FR2827095A1 - Low voltage DC power supply includes transformer with single secondary producing range of voltages by control of primary conduction times - Google Patents

Abstract

The voltage converter (2) comprises an indirect converter which consists of a transformer (24) with a secondary winding (28) from which the low voltage outputs are delivered. A switch (42) is connected to the primary winding (26), and is controlled to modify its conduction time in order to provide a range of optional output voltages.. The voltage converter (2) is intended to deliver several very low voltage security outputs, and comprises an indirect converter (6). The indirect converter consists of a transformer (24) with a primary winding and a single secondary winding (28) from which the low voltage outputs are delivered. A switch (42) is connected to the primary winding (26). A circuit (60) is provided for selecting the output of the transformer from several available low voltage outputs, and a control circuit (58) modifies the conduction time of the switch (42), under the control of the selection circuit (60), as a function of the selected voltage. A second selection circuit may be controlled according to the required output current of the device.

Description

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The invention relates to a voltage converter capable of delivering several very low safety DC voltages comprising an indirect converter.

 By very low safety voltage (SELV) we expect here, for example voltages in accordance with standard NF EN60950, ie voltages between 0 and 60V.

 In known manner, voltage converters capable of delivering several very low safety DC voltages from an alternating voltage source comprise: - an alternating voltage rectifier, - an indirect converter, also better known under the terms of converter Flyback, and - control means for selecting a DC output voltage from the possible DC output voltages of the indirect converter in response to a control signal.

 The alternating voltage rectifier is adapted to deliver a rectified direct voltage at the input of the indirect converter. The rectifier is typically connected at the input to an alternating voltage source between 80 and 250 Vac.

 The indirect converter includes a transformer fitted with several secondary windings. Each secondary winding is associated with a respective electronic output circuit so as to deliver one of the possible continuous output voltages from the voltage present at the terminals of the corresponding secondary winding. For this, each secondary winding has a different number of turns than that of the other secondary windings.

 The control means are able to select, in response to the control signal, one of the DC output voltages supplied from the secondary windings.

 Thus, when a user wishes to modify the DC output voltage of the converter, he acts on the control means to select the secondary winding delivering the DC output voltage which he wishes to obtain.

 Voltage converters comprising an indirect converter capable of delivering several very low safety voltages are formed from electronic components, with the exception of the transformer, which are in the form of small integrated circuits. Consequently, the size of these converters is mainly determined by the size of the transformer and the number of electronic output circuits corresponding to each of the secondary windings. So,

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 the size of these converters is particularly imposing because of the presence of several secondary windings. This drawback limits the possible applications of these voltage converters. For example, these converters are difficult to use for making wall sockets capable of delivering several DC voltages in the range of very low voltages from a mains supply. By standard wall outlet housing is meant, for example, a housing 60 mm in diameter and 30 mm deep.

 The invention aims to remedy this drawback by proposing a voltage converter comprising an indirect converter capable of delivering several very low DC safety voltages whose size is reduced.

 The subject of the invention is therefore a voltage converter capable of delivering at output several very low DC safety voltages comprising an indirect converter, said indirect converter comprising, on the one hand, a transformer equipped with a primary winding adapted to be connected to a source of continuous input voltage, and, on the other hand, a switch connected to the primary winding whose conduction time is controllable, the voltage converter further comprising a circuit for controlling the conduction time of the switch, characterized in that it comprises, connected to the control circuit, a first circuit for selecting a very low DC safety voltage from among said several very low DC safety voltages, in that the control circuit is adapted for modify the conduction time of the switch under the control of said first selection circuit as a function of said very low t continuous safety selected, and in that the transformer comprises a single secondary winding from which said several very low continuous safety voltages are delivered.

 According to other characteristics of the invention: - the voltage converter comprises a second circuit for selecting a maximum current intensity at the output of the voltage converter, and the control circuit is adapted to be controlled by the second circuit selection as a function of said maximum current intensity at the output of the converter; - The voltage converter comprises a rectifier connected to the input of the indirect converter and said rectifier is adapted to be connected to the output of an alternating voltage source; - the voltage converter is capable of delivering several very low continuous safety voltages between 3 Vdc and 42 Vdc from an alternating voltage of between 190 Vac and 250 Vac present at the input of the rectifier, and the

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 winding ratio m of the number of turns of the secondary winding over the number of turns of the primary winding is between 0.036 and 0.65; - the voltage converter is capable of delivering several very low DC safety voltages between 3 Vdc and 24 Vdc from an alternating voltage between 190 Vac and 250 Vac present at the input of the rectifier, and the winding ratio m the number of turns of the secondary winding over the number of turns of the primary winding is between 0.02 and 0.45.

 - the voltage converter is capable of delivering several very low continuous safety voltages between 3 Vdc and 12 Vdc from an alternating voltage between 190 Vac and 250 Vac present at the input of the rectifier, and the winding ratio m the number of turns of the secondary winding over the number of turns of the primary winding is between 0.01 and 0.2; - the voltage converter is capable of delivering several very low continuous safety voltages between 3 Vdc and 42 Vdc from an alternating voltage between 80 Vac and 150 Vac present at the input of the rectifier and the winding ratio m of the number of turns of the secondary winding on the number of turns of the primary winding is between 0.03 and 0.6.

 - the voltage converter is capable of delivering several very low continuous safety voltages between 3 Vdc and 24 Vdc from an alternating voltage between 80 Vac and 150 Vac present at the input of the rectifier, the winding ratio m of the number of turns of the secondary winding on the number of turns of the primary winding is between 0.015 and 0.35.

 - the voltage converter is capable of delivering several very low continuous safety voltages between 3 Vdc and 12 Vdc from an alternating voltage between 80 Vac and 150 Vac present at the input of the rectifier, and the winding ratio m the number of turns of the secondary winding on the number of turns of the primary winding is between 0.008 and 0.2.

- Said indirect converter comprises a diode the anode of which is connected to a first end of the secondary winding and the cathode of which is connected to a first terminal for output of very low DC safety voltages, and the voltage converter has a ratio winding m between the number of turns of the secondary winding over the number of turns of the primary winding determined from the following relationship:

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 where: - Vomm is the smallest very low continuous safety voltage at the output of the voltage converter, - Vdl is the voltage drop across the diode when it leads, - J7emm is the minimum of the DC voltage at input the indirect converter (6); - Vds is the voltage drop across the switch when this leads, -LP is the inductance of the primary winding, - Ip is the maximum current flowing through the switch; f is the operating frequency of the switch, - kip is the ratio between the maximum amplitude of the variations in the intensity of the current passing through the switch and the maximum intensity Ip of the current passing through this same switch.

 The invention also relates to a wall outlet characterized in that it comprises a voltage converter according to the invention.

 In a voltage converter such as that described above, the set of secondary windings used is replaced by a single secondary winding and a circuit for adjusting the DC voltage at the output of this single secondary winding. The size of the transformer is therefore reduced since it only has one secondary winding. In addition, the size of the adjustment circuit can easily be reduced to a smaller size than that of a secondary winding, for example by using integrated circuits. Thus, the total size of the converter described above is less than that of an equivalent converter comprising several secondary windings. 1 is therefore possible to reduce the size of such a converter to a size small enough to be accommodated. inside a standard wall outlet box.

 Other characteristics and advantages will emerge on reading the following description, given solely by way of example, and made with reference to the drawings in which: - Figure 1 is an electronic circuit of a voltage converter conforming to l invention;

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 - Figure 2 is a timing diagram of the evolution of different quantities characteristic of the operation of a converter according to the invention; and FIG. 3 is a method for determining the number of turns of the primary and secondary windings of a transformer used in a voltage converter according to the invention.

 FIG. 1 illustrates a voltage converter 2 capable of delivering several very low safety voltages. This voltage converter 2 comprises a conventional voltage rectifier 4, an indirect converter 6, and an adjustment circuit 8.

 The rectifier 4 is connected, on the one hand, to a first and a second terminals 10,12 of AC voltage input, and on the other hand, to a first and a second terminals 14,16 of input of voltage rectified. The rectified voltage present between terminals 14,16 is noted Ve. Terminals 10,12 are connected to an AC voltage source between 190 Vac and 250 Vac (not shown).

 The indirect converter 6 is capable of transforming the rectified voltage Ve present between the terminals 14 and 16 into a direct voltage, denoted Fo, different present between a first and a second terminal 18,20 for output of very low DC safety voltages.

 The general structure of the indirect converters being known, only the elements specific to the invention will be described here. For more information on the general structure of indirect converters, the reader may for example refer to the following document 01: Switching power supplies, Michel GIRARD, Edi-science, 1993.

 The indirect converter 6 essentially comprises an input circuit connected via a transformer 24 to an output circuit.

 The transformer 24 has a primary winding 26 with no turns and a single secondary winding 28 with m turns. The primary winding has first and second ends 30,32. The secondary winding 28 has only first and second ends 34,36.

d'enroulement, noté m, est défini par la relation suivante : ? K=-. ni En variante, le transformateur 24 comporte un second enroulement secondaire destiné à alimenter le circuit de réglage 8. Aucune très basse tension continue The primary and secondary windings are wound in opposite directions. Thereafter, the inductance of the primary winding 26 will be noted Lp and the ratio

winding, denoted m, is defined by the following relation:? K = -. ni As a variant, the transformer 24 includes a second secondary winding intended to supply the adjustment circuit 8. No very low direct voltage

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 safety is only issued from this second secondary winding to supply a load intended to be connected at the output of the voltage converter.

 The input circuit is connected on the one hand to the terminals 14, 16 of rectified voltage input, and on the other hand, to the first to the second ends 30, 32 of the primary winding 26.

 This input circuit includes a smoothing capacitor 40 and a controllable switch 42. The capacitor 40 is connected, on the one hand, to the rectified voltage input terminal 14 and to the end 30 of the primary winding 26 , and on the other hand to the rectified voltage input terminal 16. The switch 42 is connected between the second end 32 of the primary winding 26 and the terminal 16 of rectified voltage input.

 The switch 42 is characterized by the following quantities: - the maximum voltage between its terminals in the open state, denoted Vs; - the voltage drop between its terminals in the on state, noted Vos; - its operating frequency, noted f, that is to say the opening and closing frequency of the switch; and - the maximum intensity of the current flowing between its terminals, noted Ip.

 The switch 42 has a control input connected to the output of the adjustment circuit 8. This control input is intended to receive a control signal to adjust the conduction time per operating period of the switch 42. The signal command is here a signal whose pulse width is modulated, also called PWM (Pulse Width Modulation) control signal.

 Switch 42 is, for example, a component of the Topswich family of Power Integrations (Power Integrations, Inc., 477 N. Mathilda Avenue, Sunnyvale, CA 94086).

 The output circuit is connected to each end of the secondary winding 28 and comprises a rectifying diode 50, a smoothing capacitor 52, and a resistor 54 for measuring current.

 The anode of the diode 50 is connected to the first end 34 of the secondary winding 28 while its cathode is connected to a first terminal of the resistor 54.

 The capacitor 52 is connected, on the one hand, to the first terminal of the resistor 54 and, on the other hand, to the terminal 20 for output of very low DC safety voltages and to the second end 36 of the secondary winding 28.

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 A second terminal of the resistor 54 is connected to the terminal 18 of output of very low DC safety voltages.

 In the indirect converter 6 previously described, the current flowing from the end 30 towards the end 32 of the primary winding 26 is denoted it while the current flowing from the end 36 towards the end 34 of the secondary winding 28 is noted / 2. The current flowing at the output of the resistor 54 is denoted io.

 The adjustment circuit 8 is able to generate the control signal PWM to control the conduction time per operating period of the switch 42. This adjustment circuit 8 comprises a control circuit 58 associated with a first and a second circuit selection 60 and 62.

par période de fonctionnement de l'interrupteur 42. Ici, il comprend un circuit intégré 64, par exemple, un composant TStvhOI de la société ST Microelectronics (Worldwide Headquarters, 165, rue Edouard Branly, BP112 - Technoparc du Pays de Gex, F-01637 Saint-Genis-Pouilly cedex, France). Ce circuit intégré est apte à délivrer en sortie un signal analogique en fonction des écarts mesurés entre la tension continue de sortie Vo et une référence de tension et limite le courant continu de sortie io en fonction de la valeur d'une référence de courant. The control circuit 58 is intended to control the conduction time

by period of operation of the switch 42. Here, it includes an integrated circuit 64, for example, a TStvhOI component from the company ST Microelectronics (Worldwide Headquarters, 165, rue Edouard Branly, BP112 - Technoparc du Pays de Gex, F- 01637 Saint-Genis-Pouilly cedex, France). This integrated circuit is capable of outputting an analog signal as a function of the differences measured between the DC output voltage Vo and a voltage reference and limits the DC output current io as a function of the value of a current reference.

 For this, the integrated circuit 64 is connected as an input, on the one hand, to the terminals 18 and 20 of output of very low DC safety voltages and, on the other hand, to a voltage reference 66. The voltage reference 66 is adapted so that its value can be controlled using the first selection circuit 60.

 The integrated circuit 64 is also connected to the terminals of the current measurement resistor 54 and to a current reference 68. The current reference 68 is adapted so that its value can be controlled using the second selection circuit 62.

 The output of the integrated circuit 64 is connected to the switch 42 via an isolation circuit 70 and a PWM regulator 72 to transmit the generated control signal.

 The isolation circuit 70 is connected to the output of the integrated circuit 64. It is intended to isolate the integrated circuit 64 from the input circuit of the transformer 24.

 The PWM regulator circuit 72 is connected to the output of the isolation circuit 70 and to the control input of the switch 42. This regulator is capable of converting the analog signal delivered by the integrated circuit 64 into a PWM control signal .

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 Advantageously, such a circuit 72 is integrated with the switch 42 in the same component. This is the case when switch 42 is a switch of the "Topswitch" family from the company "Power Integrations".

 The first and second selection circuits 60, 62 are respectively intended to select the very low voltage Vo and the maximum intensity io at the output of the converter 2.

 For this, the first and second selection circuits 60,62 are respectively connected to the voltage reference 66 and to the current reference 68. For example, the first and second selection circuits 60,62 each include a switch that can be operated by an operator so that the very low output voltage Vo is the maximum current intensity io can be selected by an operator.

 Figure 2 has two timing diagrams 80.82. These timing diagrams are used here to define quantities which will be used during the description of the process of FIG. 3. The timing diagram 80 comprises two curves 84,86 representing respectively the variations of the intensity of the current il and of the current i2 during a period of operation of the switch 42, denoted T.

 At the start of period T, the intensity of the current is zero. Then during a lapse of time, noted tone, the intensity of the current increases. This lapse of time ton corresponds to the lapse of time during which the switch 42 is closed, that is to say that it corresponds to the conduction time of switch 42. At the end of the lapse of time ton l ' intensity of the current it cancels again, which corresponds to the opening of the switch 42. Then the intensity remains zero until the end of the period T, that is to say during a lapse of time noted toff.

 At the start of period T, the intensity i2 is zero as long as the switch 42 conducts, that is to say during the period of time ton. At the end of this time, the intensity i2 suddenly changes to a non-zero value. Then the intensity i2 gradually decreases from this non-zero value to a zero value for a period of time noted t. This period of time t is less than the period of time toff so that the intensity i2 vanishes before the end of the period T. In such a situation, the operation of the indirect converter 6 is said to be "in discontinuous mode".

 The timing diagram 82 comprises a curve 88 representing the variations in the intensity of the current Í1 during a period T of operation of the indirect converter 6. Unlike curve 84 the curve 88 represents the variations in the

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tensité Í1 dans le cas où le convertisseur indirect 6 fonctionne dans un mode dit"dis- continu". Dans un tel mode de fonctionnement l'intensité Í1 possède une valeur, non nulle, notée 1 m, au début de la période T. Ensuite de façon similaire à la courbe 84, l'intensité Í1 croît pendant le laps de temps ton jusqu'à une valeur maximale, notée #p.

voltage Í1 in the case where the indirect converter 6 operates in a so-called "discontinuous" mode. In such an operating mode the intensity Í1 has a non-zero value, denoted 1 m, at the start of period T. Then, similarly to curve 84, the intensity Í1 increases during the period of time ton until at a maximum value, denoted #p.

Then the evolution of the intensity Í1 is identical to that of the curve 84.

 The following quantities are defined from the preceding notations: - Ir the maximum amplitude of the variations of the current il during the conduction time ton, that is to say the difference between Ip and lm.

- A cyclic relationship, denoted D, defined according to the following relationship D = ton / T - A first relationship Krp defined according to the following relationship: Krp = Ir / Im - A second relationship, noted Kdp, defined according to the following relationship:

FIG. 3 illustrates a method for dimensioning the transformer 24 comprising seven successive steps 90, 92, 94, 96, 98, 100 and 102.

 Step 90 is a step for choosing the output power, denoted R. This step is followed by step 92 for choosing the switch 42 and the PWM regulator 72 associated with it. This choice determines the quantities f, Ip, Vds and Vsi characteristic of the switch 42.

Step 94 is a step for finding the maximum duty cycle D, denoted Dmax. For this we use the following relation:

où - n est le rendement du convertisseur 2, il est classiquement compris entre 0,7 et 0,9 ; - Venun est le minimum de la tension redressée en entrée du convertisseur indirect ;
L'étape 96 est une étape de calcul de l'inductance Lp de l'enroulement primaire 26. L'inductance Lp de l'enroulement primaire 26 est calculée en fonction du rapport cyclique maximum D.. selon la relation suivante : The maximum duty cycle when Krp is equal to 1. Consequently, the following relationship is deduced from the previous relationship allowing the maximum duty cycle Dmax to be calculated:

where - n is the efficiency of converter 2, it is conventionally between 0.7 and 0.9; - Venun is the minimum of the rectified voltage at the input of the indirect converter;
Step 96 is a step for calculating the inductance Lp of the primary winding 26. The inductance Lp of the primary winding 26 is calculated as a function of the maximum duty cycle D .. according to the following relationship:

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Step 98 is a step of determining the winding ratio m of the transformer 24.

où : - 170mm est la plus petite très basse tension continue de sécurité, c'est-à-dire la tension continue de sortie 170 obtenue pour un rapport cyclique D proche de 0 ; - vil est la chute de tension aux bornes de la diode 50 lorsque celle-ci conduit ; - Krp est le rapport défini en regard de la figure 2. Sa valeur est généralement prise égale à 0,3 car pour un rapport Krp trop petit, la taille du transformateur 24 augmente. The winding ratio m is determined according to the following relationship:

where: - 170mm is the smallest very low DC safety voltage, that is to say the DC output voltage 170 obtained for a duty cycle D close to 0; - vil is the voltage drop across the terminals of the diode 50 when the latter conducts; - Krp is the ratio defined with reference to FIG. 2. Its value is generally taken equal to 0.3 because for a Krp ratio that is too small, the size of the transformer 24 increases.

 Step 100 is a step of verifying the compatibility of the winding ratio m previously determined in step 98 with the choice of switch 42 made during step 92.

- Vemax est le maximum de la tension redressée Ve, - Vomax est la tension continue maximale réglable en sortie du convertisseur indirect 6. Indeed, it is known that the winding ratio m previously determined fixes the maximum voltage at the terminals of the switch 42. This maximum voltage must be less than the maximum voltage VSI that the switch 42 can support. For this the following inequality must be satisfied:

- Vemax is the maximum of the rectified voltage Ve, - Vomax is the maximum direct voltage adjustable at the output of the indirect converter 6.

 If the previous inequality is verified step 102 is executed. Otherwise, step 92 or step 90 is again executed by choosing components having sizes different from those previously used.

 Step 102 is a step of verifying that the undulations of the direct voltage obtained at the output of the indirect converter 6 are not too great. It has been determined experimentally that, for the amplitude of these ripples of the DC output voltage to be acceptable, it is necessary that the ratio Kdp is less than 10.

The Kdp report is calculated from the following relationships:

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- I@p est le courant maximum circulant dans la diode 50 lors du fonctionnement du convertisseur indirect 6 en mode discontinu ; - Ls est l'inductance de l'enroulement secondaire 28 ; - t est la grandeur définie en regard de la figure 2.

- I @ p is the maximum current flowing in the diode 50 during the operation of the indirect converter 6 in discontinuous mode; - Ls is the inductance of the secondary winding 28; - t is the quantity defined with reference to FIG. 2.

- S is the rate of decrease of the current i2 during the time period t;
If the calculated Kdp ratio is not less than 10, step 92 and / or step 90 is again executed for components having sizes different from those used hitherto.

- Vomm est égal à 3 Vdc ; - Vomax est égal à 24 Vdc ; - Vemm est égal à 190 Vdc ; - Vemax est égal à 340 Vdc. For example, for a voltage converter with a power of 10W capable of delivering a continuous output voltage between 3 Vdc and 24 Vdc from an alternating voltage of 230 Vac, the numerical values of the different quantities are the following :

- Vomm is equal to 3 Vdc; - Vomax is equal to 24 Vdc; - Vemm is equal to 190 Vdc; - Vemax is equal to 340 Vdc.

 - the choice of a switch whose reference is TNY 256 produced by the company Power Integrations, requires VS1 = 700 Volts; Ip = 0.45 A and f = 130 KHz; - kip is equal to 0.3; - n is equal to 0.8; - Vdl is equal to 0.5 V; - V ds is equal to 10 V; - the maximum duty cycle Dmax calculated in step 94 is equal to 0.29; - the inductance Lp calculated in step 96 is equal to 941 us; - the value of m calculated during step 98 is equal to 0, 192; the voltage at the terminals of the switch 42 is equal to 465 V, which is less than the maximum voltage VS1 that it can support; the values calculated during step 102 are the following:

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The value of Kdp is therefore equal to 1.6, which is less than 10 and therefore acceptable for the design of the converter.

 In practice the following information has been determined. This information gives the minimum and the maximum of the winding ratio m as a function of the DC output voltages VOmm and Pomax and of the alternating voltage at the input of rectifier 4.

- Si Fbmm est égal à 3 Vdc et Vomax est égal à 12 Vdc alors m est compris entre 0, 01 et 0, 2. For an alternating input voltage of rectifier 4 between 190 Vac and 250 Vac:

- If Fbmm is equal to 3 Vdc and Vomax is equal to 12 Vdc then m is between 0, 01 and 0, 2.

- If VOmm is equal to 3 Vdc and VOmax is equal to 24 Vdc then m is between 0.02 and 0.45.

- vomit is equal to 3 Vdc and VOmax is equal to 42 Vdc then m is between 0.036 and 0.65
For an AC voltage at the input of rectifier 4 between 80 and 150 Vac.

- If Fbrmn is equal to 3 <-and iomax is equal to 12 Vdc then m is between 0, 008 and 0, 2.

- If VOmm is equal to 3 Vdc and VOmax is equal to 24 Vdc then m is between 0, 015 and 0.35 - If VOmin is equal to 3 Vdc and Pomax is equal to 42 Vdc then m is between 0.03 and 0.6.

 The operation of a converter as described above will now be described with the aid of FIG. 1.

 When an operator wishes to modify the DC output voltage Vu, he selects, using the selection circuit 60, the new output voltage. In response, the selection circuit 60 modifies the value of the voltage reference 66. The integrated circuit 64 then generates, as a function of the current direct output voltage Va and of the new value of the voltage reference 66, a control signal which controls the conduction time of the switch 42 so as to obtain at the output of the converter a

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 new DC output voltage corresponding to the new value of voltage reference 66.

The conduction time of the switch 42 is a function of the duty cycle D. The duty cycle D is related to the DC output voltage Vo according to the following relationships. When the DC output voltage Fo is close to its minimum, the indirect converter 6 operates in continuous mode and the following relationship relates to the duty cycle D:

Consequently, the more the duty cycle D increases the more the voltage Fo increases.

When the voltage Va reaches a threshold for which the following relation is verified: ton + t = T, the indirect converter 6 then switches to discontinuous operating mode and the following relation links P6 to the duty cycle D:

In discontinuous operating mode, the duty cycle D is close to its maximum and the very low output voltage Vo is essentially fixed by the impedance of the load connected between the terminals 18,20. Thus, as long as the maximum power of the converter 2 is not exceeded, the adjustment circuit 8 acts on the duty cycle D essentially to regulate the output voltage Po.

 When an operator wishes to modify the limit value of the maximum intensity of the current io, he uses the selection circuit 62 to select the new maximum intensity of the current ia. Similar to what has been described for a change in the output voltage, the adjustment circuit 8 then generates a control signal taking into account the new value of the current reference 68.

 Advantageously, when the operator does not act on the selection circuits 60, 62, the adjustment circuit 8 automatically regulates the characteristics of the output voltage as a function of the value of the voltage reference 66 and of the current reference 68. The very low voltage output is thus more stable.

 The invention further relates to a method for facilitating the determination of the winding ratio m as a function of the DC voltages VOn) m and Vomax framing the desired DC voltages at the output. The winding ratio m largely determines the extent of the variation range of the DC output voltage of the indirect converter. However, it is difficult to determine a winding ratio m

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corresponding to a range of output voltages between the voltages Vom and VOmax because a value of m too small leads to a range of output voltages too low and a value of m too high cannot be achieved. Indeed, the following relation links the winding ratio m and the duty cycle D:

In the previous relation if m is close to 0 then the duty cycle D corresponding to the voltage Fbmn is close to 1. This leads to a small range of variation between a duty cycle D corresponding to the voltage VOmm and a duty cycle D corresponding to the voltage Vmax. Under these conditions VOmm and VOmax are close to each other.

 Furthermore, if the winding ratio m is large, that is to say for example greater than 0.5, the duty cycle D is close to 0. However, this is not possible since the switch 42 has a minimum conduction time to respect.

 Advantageously, the adjustment circuit 8 makes it possible to control both the DC output voltage Vo and the maximum intensity of the output current io. It is therefore possible to use such a converter 2 to recharge batteries and in particular mobile phone batteries. In addition, such a converter adapts to different types of battery.

Claims (11)

1. Voltage converter (2) capable of delivering at output several very low DC safety voltages comprising an indirect converter (6), said indirect converter (6) comprising, on the one hand a transformer (24) equipped with a winding primary (26) adapted to be connected to a source of DC input voltage, and, on the other hand, a switch (42) connected to the primary winding (26) whose conduction time is controllable, the converter voltage (2) further comprising a control circuit (58) of the conduction time of the switch (42), characterized in that it comprises, connected to the control circuit (58), a first selection circuit (60 ) a very low DC safety voltage among said several very low DC safety voltages, in that the control circuit (58) is adapted to modify the conduction time of the switch (42) under the control of said first selection circuit (60) according to said selected very low DC safety voltage, and in that the transformer (24) comprises a single secondary winding (28) from which are delivered said several very low DC safety voltages.  2. Voltage converter (2) according to claim 1, characterized in that it comprises a second selection circuit (62) of a maximum current intensity at the output of the voltage converter (2), and in that the control circuit (58) is adapted to be controlled by the second selection circuit (62) as a function of said maximum current intensity at the output of the converter.  3. Voltage converter (2) according to claim 1 or 2, characterized in that it comprises a rectifier (4) connected to the input of the indirect converter (6) and in that said rectifier (4) is suitable for be connected to the output of an alternating voltage source.  4. Voltage converter (2) according to claim 3, characterized in that it is capable of delivering several very low continuous safety voltages between 3 Vde and 42 Vde from an alternating voltage of between 190 Vae and 250 Vac present at the input of the rectifier (4), and in that the winding ratio m of the number of turns of the secondary winding (28) on the number of turns of the primary winding (26) is between 0.036 and 0.65.  5. Voltage converter (2) according to claim 3, characterized in that it is capable of delivering several very low continuous safety voltages between 3 Vde and 24 Vde from an alternating voltage between 190 Vac and 250 Vac <Desc / Clms Page number 16>  present at the input of the rectifier (4), and in that the winding ratio m of the number of turns of the secondary winding (28) on the number of turns of the primary winding (26) is between 0.02 and 0.45.  6. Voltage converter (2) according to claim 3, characterized in that it is capable of delivering several very low continuous safety voltages between 3 Vdc and 12 Vdc from an alternating voltage of between 190 Vac and 250 Vac present at the input of the rectifier (4), and in that the winding ratio m of the number of turns of the secondary winding (28) on the number of turns of the primary winding (26) is between 0 , 01 and 0.2.  7. Voltage converter (2) according to claim 3, characterized in that it is capable of delivering several very low continuous safety voltages between 3 Vdc and 42 Vdc from an alternating voltage of between 80 Vac and 150 Vac present at the input of the rectifier (4), and in that the winding ratio m of the number of turns of the secondary winding (28) on the number of turns of the primary winding (26) is between 0 , 03 and 0.6.  8. Voltage converter (2) according to claim 3, characterized in that it is capable of delivering several very low continuous safety voltages between 3 Vdc and 24 Vdc from an alternating voltage of between 80 Vac and 150 Vac present at the input of the rectifier (4), and in that the winding ratio m of the number of turns of the secondary winding (28) on the number of turns of the primary winding (26) is between 0.015 and 0.35.  9. Voltage converter (2) according to claim 3, characterized in that it is capable of delivering several very low continuous safety voltages between 3 Vdc and 12 Vdc from an alternating voltage of between 80 Vac and 150 Vac present at the input of the rectifier (4), and in that the winding ratio m of the number of turns of the secondary winding (28) on the number of turns of the primary winding (26) is between 0.008 and 0.2.  10. Voltage converter (2) according to any one of the preceding claims, characterized in that said indirect converter (6) comprises a diode (50) whose anode is connected to a first end (34) of the winding secondary (28) and the cathode of which is connected to a first terminal (18) for output of very low DC safety voltages, and in that it comprises a winding ratio m between the number of turns of the secondary winding (28) on the number of turns of the primary winding (26) determined from the following relation: <Desc / Clms Page number 17>  where: - V0min is the smallest very low continuous safety voltage at the output of the voltage converter (2), - Vdl is the voltage drop across the diode (50) when the latter conducts, - Vemm is the minimum direct voltage at the input of the indirect converter (6); - Vds is the voltage drop across the terminals of the switch (42) when the latter conducts, - Lp is the inductance of the primary winding (26), - Ip is the maximum current intensity passing through the switch (42), f is the operating frequency of the switch (42), - kip is the ratio between the maximum amplitude of the variations in the intensity of the current passing through the switch (42) and the maximum intensity Ip of the current flowing through this same switch.

 11. Wall outlet, characterized in that it comprises a voltage converter (2) according to any one of the preceding claims.