Abstract:
A bidirectional switch for switching an A.C. voltage at a load, including a monolithic component, formed in an N-type substrate, including a first vertical thyristor; a second vertical thyristor; a P-type triggering region formed opposite to the cathode of the first thyristor and an N-type triggering region formed in the P-type triggering region, the P-type triggering region being intended to receive a control signal in a negative halfwave of the A.C. voltage to trigger the first thyristor; a resistive element connected to the P-type triggering region and to the anode of the first thyristor; and a capacitor having a terminal connected to the N-type triggering region and its other terminal intended to be connected to the reference voltage.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to the field of high-voltage bidirectional switches. The present invention more specifically relates to a bidirectional switch intended to be series-connected with a load, the assembly being supplied by a high A.C. voltage, for example, the mains. 
   2. Discussion of the Related Art 
   High-voltage switches for an A.C. load of the type to which the present invention relates are used, in particular, to control household appliances, for example, washing machines, in which one or several A.C.-supplied loads (motors, pumps, heating resistors, valves) must be controlled from the machine microcontroller. 
   A bidirectional switch capable of being controlled by a low-voltage signal may be formed of a triac, the gate of which receives a low-voltage control signal. 
   A triac must be turned on at each halfwave of the supply voltage since it turns off when the current between its two power terminals disappears. 
   Some switches comprise a triac control circuit which enables, when the triac is turned on by a control signal during a halfwave, maintaining it on until the end of the next halfwave. 
   A disadvantage of such switches is that the control circuit is generally complex and expensive to form. Further, such a circuit may require use of one or several high-voltage capacitors, which components are difficult to form and are relatively expensive. 
   SUMMARY OF THE INVENTION 
   The present invention aims at providing a bidirectional switch that can be triggered by a low-voltage control signal during a halfwave of the supply voltage and remain on until the end of the next halfwave of the supply voltage, which is simple and inexpensive to form. 
   The present invention especially aims at providing a bidirectional switch which does not use a high-voltage capacitor. 
   To achieve this and other objects, the present invention provides a bidirectional switch for switching an A.C. voltage at a load, comprising a monolithic component, formed in an N-type substrate, comprising: 
   a first vertical thyristor comprising, from top to bottom, a first P-type region, the N-type substrate, a second P-type region, and a first N-type region contained in the second P-type region; 
   a second vertical thyristor comprising, from bottom to top, the second P-type region, the N-type substrate, the first P-type region, and a second N-type region contained in the first P-type region; 
   a P-type triggering region formed on the high substrate side opposite to the cathode of the first thyristor and an N-type triggering region formed in the P-type triggering region; 
   the cathode of the first thyristor and the anode of the second thyristor being intended to be connected to a terminal of the load, the anode of the first thyristor and the cathode of the second thyristor being intended to be connected to a reference voltage, and the P-type triggering region being intended to receive at least one control signal in a negative halfwave of the A.C. voltage to trigger the first thyristor; 
   a resistive element connected to the P-type triggering region and to the anode of the first thyristor; and 
   a capacitor having a terminal connected to the N-type triggering region and its other terminal intended to be connected to the reference voltage. 
   According to an embodiment of the present invention, the substrate comprises a P-type protection region surrounding the P-type triggering region and more lightly doped than the P-type triggering region, the minimum distance separating the protection region from the anode of the first thyristor being smaller than a determined distance. 
   According to an embodiment of the present invention, the protection region is in contact with the anode of the first thyristor. 
   According to an embodiment of the present invention, the capacitor is a low-voltage capacitor. 
   According to an embodiment of the present invention, the resistive element is integrated to the substrate. 
   The present invention also provides a monolithic switching device for a switch of an A.C. voltage at a load, said switching device, formed in an N-type substrate, comprising: 
   a first vertical thyristor comprising, from top to bottom, a first P-type region, the N-type substrate, a second P-type region, and a first N-type region contained in the second P-type region; 
   a second vertical thyristor comprising, from bottom to top, the second P-type region, the N-type substrate, the first P-type region, and a second N-type region contained in the first P-type region; 
   a P-type triggering region formed on the upper side of the substrate opposite to the cathode of the first thyristor and an N-type triggering region formed in the P-type triggering region; 
   the cathode of the first thyristor and the anode of the second thyristor being intended to be connected to a terminal of the load, the anode of the first thyristor and the cathode of the second thyristor being intended to be connected to a reference voltage, and the P-type triggering region being intended to be connected to the anode of the first thyristor via a resistive element and to receive at least one control signal in a negative halfwave of the A.C. voltage to trigger the first thyristor, and the N-type triggering region being intended to be connected to a terminal of a capacitor having its other terminal connected to the reference voltage. 
   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  schematically shows an example of embodiment of a high-voltage bidirectional switch according to the present invention, a portion of the switch being represented by a monolithic component in cross-section view, the other portion being represented by an electric circuit; and 
       FIGS. 2 and 3  illustrate in the form of timing diagrams the operation of the switch of FIG.  1 . 
   

   DETAILED DESCRIPTION 
   For clarity, only those elements that are necessary to the understanding of the present invention have been shown and will be described hereafter. 
     FIG. 1  shows a bidirectional switch  10  according to the present invention comprising a bidirectional switching device  12  made in monolithic form which is formed of two vertical power thyristors TH 1 , TH 2  arranged in antiparallel between two power terminals A 1  and A 2 . The two thyristors TH 1 , TH 2  are shown in dotted lines. 
   Switching device  12  is made in a substrate  14 , for example made of lightly-doped N-type silicon. An anode region  20  of first thyristor TH 1  is formed on upper surface side  21  of substrate  14 . Region  20  contains an N-type cathode region  22  of second thyristor TH 2 , more heavily doped than substrate  14 . A P-type protection region  23  more lightly-doped than region  20  at least partially surrounds region  20 . Protection region  23  conventionally enables modifying the distribution of the equipotential surfaces in substrate  14  to increase the breakdown voltage of switching device  12 . 
   Substrate  14  contains, on the side of its upper surface  21 , a first P-type triggering region  26  of first thyristor TH 1 , in the vicinity of P-type region  20 . Triggering region  26  contains a second N-type triggering region  27 , more heavily doped than substrate  14 . A P-type protection region  28  more lightly doped than triggering region  25  surrounds triggering region  25 . The minimum distance separating protection region  28  from region  20  is smaller than a determined minimum distance to ensure a proper distribution of the equipotential surfaces in substrate  14 . In particular, protection region  28  may extend to region  20 . 
   Substrate  14  comprises, on its lower surface side  30 , a P-type anode region  32  of second thyristor TH 2 . An N-type cathode region  34  of first thyristor TH 1 , more heavily doped than substrate  14 , extends into region  32  substantially opposite to the portion of region  20  which does not contain cathode region  22  and also opposite to triggering region  26 . 
   An insulating layer  36  covers upper surface  21  of substrate  14 . A metal electrode  38  is in contact with anode region  20  of first thyristor TH 1  and cathode region  22  of second thyristor TH 2 . Electrode  38  forms power terminal A 1  common to the two power thyristors TH 1 , TH 2 . 
   A metal electrode  40  is in contact with triggering region  26  and forms a first gate terminal G 1 . A metal electrode  42  is in contact with triggering region  27  and forms a second gate terminal G 2 . 
   A metal electrode  44  covers lower surface  30  of substrate  14  and is in contact with anode region  32  of second thyristor TH 2  and cathode region  34  of first thyristor TH 1 . Electrode  44  forms the second power terminal A 2  common to the two thyristors TH 1 , TH 2 . 
   Substrate  14  may comprise on its periphery a P-type protection wall  46 . Protection wall  46  preferably comprises on the upper surface side  21  of substrate  14  a P-type ring-shaped region  48  more heavily doped than wall  46 . 
   An N-type ring-shaped channel stop region  50 , more heavily-doped than substrate  14 , may be provided on the upper surface side  21  around region  20  and triggering region  26 . Region  50  enables avoiding the development of possible short-circuits between region  20  and protection wall  46 . 
   Optional metal rings  52 ,  54 , extend on the upper surface side  21  of substrate  14  and are respectively in contact with P-type ring-shaped region  48  and channel stop region  50  to improve the voltage equalization in these regions. 
   Power terminal A 1  is connected to a reference voltage of an A.C. power supply, for example, ground GND. First gate terminal G 1  is connected to power terminal A 1  by a resistor R gk . Second gate terminal G 2  is connected to a terminal of a capacitor C having its other terminal connected to reference voltage GND. 
   Power terminal A 2  is connected to a first terminal of a load R L  to be switched. An A.C. supply voltage U is applied between the second terminal of load R L  and power terminal A 1 . 
   In  FIG. 2 , curve  60  represents A.C. voltage U. It is for example the mains voltage corresponding to a sinusoidal A.C. voltage of a 50-Hz frequency and of an amplitude of several hundreds of volts. 
   According to the present invention, current pulses are applied on first gate terminal G 1 . The pulses are shown in  FIG. 2  by curve  62  superposed to curve  60  and the scale of which appears to the right of the drawing. A positive current pulse is applied to first gate terminal G 1  at the beginning of each negative halfwave of supply voltage U to cause the triggering of thyristor TH 1 . Thyristor TH 2  then automatically triggers at the next positive halfwave. 
   The operating principle of the device according to the present invention more specifically is the following. 
   On a negative halfwave of voltage U, a positive current pulse is injected onto gate terminal G 1 . The current runs through resistor R gk . As a result, the potential on gate terminal G 1  increases and the diode formed by the junction between triggering regions  26 ,  27  turns on. Carriers are then injected into the substrate and cause the triggering of thyristor TH 1 , which is biased to be able to be on. 
   During the negative halfwave, as long as thyristor TH 1  is on, the device region corresponding to the junction between triggering regions  26 ,  27 , which is close to the anode region of thyristor TH 1 , is saturated in terms of free carriers. This junction is then conductive and negative charges accumulate on the electrode of capacitor C not connected to ground. Capacitor C thus charges negatively during the negative halfwave. 
     FIG. 3  shows curve  64  of variation of voltage V C  across capacitor C for a single period of supply voltage U. Capacitor C is initially discharged. The capacitance of capacitor C is chosen so that once completely charged, the capacitor exhibits between its terminals a negative voltage V C  greater in absolute value than 0.6 volt. 
   At the end of the first negative halfwave, when the current running through first thyristor TH 1  decreases below a threshold level, first thyristor TH 1  turns off. Capacitor C triggers discharging and a negative current flows to ground through the diode formed between triggering regions  26 ,  27  and through resistor R gk . This diode is conductive since the voltage across capacitor C is greater than 0.6 volt in absolute value. 
   When the next positive halfwave of supply voltage U triggers, the potential of lower surface electrode  44  becomes positive. First, a negative current continues to flow from capacitor C to ground through the diode formed between triggering regions  26 ,  27  and through resistor R gk . The junction between triggering region  26  and substrate  14 , close to the diode formed between triggering regions  26 ,  27 , is then saturated in terms of free carriers. This junction is conductive and enables triggering of an oblique thyristor formed by second triggering region  27 , first triggering region  26 , substrate  14 , and anode region  32 . Capacitor C then completes its discharge via the oblique thyristor. The latter then causes the triggering of power thyristor TH 2 . 
   Resistance R gk  must be sufficiently large to avoid too significant a discharge of capacitor C into resistor R gk  at the end of the negative halfwave, and sufficiently small for switching device  12  not to be too sensitive to parasitic triggerings resulting from abrupt variations in the voltage thereacross (so-called dV/dt triggerings). 
   At the end of the positive halfwave, when the current running through thyristor TH 2  decreases below a threshold current, switching device  12  turns off. A current pulse must then be applied again on gate terminal G 1  at the next negative halfwave to cause the conduction of switching device  12  on the next period of voltage U. 
   The control method of the present invention enables setting the power provided to load R L  by modifying the time when the current pulse is applied on first gate terminal G 1  during a negative halfwave. 
   As an example, resistor R gk  may be of a few hundreds of ohms, for example, 300 ohms, and the capacitance of capacitor C may be of a few hundreds of nanofarads, for example, 500 nanofarads. 
   According to an alternative of the present invention, resistor R gk  connecting gate terminal G 1  to power terminal A 1  may be directly integrated to switching device  12  and be formed by a portion of P −  protection region  28  contacting anode region  20 . 
   The present invention has many advantages: 
   First, the switch according to the present invention is mainly formed of a switching device made in monolithic form, of a capacitor, and of a resistor (possibly integrated to the switching device). It thus has a particularly simple structure. Further, the switch components may be formed at low cost by conventional technologies. 
   Second, the voltages across the capacitor remain low, at most of a few volts, during the operation of the switch according to the present invention. A low-voltage capacitor which can easily be formed with a great reliability may then be selected. 
   Third, untimely triggerings of the switching device which may occur when the supply voltage abruptly varies may be avoided by setting the value of resistance R gk . 
   Of course, the present invention is likely to have various alterations, modifications, and improvements which will readily occur to those skilled in the art. 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.