Step-down converter with two magnetically coupled windings

Abstract

A buck converter in which: - a voltage source (1) has a first terminal connected to a first output terminal of the buck converter intended to be connected to a load (3), and a second terminal connected to a first terminal of a switch (K), - a second terminal of the switch (K) is connected to a first terminal of a primary coil (2p) of a transformer which also has a secondary coil (2s), - a second terminal of the primary coil (2p) is connected to a second output terminal of the buck converter intended to be connected to the load (3), - a first terminal of the secondary coil (2s) is connected to the anode of a diode (5) whose cathode is connected to the first terminal of the voltage source (1), - a second terminal of the secondary coil (2s) is connected to ground, characterized in that the second terminal of the voltage source (1) and the first terminal of the switch (K) are connected to ground.Figure for the abbreviation: figure 2.

Description

Title of the invention: Step-down converter with two magnetically coupled windings Technical field

[0001] The present invention relates to a step-down converter operating in the quadrant Vs>0 and Is>0, equipped with two magnetically coupled windings. It is applicable in all types of land, naval or air equipment using this type of converter to allow load control with regulation of the current or voltage such as the control of solenoid valves, servovalves for example. State of the prior art

[0002] The classic structure of a step-down converter, or buck converter, is illustrated in [Fig.l].

[0003] A voltage source 1 delivers a voltage Ve. A first terminal of the voltage source 1 is connected to a disconnection switch 6, itself connected to a switch K, for example made by a MOSFET type transistor, here P-channel MOSFET. The disconnection switch 6 is here connected to the source of the P-channel MOSFET transistor. A second terminal of the voltage source 1 is connected to ground.

[0004] The switch K, here more particularly the drain of the P-channel MOSFET transistor, is connected to a coil 2 having an inductance L. The coil 2 is connected to a load 3, itself connected to a second terminal of the voltage source 1.

[0005] A smoothing capacitor 4 having a capacitance C is connected in parallel with the load 3. The function of the capacitor 4 is to smooth the voltage across the terminals of the load 3.

[0006] Finally, a diode 5 has a cathode connected between the switch K, here more particularly the drain of the P-channel MOSFET transistor, and the coil 2, and an anode connected to the second terminal of the voltage source 1.

[0007] The operation of the step-down converter implies that the switch K is periodically conductive.

[0008] In a first phase where the switch K is conductive, the current supplied by the source 1 passes through the coil which stores the received energy in magnetic form. The current IL passing through the coil increases according to the slope (Ve-Vs) / L, where Vs is the voltage across the terminals of the load 3. The diode 5 is then blocked.

[0009] In a second phase where the switch K is blocked, the diode 5 is conducting. The current passing through the coil decreases according to the slope -Vs / L, the coil becomes a current source for the downstream circuit using the magnetic energy stored in the previous phase.

[0010] There are two operating modes of the step-down converter depending on whether the current flowing in coil 2 is continuous or not. In continuous conduction mode, the current through the coil never cancels. Conversely, in discontinuous conduction mode, the current through the coil cancels for part of the time at the end of the second phase.

[0011] In the event of a load short circuit, the voltage Vs becomes zero and the current in the coil can no longer decrease. It is not possible, except by adding components, to limit the instantaneous current. Consequently, in practice, it is necessary to add an inductance at the converter output in order to limit the short-circuit current, to add a very fast protection and cut-off device to limit the maximum value of the short-circuit current, to add a demagnetization circuit for the output inductance to manage the cut-off after detection of the output short-circuit, to oversize the interfaces (input filter capacitor) so that they can withstand the short-circuit current.

[0012] Furthermore, the circuit of [Fig.l] presents the classic difficulty of controlling a switch which is not referenced to ground:

[0013] - if the switch is referenced relative to the positive potential of the input voltage, this results in the need to use a P-channel (or bipolar PNP) MOSFET switch and a transposition of its gate (or base) control with respect to ground. The main constraint of this choice is linked to the fact that P-channel MOSFETs are rarer and less efficient than N-channel MOSFETs;

[0014] - if the switch is referenced with respect to the common potential between the cathode of the diode 5 and inductor 2, this potential varying cyclically between the ground potential and the input voltage, this requires an N-channel (or bipolar NPN) MOSFET switch and an auxiliary voltage source to provide a positive potential referenced to this common potential (charge pump or floating power supply). Experience also shows that it is frequently necessary to be able to saturate the control of transistor K (i.e. keep it in the conductive state permanently), which condemns the (economical) option of the charge pump.

[0015] We therefore currently have the choice between using a switch that is limited in performance and relatively rare, but easy to control, and using a high-performance switch that is very common, but whose control risks being complex.

[0016] Document EP 1 959 549 proposes a converter structure which comprises two coupled coils. Thus, in the event of a load short circuit, the second coil makes it possible to reduce the current flowing through the first. The converter is thus robust to a load short circuit.

[0017] There is, however, a need to improve such a converter, in particular to facilitate the control of the switch and thus limit the volume of electronic components dedicated to this control. Statement of the invention

[0018] The invention aims to solve the problems of the prior art by providing a step-down converter in which:

[0019] - a voltage source comprises a first terminal connected to a first terminal of output of the step-down converter intended to be connected to a load, and a second terminal connected to a first terminal of a switch,

[0020] - a second terminal of the switch is connected to a first terminal of a coil primary of a transformer which also has a secondary coil,

[0021] - a second terminal of the primary coil is connected to a second output terminal of the step-down converter intended to be connected to the load,

[0022] - a first terminal of the secondary coil is connected to the anode of a diode whose cathode is connected to the first terminal of the voltage source,

[0023] - a second terminal of the secondary coil is connected to ground,

[0024] characterized in that the second terminal of the voltage source and the first terminal of the switch are connected to ground.

[0025] Thanks to the invention, the transistor forming a switch has its source connected to ground. This simplifies its control compared to a case where the transistor is floating on the hot point, and allows direct control of the transistor by conventional control circuits allowing the use, if necessary, of advanced regulation modes, for example peak current regulation in the inductance which makes it possible to greatly simplify the regulation and increase its performance.

[0026] According to a preferred characteristic, the switch is an enrichment type power switch whose control reference is ground.

[0027] According to a preferred characteristic, a smoothing capacitor is connected between the output terminals of the step-down converter.

[0028] According to a preferred characteristic:

[0029] - a current measuring sensor is connected to the switch,

[0030] - an output of the current measuring sensor is connected to an input of a pass-filter down,

[0031] - an output of the low-pass filter is connected to a first input of a subtractor, of which a second input receives a setpoint signal,

[0032] - an output of the subtractor is connected to an input of an integrator, one output of which is connected to a first input of a comparator,

[0033] - the output of the current measuring sensor is further connected to a first input of an adder whose second input receives a ramp signal whose slope is proportional to the difference between the voltage between the terminals of the voltage source and the voltage between the output terminals of the step-down converter,

[0034] - an output of the adder is connected to a second input of the comparator, and

[0035] - an output of the comparator is connected to a control input of the switch.

[0036] According to a preferred characteristic:

[0037] - a voltage measuring device is connected to the output terminals of the converter lowering,

[0038] - an output of the voltage measuring device is connected to a first input of a subtractor, a second input of which receives a setpoint signal,

[0039] - an output of the subtractor is connected to an input of an integrator, one output of which is connected to a first input of a comparator,

[0040] - a current measuring sensor is connected to the switch,

[0041] - an output of the current measuring sensor is connected to a first input of an ad converter, a second input of which receives a ramp signal whose slope is proportional to the difference between the voltage between the terminals of the voltage source and the voltage between the output terminals of the step-down converter,

[0042] - an output of the adder is connected to a second input of the comparator, and

[0043] - an output of the comparator is connected to a control input of the switch.

[0044] The invention also relates to equipment comprising a step-down converter as previously presented.

[0045] The invention also relates to a method of operating a step-down converter as previously presented, comprising for an operating cycle

[0046] - a first phase in which the switch is closed and the diode is blocked, the step-down converter operating in continuous or discontinuous mode,

[0047] - a second phase in which the switch is open and the diode is conducting, the step-down converter operating in continuous or discontinuous mode,

[0048] - a third phase in which the switch is open and the diode is blocked, the third phase only existing for discontinuous mode.

[0049] The equipment and the method have advantages similar to those previously presented.

[0050] In a particular embodiment, the steps of the method according to the invention are implemented by computer program instructions.

[0051] Consequently, the invention also relates to a computer program on an information medium, this program being capable of being implemented in a computer, this program comprising instructions adapted to the implementation of the steps of a method as described above.

[0052] The invention also relates to an information medium readable by a computer, and comprising computer program instructions adapted to the implementation of the steps of a method as described above. Brief description of the drawings

[0053] Other characteristics and advantages will appear on reading the following description of a preferred embodiment given by way of non-limiting example, described with reference to the figures in which:

[0054] [Fig-1] illustrates a step-down converter according to the prior art,

[0055] [Fig.2] illustrates a step-down converter according to an embodiment of the invention,

[0056] [Fig.3] illustrates a first phase of an operating cycle of the converter step-down device according to one embodiment of the invention,

[0057] [Fig.4] illustrates a second phase of a converter operating cycle step-down device according to one embodiment of the invention,

[0058] [Fig.5] illustrates a third phase of a converter operating cycle step-down device according to one embodiment of the invention,

[0059] [Fig.6] illustrates timing diagrams corresponding to an operating mode continuous of the step-down converter according to one embodiment of the invention,

[0060] [Fig.7] illustrates timing diagrams corresponding to an operating mode discontinuous of the step-down converter according to an embodiment of the invention,

[0061] [Fig.8] illustrates a mode of regulation of the output current of the step-down converter according to one embodiment of the invention, and

[0062] [Fig.9] illustrates a mode of regulation of the output voltage of the converter step-down device according to one embodiment of the invention.

[0063] Identical, similar or equivalent parts of the different figures bear the same numerical references so as to facilitate the transition from one figure to another.

[0064] The different parts represented in the figures are not necessarily on a uniform scale, in order to make the figures more readable.

[0065] The different possibilities (variants and embodiments) must be understood as not being mutually exclusive and can be combined with each other.

[0066] Detailed description of particular embodiments

[0067] According to a preferred embodiment shown in [Fig.2], a step-down converter comprises a voltage source 1 delivering a voltage Ve. The step-down converter is integrated into land, naval or air electronic equipment.

[0068] A first terminal of the voltage source 1 is connected to a disconnect switch 6, itself connected to a first output terminal of the step-down converter intended to be connected to a load 3 and a second terminal of the voltage source 1 is connected to ground. The disconnect switch 6 is conventional and will not be described here.

[0069] The second terminal of the voltage source 1 is also connected to a first terminal of a switch K. The second terminal of the switch K is connected to a first terminal of a coil, or winding, 2p.

[0070] In the example shown, the switch K comprises an N-channel MOSFET type transistor. In this case, the second terminal of the voltage source 1 is connected to the source of the transistor which is therefore connected to ground. The drain of the transistor is connected to the first terminal of the coil 2p.

[0071] It should be noted that generally, the switch K comprises any type of enrichment type power switch, i.e. one to which energy must be supplied to make it conductive, the control reference of which may be ground. It may therefore be, for example, an N-channel MOSFET, NPN, IGBT, etc.

[0072] A second terminal of the primary coil 2p is connected to a second output terminal of the step-down converter intended to be connected to the load 3.

[0073] The 2p coil forms the primary circuit of a transformer which also includes a secondary circuit formed by a 2s coil. The two 2p and 2s coils are wound around a single magnetic core. The two 2p and 2s coils are wound in the same direction. The winding direction of each coil is indicated by a dot in [Fig.2],

[0074] A first terminal of the coil 2s is connected to the anode of a diode 5. The cathode of the diode 5 is connected to the first terminal of the voltage source 1. A second terminal of the coil 2s is connected to ground.

[0075] A smoothing capacitor 4 having a capacitance C is connected between the output terminals of the step-down converter, i.e. in parallel with the load 3. The function of the capacitor 4 is to smooth the voltage across the terminals of the load 3.

[0076] The operation of the step-down converter is described with reference to Figures 3 to 7.

[0077] The operation of the step-down converter is cyclical, depending on a periodic conduction state of the switch K.

[0078] There are two operating modes of the step-down converter depending on whether the current flowing in coil 2 is continuous or not. In continuous conduction mode, the current flowing through the coil never cancels out. A cycle therefore has two phases.

[0079] Alternatively, in discontinuous conduction mode, the current through the coil is zero for part of the time at the end of the second phase. A cycle therefore has three phases.

[0080] [Fig.3] represents a first phase where the switch K is closed, that is to say that the transistor is conducting. The current supplied by the source 1 passes through the primary coil 2p which stores the received energy in magnetic form. A current ILp passing through the primary coil 2p increases according to the slope (Ve-Vs) / L, where Vs is the voltage

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[0091] across load 3. Diode 5 is then blocked. A current IL also flows in load 3 and a current Ic flows in smoothing capacitor 4. [Fig.4] represents a second phase where the switch K is open, that is to say the transistor is blocked, and the diode 5 is conducting. A current ILs flows through the secondary coil 2s and the diode 5. With this topology, whatever the state of the charge, the 2p coil is demagnetized by the secondary coil according to a slope (-Ve*m / L), where m is the transformation ratio between the two coils, and the converter regulation remains operational. A current IL also flows in load 3. This current results from the discharge of smoothing capacitor 4. [Fig.5] represents a third phase which exists only for discontinuous mode. In this phase, switch K is open and diode 5 is blocked. No current flows through coils 2p and 2s. A current IL flows through load 3. This current results from the discharge of smoothing capacitor 4. [Fig.6] represents the timing diagrams corresponding to the continuous operating mode, therefore for an operating cycle comprising two phases, illustrated by figures 3 and 4. [Fig.7] represents the timing diagrams corresponding to the discontinuous operating mode, therefore for an operating cycle comprising three phases, illustrated by figures 3, 4 and 5. In Figures 6 and 7, the chronograms represent from top to bottom: - The current ILp passing through the primary coil 2p, - The current ILs passing through the secondary coil 2s, - The current IL passing through load 3, - The current Ic passing through the smoothing capacitor 4, - The voltage VLp across the primary coil 2p, - The voltage Vt across the terminals of switch K, - The voltage Vd across diode 5. [Fig.8] illustrates a mode of regulation of the output current of the step-down converter, i.e. the current IL supplied to load 3. The sum of the currents supplied to load 3 and to the smoothing capacitor 4 is equal to the current It supplied by the voltage source 1 to the switch K, i.e. to the source of the transistor forming the switch K. It should be noted that the average current in the capacitor 4 being zero, the average current in the transistor is equal to the current of the load. The regulation of the output current of the step-down converter is carried out from the instantaneous current It which flows through the switch K, and comprises two regulation loops.

[0092] A current measuring sensor 10, such as for example a measuring resistor or a current transformer, is connected in series with the source or the drain of the switch. In the example shown, the current measuring sensor 10 is connected upstream of the switch K, that is to say upstream of the source of the transistor.

[0093] Alternatively, the current measurement sensor 10 can be connected between the drain of the transistor K and the primary coil 2p.

[0094] The first control loop, or slow loop, is first described.

[0095] An output of the current measurement sensor 10 is connected to an input of a low-pass filter 11. An output of the low-pass filter 11 is connected to a first input of a subtractor 12, a second input of which receives a setpoint signal Csg.

[0096] An output of the subtractor 12 is connected to an input of an integrator 13, one output of which is connected to a positive input of a comparator 14 described below.

[0097] The low-pass filter 11 is used to average the current in the transistor, which then becomes equal to the load current. The setpoint value Csg is subtracted at 12 from the value of the filtered current, and the resulting error is injected into the integrator 13. In stabilized operation, the output voltage of the integrator is therefore proportional to the load current passing through the impedance 3.

[0098] The second control loop, or fast loop, is now described.

[0099] The output of the current measurement sensor 10 is also connected to a first input of an adder 15. A second input of the adder 15 receives a slope ramp signal (Ve - Vs) / L. The slope is proportional to the difference between the voltage between the terminals of the voltage source and the voltage between the output terminals of the step-down converter. An output of the adder 15 is connected to a negative input of the comparator 14.

[0100] The fast loop faithfully reproduces the increasing slope of the current in the primary coil 2p. This slope is proportional to Ve-Vs, so that it is possible to anticipate by this means the rapid variations of Ve and Vs.

[0101] Adder 15 makes it possible to add a compensation ramp to ensure the stability of this loop.

[0102] The output of integrator 13 is injected into the positive input of comparator 14, and the output of adder 15 is injected into the negative input of the same comparator 14, which serves as a pulse width modulator.

[0103] An output of comparator 14 is connected to a control input of switch K. In the example shown, the control input is the gate of the transistor.

[0104] Comparator 14 is synchronized by an internal oscillator and can only perform a single control pulse of transistor K per switching period, the width of which depends directly on the output voltage of the integrator and the time it takes for the image of the current in the transistor at the output of adder 15 to rise up to the value of the output voltage of integrator 13.

[0105] The operation of the down converter regulation is as follows.

[0106] The current It is measured by the current measuring sensor 10. The result of the measurement is applied to the low-pass filter 11 which delivers a SIL signal representative of the current IL passing through the load 3.

[0107] From the measurement of current It supplied to the source of the transistor, it is possible to regulate the step-down converter by peak current.

[0108] The reference signal Csg is subtracted from the signal SIL and the result is applied to the integrator 13. The integrator 13 delivers an error signal which is compared to the signal delivered by the adder 15. The comparator 14 thus delivers a control signal which is applied to the switch K, in this case to the gate of the transistor K.

[0109] When the output or input voltage varies on a transient, the slope of the ramp is modified which makes it possible to anticipate the variations and compensate for them instantly.

[0110] Alternatively, in order to compensate for subharmonic oscillations specific to the peak current mode, a fixed or variable voltage ramp is added to the voltage ramp (Ve-Vs) / L.

[0111] Alternatively, it is possible to modify the previous diagram so as to:

[0112] - Inject the output of the low-pass filter 11 into the negative input of the subtractor 12,

[0113] - Inject the Csg instruction on the positive input of subtractor 12,

[0114] - Inject the output of integrator 13 onto the negative input of the mo comparator pulse width modulation 14, and

[0115] - Inject the output of adder 15 into the positive input of comparator 14.

[0116] [Fig.9] illustrates a mode of regulating the output voltage of the step-down converter, as an alternative to the mode of regulating the output current of the step-down converter.

[0117] The regulation of the output voltage of the step-down converter comprises two regulation loops.

[0118] The first control loop, or slow loop, is first described.

[0119] A voltage measuring device 20 is connected to the output terminals of the converter. step-down, that is to say at the terminals of the load 3. An output of the device 20 is connected to a first input of a subtractor 21, a second input of which receives a reference signal Csg'.

[0120] An output of the subtractor 21 is connected to an input of an integrator 22, one output of which is connected to a positive input of a comparator 23.

[0121] The second control loop, or fast loop, is now described.

[0122] A current measuring sensor 24 is connected upstream of the switch K, that is, upstream of the transistor source.

[0123] An output of the sensor 24 is connected to a first input of an adder 25. A second input of the adder 25 receives a slope ramp signal (Ve - Vs) / L. The slope is proportional to the difference between the voltage between the terminals of the voltage source and the voltage between the output terminals of the step-down converter. An output of the adder 25 is connected to a negative input of comparator 23.

[0124] An output of comparator 23 is connected to a control input of switch K, i.e. to the gate of the transistor in the example shown.

[0125] The operation of the down converter regulation is as follows.

[0126] The voltage Vs is measured by the voltage measuring device 20. The result of the measurement is applied to the subtractor 21.

[0127] The voltage Vs is compared with the reference signal Csg' and the result of the comparison is applied to the integrator 22. The integrator 22 delivers an error signal which is compared with the signal delivered by the adder 25. The comparator 23 thus delivers a control signal which is applied to the switch K, in this case to the gate of the transistor K.

Claims

Claims

1. Step-down converter in which: - a voltage source (1) comprises a first terminal connected to a first output terminal of the step-down converter intended to be connected to a load (3), and a second terminal connected to a first terminal of a switch (K), - a second terminal of the switch (K) is connected to a first terminal of a primary coil (2p) of a transformer which also comprises a secondary coil (2s), - a second terminal of the primary coil (2p) is connected to a second output terminal of the step-down converter intended to be connected to the load (3), - a first terminal of the secondary coil (2s) is connected to the anode of a diode (5) whose cathode is connected to the first terminal of the voltage source (1), - a second terminal of the secondary coil (2s) is connected to ground,characterized in that the second terminal of the voltage source (1) and the first terminal of the switch (K) are connected to ground.,

2. A step-down converter according to claim 1, wherein the switch (K) is an enhancement-type power switch whose control reference is ground.

3. A step-down converter according to claim 1 or 2, wherein a smoothing capacitor (4) is connected between the output terminals of the step-down converter.

4. Step-down converter according to any one of claims 1 to 3, in which - a current measuring sensor (10) is connected to the switch (K), - an output of the current measuring sensor (10) is connected to an input of a low-pass filter (11), - an output of the low-pass filter (11) is connected to a first input of a subtractor (12), a second input of which receives a setpoint signal (Csg), - an output of the subtractor (12) is connected to an input of an in-

5. comparator (13) one output of which is connected to a first input of a comparator (14), - the output of the current measurement sensor (10) is further connected to a first input of an adder (15) of which a second input receives a ramp signal whose slope is proportional to the difference between the voltage between the terminals of the voltage source and the voltage between the output terminals of the step-down converter, - an output of the adder (15) is connected to a second input of the comparator (14), and - an output of the comparator (14) is connected to a control input of the switch. A step-down converter according to any one of claims 1 to 3, wherein - a voltage measuring device (20) is connected to the output terminals of the step-down converter, - an output of the voltage measuring device (20) is connected to a first input of a subtractor (21), a second input of which receives a setpoint signal (Csg'), - an output of the subtractor (21) is connected to an input of an integrator (22) one output of which is connected to a first input of a comparator (23), - a current measuring sensor (24) is connected to the switch (K), - an output of the current measurement sensor (24) is connected to a first input of an adder (25) of which a second input receives a ramp signal whose slope is proportional to the difference between the voltage between the terminals of the voltage source and the voltage between the output terminals of the step-down converter, - an output of the adder (25) is connected to a second input of the comparator (23), and - an output of the comparator (23) is connected to a control input of the switch (K).

6. Equipment comprising a step-down converter according to any one of claims 1 to 5.

7. A method of operating a step-down converter according to any one of claims 1 to 5, comprising for an operating cycle: - a first phase in which the switch (K) is closed and the diode (5) is blocked, the step-down converter operating in continuous mode or in discontinuous mode, - a second phase in which the switch (K) is open and the diode (5) is conducting, the step-down converter operating in continuous mode or in discontinuous mode, - a third phase in which the switch (K) is open and the diode (5) is blocked, the third phase only existing for discontinuous mode.

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