reference power source OCP 1 . An output from the comparator Comp 2  is supplied to the second input terminal of the AND gate AND 1 . The AND gate AND 1  provides an AND of the output of the comparator Comp 1  and the output of the comparator Comp 2  and the comparison result is sent to the reset terminal R of the flip-flop FF. 
   After the first switching element Q 1  is turned on, a current passing through the series resonant circuit may increase so that a voltage generated by the resistor Roc reaches the reference voltage Vref 1  generated by the reference power source OCP 1 . Then, the flip-flop FF is reset to turn off the first switching element Q 1 . This results in restricting the charging of the current resonant capacitor Cri, to restrict power transmitted to the secondary side. This is the overcurrent protection function that limits power supplied to the load L according to a current passing through the series resonant circuit. 
     FIG. 2  shows operational waveforms in a steady state of the switching power source apparatus of  FIG. 1 .  FIG. 3  shows operational waveforms in an overload state of the same. In  FIGS. 2 and 3 , Vds(Q 2 ) is a drain-source voltage of the second switching element Q 2 , Id(Q 1 ) is a drain current of the first switching element Q 1 , Id(Q 2 ) is a drain current of the second switching element Q 2 , and If(D 1 ) is a current passing through the diode D 1 . 
   In response to signals generated by the controller  110 , the first and second switching elements Q 1  and Q 2  alternately turn on and off with a dead time of about several hundreds of nanoseconds. During an ON period of the first switching element Q 1 , the current resonant capacitor Cri is charged through the resonant reactor Lri and the primary winding Np of the transformer T 1 . During an ON period of the second switching element Q 2 , excitation energy accumulated in an excitation inductance of the transformer T 1  is released. 
   In the ON period of the second switching element Q 2 , the primary winding Np of the transformer T 1  receives a voltage V(Np) provided by dividing a voltage V(Cri) of the current resonant capacitor Cri by an inductance of the primary winding Np and the resonant reactor Lri. The voltage V(Np) of the primary winding Np is clamped when it reaches a value defined by Vo(np/ns), where np is the number of turns of the primary winding Np, ns is the number of turns of the secondary winding Ns, and Vo is an output voltage. A resonant current generated by the current resonant capacitor Cri and resonant reactor Lri is sent to the secondary side. If the voltage V(Np) of the primary winding Np decreases below the value of Vo(np/ns), no energy is transmitted to the secondary side. Instead, the resonant current passes through the series resonant circuit having the resonant reactor Lri, the primary winding Np of the transformer T 1 , and the current resonant capacitor Cri. 
   In a normal state of the resonant switching power source apparatus according to the related art, excitation energy is accumulated in the primary winding Np of the transformer T 1  during an ON period of the first switching element Q 1 , and during an ON period of the second switching element Q 2 , the accumulated energy is reset when the primary winding Np generates a voltage V(Np) defined by Vo(np/ns). As a result, the excitation current returns to a level that is attained just before the first switching element Q 1  is turned on, as indicated with a dotted line in the waveform of a current I(Lri) passing through the resonant reactor Lri. 
   SUMMARY OF THE INVENTION 
   At the start of the power source apparatus of  FIG. 1  or at an occurrence of a load short circuit, an output voltage of the apparatus sharply drops. In this case, the primary winding Np of the transformer T 1  generates a low voltage as shown in the waveform V(Np) of  FIG. 3  to insufficiently reset excitation energy. As a result, an excitation current increases on one side (negative side in  FIG. 3 ) as indicated with a dotted line in the waveform I(Lri) of  FIG. 3 , to cause biased magnetization. This may pass a large current to break the first switching element Q 1  or the second switching element Q 2  when the switching element is turned on. 
   To prevent this, the related art of  FIG. 1  employs the overcurrent protector to limit a current by turning off the first switching element Q 1  if a current passing to the first switching element Q 1  reaches a predetermined value. If an output voltage of the power source apparatus drops to bias an excitation current to one side at the time of, for example, staring the apparatus, a current passing through the primary winding Np returns to the excitation current along a slope whose inclination is determined by the resonant reactor Lri and voltage resonant capacitor Crv. As a result, the excitation current passes until the overcurrent protector responds, so that a current to the first switching element Q 1  is unrestricted and the excitation current continuously increases. 
   Japanese Unexamined Patent Application Publication No. 2003-299351 discloses a switching power source apparatus that changes a reference voltage for an overcurrent detector if an output voltage decreases, to activate an overcurrent protector with a current corresponding to the decreased output voltage. 
   The switching power source apparatus of this disclosure includes a voltage level detection comparator to output a voltage-level-change signal of high level if the voltage level of a detection signal from an output voltage detector drops lower than a reference voltage level provided by a reference power source. The disclosed apparatus also includes a voltage level changer to decrease the absolute level of the reference voltage provided by the reference power source in response to the voltage-level-change signal from the voltage level detection comparator. In an overload state or a load short circuit state, or at the start of the apparatus, the disclosure more strongly limits currents passing through primary and secondary winding sides of a transformer, to release electric stress from parts on the primary and secondary sides. 
   This disclosure, however, gives no consideration on resetting excitation energy of the transformer, and therefore, is unable to solve the biased magnetization problem of the transformer when applied to the resonant switching power source apparatus of  FIG. 1 . Namely, the disclosure is unable to solve the problem that an excessive excitation current passes through the series resonant circuit to break any one of the first and second switching elements Q 1  and Q 2  when the switching element is turned on. 
   According to the present invention, a resonant switching power source apparatus capable of preventing an excessive current from passing through switching elements can be provided, thereby breakage of the switching elements can be prevented. 
   According to a technical aspect of the present invention, provided is a resonant switching power source apparatus having a first switching element and second switching element that are connected in series between output terminals of a DC power source and are alternately turned on and off; a series resonant circuit containing a primary winding of a transformer and a current resonance capacitor and connected in parallel with the second switching element; a controller to control ON/OFF operation of the first and second switching elements; a rectifying/smoothing circuit to rectify and smooth a voltage, which is generated on a secondary winding of the transformer during an ON period of the second switching element, and output the rectified-smoothed voltage; a current detector to detect a current passing through the series resonant circuit; and an overcurrent protector to turn off the first and second switching elements for a predetermined period so that excitation energy of the transformer is reset, if the current detector detects a predetermined current value. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a circuit diagram showing a resonant switching power source apparatus according to a related art; 
       FIG. 2  shows operational waveforms in a steady state of the apparatus according to the related art; 
       FIG. 3  shows operational waveforms in an overload state of the apparatus according to the related art; 
       FIG. 4  is a circuit diagram showing a resonant switching power source apparatus according to a first embodiment of the present invention; 
       FIG. 5  shows operational waveforms of the apparatus according to the first embodiment; 
       FIG. 6  is a circuit diagram showing a resonant switching power source apparatus according to a second embodiment of the present invention; 
       FIG. 7  shows operational waveforms of the apparatus according to the second embodiment; 
       FIG. 8  is a circuit diagram showing a resonant switching power source apparatus according to a third embodiment of the present invention; 
       FIG. 9  shows operational waveforms of the apparatus according to the third embodiment; and 
       FIG. 10  is a circuit diagram showing a resonant switching power source apparatus according to a fourth embodiment of the present invention; 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Embodiments of the present invention will be explained. 
   First Embodiment 
     FIG. 4  is a circuit diagram showing a resonant switching power source apparatus according to the first embodiment of the present invention. In addition to the controller  110  of the resonant switching power source apparatus of the related art shown in  FIG. 1 , a controller  10  of the resonant switching power source apparatus of the first embodiment includes an AND gate AND 2 , a timer  13 , a comparator Comp 3 , and a reference power source OCP 2 . 
   According to the first embodiment of the present invention, a first overcurrent detector includes a capacitor Coc, a resistor Roc, a comparator Comp 2 , and a reference power source OCP 1 . A first overcurrent protector includes an AND gate AND 1 . A second overcurrent detector includes the capacitor Coc, the resistor Roc, the comparator Comp 3 , and the reference power source OCP 2 . A second overcurrent protector includes the timer  13  and the AND gate AND 2 . 
   Among parts shown in  FIG. 4  those that are the same as those of the related art of  FIG. 1  will not be explained and different ones will be explained. 
   A first input terminal of the AND gate AND 2  is connected to an output terminal N of a flip-flop FF and a second input terminal thereof is connected to an output terminal of the timer  13 . An output terminal of the AND gate AND 2  is connected to a high-side driver  11 . 
   A non-inverting input terminal (+) of the comparator Comp 3  is connected to the reference power source OCP 2  that supplies a reference voltage Vref 2 . The reference voltage Vref 2  is higher than a reference voltage Vref 1  provided by the reference power source OCP 1 . An inverting input terminal (−) of the comparator Comp 3  is connected to a connection point between the capacitor Coc and the resistor Roc, to receive a voltage representative of a current passing through a series resonant circuit. An output terminal of the comparator Comp 3  is connected to the timer  13 . When the voltage representative of the current passing through the series resonant circuit exceeds the reference voltage Vref 2 , the comparator Comp 3  supplies a low-level signal to the timer  13 . 
   Receiving the low-level signal from the comparator Comp 3 , the timer  13  provides a low-level output and holds it for a predetermined period. The output of the timer  13  is supplied to the second input terminal of the AND gate AND 2 . In a steady state, the voltage of the resistor Roc is below the reference voltage Vref 2 , and therefore, the timer  13  provides a high-level output to achieve the same operation as that of the related art shown in  FIG. 1 . 
   If an output voltage of the power source apparatus greatly drops at the start of the apparatus or a load short circuit, an excitation current of a primary winding Np of a transformer T 1  will deviate to positive or negative side to bias magnetization. Then, a current passing during an ON period of a switching element Q 1  becomes steeper so that the overcurrent protector, i.e., the AND gate AND 1  is unable to suppress the same due to a delay time of the controller  10 . As a result, the current of the primary winding Np increases, and when the voltage of the resistor Roc reaches the reference voltage Vref 2 , the comparator Comp 3  provides a low-level output to make the timer  13  provide a low-level output for the predetermined period. 
   During the period in which the timer  13  holds the low-level output, the AND gate AND 2  keeps providing a low-level output to continuously turn off the first switching element Q 1 . After the predetermined period, the timer  13  resumes a high-level output so that the AND gate AND 2  provides a high-level output to make the first switching element Q 1  operative through the high-side driver  11 . 
     FIG. 5  shows operational waveforms of the switching power source apparatus according to the first embodiment shown in  FIG. 4 . At the start of the apparatus, an output voltage is low so that an excitation current of the primary winding Np indicated with a dotted line in  FIG. 5  is not completely reset during an ON period of a second switching element Q 2 , and therefore, gradually increases. At time t 1 , the voltage of the resistor Roc reaches the reference voltage Vref 2  and the first switching element Q 1  is kept in an OFF state for the predetermined period. During this period, the first switching element Q 1  is not turned on even at the ON timing of the first switching element Q 1  specified by the flip-flop FF. Consequently, the first and second switching elements Q 1  and Q 2  are kept off (inoperative condition) for the predetermined period. During this period, the excitation current is gradually reset through a body diode of the second switching element Q 2 . 
   After passing the predetermined period from t 1 , the first switching element Q 1  again turns on at time t 2 . At this time, the excitation current of the primary winding Np has already been reset, and therefore, no steep current flows but the current gradually increases. The above-mentioned operation is repeated to send energy to the secondary side. If the output voltage of the apparatus increases to some extent, the excitation current will be completely reset within an ON period of the second switching element Q 2 , to thereby establish a normal operation. 
   Second Embodiment 
     FIG. 6  is a circuit diagram showing a resonant switching power source apparatus according to the second embodiment of the present invention. This embodiment is based on the first embodiment shown in  FIG. 4 , and therefore, the same parts as those of the first embodiment are represented with the same reference marks to omit or simplify their explanations. Compared with the first embodiment, the second embodiment additionally has a current detection resistor Roc  2  between the source of the second switching element Q 2  and the ground and the inverting input terminal (−) of the comparator Comp 3  is connected to a connection point between the source of the second switching element Q 2  and the resistor Roc 2 . 
     FIG. 7  shows operational waveforms of the second embodiment. As explained above, an output voltage of the power source apparatus is low at the start of the apparatus, and therefore, an excitation current of the primary winding Np is not reset within an ON period of the second switching element Q 2 , to thereby gradually increase a switching current when the first switching element Q 1  is turned on. Due to the increase in the switching current of the first switching element Q 1 , a current to be passed when the second switching element Q 2  is turned on increases. 
   If the switching current of the second switching element Q 2  in operative condition increases sharply so that the voltage of the resistor Roc 2  reaches the reference voltage Vref 2  at time t 1 , the first switching element Q 1  is kept in an OFF state (inoperative condition) for a predetermined period, like the first embodiment. Namely, during this period, the first switching element Q 1  is not turned on even at the ON timing of the first switching element Q 1  specified by the flip-flop FF. Consequently, the first and second switching elements Q 1  and Q 2  are kept off for the predetermined period. During this period, the excitation current is gradually reset through a body diode of the second switching element Q 2 . 
   After passing the predetermined period from t 1 , the first switching element Q 1  again turns on at time t 2 . At this time, the excitation current of the primary winding Np has already been reset, and therefore, no steep current flows but the current gradually increases. The above-mentioned operation is repeated to send energy to the secondary side. If the output voltage of the apparatus increases to some extent, the excitation current will be completely reset within an ON period of the second switching element Q 2 , to thereby establish a normal operation. 
   Third Embodiment 
   A resonant switching power source apparatus according to the third embodiment of the present invention employs a soft start circuit to suppress a current increase at the start of the apparatus, thereby preventing an excessive current from passing to the first and second switching elements Q 1  and Q 2 . 
     FIG. 8  is a circuit diagram showing the resonant switching power source apparatus according to the third embodiment. This embodiment is based on the first embodiment shown in  FIG. 4 , and therefore, the same parts as those of the first embodiment are represented with the same reference marks to omit or simplify their explanations. The third embodiment removes the reference power source OCP 1  from the first embodiment and employs the soft start circuit including a resistor R 4 , a resistor R 5 , a capacitor C 3 , and a diode D 3 . The soft start circuit supplies a voltage divided by the resistors R 4  and R 5  instead of the reference voltage Vref 1  of the first embodiment. 
   The resistors R 4  and R 5  are connected in series and are arranged between a reference power source Vref (not shown) and the ground. A connection point between the resistors R 4  and R 5  is connected to the non-inverting input terminal (+) of the comparator Comp 2 . The capacitor C 3  is connected in parallel with the resistor R 5 . An anode of the diode D 3  is connected to the non-inverting input terminal (+) of the comparator Comp 2  and a cathode thereof is connected to the output terminal of the timer  13 . 
     FIG. 9  shows operational waveforms of the third embodiment. As explained above, an output voltage of the power source apparatus is low at the start of the apparatus, and therefore, an excitation current of the primary winding Np indicated with a dotted line in  FIG. 9  is not reset within an ON period of the second switching element Q 2  and gradually increases. 
   At time t 1 , the voltage of the resistor Roc reaches the reference voltage Vref 2 . Then, the third embodiment keeps the first switching element Q 1  in an OFF state for a predetermined period. During this period, the first switching element Q 1  is not turned on even at the ON timing of the first switching element Q 1  specified by the flip-flop FF. Consequently, the first and second switching elements Q 1  and Q 2  are kept off for the predetermined period. During this period, the excitation current is gradually reset through a body diode of the second switching element Q 2 . At the same time, the capacitor C 3  is discharged through the diode D 3  so that the reference voltage Vref 1  sharply decreases. 
   After passing the predetermined period from the time t 1 , the first switching element Q 1  again turns on at time t 2 . At this time, the excitation current of the primary winding Np has already been reset, and therefore, no steep current flows but the current gradually increases. At the time t 2 , the switching operation of the first and second switching elements Q 1  and Q 2  resumes with the decreased reference voltage Vref 1 . Thereafter, the capacitor C 3  is gradually charged with a time constant determined by the capacitor C 3  and resistor R 4 , to thereby increase the reference voltage Vref 1 . 
   After resuming the switching operation of the first and second switching elements Q 1  and Q 2 , an increasing rate of current passing to the first and second switching elements Q 1  and Q 2  is suppressed. The above-mentioned operation is repeated to send energy to the secondary side. If the output voltage of the apparatus increases to some extent, the excitation current will be completely reset within an ON period of the second switching element Q 2 , to thereby establish a normal operation. 
   Fourth Embodiment 
     FIG. 10  is a circuit diagram showing a resonant switching power source apparatus according to the fourth embodiment. This embodiment is based on the first embodiment shown in  FIG. 4 , and therefore, the same parts as those of the first embodiment are represented with the same reference marks to omit or simplify their explanations. The fourth embodiment additionally includes a soft start circuit having a capacitor C 4  and a diode D 4 . 
   The capacitor C 4  is connected in parallel with the photocoupler receiver PC 1 - 2 . An anode of the diode D 4  is connected to a first end of the capacitor C 4 , i.e., a connection point between the photocoupler receiver PC 1 - 2  and the non-inverting terminal (+) of the comparator Comp 1 . A cathode of the diode D 4  is connected to the output terminal of the timer  13 . 
   At the start of the resonant switching power source apparatus, a voltage of the capacitor C 4  gradually increases from an initial value of 0 V according to a time constant determined by the resistor R 1  and capacitor C 4 . The resistor R 1  is connected between the reference power source Vref and the capacitor C 4 . This function is equivalent to gradually increasing a voltage value of the reference power source Vref that determines an ON period of the first switching element Q 1 . Namely, this is a soft start function of gradually increasing the ON period of the first switching element Q 1  from an initial value. 
   The capacitor C 4  and timer  13  are connected through the diode D 4  to each other. When the voltage of the resistor Roc reaches the reference voltage Vref 2  at the start of the power source apparatus, the first and second switching elements Q 1  and Q 2  are kept in an OFF state for a predetermined period. During this period, an excitation current is gradually reset through a body diode of the second switching element Q 2 . 
   At the same time, the capacitor C 4  is discharged through the diode D 4 , to achieve a soft start in resuming the switching operation of the first and second switching elements Q 1  and Q 2 . Accordingly, the fourth embodiment provides the same effect as the third embodiment. 
   Effect of the present invention will be summarized. At the start of or in an overload state of the resonant switching power source apparatus of the present invention, an output voltage of the apparatus drops so that an excitation current is insufficiently reset with ON/OFF operation of the first and second switching elements. Then, the present invention turns off the first and second switching elements for a predetermined period to reset the excitation current, thereby preventing an excessive current from passing to the first and second switching elements. Namely, the present invention surely prevents breakage of the first and second switching elements. 
   The present invention is applicable to resonant switching power source apparatuses employing switching elements whose current resistivity is low. 
   This application claims benefit of priority under 35USC §119 to Japanese Patent Applications No. 2006-007879, filed on Jan. 16, 2006, the entire contents of which are incorporated by reference herein. Although the invention has been described above by reference to certain embodiments of the invention, the invention is not limited to the embodiments described above. Modifications and variations of the embodiments described above will occur to those skilled in the art, in light of the teachings. The scope of the invention is defined with reference to the following claims.