Abstract:
The switched mode power supply comprises an inductor, a switching transistor coupled in series with the inductor, and a rectifier circuit. The rectifier circuit comprises a switch coupled with the inductor for a rectification of an output voltage and a control circuit operating in a monostable mode. The switch is operated in particular in a synchronous mode with the same switching frequency as the switching transistor. The rectifier circuit can be used advantageously with a switched mode power supply having a push-pull half-bridge configuration and operating as a resonant converter or a quasi-resonant converter with soft switching for an application within a plasma television set.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a switched mode power supply comprising a transformer, a switching transistor and a rectifier circuit with a switch and a control circuit for providing a rectification of an output voltage in a synchronous rectification mode. 
       BACKGROUND OF THE INVENTION 
       [0002]    It is well known that diodes used for rectification of an AC voltage have a considerable energy dissipation because of the forward voltage drop, in particular when used for rectification of small AC voltages. For improving the efficiency of a rectifier circuit therefore a controllable switch is often used, for example a MOSFET switch with a small drain-source resistance. A disadvantage of a MOSFET rectifier circuit is that a control circuit is required for the operation of the MOSFET. 
         [0003]    A power supply with a synchronous rectifier circuit comprising a current transformer for the operation of a MOSFET switch is known from DE-A-19704604. Switching through of the switch is provided by means of a positive voltage provided by a first secondary winding of a transformer of the power supply, and the current transformer is used for sensing a current flow of a second secondary winding for blocking the switch, when the polarity of the current through the second secondary winding reverses. A rectifier circuit of this kind can be used in particular for a flyback converter. 
         [0004]    From US 2006/0187692 A1 a rectifier circuit with a MOSFET switch and a control circuit having a driver circuit with a first and a second comparator for the operation of the switch is known. The first comparator is used for controlling the voltage polarity across the switch and the second comparator is connected with a reference voltage for the regulation of the switch for providing a stabilized output voltage. 
         [0005]    A switched mode power supply with a rectifier circuit comprising a comparator being also connected with a reference voltage for providing a stabilized DC output voltage is known from EP-A-1717938. A DC rectifier circuit of this kind can used for example to provide a regulated output voltage being independent of the regulation of a further regulated output voltage of the switched mode power supply. The switched mode power supply includes a soft switching half-bridge arrangement operating in a quasi resonant mode with a feedback control from secondary side. DC/DC converters of a half bridge type with soft switching are also known from EP-A-1717940 and FR 2738417. 
         [0006]    Also known are integrated driver circuits for operating with a MOSFET switch as a synchronous rectifier circuit, for example integrated circuits STSR 3  and STSR 2 , the STSR 2  being designed for a full-wave rectifier circuit and the STSR 3  for a half-wave rectifier circuit. The STSR 3  operates over a wide operating frequency range and comprises an anticipation circuit for preventing secondary side shoot-through conditions at turn-on of the switch. Several anticipation times can be set to provide a switch-off of the switch in advance of switching on the primary side switching transistor. 
       BRIEF SUMMARY OF THE INVENTION 
       [0007]    The switched mode power supply according to the invention comprises an inductor, a switching transistor coupled in series with the inductor, a rectifier circuit with a switch being coupled with the inductor for a rectification of an output voltage and further a control circuit operating in a mono-stable mode for controlling the operation of the switch. The switch is operated in particular in a synchronous mode with the same switching frequency as the switching transistor, and the switching transistor is operated essentially with a constant switching frequency. The control gate contains for example a control circuit having a gate-on time, which is constant and in particular independent of any load condition, and which is well below the blocking time of the switching transistor. The switch is therefore operated always in a safe operating condition and any unwanted reverse currents through the switch are avoided. 
         [0008]    The rectifier circuit comprises advantageously a diode, in particular a Schottky diode, which takes over a remaining current under high load condition, when the switch is already closed and which can take over therefore a part of the dissipation losses of the rectifier circuit in case of such a high load condition. 
         [0009]    The switched mode power supply comprises in a further aspect of the invention a feedback loop for coupling the output voltage of the switched mode power supply to a driver circuit of the switching transistor. The switch-off time of the switch is determined only by the time constant of the control circuit of the switch and is therefore independent of the switching signal of the switched mode power supply. 
         [0010]    The mono-stable operation of the control circuit can be provided for example by using a digital mono-flop circuit, or by using a comparator, to which inputs a ramp generator and a threshold circuit are coupled. The control circuit for operating the switch can be designed therefore with cheap circuit components and does in particular not require a current measurement of the current flowing through the inductor. 
         [0011]    The rectifier circuit can be used advantageously with a switched mode power supply having a push-pull half-bridge configuration and operating as a resonant converter or a quasi-resonant converter with soft switching for an application within a plasma television set. The switch is in a preferred embodiment a MOSFET. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    Preferred embodiments of the invention are explained in more detail below now by way of example with reference to schematic drawings, which show: 
           [0013]      FIG. 1  a switched mode power supply including a rectifier circuit in accordance with the invention, 
           [0014]      FIG. 2  a preferred embodiment of the switched mode power supply and a rectifier circuit as shown in  FIG. 1 , 
           [0015]      FIG. 3  voltage and current diagrams of the circuit of  FIG. 2  operating in a high load condition, and 
           [0016]      FIG. 4  the circuit of  FIG. 2  operating in a standby condition. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0017]    In  FIG. 1  a switched mode power supply is shown including a rectifier circuit in accordance with the invention. The switched mode power supply comprises a transformer TR with a primary winding Lp and a secondary winding Ls 1 , the transformer being designed for providing mains isolation. A first switching transistor T 1  is coupled in series between the primary winding Lp and a DC input voltage Ve, provided by an input capacitor Ce, and a second switching transistor T 2  is arranged in parallel with the primary winding Lp and connected to ground. The switching transistors T 1 , T 2  are operated in a push-pull mode by a driver circuit IC 1  and are arranged in this embodiment as a half bridge configuration. 
         [0018]    The winding sense of the primary winding Lp 1  and secondary winding Ls 1  is indicated by dots, according to which the terminals  1  and  2  labeled with a dot have same voltage polarities during operation of the switched mode power supply. In series with the primary winding Lp, between Lp and ground in this embodiment, a resonant capacitor Cr is coupled for providing a quasi-resonant operation of the switched mode power supply. A switched mode power supply of this kind is described for example in FR 27387417 and EP 1717940. 
         [0019]    A switch T 3 , in this embodiment a MOSFET, is arranged on the secondary side in series with the secondary winding Ls 1 . An output capacitor Cs is coupled in parallel with the secondary winding Ls 1  and switch T 3  for providing a smoothed DC output voltage Vs. The switch T 3  is coupled between the secondary winding Ls 1  and ground in this embodiment. In parallel with the switch T 3  a diode Ds is arranged to allow a current i(Ds) to flow from ground in the direction of the secondary winding Ls 1 , even when the switch T 3  is closed. 
         [0020]    For a regulation of the output voltage Vs, a feedback loop FB is provided, which couples a fraction of the output voltage Vs via a voltage divider with resisters R 1 , R 2  back to the primary side to the driver circuit IC 1 . The driver circuit IC 1  operates essentially with a fixed switching frequency and provides pulse-width modulated push-pull drive signals for changing the duty cycle of the switching transistors T 1 , T 2  for a compensation of output load variations. 
         [0021]    The switch T 3  is controlled by a control circuit MC 1 , which operates according to the invention in a mono-stable mode. The control circuit MC 1  comprises an inverter I, which is coupled with an input to a junction  4  arranged between the switch T 3  and the secondary winding Ls 1  for sensing the voltage polarity at the secondary winding Ls 1 . The inverter I provides a control signal for a first input of an AND gate A and for a control gate G. 
         [0022]    The control gate G operates in a mono-stable mode and comprises for example a timer circuit. The control gate G switches from low to high at output Q, when a high pulse is received at input A, and switches to low after a defined period T=τ=constant. The control gate G comprises for example a ramp generator and a threshold detector, the ramp generator comprising a timer capacitor which is charged when an input signal is received, and when a defined threshold is reached, the threshold detector provides an off signal for discharging the timer capacitor and for providing the low signal after T=τ. Mono-stable circuits of this kind are well known in the art. 
         [0023]    The output signal of the control gate G is coupled to a second input of the AND gate A, which output is coupled to the input of a switch driver D which is coupled with an output to a control input of the switch T 3  for controlling the operation of switch T 3 . The control circuit MC 1  together with the switch T 3  operate as a synchronous rectifier circuit during operation of the switched mode power supply. 
         [0024]    The operation of the rectifier circuit is as follows: When the voltage at terminal  2  of secondary winding Ls is positive and the switch T 3  is switched through, a current i(Ls) is provided for charging capacitor Cs. When the polarity across winding Ls 1  changes, then the voltage at terminal  2  is low and at junction point  4  is high, and then the output of the inverter I is accordingly “low”. This “low” signal provides a “low” signal at the output of the AND gate A, which keeps the switch T 3  closed. 
         [0025]    When the polarity across winding Ls 1  changes again, then the voltage at terminal  2  is positive and at the junction  4  the voltage is then low, and the output of inverter I switches subsequently to “high”. A “high” signal at the output of inverter I triggers the control gate G, which provides a “one shot” signal at output Q, and hence the AND gate switches to “high” for switching through the switch T 3  via driver D, because then both input signals at the AND gate are “high”. 
         [0026]    The control gate G switches after a defined time T=τ from “high” to “low” at output Q. Then the output of the AND gate A switches from “high” to “low” and the driver D therefore switches off the switch T 3  for blocking the current through the switch T 3 . The current through secondary winding Ls may still continue to flow via diode Ds, until the polarity of the secondary winding Ls is reversed again in accordance with the switching operation of the switching transistors T 1 , T 2 . 
         [0027]    When the polarity at the secondary winding Ls is reversed again with the next switching cycle, the voltage at junction  4  is high and therefore the output of inverter I is “low”. Then the AND gate A is blocked and therefore a switching through of switch T 3  is not possible. The synchronous rectifier circuit therefore works as a half wave rectification circuit. 
         [0028]    A preferred embodiment of the rectifier circuit of  FIG. 1  is shown in  FIG. 2 , in which discrete analog components, in particular cheap circuit parts are used for realizing the control circuit MC 1  of  FIG. 1 . The primary side of the switched mode power supply with the switching transistors T 1 , T 2  and the driver circuit IC 1  corresponds essentially with the circuit as shown in  FIG. 1 , and same reference symbols are used for same circuit elements in  FIGS. 1 and 2 . 
         [0029]    At the secondary side, the secondary winding Ls 1 , diode Ds, switch T 3 , and capacitor Cs are arranged for providing an output voltage Vs, as described before with regard to  FIG. 1 . For the operation of the switch T 3  as a synchronous rectifier, a comparator CO is provided, which output controls a push-pull switching stage with transistors T 4 , T 5 , which provide the necessary current for the switching of the switch T 3 , in the embodiment of FIG.  2  being a MOSFET. The transformer TR comprises a further secondary winding Ls 2 , for providing a negative operating voltage of −2.5 V for the comparator CO and transistor T 5 , which extends the output voltage range of the comparator CO and transistors T 4 , T 5  to negative voltages for a fast switching off of the MOSFET. 
         [0030]    Between the positive input V+ of the comparator CO and ground a capacitor CP is coupled, which is charged up to a positive voltage via a diode Dp and a resistor Rp, which is connected with the output voltage Vs. Between the negative input V− of comparator CO and ground a second capacitor Cm is coupled, which is also coupled to resistor Rp, by means of a junction  5  between diode Dp and resistor Rp. Because capacitor Cp is decoupled via diode Dp from junction  5 , the voltage across Cp is not influenced by voltage drops across capacitor Cm, and remains essentially constant. The elements Rp, Dp, Cp provide therefore a threshold voltage for the operation of the comparator CO. 
         [0031]    The capacitor Cm is also charged via resistor Rp, but is periodically discharged by means of a transistor Qc and a diode Dc 1 . A current input of transistor Qc, the collector, is coupled via diode Dc 1  with capacitor Cm, and further via a diode Dc 2  to a junction  6 , which is connected with the output of comparator CO and the base terminals of transistors T 4 , T 5 , for keeping transistor T 4  switched-off, therefore also MOSFET switch T 3 , when transistor Qc is switched through. 
         [0032]    The base of transistor Qc is coupled via a diode Dc 3  and a diode Dc 4  with junction  4  such, that the transistor Qc switches through by means of Dc 4 , when the voltage at junction  4  is high, and when the voltage at junction  4  is low, transistor Qc is blocking by means of Dc 3 . 
         [0033]    The operation of the comparator CO is therefore as follows: When the voltage at junction  4  is high, the diode Dc 4  is conducting and therefore transistor Qc is switched through. 
         [0034]    Then diode Dc 1  is also conducting and discharging capacitor Cm, and diode Dc 2  is conducting, which blocks transistor T 4  and therefore switch T 3  is closed. 
         [0035]    When the voltage across secondary winding Ls 1  reverses polarity in accordance with the operation of switching transistors T 1 , T 2 , the voltage at junction  4  goes to low which blocks transistor Qc accordingly via diode Dc 3 . Then capacitor Cm is charged via resistor Rp. As long as the voltage across capacitor Cp is higher than the voltage across capacitor Cm, the output of comparator CO is high and therefore transistor T 4  is switched through and also switch T 3 . 
         [0036]    When the voltage across capacitor Cm reaches the threshold voltage of capacitor Cp, the comparator CO switches to low and therefore switch T 3  is blocked via transistors T 4 , T 5 . The switch-on time of switch T 3  is therefore independent on the load of the switched mode power supply and the switching frequency of the driver circuit IC 1 , and is only determined by the values of resistor Rp and capacitor Cm, the threshold voltage at capacitor Cp and the stabilized output voltage Vs. The switch-on time τ is in particular arranged such that the switch T 3  is closed well in advance before the polarity at terminal  2  of winding Ls 1  changes from high to low, and therefore any reverse current through switch T 3  is avoided under all operation conditions of the switched mode power supply. 
         [0037]    When the voltage at terminal  2  switches to low, the voltage at junction  4  switches to high, and then again transistor Qc is switched through for discharging capacitor Cm via diode Dc 1  and for blocking transistor T 4  via diode Dc 2 . The switch T 3  is therefore operated in correspondence with the switching transistors T 1 , T 2 . The switch on of switch T 3  is initiated via transistor Qc, which is blocked when the voltage at terminal  2  switches to high and at junction  4  to low. Transistor T 4  is then switched through by means of comparator CO. Switch T 3  is switched off after time t=, when capacitor Cm is charged to the threshold voltage as defined by the voltage across capacitor Cp. 
         [0038]    The rectifier circuit as described with regard to  FIG. 2  provides therefore a very cost efficient and reliable solution for the operation of the switch T 3 . By using a negative operating voltage as provided by secondary winding Ls 2 , in particular very low supply voltages Vs can be generated by the switched mode power supply. The start up of the rectifier circuit is accomplished by means of a resistor R 3 , via which transistor T 4  and therefore switch T 3  is switched through, when the voltage at terminal  2  is high, after switching on the switched mode power supply. The feedback loop FB comprises in this embodiment an opto-coupler Opt, for transferring a regulation signal to the driver circuit IC 1 . 
         [0039]    The operation of the switched mode power supply shown in  FIG. 2  is explained now further with regard to voltage and current diagrams shown in  FIG. 3 , which correspond with a full load operation of the switched mode power supply, and voltage and current diagrams shown in  FIG. 4 , which correspond with an operation of the switch mode power supply in a standby mode. The voltage diagram a) of  FIG. 3  shows the voltage at the gate of MOSFET T 1  and voltage diagram b) respectively the voltage at the gate of MOSFET T 2 . The driver circuit IC 1  provides a delay tm 2 , respectively tm 1 , between switching on MOSFET T 2  after MOSFET T 1  is switched off and vice versa, to avoid a short circuit situation for the supply voltage Ve. 
         [0040]    The voltage Ve at junction  3  between MOSFET T 1  and T 2  changes in accordance with the switching operation of T 1  and T 2  and represents the voltage across primary winding Lp and capacitor Cr with regard to ground,  FIG. 3   c ). The current through primary winding Lp, shown in diagram  3   d ), is therefore rising, when the voltage at junction  3  is high, and is decreasing, when the voltage at junction  3  is low. The voltage across capacitor Cr,  FIG. 3   e ), represents the resonant voltage being in phase with the switching operation of the switching transistors T 1 , T 2 . 
         [0041]    The voltage at junction  4 , at the drain of the MOSFET switch T 3 ,  FIG. 3   f ), corresponds in shape essentially with the voltage across primary winding Lp and has a voltage value depending on the input voltage Ve, winding ratio m of transformer TR, and the voltage across capacitor Cr, when the switch T 3  is closed. The voltage controlling the operation of the switch T 3 , the gate voltage for the MOSFET T 3  in this embodiment, is shown in  FIG. 3   h ). The corresponding current through the secondary winding Ls 1  under full load condition consists of the current flowing through switch T 3  and the current flowing through the diode Ds, as shown in  FIG. 3   g ). 
         [0042]    As can be seen, the switch T 3  is switched through at time t 2 , shortly after time t 1 , at which the switching transistor T 1  is blocked. The current through switch T 3  continues to flow until time t 3 , at which the switch T 3  is blocked in response to the operation of the comparator Co. The remaining energy stored in the transformer TR generates then a current flowing through diode Ds, until the polarity across secondary winding Ls 1  is reversed at time t 4 , in response to the voltage at junction  3 ,  FIG. 3   c ). 
         [0043]    The voltages at the inputs V+, V− of the comparator CO are shown in  FIG. 3   i ). As can be seen, the voltage across capacitor Cm, voltage at input V−, begins to rise at time t 2 , when the switch T 3  is switched through,  FIG. 3   h ), in response to the blocking of transistor Qc. At time t 3  the voltage at input V− reaches the voltage threshold at input V+ causing the blocking of switch T 3  by means of comparator CO. The time t 3  is well ahead the time t 4 , at which the next switching cycle of the switched mode power supply begins, providing therefore a save margin for the switching off of switch T 3 . 
         [0044]    In  FIG. 4  the same voltage and current diagrams a)-e) are shown for a situation, in which the switched mode power supply operates in a low load condition, for example in a standby mode. As can be seen in  FIG. 4   d ), the current through the primary winding Lp is significantly lower in comparison with  FIG. 3   d ), also the switching-on time of the switching transistor T 1  is shorter. The switch-off time of the switch T 3  at time t 3  is still the same, as compared with  FIG. 3   g ), because the voltage Vs, which charges capacitor Cm, is regulated by the feedback loop FB under all load conditions and therefore constant. The current through switch T 3  for charging capacitor Cs is now very small because the power consumption of the load in this operating condition is very small. The current through the diode Ds is essentially zero. Therefore, only the switch-on time of the switch T 3  is controlled in response to the operation of the switching transistors T 1 , T 2 , but not the switch-off time of the switch T 3 , which depends only on the control circuit MC 1 , respectively comparator CO, operating in a monostable mode. 
         [0045]    The present invention is not limited to the embodiments as described before with regard to the figures, and various available modifications come possible for those skilled in the art without departing from the cope of the invention. The invention as described therefore resides in the claims. The rectifier circuit can be used for example also with flyback converters operating with a pulse width modulation, or any DC/DC up-converter or down-converter. For the switch T 3  in particular a large variety of suitable semiconductor switches may be used, as known by a person skilled in the art.