Integrated switch with RF transformer control

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.

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. 1very 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 T1connected to two output terminals of a control circuit1and a secondary winding T2that controls a power switch K. In the example ofFIG. 1, switch K is a thyristor connected between two terminals2and3of the power circuit, not shown. The (cathode) gate of thyristor K is connected to a first terminal of winding T2, the other terminal of which is connected to terminal3generally representing a reference voltage (for example, the ground). On the primary side, control circuit1is 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 inFIG. 1is that a discrete transformer is required, which is thus bulky and expensive.

FIG. 2shows a second known example of a galvanic isolation control system. In the example ofFIG. 2, the crossing of isolation barrier IB is optical. An optocoupler OP, the excitation diode D of which receives a control signal from a circuit4(CTRL) and the phototransistor TO of which provides a control signal to a circuit5(PWCTRL) of control of a switch K is for example used. As previously, switch K is connected across two terminals2and3downstream of the isolation barrier. In the example ofFIG. 2, said switch again is a thyristor having its gate connected to control circuit5.

Even though the optocoupler may in some cases be integrated, the use of control electronics (circuit5) 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 ofFIG. 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; anda 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.

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. 3shows an embodiment of a control circuit10of a power switch K with a galvanic isolation (barrier IB). The representation ofFIG. 3should be compared with those of conventionalFIGS. 1 and 2.

According to the present invention, a transformer11at very high frequency (several tens of MHz at least) having a primary winding12controlled by a circuit13(OSC) is used. A secondary winding14of transformer11is used to control a power switch K downstream of isolation barrier IB. As previously, switch K is connected between two terminals2and3of the circuit to be controlled. In the example ofFIG. 3, switch K is formed of a thyristor having its (cathode) gate connected, according to the present invention, to a first terminal of winding14by a diode D1, the cathode of diode D1being connected to the gate of thyristor K. The function of diode D1is to form, with the junction capacitor of thyristor K, a peak detector of the signal in winding14.

At the primary, circuit13forms an oscillator powered by a low D.C. voltage (VCC-GND). A control terminal15receives a signal for disabling the oscillator of circuit13. An output terminal16of circuit13is connected to a first terminal of primary winding12of transformer11, the second terminal of winding12being connected to ground GND.

A feature of the present invention is to use a transformer11integrated 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 circuit13.

Another feature of the present invention is to use the isolating substrate on which is integrated transformer11to 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 5respectively show, by the equivalent electric diagram and by a top view of the isolating substrate, an embodiment of oscillating circuit13, of control circuit10, and of transformer11of the present invention.

In the example ofFIG. 4, circuit13is formed of a collpits-type oscillator. It includes an NPN-type bipolar transistor N. The collector of transistor N is connected to a terminal23of application of supply voltage VCC. Its emitter is connected, by a capacitor C1, to a first end12′ of primary winding12of transformer11. The base of transistor N is connected, by an inductance L in series with a capacitor C2, to ground terminal GND. The base of transistor N is also connected to the junction point24of two resistors R1and R2forming a polarity dividing bridge between terminals23and22. Two capacitors C3and C4in series connect the base of transistor N to ground. The midpoint25of 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 C3and C4). A resistor R3connects the emitter of transistor N to a control input26. Resistor R3biases the transistor and sets the collector (or emitter) current. The base-emitter voltage of transistor N is set by the ratio of resistance R1to the sum of resistances R2and R3. When input26is connected to ground, the oscillator is active. To deactivate the oscillator, terminal26is 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 C2, C3, and C4in series, neglecting the stray capacitances of the transistor.

The different passive components described in relation withFIG. 4can be found on the top view of FIG.5.

In the example ofFIG. 5, the substrate (schematized by dotted lines20inFIG. 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 tracks33and38of connection of central ends of planar windings12and14constitutive of transformer11. A second metallization level31defines a ground plane intended for being connected (terminal22,FIG. 4) outside of the circuit.

In the example ofFIG. 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.

Transformer11is 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 winding12, a first end12′ of which is connected, by section33in the first metallization level, to ground plane31. A second end12″ of winding12is connected to a first electrode of capacitor C1formed, for example, in the second level. A second electrode of capacitor C1, formed for example in the first level, is connected to a conductive section25′ of the second level representing midpoint25of FIG.4. Section25′ connects an emitter terminal27of transistor N to a first electrode of capacitor C3formed, for example, in the first level, to a first electrode of capacitor C4formed for example in the second level, to the second electrode of capacitor C1and to a first terminal of resistor R3. Resistor R3is, like resistors R1and R2, for example formed by a TaN (tantalum nitride) track in an opening formed in the metallization levels. The other terminal of resistor R3is connected, for example in the second level, to terminal26of application of the control signal. A second electrode of capacitor C4, formed for example in the first level, is connected to ground plane31. A second electrode of capacitor C3, formed for example in the second level, is connected to a conductive section24′ of the second level symbolized by point24of FIG.4. Section24′ connects a base terminal28of transistor N, the second electrode of capacitor C3, a first electrode of capacitor C2, formed for example in the second level in first respective terminals of resistors R1and R2. The second terminal of resistor R1is connected to a terminal29representing the collector of transistor N. The second terminal of resistor R2is connected to ground plane31. The second electrode of capacitor C2, 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 opening34of the ground plane and defining inductance L. The second (central) end of the winding of inductor L is connected, by a section35of the third level running over the winding, to ground plane31. The secondary winding of transformer11is obtained by means of a conductive pattern14of the third level, concentric to winding12, and the two ends of which are connected to pads36and37of the circuit. The central end of winding14is connected by section38of the first level to a track connecting terminal37. The different connections between conductive levels are performed by means of vias.

In the embodiment ofFIG. 5, the primary and secondary windings of transformer11have a same number of spirals. The transformation ratio thus is 1.

In the example ofFIG. 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. Transformer11is 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 inFIG. 5, connection tracks, electrodes of the capacitors, winding L and the resistors. The secondary winding of transformer11is 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 ofFIG. 5in which the dielectric of transformer11is formed of air, it could be provided to cover the front surface of glass substrate20with an isolating material (for example, a resin) which will then form the transformer dielectric.

FIG. 6shows in a very simplified view a circuit according to the present invention seen in cross-section and integrated with switch K. In the example ofFIG. 6, a silicon chip40on which is formed power switch K is placed on a substrate42, for example, a printed circuit board or package. Different conductive tracks may be present on the substrate and have been symbolized by a metallization layer41. Thyristor40is very schematically shown in FIG.6and is symbolized by an N-type substrate43, a rear surface of which (provided with a metallization) represents the anode. A track of layer41thus connects this anode to terminal2. At the front surface is present a P-type region44in which a P+-type area45is formed for the thyristor cathode (it is thus connected to terminal3), its gate being connected to region44. At the periphery of chip40, well46enabling the reverse voltage hold of the circuit has been symbolized. The representation ofFIG. 6is purely arbitrary and schematic. In particular, to simplify, diode D1has not been shown in FIG.6. In practice, it will be integrated with switch K in chip40. The forming of the power switch in itself is no object of the present invention. What matters is to have a chip40which is placed by one of its power electrodes on a metallization41of substrate42. According to the present invention, glass substrate or wafer20supporting the integrated passive components is also placed on substrate42by its rear surface (opposite to that provided with the metallizations in which the different components are formed). At the front surface of glass substrate20, the different passive components have been symbolized by a layer47from which the three conductors22,23, and26of the control circuit extend and, toward chip40, a conductor48connecting pad36(FIG. 5) to cathode3of switch K, and a conductor49connecting the other pad37of secondary winding14of transformer11to region44.

FIG. 6shows an alternative of the present invention in which transistor N (FIG. 4) is placed in the form of an integrated circuit chip50on the front surface of glass substrate20. 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 chip50is within the abilities of those skilled in the art.

The representation ofFIG. 6shows that glass substrate20not only plays the role of a galvanic isolation barrier for transformer11, but that it is also used to electrically isolate the control circuit from the tracks of printed circuit42. This is particularly advantageous since, most often, other components than thyristor K and diode D1are placed on substrate42.

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. 7shows a second embodiment of a power switch control circuit according to the present invention. In this embodiment, two thyristors Th1and Th2are used downstream of isolation barrier IB. The anode of thyristor Th1and the cathode of thyristor Th2are connected to terminal2. The cathode of thyristor Th1and the anode of thyristor Th2are connected to terminal3. The respective gates of thyristors Th1and Th2are connected to the cathodes of diodes D1and D2having their respective anodes connected to first ends51and52of secondary windings53and54of distinct isolation transformers55and56. Each isolation transformer55and56is formed in accordance with what has been discussed hereabove in relation with the first embodiment. Accordingly, respective primary windings57and58of transformers55and56are controlled by an oscillating circuit13. A single oscillating circuit13is sufficient, its output16being connected to a first end of each winding57and58, the respective second ends of which are connected to ground22on the primary side. On the secondary side, the ends of windings53and54are connected, respectively, to terminals2and3. The operation of the circuit ofFIG. 7can be deduced from that discussed hereabove in relation with FIG.4.

FIG. 8shows 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 inFIG. 8by 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 transmitter60is provided. This transmitter uses a first transformer61according to the present invention controlled by a circuit13wired as in the preceding embodiments. The modulation of the signal to be transmitted is, for example, performed by the disable control signal (terminal26) of circuit13. Primary62of transformer61is upstream of a first isolation barrier IB1which isolates the transformer control from the electric system. Secondary winding64of 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 winding64. At another point of the system, a receiver70of the very-high-frequency signals sent by transformer61is provided. On the receive side, a transformer71is used to convey these signals through a second isolation barrier IB2of the receive circuit with respect to the electric system. A primary winding72of transformer71is 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 winding72. A secondary winding74of transformer71is connected to the control terminal of a power switch K. The two power electrodes2and3of switch K are series-connected with a load or with a load to be controlled. As previously, a diode D1is interposed between the control electrode (for example, the gate of a thyristor) and winding74. As for transformer61, transformer71is a very high frequency transformer according to the present invention. Accordingly, on the side of emitter60, oscillator13and transformer61are integrated on a same glass substrate. On the receive side, transformer71is integrated on a glass substrate. Switch K and diode D1are made in the form of an integrated circuit chip placed on a same isolating substrate as transformer71.

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.