Patent Publication Number: US-10770979-B2

Title: LLC resonant converter

Description:
TECHNICAL FIELD 
     The present invention relates to a DC to DC converter (DC/DC converter) that converts one DC voltage level to another. The present invention particularly relates to changeover control of a resonant capacitor, based on an input voltage and using a switch, in an LLC resonant converter which is a typical circuit example of an isolated DC/DC converter. 
     BACKGROUND ART 
       FIG. 5  is a circuit diagram representing a schematic configuration of an LLC current resonant converter  1 , as an example of a conventional LLC resonant converter. 
     As shown in  FIG. 5 , the LLC current resonant converter  1  includes a bridge circuit  10  configured to receive a DC input voltage Vin, an LLC resonant circuit  20  connected to the bridge circuit  10 , a transformer  30  connected to the LLC resonant circuit  20 , and a rectifier circuit  40  connected to the transformer  30  and configured to send out a converted DC voltage. 
     The bridge circuit  10  has series-connected switches Q 1 , Q 2 . The states of these switches are changed in time sequence with predetermined timing. By such switching operations, the bridge circuit  10  sends out square-wave voltages at a connection point  10   a  between the switches Q 1 , Q 2  and at a GND  10   b.    
     The LLC resonant circuit  20  has a resonant capacitor Cr, an end of which is connected to the GND  10   b  of the bridge circuit  10 . The resonant capacitor Cr, as well as a magnetizing inductance Lm and a leakage inductance Lr to be described later, forms the resonant circuit. 
     The transformer  30  includes a primary winding  31  and a secondary winding  32 , which are isolated from each other. The magnetizing inductance Lm is connected in parallel to first and second ends of the primary winding  31 . The leakage inductance Lr is connected in series to the first end of the primary winding  31 . The first end of the primary winding  31  is connected to the connection point  10   a  of the switches Q 1 , Q 2 , via the leakage inductance Lr. The second end of the primary winding  31  is connected to the GND  10   b  of the bridge circuit  10 , via the resonant capacitor Cr. The secondary winding  32  is provided with a center tap  32   m.    
     The rectifier circuit  40  includes rectifier elements D 1 , D 2  whose anodes (positive electrodes) are respectively connected to first and second ends of the secondary winding  32 , a positive output terminal  42   a  connected to cathodes (negative electrodes) of the rectifier elements, a negative output terminal  42   b  connected to the center tap  32   m  of the secondary winding  32 , a current smoothing capacitor  41  connected between the pair of output terminals  42   a ,  42   b  so as to smooth the electric current. Through this rectifier circuit, a DC output voltage Vo is generated at the pair of output terminals  42   a ,  42   b.    
     This LLC current resonant converter  1  is an isolated DC/DC converter that can reduce a switching loss and noise that occur in the semiconductor devices on the primary side and the secondary side, by utilizing resonance of the single capacitance Cr and the two inductances Lm, Lr. Utilization of the leakage inductance Lr and the magnetizing inductance Lm of the transformer  30  reduces the number of elements required in the circuit configuration. 
       FIG. 6  is a graph showing an example of frequency-gain characteristics of the LLC current resonant converter  1 . 
     The LLC current resonant converter  1  converts an input voltage Vin to a desired output voltage Vo by PFM (pulse frequency modulation) at the switches Q 1 , Q 2 . The LLC current resonant converter  1  can control the gain (i.e., a conversion ratio of the output voltage Vo to the input voltage Vin) by changing the frequency in the PFM, and can thus obtain the desired output voltage Vo even when the input voltage Vin has changed. In the LLC current resonant converter  1  shown in  FIG. 5 , the output voltage and the input voltage have the following relationship:
 
output voltage  Vo =(input voltage  V in×gain)/turns ratio  N  of the transformer
 
where N is not necessarily an integer, and N may be 1 or greater, or less than 1. In  FIG. 6 , fsr represents the resonance frequency when the gain is 1, and fpr represents the resonance frequency when the gain is maximum, Gainmax.
 
     As the range from Gainmax to Gain 1 is greater (the gain is higher), the LLC resonant converter can produce a desired output voltage Vo with a lower input voltage Vin. 
       FIG. 7  is a schematic view showing a configuration example of a source circuit containing the LLC resonant converter. 
     In the source circuit  3  shown in  FIG. 7 , an alternating-current commercial power CP is applied to an PFC (Power Factor Correction: power factor improvement circuit)  2 . An isolated DC/DC converter  1 A receives an output from the PFC  2 , and produces a DC output voltage Vo. A block capacitor Cin connects two connecting lines between the PFC  2  and the isolated DC/DC converter  1 A. The LLC current resonant converter  1  according to the present invention can serve as the isolated DC/DC converter  1 A. 
     If power supply from the commercial power CP stops, the source circuit  3  should maintain the output voltage without any loss for a predetermined time (e.g., 20 ms, hereinafter called “retention time t”). During this time, power is supplied by the block capacitor Cin. The minimum capacitance Cinmin of the block capacitor Cin is obtained by the following formula. 
     
       
         
           
             
               
                 
                   
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     In this formula, P, Vc_start, and Vin_min are defined as below.
         P: maximum output power of the isolated DC/DC converter   Vc_start: charging voltage in Cin at the moment when power supply stops   Vin_min: minimum input voltage at which the isolated DC/DC converter is operable       

     As understood from this formula, the lower the minimum input voltage Vin_min, the smaller the minimum capacitance Cinmin. The minimum input voltage Vin_min can be reduced as the isolated DC/DC converter  1 A has a higher gain. Hence, a high gain enables downsizing of the block capacitor Cin, while keeping the retention time t. 
       FIG. 8( a )  shows an equivalent circuit  1 B in the LLC resonant converter including an output load resistance as a load, seen from the primary side of the transformer  30 . ZL indicates a parallel connection part in  FIG. 8( a ) .  FIG. 8( b )  shows an equivalent circuit  1 C in which a ZL inductance and a resistance are connected in series. In these circuits, Vsq represents an output voltage of the bridge circuit  10  as a square voltage source, and Req is obtained by the following formula in which the output load resistance on the secondary side of the transformer is converted to the AC resistance on the primary side. 
     In  FIG. 8( b ) , ZL(Re) and ZL(Im) represent the real part and the imaginary part of ZL, respectively. 
     
       
         
           
             
               
                 
                   
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     In this formula, Ro represents an output load resistance, and N represents an actual turns ratio (primary:secondary) of the transformer. 
     The resonance frequency fsr is obtained by the following formula. 
     
       
         
           
             
               
                 
                   
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     The resonance frequency fpr is obtained by following formula, based on the equivalent circuit  1 C of  FIG. 8( b ) . 
     
       
         
           
             
               
                 
                   
                     
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       FIG. 9  is a graph showing an example of frequency-gain characteristics and two operating regions of the LLC resonant converter. 
     As shown in  FIG. 9 , the operation of the LLC resonant converter is divided into two operating regions at its resonance frequency fpr: a capacitive region (left) and an inductive region (right). In these regions, the LLC resonant converter operates with following features. 
     &lt;Capacitive Region&gt;
         Hard switching (greater switching loss)   The higher the target gain, the higher the frequency.   A shoot-through current flows through the bridge circuit.       

     &lt;Inductive Region&gt;
         Soft switching (less switching loss)   The higher the target gain, the lower the frequency.   No shoot-through current flows through the bridge circuit.       

     Namely, operation of the LLC resonant converter in the inductive region can reduce the switching loss in the high-frequency operation, and eventually the high-frequency operation enables downsizing of the transformer. For this advantage, the LLC resonant converter is usually controlled in the inductive region. 
       FIG. 10  is a graph showing an example of frequency-gain characteristics of the LLC resonant converter, with different resonant capacitors Cr.  FIG. 11  is a graph showing an example of frequency-gain characteristics of the LLC resonant converter, with different magnetizing inductances Lm. 
     The magnetizing inductance Lm and the resonant capacitor Cr are designable parameters. A high gain is achieved either by a small Lm or a large Cr. 
     However, as indicated in  FIG. 10  and  FIG. 11 , a small Lm allows conduction of a high current and results in a greater loss (deteriorates efficiency), and a large Cr lowers the operating frequency and obstructs downsizing of the transformer. On the other hand, a large Lm or a small Cr cannot realize a high gain. Thus, a high gain conflicts with a high-frequency and a high efficiency. 
     In a conventionally proposed manner for achieving a high gain and a high efficiency, an LLC resonant converter can change the capacitance of its resonant capacitor by turning on and off the switch Q 3  in accordance with the input voltage Vin (see, for example, PTL 1). 
       FIG. 12  is a circuit diagram showing a schematic configuration of an LLC resonant converter  1 D that can change the capacitance of its resonant capacitor, similar to the LLC resonant converter disclosed in PTL 1.  FIG. 13  is a graph showing an example of frequency-gain characteristics of the LLC resonant converter  1 D. 
     The LLC resonant converter  1 D is distinguished from the LLC current resonant converter  1  shown in  FIG. 5  by a resonant capacitor changeover circuit  50  in which the switch Q 3  and a capacitor Crsw are connected in series. The resonant capacitor changeover circuit  50  is connected in parallel to the resonant capacitor Cr in the LLC resonant circuit  20 . 
     In the steady state, as shown in  FIG. 12 , the switch Q 3  is turned off, and the capacitor Crsw is disconnected from the LLC resonant circuit  20 . The LLC resonant converter  1 D operates with high-frequency characteristics, with the capacitance of the resonant capacitor Cr (“Q 3  off time” in  FIG. 13 ). 
     In contrast, when an input voltage Vin drops, the switch Q 3  is turned on. The LLC resonant converter  1 D operates with low-frequency characteristics, with the capacitances of the resonant capacitor Cr and the capacitor Crsw (“Q 3  on time” in  FIG. 13 ), thereby giving a high gain. 
     Later, when the input voltage Vin rises again, the switch Q 3  is turned off, and the capacitor Crsw is disconnected from the LLC resonant circuit  20  again. The LLC resonant converter  1 D operates, as before, with high-frequency characteristics, with the capacitance of the resonant capacitor Cr (“Q 3  off time” in  FIG. 13 ). 
     The LLC resonant converter  1 D shown in  FIG. 12 , operating as described above, can be designed with a large magnetizing inductance Lm, so that a high-frequency, high-efficiency operation can be compatible with a high gain. For such compatibility, however, transition from the operation with low-frequency characteristics to the operation with high-frequency characteristics needs to be controlled properly. 
       FIG. 14  is a graph for schematically describing an example of control in the LLC resonant converter  1 D, during the transition from low-frequency characteristics to high-frequency characteristics. 
     The LLC resonant converter disclosed in PTL 1, whose circuit configuration is similar to the one shown in  FIG. 12 , variably controls its power peak by turning on and off a switch M 1  (corresponding to Q 3  in  FIG. 12 ). 
     The LLC resonant converter which controls the transition from low-frequency characteristics to high-frequency characteristics by reducing the capacitance of the resonant capacitor is configured “to turn off the changeover switch when the input voltage reaches a changeover voltage or when the operating frequency reaches the resonance frequency fpr”. At the resonance frequency fpr, the gain reaches its peak with high-frequency characteristics. The resonance frequency fpr changes with a load. 
     However, this method causes two problems due to the transition from low-frequency characteristics to high-frequency characteristics. 
     Problem 1) The switches Q 1 , Q 2  forming the bridge circuit are broken by a shoot-through current or a surge voltage. 
     Problem 2) The output voltage Vo is uncontrollable unless the control in the capacitive region is effective. 
     Problem 1) occurs when the LLC resonant converter operates in the capacitive region. Problem 2) occurs when the LLC resonant converter is controlled in a common manner in the inductive region. 
     Problem 2) is described more specifically. When the changeover switch is turned off (from Point  1  to Point  2  in  FIG. 14 ) and the operating frequency remains unchanged, the operation with high-frequency characteristics gives a lower gain. Hence, the output voltage Vo, obtained by multiplying the input voltage Vin with the gain, falls to and below the desired voltage. Under normal control, which targets the inductive region, the operating frequency is lowered to obtain the desired output voltage Vo (from Point  2  to Point  3  in  FIG. 14 ). Such control reduces the gain further, and makes the output voltage Vo even lower. 
     CITATION LIST 
     Patent Literature 
     [PTL 1] JP 2009-514495 A 
     SUMMARY OF INVENTION 
     Technical Problem 
     In view of these problems in the conventional technology, the present invention aims to provide an LLC resonant converter that can prevent breakdown of switches in the bridge circuit due to a shoot-through current or a surge voltage and that can avoid loss of control over the output voltage when the capacitance of the resonant capacitor is changed. 
     Solution to Problem 
     In order to achieve the above object, the LLC resonant converter according to the present invention has a bridge circuit configured to receive a DC input voltage and to send out a square-wave voltage by a switching operation of a switching element; an LLC resonant circuit having at least a first capacitor and configured to resonate on receiving the square-wave voltage; a transformer having a primary side connected to the LLC resonant circuit and a secondary side isolated from the primary side; a rectifier element configured to convert an output from the secondary side of the transformer into a DC output voltage; a smoothing capacitor configured to smooth the output voltage from the rectifier element; a resonant capacitor changeover circuit having a changeover switch and a second capacitor that are connected in series to each other and connected in parallel to the first capacitor; an input voltage detection circuit configured to detect the input voltage; an output voltage detection circuit configured to detect the output voltage; an output current detection circuit configured to detect an output current fed to a load; a resonant capacitor changeover control section configured to control a state of the changeover switch in the resonant capacitor changeover circuit, based on at least one of the input voltage detected by the input voltage detection circuit, the output voltage detected by the output voltage detection circuit, and the output current detected by the output current detection circuit; and a bridge circuit control section configured to control the switching operation of the switching element by pulse frequency modulation, and to control an operating frequency of the switching element such that the detected output voltage reaches a desired voltage or in response to a command from the resonant capacitor changeover control section. The resonant capacitor changeover control section is configured to turn on the changeover switch when the detected input voltage gets lower than a predetermined changeover voltage; and to send a command to the bridge circuit control section to raise the operating frequency to the changeover frequency when the detected input voltage exceeds the changeover voltage, and thereafter to turn off the changeover switch. The changeover frequency is higher than a first resonance frequency at which a gain of the LLC resonant circuit is maximum while the changeover switch is off, the first resonance frequency being obtained from the detected output voltage and the detected output current. 
     The resonant capacitor changeover control section may be further configured: when the detected input voltage exceeds the changeover voltage, to send a command to the bridge circuit control section to suspend control of the operating frequency for bringing the detected output voltage to the desired voltage and to raise the operating frequency to the changeover frequency; then, to turn off the changeover switch; and thereafter, to send a command to the bridge circuit control section to resume the operating frequency control for bringing the detected output voltage to the desired voltage. 
     More preferably, the changeover frequency is higher than a second resonance frequency at which the gain of the LLC resonant circuit is 1 while the changeover switch is off. 
     Further, a required time between the suspension and the resumption of the operating frequency control for bringing the detected output voltage to the desired voltage needs to be shorter than at least a retention time during which an output of the LLC resonant converter can be maintained after supply of the input voltage has stopped. 
     When the input voltage rises during the operation with a low input voltage and low-frequency characteristics, the capacitance of the resonant capacitor is changed in order to shift to the operation with high-frequency characteristics. Even in this case, the LLC resonant converter having the above-described configurations can prevent breakdown of switches in the bridge circuit due to a shoot-through current or a surge voltage, and can avoid loss of control over the output voltage. In addition, the LLC resonant converter can suppress an inrush current because the difference between gains during the transition from low-frequency characteristics to high-frequency characteristics is smaller, which alleviates the stress on the switches in the bridge circuit. Eventually, the LLC resonant converter can achieve both a high-efficiency operation and a high gain, and thus can contribute to downsizing of a power unit or the like containing the LLC resonant converter. 
     Advantageous Effects of Invention 
     When the input voltage rises during the operation with a low input voltage and low-frequency characteristics, the capacitance of the resonant capacitor is changed in order to shift to the operation with high-frequency characteristics. Even in this case, the LLC resonant converter according to the present invention can prevent breakdown of switches in the bridge circuit due to a shoot-through current or a surge voltage, and can avoid loss of control over the output voltage. In addition, the LLC resonant converter can suppress an inrush current because the difference between gains during the transition from low-frequency characteristics to high-frequency characteristics is smaller, which alleviates the stress on the switches in the bridge circuit. Eventually, the LLC resonant converter can achieve both a high-efficiency operation and a high gain, and thus can contribute to downsizing of a power unit or the like containing the LLC resonant converter. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a circuit diagram showing a schematic configuration of a main part of an LLC resonant converter  100  in an embodiment according to the present invention. 
         FIG. 2  is a block diagram showing a schematic configuration of a control section in the LLC resonant converter  100 . 
         FIG. 3  is a flowchart that outlines changeover control at the control section in the LLC resonant converter  100 , between the operation with the resonant capacitor Cr alone and the operation with the parallel-connected capacitors Cr and Crsw. 
         FIG. 4  is a graph describing an example of control in the LLC resonant converter  100 , during the transition from low-frequency characteristics to high-frequency characteristics. 
         FIG. 5  is a circuit diagram representing a schematic configuration of an LLC current resonant converter  1 , as an example of a conventional LLC resonant converter. 
         FIG. 6  is a graph showing an example of frequency-gain characteristics of the LLC current resonant converter  1 . 
         FIG. 7  is a schematic view showing a configuration example of a source circuit containing the LLC resonant converter. 
         FIG. 8( a )  shows an equivalent circuit in the LLC resonant converter including an output load resistance as a load, seen from the primary side of the transformer  30 . ZL indicates a parallel connection part in  FIG. 8( a ) .  FIG. 8( b )  shows an equivalent circuit  1 C in which a ZL inductance and a resistance are connected in series. 
         FIG. 9  is a graph showing an example of frequency-gain characteristics and two operating regions of the LLC resonant converter. 
         FIG. 10  is a graph showing an example of frequency-gain characteristics of the LLC resonant converter, with different resonant capacitors Cr. 
         FIG. 11  is a graph showing an example of frequency-gain characteristics of the LLC resonant converter, with different magnetizing inductances Lm. 
         FIG. 12  is a circuit diagram showing a schematic configuration of an LLC resonant converter  1 D that can change the capacitance of its resonant capacitor, similar to the power converter disclosed in PTL 1. 
         FIG. 13  is a graph showing an example of frequency-gain characteristics of the LLC resonant converter  1 D. 
         FIG. 14  is a graph for schematically describing an example of control in the LLC resonant converter  1 D, during the transition from low-frequency characteristics to high-frequency characteristics. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     An embodiment according to the present invention is hereinafter described with reference to the drawings. 
     &lt;Schematic Configuration in the Embodiment&gt; 
       FIG. 1  is a circuit diagram showing a schematic configuration of a main part of an LLC resonant converter  100  in an embodiment according to the present invention.  FIG. 2  is a block diagram showing a schematic configuration of a control section in the LLC resonant converter  100 . 
     As shown in  FIG. 1 , the LLC resonant converter  100  includes a bridge circuit  10  configured to receive a DC input voltage Vin, an LLC resonant circuit  20  connected to the bridge circuit  10 , a transformer  30  connected to the LLC resonant circuit  20 , a rectifier circuit  40  connected to the transformer  30  and configured to send out a converted DC voltage, a resonant capacitor changeover circuit  50 , a bridge circuit control section  64 , and a resonant capacitor changeover control section  65 . 
     The bridge circuit  10  has series-connected switches Q 1 , Q 2 . The states of these switches are changed in time sequence at an operating frequency f, under control of the bridge circuit control section  64 . Namely, by such switching operations, the bridge circuit  10  sends out square-wave voltages at a connection point  10   a  between the switches Q 1 , Q 2  and at a GND  10   b.    
     The bridge circuit may be a half-bridge circuit as illustrated, or may be a full-bridge circuit instead. The switches Q 1 , Q 2  may be, for example, but are not limited to, switching elements such as field effect transistors (FETs) and IGBTs. 
     The LLC resonant circuit  20  has a resonant capacitor Cr, an end of which is connected to the GND  10   b  of the bridge circuit  10 . The resonant capacitor Cr, as well as a magnetizing inductance Lm and a leakage inductance Lr to be described later, forms the resonant circuit. 
     The transformer  30  includes a primary winding  31  and a secondary winding  32 , which are isolated from each other. The transformer  30  is an ideal transformer, and forms an equivalent circuit of a real transformer in combination with the magnetizing inductance Lm connected in parallel to first and second ends of the primary winding  31  and the leakage inductance Lr connected in series to the first end of the primary winding  31 . The first end of the primary winding  31  is connected to the connection point  10   a  of the switches Q 1 , Q 2 , via the leakage inductance Lr. The second end of the primary winding  31  is connected to the GND  10   b  of the bridge circuit  10 , via the resonant capacitor Cr. The secondary winding  32  is provided with a center tap  32   m . Similar to the LLC current resonant converter  1  shown in  FIG. 5 , the magnetizing inductance Lm and the leakage inductance Lr may be regarded as a part of the transformer  30 . 
     The rectifier circuit  40  includes rectifier elements D 1 , D 2  whose anodes (positive electrodes) are respectively connected to first and second ends of the secondary winding  32 , a positive output terminal  42   a  connected to cathodes (negative electrodes) of the rectifier elements, a negative output terminal  42   b  connected to the center tap  32   m  of the secondary winding  32 , a current smoothing capacitor  41  connected between the pair of output terminals  42   a ,  42   b  so as to smooth the electric current. Through this rectifier circuit, a DC output voltage Vo is generated at the pair of output terminals  42   a ,  42   b . The rectifier circuit  40  is not limited to the center-tapped full-wave rectifier circuit, but may be a half-wave rectifier circuit, a bridge rectifier circuit, etc. 
     The resonant capacitor changeover circuit  50  includes a switch Q 3  and a capacitor Crsw which are connected in series to each other and connected in parallel to the resonant capacitor Cr in the LLC resonant circuit  20 . The switch Q 3  turns on and off in response to a control signal from the resonant capacitor changeover control section  65 . 
     Similar to the switches Q 1 , Q 2 , the switch Q 3  may be, for example, but is not be limited to, a switching element such as a field effect transistor (FET). 
     Turning to  FIG. 2 , the control section in the LLC resonant converter  100  includes not only the bridge circuit control section  64  and the resonant capacitor changeover control section  65 , but also an input voltage detection circuit  61  for detecting an input voltage Vin (see  FIG. 1 ), an output voltage detection circuit  62  for detecting an output voltage Vo (see  FIG. 1 ), and an output current detection circuit  63  for detecting an output current Io (see  FIG. 1 ) supplied to an output load OL (see  FIG. 1 ). 
     The bridge circuit control section  64  is connected to the output voltage detection circuit  62 . Based on the output voltage Vo detected by the output voltage detection circuit  62 , the bridge circuit control section  64  controls the frequency (operating frequency f) of control signals sent to the switches Q 1 , Q 2 , and conducts pulse frequency modulation for the input voltage Vin. 
     The resonant capacitor changeover control section  65  is connected to the input voltage detection circuit  61 , the output voltage detection circuit  62 , and the output current detection circuit  63 . The resonant capacitor changeover control section  65  sends an on/off control signal to the switch Q 3 , based on the input voltage Vin, the output voltage Vo, and the output current Io detected respectively by these circuits. 
     The resonant capacitor changeover control section  65  is also connected to the bridge circuit control section  64 , and can send a command to the bridge circuit control section  64 . 
     The bridge circuit control section  64  and the resonant capacitor changeover control section  65  may be configured as a single control section. Alternatively, the functions of these control sections may be performed, for example, by a control unit for an entire device containing the LLC resonant converter  100 . As such, a proper program for performing such functions may be stored on a high-speed general-purpose CPU, etc. 
       FIG. 3  is a flowchart that outlines changeover control at the control section in the LLC resonant converter  100 , between the operation with the resonant capacitor Cr alone and the operation with the parallel-connected capacitors Cr and Crsw.  FIG. 4  is a graph describing an example of control in the LLC resonant converter  100 , during the transition from low-frequency characteristics to high-frequency characteristics. 
     If the input voltage Vin detected by the input voltage detection circuit  61  is lower than a predetermined changeover voltage, the resonant capacitor changeover control section  65  turns on the switch Q 3  in order to increase the gain as much as possible. Once the switch Q 3  is on, the LLC resonant converter  100  operates with low-frequency characteristics, with the capacitances of the resonant capacitor Cr and the capacitor Crsw. 
     In this state, as shown in  FIG. 3 , the resonant capacitor changeover control section  65  makes a comparison between the input voltage Vin detected by the input voltage detection circuit  61  (Step S 1 ) and the changeover voltage, and determines which is greater (Step S 2 ). The comparison may be repeated (No in Step S 2 ). 
     When the resonant capacitor changeover control section  65  confirms that the input voltage Vin has increased and exceeded the changeover voltage (YES in Step S 2 , Point  1  in  FIG. 4 ), the resonant capacitor changeover control section  65  calculates the resonance frequency fpr at which the gain is maximum (gain peak) while the switch Q 3  is off (high-frequency characteristics) (Step S 4 ), based on the output voltage Vo detected by the output voltage detection circuit  62  and the output current Io detected by the output current detection circuit  63  (Step S 3 ). 
     Then, the resonant capacitor changeover control section  65  sends a command to the bridge circuit control section  64 , to suspend the operating frequency control based on the detected output voltage Vo and to raise the operating frequency f instantly and sharply to the high frequency side, up to a changeover frequency that is higher than the resonance frequency fpr. Preferably, the resonant capacitor changeover control section  65  sends a command to raise the operating frequency f instantly and sharply to the high frequency side, over the resonance frequency fsr at which the gain is 1 while the switch Q 3  is off (high-frequency characteristics) (Step S 5 , from Point  1  to Point  2  in  FIG. 4 ). 
     Thereafter, the resonant capacitor changeover control section  65  turns off the switch Q 3  (Step S 6 , from Point  2  to Point  3  in  FIG. 4 ), and then sends a command to the bridge circuit control section  64  to resume the steady-state control of the operating frequency based on the detected output voltage Vo (Step S 7 ). In response to this command, the LLC resonant converter  100  returns to the high-efficient operation with high-frequency characteristics, with the capacitance of the resonant capacitor Cr alone, so that the operating frequency f gradually drops to the low frequency side (from Point  3  to Point  4  in  FIG. 4 ). 
     During the time between the suspension and the resumption of the operating frequency control based on the detected output voltage Vo in the bridge circuit control section  64 , the gain decreases as the operating frequency f increases to the high-frequency side, so that the LLC resonant converter  100  cannot provide the desired output voltage Vo by itself. Hence, the required time between the suspension and the resumption of the operating frequency control based on the detected output voltage Vo is determined in consideration of the retention time t (see the description about  FIG. 7 ) that is obtained, for example, by the capacitance of the block capacitor Cin connected to the input side of the bridge circuit  10 . Specifically, the required time between the suspension and the resumption of the operating frequency control based on the detected output voltage Vo needs to be shorter than at least the retention time t. 
     The present invention can be embodied and practiced in other different forms without departing from the gist and essential characteristics of the present invention. Therefore, the above-described embodiment is considered in all respects as illustrative and not restrictive. The scope of the invention is indicated by the appended claims rather than by the foregoing description. All variations and modifications falling within the equivalency range of the appended claims are intended to be embraced therein. 
     The present application claims priority to Japanese Patent Application No. 2017-071185, filed Mar. 31, 2017. The contents of this application are incorporated herein by reference. The contents of the document cited in this application are also entirely and concretely incorporated herein by reference. 
     INDUSTRIAL APPLICABILITY 
     The present invention is suitably applicable not only to the switched-mode power supply as described above, but also to, for example, an inverter, wireless power transfer technology, etc. 
     REFERENCE SIGNS LIST 
     
         
           1  LLC current resonant converter 
           1 A isolated DC/DC converter 
           1 B equivalent circuit 
           1 C equivalent circuit 
           1 D LLC resonant converter 
           2  PFC 
           3  source circuit 
           10  bridge circuit 
           20  LLC resonant circuit 
           30  transformer 
           31  primary winding 
           32  secondary winding 
           40  rectifier circuit 
           41  current smoothing capacitor 
           50  resonant capacitor changeover circuit 
           61  input voltage detection circuit 
           62  output voltage detection circuit 
           63  output current detection circuit 
           64  bridge circuit control section 
           65  resonant capacitor changeover control section 
           100  LLC resonant converter 
         CP commercial power 
         Cr resonant capacitor (first capacitor) 
         Crsw capacitor (second capacitor) 
         D 1  rectifier element 
         D 2  rectifier element 
         f operating frequency 
         fpr resonance frequency (first resonance frequency) 
         fsr resonance frequency (second resonance frequency) 
         Io output current 
         Lm magnetizing inductance 
         Lr leakage inductance 
         OL output load 
         Q 1  switch 
         Q 2  switch 
         Q 3  switch (changeover switch) 
         t retention time 
         Vin input voltage 
         Vo output voltage