Abstract:
The invention concerns a control circuit for controlling a power switch by means of a galvanic insulation transformer, the transformer being produced in the form of planar conductive windings on an insulating substrate ( 20 ) whereon are integrated passive components constituting a high frequency excitation oscillating circuit for a primary winding of the transformer, the transformer substrate being directly mounted on a wafer ( 24 ) whereon is mounted a circuit chip ( 40 ) integrating the power switch.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to power circuit control systems requiring galvanic isolation between the control and the power circuit. Such systems are generally used for controlling loads supplied by the A.C. power system. The galvanic isolation essentially has the function of protecting the control circuit and its user. 
   2. Discussion of the Related Art 
     FIG. 1  very schematically shows a first conventional example of a device for galvanic isolation between a control circuit and a power circuit. This device is a transformer T forming an isolation barrier IB and having a primary winding T 1  connected to two output terminals of a control circuit  1  and a secondary winding T 2  that controls a power switch K. In the example of  FIG. 1 , switch K is a thyristor connected between two terminals  2  and  3  of the power circuit, not shown. The (cathode) gate of thyristor K is connected to a first terminal of winding T 2 , the other terminal of which is connected to terminal  3  generally representing a reference voltage (for example, the ground). On the primary side, control circuit  1  is generally supplied by a low-voltage source (not shown). Either low-frequency control transformers (up to a few tens of kilohertz), or transformers excited by a synchronous pulse upon each halfwave of an A.C. supply voltage on the secondary side are provided. 
   A disadvantage of control systems of the type illustrated in  FIG. 1  is that a discrete transformer is required, which is thus bulky and expensive. 
     FIG. 2  shows a second known example of a galvanic isolation control system. In the example of  FIG. 2 , the crossing of isolation barrier IB is optical. An optocoupler OP, the excitation diode D of which receives a control signal from a circuit  4  (CTRL) and the phototransistor TO of which provides a control signal to a circuit  5  (PWCTRL) of control of a switch K is for example used. As previously, switch K is connected across two terminals  2  and  3  downstream of the isolation barrier. In the example of  FIG. 2 , said switch again is a thyristor having its gate connected to control circuit  5 . 
   Even though the optocoupler may in some cases be integrated, the use of control electronics (circuit  5 ) downstream of the isolation barrier most often is a disadvantage. 
   Other galvanic isolation barriers are known. For example, the simplest is formed of capacitors placed on each conductor to be isolated. The capacitors must then hold high voltages and are thus bulky and expensive. Further, as in the case of  FIG. 2 , they require electronic circuitry to control the thyristor or the triac that constitutes the switch downstream of the isolation barrier. 
   SUMMARY OF THE INVENTION 
   The present invention aims at providing a system for controlling a power switch that respects the constraints of a galvanic isolation between a control part and a power part and that requires no control electronics downstream of this isolation barrier. 
   The present invention also aims at providing a solution which is neither bulky nor expensive. 
   The present invention further aims at providing a solution which optimizes the control system integration. 
   To achieve these objects, the present invention provides a circuit for controlling a power switch by means of at least one galvanic isolation transformer made in the form of planar conductive windings on an isolating substrate on which passive components constitutive of a high-frequency oscillating circuit of excitation of a primary winding of the transformer are integrated, the transformer substrate being placed on a wafer on which is assembled a circuit chip integrating the power switch. 
   According to an embodiment of the present invention, an active component of the oscillating circuit for controlling the transformer is made in the form of an integrated circuit chip placed on a surface of said substrate opposite to the wafer on which this substrate is assembled. 
   According to an embodiment of the present invention, the frequency of excitation of the transformer by the oscillating circuit is greater than 40 MHz. 
   According to an embodiment of the present invention, said substrate is glass. 
   According to an embodiment of the present invention, a secondary winding of the transformer is connected to a control electrode of the power switch via a diode, the latter forming, with a stray capacitance of the power switch control electrode, a peak detector. 
   According to an embodiment of the present invention, the power switch is a thyristor or a triac. 
   According to an embodiment of the present invention, the circuit includes two transformers for controlling two power switches respectively associated with a polarity of an A.C. power supply, the two primary windings of the two transformers being controlled by a same oscillating circuit. 
   The present invention also provides a transmission-reception device of a high-frequency signal on a low-frequency supply network, including:
         a transmitter formed of a high-frequency oscillating signal of excitation of a primary winding of a first transformer having a secondary winding connected to the network conductors; and   a receiver formed of a second galvanic isolation transformer for controlling a switch, at least said first transformer being made in the form of planar conductive windings on a first isolating substrate on which are integrated passive components constitutive of the oscillating circuit.       

   According to an embodiment of the present invention, said second transformer is made in the form of planar conductive windings on a second isolating substrate, the second substrate being placed on a wafer on which is assembled a circuit chip integrating the switch. 
   The foregoing and other 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, in which: 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1 and 2 , previously described, are intended to show the state of the art and the problem to solve; 
       FIG. 3  very schematically shows an embodiment of a circuit for controlling a power switch according to the present invention; 
       FIG. 4  shows a top view of an embodiment of a circuit integrating an isolation transformer according to the present invention; 
       FIG. 5  shows an embodiment of a very high-frequency control circuit and of an isolation transformer according to the present invention; 
       FIG. 6  is a simplified cross-section view of an integrated circuit according to the present invention; 
       FIG. 7  shows an embodiment of a circuit for controlling two power switches according to the present invention; and 
       FIG. 8  shows an example of application of the circuit of the present invention to convey control signals in a power switch through an A.C. electric power system. 
   

   DETAILED DESCRIPTION 
   The same elements have been designated with the same references in the different drawings. For clarity, only those elements which are necessary to the understanding of the present invention have been shown in the drawings and will be described hereafter. In particular, the circuits to be controllable by the power switch, have not been shown and are no object of the present invention. Neither have the possible circuits providing the operating set point of the control circuit upstream of the isolation barrier. 
     FIG. 3  shows an embodiment of a control circuit  10  of a power switch K with a galvanic isolation (barrier IB). The representation of  FIG. 3  should be compared with those of conventional  FIGS. 1 and 2 . 
   According to the present invention, a transformer  11  at very high frequency (several tens of MHz at least) having a primary winding  12  controlled by a circuit  13  (OSC) is used. A secondary winding  14  of transformer  11  is used to control a power switch K downstream of isolation barrier IB. As previously, switch K is connected between two terminals  2  and  3  of the circuit to be controlled. In the example of  FIG. 3 , switch K is formed of a thyristor having its (cathode) gate connected, according to the present invention, to a first terminal of winding  14  by a diode D 1 , the cathode of diode D 1  being connected to the gate of thyristor K. The function of diode D 1  is to form, with the junction capacitor of thyristor K, a peak detector of the signal in winding  14 . 
   At the primary, circuit  13  forms an oscillator powered by a low D.C. voltage (VCC-GND). A control terminal  15  receives a signal for disabling the oscillator of circuit  13 . An output terminal  16  of circuit  13  is connected to a first terminal of primary winding  12  of transformer  11 , the second terminal of winding  12  being connected to ground GND. 
   A feature of the present invention is to use a transformer  11  integrated on an isolating substrate (preferably, made of glass). To enable integration of the transformer, the excitation frequency thereof is, according to the present invention, of several tens of MHz and, preferably, greater than 40 MHz. 
   Another feature of the present invention is to take advantage of the insulating substrate of the transformer to integrate therein all the passive components that constitute control circuit  13 . 
   Another feature of the present invention is to use the isolating substrate on which is integrated transformer  11  to isolate the passive components integrated on this substrate, with respect to the power electrodes of the switch on the secondary side. This feature of the present invention will better appear from the subsequent discussion of FIG.  6 . 
     FIGS. 4 and 5  respectively show, by the equivalent electric diagram and by a top view of the isolating substrate, an embodiment of oscillating circuit  13 , of control circuit  10 , and of transformer  11  of the present invention. 
   In the example of  FIG. 4 , circuit  13  is formed of a collpits-type oscillator. It includes an NPN-type bipolar transistor N. The collector of transistor N is connected to a terminal  23  of application of supply voltage VCC. Its emitter is connected, by a capacitor C 1 , to a first end  12 ′ of primary winding  12  of transformer  11 . The base of transistor N is connected, by an inductance L in series with a capacitor C 2 , to ground terminal GND. The base of transistor N is also connected to the junction point  24  of two resistors R 1  and R 2  forming a polarity dividing bridge between terminals  23  and  22 . Two capacitors C 3  and C 4  in series connect the base of transistor N to ground. The midpoint  25  of this series connection is connected to the emitter of transistor N. This is a conventional structure of an oscillator with a negative resistance obtained by the feedback on the transistor emitter (base-emitter junction and capacitors C 3  and C 4 ). A resistor R 3  connects the emitter of transistor N to a control input  26 . Resistor R 3  biases the transistor and sets the collector (or emitter) current. The base-emitter voltage of transistor N is set by the ratio of resistance R 1  to the sum of resistances R 2  and R 3 . When input  26  is connected to ground, the oscillator is active. To deactivate the oscillator, terminal  26  is connected to positive voltage VCC, which annuls the collector-emitter voltage of transistor N. 
   The oscillation frequency is provided by relation 1/(2π√{square root over (LC)}), where C represents the equivalent capacitance of capacitors C 2 , C 3 , and C 4  in series, neglecting the stray capacitances of the transistor. 
   The different passive components described in relation with  FIG. 4  can be found on the top view of FIG.  5 . 
   In the example of  FIG. 5 , the substrate (schematized by dotted lines  20  in  FIG. 4 ) is a glass substrate on which are deposited conductive layers to form the integrated passive components. A first metallization level is deposited on the glass substrate and is used to define tracks  33  and  38  of connection of central ends of planar windings  12  and  14  constitutive of transformer  11 . A second metallization level  31  defines a ground plane intended for being connected (terminal  22 ,  FIG. 4 ) outside of the circuit. 
   In the example of  FIG. 5 , three metallization levels separated from one another by an insulator are used. The metallization levels and especially the second one (ground plane) include openings taking part in the component forming. 
   Transformer  11  is formed by means of two planar concentric conductive tracks. These tracks are respectively formed, for example, in the second and third metallization levels. A first track defines primary winding  12 , a first end  12 ′ of which is connected, by section  33  in the first metallization level, to ground plane  31 . A second end  12 ″ of winding  12  is connected to a first electrode of capacitor C 1  formed, for example, in the second level. A second electrode of capacitor C 1 , formed for example in the first level, is connected to a conductive section  25 ′ of the second level representing midpoint  25  of FIG.  4 . Section  25 ′ connects an emitter terminal  27  of transistor N to a first electrode of capacitor C 3  formed, for example, in the first level, to a first electrode of capacitor C 4  formed for example in the second level, to the second electrode of capacitor C 1  and to a first terminal of resistor R 3 . Resistor R 3  is, like resistors R 1  and R 2 , for example formed by a TaN (tantalum nitride) track in an opening formed in the metallization levels. The other terminal of resistor R 3  is connected, for example in the second level, to terminal  26  of application of the control signal. A second electrode of capacitor C 4 , formed for example in the first level, is connected to ground plane  31 . A second electrode of capacitor C 3 , formed for example in the second level, is connected to a conductive section  24 ′ of the second level symbolized by point  24  of FIG.  4 . Section  24 ′ connects a base terminal  28  of transistor N, the second electrode of capacitor C 3 , a first electrode of capacitor C 2 , formed for example in the second level in first respective terminals of resistors R 1  and R 2 . The second terminal of resistor R 1  is connected to a terminal  29  representing the collector of transistor N. The second terminal of resistor R 2  is connected to ground plane  31 . The second electrode of capacitor C 2 , formed for example in the first level, is connected to a first end of a planar concentric winding, formed in the second level in an opening  34  of the ground plane and defining inductance L. The second (central) end of the winding of inductor L is connected, by a section  35  of the third level running over the winding, to ground plane  31 . The secondary winding of transformer  11  is obtained by means of a conductive pattern  14  of the third level, concentric to winding  12 , and the two ends of which are connected to pads  36  and  37  of the circuit. The central end of winding  14  is connected by section  38  of the first level to a track connecting terminal  37 . The different connections between conductive levels are performed by means of vias. 
   In the embodiment of  FIG. 5 , the primary and secondary windings of transformer  11  have a same number of spirals. The transformation ratio thus is 1. 
   In the example of  FIG. 5 , the second metallization level in which the ground plane is formed is the level in which are also formed most of the connection tracks. Other configurations are of course possible. 
   As an alternative, only two metallization levels are used. Transformer  11  is then formed by means of two concentric coplanar conductive tracks. These tracks are formed in the first metallization level forming the ground plane in openings of which are also formed, as in  FIG. 5 , connection tracks, electrodes of the capacitors, winding L and the resistors. The secondary winding of transformer  11  is obtained by means of an imbricated pattern concentric and coplanar to the primary winding. The connections of the central ends of the windings with peripheral elements are formed in the same level. 
   As an alternative to the representation of  FIG. 5  in which the dielectric of transformer  11  is formed of air, it could be provided to cover the front surface of glass substrate  20  with an isolating material (for example, a resin) which will then form the transformer dielectric. 
     FIG. 6  shows in a very simplified view a circuit according to the present invention seen in cross-section and integrated with switch K. In the example of  FIG. 6 , a silicon chip  40  on which is formed power switch K is placed on a substrate  42 , for example, a printed circuit board or package. Different conductive tracks may be present on the substrate and have been symbolized by a metallization layer  41 . Thyristor  40  is very schematically shown in FIG.  6  and is symbolized by an N-type substrate  43 , a rear surface of which (provided with a metallization) represents the anode. A track of layer  41  thus connects this anode to terminal  2 . At the front surface is present a P-type region  44  in which a P + -type area  45  is formed for the thyristor cathode (it is thus connected to terminal  3 ), its gate being connected to region  44 . At the periphery of chip  40 , well  46  enabling the reverse voltage hold of the circuit has been symbolized. The representation of  FIG. 6  is purely arbitrary and schematic. In particular, to simplify, diode D 1  has not been shown in FIG.  6 . In practice, it will be integrated with switch K in chip  40 . The forming of the power switch in itself is no object of the present invention. What matters is to have a chip  40  which is placed by one of its power electrodes on a metallization  41  of substrate  42 . According to the present invention, glass substrate or wafer  20  supporting the integrated passive components is also placed on substrate  42  by its rear surface (opposite to that provided with the metallizations in which the different components are formed). At the front surface of glass substrate  20 , the different passive components have been symbolized by a layer  47  from which the three conductors  22 ,  23 , and  26  of the control circuit extend and, toward chip  40 , a conductor  48  connecting pad  36  ( FIG. 5 ) to cathode  3  of switch K, and a conductor  49  connecting the other pad  37  of secondary winding  14  of transformer  11  to region  44 . 
     FIG. 6  shows an alternative of the present invention in which transistor N ( FIG. 4 ) is placed in the form of an integrated circuit chip  50  on the front surface of glass substrate  20 . The adapting of this front surface to have the contact pads of the collector, of the emitter, and of the base of transistor N appear in front of those of chip  50  is within the abilities of those skilled in the art. 
   The representation of  FIG. 6  shows that glass substrate  20  not only plays the role of a galvanic isolation barrier for transformer  11 , but that it is also used to electrically isolate the control circuit from the tracks of printed circuit  42 . This is particularly advantageous since, most often, other components than thyristor K and diode D 1  are placed on substrate  42 . 
   In practice, all the passive components of the circuit of the present invention may be integrated on a glass substrate having a side length on the order of from 5 to 10 millimeters. The value of inductance L is, preferably, smaller than some hundred nanohenries. The value of the different capacitances is, preferably, smaller than one nanofarad. The values of the different resistances are, preferably, smaller than 100 kiloohms. 
   An advantage of the present invention is that it enables integrating all the components of a power switch control circuit, without requiring low-voltage control circuits downstream of the isolation barrier with respect to the control signals. 
   Another advantage of the present invention is that at the frequencies chosen for the transformer, the capacitors of the oscillator of its control circuit remain integrable. 
   An advantage of the present invention is that the control circuit is much less bulky than conventional circuits. 
   Another advantage of the present invention, more specifically as compared to an optocoupler circuit, is that it is no longer necessary to position an optical emitter with respect to a receiver, which, even in an integrated manner, is difficult to obtain in conventional circuits. 
   Although the present invention has been described hereabove in relation with a power switch formed by a thyristor, said switch may be any power switch, for example, a triac, or an anode-gate thyristor. 
     FIG. 7  shows a second embodiment of a power switch control circuit according to the present invention. In this embodiment, two thyristors Th 1  and Th 2  are used downstream of isolation barrier IB. The anode of thyristor Th 1  and the cathode of thyristor Th 2  are connected to terminal  2 . The cathode of thyristor Th 1  and the anode of thyristor Th 2  are connected to terminal  3 . The respective gates of thyristors Th 1  and Th 2  are connected to the cathodes of diodes D 1  and D 2  having their respective anodes connected to first ends  51  and  52  of secondary windings  53  and  54  of distinct isolation transformers  55  and  56 . Each isolation transformer  55  and  56  is formed in accordance with what has been discussed hereabove in relation with the first embodiment. Accordingly, respective primary windings  57  and  58  of transformers  55  and  56  are controlled by an oscillating circuit  13 . A single oscillating circuit  13  is sufficient, its output  16  being connected to a first end of each winding  57  and  58 , the respective second ends of which are connected to ground  22  on the primary side. On the secondary side, the ends of windings  53  and  54  are connected, respectively, to terminals  2  and  3 . The operation of the circuit of  FIG. 7  can be deduced from that discussed hereabove in relation with FIG.  4 . 
     FIG. 8  shows the simplified electric diagram of an application of the present invention to the transmission of control signals through the electric power system (the mains). This system is symbolized in  FIG. 8  by two conductors P and N conveying a low-frequency A.C. voltage (for example, 220 volts, 50 Hz). At a point of the system, a transmitter  60  is provided. This transmitter uses a first transformer  61  according to the present invention controlled by a circuit  13  wired as in the preceding embodiments. The modulation of the signal to be transmitted is, for example, performed by the disable control signal (terminal  26 ) of circuit  13 . Primary  62  of transformer  61  is upstream of a first isolation barrier IB 1  which isolates the transformer control from the electric system. Secondary winding  64  of the transformer is connected by each of its ends to conductors P and N of the mains. Preferably, a coupling capacitor Ca is interposed between one of the mains conductors and winding  64 . At another point of the system, a receiver  70  of the very-high-frequency signals sent by transformer  61  is provided. On the receive side, a transformer  71  is used to convey these signals through a second isolation barrier IB 2  of the receive circuit with respect to the electric system. A primary winding  72  of transformer  71  is connected to conductors P and N of the mains. A decoupling capacitor Cb is, preferably, interposed between one of the conductors and an end of winding  72 . A secondary winding  74  of transformer  71  is connected to the control terminal of a power switch K. The two power electrodes  2  and  3  of switch K are series-connected with a load or with a load to be controlled. As previously, a diode D 1  is interposed between the control electrode (for example, the gate of a thyristor) and winding  74 . As for transformer  61 , transformer  71  is a very high frequency transformer according to the present invention. Accordingly, on the side of emitter  60 , oscillator  13  and transformer  61  are integrated on a same glass substrate. On the receive side, transformer  71  is integrated on a glass substrate. Switch K and diode D 1  are made in the form of an integrated circuit chip placed on a same isolating substrate as transformer  71 . 
   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 sizing of the different components of a control circuit according to the present invention is within the abilities of those skilled in the art based on the functional indications given hereabove and on the application. Further, other oscillator structures may be used. Many oscillator structures, preferably based on resistive, capacitive, and inductive components and on at least one bipolar transistor are available to those skilled in the art. 
   Having thus described at least one illustrative embodiment of the invention, various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. The invention is limited only as defined in the following claims and the equivalents thereto.