Patent Publication Number: US-9837886-B2

Title: Power conversion system for providing a current to meet operation of an electronic device by varying a leakage inductance thereof and method for providing the current thereof

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present disclosure relates to a power conversion system. More particularly, the present disclosure relates to a power conversion system for providing power to meet operations of an electronic device. 
     Description of Related Art 
     Since diode and Schottky diode have designated forward bias, the power loss of the power conversion system having the diode or Schottky diode to rectify power is large. Metal-oxide-semiconductor field-effect transistor (MOSFET) has advantages of low input resistance, short response time, and high input resistance, thus it replaces the diode and Schottky diode to be the main component of the rectifier. 
     In general, the power conversion system includes a plurality of synchronous rectifiers, which are driven at the same time the rectify power entering thereto. Specifically, when an electronic device electrically connected to the power conversion system is activated, the synchronous rectifiers perform synchronous rectifying procedure, and the MOSFETs of the synchronous rectifiers are switched to rectify the power entering the synchronous rectifiers; however, when the electronic device is inactivated, the synchronous rectifiers does not perform synchronous rectifying procedure. Even if the operation manner of the synchronous rectifier mentioned above is easy, the power provides by the power conversion system is a constant no matter the electronic device during non-light load condition and light load condition, thus the power loss during the electronic device under light load condition is large. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present disclosure, a power conversion system used for providing a power required for an electronic device is provided. The power conversion system includes an isolating transformer and an output-controlling device. The isolating transformer includes a primary winding and a plurality of secondary windings coupled with the primary winding, the isolating transformer has a plurality of coupling distances between the secondary windings. The output-controlling device includes a controller and a plurality of output-controlling modules electrically connected to the controller, wherein each of the secondary windings is electrically connected to one of the output-controlling devices. The controller places at least one output-controlling device in a conducting state for outputting a power. An amount of the secondary windings coupled with the primary winding is modulated for varying a leakage inductance of the power conversion system 
     According to another aspect of the present disclosure, a method for powering an electronic device includes the following steps: providing the power conversion system comprising a primary winding and a plurality of secondary windings, and there are a plurality of coupling distances between the primary winding and the secondary windings; sensing a current required for the electronic device; and modulating an amount of the secondary windings coupled with the primary winding for varying a leakage inductance of the power conversion system, thus an output current provided by the power conversion system is modulated to meet the current requirement of the electronic device. 
    
    
     
       BRIEF DESCRIPTION OF DRAWING 
       The features of the invention believed to be novel are set forth with particularity in the appended claims. The invention itself, however, may be best understood by reference to the following detailed description of the invention, which describes an exemplary embodiment of the invention, taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a circuit block diagram of a power conversion system according to a first embodiment of the present disclosure; 
         FIG. 2  is a circuit diagram of the power conversion system according to the first embodiment of the present disclosure; 
         FIG. 3  is a timing chart indicating the operations of power switches and rectifying switches shown in  FIG. 2 ; 
         FIGS. 4 a  and 4 b    are timing charts indicating operations of the power conversion system during light load condition; 
         FIGS. 5 a  and 5 b    are timing charts indicating operations of the power conversion system during normal load condition; 
         FIGS. 6 a  and 6 b    are timing charts indicating operations of the power conversion system during heavy load condition; 
         FIG. 7  is a cross sectional view of an isolating transformer according to the first embodiment of the present disclosure; 
         FIG. 8  is a diagram showing the state of leakage inductance, the magnetic flux, and temperature distribution in the isolating transformer shown in the  FIG. 7 ; 
         FIG. 9  is a diagram showing the state of leakage inductance, the magnetic flux, and temperature distribution in the isolating transformer shown in the  FIG. 7 ; 
         FIG. 10  is a diagram showing the state of leakage inductance, the magnetic flux, and temperature distribution in the isolating transformer shown in the  FIG. 7 ; and 
         FIG. 11  is a circuit diagram of a power conversion system according to a second embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference is made to  FIG. 1 , which is a circuit block diagram of a power conversion system according to a first embodiment of the present disclosure. In  FIG. 1 , the power conversion system (its reference numeral is omitted) receives an input voltage Vi and is configured to provide an output voltages Vo. The power conversion system includes a primary side and a secondary side, which are separated by an isolating transformer  30 . The isolating transformer  30  includes a primary winding  310  and a plurality of secondary windings  320   a ˜ 320   d  magnetically coupled to the primary winding  310 . The power conversion system further includes a switching module  10 , a resonant module  20 , an output-controlling device  40 , and a current sense unit  50 . The switching module  10 , the resonant module  20 , and the primary winding  310  are arranged at the primary side of the power conversion system, and the secondary windings  320   a ˜ 320   d , the output-controlling device  40 , and the current sense unit  50  are arranged at the secondary side of the power conversion system. The output-controlling device  40  includes a plurality of output-controlling modules  400   a ˜ 400   d , and the controlling modules  400   a ˜ 400   d  include a plurality of synchronous rectifying units (details are described in the following paragraphs) and a plurality of output switches (details are described in the following paragraphs). The current sense unit  50  senses a current flowing through a sensing resistor Rs electrically connected to the output-controlling device  40  and sends a current sensed signal to the controller  420  for controlling the operation of the output-controlling device  40 . 
     Reference is made to  FIG. 2 , which is a circuit diagram of a power conversion system according to the first embodiment of the present disclosure. The switching module  10  is electrically connected to the input voltage Vi and includes a first power switch QA, a second power switch QB, a third power switch QC, and a fourth power switch QD. The first to fourth switches QA˜QD are, for example, metal-oxide-semiconductor field-effect transistors (MOSFETs). The drains of the first power switch QA and the third power switch QC are connected to the input voltage Vi, the source of the first power switch QA is connected to the drain of the second power switch QB, and the source of the third power switch QC is connected to the drain of the fourth power switch QD and the primary winding  310 . The sources of the second power switch QB and the fourth power switch QD are connected to the input voltage Vi. 
     The switching module  10  may further includes a plurality of diodes D and a plurality of capacitors C electrically connected to the first to fourth power switches QA˜QD. Specifically, the diodes D are, for example, the body diodes of the first to fourth power switches QA˜QD, and the capacitors C are, for example, parasitic capacitances of the first to fourth power switches QA˜QD; the cathode of each diode D is connected to the drain of one of the first to fourth power switches QA˜QD, the anode thereof is connected to the source of one of the first to fourth power switches QA˜QD, and each of the capacitors C is electrically connected to one of the diodes D in parallel. 
     The resonant module  20  includes a resonant inductor Lr, a direct-current (DC) isolating capacitor Cb, and a magnetizing inductor. In  FIG. 2 , the resonant inductor Lr and the isolating transformer  30  are shown integrally; nevertheless, they are able to be separated in the practical manufacturing process. The DC isolating capacitor Cb, the resonant inductor Lr, and the primary winding  310  are electrically connected in series. specifically, one terminal of the DC isolating capacitor Cb is connected to the sources of the first power switch QA and second power switch QB, and the other terminal of the DC isolating capacitor Cb is connected to one terminal of the resonant inductor Lr, and the other terminal of the resonant indictor Lr is connected to the primary winding  310 . 
     The first to fourth power switches QA˜QD of the resonant module  20  are controlled using a zero-voltage-switching (ZVS) scheme to reduce switching loss. 
     The output-controlling device  40  includes a first synchronous rectifying unit  410   a , a second synchronous rectifying unit  410   b , a third synchronous rectifying unit  410   c , a fourth synchronous rectifying unit  410   d , a first output switch SW 1 , a second output switch SW 2 , a third output switch SW 3 , and a fourth power switch SW 4 . The first synchronous rectifying unit  410   a  is connected to the secondary winding  320   a  and the first output switch SW 1 , the second synchronous rectifying unit  410   b  is electrically connected to the secondary winding  320   b  and the second output switch SW 2 , the third synchronous rectifying unit  410   c  is electrically connected to the secondary winding  320   c  and the third output switch SW 3 , and the fourth synchronous rectifying unit  410   d  is electrically connected to the secondary winding  320   d  and the fourth output switch SW 4 . 
     The first synchronous rectifying unit  410   a  includes rectifying switches Q 1  and Q 2 , the second synchronous rectifying unit  410   b  includes rectifying switches Q 3  and Q 4 , the third synchronous rectifying unit  410   c  includes rectifying switches Q 5  and Q 6 , and the fourth synchronous rectifying unit  410   d  includes rectifying switches Q 7  and Q 8 . The rectifying switches Q 1  to Q 8  are, for example, MOSFETs. 
     The source of the rectifying switch Q 1  is connected to the source of the rectifying switch Q 2 , and the drains of the rectifying switches Q 1  and Q 2  are respectively connected to the secondary winding  320   a  (the drain of the rectifying switch Q 1  is connected to one terminal of the secondary winding  320   a , and the drain of the rectifying switch Q 2  is connected to the other terminal of the secondary winding  320   a ). The source of the rectifying switch Q 3  is connected to the source of the rectifying switch Q 4 , and the drains of the rectifying switches Q 3  and Q 4  are respectively connected to the secondary winding  320   b . The source of the rectifying switch Q 5  is connected to the source of the rectifying switch Q 6 , and the drains of the rectifying switches Q 5  and Q 6  are respectively connected to the secondary winding  320   c . The source of the rectifying switch Q 7  is connected to the source of the rectifying switch Q 8 , and the drains of the rectifying switches Q 7  and Q 8  are respectively connected to the secondary winding  320   d . The gates SR 1 ˜SR 8  of the rectifying switches Q 1 ˜Q 8  are electrically connected to the controller  420 , and the rectifying switches Q 1 ˜Q 8  are controlled by the controller  420  using a synchronous rectifying scheme. 
     The power conversion system further includes filters L 1 ˜L 8 , which are, for example, chokes. The filters L 1  and L 2  are arranged between the secondary winging  320   a  and the first output switch SW 1 , the filter L 3  and L 4  are arranged between the secondary winding  320   b  and the second output switch SW 2 , the filter L 5  and L 6  are arranged between the secondary winding  320   c  and the third output switch SW 3 , and the filter L 7  and L 8  are arranged between the secondary winding  320   d  and the fourth output switch SW 4 . Specifically, each secondary winding  320   a ˜ 320   d  has two terminals, one terminal of the secondary winding  320   a  is connected to the filter L 1 , and the other terminal thereof is connected to the filter L 2 ; one terminal of the secondary winding  320   b  is connected to the filter L 3 , and the other terminal thereof is connected to the filter L 4 ; one terminal of the secondary winding  320   c  is connected to the filter L 5 , and the other terminal thereof is connected to the filter L 6 ; one terminal of the secondary winding  320   d  is connected to the filter L 7 , and the other terminal thereof is connected to the filter L 8 . 
     The power conversion system still further includes a plurality of output capacitors Co. One terminal of the output capacitor is connected to synchronous rectifying unit  410   a ˜ 410   d , and the other terminal thereof is connected to one of the first to fourth output switch SW 1 ˜SW 4 . 
     It should be noted that the power conversion system is configured to provide different powers to meet the power required for the electronic device. Therefore, the controller  420  may measure the power required for the electronic device by the current sensed signal representing the current flowing through the sensing resistor Rs sent from the current sense unit  50  and place at least one of the synchronous rectifying units  410   a ˜ 410   d  or at least one of the first to fourth output switch SW 1 ˜SW 4  in a conducting state to conduct the power required for the electronic device to the electronic device. It should be noted that when the synchronous rectifying  410   a ˜ 410   d  are in the conducting state, the powers coupled to the secondary winding  320   a ˜ 320   d  are conducted to the synchronous rectifying units  410   a ˜ 410   d , and a synchronous rectifying procedure is performed. On the contrary, when the synchronous rectifying units  410   a ˜ 410   d  are not in the conducting state (or called the synchronous rectifying units  410   a ˜ 410   d  are in a non-conducting state), the power transmitted to the primary winding  310  cannot conducted to the secondary windings  320   a ˜ 320   d , and the synchronous rectifying procedure is not performed. Besides, when the first to fourth output switches SW 1 ˜SW 4  are in the conducting state, the first to fourth output switches SW 1 ˜SW 4  turn on (close), the powers with synchronous rectification are conducted to the output capacitors Co and the output Vo. One the contrary, when the first to fourth output switches SW 1 ˜SW 4  are in the non-conducting state, the first to fourth output switches SW 1 ˜SW 4  turn off (open), the powers with synchronous rectification cannot be conducted to the output capacitors Co and the output Vo. 
     The isolating transformer  30  and the resonant inductor Lr (if exist) provide a leakage inductance. In order to achieve higher efficiencies and lower electromagnetic interferences (EMI), a zero voltage switching (ZVS) mode is operated. With ZVS mode, during operation, the first to fourth power switches QA˜QD in a switching stage of the power conversion system are activated at zero crossings of their main terminal voltage to minimize turn on losses. An amount of time is required by the first to fourth power switches QA˜QD to turn off (open) and on (close). The overlap between these transitions can be referred to as dead-time (as time points between t2 and t3 and the time points between t4 and t5 shown in the  FIG. 4 a   ). A minimum amount of dead-time is needed to avoid having the first power switch QA and second power switch QB (or the third power switch QC and the fourth power switch QD) closed (turned on) at the same time. If the first power switch QA and the second power switch QB (or the third power switch QC and the fourth power switch QD) are closed at the same time, potentially destructive shoot-through current that travels directly from input voltage Vi to electronic device may result. The period of the dead-time is rises gradually as leakage inductance is increased. The gates of the first to fourth power switches QA˜QD are electrically connected to a controlling circuit (not shown), and the first to fourth power switches QA˜QD are turned off and on according to signals sent from the controlling circuit. 
     Reference is made to  FIG. 2  and  FIG. 4 a   , wherein  FIG. 4 a    is a timing chart indicating operations of the power conversion system during light load condition. In order to clarify detailed operation of the power conversion system of the present disclosure, the following example is given. It should be noted that the values given in the example are only for clarity. The values can be changed to meet system requirements. During the heavy load condition, the power (including voltage and current) provided by the power conversion system is the largest. As the load lightens, the power is reduced. During the light load condition, the power provided by the power conversion system reduces to, for example, 20%; during the normal load condition, the power provided by the power conversion system reduces to, for example, 50%. 
     Seven time points t1-t7 are shown in the  FIG. 4 a   . With the second power switch QB and the third power switch QC are closed (placed in conducting states) (wherein the first power switch QA and the fourth power switch QD are opened) to provide a conduction path between time points t1 and t2, a primary current Ip from the input voltage Vi flows through the third power switch QC, the primary winding  310 , the resonant module  20 , the second power switch QB to the ground. During this time, energy is stored in the resonant inductor Lr, and the primary current (Ip) is raised. 
     The second power switch QB is then opened at time point t2 (the third power switch QC is continuously closed), and then a short time duration later, the first power switch QA is closed (at time point t3). During this short duration, the current supported by the energy stored in the isolating transformer&#39;s leakage inductance, and optionally in resonant inductor Lr, now flows out of the capacitances C associated with the first power switch QA and the second power switch QB and into the third power switch QC (which is still closed). Specifically, when the second power switch QB is opened, the primary current Ip freewheels through the diode D associate with the first power switch QA, and the capacitor C associated with the second power switch QB is charged, and the capacitor C associated with the first power switch QA is discharged until the potential of the capacitor C associates with the second power switch QB is equal to that of the input voltage Vi. 
     When a voltage across drain-source of the first power switch QA is lower than a voltage across the forward bias of the diode D associated with the first power switch QA, the diode D associate with the first power switch QA turns on (placed in conducting states). As such, the first power switch QA is closed under zero-voltage condition (i.e., zero voltage switching). Due to that the voltage across drain-source of the first power switch QA is lower than the voltage across the diode D associate with the first power switch QA, the conduction loss is low. At this time, the primary voltage Vp of the power conversion system is zero. 
     Between time points t4 and t5, the third power switch QC is closed and the current freewheels through the diode D associated with the fourth power switch QD. During this short duration, the current supported by the energy stored in the isolating transformer&#39;s leakage inductance, and optionally in resonant inductor Lr, now flows out of the capacitances C associated with the third power switch QC and the fourth power switch QD and into the first power switch QA (which is still closed). Specifically, when the third power switch QC is opened, the primary current Ip freewheels through the diode D associate with the fourth power switch QD, and the capacitor C associated with the third power switch QC is charged, and the capacitor C associated with the fourth power switch QD is discharged until the potential of the capacitor C associates with the third power switch QC is equal to that of the input voltage Vi and a voltage across drain-source of the fourth power switch QD is dropped to zero (as curve VQ 4  shown). 
     When a voltage across drain-source of the fourth power switch QD is lower than a voltage across the forward bias of the diode D associated with the fourth power switch QD, the diode D associate with the fourth power switch QD turns on (placed in conducting states). As such, the fourth power switch QD is closed under zero-voltage condition. 
     Due to the voltage across the resonant inductor Lr is equal to the input voltage Vi, the primary current Ip is linearly decreased between time points t5 and t6. In  FIG. 4 a   , a duty cycle loss appears between time points t5 and t6 since a primary voltage Vp does not drop to negative value at time point t5, which the fourth power switch QD is closed (wherein the fourth power switch QD is closed at time point t6). The more the leakage inductance is, the more duty cycle loss is, and the duty cycle loss is given by 
     
       
         
           
             Lr 
             × 
             
               Ip 
               Vin 
             
           
         
       
     
     wherein 
     Lr is the inductance of the resonant inductor; 
     Ip is the primary current of the power conversion system; and 
     Vp is the primary voltage of the power conversion system. 
       FIG. 5 a    is a timing chart indicating operations of the power conversion system during normal load condition.  FIG. 6 a    is a timing chart indicating operations of the power conversion system during heavy load condition. The function and relative description of the power conversion system during normal load condition and during heavy load condition are the same as that of during light load condition mentioned above and are not repeated here for brevity. It should be noted that the current provided by the power conversion system is increased while the power (current) required for the electronic device is increased, and the duty cycle loss of the power conversion system is increased while the current provided by the power conversion system is increased. As such, a hold-up time depends upon the duty cycle loss is then decreased, that result in lower efficiency. Specifically, if the input voltage Vi falls below the minimum permissible voltage and adversely affects the power conversion system operation, the electronic devices that rely on the power conversion system for power could experience critical failures such as the loss of data. The length of time that the power conversion system can continue to operate in the absence of line voltage is referred to as the hold-up time. 
     If power conversion system is to provide a better efficiency, then a lower duty cycle loss will be needed. As a result, a distinctive operation of the output-controlling device  40  is required to meet the duty cycle loss required to keep the efficiency of the power conversion system within acceptable limits. 
     In general, the power provided by the power conversion system depends upon the power required for the electronic device. More particularly, the power required for the electronic device during heavy load condition is higher than that of during the light load condition. Therefore, the power (such as current) provided by the power conversion system while the electronic device operated under heavy load condition will be higher than that of operated under light load condition. 
     The output-controlling device  40  of the present disclosure is controlled to make the current provided by the power conversion system to meet the power required for the electronic device. 
     The power conversion system may provide the power to meet the required for the electronic device depends upon the operation of the first to fourth synchronous rectifying units  410   a ˜ 410   d  of the output-controlling module  400   a ˜ 400   d . Reference is made back to  FIG. 2  and  FIG. 3 . In first operation state, when a first current I 1  is required for the electronic device, the controller  420  sends signals to gates SR 1 ˜SR 8  of the rectifying switches Q 1 ˜Q 8  according to the current sensed signal sent from the current sense unit  50  for indicating that the first current I 1  is required by the electronic device, and places one of the first to fourth rectifying unit  410   a ˜ 410   d  in the conducting state for performing synchronous rectifying procedure, thus the first current I 1  is provided by the power conversion system. Specifically, the controller  420  may send pulsating signals to drive the rectifying switches Q 1  and Q 2  to interleaved turn off and on (as time points between 0˜t1 shown in  FIG. 3 ), thus a power coupled to the secondary winding  320   a  is synchronous rectified by the first synchronous rectifying unit  410   a  and the rectified power is then conducted to the output terminal (connected to the electronic device) by passing through the filters L 1  and L 2 , the first output switch SW 1 , and the output capacitor Co connected to the first output switch SW 1 . 
     In second operation state, when a second current I 2  is required for the electronic device, the controller  420  receives the current signal sent from the current sense unit for indicating that the second current I 2  is required by the electronic device, and sends signals to the gates SR 1 ˜SR 8  for placing two of the first to fourth synchronous rectifying units  410   a ˜ 410   d  in the conducting state for performing synchronous rectifying procedure, thus the second current I 2  is then provided by the power conversion system, the second current I 2  is larger than the first current I 1 . Specifically, the controller  420  may send pulsating signals to drive the rectifying switches Q 1 ˜Q 4  to interleaved turn off and on (as time points between t1 and t2 shown in  FIG. 3 ), thus powers coupled to the secondary winding  320   a  and  320   b  are synchronous rectified by the first and second synchronous rectifying units  410   a  and the  410   b , respectively, and the rectified powers are then conducted to the output terminal (connected to the electronic device) by passing through the filters L 1 ˜L 4 , the first power switch SW 1 , second output switch SW 2 , and the output capacitors Co connected to the first power switch SW 1  and the second output switch SW 2 . 
     In third operation state, when a third current I 3  is required for the electronic device, the controller  420  receives the current sensed signal sent from the current sense unit  50  for indicting that the third current is required for the electronic device, and sends signals the gates SR 1 ˜SR 8  for placing three of the first to fourth synchronous rectifying unit  410   a ˜ 410   d  in the conducting state for performing synchronous rectifying procedure, thus the third current I 3  is then provided by the power conversion system, the third current I 3  is larger than the second current I 2 . Specifically, the controller  420  may send pulsating signals to drive the rectifying switches Q 1 ˜Q 6  to interleaved turn off and on (as time points between t2 and t3 shown in  FIG. 3 ), thus powers coupled to the secondary windings  320   a ˜ 320   c  are synchronous rectified by the first to third synchronous rectifying units  410   a ˜ 410   c , respectively, and the rectified powers are then conducted to the output terminal (connected to the electronic device) by passing through the filters L 1 ˜L 6 , the first to third output switch SW 1 ˜SW 3 , and the output capacitors Co connected to the first to third output switch SW 1 ˜SW 3 . 
     In fourth operation state, when a fourth current I 4  is required for the electronic device, the controller  420  receives the current sensed signal sent from the current sense unit for indicating that the third current is required for the electronic device, and sends signals to the gates SR 1 ˜SR 8  for placing all of the first to fourth synchronous rectifying unit  410   a ˜ 410   d  in the conducting state for perform synchronous rectifying procedure, thus a fourth current I 4  is then provided by the power conversion system, the fourth current I 4  is larger than the third current I 3 . Specifically, the controller  420  may sent pulsating signals to drive the rectifying switches Q 1 ˜Q 8  to interleaved turn on and off continuously (after time point t3 shown in  FIG. 3 ), thus powers coupled to the secondary winding  320   a ˜ 320   d  are synchronous rectified by the first to fourth synchronous rectifying units, respectively, and the rectified power are then conducted to the output terminal (connected to the electronic device) by passing through the filters L 1 ˜L 8 , the first to fourth output switch SW 1 ˜SW 4  and the output capacitors Co. 
     As such, an effect of energy conservation is provided and the power loss while the electronic device operated under light load condition is reduced since the first to the fourth rectifying unit  410   a ˜ 410   d  are separately placed in the conducting state and driven to synchronous rectify the powers coupled to the secondary windings  320   a ˜ 320   d.    
     The controller  420  may selectively place the first to fourth switches SW 1 ˜SW 4  in the conducting state for conducting power required for the electronic device to the output terminal. It should be noted when the controller  420  places at least one of the first to fourth synchronous rectifying units  410   a ˜ 410   d  in the conducting state for conducting power require for the electronic device to the output terminal, the first to fourth switches SW 1 ˜SW 4  are always closed to make the rectified power(s) flowing therethrough; when the controller  420  places at least one of the first to fourth switch SW 1 ˜SW 4  in the conducting state for conducting power required for the electronic device to the output terminal, the controller  420  sends the pulsating signals to the rectifying switches Q 1 ˜Q 8  to makes the first to fourth synchronous rectifying units  410   a ˜ 410   d  perform synchronous rectifying procedure all the time. 
     Reference is made back to  FIG. 2  and  FIG. 3 . The controller  420  may place the first switch SW 1  in the conducting state for conducting a power coupled to the secondary winding  320   a  and rectified by the first synchronous rectifying unit  410   a  to the output terminal (connected to the electronic device) in first operation state, therefore the first current I 1  is provided to the electronic device (as the time points between 0 and t1 shown in the  FIG. 3 ). 
     In second operation state, the controller  420  may place the first switch SW 1  and the second switch SW 2  in the conducting state for conducting powers coupled to the secondary windings  320   a  and  320   b  and rectified by the first synchronous rectifying unit  410   a  and the second synchronous rectifying unit  410   b  to the output terminal (connected to the electronic device), therefore the second current I 2  is provided to the electronic device (as the time points between t1 and t2 shown in the  FIG. 3 ), wherein the second current I 2  is larger than the first current I 1 . 
     In third operation state, the controller  420  may place the first to third switches SW 1 ˜SW 3  in the conducting state for conducting powers coupled to the secondary windings  320   a ˜ 320   c  and rectified by the first to third synchronous rectifying units  410   a ˜ 410   c  to the output terminal (connected to the electronic device), therefore the third current I 3  is provided to the electronic device (as the time points between t2 and t3 shown in the  FIG. 3 ), wherein the third current I 3  is larger than the second current I 2 . 
     The controller  420  places the first to fourth switches SW 1 ˜SW 4  in the conducting state for conducting powers coupled to the secondary windings  320   a ˜ 320   d  and rectified by the first to fourth synchronous rectifying units  410   a ˜ 410   d  to the output terminal (connected to the electronic device), therefore the fourth current I 4  is provided to the electronic device (as the time point t3 shown in the  FIG. 3 ), wherein the fourth current I 4  is larger than the third current I 3 . 
     The arrangement of the primary winding  310  and the second windings  320   a ˜ 320   d  of the present disclosure is further controlled to lower the power loss of the power conversion system. 
     Reference is made to  FIG. 7 , which is a cross-sectional view of the isolating transformer according to the first embodiment of the present disclosure. The isolating transformer  30  further includes a bobbin  330  and a magnetic core  340 , and the magnetic core  340  is assembled with the bobbin  330 . The primary winding  310  and the secondary winding  320   a ˜ 320   d  are placed on the bobbin  330 . In  FIG. 7 , the isolating transformer  30  includes one primary winding  310  and four secondary windings  320   a ˜ 320   d , the secondary windings  320   a ˜ 320   d  are arranged at the bobbin  330  with equidistance intervals (such as inserted into slots preset on the bobbin  330  with equidistance intervals), the primary winding  310  is wound on the bobbin  330  (where the secondary windings  320   a ˜ 320   d  does not placed and across each of the secondary windings  320   a ˜ 320   d ). As a result, the primary winding  310  and the secondary windings  320   a ˜ 320   d  are arranged in a staggered manner in a side view direction, i.e., the primary winding  310  is placed at same side of each secondary winding  320   a ˜ 320   d  (wherein in  FIG. 7 , the primary winding  310  is wound on the bobbin  310  and placed at the left side of the secondary winding  320   a ˜ 320   d ). 
     Reference is made back to  FIG. 2  and  FIG. 7 , the power conversion system may provide power required for the electronic device by controlling the operation states of the output-controlling modules  400   a ˜ 400   d.    
     In one of the operation states, the controller  420  may send pulsating signals to drive the rectifying switches Q 1 ˜Q 8  to perform synchronous rectifying procedure. As a result, the power coupled to the second windings  320   a ˜ 32   d  is synchronously rectified by the first to fourth synchronous rectifying units  410   a ˜ 410   d , and the rectified powers are then conducted to the output terminal (connected to the electronic device) by passing through the conducted first to fourth switches SW 1 ˜SW 4 . Selectively, the controller  420  may drive the first to fourth switches SW 1 ˜SW 4  to close and then conduct the power coupled to the second windings  320   a ˜ 320   d  and rectified by the first to fourth synchronous rectifying units  410   a ˜ 410   d  to the output terminal (connected to the electronic device). As a result, a leakage inductance based on the magnetic coupling between the primary winding  310 , the secondary windings  320   a ˜ 320   d , and optionally in resonant inductor Lr is generated. 
     Reference is made to  FIG. 8 , the lowest leakage inductance appears at the points that each of the second windings  320   a ˜ 320   d  is close to the primary winding  310 , and the leakage inductance is increased when the coupling distance between each of the secondary windings  320   a ˜ 320   d  and the primary winding  310  is increased. The leakage inductance varies in a fixed range since the primary winding  310  and the secondary windings  320   a ˜ 320   d  are arranged in the staggered manner. 
     In another operation state, the controller  420  may send pulsating signals to the rectifying switches Q 1 ˜Q 2  to drive the first synchronous rectifying unit  410   a  perform synchronous rectifying procedure (wherein the rectifying switches Q 3 ˜Q 8  are always opened). As a result, only the power coupled to the second windings  320   a  is synchronously rectified by the first synchronous rectifying units  410   a , and the rectified power is then conducted to the output terminal (connected to the electronic device) by passing through the conducted first to fourth switches SW 1 ˜SW 4 . Selectively, the controller  420  may drive the first switch SW 1  to close and conduct the power coupled to the second windings  320   a  and rectified by the first synchronous rectifying units  410   a  to the output terminal (connected to the electronic device). Another leakage inductance based on the magnetic coupling between the primary winding  310 , the secondary windings  320   a , and optionally in resonant inductor Lr is generated. 
     Reference is made to  FIG. 9 , the lowest leakage inductance appears at the point between the second winding  320   a  and the primary winding  310 , and the leakage inductance is increased when the coupling distance between the secondary winding  320   a  and the primary winding  310  is increased. 
     In the other state, the controller  420  may send pulsating signals to the rectifying switches Q 3 ˜Q 6  to drive the second synchronous rectifying unit  410   b  and the third synchronous rectifying unit  410   c  to perform synchronous rectifying procedure (wherein the rectifying switches Q 1 , Q 2 , Q 7 , and Q 8  are always opened). As a result, only the powers coupled to the second windings  320   b  and  320   c  are synchronously rectified by the second synchronous rectifying units  410   b  and the third synchronous rectifying units  410   c , and the rectified powers are then conducted to the output terminal (connected to the electronic device) by passing through the conducted first to fourth switches SW 1 ˜SW 4 . Selectively, the controller  420  may drive the second switch SW 2  and the third switch SW 3  to close and conduct the powers coupled to the second windings  320   b  and  320   c  and rectified by the second synchronous rectifying unit  410   b  and the third synchronous rectifying units  410   c  to the output terminal (electrically connected to the electronic device). Still another leakage inductance based on the magnetic coupling between the primary winding  310 , the secondary windings  320   b  and  320   c , and optionally in resonant inductor Lr is generated. 
     Reference is made to  FIG. 10 , the lowest leakage inductance appears at the point between the second windings  320   b ,  320   c  and the primary winding  310 , and the leakage inductance is increased when the coupling distance between the secondary winding  320   b ,  320   c  and the primary winding  310  is increased. 
     In sum, the amount of the first to fourth synchronous rectifying units  410   a ˜ 410   d  performing synchronous rectifying procedure and the coupling distance between the secondary winding  320   a ˜ 320   d  performing synchronous rectifying and the primary windings  310  affects the leakage inductance of the power converting system. As such, by effectively controlling the amount of the first to fourth synchronous rectifying units  410   a ˜ 410   d  performing synchronous rectifying procedure and the coupling distance mentioned above, the power conversion system can accurately provide power required for the electronic device to the electronic device. 
     It should be noted that the power conversion system provides power to the electronic device only when the synchronous rectifying unit ( 410   a ˜ 410   d ) connected to the particular secondary winding ( 320   a ˜ 320   d ) performs synchronous rectifying procedure and the output switch (SW 1 ˜SW 4 ) connected to the synchronous rectifying ( 410   a ˜ 410   d ) is close. For example, reference is made to  FIG. 2 , when the first synchronous rectifying unit  410   a  performs synchronous rectifying procedure and the first switch SW 1  is close, the power conducted to the primary winding  310  is coupled to the secondary winding  320   a  connected to the first synchronous rectifying unit  410   a , and then the rectified power is conducted to the electronic device by passing through the filters L 1  and L 2 . In the meanwhile, a leakage inductance based on the magnetic coupling between the primary winding  310  and the secondary windings  320   a ˜ 320   d  is generated. 
     The detail data of the leakage inductance in different operation states are shown in Table 1. 
     
       
         
           
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                 1st 
                 2nd 
                 3rd 
                 4th 
                 Leakage 
               
               
                   
                 synchronous 
                 synchronous 
                 synchronous 
                 synchronous 
                 induc- 
               
               
                   
                 rectifying  
                 rectifying  
                 rectifying  
                 rectifying  
                 tance 
               
               
                   
                 unit 
                 unit 
                 unit 
                 unit 
                 (μH) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                  1st 
                 non- 
                 non- 
                 non- 
                 conducting 
                 25.19 
               
               
                 state 
                 conducting 
                 conducting 
                 conducting 
                 state 
                   
               
               
                   
                 state 
                 state 
                 state 
                   
                   
               
               
                  2nd 
                 conducting 
                 non- 
                 non- 
                 non- 
                 24.4 
               
               
                 state 
                 state 
                 conducting 
                 conducting 
                 conducting 
                   
               
               
                   
                   
                 state 
                 state 
                 state 
                   
               
               
                  3rd 
                 conducting 
                 non- 
                 non- 
                 conducting 
                 14.59 
               
               
                 state 
                 state 
                 conducting 
                 conducting 
                 state 
                   
               
               
                   
                   
                 state 
                 state 
                   
                   
               
               
                  4th 
                 non- 
                 non- 
                 conducting 
                 non- 
                 12.3 
               
               
                 state 
                 conducting 
                 conducting 
                 state 
                 conducting 
                   
               
               
                   
                 state 
                 state 
                   
                 state 
                   
               
               
                  5th 
                 non- 
                 conducting  
                 non- 
                 non- 
                 12.2 
               
               
                 state 
                 conducting 
                 state 
                 conducting 
                 conducting 
                   
               
               
                   
                 state 
                   
                 state 
                 state 
                   
               
               
                  6th 
                 non- 
                 non- 
                 conducting 
                 conducting 
                 11. 93 
               
               
                 state 
                 conducting 
                 conducting 
                 state 
                 state 
                   
               
               
                   
                 state 
                 state 
                   
                   
                   
               
               
                  7th 
                 conducting 
                 conducting  
                 non- 
                 non- 
                 10.13 
               
               
                 state 
                 state 
                 state 
                 conducting 
                 conducting 
                   
               
               
                   
                   
                   
                 state 
                 state 
                   
               
               
                  8th 
                 conducting 
                 non- 
                 conducting 
                 conducting 
                 9.95 
               
               
                 state 
                 state 
                 conducting 
                 state 
                 state 
                   
               
               
                   
                   
                 state 
                   
                   
                   
               
               
                  9th 
                 conducting 
                 non- 
                 conducting 
                 non- 
                 9.9 
               
               
                 state 
                 state 
                 conducting 
                 state 
                 conducting 
                   
               
               
                   
                   
                 state 
                   
                 state 
                   
               
               
                 10th 
                 non- 
                 conducting  
                 non- 
                 conducting 
                 9.89 
               
               
                 state 
                 conducting 
                 state 
                 conducting 
                 state 
                   
               
               
                   
                 state 
                   
                 state 
                   
                   
               
               
                 11th 
                 conducting 
                 conducting  
                 non- 
                 conducting 
                 8.29 
               
               
                 state 
                 state 
                 state 
                 conducting 
                 state 
                   
               
               
                   
                   
                   
                 state 
                   
                   
               
               
                 12th 
                 conducting 
                 conducting  
                 conducting 
                 non- 
                 8.2 
               
               
                 state 
                 state 
                 state 
                 state 
                 conducting 
                   
               
               
                 13th 
                 non- 
                 conducting  
                 conducting 
                 conducting 
                 8.12 
               
               
                 state 
                 conducting 
                 state 
                 state 
                 state 
                   
               
               
                   
                 state 
                   
                   
                   
                   
               
               
                 14th 
                 conducting 
                 conducting  
                 conducting 
                 conducting 
                 8.02 
               
               
                 state 
                 state 
                 state 
                 state 
                 state 
                   
               
               
                 15th 
                 non- 
                 conducting  
                 conducting 
                 non- 
                 8 
               
               
                 state 
                 conducting 
                 state 
                 state 
                 conducting 
                   
               
               
                   
                 state 
                   
                   
                 state 
               
               
                   
               
            
           
         
       
     
     In Table 1, “conducting state” means that the synchronous rectifying unit ( 410   a ˜ 410   d ) is places in the conducting state and performs synchronous rectifying procedure, thus the power conducted to the primary winding  310  may be coupled to particular secondary winding ( 320   a ˜ 320   d ) connected to the synchronous rectifying unit ( 410   a ˜ 410   d ) placed in the conducting state, and a rectified power is then conducted to the electronic device; nevertheless, “non-conducting state” means that the synchronous rectifying unit  410   a ˜ 410   d  is places in the non-conducting state and does not perform synchronous rectifying procedure, and the power conducted to the primary winding  310  does not coupled to the secondary winding  320   a ˜ 320   d  connected to the synchronous rectifying unit  410   a ˜ 410   d  placed in the non-conducting state. 
     Reference is made to  FIG. 4 b   , which is another timing chart indicating operations of the power conversion system during light load condition. It should be noted that the timing chart shown in the  FIG. 4 b    indicates the power conversion system which the amount of the first to fourth synchronous rectifying units  410   a ˜ 410   d  performing synchronous rectifying procedure and the coupling distance between the secondary winding  320   a ˜ 320   d  connected to the first to fourth synchronous rectifying units  410   a ˜ 410   d  performing synchronous rectifying procedure and the primary windings  310  are controlled as mentioned above. In  FIG. 4 b   , a duty cycle loss appears between time points t5 and t6′ since a primary voltage Vp does not drop to negative value at time point t5, which the fourth power switch QD is closed. Comparing to the  FIG. 4 a    (the duty cycle loss appears between time points t5 and t6), the duty cycle loss shown in the  FIG. 4 b    is reduced (the period between time points t6′ and t6 shown in the  FIG. 4 b    indicates the duty cycle loss which is eliminated from  FIG. 4 a   ). 
       FIG. 5 b    is another timing chart indicating operations of the power conversion system during normal load condition.  FIG. 6 b    is another timing chart indicating operations of the power conversion system during heavy load condition. In  FIGS. 5 b  and 6 b   , the period between time points t5 and t6 indicates duty time loss of the power conversion system which the amount of the synchronous rectifying units performing synchronous rectifying procedure and the coupling distance between the secondary winding  320   a ˜ 320   d  and the primary windings  310  are not controlled (for example, the first to fourth synchronous rectifying units  410   a ˜ 410   d  perform synchronous rectifying procedure as the same time). On the contrary, the period between time points t5 and t6′ indicates duty time loss of the power conversion system which the amount of the synchronous rectifying units performing synchronous rectifying procedure and the coupling distance between the secondary winding  320   a ˜ 320   d  and the primary windings  310  are well controlled (wherein the period between time points t6′ and t6 shown in the  FIG. 5 b    and  FIG. 6 b    indicates the duty cycle loss which is eliminated from  FIGS. 5 a  and 6 a   ). 
     In order to prevent generated heat that arises at the time of driving from being stored, the first to fourth synchronous rectifying units  410   a ˜ 410   d  may perform synchronous rectifying procedure in sequence. For example, the controller  420  may progressively increase the amount of the synchronous rectifying units ( 410   a ˜ 410   d ) performing synchronous rectifying procedure when the power required for the electronic device is gradually increased. In addition, the controller  420  may drives the synchronous rectifying units ( 410   a ˜ 410   d ) in a convergence manner when only one of the synchronous rectifying units ( 410   a ˜ 410   d ) performs synchronous rectifying procedure. More particularly, the convergence manner may first make the synchronous rectifying unit ( 410   a ˜ 410   d ) far from a central axis of the isolating transformer  30  shown in the  FIG. 7  perform synchronous rectifying procedure, and next makes the synchronous rectifying units close to the central axis of the transformer shown in the  FIG. 7  to prevent generated heat that arises at the time of driving from being stored, i.e., the controller  420  may makes the first synchronous rectifying units  410   a , the fourth synchronous rectifying units  410   d , the second synchronous rectifying units  410   b , and the third synchronous rectifying units  410   c  shown in the  FIG. 7  perform synchronous rectifying procedure in sequence. It should be noted that if the distances between two synchronous rectifying units and the central axis are the same, the two synchronous rectifying units interleaved perform synchronous rectifying procedure. 
     Reference is made back to Table 1, in 1st and 2nd states, they are only one of the synchronous rectifying units is placed in the conducting state and performs synchronous rectifying procedure. As can be shown in  FIG. 7 , the distance between the synchronous rectifying unit performing synchronous rectifying procedure in 1st state and the central axis of the isolating transformer  30  is equal to that of in 2nd state, and the leakage inductance in 1st state is close to that of in 2nd state. Therefore, the controller  420  may interleaved drive the first synchronous rectifying unit  410   a  and the fourth synchronous rectifying unit  410   d  to conduct the power coupled to the secondary winding  320   a  and  320   d  to the electronic device while the electronic device is operated under the same condition (such as light load condition) to prevent generated heat that arises at the time of driving from being stored in particular synchronous rectifying unit ( 410   a ˜ 410   d ), which is placed in the conducting state and performs synchronous rectifying procedure all the time. 
     It should be noted that the synchronous rectifying units ( 410   a ˜ 410   d ) may be interleaved placed in the conducting state (i.e. the first to fourth synchronous rectifying units  410   a ˜ 410   d  may be driven to interleaved perform synchronous rectifying procedure) according to the distance between the central axis and the synchronous rectifying units ( 410   a ˜ 410   d ), for example, the synchronous rectifying units ( 410   a ˜ 410   d ) with same distance from the central axis may be interleaved driven to perform synchronous rectifying procedure. However, that the synchronous rectifying units ( 410   a ˜ 410   d ) may be driven to interleaved perform synchronous rectifying procedure according to inductance in different operation states of the synchronous rectifying units ( 410   a ˜ 410   d ). For example, the operation states with similar leakage inductance (such as the difference in leakage inductance between the operation states is less than 5 μH) may be interleaved driven to perform synchronous rectifying procedure. 
     In sum, the power conversion system of the present disclosure performs a power conversion procedure for powering the electronic device includes step as following first, the power conversion system including a primary winding  310 , a plurality of secondary windings  320   a ˜ 320   d , and a plurality of synchronous rectifying units  410   a ˜ 410   d  is provided. There are a plurality of coupling distances between the primary winding  310  and the secondary windings  320   a ˜ 320   d.    
     Next, the operation condition (such as light load condition, normal load condition, or heavy load condition) of the electronic device is measured by measuring a current required by the electronic device. Specifically, the current required for the electronic device during light load condition may be smaller than that of during normal load condition, and the current required for the electronic device during heavy load condition may be larger than that of during normal load condition. Thereafter, the synchronous rectifying units ( 410   a ˜ 410   d ) are selectively placed in a conducting state for varying a leakage inductance of the power conversion system, thus a output current of the power conversion system is modulated to meet the current requirement of the electronic device. The output current is only provided by the synchronous rectifying units  410   a ˜ 410   d  which is placed in the conducting state, and the power conversion system has a lowest leakage inductance when all of the output-controlling modules  400   a ˜ 400   d  are placed in the conducting state, therefore a largest output current is provided. 
     The power conversion system may vary the leakage inductance by selectively place one of the synchronous rectifying units  410   a ˜ 410   d  in the conducting state at a time; however, the power conversion system may further selectively place two or more synchronous rectifying units  410   a ˜ 410   d  at a time. Besides, the leakage inductance of the power conversion system is varied when an amount of the synchronous rectifying units  410   a ˜ 410   d  placed in the conducting state changes. 
     Reference is made to  FIG. 11 , which is a circuit diagram of a power conversion system according to a second embodiment of the present disclosure. In  FIG. 11 , the power conversion system includes a switching module  10 , a resonant module  20 , a transformer  30 , and an output-controlling device  40 . The transformer  30  includes a primary winding  310  and a plurality of secondary windings  320   a ˜ 320   d  coupled with the primary winding  310 . 
     The function and relative description of switching module  10  and the resonant module  20  of the power conversion system shown in the  FIG. 11  are the same as that of first embodiment (shown in the  FIG. 2 ) mentioned above and are not repeated here for brevity, and the switching module  10  and the resonant module  20  of the power conversion system shown in the  FIG. 11  can achieve the functions as power conversion system of the first embodiment does. It should be noted that the transformer  30  and the output-controlling device  40  shown in the  FIG. 11  is different from that of the first embodiment. 
     In  FIG. 11 , the transformer  30  is a center-tapped transformer, which has an advantage of compact. However, the isolating transformer shown in  FIG. 2  has an advantage of double-current. The output-controlling device  40  is electrically connected to the secondary winding  320   a ˜ 320   d  of the transformer and includes first to fourth synchronous rectifying units  410   a ˜ 410   d , controller  420 , and first to fourth output switch SW 1 ˜SW 4 . The first synchronous rectifying unit  410   a  is connected to the secondary winding  320   a , the second synchronous rectifying unit  410   b  is connected to the secondary winding  320   b , the third synchronous rectifying unit  410   c  is connected to the secondary winding  320   c , and the fourth synchronous rectifying unit  410   d  is connected to the secondary winding  320   d.    
     The first synchronous rectifying unit  410   a  includes rectifying switches Q 1  and Q 2 , the second synchronous rectifying unit  410   b  includes rectifying switches Q 3  and Q 4 , the third synchronous rectifying unit  410   c  includes rectifying switches Q 5  and Q 6 , and the fourth synchronous rectifying unit  410   d  includes rectifying switches Q 7  and Q 8 . Specifically, the sources of the rectifying switch Q 1  and Q 2  are connected to ground, the drains thereof is connected to two taps of the second winding  320   a , and the filter L 1  is connected to the center-tap of the second winding  320   a ; the sources of the rectifying switch Q 3  and Q 4  are connected to ground, the drains thereof is connected to two taps of the second winding  320   b , and the filter L 2  is connected to the center-tap of the second winding  320   b ; the sources of the rectifying switch Q 5  and Q 6  are connected to ground, the drains thereof are connected to two taps of the second winding  320   c , and the filter L 3  is connected to the center-tap of the second winding  320   c ; the sources of the rectifying switch Q 7  and Q 8  are connected to ground, the drains thereof is connected to two taps of the second winding  320   d , and the filter L 4  is connected to the center-tap of the second winding  320   d . The gates SR 1 ˜SR 8  of the rectifying switch Q 1 ˜Q 8  and the first to fourth output switch SW 1 ˜SW 4  are connected to the controller  420 . The controller  420  sends signals the rectifying switch Q 1 ˜Q 8  to drive one of the first to the fourth synchronous rectifying units  410   a ˜ 410   d  to perform synchronous rectifying procedure. The controller  420  further sent signals to the first to fourth output switch SW 1 ˜SW 4  to makes one of the first to fourth output switch SW 1 ˜SW 4  to turn off or on, wherein when the first to fourth output switch SW 1 ˜SW 4  is turned on, the rectified power con be conducted to the electronic device. The center tap of the transformer  30  is further electrically connected to an output capacitor Co. 
     The function and relative description of other components of power conversion system of this embodiment are the same as that of first embodiment mentioned above and are not repeated here for brevity, and the power conversion of this embodiment can achieve the functions as the power conversion system of the first embodiment does. 
     Although the present disclosure has been described with reference to the foregoing preferred embodiment, it will be understood that the invention is not limited to the details thereof. Various equivalent variations and modifications can still occur to those skilled in this art in view of the teachings of the present disclosure. Thus, all such variations and equivalent modifications are also embraced within the scope of the invention as defined in the appended claims.