Abstract:
An AC-DC power converter is provided with two pairs of self-driven synchronous rectifier switches in addition to, or in place of, diode bridge rectifiers for boosting efficiency and reducing cost. An AC sensing circuit is coupled to AC input terminals, and a DC level shifting circuit applies a DC offset to an AC input signal received via the sensing circuit. A comparator circuit determines positive and negative half waves of the AC input signal relative to the DC offset value. Gate drive signals are provided for driving a first set of parallel rectifier switches during a positive half cycle of the AC input signal, and for driving a second and opposing pair of parallel rectifier switches during a negative half cycle of the AC input signal. In an embodiment, high side gate drive signals may be electrically isolated from the active rectifier control circuitry.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application claims benefit of the following patent application which is hereby incorporated by reference: U.S. Provisional Patent Application No. 61/787,923, filed Mar. 15, 2013. 
    
    
     A portion of the disclosure of this patent document contains material that is subject to copyright protection. The copyright owner has no objection to the reproduction of the patent document or the patent disclosure, as it appears in the U.S. Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever. 
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to switch-mode power converters. More particularly, the present invention relates to power supplies using synchronous rectifier circuitry in place of a diode bridge. Even more particularly, the present invention relates to control circuitry for driving synchronous rectifiers according to input voltage phase. 
     Various switch-mode power converters as conventionally known in the art include bridge rectifiers to convert an alternating-current (AC) input voltage into a rectified direct-current (DC) voltage. However, losses on such a bridge rectifier may be significant. The voltage drop for typical bridge diodes may range from about 0.7 volts to about 1.0 volts. Hence, the diode voltage drop may be as much as 2 volts as in a bridge rectifier there are two diodes conducting for every half cycle of the AC input signal. 
     BRIEF SUMMARY OF THE INVENTION 
     In a switch-mode AC-DC power converter according to the present invention, diode voltage drop losses may be significantly reduced by coupling four metal-oxide-semiconductor field-effect transistor (MOSFET) switching elements with the diodes in a conventional bridge rectifier, or by replacing the diodes with MOSFETs outright, along with control circuitry for driving the parallel MOSFET configurations. 
     A simple loss comparison between bridge rectifiers against parallel MOSFETs may be provided as follows, with certain exemplary assumptions such as a rectifier average output current of 5 amps, a rectifier diode drop of 1 volt/diode, and a MOSFET R DS(ON)  of 60 mΩ. With these parameters, the power loss for diodes is (2×V f ×I avg )=10 W, while the power loss for MOSFETs is (2×R DS(ON) ×I 2  rms)=3.7 W. Therefore, it may easily be demonstrated that a parallel MOSFET synchronous rectifier configuration of the present invention results in substantially lower losses and boosts the overall efficiency of the power converter. 
     Briefly stated, various embodiments of a power converter according to the present invention include a first AC input terminal coupled between first and second synchronous rectifier switching elements, the first and second synchronous rectifier switching elements coupled across first and second DC output terminals. A second AC input terminal is coupled between third and fourth synchronous rectifier switching elements, the third and fourth synchronous rectifier switching elements being coupled across the first and second DC output terminals. An AC input sensing circuit is coupled to the AC input terminals. A DC level shifting circuit applies a DC offset to an AC input signal received via the sensing circuit. Switching control circuitry receives the AC input signal with the DC offset and provides signals for driving the first and fourth synchronous rectifier switching elements during a positive half cycle of the AC input signal, and signals for driving the second and third synchronous rectifier switching elements during a negative half cycle of the AC input signal. 
     In one aspect of the present invention, the positive half-cycle and the negative half-cycle of the AC input signal are substantially defined with respect to the DC offset. 
     In another aspect, the synchronous rectifier switching elements are MOSFET rectifiers having parallel diodes. 
     In one embodiment, the switching control circuitry further includes a first comparator circuit that compares the AC input signal with the DC offset to a first reference voltage and generates a first control signal for turning on the first and fourth switching elements when the input signal is greater than the first reference voltage. A second comparator circuit compares the AC input signal with the DC offset to a second reference voltage and generates a second control signal for turning on the second and third switching elements when the input signal is less than the second reference voltage. 
     In one aspect of this embodiment, the first and second reference voltages are substantially equal to the DC offset. 
     In another aspect of this embodiment, a first high side driver circuit is provided to receive the first control signal and to generate an electrically isolated gate drive signal for turning on the first switching element. A second high side driver circuit is provided to receive the second control signal and to generate an electrically isolated gate drive signal for turning on the third switching element. A first buffer drive circuit is further coupled to receive the first control signal and to generate a gate drive signal for turning on the fourth switching element. A second buffer drive circuit is coupled to receive the second control signal and to generate a gate drive signal for turning on the second switching element. 
     In one embodiment of the power converter according to the present invention, a first comparator switching element is coupled to the AC sensing circuit and configured to be turned on during a positive half cycle of the AC input signal. A second comparator switching element is coupled to the sensing circuit and is configured to be turned on during a negative half cycle of the AC input signal. A first driver circuit generates drive signals to turn on and off the first and second synchronous rectifier switching elements in accordance with on-states of the first and second comparator switching elements, respectively. A second driver circuit generates drive signals to turn on and off the third and fourth synchronous rectifier switching elements in accordance with on-states of the second and first comparator switching elements, respectively. 
     In one aspect of such an embodiment, a threshold value for the comparator switching elements may define a DC offset for referencing the positive and negative half cycles of the AC input signal. 
     In another aspect, the AC input sensing circuit may include a first pair of resistors coupled on a first end to the first AC input terminal and on a second end to circuit ground, and a second pair of resistors coupled on a first end to the second AC input terminal and on a second end to circuit ground. 
     In another aspect, the first comparator switching element has a source terminal coupled to circuit ground and a gate terminal coupled to a drain terminal of the second comparator switching element and further to a node between the second pair of resistors. A second comparator switching element has a source terminal coupled to circuit ground and a gate terminal coupled to a drain terminal of the first comparator switching element and further to a node between the first pair of resistors. 
     In another aspect of any of the embodiments described above, a diode bridge rectifier may further be coupled on a first end between the AC input terminals and on a second end between the first and second DC output terminals. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  is a circuit block diagram representing an exemplary power converter and active bridge control circuitry according to an embodiment of the present invention. 
         FIG. 2  is a graphical diagram illustrating an exemplary AC input waveform as sensed by the AC sensing circuit of  FIG. 1 . 
         FIG. 3  is a graphical diagram illustrating the exemplary AC input waveform of  FIG. 2  after operation of the DC level shift-up circuit of  FIG. 1 . 
         FIG. 4  is a graphical diagram illustrating exemplary gate drive signals applied corresponding to a phase of the AC waveform with respect to the DC level shift value. 
         FIG. 5  is a circuit block diagram representing another exemplary embodiment of a power converter and active bridge control circuitry of the present invention. 
         FIGS. 6A, 6B and 6C  are graphical diagrams illustrating an AC input voltage waveform and first and second gate drive signals, respectively, in an exemplary operation of the power converter of  FIG. 5 . 
         FIG. 7  is a circuit block diagram representing another exemplary embodiment of a power converter and active bridge control circuitry of the present invention. 
         FIG. 8  is a circuit block diagram representing an embodiment of an AC sensing circuit, DC level shifting circuit and comparator circuit for the power converter of  FIG. 7 . 
         FIG. 9  is a circuit block diagram representing an embodiment of a high-side gate drive circuit for the power converter of  FIG. 7 . 
         FIGS. 10 and 11  are graphical diagrams illustrating an AC input voltage waveform and first and second gate drive signals in exemplary operations of the power converter of  FIG. 7 . 
         FIG. 12  is a circuit block diagram representing another exemplary embodiment of a power converter and active bridge control circuitry of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring generally to  FIGS. 1-12 , various embodiments of a power converter with an active bridge MOSFET rectifier, active bridge control circuitry and methods of operation may now be described. Where the various figures may describe embodiments sharing various common elements and features with other embodiments, similar elements and features are given the same reference numerals and redundant description thereof may be omitted below. 
     Referring first to  FIG. 1 , an embodiment of a power converter  10  is shown coupled on a first end to first and second AC input terminals V AC1  and V AC2  and on a second end to first and second DC terminals V DC1  and V DC2 . A first active rectifier bridge includes first and second switching elements Q 1  and Q 2  as synchronous rectifiers coupled in series across the first and second DC terminals, with the first AC terminal V AC1  coupled to a node between the first and second switching elements. A second active rectifier bridge includes third and fourth switching elements Q 3  and Q 4  as synchronous rectifiers coupled in series across the first and second DC terminals V DC1  and V DC2  and in parallel with the first active rectifier bridge, with the second AC terminal V AC2  coupled to a node between the third and fourth switching elements Q 3 , Q 4 . The synchronous rectifiers in the example shown (and generally throughout this disclosure) are MOSFETs with parallel diodes, but alternative switching elements may be used within the scope of the present invention. 
     Active rectifier control circuitry  12 ,  14 ,  16  includes an AC sensing circuit  12  coupled on a first end to the first and second AC input terminals V AC1  and V AC2  to obtain real time AC line information, a DC level shift up circuit  14  effective to apply a DC offset to the AC waveform for easy processing, and a comparator circuit  16  effective to determine whether the real time AC signal is in a positive or negative portion of the AC cycle for generating MOSFET gate drive signals. 
     The diagram of  FIG. 2  represents a scaled down real time AC line information feed into the active rectifier control circuit  12 ,  14 ,  16 . 
     The diagram of  FIG. 3  represents a DC voltage having been added to shift up the AC line information Vac_in to a DC signal Vdc_shift. 
     The diagram of  FIG. 4  represents appropriate MOSFET gate drive signals G 1 , G 2  as generated by the comparator circuit  16  for driving parallel MOSFETs, the gate drive signals G 1 , G 2  being alternately generated in accordance with positive and negative phases of the AC line waveform VAC_in, with respect to the DC signal V DC   _   SHIFT . Therefore, during a positive cycle of the AC input voltage sinewave, MOSFETs Q 1  and Q 4  are turned on and MOSFETs Q 2  and Q 3  are off, and the voltage drop across Q 1  and Q 4  is accordingly reduced. During a negative half-wave of the AC input voltage, the opposite MOSFETs Q 2  and Q 3  are turned on and MOSFETs Q 1  and Q 4  are turned off. 
     Referring now to  FIG. 5 , a more particular example of the active bridge control circuitry  12 ,  14 ,  16  and an associated operation of the power converter  10  may now be described. First and second resistors R 1 , R 2  are coupled in series between the second AC input terminal V AC2  and circuit ground  0 , and third and fourth resistors R 3 , R 4  are coupled in series between the first AC input terminal V AC1  and ground  0 . A switching element Q 5  has its source coupled to ground  0  and its drain coupled to an anode of a diode D 5 , the cathode of diode D 5  being coupled to a biasing DC voltage source V_Bias. Another switching element Q 6  has its source coupled to ground  0  and its drain coupled to an anode of diode D 6 , the cathode of diode D 6  being coupled to the biasing DC voltage source V_Bias. The gate of switching element Q 5  and the drain of switching element Q 6  are coupled to a node between the first and second resistors R 1 , R 2 . The gate of switching element Q 6  and the drain of switching element Q 5  are coupled to a node between the third and fourth resistors R 3 , R 4 . 
     In the embodiment described, resistors R 1 -R 4  form an AC sensing circuit  12 , while the switching elements Q 5 , Q 6  and diodes D 5 , D 6  form DC level shift up and comparator circuits  14 ,  16 . The switching elements Q 5 , Q 6  may be small signal MOSFETs with threshold voltages of about 3V. 
     Referring now to  FIGS. 6A, 6B and 6C , during a positive half-wave of the AC input signal V AC   _ in, through the voltage divider formed by resistors R 3 , R 4  the switching element Q 6  is turned on and switching element Q 5  is turned off. In this case, the signal G 2  is pulled low and the signal G 1  is pulled high. A first half-bridge driver  18   a  and a second half-bridge driver  18   b  are coupled to the first active rectifier bridge and the second active rectifier bridge, respectively, and are effective to generate drive signals in response to G 1  and G 2  wherein switching elements Q 1  and Q 4  are turned on and switching elements Q 2  and Q 3  are turned off. 
     Otherwise, during a negative half-wave of the AC input signal V AC   _ in, through the voltage divider formed by resistors R 1 , R 2 , the switching element Q 5  is turned on and switching element Q 6  is turned off. The signal G 1  is now pulled low and the signal G 2  is high. The first half bridge driver  18   a  and the second half bridge driver  18   b  generate drive signals in response to G 1  and G 2  wherein switching elements Q 2  and Q 3  are turned on and switching elements Q 1  and Q 4  are turned off. 
     Referring now to  FIG. 7 , in another embodiment the first and second half bridge driver circuits  18   a ,  18   b  of the power converter  10  are replaced with a first high side driver circuit  18   a  coupled to the gate of switching element Q 1 , a second high side driver circuit  18   b  coupled to the gate of switching element Q 3 , a first buffer driver circuit coupled to the gate of switching element Q 2 , and a second buffer driver circuit coupled to the gate of switching element Q 4 . 
     In an embodiment the AC sensing circuit  12  and the DC level shift up circuit  14  may have a structure as represented in  FIG. 8 . Resistors R 6  and R 7  are coupled to first and second AC input terminals (i.e., opposing ends of an AC input source Vac). A first RC circuit of resistor R 4  and capacitor C 1  are coupled in parallel with each other and further coupled on a first end to resistor R 6 . A second RC circuit of resistor R 2  and capacitor C 2  are coupled in parallel with each other and further coupled on a first end to resistor R 7  and on a second end to a second end of the first RC circuit. A third RC circuit of resistor R 5  and capacitor C 3  are coupled in parallel with each other and further coupled on a first end to a voltage source V 1  via resistor R 11 , and coupled on a second end to circuit ground. The first end of the third RC circuit is coupled to the non-inverting input (+) of an operational amplifier X 1 , while the first end of the second RC circuit is coupled to the inverting input (−) of the operational amplifier X 1 . 
     Accordingly, a DC biasing voltage is added to the AC signal, effectively shifting up the DC level of the AC wave. A resulting AC wave with DC offset is provided at node  20 . 
     In the embodiment represented in  FIG. 8 , first and second comparator circuits  16   a ,  16   b  are implemented to generate drive signals G 2 , G 1 , respectively. 
     Regarding the first comparator circuit  16   a , node  20  is coupled as an input voltage via resistor R 17  to the inverting input (−) of an operational amplifier X 3 . The non-inverting input (+) of operational amplifier X 3  is coupled to biasing voltage source V 3  via a network of resistors R 21 , R 22 , R 20  and capacitor C 12 . When the input signal (the AC waveform with DC offset at node  20 ) at the inverting input of the operational amplifier X 3  is greater than a reference voltage at the non-inverting input (generally corresponding to a positive half-wave of the AC waveform with respect to the DC offset value where for example the DC offset value is roughly equivalent to the bias voltage), gate drive signal G 2  is LOW. Alternatively, when the input signal is less than the reference voltage (generally corresponding to a negative half-wave of the AC waveform with respect to the DC offset value), gate drive signal G 2  is HIGH. 
     For the second comparator circuit  16   b  the node  20  is coupled as an input voltage via resistor R 3  to the non-inverting input (+) of operational amplifier X 2 . The inverting input (−) of operational amplifier X 2  is coupled to biasing voltage source V 4  via a network of resistors R 13 , R 14 , R 12  and capacitor C 4 . When the input signal (the AC waveform with DC offset at node  20 ) at the non-inverting input of the operational amplifier X 2  is greater than a reference voltage at the non-inverting input, gate drive signal G 1  is HIGH. Alternatively, when the input signal is less than the reference voltage, gate drive signal G 1  is LOW. 
     In an embodiment the high side driver circuit  18   a  generally represented in  FIG. 7  may have an exemplary structure as shown in  FIG. 9  for electrical isolation between an input end coupled to the comparator circuits  16   a ,  16   b  and an output end coupled to the high side synchronous rectifiers Q 1 , Q 3 . In the embodiment shown, a drive signal G 1  is provided to the gate of a first transistor S 1  via resistive network R 23 , R 25 . When the drive signal G 1  is HIGH, the transistor S 1  is turned on and a first biasing voltage (e.g., 12V) is conducted across the light emitter of an opto-coupler or equivalent isolation circuit U 1 , wherein a corresponding phototransistor is turned on. When the phototransistor is turned on, voltage from a second biasing voltage source V 5  is conducted to the control terminal of a second transistor S 2  and the second transistor S 2  is turned on to further generate an output signal G 12  as a gate drive signal to the corresponding MOSFET rectifier Q 1 . 
     It may be understood that although not shown, an equivalent high side driver circuit  18   a  may be provided for receiving the drive signal G 2 , wherein an output signal G 22  is generated as a gate drive signal to the corresponding MOSFET rectifier Q 3 . 
     In  FIGS. 10 and 11 , exemplary results may be seen for an experimental operation of the power converter with active rectifier control circuitry in an embodiment as represented in  FIGS. 7-9 . 
     Referring now to  FIG. 12 , in another embodiment a conventional diode bridge rectifier  22  may within the scope of the present invention be further coupled in parallel with the active rectifier circuit. In the example shown, the circuit configuration and operation may otherwise match the description above with respect to the embodiments in  FIGS. 7-9 . 
     The previous detailed description has been provided for the purposes of illustration and description. Thus, although there have been described particular embodiments of the present invention of a new and useful “Power Converter with Self-Driven Synchronous Rectifier Control Circuitry,” it is not intended that such references be construed as limitations upon the scope of this invention except as set forth in the following claims.