Abstract:
A circuit configuration combining a synchronous rectifier (SR) circuit for a converter with an LC snubber circuit is characterized in that the converter includes a transformer, a secondary coil of which being connected at an end to a metal oxide semiconductor field effect transistor that is parallelly connected to a resistance and a capacitance that are serially connected to each other, in order to reduce oscillation and electromagnetic interference; and that a primary coil of the transformer is serially connected at two ends to two diodes, an inductance is connected between the two diodes, and a connection end of the inductance to one of the two diodes is connected to a junction of the transformer and a main switch via a capacitance to form the LC snubber circuit. In this way, the transformer of the converter may be reset to enable regeneration of energy and upgraded efficiency of the converter.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to a circuit configuration combining a synchronous rectifier circuit for a converter with an LC snubber circuit, and more particularly to a circuit configuration in which a converter having a synchronous rectifier is provided with an LC snubber circuit to eliminate turn-off surge on the converter and to enable reset of a transformer of the converter and, accordingly, regeneration of energy, so that the converter may have upgraded efficiency.  
           [0002]    A general communication system requires a miniaturized and high-efficiency power-supply module to save space and energy. However, loss of output current at diodes during energizing period prevents the power-supply module from reaching a preset efficiency level. Recently, integrated circuits (IC) have been widely employed in computers and peripherals thereof for both industrial and domestic purposes. While a high-density IC apparatus provides more and better functions, its power density increases with the density of integrated circuits. To reduce loss of energy, such integrated circuits are designed to have a service voltage (that is, an output voltage of the power-supply module) as low as possible. In other words, a conventional concept that a computer should use low voltage and high current results in loss of energy at output diodes of the power supplier during energizing, as well as reduced efficiency of the power-supply module.  
           [0003]    The only way to solve this problem is to develop an almost ideal diode that has advantages of extremely small internal resistance and extremely low cut-in voltage. However, such ideal diode is too difficult to be realized. Nevertheless, a metal oxide semiconductor field effect transistor (MOSFET) synchronous rectifier (SR) may be used to replace a conventional diode rectifier circuit. Moreover, some active clamp circuits enable further improvement of efficiency in the use of the MOSFET SR. However, such improvement involves in the use of auxiliary active switches and complicated driving circuits.  
           [0004]    [0004]FIG. 1 shows an MOSFET SR circuit for a single-ended forward converter. The circuit includes a switch S 2  that uses a FET Q 1  and a parasitic diode D 1  thereof FET Q 2  and a parasitic diode D 2  thereof as a flywheel gear. To reduce loss of energy at the diode D 2  during energizing, a by-pass capacitance C 1  is incorporated, as shown in FIG. 2, to extend the reset time required by a transformer of the converter, and to prevent the two FETs Q 1  and Q 2  from closing at the same time. However, due to a parasitic inductance presented in the converter, the capacitance C 1  causes the converter to generate high-frequency oscillation that worsens the problem of electromagnetic interference (EMI) and increases loss of power.  
         SUMMARY OF THE INVENTION  
         [0005]    It is therefore a primary object of the present invention to provide an improved circuit configuration to eliminate drawbacks existing in the conventional MOSFET SR circuit for a converter. To achieve the above and other objects, the circuit configuration of the present invention mainly includes an MOSFET parallelly connected to a capacitance and a resistance that are connected in series, in order to reduce oscillation and EMI. The circuit configuration of the present invention also incorporates an LC snubber circuit to obtain high efficiency for the converter.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]    The structure and the technical means adopted by the present invention to achieve the above and other objects can be best understood by referring to the following detailed description of the preferred embodiments and the accompanying drawings, wherein  
         [0007]    [0007]FIG. 1 is a circuit diagram of a conventional synchronous rectifier (SR)  
         [0008]    [0008]FIG. 2 is a circuit diagram of another conventional synchronous rectifier (SR);  
         [0009]    [0009]FIG. 3 is a circuit diagram of the present invention combining an MOSFET SR circuit with an LC snubber circuit;  
         [0010]    [0010]FIG. 4 is a circuit diagram of a circuit configuration combining an MOSFET SR circuit with an RCD snubber circuit;  
         [0011]    [0011]FIG. 5 compares two load characteristic-efficiency curves separately obtained from the circuit configurations shown in FIG. 3 and FIG. 4;  
         [0012]    [0012]FIG. 6 compares two input characteristic-efficiency curves separately obtained from the circuit configurations shown in FIG. 3 and FIG. 4;  
         [0013]    [0013]FIG. 7 shows waveforms of the present invention using the LC snubber circuit;  
         [0014]    [0014]FIG. 8 shows waveforms of the present invention corresponding to different operating states of a converter thereof;  
         [0015]    [0015]FIG. 9 shows operating states of the present invention using the LC snubber circuit;  
         [0016]    [0016]FIG. 10 is a circuit diagram showing the present invention is used on a flyback converter with a synchronous rectifier circuit;  
         [0017]    [0017]FIG. 11 is a circuit diagram showing the present invention is used on a half-bridge converter with a synchronous rectifier circuit;  
         [0018]    [0018]FIG. 12 is a circuit diagram of a forward converter with a synchronous rectifier circuit, wherein the converter is provided with an energy-regenerating circuit; and  
         [0019]    [0019]FIG. 13 is a circuit diagram of a flyback converter with a synchronous rectifier circuit, wherein the converter is provided with an energy-regenerating circuit. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0020]    Please refer to FIG. 3. The present invention provides a circuit configuration that mainly includes a metal oxide semiconductor field effect transistor (MOSFET) circuit, as that shown in FIG. 1, parallelly connected to a resistance R 2  and a capacitance C 2  connected in series for reducing high-frequency oscillation and thereby eliminating electromagnetic interference (EMI) to upgrade efficiency of a converter, and an LC snubber circuit formed by incorporating an inductance L, a capacitance C, and two diodes D 3 , D 4  into a primary coil of the transformer of the converter. More specifically, in the snubber circuit of the present invention, the primary coil of the transformer is connected at an end to an end of a main switch S 1  while the primary coil and the main switch S 1  are serially connected at another ends to the diodes D 3  and D 4 , respectively; the inductance L is connected between the two diodes D 3  and D 4 ; and the capacitance C is connected between a connection end of the inductance L to one of the two diodes D 3  or D 4  (it is D 3  in the illustrated drawing) and a junction of the primary coil of the transformer and the main switch element S 1 .  
         [0021]    Movements of the LC snubber circuit of the present invention will now be described in more details as follows.  
         [0022]    When the main switch S 1  is open, energy presenting in magnetic inductance and leakage inductance would continuously guide current to flow through the diode D 3  and the capacitance C without generating a voltage surge. And, the capacitance C discharges during this period of time. When the main switch S 1  is closed, a resonance effect is produced between the capacitance C and the inductance L via the main switch S 1  and the diode D 4 , and the capacitance C charges during this period of time. The converter is reset to eliminate turn-off surge without the need of using any auxiliary active switch.  
         [0023]    Moreover, when the main switch S 1  is open, energy-stored in the main switch S 1 , the converter, and passive elements of connectors would create energy regeneration.  
         [0024]    [0024]FIG. 4 shows a resistance R 3 , a capacitance C 3 , and a diode D 5  are incorporated into the primary coil of the transformer of the converter to provide an RCD snubber circuit for protecting the main switch S 1  against turn-off surge.  
         [0025]    [0025]FIG. 5 compares two load characteristic-efficiency curves separately obtained from a converter using the LC snubber circuit of the present invention shown in FIG. 3 and another converter using the RCD snubber circuit shown in FIG. 4. Both converters include a MOSFET SR. It can be seen from FIG. 5 that, with the LC snubber circuit, the forward transformer reaches a maximum efficiency of 90.9% at 4.14A, which is higher than that of the forward transformer with the RCD snubber circuit.  
         [0026]    [0026]FIG. 6 compares two efficiency curves separately obtained from two transformers that are tested with different input voltages under two different conditions, that is, using the LC snubber circuit of the present invention and using the RCD snubber circuit of FIG. 4. Again, it can be clearly seen from FIG. 6 that the transformer using the LC snubber circuit has efficiency about 10% higher than that of the transformer using the RCD snubber circuit.  
         [0027]    [0027]FIG. 7 shows different waveforms of the present invention using the LC snubber circuit, wherein the horizontal coordinate represents time (t) while letters V and I represent voltage and current, respectively. A main waveform thereof in rest position corresponding to one cycle of switch includes seven operating states, as shown in FIG. 8. The main switch S 1  is ON in two of these seven operating states and is OFF in the other five operating states.  
         [0028]    Please refer to FIG. 9. The seven operating states of the converter using the LC snubber circuit of the present invention will now be described in details as follows.  
         [0029]    (1) State  1 :  
         [0030]    The main switch S 1  is open. The capacitance C discharges due to magnetic current in the converter and the output current. Meanwhile, energy stored in the inductance L returns to a voltage source Vi. Since a pole of the capacitance C through where voltage passes is not a positive pole, the switch S 2  keeps closed, and it is a secondary current of the converter that supplies power to a load resistance R 0 .  
         [0031]    (2) State  2 :  
         [0032]    This state starts when an induced current i L  reaches zero, and it ends when the capacitance C discharges to zero.  
         [0033]    (3) State  3 :  
         [0034]    A primary coil current i n1  continuously charges the capacitance C. When the energy is fully transferred to the capacitance C, the primary coil current i n1  should be zero. At this point, a voltage Vc of the capacitance C has been charged to a specific voltage. A secondary coil voltage |V n2 | gradually increases with the charging of the capacitance C. During the period of increasing the secondary coil voltage |V n2 |, the switch S 3  does not become ON immediately. However, the gradually increasing secondary coil voltage |V n2 | makes the diode D 2  at this point, and the FET Q 2  of the switch S 3  is opened at the same time.  
         [0035]    (4) State  4 :  
         [0036]    The diode D 3  reverses when the primary coil current i n1  reaches zero. The voltage Vc of the capacitance C slightly discharges when a reverse current of the diode D 3  in the State  4  recovers. Meanwhile, since a switch voltage V S1  is higher than the voltage source Vi, there is current flows reversely through the primary coil n 1  to the voltage source Vi to therefore recover the energy. At this stage, an energy recovery time is usually longer than a reverse recovery time of the diode D 3 . At the end of the State  4 , a value of the primary coil current i n1  is reached when a primary coil voltage V n1  is zero and the switch voltage V S1  is equal to the voltage source Vi.  
         [0037]    On the other hand, since the switch S 3  is closed, a switch voltage V S2  is equal to a negative coil voltage V n2 . Since a passive capacitivity of the capacitance C 2  and the switch S 2  may be charged by a reverse coil current i n2 , the reverse coil current i n2  would reduce to zero at the end of the State  4 .  
         [0038]    (5) State  5 :  
         [0039]    The charged capacitance C 2  may be used to extend the closed state of the FET Q 2  in the switch S 3 . That is, the converter is reset to reduce the current to zero. As a result, a flywheel current does not flow through the diode D 2  of the switch S 3  to avoid a secondary reverse voltage of the converter that has big energy loss during energizing. When the primary coil current i n1  reaches zero, a reverse voltage V n1  is induced to charge the capacitance C via the diode D 3 , causing a reverse voltage of the secondary coil voltage V n2  to rise and thereby keeps the switch S 3  closed. When the diode D 3  is made, the switch voltage V S1  is equal to a sum of the voltage Vc of the capacitance C and the voltage source Vi, and the switch voltage V S1  keeps lower than twice of the voltage of the voltage source Vi.  
         [0040]    (6) State  6   
         [0041]    When the main switch S 1  is closed, current flows from the voltage source Vi to the primary coil n 1  via the switch S 1 . Meanwhile, the diode D 4  would be forward biased, the capacitance C would discharge via the switch S 1  and the diode D 4 , and the voltage V n2  supplies power to the load resistance R 0  via the switch S 2 . After lapse of a very short time, a reverse flywheel current i S3  flows through the switch S 3 . At this point, the switch S 2  and the switch S 3  are closed at the same time.  
         [0042]    (7) State  7 :  
         [0043]    When a reverse recovery current of the diode D 2  inside the switch S 3  reaches zero, the primary current keeps flowing through the primary coil n 1  and the switch S 1 . Meanwhile, the capacitance C keeps discharging. At the secondary side, the coil voltage V n2  keeps supply of power to the load resistance R 0  and keeps reversed charge of the capacitance C via the diode D 4  and the inductance L.  
         [0044]    In addition to the above-described forward converter with SR, the present invention may also be used with a flyback converter as shown in FIG. 10 or with a half-bridge converter as shown in FIG. 11.  
         [0045]    [0045]FIG. 12 shows another embodiment of the present invention, in which an additional inductance L 1  having a secondary coil is incorporated. A primary coil of the inductance L 1  is connected to the switches S 2 , S 3 . When the switch S 2  is open, energy of possibly generated voltage surge is regenerated at the voltage source to protect the switch S 2 .  
         [0046]    [0046]FIG. 13 shows a further embodiment of the present invention, in which an additional regenerating circuit is incorporated into the flyback converter shown in FIG. 10. The regenerating circuit includes an additional inductance L 2  having a secondary coil. A primary coil of the inductance L 2  is connected to the switch S 2 . When the switch S 2  is open, energy of possibly generated voltage surge is regenerated at the voltage source to protect the switch S 2 .  
         [0047]    The present invention has been described with some preferred embodiments thereof and it is understood that many changes and modifications in the described embodiments can be carried out without departing from the scope and the spirit of the invention as defined by the appended claims.