Abstract:
This invention relates to a synchronous rectifier for LLC resonant converter. This method allows simple drive method for the synchronous rectifier MOSFETS by using the transformer secondary winding voltage and one-shot vibrator. The synchronous rectifier MOSFETs are turned on by being triggered to the transformer secondary side winding voltage and turned off after predetermined time set by one shot vibrator. The predetermined time is set by the resonant period of the resonant network.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority to and the benefit of Korean Patent Application No. 10-2008-0001331 filed in the Korean Intellectual Property Office on Jan. 4, 2008 the entire contents of which are incorporated herein by reference. 
       BACKGROUND 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a synchronous rectifier. 
         [0004]    2. Description of the Related Art 
         [0005]    A synchronous rectifier is generated by substituting a switch for a diode of a secondary coil of a transformer forming an LLC resonant converter. The synchronous rectifier provides an improved efficiency by minimizing a voltage drop across the diode. For the efficiency improvement, it is required to precisely control the on/off time of the switch associated with the secondary coil of the synchronous rectifier. 
         [0006]    In general, the synchronous rectifier senses voltages at both terminals of the switch associated with the secondary coil of the transformer, and turns on/off the switch corresponding to the sensed voltage. 
         [0007]    A current is induced by the primary coil in the secondary coil of the transformer and flows through a body diode of the switch associated with the secondary coil of the transformer a voltage corresponding to a forward voltage drop of the body diode is sensed across the terminals of the switch associated with the secondary coil. Since the voltage corresponding to the forward voltage drop of the body diode is sensed across the terminals of the switch of the secondary coil, the switch associated with the secondary coil is turned on. When the switch associated with the secondary coil is turned on, the impedance characteristics across the terminals of the switch associated with the secondary coil exhibit a constant resistance. Since the voltage across the terminals of the switch associated with the secondary coil is proportional to the current flowing through the switch, the switch is turned off when the voltage across the terminals of the switch is sensed and the current flowing through the switch is reduced below a reference value. When the switch associated with the secondary coil is turned off, the current flows through the body diode of the switch and a reverse bias is applied to the body diode of the secondary coil switch as the current induced by the primary coil in the secondary coil of the transformer is reduced to zero so that a current through the body diode does not flow any more. 
         [0008]    However, the voltage to determine the turn-off time of the secondary coil switch, sensed by the synchronous rectifier driven by the above-noted method is a low voltage (e.g., several tens of mV), it is weak relative to the noise. Further, it is difficult to control the off time of the secondary coil switch because of the influence by a parasitic component on a printed circuit board (PCB) layout. For both of these reasons, it is difficult to improve the efficiency of the above rectifier beyond a degree compared to the general LLC resonant converter. 
         [0009]    Another method for driving the synchronous rectifier has been proposed to control the on/off states of the secondary coil switch of the transformer by using a control signal for controlling the on/off states of the primary coil switch of the transformer. When the synchronous rectifier is driven by using this method, the on/off time of the secondary coil switch can be more precisely controlled. 
         [0010]    However, in order to implement the method, an additional component such as a photocoupler or a transformer is required so as to transmit a control signal of the primary coil of the transformer to the secondary coil, and hence the production cost of the synchronous rectifier in increased. Also, when the switch associated with the primary coil of the transformer is turned on/off with a frequency that is less than the resonance frequency of the transformer, resonance of the current flowing to the secondary coil of the transformer can be terminated before the switch of the primary coil of the transformer is turned off, and hence, the switch of the secondary coil of the transformer may not be turned off at an appropriate time. Accordingly, the current may flow in the reverse direction to reduce efficiency. 
         [0011]    The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art. 
       SUMMARY 
       [0012]    Briefly and generally, an embodiment of the present invention includes a synchronous rectifier including a square wave generator including a first switch and a second switch, and generating a square wave corresponding to an input voltage by alternately turning on/off the first switch and the second switch; a resonator including a first coil of a primary coil of a transformer, and generating a resonance waveform corresponding to the square wave; and an output unit including a second coil and a third coil of a secondary coil of the transformer, and outputting a first voltage corresponding to currents that are generated in the second coil and the third coil corresponding to the resonance waveform, wherein the output unit includes a third switch coupled between the second coil and a ground; a fourth switch coupled between the third coil and the ground; and a switching controller for turning on/off the third switch and the fourth switch. 
         [0013]    The switching controller maintains the third switch at the On state for a first period from a first time in which a second voltage at the third switch is reduced from a first level to a second level that is less than the first level, and it maintains the fourth switch at the On state for a second period from a second time in which a third voltage at the fourth switch is reduced from the first level to the second level. 
         [0014]    Another embodiment includes a synchronous rectifier having a square wave generator including a first switch and a second switch, and generating a square wave corresponding to an input voltage by alternately turning on/off the first switch and the second switch; a resonator including a first coil of a primary coil of a transformer, and generating a resonance waveform corresponding to the square wave; and an output unit including a second coil and a third coil of a secondary coil of the transformer, and outputting a first voltage corresponding to currents that are generated in the second coil and the third coil corresponding to the resonance waveform, wherein the output unit includes: a third switch coupled between the second coil and a ground; a fourth switch coupled between the third coil and the ground; and a switching controller for turning on/off the third switch and the fourth switch. 
         [0015]    The switching controller maintains the third switch at the On state during a first period from a first time when the current flows through a body diode of the third switch, and it maintains the fourth switch at the On state during a second period from a second time when the current flows through a body diode of the fourth switch. 
         [0016]    In some embodiments, the on/off states of the switches SR 1  and SR 2  can be precisely controlled by sensing the voltage induced to the secondary coil of the transformer without adding components such as a photocoupler or a transformer, and hence, a synchronous rectifier with a low cost, stability, and high efficiency can be realized. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  shows a configuration of a synchronous rectifier according to an exemplary embodiment of the present invention. 
           [0018]      FIG. 2  shows a waveform diagram of showing voltage and current waveforms of respective parts of a synchronous rectifier when switches Q 1  and Q 2  of a square wave generator  100  of a synchronous rectifier according to an exemplary embodiment of the present invention is driven to be turned on/off with a frequency (fs) that is less than a resonance frequency (fo) of a resonator  200 . 
           [0019]      FIG. 3  shows first to third current paths ({circle around ( 1 )}-{circle around ( 3 )}) flowing to a synchronous rectifier from the time T 11  to the time T 12  in a waveform diagram shown in  FIG. 2 . 
           [0020]      FIG. 4  shows second and fourth current paths ({circle around ( 2 )}, {circle around ( 5 )}) flowing to a synchronous rectifier from the time T 12  to the time T 13  in a waveform diagram shown in  FIG. 2 . 
           [0021]      FIG. 5  shows sixth to ninth current paths ({circle around ( 6 )}-{circle around ( 9 )}) flowing to a synchronous rectifier from the time T 13  to the time T 14  in a waveform diagram shown in  FIG. 2 . 
           [0022]      FIG. 6  shows fifth and seventh current paths ({circle around ( 5 )}, {circle around ( 7 )}) flowing to a synchronous rectifier from the time T 14  to the time T 15  in a waveform diagram shown in  FIG. 2 . 
           [0023]      FIG. 7  shows a waveform diagram of voltage and current waveforms for respective parts of a synchronous rectifier according to an exemplary embodiment of the present invention when switches Q 1  and Q 2  of a square wave generator  100  of the synchronous rectifier is driven to be turned on/off with a frequency (fs) that is greater than a resonance frequency (fo) of a resonator  200 . 
       
    
    
     DETAILED DESCRIPTION 
       [0024]    In the following detailed description, only certain exemplary embodiments of the present invention have been shown and described, simply by way of illustration. As those skilled in the art would realize, the described embodiments may be modified in various different ways, all without departing from the spirit or scope of the present invention. Accordingly, the drawings and description are to be regarded as illustrative in nature and not restrictive. Like reference numerals designate like elements throughout the specification. 
         [0025]    Throughout this specification and the claims that follow, when it is described that an element is “coupled” to another element, the element may be “directly coupled” to the other element or “electrically coupled” to the other element through a third element. 
         [0026]      FIG. 1  shows an embodiment of a synchronous rectifier. The synchronous rectifier can include a square wave generator  100 , a resonator  200 , an output unit  300 , and a feedback circuit  400 . 
         [0027]    The square wave generator  100  can include a PFM controller  110  and switches Q 1  and Q 2 , and generate a square wave corresponding to on/off states of the switches Q 1  and Q 2 . 
         [0028]    The switch Q 1  has a first terminal coupled to a node between a first terminal of an input terminal of an input voltage Vin and an inductor Lr. The switch Q 2  has a first terminal coupled to a node between a second terminal of the switch Q 1  and a capacitor Cr, and a second terminal coupled to a node between a second terminal of an input terminal of the input voltage Vin and a ground terminal. 
         [0029]    The PFM controller  10  can generate a pulse frequency modulation signal corresponding to a feedback signal Vfb input by the feedback circuit  140 , and output control signals Vgs 1  and Vgs 2  for controlling the on/off states of the switches Q 1  and Q 2 . 
         [0030]    Here, the PFM controller  110  may alternately turn on/off the two switches Q 1  and Q 2 . Also, the PFM controller  110  may drive the two switches Q 1  and Q 2  so that their duty cycles may be approximately 50%. In some embodiments, the PFM controller  110  can provide a dead time of several hundreds of nanoseconds (ns) between the control signal Vgs 1  turning on the switch Q 1  and the control signal Vgs 2  turning on the switch Q 2  so that the switch Q 1  and the switch Q 2  may not be turned on simultaneously, and hence it can prevent a penetration current caused by the arm short phenomenon and control the switches Q 1  and Q 2  to perform a zero voltage switching (ZVC) operation. Here, the dead time can represent a period for maintaining the switch Q 1  and the switch Q 2  at the turned off state. 
         [0031]    The resonator  200  can include inductors Lr and Lm, a primary coil L 1  and a capacitor Cr of the transformer, and it can transmit part of the resonance current that is generated by resonating the square wave generated by the square wave generator  100  to the output unit  300 . Here, the inductor Lm represents a parasitic inductance that is associated with the primary coil L 1  of the transformer. In other words, the inductor Lm represents magnetizing inductance, and it functions as a shunt inductor for providing a current path for controlling the current (Ip−IL 1 =Im) to flow to the capacitor Cr other than the current IL 1  that is induced to the secondary coils L 2  and L 3  of the transformer through the primary coil L 1  of the transformer from among the current Ip flowing to the inductor Lr. That is, the inductor Lm can maintain a voltage gain of the synchronous rectifier even when a load of an output terminal is changed, so as to control the output voltage Vo with an almost constant frequency without relation to a load change. 
         [0032]    The inductor Lr can have a first terminal coupled to the first terminal of the switch Q 1 . The first terminal of the primary coil L 1  of the transformer can be coupled to the second terminal of the inductor Lr. The first terminal of the capacitor Cr can be coupled to the second terminal of the primary coil L 1  of the transformer, and the second terminal can be coupled to a node between the switch Q 1  and the switch Q 2 . The inductor Lm can be formed at the primary coil L 1  of the transformer as the current flows to the primary coil L 1  of the transformer. Here, the inductance of the inductor Lm can be greater than the inductance of the inductor Lr. For example, the inductance of the inductor Lm can be three to eight times the inductance of the inductor Lr. 
         [0033]    The feedback circuit  400  can include a photo transistor PT for forming a photocoupler together with a photodiode PD of the output unit  300 , and a capacitor C 2  coupled in parallel with the photo transistor PT. The photo transistor PT can be driven by receiving the current flowing through the photodiode PD of the output unit  300 . For example, when the output voltage Vo is increased, the feedback voltage Vfb, charged in the capacitor C 2 , can be reduced, and when the output voltage Vo is reduced, the feedback voltage Vfb can be increased. The PFM controller  110  may control drive frequencies of the switches Q 1  and Q 2  according to the feedback voltage Vfb, and control the output voltage Vo to be constant through the pulse frequency modulation 
         [0034]    The output unit  300  can include secondary coils L 2  and L 3  of a transformer, switches SR 1  and SR 2 , a capacitor C 1 , a photodiode PD, a resistor R 1 , a Zener diode ZD, and a switching controller  320 , and it may outputs the output voltage Vo corresponding to the current induced to the secondary coils L 2  and L 3  of the transformer from the resonator  200 . 
         [0035]    The capacitor C 1  may have a first terminal coupled to a first terminal of the secondary coil L 2  of the transformer and a second terminal coupled to the ground. An anode of the photodiode PD can be coupled to a first terminal of the capacitor C 1 . A first terminal of the resistor R 1  can be coupled to a cathode of the photodiode PD. The Zener diode ZD can have a cathode coupled to a second terminal of the resistor R 1 , and an anode coupled to the ground. The switch SR 1  can have a first terminal coupled to a second terminal of the secondary coil L 2  of the transformer, and a second terminal coupled to ground. The switch SR 2  can have a first terminal coupled to the ground, and a second terminal coupled to a first terminal of the secondary coil L 3  of the transformer. A second terminal of the secondary coil L 3  of the transformer can be coupled to the first terminal of the secondary coil L 2  of the transformer. Here, the voltage at the capacitor C 1  is the output voltage Vo, and the current flowing to the photodiode PD can vary according to the output voltage Vo. The photodiode PD forms a photocoupler together with the photo transistor PT of the feedback circuit  400  and provides information corresponding to the output voltage Vo to the feedback circuit  400 . 
         [0036]    The switching controller  320  may include one-shot vibrators  322  and  324  and drivers  326  and  328 . 
         [0037]    The one shot vibrator  322  can generate an output signal corresponding to a voltage Vs 1  applied to the switch SR 1 . The one shot vibrator  324  can generate an output signal corresponding to a voltage Vs 2  applied to the switch SR 2 . That is, the output signals of the one shot vibrators  322  and  324  can be switched to High when the voltage Vs 1  and the voltage Vs 2  are switched from High to Low (i.e., a falling edge), the output signals are maintained at High for a predetermined time and are than switched to Low. 
         [0038]    The driver  326  can apply a control signal SRDrv 1  for turning on/off the switch SR 1  corresponding to the output signal of the one shot vibrator  322  to a control electrode of the switch SR 1 . The driver  328  may apply a control signal SRDrv 2  for turning on/off the switch SR 2  corresponding to the output signal of the one shot vibrator  324  to a control electrode of the switch SR 2 . For example, the driver  326  can be realized to output a High control signal SRDrv 1  to turn on the switch SR 1  when the output signal of the one shot vibrator  322  is High, and to output a Low control signal SRDrv 1  to turn off the switch SR 1  when the output signal of the one shot vibrator  322  is Low. In a like manner, the driver  328  can be realized to output a High control signal SRDrv 2  to turn on the switch SR 2  when the output signal of the one shot vibrator  324  is High, and to output a Low control signal SRDrv 2  to turn off the switch SR 2  when the output signal of the one shot vibrator  224  is Low. 
         [0039]    For reference, not shown in  FIG. 1 , the switches Q 1 , Q 2 , SR 1 , and SR 2  respectively can include a body diode for controlling the current to flow from the source to the drain according to the characteristic of the metal oxide semiconductor field-effect transistor, or MOSFETs. While the switches Q 1 , Q 2 , SR 1 , and SR 2  are illustrated as MOSFETs in  FIG. 1 , other types of switches are used in other embodiments. 
         [0040]    An operation of a synchronous rectifier with reference to  FIG. 1  will now be described. To start with, the current induced by the primary coil L 1  of the transformer in the secondary coils L 2  and L 3  of the transformer can be proportional to the respective turn ratios of the primary coil L 1  and the secondary coils L 2  and L 3 . 
         [0041]    On/off drive frequencies fs of the switch Q 1  and Q 2  of the square wave generator  100  can vary according to the input voltage Vin that is input through a load of an output terminal of the synchronous rectifier or an input terminal. However, the resonance frequency fo of the resonator  200  may have a constant value because it follows the characteristics of components included in the resonator  200 . 
         [0042]    First, an operation of a synchronous rectifier when the switches Q 1  and Q 2  of the square wave generator  100  are turned on/off with a frequency fs that is less than the resonance frequency fo of the resonator  200  will now be described with reference to  FIG. 2  to  FIG. 6 . A load coupled to the output terminal of the synchronous rectifier is shown as a load resistor Ro. 
         [0043]      FIG. 2  shows voltage and current waveforms of respective parts of a synchronous rectifier when switches Q 1  and Q 2  of a square wave generator  100  of a synchronous rectifier are driven to be turned on/off with a frequency fs that is less than a resonance frequency fo of a resonator  200 . 
         [0044]    A waveform from the time T 11  to the time T 13  in the waveform diagram shown in  FIG. 2  will now be described with reference to  FIG. 3  and  FIG. 4 . 
         [0045]      FIG. 3  shows first to third current paths {circle around ( 1 )}-{circle around ( 3 )} flowing in a synchronous rectifier from the time T 11  to the time T 12  according to a waveform shown in  FIG. 2 .  FIG. 4  shows second and fourth current paths {circle around ( 2 )}, {circle around ( 5 )}, flowing in a synchronous rectifier from the time T 12  to the time T 13  according to a waveform shown in  FIG. 2 . 
         [0046]    First, at the time T 11 , the switch Q 2  can be turned on. When the switch Q 2  is turned on, the current may flow from the first terminal of the input terminal of the input voltage Vin to the second terminal of the input terminal of the input voltage Vin through the first current path {circle around ( 1 )} via the inductor Lr, the primary coil L 1  of the transformer, the capacitor Cr, and the switch Q 2 . In parallel, current also flows from the first terminal of the input terminal of the input voltage Vin to the second terminal of the input terminal of the input voltage Vin through the second current path {circle around ( 2 )} via the inductor Lr, the inductor Lm, the capacitor Cr, and the switch Q 2 . Here, the current flowing through the second current path {circle around ( 2 )} is circulated in the primary coil of the transformer, and the current flowing through the first current path {circle around ( 1 )} supplies a load current to the secondary coil of the transformer. 
         [0047]    In  FIG. 2 , the first current path {circle around ( 1 )} is shown with a solid line and the second current path {circle around ( 2 )} is shown with the dotted line. The current flowing through the first current path {circle around ( 1 )}, that is, the current IL 1  flowing to the primary coil L 1  of the transformer corresponds to the difference between the current Ip flowing to the inductor Lr and the current Im flowing through the second current path {circle around ( 2 )} and it is shown in  FIG. 2 . 
         [0048]    As the current flows through the first current path {circle around ( 1 )}, a current is induced in the secondary coil L 2  of the transformer, and hence, the current flows from the first terminal of the secondary coil L 2  of the transformer to the second terminal of the secondary coil L 2  of the transformer through the third current path {circle around ( 3 )} via the capacitor C 1  and the switch S 1 . In parallel, the current also flows from the first terminal of the secondary coil L 2  of the transformer to the second terminal of the secondary coil L 2  of the transformer through a fourth current path {circle around ( 4 )} via the load resistor Ro and the switch S 1 . 
         [0049]    Since the output unit  300  of the synchronous rectifier outputs a constant voltage Vo, the capacitor C 1  can be charged with the output voltage Vo before the time T 11 . Since no current flows through the secondary coils L 2  and L 3  of the transformer before the time T 11 , the voltages at the secondary coils L 2  and L 3  of the transformer are 0V, and the voltage Vs 1  corresponds to the output voltage Vo. 
         [0050]    As the current flows through the third and fourth current paths {circle around ( 3 )}, {circle around ( 4 )}, the body diode of the switch SR 1  can be turned on. Accordingly, the voltage Vs 1  can be reduced to approximately 0V. Also, the voltage Vs 2  across the switch SR 2  can be increased from the voltage Vo to the voltage  2 Vo. 
         [0051]    The one shot vibrator  322  may output an output signal for transitioning from Low to High synchronously with a falling edge of the voltage Vs 1  and maintaining High for a predetermined time Ton. The driver  326  can turn on the switch SR 1  by receiving a High signal from the one shot vibrator  322 . 
         [0052]    When the switch SR 1  is turned on, the current flowing through the body diode of the switch SR 1  may flow through the drain from the source of the switch SR 1 . If the switch SRI is shown as an equivalent circuit, it can be shown as resistance for dropping a lesser voltage compared to the body diode, and hence, the voltage drop caused by the switch SR 1  can be less compared to the case in which the current flows through the body diode. 
         [0053]    When the output signal of the one shot vibrator  322  transitioned to High after a predetermined time Ton, the output signal of the one shot vibrator  322  can be transitioned to Low. Accordingly, the switch SR 1  can be turned off so that the current flowing from the source of the switch SR 1  through the drain flows through the body diode of the switch SR 1 . 
         [0054]    The resonance frequency fo of the resonator  200  can have a constant value, and in  FIG. 2 , resonance generated by the resonator  200  may start from the time T 11  and finish at the time T 12 . The period Ton during which the output signal of the one shot vibrator  322  maintains High is set to be finished before the T 12 . That is, the switch SR 1  is set to be turned off about the time when resonance by the resonator  200  is terminated, and hence, the period during which the current flows through the body diode of the switch SR 1  can be very short, and the voltage drop caused by the switch SR 1  can be minimized. 
         [0055]    The time T 12  represents the time when the resonance between the inductor Lr and the capacitor Cr of the resonator  200  is terminated. 
         [0056]    When the resonance between the inductor Lr and the capacitor Cr of the resonator  200  is terminated at the time T 12 , the current of the primary coil of the transformer flows through the second current path {circle around ( 2 )}. Accordingly, no current is induced by the primary coil L 1  of the transformer in the secondary coil L 2  of the transformer, and thus no current flows through the third and fourth current paths {circle around ( 3 )}, {circle around ( 4 )}. Accordingly, since the current flowing through the secondary coils L 2  and L 3  of the transformer becomes 0 A, the voltages across the secondary coils L 2  and L 3  of the transformer become 0V, and hence, the voltage Vs 1  is increased to the voltage Vo, and the voltage Vs 2  is reduced from the voltage  2 Vo to the voltage Vo. 
         [0057]      FIG. 4  shows that at this time the current can freewheel by the voltage Vo charged in the capacitor C 1  through a fifth current path {circle around ( 5 )} via the first terminal of the capacitor C 1 , the load resistor Ro, and the second terminal of the capacitor C 1 . 
         [0058]    Waveforms in  FIG. 2  from the time T 13  to the time T 15  will now be described with reference to  FIG. 5  and  FIG. 6 . 
         [0059]      FIG. 5  shows sixth to ninth current paths {circle around ( 6 )}-{circle around ( 9 )} flowing through the synchronous rectifier from the time T 13  to the time T 14 .  FIG. 6  shows fifth and seventh current paths {circle around ( 5 )}, {circle around ( 7 )} flowing through the synchronous rectifier from the time T 14  to the time T 15 . 
         [0060]    The time T 13  can represent the time when the switch Q 1  is turned on. As the switch Q 1  is turned on, the current can flow from the first terminal of the input terminal of the input voltage Vin to the first terminal of the input terminal of the input voltage Vin through the sixth current path {circle around ( 6 )} via the switch Q 1 , the capacitor Cr, the primary coil L 1  of the transformer, and the inductor Lr. At this time the current can flow through the seventh current path {circle around ( 7 )} via the first terminal of the input terminal of the input voltage Vin, the switch Q 1 , the capacitor Cr, the inductor Lm, the inductor Lr, and the first terminal of the input terminal of the input voltage Vin. Here, the current flowing through the seventh current path {circle around ( 7 )} is circulated in the primary side of the transformer, and the current flowing through the sixth current path {circle around ( 6 )} supplies a load current to the secondary coil of the transformer. 
         [0061]    In  FIG. 5  the sixth current path {circle around ( 6 )} is shown with a solid line and the seventh current path {circle around ( 7 )} is shown with a dotted line. In this case, the current flowing through the sixth current path {circle around ( 6 )}, that is, the current IL 1  flowing to the primary coil L 1  of the transformer corresponds to the difference between the current Ip flowing to the inductor Lr and the current Im flowing through the seventh current path {circle around ( 7 )}, and it is shown in  FIG. 2 . 
         [0062]    As the current flows through the sixth current path {circle around ( 6 )}, the current is induced in the secondary coil L 3  of the transformer, and the current flows from the first terminal of the secondary coil L 3  of the transformer to the second terminal of the secondary coil L 3  of the transformer through the eighth current path {circle around ( 8 )} via the capacitor C 1  and the body diode of the switch SR 2 . In this case, the current flows through the ninth current path {circle around ( 9 )} via the first terminal of the secondary coil L 3  of the transformer, the photodiode PD, the resistor R 1 , the Zener diode ZD, the body diode of the switch SR 2 , and the second terminal of the secondary coil L 3  of the transformer. 
         [0063]    At the time T 13 , as the current flows through the eighth and ninth current paths {circle around ( 8 )}, {circle around ( 9 )}, the body diode of the switch SR 2  is turned on, and the voltage Vs 2  at the switch SR 2  maintains 0V. Also, the voltage Vs 1  across the switch SR 1  can be increased from the voltage Vo to the voltage  2 Vo. 
         [0064]    The one shot vibrator  324  may output an output signal for transitioning from Low to High synchronously with a falling edge of the voltage Vs 2  and maintaining High for a predetermined time Ton. The driver  328  can turn on the switch S 2  by receiving a High signal from the one shot vibrator  324 . 
         [0065]    When the switch S 2  is turned on, the current flowing through the body diode of the switch S 2  flows from the source of the switch S 2  through the drain. If the switch S 2  is represented with an equivalent circuit, it can be shown with a resistance for dropping a lesser voltage compared to the body diode, and hence, the voltage drop caused by the switch S 2  becomes smaller compared to the case in which the current flows through the body diode. 
         [0066]    The output signal of the one shot vibrator  324  can transition to High. After a predetermined time Ton passes, the output signal of the one shot vibrator  324  can transition to Low and the switch S 2  is turned off. When the switch S 2  is turned off, the current flowing from the source of the switch S 2  through the drain flows through the body diode of the switch S 2  so that the current flows through the eighth and ninth current paths {circle around ( 8 )}, {circle around ( 9 )}. 
         [0067]    The resonance frequency fo of the resonator  200  can have a constant value, and in  FIG. 2 , a resonance generated by the resonator  200  is shown to start at the time T 13  and terminate at the time T 14 . In this case, the period Ton during which the output signal of the one shot vibrator  322  maintains High can be set to be terminated before the time T 14 . That is, the switch S 2  is set to be turned off at about the time when the resonance by the resonator  200  is terminated, and hence the period in which the current through the body diode of the switch S 2  can be realized to be very short, and the voltage drop caused by the switch S 2  can be minimized. 
         [0068]    The time T 14  represents the time when resonance between the inductors Lr and Lm and the capacitor Cr of the resonator  200  is finished. 
         [0069]    When resonance between the inductors Lr and Lm and the capacitor Cr of the resonator  200  is finished at the time T 14 , the current of the primary coil of the transformer flows through the seventh current path {circle around ( 7 )}. Accordingly, the current is induced by the primary coil L 1  of the transformer in the secondary coil L 3  of the transformer, and no current flows through the eighth and ninth current paths {circle around ( 8 )}, {circle around ( 9 )}. As a result, since the current flowing through the secondary coils L 2  and L 3  of the transformer is 0 A, the voltages at the secondary coils L 2  and L 3  of the transformer become 0V so that the voltage Vs 2  is increased to the voltage Vo, and the voltage Vs 1  is reduced from the voltage  2 Vo to the voltage Vo. 
         [0070]    In this case, the current is freewheeled through the fifth current path {circle around ( 5 )} via the first terminal of the capacitor C 1 , the photodiode PD, the resistor R 1 , the Zener diode ZD, and second terminal of the capacitor C 1  by the voltage Vo charged in the capacitor C 1 . 
         [0071]    An operation of the synchronous rectifier after the time T 15  corresponds to an operation after the time T 11 , and hence, it will not be described. 
         [0072]    Up to now the operation of the synchronous rectifier when the switches Q 1  and Q 2  of the square wave generator  100  are turned on/off with the frequency fs that is less than the resonance frequency fo of the resonator  200  has been described. 
         [0073]    Hereinafter, an operation of the synchronous rectifier when the switches Q 1  and Q 2  of the square wave generator  100  are turned on/off with the frequency fs that is greater than the resonance frequency fo of the resonator  200  will now be described with reference to  FIG. 7 . 
         [0074]      FIG. 7  shows voltage and current waveforms for respective portions of a synchronous rectifier in some embodiments when switches Q 1  and Q 2  of a square wave generator  100  of the synchronous rectifier are driven to be turned on/off with a frequency fs that is greater than a resonance frequency fo of a resonator  200 . 
         [0075]      FIG. 7  shows the case in which the time when an output signal of the one shot vibrator  322  transitioned to Low corresponds to the time when levels of the control signals Vgs 1  and Vgs 2  transitioned when the switches Q 1  and Q 2  are turned on/off with a frequency fs that is greater than the resonance frequency fo of the resonator  200 . 
         [0076]    The time T 21  represents the time when the control signal Vgs 2  transitioned from Low to High and the switch Q 2  is accordingly turned on. 
         [0077]    As the switch Q 2  is turned on, the current flows through the first and second current paths {circle around ( 1 )}, {circle around ( 2 )} shown in  FIG. 3 . In this case, the current flowing to the first current path {circle around ( 1 )}, that is, the current IL 1  flowing to the primary coil L 1  of the transformer corresponds to the difference between the current Ip flowing to the inductor Lr and the current Im flowing to the second current path {circle around ( 2 )}, which is shown in  FIG. 7 . 
         [0078]    The current corresponding to the current flowing through the first current path {circle around ( 1 )} is induced in the secondary coil L 2  of the transformer, and hence, the current flows through the body diode of the switch SR 1  and the current flows through the third and fourth current paths {circle around ( 3 )}, {circle around ( 4 )}. As the current flows through the third and fourth current paths {circle around ( 3 )}, {circle around ( 4 )}, the body diode of the switch SR 1  is turned on and the voltage Vs 1  is steeply reduced to 0V. Also, the Vs 2  voltage at the switch SR 2  can be increased to  2 Vo. 
         [0079]    The one shot vibrator  322  can output an output signal for transitioning from Low to High synchronously with a falling edge of the voltage Vs 1  and for maintaining High for a predetermined time Ton. The driver  326  can turn on the switch SR 1  by receiving a High signal from the one shot vibrator  322 . 
         [0080]    When the switch SR 1  is turned on, the current flowing through the body diode of the switch SR 1  can flow from the source of the switch SR 1  through the drain. If the switch SR 1  is represented as an equivalent circuit, it can be expressed as resistance having a lesser voltage compared to the body diode, and hence, the voltage drop caused by the switch SR 1  can become less compared to the case in which the current flows through the body diode. 
         [0081]    The output signal of the one shot vibrator  322  transitions to High, stays High for a predetermined time Ton and then transitions to Low. Accordingly, the switch SR 1  is turned off and the current flowing from the source of the switch SR 1  through the drain flows through the body diode of the switch SR 1 . 
         [0082]    The period Ton in which the output signal of the one shot vibrator  322  is High is typically shorter than the period in which the control signals Vgs 1  and Vgs 2  are High. Accordingly, the period Ton is less than half of the on/off drive period of the switches Q 1  and Q 2 : Ton=0.5/fo. Since the resonance frequency of the switches Q 1  and Q 2  varies with the input voltage Vin or the load of the output terminal, the period Ton varies accordingly. 
         [0083]    The time T 22  represents the time at which the control signal Vgs 2  transitions from High to Low and the switch Q 2  is turned off. 
         [0084]    The switch Q 2  is turned off at the time T 22  before the resonance between the inductor Lr and the capacitor Cr that started when the switch Q 2  was turned on at the time T 21 , is terminated. 
         [0085]    The above represents the turn on/off operation of the switches Q 1  and Q 2  of the synchronous rectifier with the frequency fs that is greater than the resonance frequency fo of the resonator  200 . 
         [0086]    When the switch Q 2  is turned off, the current flowing through the primary coil of the transformer can be substantially reduced, and hence, as shown in  FIG. 7 , the waveform of the current Ip flowing to the inductor Lr at the time T 22  can change in a non-linear manner. 
         [0087]    Since the current Ip flowing to the inductor Lr is steeply reduced, the current flowing through the first current path {circle around ( 1 )} can be steeply reduced, and hence, the current induced in the secondary coil L 2  of the transformer can also be steeply reduced. 
         [0088]    The time T 23  represents the time when the control signal Vgs 1  transitioned from Low to High and the switch Q 1  is turned on. 
         [0089]    When the switch Q 1  is turned on, the current flowing from the primary coil of the transformer through the first and second current paths {circle around ( 1 )}, {circle around ( 2 )} flows through the sixth and seventh current paths {circle around ( 6 )}, {circle around ( 7 )} shown in  FIG. 5 . In this case, the current flowing through the sixth current path {circle around ( 6 )}, that is, the current IL 1  flowing through the primary coil L 1  of the transformer corresponds to the difference between the current Ip flowing to the inductor Lr and the current Im flowing through the seventh current path ({circle around ( 7 )}), which is shown in  FIG. 7 . 
         [0090]    Since the current flows through the sixth current path {circle around ( 6 )}, the current is induced in the secondary coil L 3  of the transformer, and hence, the current flows through the eighth and ninth current paths {circle around ( 8 )}, {circle around ( 9 )} shown in  FIG. 5  and the body diode of the switch SR 2  is turned on. 
         [0091]    When the body diode of the switch SR 2  is turned on, the voltage Vs 2  is steeply reduced to 0V, and the Vs 1  voltage at the switch SR 1  is increased to be the voltage  2 Vo. 
         [0092]    The one shot vibrator  324  can output an output signal for transitioning from Low to High synchronously with a falling edge of the voltage Vs 2  and maintaining High for a predetermined time Ton. The driver  328  can turn on the switch SR 2  by receiving a High signal from the one shot vibrator  324 . 
         [0093]    When the switch SR 2  is turned on, the current flowing through the body diode of the switch SR 2  flows from the source of the switch SR 2  through the drain. If the switch SR 2  in this case is represented as an equivalent circuit, it can be expressed as resistance for dropping a lesser voltage compared to the body diode, and hence, the voltage drop caused by the switch SR 2  compared to the case in which the current flows through the body diode can become smaller. 
         [0094]    The output signal of the one shot vibrator  324  transitions to High, stays High for a predetermined time Ton and then transitions to Low. Accordingly, the switch SR 2  is turned off and the current flowing from the source of the switch SR 2  through the drain flows through the body diode of the switch SR 2 . 
         [0095]    The time T 24  represents the time when the control signal Vgs 1  transitioned from High to Low and the switch Q 1  is turned off. 
         [0096]    The switch Q 1  is turned off at the time T 24  before the resonance between the inductor Lr and the capacitor Cr that started when the switch Q 1  is turned on at the time T 23 , which turns on/off the switches Q 1  and Q 2  of the synchronous rectifier with the frequency fs that is greater than the resonance frequency fo of the resonator  200 . 
         [0097]    When the switch Q 1  is turned off, the current flowing to the primary coil of the transformer is steeply reduced, and hence, as shown in  FIG. 7 , the waveform of the current Ip flowing to the inductor Lr at the time T 24  can change in a non-linear manner. 
         [0098]    Since the current Ip flowing to the inductor Lr is steeply reduced, the current flowing through the sixth current path {circle around ( 6 )} is steeply reduced, and hence, the current induced to the secondary coil L 3  of the transformer is steeply reduced. 
         [0099]    An operation of the synchronous rectifier after the time T 25  corresponds to the above-described operation after the time T 21  and hence it will not be described. 
         [0100]    The synchronous rectifier can sense the voltage induced to the secondary coil of the transformer, and precisely control the on/off states of the switches SR 1  and SR 2 , thereby improving efficiency. Also, the production cost of the synchronous rectifier may be smaller since there is no need to add a photo coupler or a transformer. 
         [0101]    While this invention has been described in connection with what is presently considered to be practical exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.