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.

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.

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. 1shows a bidirectional switch10according to the present invention comprising a bidirectional switching device12made in monolithic form which is formed of two vertical power thyristors TH1, TH2arranged in antiparallel between two power terminals A1and A2. The two thyristors TH1, TH2are shown in dotted lines.

Switching device12is made in a substrate14, for example made of lightly-doped N-type silicon. An anode region20of first thyristor TH1is formed on upper surface side21of substrate14. Region20contains an N-type cathode region22of second thyristor TH2, more heavily doped than substrate14. A P-type protection region23more lightly-doped than region20at least partially surrounds region20. Protection region23conventionally enables modifying the distribution of the equipotential surfaces in substrate14to increase the breakdown voltage of switching device12.

Substrate14contains, on the side of its upper surface21, a first P-type triggering region26of first thyristor TH1, in the vicinity of P-type region20. Triggering region26contains a second N-type triggering region27, more heavily doped than substrate14. A P-type protection region28more lightly doped than triggering region25surrounds triggering region25. The minimum distance separating protection region28from region20is smaller than a determined minimum distance to ensure a proper distribution of the equipotential surfaces in substrate14. In particular, protection region28may extend to region20.

Substrate14comprises, on its lower surface side30, a P-type anode region32of second thyristor TH2. An N-type cathode region34of first thyristor TH1, more heavily doped than substrate14, extends into region32substantially opposite to the portion of region20which does not contain cathode region22and also opposite to triggering region26.

An insulating layer36covers upper surface21of substrate14. A metal electrode38is in contact with anode region20of first thyristor TH1and cathode region22of second thyristor TH2. Electrode38forms power terminal A1common to the two power thyristors TH1, TH2.

A metal electrode40is in contact with triggering region26and forms a first gate terminal G1. A metal electrode42is in contact with triggering region27and forms a second gate terminal G2.

A metal electrode44covers lower surface30of substrate14and is in contact with anode region32of second thyristor TH2and cathode region34of first thyristor TH1. Electrode44forms the second power terminal A2common to the two thyristors TH1, TH2.

Substrate14may comprise on its periphery a P-type protection wall46. Protection wall46preferably comprises on the upper surface side21of substrate14a P-type ring-shaped region48more heavily doped than wall46.

An N-type ring-shaped channel stop region50, more heavily-doped than substrate14, may be provided on the upper surface side21around region20and triggering region26. Region50enables avoiding the development of possible short-circuits between region20and protection wall46.

Optional metal rings52,54, extend on the upper surface side21of substrate14and are respectively in contact with P-type ring-shaped region48and channel stop region50to improve the voltage equalization in these regions.

Power terminal A1is connected to a reference voltage of an A.C. power supply, for example, ground GND. First gate terminal G1is connected to power terminal A1by a resistor Rgk. Second gate terminal G2is connected to a terminal of a capacitor C having its other terminal connected to reference voltage GND.

Power terminal A2is connected to a first terminal of a load RLto be switched. An A.C. supply voltage U is applied between the second terminal of load RLand power terminal A1.

InFIG. 2, curve60represents 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 G1. The pulses are shown inFIG. 2by curve62superposed to curve60and the scale of which appears to the right of the drawing. A positive current pulse is applied to first gate terminal G1at the beginning of each negative halfwave of supply voltage U to cause the triggering of thyristor TH1. Thyristor TH2then 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 G1. The current runs through resistor Rgk. As a result, the potential on gate terminal G1increases and the diode formed by the junction between triggering regions26,27turns on. Carriers are then injected into the substrate and cause the triggering of thyristor TH1, which is biased to be able to be on.

During the negative halfwave, as long as thyristor TH1is on, the device region corresponding to the junction between triggering regions26,27, which is close to the anode region of thyristor TH1, 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. 3shows curve64of variation of voltage VCacross 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 VCgreater in absolute value than 0.6 volt.

At the end of the first negative halfwave, when the current running through first thyristor TH1decreases below a threshold level, first thyristor TH1turns off. Capacitor C triggers discharging and a negative current flows to ground through the diode formed between triggering regions26,27and through resistor Rgk. 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 electrode44becomes positive. First, a negative current continues to flow from capacitor C to ground through the diode formed between triggering regions26,27and through resistor Rgk. The junction between triggering region26and substrate14, close to the diode formed between triggering regions26,27, is then saturated in terms of free carriers. This junction is conductive and enables triggering of an oblique thyristor formed by second triggering region27, first triggering region26, substrate14, and anode region32. Capacitor C then completes its discharge via the oblique thyristor. The latter then causes the triggering of power thyristor TH2.

Resistance Rgkmust be sufficiently large to avoid too significant a discharge of capacitor C into resistor Rgkat the end of the negative halfwave, and sufficiently small for switching device12not 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 TH2decreases below a threshold current, switching device12turns off. A current pulse must then be applied again on gate terminal G1at the next negative halfwave to cause the conduction of switching device12on the next period of voltage U.

The control method of the present invention enables setting the power provided to load RLby modifying the time when the current pulse is applied on first gate terminal G1during a negative halfwave.

As an example, resistor Rgkmay 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 Rgkconnecting gate terminal G1to power terminal A1may be directly integrated to switching device12and be formed by a portion of P−protection region28contacting anode region20.

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 Rgk.

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.