Abstract:
A method for controlling an SCR-type switch, consisting of applying on the switch gate several periods of an unrectified high-frequency voltage, the power of one HF halfwave being insufficient to start the SCR-type switch.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention generally relates to the control of SCR-type switches. The term “SCR-type switches” is here used to designate components such as thyristors, triacs, and controlled bidirectional switches of various types comprising four or five alternated semiconductor layers. The present invention especially applies to the case where the switches are medium-power switches intended to switch loads connected to the A.C. supply network (at 50 or 60 Hz).  
           [0003]    2. Discussion of the Related Art  
           [0004]    Generally, in the control of multiple switches and/or of switches supplied by the A.C. network, the problem of the isolation between the control circuit and the switch(es) to be controlled is posed, and a galvanic isolation circuit such as a transformer or an optocoupler is generally provided, which has the disadvantage of being expensive and difficult to integrate.  
           [0005]    It has been provided, for example, in PCT patent application WO/0250850 of the applicant (B4882), to control power switches with high-frequency signals. In this patent application, the high frequency is used for the advantage that it has of enabling use of a small transformer, which enables galvanic isolation at lesser cost but however requires a transformer. After transformation, this high frequency is applied to the gate of the component to be switched via a rectifying diode, that is, in fact, the control is performed by signals in the form of D.C. pulses.  
         SUMMARY OF THE INVENTION  
         [0006]    The present invention provides a novel SCR-type switch control method.  
           [0007]    More specifically, the present invention provides such a method and an adapted device, especially enabling solving in a particularly simple fashion the problem of the isolation between the control circuit and the switch to be controlled.  
           [0008]    To achieve these objects, the present invention provides a method for controlling an SCR-type switch, consisting of applying on the switch gate several periods of an unrectified high-frequency voltage, the power of an HF halfwave being insufficient to start the SCR-type switch.  
           [0009]    According to an embodiment of the present invention, the HF voltage oscillates at a selected frequency between 10 kHz and a few GHz.  
           [0010]    According to an embodiment of the present invention, the high frequency is applied via an insulating layer formed above a sensitive area of the component.  
           [0011]    According to an embodiment of the present invention, the high frequency is applied above a gate region of a thyristor.  
           [0012]    According to an embodiment of the present invention, the high frequency is applied above a gate region of a triac.  
           [0013]    According to an embodiment of the present invention, the high frequency is applied via a high-frequency line.  
           [0014]    According to an embodiment of the present invention, the high frequency is applied via a winding.  
           [0015]    The present invention also provides an SCR-type switch component, comprising two main electrodes and at least one control electrode formed on an insulating layer and arranged above a starting region of the component, said control electrode being intended to be connected to an unrectified HF power supply.  
           [0016]    According to an embodiment of the present invention, the control electrode is arranged above a gate region of a thyristor.  
           [0017]    According to an embodiment of the present invention, the control electrode is arranged above a gate region of a triac.  
           [0018]    According to an embodiment of the present invention, the control electrode is a high-frequency line.  
           [0019]    According to an embodiment of the present invention, the high frequency is applied via a winding.  
           [0020]    According to an advantage of the present invention, the application of an HF signal in the control area of an SCR-type component may be performed without for a metallization to be in contact with the sensitive semiconductor area, which simplifies the design and structure of the switch and solves all the isolation and reference problems which are generally posed upon control by a same control circuit of various thyristors and/or triacs, or other bidirectional switches.  
           [0021]    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  
       [0022]    [0022]FIG. 1 shows a principle diagram of the control of a thyristor;  
         [0023]    [0023]FIG. 2 shows the shape of the current settling in a thyristor for an A.C. voltage applied at a 1-MHz frequency;  
         [0024]    [0024]FIG. 3 shows the shape of the current settling in a thyristor controlled by an A.C. signal at a frequency on the order of 20 MHz; and  
         [0025]    [0025]FIGS. 4, 5,  6 A, and  7  show various examples of implementation of the present invention, FIG. 6B being an equivalent diagram of the embodiment of FIG. 6A. 
     
    
     DETAILED DESCRIPTION  
       [0026]    [0026]FIGS. 4, 5,  6 A, and  7  are partial simplified cross-section views and are only intended to simplify the understanding of the following description. Especially, for simplification, some elements have been shown in a same cross-section plane while they are in fact in different planes. It will be within the abilities of those skilled in the art to form practical devices, using known techniques. Further, as usual in the field of semiconductor component representation, the dimensions of the various layers and regions are not drawn to scale.  
         [0027]    The basic idea of the present invention is to directly control an SCR-type switch with an unrectified HF voltage. It should be noted that this approach differs from conventional SCR-type switch control modes according to which a D.C. voltage or voltage pulse is applied on the gate electrode, connected to a gate area of this component. In the case where a voltage pulse is applied, said pulse must be of sufficient amplitude to turn on a junction and of sufficient intensity to have a sufficient current flow through this junction. In other words, the pulse must have a given minimum power.  
         [0028]    A priori, when an A.C. signal is applied to the gate of a thyristor such that the power of each halfwave is insufficient to turn on the thyristor and that the duration of each halfwave is shorter than the component priming time, the effect of positive and negative halfwaves annuls and the A.C. signal has no switch starting effect.  
         [0029]    The applicant has however tried the experiment in a diagram of the type in FIG. 1, in which an HF signal is applied between gate G and cathode A of a thyristor. A D.C voltage VAK of appropriate biasing is applied across the series assembly of a load L and of thyristor TH. It is considered that cathode K of the thyristor is grounded.  
         [0030]    In FIG. 2, an HF voltage at a frequency of approximately 1 megahertz applied between the gate and the cathode has been shown by a curve  10  and the observed anode current has been shown by a curve  11 . It should be noted that, after approximately three halfwaves of the high-frequency A.C. voltage, the thyristor conduction settles. Then, as conventional with a thyristor, the HF power supply can be interrupted and the thyristor remains conductive.  
         [0031]    [0031]FIG. 3 shows two similar curves for an A.C. voltage at a frequency on the order of 20 MHz. More specifically, in FIG. 3, curve  20  shows the gate/cathode voltage and curve  21  shows the anode current. It can be seen that the anode current progressively increases before settling at a sufficient value for the switch to remain on if the high-frequency voltage is interrupted. The settling duration of the conduction extends over from approximately 40 to 50 high-frequency periods.  
         [0032]    Thus, unexpectedly, when a high-frequency control voltage is applied to the gate of a thyristor, and more generally of an SCR-type switch, said switch is switched on while each halfwave of the A.C. voltage has a power and/or a duration insufficient to ensure the switching of the considered SCR-type component.  
         [0033]    A significant advantage of the high-frequency control is that the high-frequency voltage may be applied to the gate terminal via a coupling capacitor which exhibits a very small impedance for high frequencies and which blocks the D.C. voltage or even the A.C. voltage at the network frequency (50 or 60 hertz), to which are generally connected the control terminals of a mono- or bidirectional switch. The possibility of inserting such a coupling capacitor result in that problems to be generally overcome of isolation of the control circuit with respect to the main circuit of a power switch are simply solved.  
         [0034]    The present invention also provides various SCR switch structures capable of implementing the present invention.  
         [0035]    A common point of these various structures is that, as usual, the main electrodes of the power switching component are normally connected to appropriate conductive areas, but that no gate metallization is provided any longer in contact with the semiconductor gate area. Instead of this gate contact, various means are provided to inject an HF signal via an insulating layer towards a starting area of the power component. In other words, a capacitive coupling is performed through the insulating layer towards a sensitive area of the semiconductor component. The insulating layer may be one of the silicon oxide layers currently used in silicon semiconductor components.  
         [0036]    [0036]FIG. 4 shows an example of an HF-controlled thyristor according to the present invention. This component of vertical type conventionally comprises an N-type cathode region  41 , formed in a P-type well  42 , itself formed in a lightly-doped N-type layer  43 , a P-type anode region  44  being present on the rear surface side of the component. N-type region  41  is in contact with a cathode metallization MK and is connected to a cathode terminal K. The rear surface of the component is coated with an anode metallization MA and is connected to an anode terminal A. It should be reminded that, conventionally, a gate terminal is formed by a metallization in contact with a portion of layer  42 . According to the present invention, region  42  is uniformly coated with an insulating layer  45  above which is formed a metallization MG to which it is provided to issue a high-frequency starting signal. It will again be underlined that metallization MG is completely insulated with respect to the main thyristor circuit. The HF frequency may be applied between terminal G and one or the other of terminals A and K which, being at set voltages, are considered as grounds from the viewpoint of high frequencies.  
         [0037]    [0037]FIG. 5 shows an application of the present invention to the control of a triac. This triac comprises the same layers  41  to  44  as those previously described for the thyristor of FIG. 4. In rear surface P-type layer  44  is further formed an N-type region  51  in contact with the rear surface metallization and, conventionally, a P-type region  52  is formed on the front surface side in N-type region  43 , corresponding to a gate region. Preferably, a more heavily-doped N-type region  53  is provided in the vicinity of region  52 . Metallizations MG 1  and MG 2  are respectively formed above P-type region  52  and N-type region  53 . A high-frequency signal may for example be applied between terminals G 1  and G 2  connected to metallizations MG 1  and MG 2  and it can be acknowledges that this starts the triac in the same way as the thyristor was previously started. It should be noted that terminal G 2  is optional and that an HF control signal could be applied on terminal G 1  only. A starting of the triac is then also acknowledged.  
         [0038]    [0038]FIG. 6A shows an alternative embodiment of the structure shown in FIG. 5. Instead of providing two metallizations MG I and MG 2  between which a high-frequency signal is applied, a high-frequency line  61 , between the terminals of which a high-frequency voltage HF is applied, is provided above regions  52  and  53 . Again, by capacitive coupling, as illustrated in FIG. 6B, a current starts flowing in the underlying layers and it can be acknowledged that this results in a turning-on of the main component.  
         [0039]    According to another alternative of the present invention, as schematically illustrated in FIG. 7, the high frequency may be applied to a sensitive area of the switch via a winding  71  formed on an insulating layer  72  coating substrate  73  in which is formed the power component, as illustrated in FIG. 4 or  5 . Various applications of such an HF injection by a coil may be used. A secondary winding may be formed in the semiconductor, for example from a diffused layer or an area filled with a conductive material such as polysilicon formed in insulated grooves formed in a portion of the semiconductor substrate, the HF being then present at the ends of the secondary winding. It can also be provided for winding  71  to generate a magnetic field which creates in the conductive material eddy currents turning on sensitive junctions as indicated previously. In such an application, the frequencies can be very high, on the order of from several megahertz to several gigahertzes.  
         [0040]    It should be understood that the present invention is likely to have various alterations and modifications, the base of the present invention being a control by a high-frequency signal of a semiconductor component by high frequency injection in a sensitive area of this semiconductor component. This sensitive area may be distinct from the conventional gate areas of SCR-type components; it is enough for it to cause the turning-on of a junction which generates charges in the vicinity of the blocking junction of properly-biased SCR-type component. The high-frequency range will be adapted to the context of the application. High frequencies within a range from 10 kHz to several GHz may for example be envisaged according to the application. As indicated previously, each of the high-frequency halfwaves does not need be very energetic since, as shown in relation with FIGS. 2 and 3, an accumulation phenomenon which causes a progressive starting of the controlled semiconductor component occurs.  
         [0041]    Further, although the present invention has been described in relation with simplified examples of specific thyristors and triacs, it applies to any SCR-type component and in particular to any type of bidirectional component.  
         [0042]    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.