CONVERTER DEVICE AND UNINTERRUPTIBLE POWER SUPPLY EQUIPPED WITH SUCH A DEVICE

BACKGROUND OF THE INVENTION

The invention relates to the field of converters such as inverters, for example those used in uninterruptible power supplies, in particular high-power uninterruptible power supplies, i.e. with a power that is generally comprised between about 100 and 500 kVA.

The invention more particularly relates to a converter device enabling an AC voltage and current to be supplied by filtering pulses obtained on a modulated signal output from three substantially DC voltages available on a reference voltage line and on two voltage inputs of opposite signs, said device comprising two switching units connected between said reference voltage line and respectively one and the other of said inputs, each switching unit comprising first switching means connected between the input to which said switching unit is connected and a switching output of said switching unit to supply pulses having the same sign as that of the voltage available on said input by main turn-ons and turn-offs of said first switching means, said device comprising, for each switching unit, second switching means associated with said switching unit and connected between said switching unit and said modulated signal output, and first control means acting on said second switching means to establish turn-on of said second switching means when the sign of said AC voltage is the same as that of the voltage on the input to which said switching unit is connected.

The invention also relates to an uninterruptible power supply comprising a power supply input on which an AC input voltage is applied, a rectifier connected to said input, two substantially DC voltage lines of opposite signs connected on output of said rectifier, an inverter connected to said voltage lines of substantially DC voltage and comprising an output designed to supply a backed-up voltage.

STATE OF THE ART

Uninterruptible power supplies are commonly deve10ped to improve their efficiency and to reduce audible nuisances generated by switching frequencies that are often 10w, i.e. about a few thousand hertz. In this context, it has been shown that it was interesting to use uninterruptible power supplies presenting topo10gies on several levels, generally three levels, using components with enhanced performances enabling the problem evoked above to be palliated.

With reference to figure 1, such an uninterruptible power supply 11 comprises a mains supply input 12 to which an electrical mains supply is connected and which enables a variable input voltage which is more often than not AC to be applied to said uninterruptible power supply 11. The uninterruptible power supply also comprises a mains supply output 13 to which 10ads are connected and which enables an electric power supply called backed-up power supply i.e. an electric power supply for which the voltage and frequency are controlled, to be supplied,. The uninterruptible power supply 11 comprises a rectifier or an AC/DC converter 15 connected to the mains supply input 12, substantially DC voltage lines 16, 17, and a voltage reference 18 connected on output of the rectifier. Uninterruptible power supply 11 also comprises a DC/DC converter 19 comprising electric power storage means 20, said converter and said storage means being connected to substantially DC voltage lines 16, 17. Uninterruptible power supply 11 further comprises decoupling capacitors 21, 22 connected between voltage reference 18 and substantially DC voltage lines 16, 17, and an inverter or reversible DC/AC converter 23 connected between said lines 16, 17 and mains supply output 13. Converter 23 of uninterruptible power supply 11 comprises six switching cells. More precisely, converter 23 comprises two switching cells for each of the three phases, one dedicated to the positive half-waves and the other dedicated to the negative half-waves.

As can be seen in figure 1, uninterruptible power supply 11 presents a topo10gy on three levels, i.e. rectifier 15 supplies a substantially DC voltage on three levels, that is to say a positive level on line 16, a negative level on line 17 and a reference level on voltage reference 18. In parallel, DC/AC converter 23 supplies an AC voltage from these three DC voltage levels. The positive and negative levels generally present the same electric potential in absolute value substantially equal to half the voltage U between Lines 16 and 17.

With reference to figure 2, two cells of DC/AC converter 23 are represented for a given phase. The converter device thus partially represented supplies an AC voltage VS and an AC current IS on a phase line. AC voltage VS and current IS are obtained by filtering pulses obtained on a modulated signal output SM from three substantially DC voltage levels -U/2, UREF, U/2 available on a reference voltage line REF and on two voltage inputs P, N of opposite signs. The filtering means used comprise an inductance L connected between the modulated signal output SM and AC voltage VS and current IS output. The filtering means further comprise a capacitor C connected between said AC voltage VS and current IS output and a voltage reference point presenting the same electric potential as reference voltage line REF.

The converter device represented in figure 2 comprises two switching units UCl, UC4 controlled by means of a control unit CD l represented separately in figure 3. Switching units UC1, UC4 are connected between reference voltage line REF and respectively one and the other of said inputs P, N. Each switching unit UC1, UC4 comprises first switching means, i.e. a transistor Tl, T4, connected between voltage input P, N to which said switching unit is connected and a switching output SI, S4 of said switching unit. Transistors Tl, T4 can also be called main transistors. By means of this set-up, the pulses on modulated signal output SM are obtained by a succession of main turn-ons and turn-offs of transistors Tl, T4 performed by means of control unit CD1. When transistor Tl, T4 of a switching unit UCl, UC4 is in main turned-on state, the voltage on switching output SI, S4 of said switching unit is equal to the DC voltage -U/2, U/2 of voltage input P, N to which said switching unit is connected. Each switching unit UCl, UC4 further comprises a diode DC2, DC3 connected between reference voltage line REF and switching output SI, S4 of said switching unit to establish a voltage equal to said reference voltage UREF on said switching output when a main turn-off takes place. In this way, transistors Tl, T4 of each switching unit UCl, UC4 enable pulses having the same sign as that of the voltage available on voltage input P, N of said switching unit to be supplied on their respective switching outputs SI, S4.

As can be seen in figure 3, transistors Tl, T4 are controlled from control signals Fl, F2.

The latter are obtained from AC voltage VS using well known pulse width modulation techniques.

More precisely, control signal Fl, respectively F2 is applied to the control input of transistor Tl, respectively T4.When the amplitude of control signal Fl, F2 applied on the control input of a transistor Tl, T4 is equal to zero, said transistor is turned-off, and when this amplitude is equal to one, said transistor is turned-on. When AC voltage VS is positive, respectively negative, main turn-on of transistor Tl, respectively T4, enables a voltage with an amplitude that is equal to positive DC voltage +U/2, respectively negative DC voltage -U/2, to be supplied on switching output SI, respectively S4. In the same way, when AC voltage VS is positive, respectively negative, main turn-off of transistor Tl, respectively T4, enables diode DC2, respectively DC4, to be turned-on, which enables a voltage with an amplitude that is equal to zero to be supplied on switching output SI, respectively S4. This succession of main turn-ons and turn-offs applied on transistor Tl, respectively T4, thereby enables pulses of variable width having an amplitude substantially equal to DC voltage U/2 and having a positive sign, respectively a negative sign, to be obtained on switching output SI, respectively S4.

For each switching unit UCl, UC4, the conversion device represented in figure 2 further comprises second switching means, i.e. transistors T2, T3 corrected between said switching unit and modulated signal output SM. Transistors T2, T3 enable switching unit UCl, UC4 to be connected to modulated signal output SM according to the sign of AC voltage VS, i.e. they enable switching output SI, S4 of said switching unit UCl, UC4 to be cormected to modulated signal output SM.

As can be seen in figure 3, control signal Fl, respectively F2, is inverted by means of first control means, i.e. an inverter 52, respectively 51. The signal on output from inverter 52, respectively 51, is applied to the control input of transistor T3, respectively T2. When AC voltage VS is positive, respectively negative, control signal F2, respectively Fl, is equal to zero and the signal on output from inverter 51, respectively 52, is therefore equal to one. This results in transistor T2 being turned-on, when AC voltage VS is positive, so that switching output SI of switching unit UCl is connected to modulated signal output SM. In the same way, when AC voltage VS is negative, it is transistor T3 that is turned-on so that switching output S4 of switching unit UC4 is connected to modulated signal output SM. By means of these first control means 51, 52, it is possible to supply pulses of variable width, having an amplitude substantially equal to DC voltage U/2 and having a sign identical to the sign of AC voltage VS, on modulated signal output SM. In other words, first control means 51, 52 enable switching output SI, S4 of switching unit UCl, UC4 to be connected to modulated signal output SM when the sign of AC voltage VS is the same as that of the voltage available on the voltage input of said switching unit. Consecutive filtering of these pulses on modulated signal output SM, by means of inductance L and capacitance C, thereby enables AC voltage VS to be supplied.

As described above, the converter device represented in figure 2 enables operation during the active phases, that is to say when AC voltage VS and current IS are of the same sign. During the active phases, the voltage pulses on switching outputs SI, S4 are therefore obtained by means of switching units UCl, UC4 described above. This same converter device also comprises additional means described in the fol10wing enabling operation during the reactive phases, that is to say when AC voltage VS and current IS are of opposite signs.

In the converter device represented in figure 2, the means enabling operation during the reactive phases comprise diodes referenced Dl, D2, D3, D4 connected in parallel with the transistors respectively referenced Tl, T2, T3, T4. More precisely, each diode presents a cathode and an anode respectively connected to the emitter and collector of the transistor to which it is connected in parallel. These diodes Dl, D2, D3, D4 are thus often referred to as antiparallel diodes.

When AC voltage VS is positive and AC current IS is negative, switching is performed by means of second switching means associated with switching unit UC4, i.e. transistor T3, and also by means of diodes Dl, D2 connected in parallel with the first and second switching means associated with the cell of switching unit UCl, i.e. connected in parallel with transistors Tl and T2 respectively. When AC voltage VS is negative and AC current IS is positive, switching takes place on the one hand by means of the second switching means associated with switching unit UCl, i.e. transistor T2, and also by means of diodes D4, D3 connected in parallel with the first and second switching means associated with switching unit UCl, i.e. connected in parallel with transistors T4 and T3 respectively.

More precisely, when transistor T3, respectively T2, is turned-on, AC current IS f10ws via said transistor and diode DC3, respectively DC2. This results in the voltage on modulated signal output SM being substantially equal to reference voltage REF. Inversely, when transistor T3, respectively T2, is turned-off, AC current IS f10ws via diodes D2, Dl, respectively diodes D3, D4, which results in the voltage on modulated signal output SM being substantially equal to DC voltage U/2, respectively -U/2.

Thus, during the active phases, to establish pulses on modulated signal output SM, first switching means Tl, T4 of switching units UC1, UC4 are essentially used. During the reactive phases, establishment of pulses on modulated signal output SM essentially uses second switching means T3, T2 associated with switching units UC4, UCl.

When uninterruptible power supply 11 represented in figure 1, and in particular the converter device represented in figures 2 and 3, is in use, the switching speeds of transistors Tl to T4 and the high currents f10wing in the latter impose considerable structural constraints. This results in the switching 10sses in these active power electronics components limiting the increase of the switching frequency. One technical problem is to limit these switching 10sses during the active phases of operation of the converter device, while at the same time ensuring satisfactory operation during the reactive phases.

SUMMARY OF THE INVENTION

The object of the invention is to provide a solution to the problems of converter devices of the prior art by proposing a converter device enabling an AC voltage and an AC current to be supplied by filtering pulses obtained on a modulated signal output from three substantially DC voltages available on a reference voltage line and on two voltage inputs of opposite signs, said device comprising two switching units connected between said reference voltage line and respectively one and the other of said inputs, each switching unit comprising first switching means connected between the input to which said switching unit is collected and a switching output of said switching unit to supply pulses having the same sign as that of the voltage available on said input by main turn-one and turn-offs of said first switching means, said device comprising, for each switching unit, second switching means associated with said switching unit and collected between said switching unit and said modulated signal output, and first control means acting on said second switching means to establish turn-on of said second switching means when the sign of said AC voltage is the same as that of the voltage on the input to which said switching unit is connected, said device being characterized in that, for each switching unit, it comprises a switching aid circuit of said switching unit connected between the input to which said switching unit is connected and the switching output of said switching unit to establish a switching voltage of said first switching means that is substantially equal to zero, before any main turn-on of the first switching means of said switching unit, and that for each switching unit said device comprises second control means acting on the second switching means associated with the switching unit that is connected to the voltage input of the same sign as that of said AC voltage to establish turn-off of said second switching means when said AC voltage and said AC current are of opposite signs.

Each switching unit preferably further comprises a diode connected between the reference voltage line and the switching output of said switching unit to establish a voltage equal to said reference voltage on said switching output when main turn-off takes place.

The second switching means of each switching unit are preferably connected between the switching output of said switching unit and thé modulated signal output. Alternatively, the switching output of each switching unit is connected directly to the modulated signal output, and the second switching means of each switching unit are connected in series between the diode and said modulated signal output.

Each switching unit is preferably controlled by means of a pulse width modulation control signal the amplitude of which is maintained at a value substantially equal to zero when the sign of the AC voltage is the opposite of the sign of the voltage on the input to which said control unit is connected, and the first control means acting on the second switching means associated with one of the switching units comprise an inverter connected between the control input of said second switching means and an input point of the control signal of the other switching unit. Advantageously, the second control means acting on the second switching means associated with a switching unit comprise between the control input of said second switching means and the input point of the control signal of the other switching unit:

- means for testing the sign of the AC current with respect to that of the voltage on the input to which said first switching unit is connected, and

- a 10gic "AND" Boolean operator provided with two inputs connected to an output of the inverter of the first control means of said second switching means and to an output of said means for testing and with an output to establish turn-off of said second switching
means when said AC current is of opposite sign to that of the voltage on the input to
which said first switching unit is connected.

The switching aid circuit of each switching unit preferably comprises inductive means, branch-off means for shunting a current from the switching output to divert said current to said inductive means before main turn-on, and energy storage means connected in parallel on the diode of said switching unit to establish a resonance of said current in said inductive means before main turn-on. Advantageously, the inductive means of the switching aid circuit of each switching unit are essentially formed by a transformer connected to the switching output of said switching unit and comprising reverse-wound windings, and said branch-off means comprise auxiliary switching means directly connected between said inductive means and the voltage input to which said switching unit is connected. The transformer of the switching aid circuit of each switching unit advantageously comprises:

- a first winding connected between the switching output of said switching unit and the branch-off means of said switching aid circuit, and

- a second winding magnetically coupled to said first winding and connected between said switching output and the reference voltage line.

The transformer preferably presents a transformation ratio of less than one.

The switching aid circuit of each switching unit preferably comprises at least a first b10cking diode connected between the first winding and the reference voltage line. Advantageously, the switching aid circuit of each switching unit comprises a second b10cking diode connected between the second winding and the reference voltage line.
The converter device preferably comprises third control means acting on the first switching means of each switching unit, said third control means being connected between the input point of the pulse width modulation control signal of said switching unit and the control input of said first switching means, said third control means enabling a succession of main turn-ons and turn-offs of said first switching means to be commanded from said control

signal, said third control means comprising a delay module designed to establish a delayed main turn-on after a period greater than a preset period. The converter device advantageously comprises fourth control means acting on the auxiliary switching means of the switching aid circuit of each switching unit, said fourth control means being connected between the input point of the pulse width modulation control signal of said switching unit and the control input of said auxiliary switching means, said fourth control means comprising a module designed to establish turn-on of said auxiliary switching means during a preset period.

The invention also relates to an uninterruptible power supply comprising a power supply input on which an AC input voltage is applied, a rectifier connected to said input, two substantially DC voltage lines of opposite signs connected on output of said rectifier, an inverter connected to said substantially DC voltage lines and comprising an output designed to supply a backed-up voltage, said power supply being characterized in that said inverter is a converter device as described in the above and supplies a backed-up AC voltage from substantially DC voltages.

BRIEF DESCRIPTION OF THE DRAWINGS

Other advantages and features will become more clearly apparent from the fol10wing description of particular embodiments of the invention, given as non-restrictive examples only and represented in the accompanying drawings.

Figure 1 represents an uninterruptible power supply according to the prior art.

Figure 2 represents a converter device of DC voltage into AC voltage according to the prior art.

Figure 3 represents the control unit of the converter device represented in figure 2.

Figure 4 schematically represents a converter device according to the invention.

Figure 5 represents the control unit able to be implemented on the converter device represented in figure 4, and also the converter devices represented in figures 6 and 7.

Figure 6 represents an embodiment of a converter device according to the invention using the control unit represented in figure 5.

Figure 7 represents another embodiment of a converter device according to the invention using the control unit represented in figure 5.

Figures 8A to 8H are timing diagrams illustrating operation of the converter device represented in figure 6 or 7 when the AC voltage and current are of opposite signs and in the particular case where a control unit according to the prior art as represented in figure 3 is used.

Figures 9A to 9H are timing diagrams illustrating operation of the converter device represented in figure 6 or 7 with a control unit according to the invention as represented in figure 5, and when the AC voltage and current are of opposite signs.

Figures l0A to l0L are timing diagrams illustrating operation of the converter device according to the invention represented in figure 6 or 7 when the AC voltage and current are of the same sign.

Figures 11A to 11K are timing diagrams illustrating operation in rectifier mode of the converter device according to the invention represented in figure 6 or 7, when the AC voltage and current are of the same sign, at the beginning and at the end of a half-wave of said AC voltage.

Figure 12 represents an uninterruptible power supply according to the invention

DETAILED DESCRIPTION OF AN EMBODIMENT

With reference to figure 4, the converter device comprises a large number of elements that have already been described and which bear the same reference numbers. The converter device further comprises, for each switching unit UCl, UC4, a switching aid circuit Al, A4 of said switching unit connected between voltage input P, N to which said switching unit is connected and switching output SI, S4 of said switching unit. These switching aid circuits Al, A4 of each switching unit UCl, UC4 enable a switching voltage of first switching

means Tl, T4 of said switching unit that is substantially equal to zero to be established before main turn-on.

In the fol10wing, turn-on of switching means could also be qualified as triggering of said switching means. The term "main", associated with the terms turn-on, triggering or turn-off, can be used with reference to switching of the first switching means Tl, T4 with which the switching aid circuits interact to soften the switching. The term "main" also enables the distinction to be made between switching of the first switching means Tl, T4 and those of second switching means T2, T3 or those of auxiliary switching means TXl, TX4.

Generally speaking, to establish pulses on modulated signal output SM, the switching giving rise to 10sses in second switching means T2, T3 are less frequent than the switching of first switching means Tl, T4. The switching aid circuits are therefore generally designed to establish a switching voltage substantially equal to zero for switching of first switching means Tl, T4 only, i.e. during the active phases of operation.
To establish a switching voltage substantially equal to zero before main turn-on or triggering, switching aid circuit Al, A4 of switching unit UCl, UC4 involved generally comprises means for diverting a current IRP, IRN from switching output SI, S4 of said switching unit and for establishing a resonance of this current. These switching aid circuits Al, A4, generally designed for the active phases of operation, can in f act interact in unfavourable manner with second switching means T3, T2, during the reactive phases of operation.

For example, as described in detailed manner in the fol10wing, switching aid circuits Al, A4 can comprise energy storage means which act, during the reactive phases of operation, on the current f10wing in second switching means T2, T3. More precisely, these energy storage means are generally arranged in such a way that, when turn-on of second switching means T2, T3 takes place, discharging of said energy storage means takes place via the whole of said second switching means, which requires over-dimensioning of these second switching means and of the energy storage means.

It has been found that by modifying control unit CDl, it was possible to use a switching aid circuit avoiding any over-dimensioning of second switching means T2, T3. For this, second control means described further on in detailed manner were added to control unit CDl to act on second switching means T2, T3 associated with switching unit UCl, UC4 that is connected to input P, N. These second control means thereby enable turn-off of second switching means T2, T3 associated with switching unit UCl, UC4 that is connected to the voltage input of opposite sign from the sign of said AC voltage to be established when AC voltage VS and current IS are of opposite signs, i.e. during the reactive phases. In other words, when AC voltage VS and current IS are of opposite signs, the second control means act on second switching means T2, T3 associated with switching unit UCl, UC4 that is connected to input P, N to protect said switching unit from modulated signal output SM.

A modified control unit CD2 including the second control means is represented in figure 5. As in control unit CDl represented in figure 3, control of each switching unit is performed by means of a pulse width modulation control signal Fl, F2 of said switching unit. As described in the foregoing, the amplitude of control signal Fl, F2 of a switching unit UCl, UC4 is maintained equal to zero when the sign of AC voltage VS is the opposite of the sign of the voltage on input P, N to which said switching unit is connected. In other words, the amplitude of signal Fl is maintained equal to zero when the sign of AC voltage VS is negative, and the amplitude of signal F2 is maintained equal to zero when the sign of AC voltage VS is positive. As in control unit CDl represented in figure 3, the first control means acting on second switching means T2, T3 associated with one of switching units UCl, UC4 comprises an inverter 51, 52 connected between the control input of said second switching means and an input point of control signal F2, Fl of the other switching unit UC4,UC1.

Control unit CD2 further comprises, for each switching unit, second control means also acting on second switching means T2, T3 associated with switching unit UCl, UC4 that is connected to the voltage input of the same sign as that of AC voltage VS to establish turn-off of said second switching means when said AC voltage VS and AC current IS are of opposite signs. In other words, the second control means act on second switching means T2, respectively T3, associated with switching unit UCl, respectively UC4, to establish turn-off of said second switching means when AC current IS is negative, respectively positive.

The second control means acting on second switching means T2, T3 associated with a first switching unit UCl, UC4 comprise between the control input of said second switching means T2, T3 and the input point of control signal F2, Fl of the other switching unit UC4, UCl:

- means for testing 61, 62 the sign of AC current IS with respect to that of the voltage on input P, N to which said first switching unit UCl, UC4 is connected, and a 10gic "AND" boolean operator 65, 66 provided with two inputs connected to an output of inverter 51 of the first control means of said second switching means T2, T3 and to an output of said means for testing 61, 62 and with an output to establish turn-off of said second switching means when said AC current IS is of opposite sign to that of the voltage on the input to which said first switching unit is connected.

An example of a switching aid circuit Al, A4 able to be implemented for each switching unit UCl, UC4 is represented in figure 6. The converter device represented in figure 6 comprises a certain number of the elements already described in the foregoing and indicated by the same reference numbers. As in figures 2 and 4, only the two switching units associated with one of the three phases have been represented.

With reference to figure 6, switching aid circuit Al, A4 of each switching unit UCl, UC4 comprises inductive means, branch-off means for tapping current ERP, IRN from switching output SI, S4 to divert said current to said inductive means before main triggering takes place, and a capacitor C2, C3 connected in parallel on diodes DC2, DC3 of said switching unit to establish a resonance of said current IRP, IRN in said inductive means before main triggering takes place.

The inductive means of switching aid circuit Al, A4 of each switching unit UCl, UC4 are essentially formed by a transformer TP, TN connected to switching output SI, S4 of said switching unit and comprising reverse-wound windings. In other words, the two windings of the transformer are directly connected to switching output SI, S4. In the converter
device represented in figure 6, transformer TP, TN is directiy connected to switching output SI, S4 of said switching unit. Due to the fact that the inductive means of each switching aid circuit Al, A4 are essentially formed by a transformer, and that the latter is directiy connected to switching output SI, S4, the topo10gy of the converter device and of its switching aid circuits is simplified.

The branch-off means of switching aid circuit Al, A4 of each switching unit UCl, UC4 for their part comprise auxiliary switching means in the form of auxiliary transistors TXl, TX4, directly connected between transformer TP, TN and voltage input F, N to which said switching unit is connected. What is meant by directly connected is that the connection means between the auxiliary transistor and voltage input P, N and between this same auxiliary transistor and transformer TP, TN are essentially formed by electrical conductors. Auxiliary transistors TXl, TX4 participate in establishing a branch-off of AC current IS in transformer TP, TN before main triggering.

More precisely, transformer TP, TN of switching aid circuit Al, A4 of each switching unit UCl, UC4 comprises a first winding 71, 72 connected between switching output SI, S4 of said switching unit and branch-off means TXl, TX4 of said switching aid circuit. Transformer TP, TN comprises a second winding 73, 74 magnetically coupled to first winding 71, 72 and connected between switching output SI, S4 and reference voltage line REF via diode DA2, DAS. This second winding 73, 74 is moreover reverse-wound with respect to first winding 71, 72.

This configuration of transformer TP, TN enables more current to be diverted in each of the windings of transformer TP, TN when auxiliary transistors TXl, TX4 are triggered. Due to the reverse coiling of the windings and to connection of the contiguous ends of said windings to the power supply input, AC current IS is in fact diverted to be shared in each of the windings. Input current IRP, IRN is thereby amplified by mutual induction. This enables the current rating of auxiliary transistor TXl, TX4 to be reduced. After turn-off of diode DC2, DC3, voltage V2, V3 at the terminals of main transistor Tl, T4 decreases to a value substantially equal to zero, and diode Dl, D4 turns on, which enables said main transistor to be triggered under zero voltage.

In the embodiment represented in figure 6, switching aid circuit Al, A4 of each switching unit UCl, UC4 comprises a first b10cking diode DX2, DX3 connected between first winding 71, 72 and reference voltage line REF. When auxiliary transistor TXl, TX4 is turned-off, this diode DX2, DX3 enables the current to f10w in first winding 71, 72 in a single direction. This diode also enables the voltage at the terminals of auxiliary transistor TXl,TX4 to be limited.

In the embodiment represented in figure 6, switching aid circuit Al, A4 of each switching unit UCl, UC4 further comprises a second b10cking diode DA2, DA3 connected between second winding 73, 74 and reference voltage line REF. This diode enables the current to f10w in a single direction in this second winding. The presence of these diodes DA2, DA3 prevents any reversible operation of the switching aid circuits and enables transformer TP, TN to be demagnetized. This unidirectional operation of switching aid circuit Al, A4 is of interest as it limits the operating time of said switching aid circuit Al, A4 and therefore limits the 10sses in said circuit.

This configuration of switching aid circuit Al, A4, once main transistor Tl, T4 has been triggered, enables transformer TP, TN to be demagnetized, i.e. there is no 10nger any current f10wing in the transformer windings. This prevents energy which would result in destruction of the converter device from accumulating in the transformer. This demagnetization is made possible by diode DX2, DX3 which enables the reference voltage to be applied on winding 71, 72 when auxiliary transistor TXl, TX4 is off and when turn-on of said diode is performed, and by diode DA2, DA3 which withstands the voltage present on winding 73, 74.
Transformer TP, TN generally presents magnetic leaks on each of the windings which can generally not be ignored. An equivalent inductance can thus be defined created by the leaks and this inductance be linked to an equivalent resonance inductance. This resonance inductance determines the rising and falling s10pe of the current in the transformer windings. Advantageously, transformer TP, TN comprises an electrically insulating material separating the windings. The choice of thickness of this insulating material, among other things, enables the leakage inductance of the transformer and therefore the current s10pe to be adjusted.

The duty cycle to generate control signals Fl, F2 is generally chosen taking account of the demagnetization time of transformer TP, TN, which is generally about one half the triggering time. This prevents saturation of these transformers.

In the embodiment represented in figure 6, second switching means T2, T3 of each switching unit UCl, UC4 are connected between switching output SI, S4 of said switching unit and modulated signal output SM.

In the embodiment represented in figure 7, the converter device comprises a certain number of the element already described in the foregoing and indicated by the same reference numbers. Unlike the converter device of figure 6, switching output SI, S4 of each switching unit UCl, UC4 is connected to modulated signal output SM. Furthermore, second switching means T2, T3 of each switching unit UCl, UC4 are connected in series between diode DC2, DC3 and modulated signal output SM. This embodiment operates in essentially the same way as that of figure 6 using control unit CD2 represented in figure 5.

The converter devices represented in figures 6 or 7 can generally be used in reversible manner. In other words, the converter device can operate in a rectifier mode enabling a substantially DC voltage to be obtained on DC voltage inputs P, N from AC voltage VS, i.e. as an AC/DC converter.

In the embodiments represented in figures 6 and 7, transistor Tl, T4 of each switching unit UCl, UC4 can be used in dual thyristor mode, i.e. triggering takes place in natural manner. In general, main triggering takes place in natural manner when the switching voltage becomes substantially equal to zero and antiparallel diode Dl, D4 turns on. For this, control unit CD2 represented in figure 5 comprises a comparator 81, 82 for detecting zero Crossing of the voltage at the terminals of first switching means Tl, T4. The output of this comparator 81, 82 is connected to an input of a first 10gic "AND" boolean operator referenced 83, 84. What is meant by 10gic "AND" boolean operator is also a product of 10gic inputs or a conjunctive binary operator, each of said 10gic inputs being able to be equal to zero or to one. Another input of this operator is connected to the input point of control signal Fl, F2. Zero crossing of the voltage at the terminals of transistor Tl, T4 and the simultaneous presence of a pulse of control signal Fl, F2 thereby enables the output of this boolean operator 83, 84 to be activated and transistor Tl, T4 to be triggered.

However, in rectifier mode and in the case where the intensity of AC current IS is too 10w, i.e. for an amplitude of AC current IS less than about 10% of its maximum value, which generally corresponds to the beginning or end of a half-wave of said current, the voltage on switching output SI, 34 does not have time to reach the required value of line voltage P, N and natural triggering of main transistors Tl, T4 is not possible. Indeed, in this case, capacitors CR2, CR3 do not have time to charge and it is difficult to obtain a resonance of the current entering the inductive means cancelling the voltage at the terminals of main transistors Tl, T4.

To remedy this drawback, control unit CD2 represented in figure 5 comprises a delay module 91, 92 designed to force delayed main triggering after a period greater than a preset period TMAX. This forced operation mode is implemented in inverse mode, mainly at the beginning and end of the half-wave of AC voltage VS, when the value of AC current IS is not sufficient to charge capacitors CR2, CR3. The output of operator 83, 84 is connected to a second 10gic "OR" boolean operator referenced 93, 94, the output whereof is connected to the control input of main transistor Tl, T4. What is meant by 10gic "OR" boolean operator is also a disjunctive binary operator, each of said 10gic inputs being able to be equal to zero or to one. Thus, in normal operation, when the output of "AND" operator 83, 84 is activated, the output of operator 93, 94 is also activated, which enables triggering of main transistor Tl, T4 to be commanded at the moment the voltage at the terminals of said transistor crosses zero.

Control unit CD2 represented in figure 5 further comprises a module 95, 96 enabling auxiliary transistor TXl, TX4 to be triggered during a preset period TMAX'. This period runs from the rising front of control signal Fl, F2. In normal operation and during this period TMAX', auxiliary transistor TXl, TX4 can therefore be triggered, which enables the switching voltage to be cancelled to trigger main transistor Tl, T4.

With reference to figures 8A to 8H and for the purposes of comparison with the converter device according to the invention, operation of the converter device represented in figure 6 associated with control unit CDl represented in figure 3 (prior art) is described in the fol10wing in the case where AC voltage VS is positive and AC current IS is negative. It
should be noted that the description of this operation also applies to the converter device represented in figure 7.

So 10ng as control signal Fl is equal to one, transistor Tl is in on state and transistor T3 is maintained in off state by means of inverter 51. AC current IS f10ws in diodes Dl and D2 (figures 8E and 8G). Voltage VCR2 at the terminals of capacitor CR2 is for its part equal to voltage U/2 on input P.

At a time tl, control signal Fl goes from one to zero, transistor Tl is turned-off and transistor T3 turns on. It should be noted that transistor T2 is kept turned-on throughout the positive half-wave due to the presence of inverter 52 and to the f act that control signal F2 on input of said inverter is kept equal to zero throughout the positive half-wave. This results in voltage VCR2 at the terminals of capacitor CR2 being cancelled by discharging of said capacitor CR2 through transistor T2. The value of current IT3 £ind IT2 f10wing in transistor T3 and T2 thereby becomes very high (figure 8C) which may be detrimental to their integrity. Transistor T2 being turned-on throughout period during which AC current IS is negative and AC voltage VS is positive, voltage VDl at the terminals of Dl is maintained equal to voltage U/2 on input P (figure 8D). It is therefore diode Dl that withstands the whole of voltage U/2 of input P. The current therefore f10ws in transistor T3 (figure 8C) and in diode DC3.

At a time t2, control signal Fl goes from zero to one, transistor Tl turns on and transistor T3 turns off This results in the current being diverted to capacitor CR2 via diode D2 (figure 8G), which s10ws down the rise of voltage VT3 at the terminals of transistor T3 (figure 8B).

At a time t3, capacitor CR2 is charged and presents a voltage VCR2 at its terminals equal to voltage U/2 on input P. AC current IS f10ws in diodes Dl and D2 (figures 8E and 8G).
With reference to figures 9A to 9H, operation of the converter device represented in figure 6 associated with control unit CD2 represented in figure 5 is described in the fol10wing, still in the case where AC voltage VS is positive and AC current IS is negative, i.e. during the reactive phases. This operation can be transposed to the case where voltage VS is negative and current IS is positive, T2 then operating as T3 in the fol10wing and vice-versa.

It should be noted that the description of this operation also applies to the converter device represented in figure 7.

So 10ng as control signal Fl is equal to one, transistor Tl is on and transistor T3 is kept off by means of inverter 51. AC current IS f10ws in diodes Dl and D2 (figures 9E). Voltage VCR2 at the terminals of capacitor CR2 is for its part equal to voltage U/2 on input P (figure 9G).

At time tl, control signal Fl goes from one to zero, transistor Tl is turned off and transistor T3 turns on as was the case previously. Moreover, the output of inverter 52 is kept equal to one due to the fact that, throughout the positive half-wave of voltage VS, control signal F2 is kept equal to zero. In parallel, the sign of the AC current enables the output of comparator 61 to be kept at zero. This results in the output of 10gic "AND" boolean operator 65 being equal to zero and transistor T2 being turned-off. Second control means 61, 65 therefore enable transistor T2 to be turned off, which prevents the current originating from discharging of capacitor CR2 from f10wing. Capacitor CR2 therefore remains charged (figure 9G) and the voltage at the terminals of diode Dl is maintained equal to zero (figure 9E). It is therefore diode D2 that withstands the whole of voltage U/2 of input P when transistor T3 is on (figure 9F).

As previously, at time t2, control signal Fl goes from zero to one, transistor Tl turns on and transistor T3 turns off. The current is diverted in diode D2.

At time t2, AC current IS f10ws in diodes Dl and D2 (figures 9E).

The use of control unit CD2 represented in figure 5, in particular of second control means 61, 62, 65, 66 of said control unit, enables current IT3, IT2 f10wing in transistor T3, T2 to be limited preventing discharging of capacitor CR2, CR3. Transistor T3 therefore does not need to be over-dimensioned to withstand a current to which a discharge current of capacitor CR2, CR3 would be added.

With reference to the timing diagrams of figures 10A to 10L, operation of the converter device represented in figure 6 or in figure 7 is described in the fol10wing. It should be noted that these timing diagrams extend over a period during which AC voltage and current VS, IS can be considered as being continuous. The fol10wing description is limited to operation during the positive half-waves of AC voltage VS, i.e. essentially to operation of switching unit UCl and of switching aid circuit Al. Operation of the converter device during the negative half-waves of AC voltage VS can be easily derived there from by the person skilled in the trade. The fol10wing description is made for the case where AC voltage VS and AC current IS are of the same sign, i.e. when the switching aid circuit is used to obtain soft switching of main transistors Tl, T4. Furthermore, the fol10wing description applies to rectifier mode of the converter device, i.e. to the DC/AC mode of operation, a condition also being that the intensity of AC current IS is sufficient to obtain natural turn-on of transistors Tl, T4. In other words, the operation described hereafter to a certain extent excludes rectifier mode for the beginning and end of the half-waves of AC voltage VS.

Main transistor Tl is initially in a triggered or on state, which is indicated by the presence of a bold line in figure l0B. Auxiliary transistor TXl is for its part in off state, which is indicated by the absence of a bold line in figure I0C. As can be seen in figure l0G, diode DC2 is turned-off. Transistor Tl sees a current ITI f10w, represented in figure l0F, that is substantially equal to AC current IS. Voltage VI at the terminals of transistor Tl is thereby substantially equal to zero, and voltage VCR2 at the terminals of capacitor CR2 is substantially equal to voltage U/2 on input P (figure l0E). Diode DA2 does not see any current f10w as is represented in figure l0H and is in off state. Voltage VDA2 at its terminals represented in figure 101 is therefore substantially equal to the value of voltage U/2 on input P.

At time tl, transistor Tl is off (Figure 10B), and AC current IS is diverted in capacitor CR2. Voltage VI at the terminals of main transistor Tl starts to increase progressively discharging capacitor CR2, as can be seen in figure 10E. Diode DA2 is still in off state and voltage VDA2 at its terminals starts to decrease (figure 101) until it reaches a zero value. At the same time, as can be seen in figure 10L, voltage VTXl at the terminals of auxiliary transistor TXl increases to the value of voltage U/2 on input P.

At time t2, voltage VCR2 at the terminals of capacitor CR2 reaches the value of the reference voltage (Figure 10E) and diode DC2 starts to conduct a current IDC2 the value of which is substantially equal to the value of current IS represented in Figure 100G.

At time t3, auxiliary transistor TXl is triggered (Figure 10C), which will result in a decrease of current E)C2 in diode DC2 (Figure 10G) which is diverted to auxiliary transistor TXl which has turned on. As can be seen in figure 10J, auxiliary transistor TXl therefore sees a current ITXl that increases progressively. Current IRP in transformer TP, represented in figure 10D, will therefore increase at the same time as current IDC2 decreases. After triggering of diode DA2, this current IRP results from the sum of current ITXl in first winding 71 of transformer TP (figure 10J) and of current IDA2 in second winding 73 of this same transformer TP (figure 10H). As soon as diode DA2 is triggered, voltage U/2 on input P is applied to the two windings 71, 73 of transformer TP. On account of the magnetic 10sses of this transformer, winding 73 will be subjected to a voltage at its terminals that is substantially equal to voltage U/2 on input P. The transformation ratio of transformer TP being very c10se to one, current ITXl in winding 71 represented in figure 10J and current IDA2 in winding 73 represented in figure 10H are substantially equal to half the value of current IRP entering transformer TP, i.e. equal to half of AC current IS.

At time t4, there is no 10nger any current f10wing in diode DC2, which results in the latter being turned off (figure 10G). Voltage V2 at the terminals of capacitor CR2 (figure 10E) therefore starts to increase by resonance phenomenon with transformer TP. At the same time, as can be seen in figures 10D, 10H, and 10J, current IRP on the input of transformer TP and currents IDA2 and ITXl in each winding will increase. In this way, current IRP in the transformer will enter into resonance. Indeed, at time t4, capacitor CR2 which is discharged will charge progressively as voltage V2 at its terminals increases to reach the voltage of input P.

Between times t4 and t5, when voltage V2 at the terminals of capacitor CR2 is substantially equal to half the voltage U/2 on input P, current IRP entering transformer TP will reach a resonance peak (figures 10D and 10E). During this lapse of time, the voltage at the terminals of winding 71 of transformer TP will decrease and the voltage at the terminals of winding 73 of this same transformer will increase. In other words, due to the variation of voltage V2, voltage U/2 on input P will simultaneously switch from winding 71 to winding 73.

At time t5, whereas voltage V2 at the terminals of capacitor CR2 is equal to voltage U/2 on input P (fïgure 10E), a weak current will f10w in reverse-connected diode Dl parallel to transistor Tl. This can be seen in figure 10F representing current m f10wing in the module formed by main transistor Tl and diode Dl. Main transistor Tl is triggered between time t5 and time t6 with a voltage at its terminals that is therefore substantially equal to zero (figure 10E). The power dissipated when this triggering takes place is therefore minimized.
At time t6, current ITl in main transistor Tl increases progressively (figure 10F) and at the same time the intensity of currents ITXl and IDA2 in respectively first and second winding 71, 73 decrease (figures 10J and 10H).

At time t7, there is no 10nger any current f10wing in diode DA2 and in second winding 73 of transformer TP (figure 10H), which results in turn-off of said diode. A current of 10w intensity IMAG represented in figure 10J, due to magnetization of transformer TP, continues to f10w in transistor TXl and in first winding 71 of said transformer. Between time t7 and time t8, the voltages at the terminals of windings 71, 73 of transformer TP being substantially equal to zero, the value of this current IMAG remains substantially constant.

At time t8, transistor TXl is commanded to off state (Figure 10C) and diode DX2 enables magnetization current IMAG f10wing in first winding 71 to be completely evacuated. Full demagnetization of transformer TP thus takes place before main turn-off of main transistor Tl. As can be seen in figure 10L, the value of the voltage at the terminals of transistor TXl is substantially equal to voltage U/2 on input P. As can be seen in figure 101, the voltage at the terminals of diode DA2 is for its part substantially equal to twice the value of voltage U/2 on input P. During demagnetization of transformer TP, voltage VTXl at the terminals of auxiliary transistor TXl is therefore twice as 10w as voltage VDA2 at the terminals of diode DA2. It is therefore diode DA2 that absorbs a high demagnetization voltage instead of auxiliary transistor TXl, which enables a transistor TXl of 10wer rating to be chosen, which is therefore less costly and which operates with a 10wer power consumption.

At time t9, transformer TP is fully demagnetized, i.e. the mean value of the voltage at its terminals is zero. Current MAG therefore becomes zero and diode DX2 turns off. The initial situation preceding time tl is thus reverted to.

With reference to the timing diagrams of figures 11A to 1 IK, operation in rectifier mode of the converter device represented in figure 6 or in figure 7, i.e. DC/AC operating mode of said converter, is described in the fol10wing in the case where the intensity of AC current IS is not sufficient to obtain natural turn-on of transistors Tl, T4. The operation described hereafter is therefore applicable to the beginning and end of the half-wave of AC current IS. It should be noted that these timing diagrams extend over a period during which AC voltage and current VS, IS can be considered as being continuous. The fol10wing description is limited to operation during the positive half-waves of AC voltage VS, operation during the negative half-waves of said AC voltage VS being able to be easily derived there from by the person skilled in the trade.

At the outset, transistor Tl is triggered or on, as can be seen in figure 11A. Transistor Tl conducts a current ITl represented in figure IIE the value of which is substantially equal to that of AC current IS. As can be seen in figures 1ID and 1IF, the value of voltage VCR2 at the terminals of capacitor CR2 is almost zero and diode DC2 is in off state.

At time tl, main transistor Tl goes from on state to off state (Figure IIA) and AC current IS is diverted in capacitor CR2. Voltage VCR2 at the terminals of capacitor CR2 starts to decrease progressively discharging capacitor CR2, and voltage VI at the terminals of main transistor Tl increases progressively, as can be seen in figure IID. The intensity of AC current IS being too 10w, voltage VI at the terminals of transistor Tl increases very s10wly and does not manage to reach the value of voltage U/2 on input P. Diode DC2 can therefore not be triggered and therefore does not conduct (figure HF).
At time t2, auxiliary transistor TXl is triggered (Figure UB). As can be seen in figure 111, auxiliary transistor TXl therefore sees a current ITXl which increases progressively. In the same way, current IRP in transformer TP (figure IIC) and current IDA2 in diode DA2 (figure 11G) increase. Current IRP in transformer TP will then enter a resonance phase. Capacitor CR2 which is initially charged will in fact discharge progressively as voltage VI at the terminals of main transistor Tl decreases to zero. Current IRP in transformer TP will then reach a resonance peak (figure 11C) which will then continue with a drop. As can be seen in figures 11C, 11D, 11G, 11H, 111 and IIK, the resonance phase results in oscillations without voltage VI at the terminals of main transistor Tl being able to be cancelled. Transistor Tl can therefore not trigger due to the f act that the outputs of comparator 81 and of 10gic "AND" boolean operator 83 of control unit CD2 remain in an inactive state.

At time t3, after the lapse of time TMAX defined by delay module 91 of control means CD2, main transistor Tl is triggered automatically (figure IIA) due to the f act that the output of 10gic "OR" boolean operator 93 switches to active state. At the same time, voltage VI at the terminals of main transistor Tl is sharply brought to zero (Figure IID), which generates a current peak in main transistor Tl (Figure 1 IE). Current IRP decreases (Figure IIC) and diode DA2 reverts to off state (Figure IIG). Only a magnetizing current MAG f10ws in transistor TXl (Figure 1II).

At time t4, after a period TMAX' defined by module 95 of control unit CD2 and usually greater than the time TMAX, auxiliary transistor TXl is turned off (Figure IIB). Diode DX2 enables full demagnetization of transformer TP to be achieved at time t5 (Figures 11H, 111 and 11J).

At time t5, transformer TP is fully demagnetized. Current IMAG therefore becomes zero and diode DX2 turns off (figure 1 IJ). The initial situation preceding time tl is therefore reverted to.

The converter devices described above can be used in an uninterruptible power supply 501 as represented in figure 12. This uninterruptible power supply comprises a power supply input 502 on which a variable input voltage from a first three-phase power system is applied. The uninterruptible power supply comprises a rectifier 503, said rectifier being connected between power supply input 502 on the one hand and two substantially DC output lines 504 or voltage busses on the other hand. The uninterruptible power supply comprises an inverter 506 corresponding to one of the converter devices described above, said inverter being connected between output lines 504 and an output 507 designed to supply a backed-up three-phase AC voltage to a 10ad 508. DC voltage bus 504 is also connected to a battery 509 via a DC/DC converter 510.

As can be seen in figure 12, static switches 511 and 512 enable selection of either power supply input 502 of the first three-phase power system or a power supply input 513 of a second power system that is also three-phase. It is thus possible to supply the 10ad by means of the backed-up first power system via uninterruptible power supply 501, and to switch over to the second power system if required.

CLAIMS

1. Converter device enabling an AC voltage (VS) and current (IS) to be supplied by filtering pulses obtained on a modulated signal output (SM) from three substantially DC voltages (-U/2, UREF, U/2) available on a reference voltage line (REF) and on two voltage inputs (P, N) of opposite signs,

said device comprising two switching units (UCl, UC4) connected between said reference voltage line and respectively one and the other of said inputs, each switching unit comprising first switching means (Tl, T4) connected between the input to which said switching unit is connected and a switching output (SI, S4) of said switching unit to supply pulses having the same sign as that of the voltage available on said input by main turn-ons and turn-offs of said first switching means,

said device comprising, for each switching unit, second switching means (T2, T3) associated with said switching unit and connected between said switching unit and said modulated signal output, and first control means (51, 52) acting on said second switching means to establish turn-on of said second switching means when the sign of said AC voltage is the same as that of the voltage on the input (P, N) to which said switching unit is connected,

characterized in that, for each switching unit, said device comprises a switching aid circuit (Al, A4) of said switching unit connected between the input to which said switching unit is connected and the switching output of said switching unit to establish a switching voltage of said first switching means that is substantially equal to zero, before any main turn-on of the first switching means of said switching unit, and that for each switching unit said device comprises second control means acting on the second switching means associated with the switching unit that is connected to the voltage input (F, N) of the same sign as that of said AC voltage to establish turn-off of said second switching means when said AC voltage and said AC current are of opposite signs.

2. Device according to claim 1, characterized in that each switching unit (UC1, UC4)
further comprises a diode (DC2, DC3) connected between the reference voltage line
(REF) and the switching output (SI, S4) of said switching unit to establish a voltage
equal to said reference voltage on said switching output when main turn-off takes place.

3. Device according to claim 2, characterized in that the second switching means (T2, T3) of each switching unit (UCl, UC4) are connected between the switching output (SI, S4) of said switching unit and the modulated signal output (SM).

4. Device according to claim 2, characterized in that the switching output (SI, S4) of each switching unit (UCl, UC4) is directly connected to the modulated signal output (SM), and that the second switching means (T2, T3) of each switching unit (UCl, UC4) are connected in series between the diode (DC2, DC3) and said modulated signal output (SM).

5. Device according to any one of claims 3 or 4, characterized in that each switching unit (UCl, UC4) is controlled by means of a pulse width modulation control signal (Fl, F2) the amplitude of which is maintained at a value substantially equal to zero when the sign of the AC voltage (VS) is opposite to the sign of the voltage on the input (P, N) to which said control unit is connected, and

that the first control means acting on the second switching means (T2, T3) associated with one of the switching units (UCl, UC4) comprise an inverter (51, 52) connected between the control input of said second switching means (T2, T3) and an input point of the control signal (F2, Fl) of the other switching unit.

6. Device according to claim 5, characterized in that the second control means acting on the second switching means (T2, T3) associated with a switching unit (UCl, UC4) comprise between the control input of said second switching means (T2, T3) and the input point of the control signal (F2, Fl) of the other switching unit (UC4, UCl):

- means for testing (61, 62) the sign of the AC current (IS) with respect to that of the voltage on the input (P, N) to which said first switching unit is connected, and

- a logic "AND" boolean operator (65, 66) provided with two inputs connected to an output of the inverter (51, 52) of the first control means of said second switching means and to an output of said means for testing (61, 62) and with an output to

establish turn-off of said second switching means when said AC current is of opposite sign to that of the voltage on the input (P, N) to which said first switching unit is connected.

7. Device according to any one of claims 2 to 6, characterized in that the switching aid circuit (Al, A4) of each switching unit (UCl, UC4) comprises inductive means, branch-off means for shunting a current (IRP, IRN) from the switching output (SI, S4) to divert said current to said inductive means before main turn-on, and energy storage means (CR2, CR3) connected in parallel on the diode (DC2, DC3) of said switching unit to establish a resonance of said current (IRP) in said inductive means before main turn-on.

8. Device according to claim 7, characterized in that the inductive means of the switching aid circuit (Al, A4) of each switching unit are essentially formed by a transformer (TP, TN) connected to the switching output of said switching unit and comprising reverse-wound windings, and that said branch-off means comprise auxiliary switching means (TXl, TX4) directly connected between said inductive means and the voltage input (P, N) to which said switching unit is connected.

9. Device according to claim 8, characterized in that the transformer (TP, TN) of the switching aid circuit (Al, A4) of each switching unit (UCl, UC4) comprises:

- a first winding (71, 72) connected between the switching output (SI, S4) of said switching unit and the branch-off means (TXl, TX4) of said switching aid circuit, and

- a second winding (73, 74) magnetically coupled to said first winding and connected between said switching output and the reference voltage line (REF).

10. Device according to one of claims 8 or 9, characterized in that the transformer (TP, TN) presents a transformation ratio of less than one.

11. Device according to any one of claims 9 or 10, characterized in that the switching aid circuit (Al, A4) of each switching unit (UCl, UC4) comprises at least a first b10cking
diode (DX2, DX3) connected between the first winding (71, 72) and the reference voltage line (REF).

12. Device according to claim 11, characterized in that the switching aid circuit (Al, A4) of each switching unit (UCl, UC4) comprises a second b10cking diode (DA2, DA3) connected between the second winding (73,74) and the reference voltage line (REF).

13. Device according to any one of claims 8 to 12, characterized in that said device comprises third control means acting on the first switching means (Tl, T4) of each switching unit (UCl, UC4), said third control means being connected between the input point of the pulse width modulation control signal (Fl, F2) of said switching unit and the control input of said first switching means (Tl, T4), said third control means enabling a succession of main turn-ons and turn-offs of said first switching means to be commanded from said control signal (Fl, F2), said third control means comprising a delay module (91, 92) designed to establish a delayed main turn-on after a period greater than a preset period (TMAX).

14. Device according to any one of claims 8 to 13, characterized in that said device comprises fourth control means acting on the auxiliary switching means (TXl, TX4) of the switching aid circuit of each switching unit (UCl, UC4), said fourth control means being connected between the input point of the pulse width modulation control signal (Fl, F2) of said switching unit and the control input of said auxiliary switching means (TXl, TX4), said fourth control means comprising a module (95, 96) designed to establish turn-on of said auxiliary switching means during a preset period (TMAX').

15. Uninterruptible power supply (301) comprising a power supply input (302) on which an AC input voltage is applied, a rectifier (303) connected to said input, two substantially DC voltage lines of opposite signs connected on output of said rectifier, an inverter (306) connected to said substantially DC voltage lines and comprising an output (307) designed to supply a backed-up voltage, characterized in that said inverter is a converter device according to one of the foregoing claims and supplies a backed-up AC voltage from substantially DC voltages.