Abstract:
The disclosed embodiments relate to apparatus and method for reducing power losses in a power supply. There is provided an apparatus comprising means for coupling a first signal (S 2 ) to a reference level (ground) when the coupling means is conductive, means for placing the coupling means in a conductive state during a duration of a portion of a period of a second signal (S 3 ), and means for altering the duration of conduction of the coupling means in response to an amplitude of the second signal.

Description:
FIELD OF THE INVENTION 
     This application claims the benefit under 35 U.S.C. §365 of International Application PCT/US2005/034990, filed Sep. 28, 2005, which was published in accordance with PCT article 21(2) on Apr. 12, 2007 in English. 
     BACKGROUND OF THE INVENTION 
     As shown in  FIG. 1 , a typical switch-mode power supply (SMPS) includes primary side components  150  and secondary side components. Primary side, also referred to as “hot side” components comprise switch-mode controller  106 , a switch-mode metal-oxide-semiconductor (MOSFET)  108 , MOSFET heatsink  110 , current sensing resistor  112 , surge suppression capacitor  114 , a transformer  116  having primary winding  120 , secondary winding  118 , rectifier diode  102 , filter capacitor  104  and an opto-isolator  126 . Secondary or “cold-side” components comprise secondary transformer windings  122  and  124 , rectifier diodes  128  and  136  with their respective heatsinks  130  and  138  and filter capacitors  132  and  134 . The entire switch-mode supply is powered from an unregulated voltage source  100 . The controller  106  provides a drive signal V D  to MOSFET  108  to produce current flow in primary winding  120  of transformer  116 . Secondary winding  118  of transformer  116  provides a source of voltage, which when rectified and filtered by diode  102  and capacitor  104  respectively, provide supply voltage V DD  to controller  106 . Feedback signal V FB  is developed from a rectified and filtered secondary supply +12V and fed back to controller  106  through opto-isolator  126 , thus establishing a feedback loop to control the switching on and off of MOSFET  108 . By comparison of feedback signal V FB  to a reference value in controller  106  and variation of the conduction cycle of MOSFET  108  in response to differences between the feedback signal and the reference level, regulation of operating levels in the SMPS can be realized. Resistor  112  senses the primary current flowing in MOSFET  108  which serves as the current feedback signal to current-mode controller  106 . Using current-mode control prevents excessive current to be drawn from the switch-mode supply under overload conditions. By rectifying a signal from transformer  116  secondary windings  122  and  124  by diodes  128  and  136  respectively, regulated output voltages +6.5V and +12V are developed and filtered by capacitors  132  and  134  respectively. Rectification of the signals developed across windings  122  and  124  may be accomplished by diodes in series with their respective windings between ground and the supply outputs. In this described typical SMPS, one of the diodes,  128 , is placed with its cathode connected to the positive output of its particular supply, thus causing both anode and cathode of diode  128  to be remote from ground. In the exemplary +12V supply, diode  136  is placed such that its anode is connected to ground. In the type rectifiers described in this exemplary switch-mode supply, an often large source of inefficiency is the voltage drop across the rectifier diodes. In higher power supplies the inefficiency introduced by the voltage drop across the diodes can be significant, thus requiring heat sinking and possibly active measures such as forced air cooling. 
     In order to improve the rectifier efficiency, a transistor, usually a MOSFET may be used as a low voltage-drop switch to replace a diode. This technique is referred to as synchronous rectification. Synchronous rectification requires control of the drive to the synchronous rectifier to turn the MOSFET on or off during the appropriate portions of the signal being rectified. Integrated circuit controllers are often used to control conduction of the MOSFET. These integrated circuits, such as the ST Microelectronics STS-R3, or Anachip AP436 are moderately expensive and require an additional 4 to 8 external components. These ICs often include clock generation circuits and other sophisticated methods to determine on/off control of synchronous rectifier MOSFETS. The present invention involves a less complex control circuit that may use discrete components to provide a low-cost implementation of synchronous rectification in a switch-mode power supply. 
     SUMMARY OF THE INVENTION 
     Certain aspects commensurate in scope with the originally claimed invention are set forth below; however the invention may encompass a variety of aspects that may not be set forth below. 
     The disclosed embodiments relate to an apparatus comprising a first device, possibly a transistor, configured to couple a first signal to a reference level when the first device is made conductive, a second device, possibly a differentiator or a high-pass filter, responsive to a second signal which may be out of phase with the first signal, the second device configured to control conduction of the first device during a portion of a period of the second signal, and a detector, possibly a diode peak detector, responsive to an amplitude of the second signal, the detector configured to alter a duration of conduction of the first device. In this apparatus the detector may reduce the duration of conduction of the first device in response to an increase in the amplitude of the second signal. 
     A further embodiment includes means for coupling a first signal to a reference level when said coupling means is conductive, means for placing said coupling means in a conductive state during a duration of a portion of a period of a second signal, and means for altering said duration of conduction of said coupling means in response to an amplitude of said second signal. 
     A yet further embodiment is a method comprising the steps of coupling a first signal to a reference level by conduction of a device, differentiating a second signal, controlling conduction of the device in response to the differentiated second signal, and altering conduction of the device in response to an amplitude of the second signal. Variations of this method may include reducing the duration of conduction of the device in response to an increase in the amplitude of the second signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present invention will be described below in more detail, with reference to the accompanying drawings in which similar elements in each figure have the same reference designator: 
         FIG. 1  is a block diagram of a typical switch-mode power supply; 
         FIG. 2  is a schematic diagram of an embodiment of the present invention; 
         FIG. 3  is a representative waveform of the drain voltage of the main switching MOSFET ( 108 ); and 
         FIG. 4  shows representative waveforms of the drain voltage and gate voltage of a synchronous rectifier MOSFET in an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     An embodiment of the discrete control circuit shown in  FIG. 2  satisfies the need for a low-cost, synchronous rectifier controller.  FIG. 2  shows a representative switch-mode power supply used in electronic equipment applications. The primary side circuit  150  is typical of switch-mode power supplies, is well known to those skilled in the art and is similar to that described previously. Switch-mode transformer  116  has multiple secondary windings,  122  and  124 , to develop different supply voltages. Diode  128  is used in this system as a conventional rectifier. Use of this high-side rectifier for the +6.5V supply serves the dual purposes of rectifying the signal S 3 , from winding  122  to generate the +6.5V supply and so that the AC signal, S 3 , at the anode of diode  128  will be available to develop a switching control signal to drive synchronous rectifier MOSFET  214 . MOSFET transistors can be used as rectifiers (synchronous rectification) by controlling the conduction time of the MOSFET, making the conduction coincide with the desired portion of the pulse waveform. Since the MOSFET can have much lower voltage drop than even a Schottky diode, the efficiency of the power supply can be improved. In most cases, when synchronous rectification is used, it is possible to eliminate the large heatsink normally used to cool the diodes. In the exemplary embodiment shown in  FIG. 2 , MOSFET  214  is arranged with its source connected to the secondary side ground. This configuration simplifies generating the drive signal to MOSFET  214 . The control voltage for the gate drive of synchronous rectifier  214  is developed from the signal S 3 . The pulse signal S 3  is of the opposite polarity to signal S 2  that appears at the drain of MOSFET  214 . This phase reversal is determined by the phasing of windings  122  and  124  and is done so that the polarity of signal S 3  is of the phase needed to turn on the MOSFET gate when signal S 2  is at its most negative level. Conduction from the drain to source of MOSFET  214  when signal S 2  is at its most negative level clamps signal S 2  to ground, thus rectifying signal S 2  to produce the +12V output. Controller  106  is designed such that signals S 2  and S 3  can have variable duty cycles; with the positive portion of signal S 3  increasing in duration at higher line voltage. As a result, means are necessary for shortening the duration of the pulse at MOSFET  214  gate so as to provide the proper conduction time for the MOSFET to assure that MOSFET  214  conducts only when signal S 2  is at its negative level. Capacitor  202  and resistor  204  form a high-pass filter that differentiates the waveform on signal S 3  to produce the drive waveform for the MOSFET  214  gate. Differentiation of the waveform aids to reduce the conduction time of MOSFET  214  so that MOSFET  214  turns on at or after the voltage at its drain is negative and turns off at or before its drain voltage goes high. A low power (by comparison to a conventional rectifier diode such as diode  128 ) diode  136  conducts during the negative excursion of signal S 2  during which time the MOSFET may not be turned on due to the gate drive interval to MOSFET  214  possibly being of lesser duration than the negative excursion of signal S 2 . The function just described for diode  136  could also be performed by an internal parasitic diode in MOSFET  214 . Diode  206  and capacitor  208  rectify signal S 3  to develop a negative bias voltage that is proportional to the value of unregulated voltage source  100  (and to the AC line input voltage). As the negative bias increases it reduces the average voltage at the gate of MOSFET  214  and thus reduces the conduction time of the MOSFET. The voltage divider formed by resistors  210  and  204  scale the negative bias developed by diode  206  and capacitor  208  to establish the desired range of negative bias added to the gate drive. This negative bias on the gate of MOSFET  214  prevents excess conduction (and increased losses) at high line voltages. Resistor  200  provides current limiting for the negative supply formed by diode  206 . Resistor  212  reduces the rise time of the drive voltage to MOSFET  214  gate to minimize radiated noise due to fast switching transients. 
     The waveform depicted in  FIG. 3  shows the signal S 1  at the drain of switch-mode MOSFET  110 , this voltage being the primary voltage on winding  120  of transformer  116 . The upper trace of  FIG. 4  depicts the signal voltage S 2  which is induced in secondary winding  124 , this being also the voltage on the drain of synchronous rectifier MOSFET  214 . The lower trace of  FIG. 4  depicts the gate drive to MOSFET  214 . The conduction threshold of MOSFET  214  is approximately 2.5 volts to 3.0 volts, as shown by R 1  and R 2  in  FIG. 4 . Projecting the point in time of the gate drive signal passing the conduction threshold onto the drain voltage waveform in the upper plot of  FIG. 4  indicates that the MOSFET  214  conduction is well within the interval of the negative excursion of signal S 2 . It can also be seen from the gate drive waveform in  FIG. 4  that increasing the negative bias applied to the gate of MOSFET  214  through detector  206 , as occurs at higher line voltage, will reduce the period of time that the gate drive voltage is above the conduction threshold R 1  or R 2 . 
     While the present invention has been described with reference to the preferred embodiments, it is apparent that various changes may be made in the embodiments without departing from the spirit and the scope of the invention.