Control of an anode-gate thyristor

A circuit for controlling an anode-gate thyristor includes a first transistor that couples a thyristor gate to a first terminal to receive a potential lower than a potential of a second terminal connected to the thyristor anode. A control terminal of the first transistor is driven by a control signal which is positive with respect to the potential of the first terminal.

PRIORITY CLAIM

This application claims the priority benefit of French Application for Patent No. 1850069, filed on Jan. 5, 2018, the content of which is hereby incorporated by reference in its entirety to the maximum extent allowable by law.

TECHNICAL FIELD

The present disclosure generally relates to electronic circuits and, more specifically, to the control of thyristor-type power switches. The present disclosure more particularly applies to the control of a thyristor connected to an AC voltage, such as the control of an AC load or the control of a thyristor of a controllable rectifying bridge.

BACKGROUND

The control of a thyristor-type or triac-type power switch requires extracting or injecting a current from or into the gate thereof. The generation of this current may involve using a specific circuit to generate a current galvanically isolated from upstream circuits, which operate under a different voltage or with a different potential reference. This is particularly true in power applications where the switch controls an AC load or is connected to terminals of an AC voltage, as is the case for a rectifying bridge.

Other applications avoid an insulation transformer at the cost of utilizing a pulse control of the thyristors. For example, U.S. Pat. No. 9,525,361 (incorporated by reference) describes a rectifying bridge having two anode-gate thyristors provided in the lower portion of the bridge, that is, with the anodes connected to the most negative potential of the rectified voltage (typically, a reference potential or ground).

SUMMARY

It is an aim of embodiments of the present description to at least partially address one or more previously unaddressed issues.

An embodiment overcomes all or part of the disadvantages of thyristor control circuits.

An embodiment provides an approach avoiding using an isolation transformer or an optocoupler.

An embodiment provides an approach avoiding a pulse control of the thyristors.

An embodiment provides an approach particularly adapted to the control of an anode-gate thyristor.

An embodiment provides an approach particularly adapted to the control of an AC load by an anode-gate thyristor.

An embodiment provides an approach particularly adapted to the control of anode-gate thyristors of a controllable rectifying bridge.

Thus, an embodiment provides a circuit for controlling an anode-gate thyristor. The circuit includes a first transistor coupling the thyristor gate to a first terminal applying a potential lower than the potential of a second terminal having the thyristor anode connected thereto, and a control terminal of the first transistor is intended to receive a control signal which is positive with respect to the potential of the first terminal.

According to an embodiment, the control terminal of the first transistor is coupled, by a second transistor, to a third terminal for supplying a potential which is positive with respect to the potential of the second terminal.

According to an embodiment, the third and first terminals are intended to receive a DC voltage.

According to an embodiment, the second transistor is controlled by a signal referenced to the potential of the second terminal.

According to an embodiment, the second transistor is directly controlled by a microcontroller powered between the third and second terminals.

According to an embodiment, the second transistor is controlled by a circuit for detection of the direction of the halfwave of an AC voltage applied across the thyristor.

According to an embodiment, the second transistor is a bipolar transistor.

An embodiment provides a rectifying bridge comprising at least one anode-gate thyristor and at least one control circuit.

According to an embodiment, the rectifying bridge comprises two anode-gate thyristors, each associated with a first transistor and with a second transistor.

An embodiment provides a circuit for controlling an AC load comprising an anode-gate thyristor and a control circuit.

DETAILED DESCRIPTION

The same elements have been designated with the same reference numerals in different drawings.

For clarity, only those steps and elements which are useful to the understanding of the embodiments which will be described have been shown and will be detailed. In particular, the applications of the described rectifying bridge or of the controlled AC load have not been detailed, the described embodiments being compatible with usual applications and loads for which a controllable rectifying bridge or a thyristor are desired to be used to control an AC load. The generation of the thyristor control orders according to the needs of the load or to other criteria relating to the application has not been detailed either, the described embodiments being here again compatible with known techniques.

Throughout the present disclosure, the term “connected” is used to designate a direct electrical connection between circuit elements, whereas the term “coupled” is used to designate an electrical connection between circuit elements that may be a direct electrical connection, or may be connected via one or more intermediate elements such as resistors, capacitors, transistors or buffers. Unless indicated otherwise, when the term “coupled” is used, the connection can be implemented by a direct connection.

The terms “approximately”, “about”, and “on the order of” are used herein to designate a tolerance of plus or minus 10%, and preferably of plus or minus 5%, of the value in question.

FIG. 1is a simplified representation, partially in the form of blocks, of an embodiment of a controllable rectifying bridge comprising anode-gate thyristors.

This rectifying bridge comprises two parallel branches between two terminals21and22delivering a rectified voltage Vout. Each branch comprises a diode D1, D2, connected to a thyristor T1, T2, between terminals21and22, the thyristor anodes being connected to terminal22and the diode cathodes being connected to terminal21. The respective midpoints of the two branches define terminals23and24to which the AC voltage Vac to be rectified is applied, terminal23being coupled to the anode of diode D1and to the cathode of thyristor T1, terminal24being coupled to the anode of diode D2and to the cathode of thyristor T2. Thyristor T2is intended to be turned on during all or part of the positive halfwaves of voltage Vac. Thyristor T1is intended to be turned on during all or part of the negative halfwaves of voltage Vac. Thyristors T1and T2are anode-gate thyristors. They are thus turned on by extraction of a current from their gate.

In the embodiment ofFIG. 1, signals for controlling thyristors T1and T2are produced by a digital circuit2, for example, a microcontroller (MCU). Microcontroller2is powered with a DC voltage Vcc1referenced to the system ground, that is, reference terminal22of rectified voltage Vout. Since this reference corresponds to the potential of the anodes of thyristors T1and T2and the thyristors are anode-gate thyristors, a potential lower than that of terminal22is used to extract the gate current.

According to the described embodiments, ground22is used as an intermediate potential between two DC voltages obtained from an auxiliary voltage source (not shown) supplying a DC voltage Vcc. Voltage Vcc is applied between two terminals25and26, different from terminals21,22,23, and24. Voltage Vcc is used to supply voltage Vcc1for powering circuit2and is used to supply a voltage Vcc2having its most positive potential corresponding to ground22. Thus, ground22is considered as an intermediate midpoint of voltage Vcc.

In the example ofFIG. 1, terminal25is connected to input IN of a linear regulator3having an output terminal27OUT supplying the most positive potential of power supply voltage Vcc1of microcontroller2, and referenced to ground, and reference terminal GND of regulator3is coupled to terminal22. A capacitor C1connected between terminals27and22smooths voltage Vcc1. As a variation, regulator3is replaced with a Zener diode (not shown) connected between terminals25and22. A capacitor C2is connected between terminals26and22.

To obtain voltage Vcc2, a Zener diode DZ is, for example, connected between terminals22and26, its anode being on the side of terminal26. A resistor29(typically of a few hundreds of ohms to avoid dissipating excess power) couples terminals25and22. A capacitor connected between terminals22and26smooths voltage Vcc2.

A voltage Vcc1for powering microcontroller2, and a voltage −Vcc2negative with respect to terminal22enabling drawing of a gate current from thyristors T1and T2, are thus available.

For example, the gate of thyristor T1is coupled to terminal26by an NPN-type bipolar transistor31, the collector of transistor31being coupled, preferably by a protection resistor32, to the gate of thyristor T1, and its emitter being connected to terminal26. The base of transistor T1is coupled, by a voltage-to-current conversion or biasing resistor33in series with a MOS transistor34(for example, a PMOS), to terminal27. Transistor34is controlled by microcontroller2and has its source on the side of terminal27. Thus, when microcontroller2switches the transistor34to the on state by applying to its gate a low digital level (potential lower than the potential of terminal27), a current flows from terminal27, through transistor34and through resistor33. The base current applied to transistor31turns it on, which causes the extraction of a gate current from thyristor T1, causing it to be properly biased (positive anode-to-cathode voltage). The on state of thyristor T1is maintained until the current flowing therethrough disappears, that is, on inversion of the biasing of voltage Vac.

Similarly, the gate of thyristor T2is coupled to terminal26by an NPN-type bipolar transistor35, the collector of transistor35being coupled, preferably by a protection resistor36, to the gate of thyristor T2and its emitter being connected to terminal26. The base of transistor T2is coupled, by a voltage-to-current conversion or biasing resistor37in series with a MOS transistor38(for example, a PMOS) to terminal27. Transistor38is controlled by microcontroller2and has its source on the side of terminal27. Thus, when microcontroller2turns the transistor38to the on state by applying to its gate a low digital level (potential lower than the potential of terminal27), a current flows from terminal27, through transistor38, and through resistor37. The base current applied to transistor35turns it on, which causes the extraction of a gate current from thyristor T1, causing it to be properly biased (positive anode-to-cathode voltage). The on state of thyristor T2is maintained until the current flowing therethrough disappears, that is, on inversion of the biasing of voltage Vac.

FIGS. 2A, 2B, 2C, and 2Dillustrate, in timing diagrams, the operation of the rectifying bridge ofFIG. 1.

FIG. 2Ashows an example of the shape of voltage Vac.FIG. 2Bshows an example of drain signal V38of transistor38.FIG. 2Cshows an example of a drain signal V34of transistor34.FIG. 2Dshows the corresponding shape of voltage Vout. For simplification, a steady state is considered and it is assumed that the load connected to terminals21and22is neither inductive (no phase shift between voltages Vout and Vac), nor capacitive (lack of smoothing of voltage Vout). Further, the time and amplitude effects of voltage drops in switches in the on state are neglected.

FIGS. 2A to 2Dillustrate a particularly simple control of transistors34and38, corresponding to a simple inversion of the signals applied on their gates. During positive halfwaves of voltage Vac, transistor38is turned on to turn on thyristor T2. During negative halfwaves of voltage Vac, transistor34is turned on to turn on thyristor T1. The control by microcontroller2is compatible with a phase angle control of thyristors T1and T2, enabling adjustment of the output power. In particular, the limitation of the inrush current is obtained by phase-angle controlling thyristors T1and T2.

It can be seen that the control signals supplied by microcontroller2are of same nature, that is, are digital signals. Further, these signals may remain active (i.e., are time-invariant) during the entire period of voltage Vac having the corresponding thyristor assigned thereto (i.e., the corresponding halfwave), and thus do not need to be pulse signals with respect to the frequency of voltage Vac.

For the assembly shown inFIG. 1to operate properly, auxiliary voltage Vcc has an amplitude at least equal to the sum of voltage Vcc1used to power the microcontroller, and the voltage Vcc2set by Zener diode DZ, and the minimum voltage drop in linear regulator3.

The threshold voltage of Zener diode DZ does not need to be particularly high. It just has to be greater than the voltage drop in resistor32or33sufficient to impose a current greater than the turn-on current of thyristor T1or T2, plus the corresponding collector-to-emitter voltage drop in transistor31or35. As a specific embodiment, a Zener diode of a few volts, for example, in the order of 3 volts, may be used. As a variation, Zener diode DZ may be replaced with another regulator.

Auxiliary voltage Vcc may be obtained in several ways. For example, it may be a voltage otherwise available in the electronic device containing the controlled rectifying circuit. According to another example, voltage Vcc is extracted from voltage Vac by means of a switched-mode power supply with a galvanic isolation.

The circuit described in relation withFIG. 1avoids using conversion elements of an optocoupler or galvanic isolation transformer type to apply the control signals to the thyristors. This considerably simplifies the forming of a controllable rectifying bridge and decreases the cost thereof. Further, the need for a pulse control at a frequency greater than the frequency of voltage Vac such as described in above-mentioned U.S. Pat. No. 9,525,361 is avoided. This avoids risks for the turning on not to occur and simplifies the manufacturing.

It should be noted that it is not disturbing, once thyristor T1has been turned on, for its anode potential to fall below the potential of terminal26. Indeed, it remains on until the current that it conducts disappears, that is, until the end of the negative halfwave.

FIG. 3is a simplified representation, partially in the form of blocks, of another embodiment of a controllable rectifying bridge with anode-gate thyristors T1, T2.

As compared with the assembly ofFIG. 1, the microcontroller is replaced with a switching circuit supplying the control signals of the gates of transistors31and35, from a detection of the direction of the halfwave of AC voltage Vac.

Further,FIG. 3illustrates the case of a mixed bridge, that is, two diodes D3and D4respectively couple terminals23and24to terminal22via a resistor R. The function of resistor R is to limit the inrush current at the starting of the system (transient state), that is, before thyristors T1and T2are controlled. This variation may also be implemented in the embodiment ofFIG. 1.

It also uses the principle of a voltage Vcc applied between two terminals25and26, with a ground22of the assembly between the potentials of terminals25and26, and of the use of a voltage regulator3and a Zener diode DZ to determine voltages Vcc1and Vcc2. It also comprises the assemblies with transistors31and35(with resistors32,33,36,37). Capacitors C1and C2and resistor29are also present.

Circuit4comprises two P-type MOS transistors34and38, having their respective drains, as inFIG. 1, coupled to resistors33and37. The sources of transistors34and37are coupled via a starting circuit4. Circuit4comprises a switch42(for example, a PMOS) coupling the sources of transistors34and37to terminal27. Transistor42is controlled by a turn-on (TO) circuit44, for example, an amplitude detector which detects the end of the transient period to only activate circuit44once the inrush current has passed. As long as transistor42is not on, the bridge operation is in inrush current limitation by resistor R. A power factor correction circuit which comprises an output capable of controlling a relay shorting resistor R may be used.

In steady state, transistor42is on and transistors34and38are thus powered. The control of transistors34and38involves detecting the current halfwave of voltage Vac.

To achieve this, a circuit5for detecting the direction of the halfwave of voltage Vac is provided. For example, detecting of negative halfwaves is performed with a bipolar transistor52, for example, of NPN type, having its emitter coupled to the junction point of a resistive divider (resistors54and56in series) between terminal27and terminal22, having its collector connected to terminal22, and having its base coupled, by a resistor58, to terminal24. The junction point of resistors54and56(and thus the collector of transistor52) is connected to the gate of transistor34. Transistor52is on during negative halfwaves of voltage Vac (positive base-emitter voltage), which makes the gate voltage of transistor34drop and causes the turning-on thereof (transistor42being on in steady state). As for the assembly ofFIG. 1, the turning-on of transistor34causes, via transistor31, the turning-on of thyristor T1.

The gate of transistor38is connected to the junction point of two MOS transistors62and64, respectively a PMOS and an NMOS, connected between terminals27and22, with their gates being connected to the collector of transistor52. Transistors62and64form a push-pull stage, transistor62being on when transistor52is on and transistor64being on when transistor52is off. Thus, transistor38is turned on during positive halfwaves of voltage Vac (transistor52off) since its gate is taken to a low potential (ground22to within the voltage drop in transistor64). In the same way as for the assembly ofFIG. 1, the turning-on of transistor38causes, via transistor35, the turning-on of thyristor T2. Preferably, a diode D52couples the base and the emitter of transistor52(anode on the emitter side) to protect the emitter of transistor52during positive halfwaves.

FIGS. 4A, 4B, 4C, 4D, 4E, and 4Fillustrate in timing diagrams the operation of the mixed bridge ofFIG. 3.FIG. 4Ashows an example of the shape of voltage Vac.FIG. 4Bshows an example of the shape of signal V44supplied by circuit44for turning on transistor42.FIG. 4Cshows the shape of emitter signal V52of transistor52.FIG. 4Dshows an example of drain signal V34of transistor34.FIG. 4Eshows an example of drain signal V38of transistor38.FIG. 4Fshows the corresponding shape of voltage Vout. In the same way as forFIGS. 2A to 2C, for simplification, a steady state is considered and it is assumed that the load connected to terminals21and22is neither inductive (no phase shift between voltages Vout and Vac), nor capacitive (lack of smoothing of voltage Vout). Further, the effects in terms of time and amplitude of the voltage drops of the switches in the on state are neglected.FIGS. 4A to 4Fillustrate, in their left-hand portion, the operation in transient state (limitation of the inrush current) and, in their right-hand portion, the operation in steady state.

During the transient state, transistor42is off (signal V44in the high state) and transistors34and38are thus non-conductive independently from AC voltage Vac. Their drains are thus unconnected and transistors31and35cannot be controlled. The rectification is performed by diode bridge D1, D2, D3, D4and the current is limited by resistor R.

In steady state, transistor42is on and transistors34and38are thus alternately controlled according to the sign of voltage Vac detected by transistor52.

An advantage of the assembly ofFIG. 3is that it is particularly simple to form and requires no microcontroller.

Other circuits than circuits4and5may be provided, and may be provided to effect the described functionalities.

FIG. 5is a simplified representation, partially in the form of blocks, of an embodiment of an AC load control circuit.

According to this embodiment, a load8to be powered under an AC voltage Vac is series-connected with a parallel association of two thyristors T1and T3, respectively with an anode gate and a cathode gate, between terminals23and24of application of AC voltage Vac. Load8is on the side of terminal23, and terminal24forms the voltage reference (ground22) of the assembly. The assembly comprises microcontroller2, regulator3, and the elements described in relation withFIG. 1, in particular capacitors C1and C2, Zener diode DZ, and resistor29. However, cathode-gate thyristor T3, intended to be on during positive halfwaves of voltage Vac, is directly controlled by microcontroller2, has its gate coupled by a resistor39to an output terminal of microcontroller2. Indeed, since microcontroller2has the same voltage reference as the cathode of thyristor T3, it may directly sample therefrom a gate current to turn it on. Transistors38and35ofFIG. 1and their corresponding assemblies are thus not necessary. Transistor34and the assembly of bipolar transistors31(base resistor33coupled to transistor34and collector resistor32coupled to the gate of thyristor T1) are, however, required for the control of thyristor T1during negative halfwaves of voltage Vac. The operation of the rest of the circuit ofFIG. 5can be deduced from the operation discussed in relation withFIG. 1.

An advantage of the described embodiments is that they allow a control of anode-gate thyristors having their anodes interconnected and being powered with an AC voltage.

Another advantage of the described embodiments is that they avoid a pulse control of thyristors with a frequency greater than that of the AC voltage and thus avoid the risk for the thyristor turn-on current not to be reached.

Another advantage of the described embodiments is that they are compatible with an embodiment with no microcontroller.

Various embodiments have been described. Various modifications will occur to those skilled in the art. In particular, bipolar transistors31and35may be replaced with MOS transistors by way of a few modifications within the abilities of those skilled in the art to take into account the fact that a MOS transistor is voltage-controlled and not current-controlled. In particular, it will be ascertained that the MOS transistors withstand a gate voltage equal to the sum of voltages Vcc1and Vcc2. Otherwise, the MOS transistors are protected by a Zener diode in parallel with a resistor between the gate and the source of the transistors to limit the gate voltage. Further, the sizing of the different components depends on the application and their determination is within the abilities of those skilled in the art according to the needs of the application. Further, the practical implementation of the embodiments which have been described is within the abilities of those skilled in the art based on the functional indications given hereabove.