1.  Field on the Invention
The present invention relates to A.C. voltage rectifying elements.  The present invention more specifically relates to the implementation of a rectifying function (diode) by means of a MOS transistor. 
2.  Discussion of the Related Art
FIG.  1 very schematically shows a first example of a circuit using rectifying elements of the type to which the present invention may apply.  In this example, it is a so-called forward-type converter.  Such a converter is essentially formed of a transformer T having a primary winding T1 receiving a D.C. voltage provided by a capacitor C1 connected to the output of a diode bridge B supplied with an A.C. voltage.  Winding T1 is grounded by a switch K. Switch K is controlled in pulse-width modulation by a PWM signal (high-frequency with respect to the A.C. power supply) set to regulate a D.C. voltage Vout provided by the converter.  Voltage Vout is provided on the secondary side T2 across a capacitor C2 storing the power transferred from primary T1 to secondary T2 of the transformer during periods when switch K is on.  A first end of secondary winding T2 is connected, by a diode D1 in series with an inductance L1, to a first electrode of capacitor C2 defining a positive output terminal, while its other end is directly connected to the other electrode of capacitor C2 defining ground M2 on the side of voltage Vout.  A free wheel diode D2 connects the junction point of diode D1 and of inductance L1 to ground M2, the anode of the diode being on the ground side.  The operation of such a converter is known. 
The rectifying elements are here formed of diodes D1 and D2 which have the disadvantage of exhibiting a threshold voltage on the order of from 0.3 to 1.5 volts, which adversely affects the converter operation, especially in low-voltage applications. 
FIG.  2 partially and schematically illustrates a modification applied to a PWM converter of FIG.  1 to decrease the threshold voltage of the rectifying elements.  In FIG.  2, only a portion of the secondary has been shown, the rest being similar to FIG.  1.  To decrease the threshold voltage of diodes D1 and D2, said diodes are replaced with two N-channel MOS transistors N1 and N2 which are adequately controlled by a specific circuit CTRL.  For voltage reference reasons, transistor N1 replacing diode D1 must however be placed on the ground branch of the converter, while transistor N2 can be placed in the same way as diode D2 of FIG.  1.  Control circuit CTRL further receives a supply voltage SUPPLY as well as a signal SYNCH of synchronization with respect to the switching of the D.C. voltage performed on the primary side, to synchronize the respective turn-off and turn-on times of transistors N1 and N2 with the turn-off and turn-on times of switch K (not shown in FIG.  2). 
A disadvantage of the synchronous rectifying circuit of FIG.  2 is that transistors N1 and N2 cannot have an autonomous operation.  They need a synchronization signal coming from the primary as well as a supply voltage. 
Another disadvantage is the presence of a MOS transistor on the ground line and not on the high line on the secondary side. 
FIG.  3 illustrates another example of a voltage converter to which the present invention applies.  It is a D.C./D.C. converter having the function of raising an output voltage Vout with respect to the level of an input voltage V1 provided, for example, by a battery.  The positive electrode of battery V1 is connected to a first end of an inductance L having its other end connected, by a first MOS transistor N1, to a first electrode of an output capacitor across which is sampled output voltage Vout.  The junction point of inductance L and of transistor N1 is further connected, by a transistor N2, to the ground defined by the negative electrode of battery V1 to which the second electrode of capacitor C is connected.  In such an application, the control of transistors N1 and N2 is particularly difficult since it requires a level shifter to control transistor N1 which has no ground reference. 
It would be desirable to have a rectifying element with a low threshold voltage, which does not pose the problems of MOS transistor control in conventional configurations.