ISOLATED DC/DC CONVERTER

Abstract

The invention relates to an isolated DC/DC converter (1) comprising: a first branch (A) comprising series-connected switches (MA1, MA2), the first branch (A) being connected to the input of the converter; a second branch (B) comprising series-connected switches (MB1, MB2); an inductance (L2) connected between the midpoints of the first and the second branch (B); a capacitor connected across the end terminals of the second branch (B); a third branch (C) comprising a magnetic component and connected to the midpoint of the second branch (B); wherein a series of opening and shutting actions of the switches (MA 1, MA2, MB1, MB2) converts an input voltage (Ue) into an output voltage (Uout) by means of the magnetic component.

Claims

1. An isolated DC/DC converter comprising: a first arm comprising switches in series, the first arm being connected to the input of the converter; a second arm comprising switches in series; an inductance connected between the centre points of the first and the second arm; a capacitance connected between the end terminals of the second arm; a third arm comprising a magnetic component, the third arm being connected to the centre point of the second arm; in which successions of opening and closing operations of the switches allow an input voltage to be converted into an output voltage by means of the magnetic component.

2. The converter according to claim 1, in which the first arm is configured to control the output voltage of the isolated DC/DC converter by modifying an electrical parameter of a signal flowing through the inductance, the duty cycle of the second arm remaining substantially constant.

3. The converter according to claim 2, in which the second arm is configured so that its duty cycle is substantially equal to 50%.

4. The converter according to claim 1, comprising a circuit that is intended to implement a first loop so as to enslave a setpoint for an electrical parameter of a signal flowing through the inductance connected between the first and second arms to a difference between the value of the output voltage of the isolated DC/DC converter and a setpoint output voltage for the isolated DC/DC converter.

5. The converter according to claim 1, in which the magnetic component has a primary circuit and a secondary circuit that are separated by an electrical isolation barrier, said magnetic component being configured so as, during the conversion of an input voltage of the isolated DC/DC converter into an output voltage, to operate as a transformer from the primary circuit to the secondary circuit and as an impedance that stores energy in the primary circuit.

6. The converter according to claim 5, in which the magnetic component is configured so that: over a first portion of an operating period of the converter, a first portion of the primary circuit transfers an energy to a first portion of the secondary circuit and a second portion of the primary circuit provides an inductance storing energy; over a second portion of the operating period of the converter, the second portion of the primary circuit transfers an energy to a second portion of the secondary circuit, and the first portion of the primary circuit provides an inductance storing energy.

7. The converter according to claim 5, in which the primary circuit of the magnetic component comprises a primary winding and the secondary circuit of the magnetic component comprises at least one first secondary winding and at least one second secondary winding that are not magnetically coupled to one another, said first and second secondary windings being magnetically coupled to the primary winding.

8. The converter according to claim 7, in which the magnetic component is configured to act as a transformer from the primary winding, either to the first secondary winding(s) or to the second secondary winding(s); while operating as an impedance that stores energy in the primary winding.

9. The converter according to claim 6, in which the magnetic component comprises at least a first and a second transformer in series, in which transformers: the primary of the first transformer forms the first portion of the primary circuit and the secondary of the first transformer forms the first portion of the secondary circuit; the primary of the second transformer forms the second portion of the primary circuit and the secondary of the second transformer forms the second portion of the secondary circuit.

10. A device for converting voltage comprising a combination of at least two isolated DC/DC converters, each isolated DC/DC converter comprising: a first arm comprising switches in series, the first arm being connected to the input of the converter; a second arm comprising switches in series; an inductance connected between the centre points of the first and the second arm; a capacitance connected between the end terminals of the second arm; a third arm comprising a magnetic component, the third arm being connected to the centre point of the second arm; in which successions of opening and closing operations of the switches allow an input voltage to be converted into an output voltage by means of the magnetic component; and in which the first respective arms of the isolated DC/DC converters are configured to operate with a phase shift of 2π/n, and the second respective arms of the converters are configured to operate with a phase shift of π/n, n being the number of isolated DC/DC converters.

11. The device according to claim 10, in which the isolated DC/DC converters share a single circuit that is intended to implement a first loop so as to enslave a setpoint for an electrical parameter of a signal flowing through the inductance connected between the first and second arms to a difference between the value of the output voltage of the isolated DC/DC converter and a setpoint output voltage for the isolated DC/DC converter, so that the first respective arms of the isolated DC/DC converters receive the same setpoint.

12. A method for converting voltage comprising the steps of: providing at least one isolated DC/DC converter comprising: a first arm comprising switches in series, the first arm being connected to the input of the converter; a second arm comprising switches in series; an inductance connected between the centre points of the first and the second arm; a capacitance connected between the end terminals of the second arm; a third arm comprising a magnetic component, the third arm being connected to the centre point of the second arm; in which successions of opening and closing operations of the switches allow an input voltage to be converted into an output voltage by means of the magnetic component; performing successions of opening and closing operations of the switches allowing an input voltage to be converted into an output voltage by means of the magnetic component of the isolated DC/DC converter.

13. The method according to claim 12, in which the performance of the successions of opening and closing operations of the switches comprises modification of an electrical parameter of a signal flowing through the inductance, the duty cycle of the second arm remaining substantially constant.

14. The method according to claim 13, in which the duty cycle of the second arm is substantially equal to 50%.

15. The method according to claim 12, in which the conversion of the input voltage into an output voltage comprises a first loop enslaving the setpoint of an electrical parameter of a signal flowing through the first arm to a difference between the value of the output voltage of the isolated DC/DC converter and a setpoint output voltage for the isolated DC/DC converter.

16. The method according to claim 12 comprising: the provision of a plurality of isolated DC/DC converters; and in which: the second respective arms of the isolated DC/DC converters operate with a phase shift of π/n, n being the number of isolated DC/DC converters; and the first respective arms of the isolated DC/DC converters operate with a phase shift of 2π/n.

17. The method according to claim 16, in which the conversion of the input voltage into an output voltage is performed with the same setpoint delivered by a single first arm that is shared between the isolated DC/DC converters.

18. The converter according to claim 2, comprising a circuit that is intended to implement a first loop so as to enslave a setpoint for an electrical parameter of a signal flowing through the inductance connected between the first and second arms to a difference between the value of the output voltage of the isolated DC/DC converter and a setpoint output voltage for the isolated DC/DC converter.

19. The converter according to claim 4, in which the magnetic component has a primary circuit and a secondary circuit that are separated by an electrical isolation barrier, said magnetic component being configured so as, during the conversion of an input voltage of the isolated DC/DC converter into an output voltage, to operate as a transformer from the primary circuit to the secondary circuit and as an impedance that stores energy in the primary circuit.

20. The converter according to claim 6, in which the primary circuit of the magnetic component comprises a primary winding and the secondary circuit of the magnetic component comprises at least one first secondary winding and at least one second secondary winding that are not magnetically coupled to one another, said first and second secondary windings being magnetically coupled to the primary winding.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0059] The invention will be better understood with reference to the drawings, in which:

[0060] FIG. 1 illustrates an example of an isolated DC/DC converter according to the prior art;

[0061] FIGS. 2 and 3 each illustrate an example of an isolated DC/DC converter according to the invention;

[0062] FIG. 4 illustrates an example of a method for controlling a conversion device comprising interlacing of converters according to the invention;

[0063] FIG. 5 illustrates a variant of the magnetic component of the converter in FIGS. 2 and 3;

[0064] FIGS. 6a to 6e illustrate exemplary embodiments of the magnetic component illustrated in FIG. 5.

DETAILED DESCRIPTION OF THE DRAWINGS

[0065] The converter according to the invention will be better understood with reference to FIG. 2, which presents an example of an isolated DC/DC converter according to the invention.

[0066] The isolated DC/DC converter 1 comprises a first arm A of switches in series and a second arm B of switches in series. The arms A, B comprise switches MA1, MA2, MB1, MB2, in which a succession of opening and closing operations allows the output of the isolated DC/DC converter 1 to be controlled. These switches may be transistors, such as MOSFET, IGBT or other transistors. A portion, notably the switches of the second arm, or the whole of the isolated DC/DC converter 1 can be produced from a semiconductor material such as silicon (Si), gallium nitride (GaN), silicon carbide (SiC) or any other semiconductor material.

[0067] The first arm A comprises two switches MA1, MA2 in series. The switch MA1, referred to as the high-side switch, is connected to the high terminal of a voltage source (not shown). The switch MA2, referred to as the low-side switch, is connected to the low terminal of the voltage source. This low terminal corresponds notably to a first earth GND1 of the isolated DC/DC converter 1. Each switch MA1, MA2 can comprise a transistor in parallel with a freewheeling diode.

[0068] An inductance L2 has a first terminal connected to the centre point of the two switches MA1, MA2 of the first arm A, and a second terminal connected to the centre point of the second arm B.

[0069] The second arm B comprises two switches MB1, MB2 in series, which are preferably identical. The switches MB1, MB2 comprise diodes that are similar to the diodes described above for the switches MA1, MA2 of the first arm A. Moreover, each switch MB1, MB2 comprises a capacitance CB1, CB2 in parallel. These capacitances CB1, CB2 are used for zero voltage switching or ZVS when the switches MB1, MB2 open. During the opening of a switch MB1, MB2, energy stored in an inductance, notably a leakage inductance of a magnetic component described later, is recovered in order to discharge the capacitance CB1, CB2 that is at the terminals of the switch. Once the voltage is close to 0V, the switch is controlled and thus zero voltage switching is produced, which greatly reduces switching losses. These capacitances CB1, CB2 may be inherently present in the structure of the semiconductor making up the switches MB1, MB2, as parasitic elements. The parasitic capacitances of the switches MB1, MB2 may therefore suffice for producing zero voltage switching without the addition of supplementary capacitances. The first arm A could likewise comprise capacitances for soft switching of its switches MA1, MA2. However, this would bring about current ripples in the inductance L2 that are likely to result in losses. The result of this would be that the advantage of the soft switching of the switches MA1, MA2 of the first arm A would be lost.

[0070] A capacitance C1 is connected between the end terminals of the second arm B. In particular, the capacitance C1 is connected to the high-side switch MB1 of the second arm B and to the low-side switch MB2 of the second arm, at a respective terminal that is different from the centre point of the second arm B.

[0071] The centre point between the two switches MB1, MB2 of the second arm B is connected to a third arm C that comprises two isolation transformers T1, T2 in series. Each transformer T1, T2 comprises a primary L11, L21 and a secondary L12, L22. The primaries L11, L21 and the secondaries L12, L22 are respectively in series. The centre point of the switches MB1, MB2 is connected to the primaries L11, L21. A capacitance C33 is in series with the transformers T1, T2. However, the isolated DC/DC converter 1 could do without this capacitor. The capacitance C33 allows the DC component of the signal transmitted by the transformers T1, T2 to be eliminated, notably in the case of a half-bridge structure. The capacitance C33 can be eliminated in a full-bridge structure. The secondaries L12, L22 are in series, with their centre point connected to a second earth GND2 of the isolated DC/DC converter 1.

[0072] Diodes D31, D32 are connected to the secondaries L12, L22 in order to rectify the signal from the transformers T1, T2. To this end, a diode D31 has its anode connected to a terminal of one secondary L12 and the other diode D32 has its anode connected to a terminal of the other secondary L22, these terminals being different from the centre point of the two secondaries L12, L22. The output of the isolated DC/DC converter 1 is taken between the terminal of the diodes D31, D32 that is not connected to the secondaries L12, L22 and that is common to the two diodes D31, D32 and the second earth GND2. The high output is therefore taken from the common terminal of the diodes D31, D32.

[0073] In a variant, each diode D31, D32 has its respective cathode connected to a terminal of the secondary L12, L22 that is different from the centre point. The high output is taken from the centre point of the secondaries L12, L22. The centre point of the secondaries L12, L22 is therefore not connected to the second earth GND2. The terminal that is common to the two diodes D31, D32 is connected to the second earth GND2.

[0074] The diodes D31, D32 could advantageously be replaced by switches, notably transistors, such as MOSFET, IGBT or other transistors, in order to obtain synchronous rectification at the output of the transformers T1, T2, for example. For high-current applications on the secondary, the use of transistors instead of the diodes allows the overall output from the isolated DC/DC converter 1 to be improved.

[0075] The isolated DC/DC converter 1 comprises a capacitance CF for filtering the output signal.

[0076] The switches MB1, MB2 of the second arm B have a duty cycle that allows an energy to be transferred through the transformers T1, T2. In particular, over a first portion of an operating period, the switch MB1 is closed and the switch MB2 is open. A magnetizing inductance of the primary L11 of the first transformer T1 stores energy and the primary L21 of the second transformer T2 transfers an energy to the secondary L22 of the second transformer T2.

[0077] Over a second portion of the operating period, a magnetizing inductance of the primary L21 of the second transformer T2 stores energy, and the primary L11 of the first transformer T1 transfers an energy to the secondary L12 of the first transformer T1. The durations of the first and second portions of operation are defined by the duty cycle of the switches MB1, MB2.

[0078] In the prior art illustrated in FIG. 1, the transmission of energy through the transformers T1, T2 is controlled by the switches Q1, Q2 in series. In the isolated DC/DC converter according to the invention, the second arm B likewise allows this energy transmission to be controlled. However, in the prior art, the voltage at the terminals of the branch comprising the two switches Q1, Q2 is equal to the input voltage Ue of the converter. By contrast, in the isolated DC/DC converter 1 according to the invention, the voltage VC1 at the terminals of the second branch B, that is to say at the terminals of the capacitance C1, is provided by the expression

[00001]$\mathrm{VC}.Math..Math.1=\frac{{\alpha }_A}{{\alpha }_B}\mathrm{Ue},$

[0079] where α.sub.A is the duty cycle of the first arm A and α.sub.B is the duty cycle of the second arm B.

[0080] Thus, in the isolated DC/DC converter 1, the duty cycle α.sub.A of the first arm A constitutes, in relation to the prior art, a supplementary parameter in the control of the transfer of energy trough the transformers T1, T2. Control of the isolated DC/DC converter 1 is therefore refined in relation to the prior art.

[0081] Moreover, the range of values that can be accessed by the voltage VC1 at the terminals of the second branch B is above the range of values that can be accessed by the voltage at the terminals of the branch of the switches Q1, Q2 in the prior art. The reason for this is that if the ratio α.sub.A/α.sub.B is above 1, then the voltage VC1 at the terminals of the second branch B is above the input voltage Ue. In particular, the voltage VC1 may be above a maximum value Ue.sub.max of the input voltage Ue. The voltage VC1 at the terminals of the branch B may therefore be higher than the input voltage Ue of the isolated DC/DC converter 1, in contrast to the prior art. Equally, if the ratio α.sub.A/α.sub.B is below 1, then the voltage VC1 at the terminals of the second branch B is below the input voltage Ue. In particular, the voltage VC1 may be below a minimum value Ue.sub.min of the input voltage Ue. The voltage VC1 at the terminals of the second branch B may therefore be lower than the input voltage Ue of the isolated DC/DC converter, in contrast to the prior art.

[0082] It may be noted that this property of lowering or raising the input voltage may be implemented by adding a supplementary stage to the converter of the prior art illustrated in FIG. 1. By way of example, the supplementary stage may be a step-up/step-down (“buck-boost”) converter connected to the primary side of the converter. The converter obtained would therefore have two supplementary arms of switches in relation to the arms of switches Q1, Q2. The total number of arms of switches would therefore be 3 on the primary side of the converter. By contrast, in the converter according to the invention, the property of lowering or raising the input voltage is obtained with two arms A, B of switches MA1, MA2, MB1, MB2 on the primary side of the converter 1.

[0083] In particular, the switches MB1, MB2 of the second arm B operate with a duty cycle α.sub.B that does not vary, that is to say that remains constant over the course of time. During the operation of the isolated DC/DC converter 1, the output voltage Vout is controlled by the current flowing in the inductance L2. This current is controlled by the first arm A. To this end, the isolated DC/DC converter 1 comprises a control unit 5 for the first arm A. The control unit 5 delivers a pulse width modulation or PWM signal S2 that controls the opening and closing of the switches MA1, MA2 of the first arm A in order to control the current flowing in the inductance L2. The switches MA1, MA2 of the first arm A are controlled so that the current flowing in the inductance L2 allows a desired voltage value to be obtained at the output of the isolated DC/DC converter 1. Thus, in contrast to the prior art, it is not necessary to vary the duty cycle α.sub.B of the switches MB1, MB2 that are connected to the transformers T1, T2. The second arm B can therefore operate at its most advantageous duty cycle α.sub.B for the transmission of energy by the transformers T1, T2, notably at 50%.

[0084] The voltage stresses at the terminals of the diodes D31, D32 are dependent on the duty cycle α.sub.B of the second arm B, and are provided by the following expressions:

V(D31)=Vout/(1 −α.sub.B) and V(D32)=Vout/α.sub.B

[0085] The duty cycle α.sub.B is preferably equal to 50%. Thus, the voltage stresses at the terminals of two diodes D31, D32 are equal, and the wear is the same between the diodes D31, D32. Moreover, at a duty cycle of 50%, the current ripples owing to the magnetizing inductances of the transformers T1, T2 are compensated for among one another. Thus, the current on the secondaries L12, L22 is continuous.

[0086] The voltage VC1 at the terminals of the second branch B is then equal 2α.sub.AUe. With the duty cycle α.sub.A of the first arm A, the voltage VC1 at the terminals of the second arm B can be varied. If the duty cycle α.sub.A of the first arm A is below 0.5, the voltage VC1 at the terminals of the second arm B is below 2Ue. If the duty cycle α.sub.A of the first arm A is above 0.5, the voltage VC1 at the terminals of the second arm B is above 2Ue. A duty cycle α.sub.B of 0.5 for the second arm B therefore allows simple control of the isolated DC/DC converter 1.

[0087] In particular, when the input voltage Ue of the voltage converter 1 varies, the first arm A makes it possible to ensure that the output voltage Vout keeps a desired value. Thus, if the input voltage Ue of the isolated DC/DC converter 1 changes value, the control unit 5 modifies the control of the duty cycles α.sub.A of the switches MA1, MA2 of the first arm A in corresponding fashion in order to maintain the current flowing through the coil L2 at a desired value. This is particularly advantageous in an electric vehicle, where the level of charge of a battery can vary over the course of time.

[0088] More particularly, the control unit 5 produces a first feedback loop that enslaves the current flowing through the inductance connected between the first A and second B arms to a difference between the value Vout mes of the output voltage of the isolated DC/DC converter 1 and a desired voltage Vout at the output of the isolated DC/DC converter 1. To this end, the control unit 5 receives the voltage Vout_mes measured at the output of the isolated DC/DC converter, possibly multiplied by a gain K1. The control unit 5 then compares a setpoint voltage V* with the measured voltage Vout_mes. The setpoint voltage V* corresponds to the voltage Vout desired at the output of the isolated DC/DC converter 1. According to the result of the comparison, a controller 51 delivers to the first arm A a setpoint current I2cons that has to flow through the inductance L2.

[0089] The setpoint current I2cons can be transmitted directly to a controller 52 that delivers to the first arm A the PWM signal S2 from the setpoint current I2cons. However, the control unit 5 can produce a second loop that enslaves the current flowing through the inductance L2 to a difference between the value I2mes of the current flowing through the inductance L2 and the setpoint current I2cons. In particular, the control unit 5 compares the setpoint current I2cons that is output by the first loop with the current I2mes measured on the inductance L2. The current I2cons is possibly multiplied by a gain K2 before the comparison. According to the result of this comparison, the controller 52 determines the signal S2 for controlling the duty cycle α.sub.A of the switches MA1, MA2 of the first arm A so as to adjust the current flowing through the inductance L2. Voltage loops could be used. However, the current loop is easier to implement because, as a small signal, the current loop makes it possible to have a transfer function of the first order, whereas the voltage loop is of the second order. Moreover, the isolated DC/DC converter 1 could implement the first loop without using the second loop.

[0090] The isolated DC/DC converter 1 according to the invention can be designed to cover an operating range. The operating range corresponds to an input voltage Ue of the isolated DC/DC converter 1 that is between a minimum value Ue.sub.min1 and a maximum value Ue.sub.max1; and to an output voltage Vout that is between a minimum value Vout.sub.min1 and a maximum value Vout.sub.max1 By way of example, the input voltage Ue is between 170 and 450V, and the target voltage Vout at the output of the isolated DC/DC converter 1 is between 12 and 16V. By way of example, the minimum value Vout.sub.min1 of the output voltage is between 8 and 14V and the maximum value Vout.sub.max1 of the output voltage is between 15 and 16V.

[0091] In the examples illustrated in FIGS. 2 and 3, the magnetic component of the isolated DC/DC converter 1 comprises a first T1 and a second T2 transformer in series. The magnetic component may be replaced by a magnetic component 31 that is illustrated in FIG. 5. The magnetic component 31 comprises a primary circuit with a single primary winding 33 connected to the capacitance C33 and a secondary circuit with two secondary windings 35a and 35b. The two secondary windings 35a and 35b are magnetically coupled to the primary winding 33 but are not magnetically coupled to one another. Such a magnetic component 31 allows not only a reduction in the cost of the converter by reducing the number of components comprising ferrite but also a reduction in the bulk of the converter by allowing a more compact converter to be obtained.

[0092] The operation of the isolated DC/DC converter 1 remains the same. The magnetic component 31 operates in a similar manner to two perfect transformers in series. Over the first portion of the modulation period, a first portion of the primary winding 33 provides an inductance, and a second portion of the primary winding 33 transfers the energy to the second secondary winding 35a. Over the second portion of the modulation period, the first portion of the primary winding 33 transfers the energy to the first secondary 35b, and the second portion of the primary winding 33 provides an inductance.

[0093] Various configurations allowing a magnetic component 31 to be obtained that allows magnetic coupling between the primary winding 33 and the secondary windings 35a and 35b without there being any magnetic coupling between the secondary windings 35a and 35b are illustrated in FIGS. 6a to 6c.

[0094] FIGS. 6d, 6e illustrate examples of a magnetic component 31 that comprise at least two first secondary windings 35a in parallel and at least two second secondary windings 35b in parallel. These configurations are advantageous in applications in which the current flowing in the isolated DC/DC converter 1 is high, for example above 100 A, or even above 200 A. The isolated DC/DC converter 1 then comprises a plurality of diodes D31, each connected to a first respective secondary winding 35a; and a plurality of diodes D32, each connected to a second respective secondary winding 35b. As in the examples illustrated in FIGS. 2 and 3, the diodes D31, D32 could be replaced with switches.

[0095] The components 31 illustrated in FIGS. 6a to 6e are described in more detail in French patent application 1458573, the content of which is incorporated in the present application.

[0096] In the example illustrated in FIG. 2, the second arm B and the primaries of the transformers T1, T2 form a half-bridge structure. The example illustrated in FIG. 3 is identical to the example in FIG. 2 except that, in FIG. 3, the second arm B and the primaries of the transformers T1, T2 form a full-bridge structure with a fourth arm D. The switches of the fourth arm D are preferably identical to those of the second arm B.

[0097] For power applications, it may be advantageous to combine a plurality of isolated DC/DC converters 1 that are illustrated in FIGS. 2 and 3. Isolated DC/DC converters 1 can be placed in parallel and combined in order to limit current ripples and reduce the value of the filtering capacitance CF at the output of the isolated DC/DC converter 1. In each isolated DC/DC converter 1, owing to the first arm A, the duty cycle α.sub.B of the second arm B remains constant.

[0098] FIG. 4 illustrates operation of a conversion device 10 that comprises a combination of isolated DC/DC converters 1. Preferably, the first feedback loop is common to all the isolated DC/DC converters 1. Thus, the first arms A receive the same setpoint current I2cons. To this end, the device 10 can comprise a single controller 51 delivering a single setpoint current I2cons to all the first arms A of the isolated DC/DC converters 1. Thus, current balancing between the converters is ensured.

[0099] Preferably, the isolated DC/DC converters 1 operate with a phase shift. In particular, the first arms A operate with a phase shift of 2π/n, where n is the number of isolated DC/DC converters 1, which allows fluctuations of the output of the device 10 and electromagnetic compatibility problems to be limited. The second arms B of the isolated DC/DC converters 1 operate with a phase shift of π/n, which allows ripples at the output of the device 10 to be limited. These ripples can be caused by parasitic elements, such as primary-side or secondary-side parasitic inductances.

[0100] The invention is not limited to the examples described. In particular, the voltage loops can be replaced by current loops. The isolated DC/DC converter can also be used in an AC/DC converter that is configured to convert an AC voltage into a DC voltage or vice versa, or in an AC/AC converter. Advantageously, the isolated DC/DC converter is then complemented by an AC/DC converter upstream of the first arm and/or a DC/AC converter downstream of the isolated DC/DC converter.