BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention generally relates to the protection of switches. More specifically, the present invention relates to the protection of a bidirectional switch upon occurrence of an overcurrent, resulting for example from a short-circuit in a load controlled in A.C. mode by the switch. 
   2. Discussion of the Related Art 
     FIGS. 1A and 1B  illustrate a known method for protecting a bidirectional switch for controlling an A.C. load against overcurrents. 
     FIG. 1A  schematically and partially illustrates a load  1  series-connected with a bidirectional switch  2  across an A.C. voltage source  3 . Bidirectional switch  2  is formed by the anti-series connection, between two terminals A and C, of two insulated-gate bipolar transistors (IGBT)  4  and  5 . “Anti-series” means that IGBT transistors  4  and  5  are in series and that their junction point is a common emitter point. Each terminal A and C is then connected to the collector of one of transistors  4  and  5 . For clarity, the collector of transistor  4  is designated in the following description of  FIG. 1  as “A” and the collector of transistor  5  is designated as “C”. The emitter of each of transistors  4  and  5  is connected to its respective collector A, C by a respective diode D 1 , D 2 . Each diode D 1 , D 2  is in antiparallel with the junction of its associated transistor  4 ,  5 . Collector A of transistor  4  is connected to a supply terminal of load  1 . Collector C of transistor  5  is connected to a terminal of source  3 , non-connected to load  1 . A circuit  6  for controlling and protecting transistor  4  is connected between its gate G 1  and its collector A. Similarly, for transistor  5 , another control and protection circuit  7  is connected between its gate G 2  and its collector C. The two protection circuits  6  and  7  are identical. 
     FIG. 1B  schematically and partially illustrates a protection circuit ( 6  or  7 ,  FIG. 1A ) connected between a collector A or C and a gate G 1  or G 2 . A control block  10  (CTRL) comprises two supply terminals respectively connected to a high supply rail Vcc and a low supply or ground rail GND connected to the emitter of protected transistor  4  or  5 . An output terminal of block  10  is connected to an end of a resistor  11  having another end forming the output terminal of protection circuit  6 , respectively  7 , connected to gate G 1 , respectively, G 2 , of transistor  4 , respectively  5 . An input terminal of block  10  is connected to an output terminal OUT of a comparator  12 . Two supply terminals of comparator  12  are respectively connected to high supply rail Vcc and low supply rail GND. An inverting input IN 1  of comparator  12  is connected to a reference D.C. voltage source (V)  13 . A non-inverting input IN 2  of comparator  12  is connected to high supply rail Vcc, via a biasing resistor  14 . Non-inverting input IN 2  is also connected to the anode of a diode  15  having its cathode connected to collector A, respectively C, of protected unidirectional switch  4 , respectively  5 . Circuits  6 , respectively  7 , ensure their protection function by controlling gate G 1 , respectively G 2 , of transistor  4 , respectively  5 , according to the result of the comparison, by comparator  12 , of the current value of the collector-emitter voltage with voltage reference V provided by source  13 . 
   The protection circuit of  FIG. 1B  enables controlling collector-emitter voltage Vce across protected transistor  4  or  5  by means of comparator  12 . Given the current-vs.-voltage characteristic of a transistor, a voltage Vce unusually high as compared to the reference set by source  13  corresponds to the occurrence of an overcurrent, linked to a malfunction of load  1  or of source  3 . Diode  15  is a protection diode intended to protect non-inverting (+) input IN 2  of comparator  12 , especially when protected transistor  4  or  5  is off. 
   A disadvantage of the structure previously described in relation with  FIGS. 1A and 1B  is the need to repeat twice a protection circuit of a one-way switch to obtain a bidirectional switch protection circuit. 
   Another disadvantage of the previous structure is that protection diodes  15  of circuits  6 ,  7  must be able to hold a high voltage, especially when switch  2  is off. High-voltage diodes are relatively complex and bulky to make in integrated form. 
   It has previously been considered that bidirectional switch  2  is formed of the anti-series connection of two IGBT transistors, each being associated with a free wheel diode in anti-parallel. However, the same disadvantages are encountered if the transistors are of MOS type. 
     FIG. 2  schematically and partially illustrates another known embodiment in which bidirectional switch  2  is formed of the antiparallel connection of two IGBT or MOS transistors of same conduction type T 1  and T 2 , each transistor T 1 , T 2  being in series with a respective rectifying diode D 3 , D 4 . For clarity, the connection of switch  2  in series with load  1  across A.C. source  3  described in relation with  FIG. 1A  is indicated only by the mentioning of terminals A and C in  FIG. 2 . Terminal A is connected to the anode of diode D 3 , the cathode of which is connected to the collector of transistor T 1 . Terminal A is also connected to the cathode of diode D 4 , the anode of which is connected to the emitter of transistor T 2 . Terminal C is connected to the emitter of transistor T 1  and to the collector of transistor T 2 . 
   Protection circuit  17  of switch  2  here is comprised of two separate comparators  121  and  122 . Non-inverting (+) input IN 21  of comparator  121  is connected to the anode of a diode  151  having its cathode connected to the collector of transistor T 1  (cathode of diode D 3 ). The inverting (−) input IN 11  of comparator  121  receives a reference voltage Vref+, positive with respect to the ground defined by one of the two terminals of switch  2 , for example, terminal C, and provided by a voltage source  131 . 
   Non-inverting (+) input IN 22  of comparator  122  is connected to the cathode of a diode  152  having its anode connected to the emitter of transistor T 2  (anode of diode D 4 ). Inverting (−) input IN 12  of comparator  121  receives a reference voltage Vref−, negative with respect to ground GND and provided by a second voltage source  132 . 
   The respective outputs OUT 1  and OUT 2  of comparators  121 ,  122  are connected to input terminals of a control circuit (not shown) driving, generally via resistors (not shown), gates G 1  and G 2  of transistors T 1  and T 2 . 
   The supply of comparator  121  is ensured by a source  133  of a positive supply voltage +Vcc connected between a supply terminal of comparator  121  and ground GND. Similarly, a source  134  of a negative supply voltage −Vcc is connected between a supply terminal of comparator  122  and ground GND. 
   The operating principle of protection circuit  17  of  FIG. 2  is similar to that of a protection circuit  6 ,  7  of  FIGS. 1A and 1B , voltage Vce of each transistor being compared with a respective reference Vref+, Vref− set by respective source  131  or  132 . Diode  151 ,  152  of each portion of circuit  17  dedicated to the protection of one of the two switches unidirectional in current T 1 , D 3  and T 2 , D 4  is homologous to diode  15  of each circuit  6 ,  7  of  FIG. 1 . 
   A disadvantage of such a structure is the need to provide two voltage reference supply sources  133  and  134 . 
   Another disadvantage of such a structure is the presence of high-voltage diodes  151  and  152 . 
   According to another known method, a read resistor is introduced in series with the load and the bidirectional switch and the occurrence of overcurrents across this resistor is detected. As compared to the diagram of  FIG. 2 , the two non-inverting inputs of comparators  121  and  122  are then connected to the junction point of the switch and the detection resistor, the other terminal of this resistor being connected to ground, which corresponds to one of the terminals of application of the A.C. supply voltage. Diodes  151  and  152  are then no longer necessary. 
   SUMMARY OF THE INVENTION 
   An object of the present invention is to provide a circuit for protecting a bidirectional switch which overcomes the disadvantages of conventional circuits and which, especially, is more easily integrable. 
   The present invention also aims at providing a circuit such that it does not use a negative independent power supply. 
   The present invention also aims at providing such a circuit that is common to the two one-way switches forming it. 
   To achieve these and other objects, the present invention provides a circuit for detecting an overcurrent in an element run through by an A.C. supply current, comprising detecting a variation in the voltage across the element beyond two predetermined thresholds, said circuit comprising: 
   a first comparator, assigned to the halfwaves of a first sign of the A.C. power supply, receiving on a reference input a first reference voltage setting a first one of said thresholds; 
   a second comparator, assigned to the halfwaves of a second sign of the A.C. power supply, receiving on a reference input a second reference voltage setting a second one of said thresholds; and 
   an input stage providing, to respective interconnected read inputs of the comparators, a voltage proportional to said voltage across said element, said stage comprising at least one first resistive element introducing a voltage drop between a first one of the terminals of the element and said read inputs. 
   According to an embodiment of the present invention, the circuit is supplied between a high supply rail and a ground to which is connected one of said terminals of the element not connected to said first resistive element. 
   According to an embodiment of the present invention, the input stage comprises: 
   a first series connection of two low-voltage diodes, between said high rail and the ground, the anode of a first diode being grounded while the cathode of a second diode is connected to the high rail; and 
   a second series connection, between said high rail and said ground, of at least two resistive elements, the midpoints of said first and second series connection being interconnected to said read inputs of said first and second comparators. 
   According to an embodiment of the present invention, said first and second reference voltages are set by at least one resistive dividing bridge formed between said high rail and the ground. 
   According to an embodiment of the present invention, said first and second reference voltages are set by a single resistive dividing bridge formed of a series connection, between said high rail and said ground, of three resistive elements, said predetermined thresholds being respectively sampled across the intermediary resistor of the bridge. 
   According to an embodiment of the present invention, outputs of the first and second comparators are combined. 
   According to an embodiment of the present invention, the outputs are combined by a logic two-input OR gate. 
   According to an embodiment of the present invention, the that conducts an A.C. supply current is a bidirectional switch. 
   According to an embodiment of the present invention, the element conducting an A.C. supply current is a resistor. 
   The present invention also provides a circuit of protection against an overcurrent of a bidirectional switch in the on state, run through by an A.C. supply current. 
   According to an embodiment of the present invention, said resistor is in series with said switch. 
   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 
       FIGS. 1A and 1B , previously described, illustrate a first conventional mode of switch protection against overcurrents; 
       FIG. 2 , previously described, illustrates another conventional mode of switch protection against overcurrents; 
       FIG. 3  schematically and partially illustrates an embodiment of the protection of a bidirectional A.C. load control switch against overcurrents according to the present invention; and 
       FIG. 4  schematically and partially illustrates another embodiment of the of a bidirectional A.C. load control switch against overcurrents according to the present invention. 
   


   DETAILED DESCRIPTION 
     FIG. 3  schematically and partially illustrates an embodiment of the present invention. A load  21  is connected in series with a bidirectional switch  22  across an A.C. voltage source  23 . Bidirectional switch  22  is a switch with two input/output terminals A and C and two control terminals G 1  and G 2 . For example, terminal A is connected to a supply terminal of load  21 , terminal C is connected to a terminal of source  23 , another terminal of which is connected to another supply terminal of load  21 . Hereafter, terminal C forms the low voltage reference or ground point. The structure of switch  22  may be identical to that of switch  2  of  FIG. 1A . 
   According to the embodiment illustrated in  FIG. 3 , switch  22  is formed of the anti-parallel connection of two switches unidirectional in current. Each unidirectional switch is formed of the series connection, between terminals A and C, of a diode D 3 , D 4  and of a transistor T 1 , T 2 . 
   The series connection in each branch of switch  22  of a transistor T 1 , T 2  with a respective diode D 3 , D 4  may be similar to that of  FIG. 2 . 
   According to a preferred embodiment illustrated in  FIG. 3 , each collector of a transistor T 1 , T 2  is connected to the cathode of its respective rectifying diode D 3 , D 4 . 
   Transistors T 1 , T 2  of the two one-way switches are MOS transistors of the same conduction type or, for example, insulated-gate bipolar transistors (IGBT). 
   According to the present invention, switch  22  is protected against overcurrents by a single protection circuit  24  with two inputs, respectively connected to terminals A and C, and two output terminals, respectively connected, preferably via a respective protection resistor  46 ,  47 , to control terminals G 1  and G 2  of switch  22 . More specifically, circuit  24  comprises a detection circuit  25  and a control circuit (CTRL)  26 . 
   According to the present invention, detection circuit  25  comprises a resistor  30  having one end forming the input of protection circuit  24  connected to terminal A. Another end of resistor  30  is connected to the midpoint  31  of a series connection, between a high supply rail Vdd and the ground, of two low-voltage diodes  32  and  33 . The anode of diode  33  is grounded while the cathode of diode  32  is connected to rail Vdd. 
   Midpoint  31  is connected to the midpoint of a series connection, also between high rail Vdd and the ground, of two resistors  34  and  35 . Midpoint  31  is also connected, on the one hand, to an inverting (−) read input  36  of a first comparator  37  and, on the other hand, to a non-inverting (+) read input  38  of a second comparator  39 . 
   A non-inverting (+) input  40  of comparator  37  forms a reference input connected, via a resistor  41 , to high rail Vdd. Output COMP 1  of comparator  37  is connected to a first one of two inputs of an OR gate  42 . An inverting (−) input  43  of comparator  39  forms a reference input connected, via a resistor  44 , to ground. Output COMP 2  of second comparator  39  is connected to a second input of gate  42 . 
   Preferably, an intermediary resistor  45  is connected between the non-inverting (+) input  40  of first comparator  37  and the inverting (−) input  43  of second comparator  39 . The values of resistors  41 ,  44 , and  45  of reference inputs  40 ,  43  are adjusted so that the high threshold of first comparator  37  is greater than the low threshold of second comparator  39 . As an alternative, series resistors  41 ,  44 , and  45  may be replaced with resistive voltage dividers respectively assigned to inputs  40  and  43  of comparators  37  and  39 . An advantage of the embodiment of  FIG. 3  however is to link together the operating range thresholds so that they undergo the same possible drifts, resistor  45  guaranteeing the separation between thresholds. 
   The output of gate  42  forms the output of detection circuit  25  and is connected to an input of control circuit  26 . 
   For simplification and clarity, as will be understood by those skilled in the art, it has been omitted to show the supplies Vcc of comparators  37  and  39  and of control circuit  26  in  FIG. 3 . 
   In normal operation, load  21  conducts an A.C. current predetermined by its nature and/or its operating mode, the A.C. voltage between terminals A and C of on switch  22  is very small (due to the series resistance of switch  22  in the on state) as compared to the A.C. voltage provided by source  23 . The values of the different input resistances  30 ,  34 , and  35  are set so that the voltage signal applied on inverting  36  and non-inverting  38  input of the first  37  and second  39  comparators, respectively remains within a voltage range between the thresholds set by resistors  41 ,  44 , and  45 . Then, the voltage signal transmitted by input resistor  30  is such that, for both comparators  37  and  39 , the low-voltage signal on their non-inverting input is always (in normal operation) greater than the low-voltage signal on their inverting input. Outputs COMP 1  and COMP 2  then take a same logic state. Detection circuit  25  thus provides control circuit  26  with a signal of a first logic value. Control circuit  26  is designed to ensure, as a response to this first logic value, the holding of switch  22  in the on state. 
   Upon occurrence of an overcurrent, generally linked to a malfunction of load  21  or of source  23 , as soon as the image of the voltage between terminals A and C of switch  22  comes out of the acceptable voltage range set by the thresholds of comparators  37  and  39  and an attenuation coefficient linked to the presence of resistors  30 ,  34 , and  35 , one of comparators  37  and  39  switches states. This low-voltage image is obtained due to the arrangement of the input stage formed of resistors  30 ,  34 , and  35  and of diodes  32  and  33  which are used to limit the voltage at midpoint  31  within a range from Vdd+VD 32  to GND−VD 33 , when switch  22  is off, VD 32  and VD 33  begin the voltage drop introduced by the respective diode  32  and  33 . In the case of a positive halfwave, the detection is performed in the case where the high threshold of reference input (+)  40  is exceeded. In the case of a negative halfwave, the detection is performed in the case where it is fallen below the low threshold of the reference input  43  (−). The corresponding switching of a single input of gate  42  causes a switching of the output of this gate. Control circuit  26  then receives a second logic value complementary to the first one. Control circuit  26  is designed to modify, as a response to this second logic value, the control of gates G 1  and G 2  to turn off switch  22 . 
   An advantage of the present invention is to provide a circuit of protection against overcurrents of a bidirectional switch controlling an A.C. load easier to integrate than known circuits. Indeed, the detection circuit according to the present invention, conversely to known circuits, requires no additional negative power supply. 
   Further, one and the same circuit connected across a bidirectional switch advantageously enables protecting two controllable one-way switches forming it. 
   Another advantage of the present invention is that the detection and protection circuits according to the present invention are advantageously usable with anti-parallel type switches as well as with anti-series type switches. For an anti-series assembly, a shunt should however be used (for example, a resistor) between the two switches. 
   Further, the detection circuit according to the present invention may advantageously be used with a so-called MBS-type bidirectional switch which exhibits the antiparallel structure of the drawing, in which the cathode of diode D 3 , D 4  is connected to the collector of the associated transistor T 1 , T 2 . On the contrary, the known detection and protection circuit of  FIG. 2  could not be used with such an MBS bidirectional switch. Indeed, for protection circuit  17  of  FIG. 2 , input terminal IN 22  of comparator  122  must be connected to the emitter of transistor T 2 . In the case of a bidirectional MBS switch, such a connection results in directly connecting terminal A and the anode of diode  152 . As illustrated in  FIG. 2 , input IN 22  must be protected by interposing diode D 4  between terminal A and the anode of diode  152 . This has a double advantage, on the one hand, that the circuit according to the present invention may be used with more switches than in the state of the art. On the other hand, MBS-type switches are easier to form in terms of integration. 
   The values of the different resistances  30 ,  34 ,  35 ,  41 ,  44 , and  45  are set, on the one hand, to enable detection of an overcurrent according to the previously-discussed principles. On the other hand, the values of resistances  30 ,  34 , and  35  are also set to limit, when switch  22  is off, the parallel leakage current of switch  22  as well as the power dissipated in resistors  30 ,  34 ,  35 ,  41 ,  44 , and  45 . 
   As a specific example of implementation, the following values will be set for the resistors: 
   resistor 30: 1. 10 6 Ω; 
   resistor 34: 1.3. 10 6 Ω; 
   resistor 35: 360. 10 3 Ω; 
   resistor 41: 47. 10 3 Ω; 
   resistor 44: 4.3. 10 3  Ω; and 
   resistor 45: 13. 10 3 Ω. 
   In this case, for an A.C. voltage supplied by source  23  of 220 V, the thresholds of comparators  37  and  39  are on the order of 4 volts and 1 volt, respectively, which enables detecting the occurrence of an overcurrent as soon as the collector-emitter voltage Vce across a transistor T 1  or T 2  exceeds, in absolute value, 7 volts. 
   Further, diodes  32  and  33  are low-voltage diodes. They are thus easier to form and less bulky in integrated form than homologous high-voltage diodes ( 15 ,  FIG. 1B ) of known circuits. Indeed, the diodes according to the present invention are connected to low voltage power supply Vdd. Conversely to protection diodes of known circuits, they are not intended to protect the protection circuits when the switch is off, that is, when the voltage thereacross is high. Further, upon occurrence of an overcurrent, diodes  32  and  33  are protected in current by input resistor  30 . 
   As will be understood by those skilled in the art, the detection circuit of according to the present invention is not limited to a detection of an overcurrent across a bidirectional switch. 
   Thus,  FIG. 4  schematically and partially illustrates another embodiment of the protection of a bidirectional switch  22  for controlling an A.C. load  21  supplied by an A.C. source  23 . This embodiment differs from that of  FIG. 3  in that protection circuit  24  according to the present invention now is connected across a resistor  70  series-connected with switch  22  and load  21  across source  23 . The ground indicated in  FIG. 4  as corresponding to terminal C may however, as an alternative, correspond to terminal A. 
   Of course, the present invention is likely to have various alterations, modifications, and improvements which will readily occur to those skilled in the art. In particular, the present invention has been described hereabove as applied to a specific bidirectional switch. However, the present invention also applies to a bidirectional switch formed of the anti-series connection of two controllable switches unidirectional in current connected in anti-parallel to respective free wheel diodes. Further, it has been considered in the foregoing description that the one-way switches are IGBT transistors. The one-way switches may however be N- or P-channel MOS transistors, connected in series or in antiparallel. Further, specific circuit elements may be replaced with functionally equivalent elements. In particular, it will be within the abilities of skilled in the art to select a control circuit  26  capable of appropriately driving of the control terminals of switch  22 . Similarly, the comparison function described in relation with comparators  37  and  39  may be carried out by any appropriate circuit. 
   Further, it has been considered in the description of the present invention that the switch controls the supply of a load placed in series with the switch. However, the switch could be placed in parallel with the load and control its supply according to a predetermined cycle, for example, according to the voltage thereacross. 
   Moreover, load  21  may be any A.C.-supplied bidirectional load. In particular, the load may especially be a resistive element, for example, in lighting or heating devices. 
   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.
A circuit for detecting an overcurrent in an element in which an A.C. Supply current flows, including a first comparator, assigned to the halfwaves of a first sign of the A.C. Power supply, receiving on a reference input a first reference voltage setting a first one of the thresholds, a second comparator, assigned to the halfwaves of a second sign of the A.C. Power supply, receiving on a reference input a second reference voltage setting a second one of the thresholds, and an input stage providing, to respective interconnected read inputs of the comparators, a voltage proportional to the voltage across the element, the stage including at least one first resistive element introducing a voltage drop between a first one of the terminals of the element and the read inputs.