Bidirectional AC/DC H-bridge power converter

This application claims the priority benefit of French patent application number 17/50209, the content of which is hereby incorporated by reference in its entirety to the maximum extent allowable by law.

The present disclosure relates to power converters, and more particularly aims at an AC/DC converter, that is, a converter capable of converting an AC voltage into a DC voltage.

An AC/DC converter conventionally comprises a stage for rectifying the AC voltage applied at its input, comprising one or a plurality of diodes, followed by a stage of low-pass filtering of the rectified AC voltage delivered by the rectifying stage, followed by a stage of DC/DC conversion of the DC voltage delivered by the low-pass filtering stage, typically a switched-mode conversion stage comprising one or a plurality of controlled switches and an isolation transformer.

A disadvantage of converters of this type lies in their relatively high cost and bulk, resulting from the high number of components that they comprise and from the size of these components.

In particular, the low-pass filtering stage conventionally comprises a large capacitor, typically, a capacitor of several tens of microfarads and of several hundreds of volts for a converter intended to receive on its AC input a 220-V AC voltage, for example, the mains voltage.

Further, to limit switching losses in the DC/DC conversion stage, the operating frequency of the conversion stage (or cut-off frequency) is generally limited to a few hundreds of kHz, which limits possibilities of miniaturizing the DC/DC conversion stage, and in particular the isolation transformer.

Another disadvantage of converters of this type is that the presence of the rectifying stage comprising diodes significantly limits the converter efficiency.

Further, in certain applications where the converter is used to power an active DC load, for example, a battery, there is a need for a reversible converter, that is, capable not only of converting the AC input voltage into a DC output voltage to power the load (typically to charge a battery from an AC voltage), but also of converting a DC voltage delivered by the load into an AC voltage supplied on the AC input terminals of the converter (typically to inject back electric power drawn from the battery onto the AC network). Due to their architecture, and particularly due to the presence of the rectifying stage comprising diodes and of the low-pass filtering stage, converters of the type described hereabove do not provide such a reversibility.

U.S. Pat. No. 6,424,548 describes a so-called direct conversion switched-mode AC/DC converter, that is, which comprises no rectifying stage comprising diodes and no low-pass filtering stage upstream of the switched-mode conversion stage. More particularly, this converter comprises four bidirectional switches forming a first controlled H bridge, followed by an isolation transformer comprising a primary winding and a secondary winding, magnetically coupled, followed by four nearly bidirectional switches forming a second controlled H bridge. The input of the first H bridge directly receives the AC voltage to be converted, the primary winding of the transformer is connected between the output terminals of the first H bridge, the secondary winding of the transformer is connected between the input terminals of the second H bridge, and the DC load to be powered is connected between the output terminals of the second H bridge.

An advantage of this converter is that it comprises no rectifying stage comprising diodes and no low-pass filtering stage upstream of the switched-mode conversion stage, which enables to decrease the cost and the bulk and to improve the efficiency as compared with conventional solutions. Further, this converter is reversible.

However, as in conventional converters, switching losses in the switched-mode conversion stage in practice prevent the increase of the switching frequency of the switches beyond a few hundreds of kHz, which limits possibilities of miniaturizing the system, and in particular the isolation transformer.

It would be desirable to have an AC/DC converter overcoming all or part of the above-mentioned disadvantages of existing converters.

Thus, an embodiment provides a AC-voltage-to-DC-voltage conversion circuit, comprising:

four first bidirectional switches forming a first H bridge, first and second input nodes of the bridge being respectively coupled to first and second nodes of application of the AC voltage;

a transformer comprising a primary winding and a secondary winding, magnetically coupled, first and second ends of the primary winding being respectively coupled to first and second output nodes of the first bridge;

a first inductance series-connected with the first bridge between the first input node of the bridge and the first node of application of the AC voltage;

a second inductance series-connected with the first bridge, between the first output node of the bridge and the first end of the primary winding; and

a control circuit capable of controlling the first bridge alternately to a first configuration where the switches of a first diagonal of the bridge are on and the switches of a second diagonal of the bridge are off, and to a second configuration where the switches of the first diagonal are off and the switches of the second diagonal are on,

the control circuit being capable, during a phase of transition between the first and second configurations, of successively:

turning on a first switch of the second diagonal;

turning off a first switch of the first diagonal when the current flowing through this switch takes a zero value;

turning on the second switch of the second diagonal; and

turning off the second switch of the first diagonal when the current flowing through this switch takes a zero value.

According to an embodiment, the conversion circuit further comprises four second switches forming a second H bridge, first and second input nodes of the second bridge being respectively coupled to first and second ends of the secondary winding of the transformer, and first and second output nodes of the second bridge being respectively coupled to first and second DC voltage supply nodes.

According to an embodiment, the control circuit is capable of controlling the second bridge alternately to a first configuration where the switches of a first diagonal of the bridge are on and the switches of a second diagonal of the bridge are off, and to a second configuration where the switches of the first diagonal are off and the switches of the second diagonal are on.

According to an embodiment, the control circuit is configured to switch the first bridge between its first and second configurations and to switch the second bridge between its first and second configurations substantially at the same frequency.

According to an embodiment, the control circuit is configured to switch the first bridge between its first and second configurations at a frequency greater than or equal to 1 MHz.

According to an embodiment, each first switch is capable of being controlled to the on state by the control circuit and to automatically turn off when the current flowing therethrough takes a zero value.

According to an embodiment, each first switch is equivalent to an anti-series association of first and second MOS transistors connected by their drains, the sources of the first and second transistors respectively forming the conduction nodes of the switch, and the gates of the first and second MOS transistors forming first and second switch control nodes.

According to an embodiment, the control circuit is configured to, when it controls a switch of the first bridge to the on state, apply a turn-on control signal to the gate of one of the first and second transistors of the switch and hold a blocking signal on the gate of the other transistor.

According to an embodiment, the control circuit is configured to, when it controls a switch of the first bridge to the on state, apply a turn-on control signal to the gate of the first transistor and a turn-off signal to the gate of the second transistor when the current to be conducted by the switch has a first biasing, and apply a turn-on control signal to the gate of the second transistor and a turn-off signal to the gate of the first transistor when the current to be conducted by the switch has a second biasing opposite to the first biasing.

According to an embodiment, the first switches are gallium nitride switches.

Another embodiment provides AC-voltage-to-DC-voltage conversion circuit, comprising:

four first bidirectional switches forming a first H bridge, first and second input nodes of the bridge being directly coupled respectively to first and second nodes of application of the AC voltage;

a first capacitor connected in parallel with the first bridge between the first and second input nodes of the bridge;

a second capacitor connected in parallel with the first bridge between first and second output nodes of the bridge; and

a control circuit capable of controlling the bridge alternately to a first configuration where the switches of a first diagonal of the bridge are on and the switches of a second diagonal of the bridge are off, and to a second configuration where the switches of the first diagonal are off and the switches of the second diagonal are on,

the control circuit being capable, during a phase of transition between the first and second configurations, of successively:

turning off the switches of the first diagonal; and

for each switch of the second diagonal, turning on the switch only when the voltage thereacross takes a zero value.

According to an embodiment, the conversion circuit further comprises four additional capacitors respectively connected across the four first switches.

According to an embodiment, the conversion circuit further comprises a transformer comprising a primary winding and a secondary winding, magnetically coupled, first and second ends of the primary winding being respectively coupled to first and second output nodes of the first bridge.

According to an embodiment, the conversion circuit further comprises four second switches forming a second H bridge, first and second input nodes of the second bridge being respectively coupled to first and second ends of the secondary winding of the transformer, and first and second output nodes of the second bridge being respectively coupled to first and second DC voltage supply nodes.

According to an embodiment, the control circuit is capable of controlling the second bridge alternately to a first configuration where the switches of a first diagonal of the bridge are on and the switches of a second diagonal of the bridge are off, and to a second configuration where the switches of the first diagonal are off and the switches of the second diagonal are on.

According to an embodiment, the control circuit is configured to switch the first bridge between its first and second configurations and to switch the second bridge between its first and second configurations substantially at the same frequency.

According to an embodiment, the control circuit is configured to switch the first bridge between its first and second configurations at a frequency greater than or equal to 1 MHz.

According to an embodiment, the control circuit comprises, for each first switch:

a first circuit capable of detecting that the voltage across the switch has taken a zero value and of supplying a logic output signal corresponding to the result of this detection;

a second circuit capable of performing a logic operation between the signal supplied by the first circuit and an external switch control signal, and of supplying a logic switch control signal corresponding to the result of this operation; and

a third circuit capable of adapting the level of the signal supplied by the second circuit to accordingly control the switch.

According to an embodiment, each first switch is equivalent to an anti-series association of first and second MOS transistors connected by their sources, the drains of the first and second transistors respectively forming the conduction nodes of the switch, and the gates of the first and second MOS transistors being connected to a same switch control node.

According to an embodiment, the first circuit comprises a voltage comparator, a first voltage dividing bridge coupled between the drain and source nodes of the first transistor and having an output node coupled to a positive input node of the comparator, and a second voltage dividing bridge coupled between the drain and source nodes of the second transistor and having an output node coupled to a negative input node of the comparator.

According to an embodiment, the first switches are gallium nitride switches.

The foregoing and other features and advantages will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings.

FIG. 1 is a wiring diagram of an example of an AC/DC converter according to a first embodiment;

FIG. 2 is a timing diagram illustrating the operation of the converter of FIG. 1 ;

FIG. 3 is a block diagram illustrating an embodiment of a switch control circuit of the converter of FIG. 1 ;

FIG. 4 is a more detailed diagram of an embodiment of a switch of the converter of FIG. 1 and of the circuit for controlling this switch;

FIG. 5 is a wiring diagram of an example of an AC/DC converter according to a second embodiment;

FIG. 6 is a timing diagram illustrating the operation of the converter of FIG. 5 ;

FIGS. 7A and 7B illustrate an embodiment of a switch of the converter of FIG. 5 ;

FIG. 8 schematically and partially illustrates an embodiment of a control circuit of the converter of FIG. 5 ; and

FIG. 9 is a more detailed electric diagram of an embodiment of the control circuit of FIG. 8 .

The same elements have been designated with the same reference numerals in the various drawings and, further, the various drawings are not to scale. For clarity, only those steps and elements which are useful to the understanding of the described embodiments have been shown and are detailed. In particular, the applications which may be made of the described AC/DC converters have not been detailed, the described embodiments being compatible with usual AC/DC converter applications. Further, the switch control circuits of the described converters are only partially detailed, the total forming of such control circuits being within the abilities of those skilled in the art based on the indications of the present description. Unless otherwise specified, expressions “approximately”, “substantially”, and “in the order of” mean to within 10%, preferably to within 5%. Further, term “connected” is used to designate a direct electric connection, with no intermediate electronic component, for example, by means of one or a plurality of conductive tracks or conductive wires, and term “coupled” or term “linked” is used to designate an electric connection which may be direct (then meaning “connected”) or indirect (that is, via one or a plurality of intermediate components).

First Embodiment—Capacitive Structure

FIG. 1 is a wiring diagram of an example of an AC/DC converter 100 according to a first embodiment.

Converter 100 comprises a first controlled H bridge H 1 , or primary bridge, followed by an isolation transformer T, followed by a second controlled H bridge H 2 , or secondary bridge.

Bridge H 1 is formed of four switches, bidirectional in terms of current and of voltage, S 1 , S 2 , S 3 , and S 4 , for example, identical (to within manufacturing dispersions), each comprising two main conduction nodes and at least one control node. Switches S 1 and S 4 are series-coupled, by their conduction nodes, between input nodes C and D of the bridge. Switches S 2 and S 3 are series-coupled, by their conduction nodes, between nodes C and D, in parallel with the branch comprising switches S 1 and S 4 . Junction point E of switches S 1 and S 4 defines a first output node of the bridge, and junction point F of switches S 2 and S 3 defines a second output node of the bridge. More particularly, in the shown example, switch S 1 has a first conduction node connected to node C and a second conduction node connected to node E, switch S 4 has a first conduction node connected to node E and a second conduction node connected to node D, switch S 2 has a first conduction node connected to node C and a second conduction node connected to node F, and switch S 3 has a first conduction node connected to node F and a second conduction node connected to node D.

Input nodes C and D of bridge H 1 are directly coupled, that is, with no intermediate rectifying stage comprising diodes and no low-pass filtering stage, to nodes A and B of application of the AC input voltage of the converter. Low-pass filtering stage here means a low-pass filtering stage having a cut-off frequency approximately equal to or smaller than the frequency of the AC input voltage of the converter. Low-pass filtering elements with a higher cut-off frequency may however be provided, particularly to filter possible parasitic signals generated during the switching of the switches of bridge H 1 . More particularly, in this example, node C is coupled to node A via an inductance 11 , and node D is connected to node B. Inductance 11 has the function of filtering possible parasitic voltage peaks generated during the switching of the switches of bridge H 1 . In the shown example, inductance 11 has a first end connected to node A and a second end connected to node C. As a variation, inductance 11 may be omitted, node C then being connected to node A.

According to an aspect of the first embodiment, primary bridge H 1 is purely capacitive at its input and at its output. More particularly, converter 100 comprises a capacitor CI having its electrodes respectively connected to input nodes C and D of bridge H 1 , and a capacitor CO having its electrodes respectively connected to output nodes E and F of bridge H 1 . It should be noted that such a layout is unusual. Indeed, in power electronics, passive electric power exchange elements of different natures are generally provided at the input and at the output of a same H bridge, to respect source association rules.

In this example, converter 100 further comprises four capacitors C 1 , C 2 , C 3 , C 4 , for example, identical (to within manufacturing dispersions), respectively coupled in parallel with switches S 1 , S 2 , S 3 , S 4 of bridge H 1 . More particularly, each capacitor Ci, i being an integer in the range from 1 to 4, has its electrodes respectively connected to the conduction nodes of the switch Si having the same index i.

Transformer T comprises a primary winding W 1 and a secondary winding W 2 , magnetically coupled.

Ends G and H of primary winding W 1 are respectively coupled to output nodes E and F of bridge H 1 . More particularly, in the shown example, end G of primary winding W 1 is coupled to node E via an inductance 12 , and end H of primary winding W 1 is connected to node F. Inductance 12 has the function of filtering possible parasitic voltage peaks. In the shown example, inductance 12 has a first end connected to node G and a second end connected to node E. As a variation, inductance 12 may be omitted, node G then being connected to node E.

Bridge H 2 is formed of four controlled switches S 5 , S 6 , S 7 , and S 8 , for example, identical (to within manufacturing dispersions), each comprising two main conduction nodes and at least one control node. Switches S 5 , S 6 , S 7 , and S 8 are for example nearly bidirectional switches, that is, switches capable of conducting current in both directions, but only enabling to control the current in one direction, that is, which can only be controlled to the off state when a voltage having a determined biasing is applied between their conduction nodes (in other words, switches which are bidirectional in terms of current but unidirectional in terms of voltage). As a variation, switches S 5 , S 6 , S 7 , S 8 are bidirectional in terms of current and of voltage. Switches S 5 and S 8 are series-coupled, by their conduction nodes, between input nodes K and L of the bridge. Switches S 6 and S 7 are series-coupled, by their conduction nodes, between nodes K and L, in parallel with the branch comprising switches S 5 and S 8 . Junction point M of switches S 5 and S 8 defines a first output node of the bridge, and junction point N of switches S 6 and S 7 defines a second output node of the bridge. More particularly, in the shown example, switch S 5 has a first conduction node connected to node L and a second conduction node connected to node M, switch S 8 has a first conduction node connected to node M and a second conduction node connected to node K, switch S 6 has a first conduction node connected to node L and a second conduction node connected to node N, and switch S 7 has a first conduction node connected to node N and a second conduction node connected to node K.

Input nodes K and L of bridge H 2 are respectively coupled to ends I and J of secondary winding W 2 of transformer T. In the shown example, end I of winding W 2 is connected to node K and end J of winding W 2 is connected to node L.

Converter 100 further comprises an output filtering capacitor CF, at the output of bridge H 2 . In the shown example, the electrodes of capacitor CF are respectively connected to output nodes M and N of bridge H 2 .

Output nodes M and N of bridge H 2 are respectively coupled to nodes O and P supplying the DC output voltage of converter 100 . In the shown example, node M is connected to node O and node N is connected to P.

Converter 100 further comprises a circuit 101 (not detailed) for controlling switches S 1 , S 2 , S 3 , S 4 , S 5 , S 6 , S 7 and S 8 of bridges H 1 and H 2 .

In operation, a load L to be powered, for example, an electric battery, may be connected between output nodes O and P of the converter.

Converter 100 operates as follows: Circuit 101 controls primary bridge H 1 alternately to a first configuration where switches S 1 and S 3 , defining a first diagonal of the bridge, are on and switches S 2 and S 4 , defining a second diagonal of the bridge, are off, and to a second configuration where switches S 1 and S 3 are off and switches S 2 and S 4 are on. The frequency of the switching of bridge H 1 between the first and second configurations, called cut-off frequency, is preferably selected to be much higher than the frequency of the AC voltage to be converted, for example, in the range from 200 kHz to 20 MHz for an input frequency in the range from 20 to 100 Hz. Duty cycle ϕH 1 of the switching between the first and second configurations of bridge H 1 is for example approximately equal to 0.5.

Circuit 101 further controls secondary bridge H 2 alternately to a first configuration where switches S 5 and S 7 , defining a first diagonal of the bridge, are on and switches S 6 and S 8 , defining a second diagonal of the bridge, are off, and to a second configuration where switches S 5 and S 7 are off and switches S 6 and S 8 are on. The frequency of the switching of bridge H 2 between the first and second configurations is substantially equal to the switching frequency of bridge H 1 . On switching from the first configuration to the second configuration, circuit 101 may further control bridge H 2 to a first intermediate configuration where switches S 5 and S 8 are on and switches S 6 and S 7 are off. Further, on switching from the second configuration to the first configuration, circuit 101 may control bridge H 2 to a second intermediate configuration where switches S 5 and S 8 are off and switches S 6 and S 7 are on.

Duty cycle ϕH 2 of the switching between the first and second configurations of bridge H 2 may however be different from duty cycle ϕH 1 of the primary bridge. Further, a phase shift δH 1 H 2 may be provided between the control sequence of primary bridge H 1 and the control sequence of secondary bridge H 2 .

By varying parameters ϕH 2 and δH 1 H 2 , circuit 101 regulates the DC output voltage of converter 100 , and/or the current sampled from the source of AC converter power supply voltage, for example, to ensure a sinusoidal absorption of the current supplied by the AC source.

Further, the converter of FIG. 1 is reversible. Thus, in the case where an active load L, for example, a battery, is connected to the converter output, circuit 101 may, by varying the above-mentioned parameters ϕH 1 , ϕH 2 and/or δH 1 H 2 , control a DC/AC electric power transfer from DC load L to the AC input of the converter.

According to an aspect of the first embodiment, control circuit 101 is configured to, during phases of transition between the first configuration (diagonal S 1 , S 3 conductive and diagonal S 2 , S 4 non-conductive) and the second configuration (diagonal S 1 , S 3 non-conductive and diagonal S 2 , S 4 conductive) of primary bridge H 1 , only turn on switches S 2 and S 4 when the voltage thereacross takes a value zero and, during phases of transition between the second configuration (diagonal S 1 , S 3 non-conductive and diagonal S 2 , S 4 conductive) and the first configuration (diagonal S 1 , S 3 conductive and diagonal S 2 , S 4 non-conductive) of bridge H 1 , only turn on switches S 1 and S 3 when the voltage thereacross takes a zero value.

This enables to significantly limit switching losses in primary bridge H 1 . The cut-off frequency of the converter may then be selected to be relatively high, for example, greater than 1 MHz or even greater than 10 MHz, which enables to significantly decrease the dimensions of isolation transformer T.

In a preferred embodiment, switches S 1 , S 2 , S 3 , S 4 of the primary bridge H 1 and switches S 5 , S 6 , S 7 , S 8 of secondary bridge H 2 are gallium nitride switches, for example, switches of the type described in patent application EP2736078 previously filed by the applicant, or in the article entitled “The single reference Bi-Directional GaN HEMT AC switch” by D. Bergogne et al. ( Power Electronics and Applications (EPE′15 ECCE-Europe). Gallium nitride switches are indeed capable of operating at high switching frequencies with no risk of damage. The described embodiments are however not limited to this specific example.

FIG. 2 is a timing diagram illustrating the operation of converter 100 of FIG. 1 during a phase of transition from the first configuration (diagonal S 1 , S 3 conductive and diagonal S 2 , S 4 non-conductive) to the second configuration (diagonal S 1 , S 3 non-conductive and diagonal S 2 , S 4 conductive) of primary bridge H 1 .

More particularly, the timing diagram of FIG. 2 comprises a first curve V CO showing the time variation of voltage V CO across output capacitor CO of primary bridge H 1 , a second curve V S1 showing the time variation of voltage V S1 across switch S 1 , and a third curve V S2 showing the time variation of voltage V S2 across switch S 2 .

The example of FIG. 2 considers an initial time t 0 at which primary bridge H 1 is in the first configuration. Switches S 1 and S 3 are then on and switches S 2 and S 4 off, and voltage V CO across capacitor CO is substantially equal to value V e of the AC input voltage of the converter at the considered time, in the order of 200 volts in the shown example. Given that the converter cut-off frequency is much higher than the AC input voltage of the converter, the AC input voltage is considered as constant during the entire duration of a phase of transition between the first and second configurations of bridge H 1 . At time t 0 , voltage V S1 across switch S 1 is substantially zero (switch S 1 on) and voltage V S2 across switch S 2 is substantially equal to value V e (switch S 2 off).

At a time t 1 subsequent to time t 0 , marking the beginning of a phase of transition between the first and second configurations of primary bridge H 1 , circuit 101 controls switches S 1 and S 3 to the off state, while holding switches S 2 and S 4 in the off state. To ensure the continuity of the current flowing through inductance 12 and/or through the inductance formed by winding W 1 of transformer T, capacitor CO then fully discharges, to a zero value, and then negatively charges to value −V e . During the phase of discharge and then of negative charge of capacitor CO, voltage V S1 across switch S 1 increases to reach value V e and voltage V S2 across switch S 2 decreases to a zero value.

Control circuit 101 is configured to detect that voltage V S2 across switch S 2 has taken a zero value after time t 1 of turning off of switches S 1 and S 3 , and to control switch S 2 to the on state when it is detected that voltage V S2 has taken a zero value.

Thus, at a time t 2 subsequent to time t 1 , corresponding to the time at which the voltage across switch S 2 has taken a zero value, circuit 101 controls switch S 2 to the on state.

It should be noted that voltages V S3 and V S4 across switches S 3 and S 4 have not been shown in FIG. 2 , these voltages having substantially the same variation as voltages V S1 and V S2 , respectively.

Time t 2 marks the end of the phase of transition between the first and second configurations of primary bridge H 1 . Switches S 2 and S 4 are then on and switches S 1 and S 3 are off, and voltage V CO across capacitor CO is substantially equal to value −V e , in the order of −200 Volts in the shown example. At time t 2 , voltage V S2 across switch S 2 is substantially zero and voltage V S1 across switch S 1 is substantially equal to value V e .

The operation of converter 100 during phases of transition from the second configuration (diagonal S 1 , S 3 off and diagonal S 2 , S 4 on) to the first configuration (diagonal S 1 , S 3 on and diagonal S 2 , S 4 off) of primary bridge H 1 is similar to which has just been described, that is, circuit 101 first controls switches S 2 and S 4 to the off state, while holding switches S 1 and S 3 in the off state, and then monitors the voltage across switches S 1 and S 3 to only control switches S 1 and S 3 to the off state when the voltage thereacross takes a zero value.

FIG. 3 schematically and partially illustrates an embodiment of control circuit 101 of converter 100 of FIG. 1 . FIG. 3 more particularly illustrates a switch S 1 of primary bridge H 1 of the converter, as well as a control circuit associated with this switch. The control circuit shown in FIG. 3 corresponds to a portion of control circuit 101 of FIG. 1 . As an example, each of switches S 1 , S 2 , S 3 , S 4 of primary bridge H 1 of the converter is associated with a specific control circuit of the type described in relation with FIG. 3 .

The control circuit of FIG. 3 comprises, coupled to switch S 1 by its main conduction nodes, a circuit 301 for detecting the switching to a zero value or to a value considered as zero of the voltage across switch Si. Stage 301 supplies a logic output signal indicating whether the voltage across switch S 1 is or not considered as zero.

The control circuit of FIG. 3 further comprises a circuit 303 receiving the logic output signal of stage 301 , and further receiving an external control signal CMD EXT , supplied by a centralized control circuit (not detailed) of circuit 101 of FIG. 1 . Circuit 303 carries out a logic operation between the output signal of circuit 301 and external control signal CMD EXT , and supplies a logic signal for controlling switch Si, corresponding to the result of this operation. As an example, to switch bridge H 1 from the first configuration (diagonal S 1 , S 3 conductive and diagonal S 2 , S 4 non-conductive) to the second configuration (diagonal S 1 , S 3 non-conductive and diagonal S 2 , S 4 conductive), the centralized control circuit of circuit 101 simultaneously sends an order to turn off switch S 1 , an order to allow the turning on of switch S 2 , an order to turn off switch S 3 , and an order to allow the turning on of switch S 4 , via inputs CMD EXT of circuits 303 associated with switches S 1 , S 2 , S 3 , and S 4 , respectively. Similarly, to switch bridge H 1 from the first configuration (diagonal S 1 , S 3 non-conductive and diagonal S 2 , S 4 conductive) to the second configuration (diagonal S 1 , S 3 non-conductive and diagonal S 2 , S 4 conductive), the centralized control circuit of circuit 101 simultaneously sends an order to turn on switch S 1 , an order to turn off switch S 2 , an order to allow the turning on of switch S 3 , and an order to allow the turning off of switch S 4 , via inputs CMD EXT of the circuits 303 associated with switches S 1 , S 2 , S 3 , and S 4 , respectively.

For each switch Si, when the circuit 303 associated with the switch receives an order to turn off the switch, it directly applies a turn-off control signal on its output, without taking into account the output signal of circuit 301 .

When the logic circuit 303 associated with a switch Si receives an order to allow the tuning on of switch, it starts by holding on its output a signal for controlling the switch to the off state as long as the output signal of circuit 301 does not indicate that the voltage across the switch has taken a zero value, after which it applies on its output a signal for controlling switch Si to the on state only when the output signal of circuit 301 indicates that the voltage across the switch has taken a zero value.

The control circuit of FIG. 3 further comprises a circuit 305 for adapting the level of the control signal of switch Si, for example, a voltage and/or current amplifier (to be able to charge the gate in the case of a MOS switch), having its input coupled to the output of circuit 303 and having its output coupled to a control node of switch Si. Circuit 305 is capable of transposing the logic control signal supplied by circuit 303 into a signal having a level capable of effectively controlling the switch.

FIG. 4 is a more detailed diagram of an embodiment of the circuit of FIG. 3 . The circuit of FIG. 4 shows the same elements as the circuit of FIG. 3 , and illustrates in further detail an embodiment of switch Si and of circuit 301 for detecting that the voltage across switch Si has taken a zero value.

In this example, a switch Si equivalent to two MOS transistors M 1 and M 2 connected in anti-series by their sources (s) is considered. The drains (d) of transistors M 1 and M 2 respectively form the two conduction nodes K 1 and K 2 of switch Si. The gates (g) of transistors M 1 and M 2 are connected at a node CMD forming the control node of switch Si. Node REF, common to the sources (s) of transistors M 1 and M 2 , forms a reference node of the switch, having the control signal applied to node CMD referenced thereto.

Circuit 301 comprises a first resistive voltage dividing bridge comprising a resistor r 1 in series with a resistor r 2 , supplying an attenuated image of the drain-source voltage of transistor M 1 , and a second resistive voltage dividing bridge comprising a resistor r 1 ′ in series with a resistor r 2 ′, supplying an attenuated image of the drain-source voltage of transistor M 2 . In this example, resistor r 1 has a first end connected to node K 1 and a second end connected to a node Q, and resistor r 2 has a first end connected to node Q and a second end connected to node REF. Further, in this example, resistor r 1 ′ has a first end connected to node K 2 and a second end connected to a node R, and resistor r 2 ′ has a first end connected to node R and a second end connected to node REF. As an example, resistors r 1 and r 1 ′ are identical, and resistors r 2 and r 2 ′ are identical (to within manufacturing dispersions).

Circuit 301 further comprises a voltage comparator 421 referenced to node REF. Output node Q of the first voltage dividing bridge (r 1 , r 2 ) is coupled to the positive input (+) of comparator 421 , and output node R of the second voltage dividing bridge (r 1 ′, r 2 ′) is coupled to the negative input (−) of comparator 421 .

The output of comparator 421 forms the output of circuit 301 , coupled to the input of circuit 303 (also referenced to node REF). As previously described in relation with FIG. 3 , the output of circuit 303 is coupled to the input of amplification stage 305 , the output of amplification stage 305 being coupled to control node CMD of switch Si.

It should be noted that in the example of FIG. 4 , the entire chain of control of switch Si, from the detection of the zero crossing of the voltage across the switch to the effective control of the switch, has the same reference potential, that is, the potential of reference node REF of the switch. Thus, the control circuit of FIG. 4 comprises no isolation element, which enables the circuit to be highly responsive. In particular, this enables to control with no significant delay the turning on of switch Si when the voltage thereacross takes a zero value, and thus to maintain switching losses in the switch at a very low level.

It should be noted that in the embodiment of FIG. 1 , capacitors C 1 , C 2 , C 3 , C 4 enable to slow down voltage variations across switches S 1 , S 2 , S 3 , S 4 of primary bridge H 1 during switching operations, to ease the detection of the zero crossing of the voltage thereacross. As a variation, these capacitors may be omitted.

Second Embodiment—Inductive Structure

FIG. 5 is a wiring diagram of an example of an AC/DC converter 500 according to a second embodiment.

Converter 500 comprises a first controlled H bridge H 1 , or primary bridge, followed by an isolation transformer T, comprising a primary winding W 1 and a secondary winding W 2 , magnetically coupled, followed by a second controlled H bridge H 2 , or secondary bridge.

Bridge H 1 is formed of four switches, bidirectional in terms of current and of voltage, S 1 , S 2 , S 3 , and S 4 , for example, identical (to within manufacturing dispersions), each comprising two main conduction nodes and at least one control node. Switches S 1 and S 4 are series-coupled, by their conduction nodes, between input nodes C and D of the bridge. Switches S 2 and S 3 are series-coupled, by their conduction nodes, between nodes C and D, in parallel with the branch comprising switches S 1 and S 4 . Junction point E of switches S 1 and S 4 defines a first output node of the bridge, and junction point F of switches S 2 and S 3 defines a second output node of the bridge. More particularly, in the shown example, switch S 1 has a first conduction node connected to node C and a second conduction node connected to node E, switch S 4 has a first conduction node connected to node E and a second conduction node connected to node D, switch S 2 has a first conduction node connected to node C and a second conduction node connected to node F, and switch S 3 has a first conduction node connected to node F and a second conduction node connected to node D.

Input nodes C and D of bridge H 1 are directly coupled, that is, with no intermediate rectifying stage comprising diodes and no low-pass filtering stage, to nodes A and B of application of the AC input voltage of the converter.

Output nodes E and F of bridge H 1 are coupled to ends G and H of primary winding W 1 of transformer T.

According to an aspect of the second embodiment, primary bridge H 1 is purely inductive at its input and at its output. More particularly, converter 500 comprises an inductance LI series-connected with the bridge, via input nodes C and D of the bridge, between nodes A and B of application of the AC input voltage of the converter, and an inductance LO series-connected with the bridge, via output nodes E and F of the bridge, between ends G and H of primary winding W 1 of transformer T. In the shown example, inductance LI has a first end connected to node C and a second end connected to node A, and node D is connected to node B. Further, in this example, inductance LO has a first end connected to node E and a second end connected to node G, and node F is connected to node H. In the embodiment of FIG. 5 , the converter comprises no capacitive element connected at the input of bridge H 1 , between nodes C and D, and comprises no capacitive element connected at the output of bridge H 1 , between nodes, E and F, either. It should be noted that here again, such a layout is unusual. Indeed, in power electronics, passive electric power exchange elements of different natures are generally provided at the input and at the output of a same H bridge, to respect source association rules.

Bridge H 2 is formed of four controlled switches S 5 , S 6 , S 7 , and S 8 , for example, identical (to within manufacturing dispersions), each comprising two main conduction nodes and at least one control node. Switches S 5 , S 6 , S 7 , and S 8 are for example nearly bidirectional switches, that is, switches capable of conducting current in both directions, but only enabling to control the current in one direction (in other words, switches which are bidirectional in terms of current but unidirectional in terms of voltage). As a variation, switches S 5 , S 6 , S 7 , S 8 are bidirectional in terms of current and of voltage. Switches S 5 and S 8 are series-coupled, by their conduction nodes, between input nodes K and L of the bridge. Switches S 6 and S 7 are series-coupled, by their conduction nodes, between nodes K and L, in parallel with the branch comprising switches S 5 and S 8 . Junction point M of switches S 5 and S 8 defines a first output node of the bridge, and junction point N of switches S 6 and S 7 defines a second output node of the bridge. More particularly, in the shown example, switch S 5 has a first conduction node connected to node L and a second conduction node connected to node M, switch S 8 has a first conduction node connected to node M and a second conduction node connected to node K, switch S 6 has a first conduction node connected to node L and a second conduction node connected to node N, and switch S 7 has a first conduction node connected to node N and a second conduction node connected to node K.

Input nodes K and L of bridge H 2 are respectively coupled to ends I and J of secondary winding W 2 of transformer T. In the shown example, end I of winding W 2 is connected to node K and end J of winding W 2 is connected to node L.

Converter 500 further comprises an output filtering capacitor CF, at the output of bridge H 2 . In the shown example, the electrodes of capacitor CF are respectively connected to output nodes M and N of bridge H 2 .

Output nodes M and N of bridge H 2 are respectively coupled to nodes O and P supplying the DC output voltage of converter 500 . In the shown example, node M is connected to node O and node N is connected to node P.

Converter 500 further comprises a circuit 501 (not detailed) for controlling switches S 1 , S 2 , S 3 , S 4 , S 5 , S 6 , S 7 and S 8 of bridges H 1 and H 2 .

In operation, a load L to be powered, for example, an electric battery, may be connected between output nodes O and P of the converter.

Converter 500 operates as follows: Circuit 501 controls primary bridge H 1 alternately to a first configuration where switches S 1 and S 3 , defining a first diagonal of the bridge, are on and switches S 2 and S 4 , defining a second diagonal of the bridge, are off, and to a second configuration where switches S 1 and S 3 are off and switches S 2 and S 4 are on. As in the first embodiment, the frequency of the switching of bridge H 1 between the first and second configurations, or cut-off frequency, is preferably selected to be much higher than the frequency of the AC voltage to be converted, for example, in the range from 200 kHz to 20 MHz for an input frequency in the range from 20 to 100 Hz. Duty cycle ϕH 1 of the switching between the first and second configurations of bridge H 1 is for example approximately equal to 0.5.

Circuit 501 further controls secondary bridge H 2 alternately to a first configuration where switches S 5 and S 7 , defining a first diagonal of the bridge, are on and switches S 6 and S 8 , defining a second diagonal of the bridge, are off, and to a second configuration where switches S 5 and S 7 are off and switches S 6 and S 8 are on. The frequency of the switching of bridge H 2 between the first and second configurations is substantially equal to the switching frequency of bridge H 1 . Duty cycle ϕH 2 of the switching between the first and second configurations of bridge H 2 may however be different from duty cycle ϕH 1 of the primary bridge. Further, a phase shift δH 1 H 2 may be provided between the control sequence of primary bridge H 1 and the control sequence of secondary bridge H 2 .

As in the first embodiment, by varying parameters ϕH 2 and δH 1 H 2 , circuit 501 regulates the DC output voltage of converter 500 , and/or the current sampled from the converter AC power supply voltage source, for example, to ensure a sinusoidal absorption of the current supplied by the AC source.

Further, as in the first embodiment, the converter of FIG. 5 is reversible. Thus, in the case where an active load L, for example, a battery, is connected to the converter output, circuit 501 may, by varying the above-mentioned parameters ϕH 1 , ϕH 2 and/or δH 1 H 2 , control a DC/AC electric power transfer from DC load L to the AC input of the converter.

According to an aspect of the second embodiment, control circuit 501 is configured to, during phases of transition between the first configuration (diagonal S 1 , S 3 conductive and diagonal S 2 , S 4 non-conductive) and the second configuration (diagonal S 1 , S 3 non-conductive and diagonal S 2 , S 4 conductive) of primary bridge H 1 , only turn off switches S 1 and S 3 when the current flowing therethrough takes a zero value and, during phases of transition between the second configuration (diagonal S 1 , S 3 non-conductive and diagonal S 2 , S 4 conductive) and the first configuration (diagonal S 1 , S 3 conductive and diagonal S 2 , S 4 non-conductive) of bridge H 1 , only turn off switches S 2 and S 4 when the current flowing therethrough takes a zero value.

This enables to significantly limit switching losses in primary bridge H 1 . The cut-off frequency of the converter may then be selected to be relatively high, for example, greater than 1 MHz or even greater than 10 MHz, which enables to significantly decrease the dimensions of isolation transformer T.

In a preferred embodiment, switches S 1 , S 2 , S 3 , S 4 of the primary bridge H 1 and switches S 5 , S 6 , S 7 , S 8 of secondary bridge H 2 are gallium nitride switches. The described embodiments are however not limited to this specific example.

FIG. 6 is a timing diagram illustrating the operation of converter 500 of FIG. 5 during a phase of transition from the first configuration (diagonal S 1 , S 3 conductive and diagonal S 2 , S 4 non-conductive) to the second configuration (diagonal S 1 , S 3 non-conductive and diagonal S 2 , S 4 conductive) of primary bridge H 1 .

More particularly, the timing diagram of FIG. 6 comprises a first curve I LO showing the time variation of current I LO flowing through output inductance LO of primary bridge H 1 , a second curve I S1 showing the time variation of current I S1 flowing through switch S 1 , and a third curve I S2 showing the time variation of current I S2 flowing through switch S 2 .

The example of FIG. 6 considers an initial time t 0 at which primary bridge H 1 is in the first configuration. Switches S 1 and S 3 are then on and switches S 2 and S 4 are off, and current I LO flowing through inductance LO is substantially equal to the current I e flowing through input inductance LI of bridge H 1 at the considered time. Given that the converter cut-off frequency is much higher than the AC input voltage of the converter, the AC input current I e is considered as constant during the entire duration of a phase of transition between the first and second configurations of bridge H 1 . At time t 0 , current I S1 in switch S 1 is substantially equal to input current I e (switch S 1 on), and current I S2 in switch S 2 has a substantially zero value (switch S 2 off). Further, current I S3 (not shown) in switch S 3 is substantially equal to input current I e , and current I S4 (not shown) in switch S 4 has a substantially zero value.

At a time t 1 subsequent to time t 0 , marking the beginning of a phase of transition between the first and second configurations of primary bridge H 1 , circuit 501 controls switch S 2 to the on state, while holding switches S 1 and S 3 in the on state and switch S 4 in the off state. Inductance LO then discharges until the current flowing through primary winding W 1 of the transformer, and thus current I S1 flowing through switch S 1 , takes a zero value. At the same time, current I S2 increases to reach value I e .

Switch S 1 is configured to automatically turn off when the current flowing therethrough takes a zero value. At a time t 2 subsequent to time t 1 , switch S 1 turns off.

At a time t 3 subsequent to time t 2 , circuit 501 controls switch S 4 to the on state, while holding switches S 2 and S 3 in the on state and switch S 1 in the off state. Inductance LO then negatively charges until the current flowing therethrough reaches value −I e . At the same time, current I S4 flowing through switch S 4 increase to reach value I e , and current I S3 flowing through S 3 decreases until it takes a zero value.

Switch S 3 is configured to automatically turn off when the current flowing therethrough takes a zero value. Thus, at a time t 4 subsequent to time t 3 , switch S 3 turns off.

Time t 4 marks the end of the phase of transition between the first and second configurations of primary bridge H 1 . Switches S 2 and S 4 are then conductive and switches S 1 and S 3 are non-conductive.

The operation of converter 500 during phases of transition from the second configuration (diagonal S 1 , S 3 non-conductive and diagonal S 2 , S 4 conductive) to the first configuration (diagonal S 1 , S 3 conductive and diagonal S 2 , S 4 non-conductive) of primary bridge H 1 is similar to what has just been described, that is, circuit 501 first controls switch S 1 to the on state while holding switches S 2 and S 4 in the off state and switch S 3 in the off state and then, after the automatic turning off of switch S 2 , it controls switch S 3 to the on state while holding switches S 1 and S 4 in the on state and switch S 2 in the off state, until the automatic turning off of switch S 4 .

Thus, in the example of operation described in relation with FIG. 6 , each of switches S 1 of primary bridge H 1 should be controllable to the on state by control circuit 501 and should be capable, after having turned on, of automatically turning back off when the current flowing therethrough takes a zero value.

FIGS. 7A and 7B schematically illustrate an embodiment of a switch S 1 of primary bridge H 1 adapted to the operating mode described in relation with FIG. 6 .

In this example, a switch Si equivalent to two MOS transistors M 1 and M 2 connected in anti-series by their drains (d) is considered. The sources (s) of transistors M 1 and M 2 respectively form the two conduction nodes K 1 and K 2 of switch Si. The gate (g) of transistor M 1 is connected to a node CMD 1 forming a first control node of switch Si. The gate (g) of transistor M 2 is connected to a node CMD 2 forming a second control node of switch Si.

Control circuit 501 of converter 500 is configured to, when it controls a switch Si of primary bridge H 1 to the on state, start the switch so that it behaves as a diode, and thus that it can naturally turn off when the current between its conduction nodes K 1 and K 2 takes a zero value. To achieve this, on turning on of switch Si, circuit 501 applies a turn-on control signal to the gate of one of the two transistors M 1 and M 2 and maintains a turn-off signal on the gate of the other transistor.

More particularly, in the shown example, in the case where a negative current should flow between conduction nodes K 1 and K 2 of the switch, a positive gate-source voltage is applied to transistor M 1 (voltage V 1 between node CMD 1 and node K 1 ), and a zero or negative gate-source voltage is applied to transistor M 2 (voltage V 2 between node CMD 2 and node K 2 ). In this case, switch Si behaves as a diode having its anode connected to node K 2 and its cathode connected to node K 1 . Such a configuration is illustrated in FIG. 7A .

In the case where a positive current should flow between conduction nodes K 1 and K 2 of the switch, a negative or zero gate-source voltage is applied to transistor M 1 (voltage V 1 between node CMD 1 and node K 1 ), and a positive gate-source voltage is applied to transistor M 2 (voltage V 2 between node CMD 2 and node K 2 ). In this case, switch Si behaves as a diode having its anode connected to node K 1 and its cathode connected to node K 2 . Such a configuration is illustrated in FIG. 7B .

In the example described hereabove in relation with FIGS. 6 and 7A-7B , switches Si are controlled to the on state and are configured to automatically turn back off when the current flowing therethrough takes a zero value. As a variation, control circuit 501 is configured to detect that the current has taken a zero value in each switch Si and to control the switch to the off state when it is detected that current I Si in the switch has taken a zero value.

As an example, referring again to FIG. 6 , at time t 1 , circuit 501 controls switch S 2 to the on state, while holding switches S 1 and S 3 in the on state and switch S 4 in the off state.

Control circuit 501 is configured to detect that current I S1 in switch S 1 has taken a zero value after time t 1 of turning on of switch S 2 , and to control switch S 1 to the off state when it is detected that current I S1 has taken a zero value.

Thus, at a time t 2 subsequent to time t 1 , corresponding to the time when the current in switch S 1 has taken a zero value, circuit 501 controls switch S 1 to the off state.

At a time t 3 subsequent to time t 2 , circuit 501 controls switch S 4 to the on state, while holding switches S 2 and S 3 in the on state and switch S 1 in the off state.

Control circuit 501 is configured to detect that current I S3 in switch S 3 has taken a zero value after time t 3 of turning on of switch S 4 , and to control switch S 3 to the off state when it is detected that current I S3 has taken a zero value.

Thus, at a time t 4 subsequent to time t 3 , corresponding to the time when the current in switch S 3 has taken a zero value, circuit 501 controls switch S 3 to the off state.

FIG. 8 schematically and partially illustrates an embodiment of control circuit 501 of converter 500 of FIG. 5 . FIG. 8 more particularly illustrates a switch Si of primary bridge H 1 of the converter, as well as a control circuit associated with this switch. The control circuit shown in FIG. 8 corresponds to a portion of control circuit 501 of FIG. 5 . As an example, each of switches S 1 , S 2 , S 3 , S 4 of primary bridge H 1 of the converter is associated with a specific control circuit of the type described in relation with FIG. 8 .

The control circuit of FIG. 8 comprises, coupled with switch Si, for example, in series with switch Si, a circuit 801 for detecting that the current in switch Si has taken a zero value.

Stage 801 supplies a logic output signal indicating whether the current in switch Si is or not considered as having a zero value.

The control circuit of FIG. 8 further comprises a circuit 803 receiving the logic output signal of stage 801 , and further receiving an external control signal CMD EXT , supplied by a centralized control circuit (not detailed) of circuit 501 of FIG. 5 . Circuit 803 carries out a logic operation between the output signal of circuit 801 and external control signal CMD EXT , and supplies a logic signal for controlling switch Si, corresponding to the result of this operation. As an example, to switch bridge H 1 from the first configuration (diagonal S 1 , S 3 conductive and diagonal S 2 , S 4 non-conductive) to the second configuration (diagonal S 1 , S 3 non-conductive and diagonal S 2 , S 4 conductive), the centralized control circuit of circuit 501 simultaneously sends, at time t 1 , an order to turn on switch S 2 and an order to allow the turning off of switch S 1 , via inputs CMD EXT of the circuits 803 associated with switches S 2 and S 1 , respectively. At time t 3 , the centralized control circuit of circuit 501 simultaneously sends an order to turn on switch S 4 and an order to allow the turning off of switch S 3 , via inputs CMD EXT of the circuits 803 associated with switches S 3 and S 4 , respectively.

For each switch Si, when the circuit 803 associated with the switch receives an order to turn on the switch, it directly applies a turn-on control signal on its output, without taking into account the output signal of stage 801 .

When the logic circuit 803 associated with a switch Si receives a turn-off authorization order, it starts by holding on its output a signal for controlling the switch to the on state as long as the output signal of circuit 801 does not indicate that the current in switch Si has taken a zero value, after which it applies on its output a signal for controlling switch Si to the off state only when the output signal of circuit 801 indicates that the current in the switch has taken a zero value.

The control circuit of FIG. 8 further comprises a circuit 805 for adapting the level of the control signal of switch Si, for example, a voltage and/or current amplifier (to be able to charge the gate in the case of a MOS switch), having its input coupled to the output of circuit 803 and having its output coupled to a control node of switch Si. Circuit 805 is capable of transposing the logic control signal supplied by circuit 803 into a signal having a level capable of effectively controlling the switch.

FIG. 9 is a more detailed diagram of an embodiment of the circuit of FIG. 8 . The circuit of FIG. 9 shows the same elements as the circuit of FIG. 8 , and illustrates in further detail an embodiment of circuit 801 for detecting that the current in switch Si has taken a zero value and of logic circuit 803 for generating the control signal of switch Si.

Circuit 801 comprises a device 901 for measuring the current flowing through the branch of bridge H 1 comprising switch Si, that is, referring to FIG. 5 , between nodes C and E for switch S 1 , between nodes C and F for switch S 2 , between nodes F and D for switch S 3 , and between nodes E and D for switch S 4 . In this example, current measurement device 901 supplies a voltage V ISi representative of current I Si . In particular, in this example, the sign of voltage V ISI is representative of the sign of current I Si . As an example, voltage V ISI is proportional to current I Si . Current measurement device 901 is for example a Hall-effect sensor, or any other adapted current sensor. Circuit 801 further comprises, in this example, a voltage comparator 903 having a first input (+) coupled to the output of current measurement device 901 and having a second input (−) coupled to a node GND of application of a reference potential, for example, the ground. In this example, output voltage V ISI of current measurement device 901 is referenced with respect to node GND. Thus, the output signal of comparator 903 is in a first state, for example, a high state, when current I Si flows in a first direction, and in a second state, for example, a low state, when current I Si flows in the opposite direction. In this example, the output signal of stage 801 corresponds to the output signal of comparator 801 .

In the example of FIG. 9 , logic circuit 803 comprises two D flip-flops 911 and 913 . Each D flip-flop comprises a first input node d, a second input node ck, a third input node set, and an output node q. Each D flip-flop operates as follows. When the binary signal applied to the set input node of the flip-flop is in a first state, for example, in the low state, for each rising edge of the binary signal applied to input node ck, the binary signal applied to input node d is copied and stored on the q output node. When the binary signal applied to the set input node of the flip-flop is in a second state, for example, in the high state, the signal supplied on the q output node is forced to a predetermined state, for example, to the high state, independently from the states of the signals applied to input nodes d and ck.

In this example, a binary signal in the low state (‘0’) is permanently applied to the d input nodes of flip-flops 911 and 913 . As an example, the d input nodes of flip-flops 911 and 913 are coupled to a node of application of a reference potential, for example, node GND. The ck input node ck of flip-flop 911 is directly coupled to the output of comparator 903 , while the ck input node of flip-flop 913 is coupled to the output of comparator 903 via an inverter. The set input nodes of flip-flops 911 and 913 are coupled to a node of application of external control signal CMD EXT .

Logic circuit 803 of FIG. 9 further comprises an AND logic gate 915 with two inputs and one output. The first and second inputs of the AND gate are respectively coupled to the q output node of D flip-flop 911 and to the q output node of D flip-flop 913 . The output of the AND gate forms the output of circuit 803 and is coupled to the input of circuit 805 .

The control circuit of switch Si of FIG. 9 operates as follows. To control switch Si to the on state, control signal CMD EXT is set to the high state. The binary signal supplied to the q output nodes of flip-flops 911 and 913 then are in the high state. Thus, the output signal of AND gate 915 is in the high state, and the switch is held in the on (conductive) state. To allow the automatic turning back off of switch Si, signal CMD EXT is set to the low state. First, the q outputs of flip-flops 911 and 913 remain in the high state, so that switch Si remains on. When current I Si in switch Si reverses, the output signal of comparator 903 switches state. As a result, the output signal of one of the two flip-flops 911 and 913 switches to the low state. Thus, the output signal of AND gate 915 switches to the low state, which causes the turning off of switch Si.

Specific embodiments have been described. Various alterations, modifications, and improvements will occur to those skilled in the art. In particular, the described embodiments are not limited to the examples of numerical values mentioned as an example in the present description.

It should further be noted that the control circuit described in relation with FIGS. 8 and 9 , enabling to control a switch to either force its turning on or have it automatically turn off when the current flowing therethrough takes a zero value, may advantageously be used for other applications than the AC/DC converter described in relation with FIGS. 5 and 6 . As an example, such a circuit may be used to control a switch of a three-phase inverter, or more generally for any application capable of taking advantage of the automatic turning off of a switch when the current flowing therethrough takes a zero value.

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.