Abstract:
A synchronous rectifier, comprising an input unit outputting a process signal in response to an AC input signal, a control unit electrically connected to the input unit and including a pulse-time control circuit, producing a first driving signal and a second driving signal, and an output unit electrically connected to the input unit and the control unit. And the output unit has a first switch control circuit and a second switch control circuit in response to the first driving signal and the second driving signal respectively for transforming the process signal into an output signal while the first switch control circuit and the second switch control circuit are free for a cross conduction.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a synchronous rectifier, and more particularly, to a synchronous rectifier utilizing a transformer to drive synchronous rectifying transistors.  
         BACKGROUND OF THE INVENTION  
         [0002]    In present power supply products, the synchronous rectifier often utilizes a transformer to drive synchronous rectifying transistors for achieving efficient rectifying operation. As shown in the FIG. 1, it is a schematic diagram of a conventional synchronous rectifier  10 . In FIG. 1, the synchronous rectifier  10  includes: a input unit  11 , a control unit  12  and an output unit  13 . Meanwhile, the input unit  11  further includes a signal detecting circuit  111 , a rectifying circuit  112 , a signal amplified circuit  113 , a first transformer T 1 , a second transformer T 2 , a third transformer T 3  and a bridge rectifying circuit constructed by four transistors, Qa, Qb, Qc and Qd. Furthermore, the output unit  13  includes a first rectifying inductor L 1 , a second rectifying inductor L 2 , a rectifying capacitor C, a fourth transformer T 4 , a first and a second switch control circuit  131 ,  132 , and a third and a fourth switch circuit  133 ,  134 . Of course, in the first switch control circuit  131  further includes a transistor Q 1 , a first diode D 1 , a first resistor R 1 , and a first induction coil L 11 . And the second switch control circuit  132  further includes a transistor Q 2 , a second diode D 2 , a second resistor R 2 , and a second coil L 22 . And then the third and the fourth switch control circuits  133 ,  134  can be a transistor Q 3  and Q 4 . The first transformer T 1  further includes a first side coil T 11 , and a second side coils T 12 , T 13 . And the second transformer T 2  further includes a second side coil T 21 , and a second side coils T 22 , T 23 . Besides, the third transformer T 3  includes a first side coil T 31  and a second side coil T 32 . And the fourth transformer T 4  includes a first side coil T 41  and a second side coil T 42 . The theory and the drawbacks of the conventional synchronous rectifier now represent as below.  
           [0003]    After an AC input current Iin detected by the detecting circuit  111 , then the AC input current Iin is transformed to the second coil T 32  through the first side coil T 31  of the third transformer T 3 . Meanwhile, the control unit  12  produces a rectifying control signal In to the input unit  11  to control the conducting sequences of the transistor Qa, Qb, Qc and Qd, and to proceed the power transmission. The rectifying control signal In is amplified by the signal amplifying circuit  113  and then have a control signal Iac and Ibd.  
           [0004]    Because the control signal Iac is transformed to the second coil T 12  and T 13  through the first coil T 11  of the first transformer T 1 , the gate electrode of the transistors Qa and Qc, which are electrically connected to the second side coil T 12  and T 13 , can generate a gate voltage Vag and a gate voltage Vcg. Therefore, the control signal Iac can control the transistors Qa and Qc to be in a turn on state or a turn off state. By the same reason, the control signal Ibd can make the transistor Qb and Qd to generate a gate voltage Vbg and a gate voltage Vdg on the gate electrodes thereof through the second transformer T 2 . And make the transistor Qb and Qd to be in a turn on state or a turn off state. Therefore, by the bridge switch circuit consisted of the four transistors Qa, Qb, Qc and Qd, the direct input current Vin can be transformed to the output unit  13  through the fourth transformer T 4 .  
           [0005]    Furthermore, the current signal Ip 1  and the voltage signal Vp 1  are inputted into the fourth transformer T 4  of the output unit  13 , and then be transformed by the first side coil T 41  of the fourth transformer T 4 , and then the second side coil T 42  produces another one current signal Ip 2  and voltage signal Vp 2 . The first and second switch control circuits  131 ,  132  are electrically connect to the second side coil T 42 . So that the first induction coil L 11  of the first switch control circuit  131  can produce a current Ip 21  according to the current signal Ip 2 . Besides, the transistor Q 1 &#39;s gate electrode forms a gate voltage V 1   g  to control the transistor Q 1  in the turn on or the turn off state. Of course, according to the current signal Ip 2 , the second induction coil L 22  of the second switch control circuit  132  can produce a induction current Ip 22 . And the transistor Q 2 &#39;s gate electrode forms a gate voltage V 2   g  to control the transistor Q 2  to be turned on or turned off.  
           [0006]    So, depends on the switching between the turned-on states and the turned-off states of the transistors Q 1  and Q 2 , and co-operates with the first and second rectifying inductors L 1  and L 2 , the rectifying action of the rectifying capacitor C can transform the current signal Ip 2  into a rectifying output signal for outputting itself. Certainly, the rectifying output signal includes a rectifying output current lout and a rectifying output voltage Vout.  
           [0007]    According to the above explanation, the conventional synchronous rectifier&#39;s  10  control scheme, which is for controlling the conduction state of the transistors Q 1  and Q 2 , is mainly depended on the induction currents Ip 21  and Ip 2  produced by the first and second induction coils L 11 , L 23 , and depended on the gate voltages V 1   g , V 2   g  formed in the transistor Q 1 , Q 2 , to drive the transistors Q 1  and Q 2 . However, by the restriction of the leaking induction phenomenon, the conventional synchronous rectifier  10  is can&#39;t constructs an accurate driving control signal, e.g. the gate voltages V 1   g , V 2   g . Therefore, the goodwill for raising the efficiency of rectifying is not so well as the prediction of the conventional synchronous rectifier  10 .  
           [0008]    Furthermore, please refer to the FIG. 2( a ), which is the wave form drawing of the driving control signal of the conventional synchronous rectifier  10  for controlling the first and second switch control circuits  131 ,  132 . The FIG. 2( a ) includes the wave forms of the gate voltages V 1   g , V 2   g  for driving the transistors Q 1 , Q 2 , and the wave form of the voltage signal Vp 2 , which is transformed form the voltage signal Vp 1  by the first side coil T 41  of the fourth transformer T 4 , and then form in the second side coil T 42 . It is obviously that the gate voltages V 1   g , V 2   g  can change its level follow the exchanges between the high level H and the low level L of the voltage signal Vp 2 . Furthermore, FIG. 2( a ) shows the transient response of the gate voltages V 1   g , V 2   g  in different times t1˜t8. But, according to the FE1 which represents the falling edge waveform of the gate voltage V 1   g  in time t5 and the FE2 which represents the falling edge waveform of the gate voltage V 2   g  in time t7, it can be understood that while the gate voltage V 1   g  is from a high level H to a low level L, the gate voltage V 2   g  varies at the same time. However, because of the leaking induction phenomenon, the gate voltage V 2   g  should passes a raising period that it can completes a transformation from a low level L to a high level H. By the same reason, while the gate voltage V 2   g  is from a high level H to a low level L, the gate voltage V 1   g  should passes a raising period that it can completes a transformation from a low level L to a high level H. Therefore, the transistors Q 1 , Q 2  is controlled by the gate voltages V 1   g , V 2   g  so that the cross conduction phenomenon is hard to overcome.  
           [0009]    And in the FIG. 1, the functions of the third, fourth switch control circuits  133 ,  134 , which are individually connect to the gate electrodes of transistors Q 1 , Q 2 , should cooperate with the FIG. 2( b ) which shows the conventional synchronous rectifier  10  electrically connected to another synchronous rectifier  50 . In the FIG. 2( b ) the conventional synchronous rectifier  10  is parallelly connected to another power supply  5 . It is often to provide a output current detector S connected to a common output terminal of the power supplies  1  and  5 , for preventing a reverse current Ir, which is produced by the input unit  51  of the synchronous rectifier  50 , passing the aforesaid parallel connection to destroy the power supply  1  or make it malfunction. While the output current detector S detects the reverse current Ir, the output current detector S creates a sensing signal Is to the control unit  12  of the synchronous rectifier  10 , and then the control unit  12  produces a voltage signals V 3   g  and V 4   g , and separately sends the voltage signal V 3   g  and V 4   g  to the gate electrode of the transistors Q 3  and Q 4 . So the transistors Q 1  and Q 2  can be forced to cut off the connection with the transistors Q 3  and Q 4  for preventing the power supply  1  from being destroyed by the reverse current Ir.  
           [0010]    However, the weakness of the prior art is that although the output current detector S connect with the output terminal of the synchronous rectifier  10  can solve the problem from the reverse current Ir, but the electric power loss will increase when the electric load getting high. So, the prior art cannot really effectiveness to increase the transportation efficiency of the electric power.  
         SUMMARY OF THE INVENTION  
         [0011]    It is therefore an object of the present invention to provide a synchronous rectifier which can solve the cross conduction problem.  
           [0012]    It is another object of the present invention to provide a synchronous rectifier preventing the reverse current to destroy the power supply in the parallel connection without increasing the power loss.  
           [0013]    According to as aspect embodiment of the present invention, a synchronous rectifier includes an input unit outputting a process signal in response to an AC input signal, a control unit electrically connected to the input unit and including a pulse-time control circuit for producing a first driving signal and a second driving signal, and an output unit electrically connected to the input unit and the control unit, and having a first switch control circuit and a second switch control circuit in response to the first driving signal and the second driving signal respectively for transforming the process signal into an output signal while the first switch control circuit and the second switch control circuit are free for a cross conduction.  
           [0014]    Preferably, the input unit further includes a signal detecting circuit for detecting and inputting the AC input signal, and a rectifying circuit electrically connected to the signal detecting circuit and the control unit for rectifying the AC input signal in response to a rectifying signal from the control unit so as to output the process signal.  
           [0015]    Preferably, the signal detecting circuit is a current detecting circuit  
           [0016]    Preferably, the rectifying circuit includes a plurality transformer and a plurality of transistors.  
           [0017]    Preferably, the rectifying circuit includes a first, a second and a third transformer, and a bridge rectifier having four transistors.  
           [0018]    Preferably, the first and the second transformer transfers the rectifying signal to a gate terminal of the four transistors for producing a gate control voltage.  
           [0019]    Preferably, the third transformer transforms the AC input signal to the rectifying circuit.  
           [0020]    Preferably, the four transistors are MOSFETs.  
           [0021]    Preferably, the input unit further includes a signal amplifying circuit electrically connected to the control unit and the rectifying circuit for amplifying and outputing the rectifying signal to the rectifying circuit.  
           [0022]    Preferably, the signal amplifying circuit is a current amplifier.  
           [0023]    Preferably, the pulse time control circuit includes a pulse width modulator and an adjustable time-pushing circuit.  
           [0024]    Preferably, the adjustable time-pushing circuit cooperates with the pulse width modulator to produce the first and the second drive signals, and the rectifying signal, and adjusts and sets periods of the first and the second pulse time.  
           [0025]    Preferably, the control circuit further comprises a signal cut-off circuit electrically connected to the signal detecting circuit and the pulse time control circuit for producing the first and the second drive signals in response to the AC input signal in a specific signal state, thereby the first and the second switch control circuits both being introduced to a cut-off state for preventing the synchronous rectifier from an external signal.  
           [0026]    Preferably, the signal cut-off circuit is a low current cut-off circuit.  
           [0027]    Preferably, the low current cut-off circuit comprises plural voltage comparators.  
           [0028]    Preferably, the specific signal state is a low current state.  
           [0029]    Preferably, the external signal is an inversed current produced by an dditional synchronous rectifier parallelly connected to the synchronous rectifier.  
           [0030]    Preferably, the first and the second switch control circuits are MOSFETs.  
           [0031]    Preferably, the first and the second drive signals are respectively inputted into gates of the first and the second MOSFETs for producing gate control voltages.  
           [0032]    Preferably, the output unit further comprises a first filtering inductor circuit and a second wave filtering inductor circuits individually connected to drain terminals of the first and the second MOSFETs and a filtering capacitor circuit electrically connected to the first and the second filtering inductor circuits, and source terminals of the first and the second MOSFETs, wherein the first and the second filtering inductor circuits and the filtering capacitor circuit rectify and transform the process signal into the output signal in response to the conducted state and the non-conducted state of the first and the second MOSFETs.  
           [0033]    Preferably, the first and the second filtering inductor circuits are filtering inductors.  
           [0034]    Preferably, the wave filter capacitor circuit is a wave filter capacitor.  
           [0035]    Preferably, the output unit further comprises a fourth transformer electrically connected to the rectifying circuit, and the first and the second MOSFETs for transforming the process signal to the drain terminals of the first and the second MOSFETs.  
           [0036]    Preferably, the first driving signal and the second driving signal both have a first and a second state.  
           [0037]    Preferably, the second driving signal is retained in the second state for a period of a first pulse time and then transformed into the first state while the first driving signal is transformed from the first state into the second state.  
           [0038]    Preferably, the first driving signal is retained in the second state for a period of a first pulse time and then transformed into the first state while the first driving signal is transformed from the first state into the second state.  
           [0039]    Preferably, the first state is a high level and the second state is a low level.  
           [0040]    Preferably, the first state is a low level and the second state is a high level.  
           [0041]    Preferably, while the process signal is in the first state, the first driving signal is in the second state and the second driving signal is in the first state  
           [0042]    Preferably, while the process signal is in the second state, the first driving signal is in the first state and the second driving signal is in the second state.  
           [0043]    Preferably, the first switch control circuit is set in one of a conducted state and a non-conducted state according to one of the first state and the second state of the second driving signal.  
           [0044]    Preferably, the second switch control circuit is set in one of a conducted state and a non-conducted state according to one of the first state and the second state of the first driving signal.  
           [0045]    According to another preferred embodiment of the present invention, a synchronous rectifier, comprising an input unit outputting a process signal in response to an AC input signal, a control unit electrically connected to the input unit and including a pulse-time control circuit, producing a first driving signal and a second driving signal, and an output unit electrically connected to the input unit and the control unit, and having a first switch control circuit and a second switch control circuit in response to the first driving signal and the second driving signal respectively for transforming the process signal into an output signal while the first switch control circuit and the second switch control circuit are free for a cross conduction, wherein the input unit further comprises a signal detecting circuit for detecting and inputting the AC input signal, and a rectifying circuit electrically connected to the signal detecting circuit and the control unit for rectifying the AC input signal in response to a rectifying signal from the control unit, so as to output the process signal.  
           [0046]    The foregoing and other features and advantages of the present invention will be more clearly understood through the following descriptions with reference to the drawings, wherein: 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0047]    [0047]FIG. 1 is a schematic view showing block structure diagram of the conventional synchronous rectifier;  
         [0048]    [0048]FIG. 2( a ) is a wave diagram showing the wave form of the driving control signal of the switch control circuits of the conventional synchronous rectifier;  
         [0049]    [0049]FIG. 2( b ) is a schematic view showing the conventional synchronous rectifier is parallel connecting to another synchronous rectifier;  
         [0050]    [0050]FIG. 3 is a schematic view showing block structure diagram of the preferred embodiment of the synchronous rectifier of the present invention;  
         [0051]    [0051]FIG. 4 is a schematic view showing the detail electrical connections between the control unit, the input unit and the output unit of a preferred embodiment of the present invention; and  
         [0052]    [0052]FIG. 5 is a wave diagram showing the wave forms of the first and second driving control signals of the first and second switch control circuits of a preferred embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0053]    The present invention will now described more specifically with reference to the following embodiments. Please refer to FIG. 3. FIG. 3 shows the block structure according to a preferred embodiment of the present invention. It shows clearly in the drawing that the present invention&#39;s synchronous rectifier  20  has an input unit  21 , a control unit  22  and an output unit  23 . The input unit  21  includes a signal detecting circuit  211 , a rectifying circuit  212 , signal amplifying circuit  213 , a first transformer T 1 , a second transformer T 2 , a third transformer T 3  and a bridge rectifier having four transistors Qa, Qb, Qc and Qd. And, the output unit  13  includes a first filtering inductor circuit L 1  (ex: a first filtering inductor), a second filtering inductor circuit L 2  (ex: a second filtering inductor), and a filtering capacitor circuit C, a fourth transformer unit T 4 , a first and a second switch control circuits  131 , 132 . Certainly, the first switch control circuit can be a transistor Q 1 , and the second switch control circuit can be a transistor Q 2 . The transistors Q 1  and Q 2  can be MOSFETs.  
         [0054]    The control unit  22  includes a pulse-time control circuit  221  and a signal cut-off circuit  222 . And the pulse-time control circuit  221  further comprises a pulse width modulation  2211  (PWM) and an adjustable time-pushing circuit  2212 . Because of respecting an alternative input current Iin, the pulse-time control circuit  221  can produce a first and a second driving signals V 1   g  and V 2   g  inputted individually to the gate electrodes of the transistors Q 1  and Q 2 . And the signal cut-off circuit  222  can also produce the first and the second driving signals V 1   g  and V 2   g  because of respecting the alternative input current Iin thereby.  
         [0055]    Besides, the first transformer T 1  further includes a first side coil T 11  and two second side coils T 12 , T 13 . And the second transformer T 2  has a first side coil T 21  and two second side coils T 22 , T 23 . Furthermore, the third transformer T 3  includes a first side coil T 31  and a second side coil T 32 . And the fourth transformer T 4  includes a first side coil T 41  and a second side coil T 42 .  
         [0056]    The working theory of the embodiment in the FIG. 3 now will be explained as hereafter. The signal detecting circuit  211  can make the alternative input signal Iin inputted therein be transformed by the first side coil T 31  of the third transformer T 3 , to the second side coil T 32 . Meanwhile, the control unit  22  produces the rectifying signal In to the input unit  21  to make the transistors Qa, Qb, Qc and Qd rectifying the AC input signal Iin. Wherein, the rectifying signal In will include a control signal Iac and Ibd after be amplified by the signal amplifying circuit  213 .  
         [0057]    Because, the control signal Iac is transformed to the second side coils T 12 , T 13  by the first side coil T 11  of the transformer T 1 , the gate electrodes of the transistors Qa, Qc, which electrical connect to the second side coils T 12 , T 13 , can individually form a gate voltages Vag and Vcg. Therefore, the control signal Iac can control the transistors Qa and Qc in the conducted state or a non-conducted state. By the same reasons, through the second transformer T 2 , the control signal can individually produces gate voltages Vbg, Vdg on the gate electrodes of the transistors Qb and Qd, and then makes the transistors Qb and Qd in the conducted state or the non-conducted state. So, by the bridge rectifying circuit constructed by the four transistors Qa, Qb, Qc and Qd, the alternative input signal Iin can be rectified, and then includes a current signal Ip 1  and voltage signal Vp 1 , then be outputted to the output unit  23 .  
         [0058]    The current signal Ip 1  is outputted into the fourth transformer T 4  of the output unit  23 . Through the transformation of the first side coil T 41  of the fourth transformer T 4 , the second side coil T 42  occurs another one current signal Ip 2  and one voltage signal Vp 2  Besides, through the first and the second driving signal V 1   g  and V 2   g  produced by the control unit  23 , the transistors Q 1  and Q 2  can exchanging the conducted/non-conducted state therebetween. So, by corresponding with the rectifications of the first and the second filtering inductors L 1  and L 2 , and the filtering capacitor C, the current signal Ip 2  can be changed into a filtering output signal then be outputted. Certainly, the filtering output signal includes a filtering output current Iout and filtering output voltage Vout. Wherein, the filtering inductors L 1 , L 2  are individual connect to the drain electrodes of the transistors Q 1 , Q 2 . And the filtering capacitor C electrically connects to the source electrodes of the first and the second filter inductors L 1 , L 2  and transistors Q 1 , Q 2 .  
         [0059]    According to the above explanation, the embodiment of the FIG. 3 has no similar elements like the third and fourth switch control circuit  133  and  134 , the schema for controlling the transistors Q 1  and Q 2  in the conducted state or the non-conducted state is taken on by the control unit  22  in the present invention. So the preferred embodiment of the present invention will not occurs the unconquerable cross-conduction phenomenon like the conventional synchronous rectifier  10 &#39;s control schema occurs.  
         [0060]    Please refer to the FIG. 4, it shows the detail electric structures between the control unit  22 , input unit  21 , and the output unit  23 . In FIG. 4, the alternative input signal Iin detected by the signal detecting circuit  211  can be inputted into the PWM  2211 , and by cooperating with the adjustable time-pushing circuit  2212 , the PWM  2211  can produce the first and the second drive signal V 1   g  and V 2   g . Of course, the driving circuits P 1 , P 2  of the transistor Q 1 , Q 2  of the adjustable time-pushing circuit  2212 , are the driving step for producing the first and the second drive signal V 1   g  and V 2   g . Besides, by changing the parameters of the PWM  2211 , and the resistor value and the capacitor value of the adjustable time-pushing circuit  2212 , the time sequences of the first and the second drive signal V 1   g  and V 2   g  can be changed respectively. Therefore, it is clear that the PWM  2211  provides a flex method to control or adjust the time sequences to the first and the second drive signal V 1   g  and V 2   g.    
         [0061]    In FIG. 4, the signal cut-off circuit  222  is provided, which includes the first, second and third voltage comparing circuits VC 1 , VC 2  and VC 3 , and a transistor Q 5 . The first, second and third voltage comparing circuits VC 1 , VC 2  and VC 3 , and a transistor QS produce the first and the second drive signals V 1   g  and V 2   g , and to force the first and the second switch control circuit  231 ,  232  (FIG. 3) into a cut-off state, while the alternative input signal Iin (please refer to the FIG. 3, the first side coil T 31  of the third transformer T 3 ) is in a special state (ex: a low current state), for preventing an external signal (ex: the reverse current Ir of the FIG. 2( b )) reversal inputted from the output unit  23  and destroys synchronous rectifier  20 . Furthermore, when a power supply, which has the synchronous rectifier  20 , connects to another power supply which also has another synchronous rectifier, while the first and the second voltage comparing circuits VC 1  and VC 2  find out that the alternative input signal Iin is in a low current state, the third voltage comparing circuit VC 3  produces the first and the second drive signal V 1   g  and V 2   g  to force the first and the second switch control circuit  231 ,  232  to cut-off, for preventing the invasion of the reverse current Ir. On the other sides, for conforming that the first and the second switch control circuit  231 ,  232  are really cut-off, the first and the second voltage comparing circuits VC 1  and VC 2  will cut-off the transistor QS when the AC input signal Iin is in the low current state. The reason is because the collector of the transistor Q 5  is electrical connects with the drive control circuits P 1  and P 2  of the transistors Q 1  and Q 2  (the connection is signed as+Vcc_d). So, while the transistor Q 5  is cut-off by the first and the second voltage comparing circuits VC 1  and VC 2 , will also bring the drive control circuits P 1  and P 2  of the transistors Q 1  and Q 2  in a cut-off state. Therefore, it can be confirmed that the first and the second drive signals V 1   g  and V 2   g  produced by the drive control circuits P 1  and P 2  of the transistors Q 1  and Q 2 , can force the first and the second switch control circuit  231 ,  232  in a cut-off state. Simply speaking, the present invention will not increases the electric power loss occurred by the output current detector S of the conventional synchronous rectifier  10 .  
         [0062]    Besides, the control unit  22  and the output unit  23  can be used to solve the cross-conduction occurred by the conventional first and second switch control circuit  131 , and  132 . Please refer to the FIG. 5, showing the wave forms of the first and second driving control signals V 1   g  and V 2   g  of the first and second switch control circuits  231  and  232  of a preferred embodiment of the present invention. The FIG. 5 displays the wave forms of the first and second driving control signals V 1   g  and V 2   g  produced by the control unit  22 , and the wave form of the voltage signal Vp 2  which is transformed by first side coil T 41  of the fourth transformer T 4 , and then produced at the second side coil T 42 . It is clearly like the FIG. 2( a ), the first and second driving control signals V 1   g  and V 2   g  must change the electric levels thereof by following the first state (ex: a low level L) or the second state (ex: a high level H) of the voltage signal Vp 2 , and have different transient states at different times from t1 to t8 in the FIG. 2( a ). However, the differences between the FIG. 2( a ) and FIG. 5 is that the gate voltages V 1   g  and V 2   g  in FIG. 2( a ) proceed with the transient states at the same time. Therefore, in the FIG. 1, the transistors Q 1  and Q 2  occur the cross conduction because influences of the gate voltages V 1   g  and V 2   g . However, in the FIG. 5, the transients of the first and second driving control signals V 1   g  and V 2   g  do not occurr at the same time. On the contrary, during the transient state, there are time-laggings td1, td2 between the first and second driving control signals V 1   g  and V 2   g . For examples, while the PWM 2211 transform the first driving control signal V 1   g  from the fist state (a low level state L) into the second state (a high level state H), the PWM 2211 still maintains the second driving control signal V 2   g  in the second state for the time-lagging td, and then transform the second driving control signal V 2   g  into the first state. Or on contrary, while the PWM 2211 transform the second driving control signal V 2   g  from the first state into the second state, the PWM 2211 still maintains the first driving control signal V 1   g  in the second state for the time-lagging td2, and then transform the first driving control signal V 1   g  into the first state. So, the first and the second switch control circuit  231 ,  232  controlled by the first and second driving control signals V 1   g  and V 2   g  will not occur the cross conductions.  
         [0063]    Therefore, by the application of the disclosures of the preferred embodiments of the present invention, the cross conduction problem occurred by the conventional product. Moreover, the present invention can not only prevent the reverse current reverse flows from a output terminal of the power supplies in parallel connection to destroy the power supplies or make them to be malfunction, and but also won&#39;t increasing of the power loss. So, the present invention has a highly commercial application.  
         [0064]    While the invention has been described in terms of what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention needs not be limited to the disclosed embodiments. On the contrary, it is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims which are to be accorded with the broadest interpretation so as to encompass all such modifications and similar structures.