Abstract:
A control circuit for switching power converters with synchronous rectifiers is disclosed for providing start-up and shut-down protection. The control circuit for switching power converters with synchronous rectifiers includes a device for blocking the driving signals to the synchronous rectifiers, a voltage sampling circuit, a reference voltage, and a comparator. The comparator compares a sample voltage to a reference voltage to determine when to block and when to admit driving signals to the synchronous rectifiers. The control circuit for switching power converters with synchronous rectifiers is particularly useful for minimizing component damage due to start-up and shut-down transients.

Description:
FIELD OF THE INVENTION 
     The present invention relates to control circuits for synchronous rectifiers and is particularly concerned with providing start-up and shut-down protection for such rectifiers. 
     BACKGROUND OF THE INVENTION 
     The use actively-controlled switches such as transistors, usually power MOSFETs or power BJTs, to replace diodes is a technique for improving the efficiency of rectification in power supplies. 
     In low voltage converters, for example around 10 volts and less, the voltage drop of a silicon rectifying diode (typically around 0.7 to 1 volt for a silicon diode at its rated current) has an adverse effect on efficiency. One solution known in the art is to replace standard silicon diodes with Schottky diodes, which exhibit very low voltage drops (as low as 0.5 volts). However, even Schottky rectifiers can be exhibit significant losses, notably at high currents and low voltages. 
     In power supplies using synchronous rectification, power switches replace the rectification diodes and are driven synchronously with the waveform to be rectified so that the switch is conducting during a portion of the waveform and blocking during the remaining portion of the waveform. 
     Several methods for driving synchronous rectifiers are known in the art. The basic categorization involves directly driven methods and self-driven methods. 
     With the directly driven methods, the driving signal is generated by an integrated controller device typically circuitry contained in an integrated circuit chip. The cost of the integrated controllers may be significant. 
     With the self-driven methods, the driving signal is generated using discrete components and extra transformer windings which produce the necessary signals for driving the synchronous rectifying devices. 
     By way of example of an existing solution, referring to  FIG. 1  there may be seen an electrical schematic diagram of a power supply having self-driven synchronous rectifying elements. Primary input voltage is provided at  101  to one side of the primary winding  103  of power transformer  102 . Coupled to the other side of primary winding  103  is capacitor  107  and first primary drive field effect transistor (FET)  109 . Second primary drive FET transistor  108  is connected to the other side of capacitor  107  and then to the other primary input voltage leads  111 . First primary FET transistor also connects to the other primary input voltage lead  111 . Gate drive for FET transistors  108  and  109  is provided at  110  and  112  respectively and consists of appropriate out-of-phase drive signals which alternately turn on and off FETs  108  and  109  to produce an alternating current in primary winding  103  of transformer  102 . 
     Secondary winding  104  of transformer  102  has two synchronous rectifying FET transistors, FET  114  and FET  115  which are driven by circuitry described below so as to conduct at appropriate times to rectify the voltage waveform produced across secondary winding  104 . Filter inductor  113  and filter capacitor  116  act to smooth voltage and current variations in the output current and voltage respectively. Resistor  118  and capacitor  117  are illustrative of loads on the power supply, while a secondary side ground reference point may be seen at  119   a.    
     Tertiary windings  105  and  106  of transformer  102  serve to produce the driving voltages for the synchronous rectifying FETs  114  and  115 . Voltage pulses produced at windings  105  and  106  due to the variations in current in the primary winding  103  are passed through capacitors  120  and  123  respectively, to the gates of FETs  114  and  115  respectively. Diodes  121  and  124  serve to provide a current path during the reverse voltage cycles of windings  105  and  106 , while resistors  122  and  125  ensure that the gates of FETs  114  and  115  will be turned off when no driving voltage is present. A secondary side ground reference point may be seen at  119   b , and in this example is conductively continuous with  119   a.    
     This solution may contain several disadvantageous operational characteristics. Firstly, during the converter start-up operation, the synchronous rectifier driving circuit can pull current out from the circuit load. Since the magnitude of this current for the driving is essentially uncontrollable, the current flowing from the circuit load represents significant risk of component failures on both sides, i.e. on the side of power converter and on the side of circuit load. In the industry, this problem is frequently referred to as the pre-biased output start-up problem. Secondly, during the converter shut-down operation, the driving circuit may generate driving voltages which can exceed the synchronous rectifier ratings introducing risk of the rectifiers and overall power converter failure. 
     In view of the foregoing, it would be desirable to provide a means for enabling such a synchronous rectifier power supply to operate reliably and fault-free during start-up and shut-down transients. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a method and apparatus for enabling a synchronous rectifier power supply to operate reliably and fault-free during start-up and shut-down transients. 
     According to a first aspect of the invention there is disclosed a power converter fed by a primary voltage, the power converter having synchronous rectifying elements and producing a secondary voltage, the power converter having driving circuitry for the synchronous rectifying elements; blocking circuitry connected between the driving circuitry and the synchronous rectifying elements; secondary voltage sampling circuitry; and control circuitry which activates the blocking circuitry in the event that the secondary voltage sampling circuitry indicates a secondary voltage below a preconfigured value. 
     In some embodiments of this aspect the control circuitry has a comparator; the voltage sampling circuit is connected to a first input of the comparator; a reference voltage circuit is connected to a second input of the comparator; and the output of the comparator is connected to the blocking circuitry for controlling the blocking operation. 
     In some of these embodiments the power converter has a power transformer having primary, secondary and tertiary windings; the secondary windings producing the secondary voltage; and the tertiary windings connected to the driving circuitry. In some of these embodiments there is a local power supply circuit connected to the tertiary windings. In some of these embodiments the local power supply circuit has a rectifying diode having an anode connected to the tertiary windings; a filter capacitor connected to the rectifying diode; and a load resistor. 
     In some embodiments the reference voltage circuit has a Zener diode and a resistor in series; the resistor connected to the local power supply circuit at a proximate end; the resistor connected at a distal end to the Zener diode&#39;s cathode; the Zener diode connected to a local circuit ground at the anode end; and the junction of the Zener diode and the resistor connected to the comparator input. 
     According to another aspect of the invention there is disclosed a method of controlling a power converter fed by a primary voltage and producing a secondary voltage; the power converter having synchronous rectifying elements controlled by a control voltage; the method having the steps of sensing the secondary voltage, and in the event that the secondary voltage is below a predetermined value, blocking the control volt from reaching the synchronous rectifying element. 
     In some of these embodiments the synchronous rectifying elements are Field Effect Transistors. In some embodiments of this aspect of the invention the sensing step is performed by a comparator comparing the secondary voltage to a reference voltage. In some embodiments of this aspect of the invention, the blocking step is performed by bipolar transistors controlled by the output of the comparator. 
     Note: in the following the description and drawings merely illustrate the principles of the invention. It will thus be appreciated that those skilled in the art will be able to devise various arrangements that, although not explicitly described or shown herein, embody the principles of the invention and are included within its spirit and scope. Furthermore, all examples recited herein are principally intended expressly to be only for pedagogical purposes to aid the reader in understanding the principles of the invention and the concepts contributed by the inventor(s) to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions. Moreover, all statements herein reciting principles, aspects, and embodiments of the invention, as well as specific examples thereof, are intended to encompass equivalents thereof. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be further understood from the following detailed description of embodiments of the invention, with reference to the drawings in which like reference numbers are used to represent like elements, and: 
         FIG. 1  illustrates an exemplary self-driving synchronous power supply circuit diagram according to the prior art; 
         FIG. 2  illustrates an exemplary self-driving synchronous power supply circuit diagram with new elements according to an embodiment of the invention; 
         FIG. 3  illustrates an exemplary self-driving synchronous power supply circuit diagram with specific circuit elements according to an embodiment of the invention; and 
         FIG. 4  illustrates series of voltage waveforms which may be found in association with the operation of the self-driving synchronous power supply of  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, numerous specific details are set forth. However, it is understood that embodiments of the invention may be practiced without these specific details. In other instances, well-known circuits, structures and techniques have not been shown in detail in order not to obscure the understanding of this description. It will be appreciated, however, by one skilled in the art that the invention may be practiced without such specific details. Those of ordinary skill in the art, with the included descriptions, will be able to implement appropriate functionality without undue experimentation. 
     References in the specification to “one embodiment”, “an embodiment”, “an example embodiment”, etc., indicate that the embodiment described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is submitted that it is within the knowledge of one skilled in the art to effect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described. 
     In the following description and claims, the terms “coupled” and “connected,” along with their derivatives, may be used. It should be understood that these terms are not intended as synonyms for each other. “Coupled” is used to indicate that two or more elements, which may or may not be in direct physical or electrical contact with each other, cooperate or interact with each other. “Connected” is used to indicate the establishment of communication between two or more elements that are coupled with each other. 
     In the following figures, like reference numbers are used to represent like elements. 
     Referring to  FIG. 2  there may be seen a circuit diagram of a self-driven synchronous power supply with new elements according to an embodiment of the invention. Primary input voltage is provided at  201  to one side of the primary winding  203  of power transformer  202 . Coupled to the other side of primary winding  203  is capacitor  207  and first primary drive field effect transistor (FET)  209 . Second primary drive FET  208  is connected to the other side of capacitor  207  and then to the other primary input voltage lead  211 . First primary drive FET also connects to the other primary input voltage lead  211 . Gate drive for FETs  208  and  209  is provided at  210  and  212  respectively and consists of appropriate out-of-phase drive signals which alternately turn on and off FETs  208  and  209  to produce an alternating current in primary winding  203  of transformer  202 . 
     Secondary winding  204  of transformer  202  has two synchronous rectifying FETs, FET  214  and FET  215  which are driven by circuitry described below so as to conduct at appropriate times to rectify the voltage waveform produced across secondary winding  204 . Filter inductor  213  and filter capacitor  216  act to smooth voltage and current variations in the output current and voltage respectively. Resistor  218  and capacitor  217  are illustrative of loads on the power supply, while a secondary side ground reference point may be seen at  219   a.    
     Tertiary windings  205  and  206  of transformer  202  serve to produce the driving voltages for the synchronous rectifying FETs  214  and  215 . Voltage pulses produced at windings  205  and  206  due to the variations in current in the primary winding  203  are passed through capacitors  220  and  223  respectively, to the gates of FETs  214  and  215  respectively through the blocking circuits  258  and  256 . Diodes  221  and  224  serve to provide a current path during the reverse voltage cycles of windings  205  and  206 , while resistors  222  and  225  ensure that the gates of FET transistors  214  and  215  will be turned off when no driving voltage is present. A secondary side ground reference point may be seen at  219   b , and in this example is conductively continuous with  219   a.    
     Power supply block  250  derives a supply voltage from the tertiary windings  205  and  206  of power transformer  202  through capacitors  220  and  223 . The power supply voltage of power supply block  250  is used to power comparator circuitry  254  which is provided with a sample  252  of the secondary voltage and a reference voltage  235 . In operation, insofar as the sampled voltage of the secondary voltage is some amount below the reference voltage  235 , the comparator will operate to activate the two blocking circuits  256  and  258 . These blocking circuits act to block the synchronous drive signals from capacitors  220  and  223  during portions of the start-up and shut-down operation of the power supply. More specifically, these blocking circuits can act to block the synchronous drive signals when the secondary output voltage is below a pre-established amount. 
     The various blocks of power supply  250 , comparator  254 , blocking circuits  256  and  258 , and voltage sample block  252  may be realized in alternative ways evident to one skilled in the art.  FIG. 3  provides a circuit schematic of one exemplary way to realize these circuit blocks. 
     Referring to  FIG. 3  there may be seen a circuit diagram of a self-driven synchronous power supply with elements having reference numbers similar to corresponding elements in  FIG. 2 . Primary input voltage is provided at  301  to one side of the primary winding  303  of power transformer  302 . Coupled to the other side of primary winding  303  is capacitor  307  and first primary drive field effect transistor (FET)  309 . Second primary drive FET  308  is connected to the other side of capacitor  307  and then to the other primary input voltage lead  311 . First primary drive FET also connects to the other primary input voltage lead  311 . Gate drive for FETs  308  and  309  is provided at  310  and  312  respectively and consists of appropriate out-of-phase drive signals which alternately turn on and off FETs  308  and  309  to produce an alternating current in primary winding  303  of transformer  302 . 
     Secondary winding  304  of transformer  302  has two synchronous rectifying FETs, FET  314  and FET  315  which are driven by circuitry described below so as to conduct at appropriate times to rectify the voltage waveform produced across secondary winding  304 . Filter inductor  313  and filter capacitor  316  act to smooth voltage and current variations in the output current and voltage respectively. Resistor  318  and capacitor  317  are illustrative of loads on the power supply, while a secondary side ground reference point may be seen at  319   a.    
     Tertiary windings  305  and  306  of transformer  302  serve to produce the driving voltages for the synchronous rectifying FETs  314  and  315 . Voltage pulses produced at windings  305  and  306  due to the variations in current in the primary winding  303  are passed through capacitors  320  and  323  respectively, to blocking circuits  356  and  358  respectively and then to the gates of FET transistors  314  and  315  respectively. Diodes  321  and  324  serve to provide a current path during the reverse voltage cycles of windings  305  and  306 , while resistors  322  and  325  ensure that the gates of FET transistors  314  and  315  will be turned off when no driving voltage is present. Resistors  322  and  325  are terminated at secondary ground reference points  319   c  and  319   d  respectively, which are continuous with  319   a . A secondary side ground reference point may also be seen at  319   b , and is likewise conductively continuous with  319   a.    
     Power supply block  350  derives a supply voltage from the tertiary windings  305  and  306  of power transformer  302  through capacitors  320  and  323 . Diodes  326  and  327  rectify the supplied voltage pulses while filter capacitor  328  and load resistor  329  smooth the provided voltage so that it may serve as a supply rail to comparator  330 . Resistor  329  has a value that provides the desired hold-up time for the power supply block. Power supply block  350  is referenced to secondary side ground  319   e  which is conductively continuous with the other secondary side ground references. 
     Comparator  330  is powered, as described, by power supply block  350  and is further referenced to secondary side ground  319   f  which is conductively continuous with the other secondary side ground references for its other power rail. Comparator  330  is provided by a reference voltage  335  at its non-inverting input. This reference voltage may be derived by any convenient means known in the art, for example via a resistor-Zener diode pair or a bandgap voltage reference source. The inverting input of comparator  330  is supplied with a voltage sample  334  generated by the resistor stack of resistors  332  and  333 . The voltage sample  334  will be a fraction of the secondary voltage, the fraction defined by the ratio of resistors  332  and  333 . The output of comparator  330  is fed through resistor  331  to blocking circuits  356  and  358 . 
     Blocking circuit  356  has diode  346 , PNP transistor  347 , resistor  348 , and diode  349  connected as shown. Likewise, blocking circuit  358  has diode  336 , PNP transistor  337 , resistor  338 , and diode  339  connected as shown. In order for synchronous pulses from capacitors  320  and  323  to reach the gates of FETs  314  and  315  respectively, the output of comparator  330  must be pulling the cathodes of diodes  346  and  336  low so that transistors  347  and  337  will be rendered non-blocking. 
     Other gate current control circuits than are depicted in  FIGS. 2 and 3  may be used between synchronous rectifier transistor gate (e.g. gate of FET transistor  315 ) and driver output (e.g. collector of transistor  347 ). For example, two parallel resistors of different values, one connected directly between the synchronous rectifier transistor gate and driver output and the second one in series with a diode with the diode&#39;s anode is connected to the synchronous rectifier FET transistor gate. Alternatively, capacitors connected between the synchronous rectifier transistor gate and ground, are another example of gate control circuits. 
     The operation of the circuit will now be described in conjunction with the exemplary component arrangement of  FIG. 3  and the voltage waveforms depicted in  FIG. 4 . 
     In operation, the synchronous activity occurs repetitively with respect to a switching cycle with period T. The voltage waveform of the primary winding  303 , designated as V p    460  in  FIG. 4 , produces voltages in  302  transformer tertiary windings  305  and  306 , which couple with voltages across capacitors  320  and  323  respectively, generate driving voltages designated as V df    262  and V dr    264  in  FIG. 2 ; V df    362  and V dr    364  in  FIG. 3 ; and V df    462  and V dr    464  in  FIG. 4 , respectively. 
     Diodes  326 ,  327  and capacitor  328  generate proper supply voltage for comparator  330  relatively quickly upon initial start-up from the driving signals V df    462  and V dr    464 . As a result of the rapid power-up the comparator is ready to control the synchronous rectifier driving circuit via the blocking circuits almost immediately upon driving signals occurring upon the tertiary windings. 
     During the start-up or shut-down time intervals, when the secondary output voltage is such that the voltage reference derived from the resistor divider  332  and  333  is below the reference voltage  335 , the comparator  330  sets its output high and through resistor  331  turns the transistors  337  and  347  off. Consequently, the gates of the synchronous rectifiers  314  and  315  are pulled low via resistors  322  and  325 . The synchronous rectifiers are disabled. While the synchronous rectifiers  314  and  315  are disabled, the intrinsic diodes  314   i  and  315   i  allows the converter to operate. However, the current cannot flow from the load capacitor  317  back to the converter. 
     When, during the start-up or shut-down time, the secondary output voltage rises such that the voltage reference derived from the resistor divider  332  and  333  is above the reference voltage  335 , the comparator  330  sets its output low and through resistor  331  turns the transistors  337  and  347  on. The synchronous rectifiers are enabled to operate. 
     During the shut-down operation in a self-driven synchronous power supply, the power supply circuits will see collapsing voltages and currents. At certain point, the power supply control circuitry will stop operating however, the energy stored in inductances and capacitors may still be significant. Consequently, the power supply may enter into an undesirable oscillation state resulting in excessive tertiary winding voltages V df    462  and V dr    464 . In some cases, these voltages may reach a magnitude of several tens of volts significantly exceeding the gate-to-source voltage rating of the synchronous rectifier transistors  314  and  315 . As a result, the synchronous rectifier transistors  314  and  315  may fail. 
     As embodiments of this invention essentially disconnects the driving voltages V df    462  and V dr    464  from the synchronous rectifiers transistors  314  and  315 , protection is provided during this vulnerable time period. In order for reliable operation, it is preferable that the blocking circuit components satisfy the following component ratings, where: 
     V ce,e  is the collector-to-emitter rating of the transistors  337  and  347 ; 
     V r,s  is the reverse voltage rating of the diodes  339  and  349 ; and 
     V gs,sr  is the gate-to-source voltage rating of the synchronous rectifiers  315  and  315 . 
     And the conditions to be satisfied are:
 
V ce,e &gt;&gt;V gs,sr  
 
V r,s &gt;&gt;V gs,sr  
 
     Therefore what has been disclosed is a means for enabling a synchronous rectifier power supply to operate reliably and fault-free during start-up and shut-down transients. 
     Numerous modifications, variations and adaptations may be made to the embodiment of the invention described above without departing from the scope of the invention, which is defined in the claims.