Abstract:
A controllable rectifying element, comprising a bipolar transistor having a current input terminal connected to a control terminal by a first switch and having a current output terminal connected to the control terminal by a second switch, the turn-off and turn-on phases of the first and second switches being complementary and depending on the state desired for the rectifying element.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to the forming in integrated form of a rectifying element or rectifier. 
   2. Discussion of the Related Art 
   In the present description, an element performing a function identical to that of a bipolar diode, for example, used in D.C./D.C. converters of voltage step-down or step-up type, is designated as a rectifier. 
     FIG. 1  shows a conventional example of a D.C./D.C. voltage step-up converter  1  (CONV) supplying a load  2  (Q). Converter  1  provides a D.C. voltage Vout between terminals S and M of a storage capacitor  3  from a D.C. supply voltage Vin of lower level, applied between two input terminals E and M, terminal M forming, for example, a common ground with load  2 . Load  2  is connected between terminals S and M of capacitor  3 . For example, load  2  is formed of two light-emitting diodes of a backlighted screen. 
   Converter  1  comprises, in series between terminals E and S, an inductance  4  and a rectifying element  5  (RECTIF). Rectifier  5  is oriented to enable flowing of a current from inductance  4  to capacitor  3 . A controllable switch  6  is connected between input terminal  7  of rectifier  5  and reference rail M. Generally, switch  6  is a MOS transistor having its control terminal formed by its gate receiving a pulse-width modulated signal PWM intended to regulate voltage Vout from either a reference voltage, or the load needs. The PWM signal, depending on the application, is provided by a control circuit not shown. 
   The rectifier is intended to act as a diode, that is, to block the reverse current when voltage Vout across the load becomes greater than the voltage across switch  6 . 
   In the simplest applications, rectifier  5  is formed of a simple diode, generally of a Schottky diode. Such a Schottky diode has the disadvantage of being uneasily integrable. 
   Further, in many applications and especially in the case of a switched-mode application such as illustrated in  FIG. 1 , it is desired to be able to turn off the load supply circuit, either due to the presence of an electric problem therein (for example, a short-circuit), or for other reasons. Such a function of a rectifier element is generally designated as a true shutdown. Such a function especially enables, if switch  6  is off, preventing supply voltage Vin present between terminals E and M from recharging capacitor  3  while this is not desired. This especially enables performing a protection function by preventing overcurrents in case of a short-circuit on the load side (between terminals S and M). 
   To perform this function, a rectifier  5  formed of a diode D in series with a switch  8  is thus generally used, as illustrated in  FIG. 1 . Switch  8  is controlled by a circuit  9  (CTRL) receiving an enable signal EN from a control organ of the converter. Enable signal EN is used, for example, to interrupt the converter operation by the turning-off of switch  8  in case of a short-circuit in the load (detected by means not shown), in case of a problem on the side of the switched-mode power supply (PWM) control circuit, or more generally as soon as a turning-off of the converter is desired to be guaranteed. In practice, switch  8  is generally formed of a MOS transistor. 
   A disadvantage of rectifier  5  of  FIG. 1  is the addition, in normal operation, of a voltage drop due to the on-state series resistance of switch  8  (Ron of the MOS transistor). 
   Another disadvantage is linked to the bulk of the high-voltage MOS transistor forming switch  8 . 
   It has also been provided to form the rectifier in the form of a single MOS transistor by using its parasitic bulk-source diode. Such a solution requires being able to control the bulk connection to ground to control the parasitic diode. When a circuit shutdown is desired, the polarities of the MOS transistor and of its bulk are reversed to reverse the polarity of its parasitic diode. Another disadvantage is that the actual transistor must be controlled at the frequency of the PWM signal. In normal operation, the transistor is controlled in reverse fashion with respect to switch  6 . 
   In addition to the difficulty linked to the need for controlling the transistor at the frequency of the PWM signal, the source of the transistor is floating, which poses problems of control reference of its gate signal. This generally results in using level-shifting circuits, which makes the circuit particularly complex, especially for a high-voltage operation. 
   The series association of diode D with switch  8  could have been devised to be replaced by a thyristor. However, such a solution is incompatible with switched-mode power supplies. Indeed, a thyristor would require being controlled to be turned off each time switch  6  is off. Now, the frequencies of the trains of width-modulated pulses are generally of several tens, or even hundreds of kilohertz, which is incompatible with switching rates currently available for thyristors. 
   SUMMARY OF THE INVENTION 
   The present invention aims at providing a novel controllable rectifier which has advantages over known solutions. 
   The present invention especially aims at providing a controllable resistor capable of isolating, at its turning-on, the elements that it separates. 
   The present invention also aims at providing a rectifier in which the series voltage drop in normal operation is minimized. 
   The present invention also aims at providing a solution compatible with an integration and which is of minimum bulk. 
   To achieve these and other objects, the present invention provides a controllable rectifying element, comprising a bipolar transistor having a current input terminal connected to a control terminal by a first switch and having a current output terminal connected to the control terminal by a second switch, the turn-off and turn-on phases of the first and second switches being complementary and depending on the state desired for the rectifying element. 
   According to an embodiment of the present invention, the rectifying element further comprises a circuit for controlling the first and second switches according to the state of a signal for enabling/disabling the rectifying element. 
   According to an embodiment of the present invention, said switches are formed of P-channel MOS transistors having their respective gates connected to the control terminal of the bipolar transistor via current sources. 
   According to an embodiment of the present invention, the control circuit comprises two N-channel MOS transistors connecting the respective gates of the P-channel MOS transistors to ground, said N-channel MOS transistors being respectively controlled by the enable signal and by its inverse. 
   According to an embodiment of the present invention, said bipolar transistor is a PNP transistor. 
   According to an embodiment of the present invention, said bipolar transistor is an NPN transistor. 
   The present invention also provides a voltage converter of D.C./D.C. type comprising a rectifying element. 
   The foregoing objects, features, and advantages of the present invention will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1 , previously described, schematically and partially illustrates a D.C./D.C. converter of voltage step-up type comprising a known rectifying element; 
       FIG. 2  partially and schematically shows an embodiment of a voltage step-up converter comprising a controllable rectifying element according to the present invention; and 
       FIG. 3  partially and schematically shows a detailed embodiment of the rectifying element of  FIG. 2 . 
   

   DETAILED DESCRIPTION 
   Same elements have been designated with same reference numerals in the different drawings. For clarity, only those elements that are necessary to the understanding of the present invention have been shown in the drawings and will be described hereafter. In particular, the generation of the different control signals has not been shown and is no object of the present invention. The present invention applies whatever the reason for which the rectifying element is desired to be made controllable. The generation of the enable/disable signal, be it based on a detection of a short-circuit in the load or of another malfunction, remains conventional. 
   Further, the present invention will be described hereafter in relation with an example of application to a D.C./D.C. converter of switched mode power supply type. It should however be noted that it more generally applies to the forming of a controllable rectifying element whatever the application. 
     FIG. 2  very schematically shows in a view to be compared with that of previously-described  FIG. 1  an embodiment of a D.C./D.C. converter  10  of switched-mode power supply type using a rectifier  15  specific to the present invention. As previously, the converter comprises two input terminals E and M between which is applied a voltage Vin and two output terminals S and M (terminal M forming for example a common ground terminal) for providing a voltage Vout to a load  2  (Q). Converter  10  comprises, in series between terminals E and S, an inductance  4  and rectifying element  15 , and between terminals S and M, a storage capacitor  3 . A switch  6  controlled by a pulse-width modulation signal (PWM) of high frequency (generally several tens of kilohertz) connects an input terminal  7  of rectifying element  15  to ground M. 
   Converter  10  of  FIG. 2  differs from that of  FIG. 1  only by the structure of its rectifier  15 . 
   According to this embodiment of the present invention, rectifier  15  is formed of a PNP-type bipolar transistor  18  having its emitter forming input terminal  7  of rectifier  15  and having its collector forming the output terminal connected to terminal S. 
   According to the present invention, the base of transistor  18  is connected to its emitter by a switch  11  and to its collector by a switch  12 . Switches  11  and  12  are controlled by a circuit  19  (CTRL) receiving an enable signal EN of the rectifier. Signal EN originates, like for a conventional controllable rectifier, from an anomaly detection circuit (short-circuit or other) of the control circuit of switch  6  of the switched-mode power supply, etc. 
   The function of control circuit  19  is to control switches  11  and  12  by making sure, among others, that they are not simultaneously on. 
   In normal operation, switch  12  is on while switch  11  is off. Transistor  18  is thus diode-connected, which implies that it operates at the limit of the linear state. Thus, the voltage drop thereacross is minimum and is equivalent to that of a forward-biased bipolar diode (forward-biased base-emitter junction). In normal operation, switches  11  and  12  remain in this state (no switching) and the converter operates by control of switch  6 . 
   When enable signal EN is switched to a low state indicative of a disabling of the circuit, control circuit  19  turns on switch  11  and turns off switch  12 . Transistor  18  is then equivalent to its base-collector junction which, it being a PNP transistor, is effectively reverse-biased if input voltage Vin is greater than voltage Vout across capacitor  3 . 
   An advantage which already appears from the description of  FIG. 2  is that the voltage drop of controllable rectifying element  15  in the on state is smaller than that of conventional circuit  5 . According to the present invention, the voltage drop of an on-state diode is reproduced. 
   Another advantage is that switches  11  and  12 , although having to stand a high voltage between their terminals, only see the flowing of a very small current in the on state. Indeed, whatever the operating mode and due to the fact that transistor  18  is not saturated, the current likely to run through switch  11  and  12  is, with respect to the current flowing between the emitter and collector of transistor  18 , divided by the gain (β) of the transistor. 
   In practice, this results in permitting the surface area of each switch  11  and  12  to be small, in an integrated implementation, to approximately correspond to the surface area required to form transistor  18 , divided by the gain thereof. 
     FIG. 3  shows an embodiment of a control circuit  19  for controlling switches  11  and  12  of a rectifier  15  according to the present invention. In this example, switches  11  and  12  are formed of P-channel MOS transistors. The respective gates of each transistor are connected to ground M by an N-channel MOS transistor, respectively,  21 ,  22 , and base  23  of transistor  18  is connected to the gates of transistors  11  and  12  by current sources, respectively  24 ,  25 . The respective gates of transistors  21  and  22  receive enable signal EN, via an inverter  26  for one of the transistors (for example, transistor  21 ). It is here assumed that signal EN is active in the high state. 
   In normal operation, signal EN is thus high and imposes the turning-on of transistor  22  and the turning-off of transistor  21 . Since transistor  22  is on, the gate of transistor  12  is pulled to ground, which turns it off and short-circuits terminals  23  and S. As for transistor  11 , it remains off, its gate being in the air. 
   Upon switching of signal EN to the low state, transistor  21  turns on while transistor  22  turns off. In the absence of current source  25 , the turning-off of the transistor would make the gate of transistor  12  floating. Current source  25  thus enables carrying the charges away therefrom to guarantee its turning-off. Similarly, current source  24  is used to evacuate the charges from MOS transistor  11  upon turning-off of transistor  21  when signal EN switches high. Current sources  25  and  24  are formed, in the simplest fashion, of a resistor or, as an alternative, of any element performing this function, for example, MOS transistors adequately controlled by signal EN. 
   The embodiment of  FIG. 3  uses P-channel MOS transistors for switches  11  and  12 , which is preferable since this avoids use of a level shifter to control an N-channel MOS transistor. 
   An advantage of the present invention is that the integration surface necessary to form rectifier  15  is considerably reduced as compared to the conventionally formed rectifier. In particular, in a technology where MOS transistors are formed vertically, P-channel MOS transistors  11  and  12 , which must stand the high voltage but only need letting through a small current, take up a small surface area (proportional to the current that they must stand). 
   Another advantage is that the rectifying element is easily integrable, if need be with its control circuit. 
   Another advantage, induced by the circuit of the present invention and the use of a bipolar transistor, is that the current flowing through the rectifying element is automatically limited. Indeed, the base current of bipolar transistor  18  is a function of the current between its emitter and its collector, and thus of the current in inductance  4 . Accordingly, a variation of the current in inductance  4  translates as a variation in the operating point of transistor  18 . 
   Of course, the present invention is likely to have various alterations, modifications, and improvement which will readily occur to those skilled in the art. In particular, it has been assumed hereabove that transistor  18  is a PNP-type transistor. It may however be an NPN-type transistor. The modification of the connections to enable use of an NPN-type transistor is within the abilities of those skilled in the art based on the above functional description. In particular, switches  11  and  12  are then adapted to enable injection of a base current in normal operation. 
   Further, although the present invention has been described in relation with an application to a voltage step-up converter, it also applies to voltage step-down converters and, more generally, as soon as a controllable rectifying element is desired to be used. 
   Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and the scope of the present invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. The present invention is limited only as defined in the following claims and the equivalents thereto.