FR3102021A1 - Synchronous rectification voltage converter - Google Patents

Abstract

Synchronous rectification voltage converter comprising a primary circuit (31) and a secondary circuit (32), the primary circuit (31) comprising a primary winding (33), the secondary circuit (32) comprising a secondary winding (35) facing to the primary winding (33) to form a transformer, the secondary circuit (32) comprising a rectifier transistor (40) connected to a terminal (36) of the secondary winding (35) and of which a gate driver (VGATE ) defines the on or off state of the rectifier transistor (40), the secondary circuit (32) comprising a current sensor (50) connected in series to a terminal (37) of the secondary winding (35), the sensor of current (50) being configured to deliver a measurement of the current (Vinfo) flowing in the secondary winding (35), the gate control (VGATE) of the rectifying transistor (40) being a function of the current measurement. Figure for the abstract: figure 3

Description

Synchronous rectification voltage converter

The present invention relates to the field of on-board electrical equipment and more specifically to a switched-mode power supply system, in particular of the flyback converter type, equipped with synchronous rectification.

A switching power supply is a power supply whose regulation is ensured by electronic power components (windings, diode, transistors, and passive elements) used in switching, generally in order to modify an input voltage into an output voltage of different and stabilized amplitude. We therefore commonly speak of a voltage converter, or more simply of a converter. Cutting the power supply requires rectifying the output in order to allow a direct current supply.

A flyback converter comprises two circuits: a first so-called primary circuit, and a second so-called secondary circuit. Each circuit comprises an inductance, and the two inductors placed close to each other form coupled inductors playing the role of a transformer.

A flyback converter operates in two phases: a first energy storage phase in the coupled inductors, via a current flowing in the primary circuit, the current of the secondary circuit inductance being blocked; a second phase of energy restitution by a current flowing in the secondary circuit, the primary circuit being blocked.

The most classic solution for blocking the secondary circuit (and therefore rectifying the voltage) is to use a diode. This is the example of Figure 1. In this simplified example, the primary circuit 1, with an input voltage V _IN , is provided with a first winding 3 forming one of the coupled inductors and a switch 4 ( typically a transistor) controlled by a control signal. The secondary circuit 2, with an output voltage V _OUT , is provided with a second winding 5 forming the other of the coupled inductances, an output capacitor 6 in parallel with the output, and a diode 7 placed between the second winding 5 and the output capacitor 6. The diode 7 imposes the direction of the current in the second winding 5. Thus, during the storage phase, the voltage across the terminals of the second winding 5 is negative, thus blocking the diode 7 It is the output capacitor 6 which supplies the energy demanded by the load connected to the output.

Although this solution has the advantage of simplicity, it also has several disadvantages. In particular, the use of a diode leads to a significant drop in efficiency, in particular due to the high current passing through it. This poor efficiency is particularly detrimental for the supply of electronic devices on board a vehicle, due to the limitation of the quantity of energy and the problems of heat dissipation. This drop in efficiency is all the greater for low voltages, which are precisely the most common voltages for supplying on-board electrical systems. By way of illustration, losses of approximately 6% for an output voltage V _OUT of 12 V become losses of approximately 12% for an output voltage V _OUT of 5 V.

Other solutions have therefore been developed, with so-called secondary synchronous rectification with a MOSFET transistor (for English "metal-oxide-semiconductor field-effect transistor" meaning insulated-gate field-effect transistor). Figure 2 gives an example of such a voltage converter. There is a primary circuit 11 and a secondary circuit 12. As before, the primary circuit 11, with an input voltage V _IN , is provided with a first winding 13 forming one of the coupled inductors and a switch 14 ( here a transistor) controlled by a control signal. This control signal is sent by a controller 10 connected to the two terminals of the first winding 13. The primary circuit 11 can comprise other elements, such as for example a capacitor 18 acting as a low-pass filter.

Instead of a diode, the secondary circuit comprises an N-type MOSFET rectifier transistor 20 connected to one terminal of the secondary winding 15, and whose gate is controlled by a control voltage V _GATE supplied by a secondary controller 22 from the drain voltage V _DRAIN to the drain of the rectifying transistor 20 which is connected to the secondary winding 15. It is the drain voltage V _DRAIN which constitutes the voltage information relating to the waveform which allows recovery. By the control voltage V _GATE , the rectifying transistor 20 is turned on or off, allowing the passage from a storage phase to a restitution phase, and vice versa. Secondary circuit 12 may include other elements, such as capacitors 24 acting as low-pass filters.

The use of a rectifying transistor 15 controlled by the secondary controller 16 makes it possible to significantly improve the yield. However, due to the use of the drain voltage V _DRAIN the secondary controller 16 then has a complex and sensitive architecture, which makes the converter susceptible to major malfunctions in the presence of resonances linked to the imperfections of the components of the converter (inductors leakage, stray capacitances, etc.), which result in particular in disturbances of the drain voltage V _DRAIN . Short-circuits are also likely to occur if the voltage changes too quickly. However, an on-board system must have very high reliability, and can be subjected to very significant constraints, for example in terms of sudden voltage variations.

Presentation of the invention

The invention aims to allow voltage conversion via a switched-mode power supply whose output is correctly rectified, with high efficiency, low heat dissipation, and more robust than the voltage converters of the state of the art.

To this end, the invention proposes a voltage converter with synchronous rectification comprising a primary circuit and a secondary circuit, the primary circuit comprising a primary winding, the secondary circuit comprising a secondary winding facing the primary winding to form a transformer , the secondary circuit comprising a rectifying transistor connected to a terminal of the secondary winding and of which a gate control defines the on or off state of the rectifying transistor, in which the secondary circuit comprises a current sensor connected in series to a terminal of the secondary winding, the current sensor being configured to deliver a measurement of the current flowing in the secondary winding, the gate control of the rectifying transistor being a function of the current measurement.

The synchronous rectification voltage converter according to the invention makes it possible, by taking into account a measurement of the current flowing in the secondary winding to determine the gate control of the rectifying transistor, rather than a measurement of voltage at the terminals of the secondary winding, makes it possible to obtain a more reliable power supply, robust to possible disturbances of the electrical circuits, with in addition a reduced complexity.

The invention is advantageously supplemented by the following different characteristics taken alone or according to their different possible combinations:
- the current sensor comprises a first winding and a second winding which form two coupled inductors, the first winding being connected in series to the second terminal of the secondary winding and to the output of the converter, the second winding of the current sensor being connected to a measurement adaptation circuit configured to deliver the current measurement;
- the second winding of the current sensor comprises at least fifty times more turns than the first winding of the current sensor;
- the measurement adaptation circuit comprises a voltage limiter comprising at least one Zener diode in a direct direction between ground and an output node of the current sensor;
- voltage limiter comprises another diode in a direct direction between the output node of the current sensor and ground;
- the measurement adaptation circuit comprises two branches connected to ground, in parallel but separated by a diode, each branch comprising a resistor;
- the secondary circuit comprises a secondary controller, the output node of the current sensor being connected to the secondary controller, a gate control output of the secondary controller being connected to the gate of the rectifying transistor;
- the secondary controller is configured to control, by the gate control, the on state of the rectification transistor when the measurement of the current is greater than a threshold voltage, and the off state of the rectification transistor when the measurement of the current is below a threshold voltage;
- the current measurement provided by the current sensor to the secondary controller has a profile with a plateau followed by a decreasing part;
- the converter is configured so that the threshold voltage is reached by measuring the current at a point anticipating by at least 150 ns a zero crossing of the decreasing part of the profile of the current measurement.

Presentation of figures

Other characteristics, objects and advantages of the invention will emerge from the description which follows, which is purely illustrative and not limiting, and which must be read in conjunction with the appended drawings in which:

FIG. 1, already commented on, schematically shows a converter of the flyback type;

FIG. 2, already commented on, schematically shows a prior art flyback converter;

FIG. 3 schematically illustrates a voltage converter according to one possible embodiment of the invention;

FIG. 4 schematically illustrates a current sensor according to one possible embodiment of the invention;

Figure 5 schematically illustrates an example voltage profile of current measurement and gate drive.

detailed description

Referring to Figure 3, the voltage converter comprises a primary circuit 31 with an input voltage V _IN , and a secondary circuit 32, with an output voltage V _OUT . The voltage converter is a direct current - direct current, or DC-DC (from the English " Direct Current-Direct Current" ) voltage converter. The primary circuit 31 is provided with a first winding 33 forming one of the coupled inductors and with a transistor 34, controlled by a chopping signal sent by a primary controller 30, for example connected to the two terminals of the first winding 33. The primary circuit 31 may include other elements, such as for example a capacitor 38 acting as a low-pass filter. Thus, the primary circuit 1 of this example is similar to that presented with reference to FIG. 2. In fact, the invention does not require modifying the primary circuit 31 of the voltage converters of the prior art, and all variations of these can be used.

The secondary circuit 32, with an output voltage V _OUT , is provided with a second winding 35 forming the other of the coupled inductors. The first winding 33 and the second winding 35 are arranged close so as to generate a magnetic coupling between them. Secondary circuit 32 also includes a rectifying transistor 40 connected to one terminal of secondary winding 35, and a secondary controller 42 connected to the gate of rectifying transistor 40, and which actively controls it. It is therefore a converter with synchronous rectification. The rectifying transistor 40 is typically a MOSFET, in particular of the N type in the example presented. Secondary controller 42 is configured to control the gate of rectifying transistor 40 by means of a control voltage V _GATE . By the control voltage V _GATE , the rectifying transistor 40 is turned on or off, allowing the transition from a storage phase to a restitution phase, and vice versa. Secondary circuit 32 may include other elements, such as capacitors 44 acting as low pass filters to filter signals.

The secondary controller 42 determines the control voltage from a voltage representative of a measurement of the current flowing in the secondary winding 35. This voltage representative of a measurement of the current is supplied by a current sensor 50 disposed at a terminal of the secondary winding 35. The current sensor 50 is therefore electrically in series with the transformer of the converter. In the example illustrated, the current sensor is placed between the second terminal 37 of the secondary winding and the output terminal 64 of the voltage converter.

Referring to Figure 4, the current sensor 50 comprises a first winding 51 of an inductor L1 and a second winding 52 of an inductor L2 which form two coupled inductors. Being a current sensor, the ratio between L1 and L2 is chosen in order to minimize the impact of the current sensor on the output voltage V _OUT . Consequently, this ratio is preferably chosen to be greater than 50, and even more preferably greater than 80. In order not to reduce the current measurement too much, this ratio between L1 and L2 is preferably chosen to be less than 300. The second winding 52 of the current sensor thus typically has 50 to 300 times more turns than the first winding 51 of the current sensor. The first winding 51 is connected in series to the second terminal 37 of the secondary winding 35 and to the output of the voltage converter which delivers the output voltage V _OUT . The second winding 52 is connected to a measurement matching circuit 54.

The measurement adaptation circuit 54 comprises two branches 56, 58 connected to ground, in parallel but separated by a diode D3. The first branch 56 includes a resistor R3, and the second branch 58 includes a resistor R4. The first branch 56 is connected to the second winding 52 of the current sensor 50, directly or through an optional resistor R2. The second branch 58 is connected to the output node 62 of the current sensor 50, which delivers the current measurement V _info . This part of the adaptation circuit 54 makes it possible to translate into voltage the measurement of the current flowing in the second winding 32 and therefore in the current sensor 50. The diode D3 makes it possible to ensure that the voltage across the terminals of the resistor R4 remains positive, even in the event of disturbances. Resistor R3 has a resistance value at least 100 times greater than the value of resistor R4, and preferably at least greater than 1000 times the value of resistor R4, in order to allow the voltage across the terminals of the second winding 52 of the current sensor 50 to return to a zero value at the end of the cycle.

The measurement adaptation circuit 54 comprises a voltage clipper 60, configured to clip the current measurement, connected in parallel with the second branch 58 carrying the resistor R4. Voltage limiter 60 comprises at least one Zener diode D1 in a forward direction between ground and output node 62 to which second branch 58 and diode D3 are connected. When the measurement voltage reaches the reverse voltage of the Zener diode D1, the Zener diode D1 becomes conductive in its indirect direction in the direction of ground, thereby clipping the waveform of the measurement voltage. Being a discontinuous mode converter, the output wave whose current is measured by the current sensor 50 takes the form of a triangle presenting very spread values. However, the measurement of high voltage or current values at the start of conduction in the secondary circuit 32 is of no interest for the control of the rectification, and risks causing the saturation of the current sensor 50 or of the secondary controller 42. Clipping therefore allows a sizing of the components allows a finer analysis of the lower values occurring at the end of conduction. Since the voltage limiter 60 is in parallel with the second branch 58 carrying the resistor R4, the voltage across the terminals of the voltage limiter 60 directly depends on the value of this resistor R4. The choice of resistor R4 therefore makes it possible to determine the value of the clipping. Clipping can for example be at 5V.

However, the constraints imposed on the Zener diode D1 cause it to have a high parasitic capacitance, typically of the order of 1 nf. In order to reduce the parasitic capacitance of the voltage limiter, the voltage limiter 60 preferably comprises a second diode D4 in a direct direction between the output node 62 and ground, that is to say in the direction opposed to the Zener diode D1. This second diode D4 has a parasitic capacitance at least 100 times lower than the parasitic capacitance of the Zener diode D1, for example of the order of 2 pf.

The output 62 of the current sensor, after the measurement adaptation circuit 54, delivers a measurement voltage V _info representative of the measurement of the current flowing in the second winding 52 of the current sensor, which depends on the current flowing in the secondary winding 35. The output 62 of the current sensor 50 is connected to the secondary controller 42, which determines the control of the gate of the rectifying transistor 40 according to the current measurement V _info .

The function of the secondary controller 42 is first to amplify the measurement voltage so that its value is sufficient to drive the rectifying transistor 40. The secondary controller, in its simplest version, can thus be an amplifier. The secondary controller 42 can also implement, via the determination of the gate command V _GATE , additional functions making it possible to improve the robustness of the converter. In particular, the secondary controller 42 can anticipate the voltage drop thanks to the knowledge of the current flowing in the secondary winding 35, in order to ensure a safety margin for the gate control V _GATE .

FIG. 5 illustrates an example of determining the gate command V _GATE from the current measurement V _info . The upper curve 65 shows the actual waveform of the current, which appears as a triangular wave (substantially in the shape of a right triangle), according to an arbitrary unit. The median curve 70 of FIG. 5 which represents the profile of the measurement of the current V _info , expressed in voltage, which takes up the triangular waveform of the current truncated by the clipping carried out by the measurement adaptation circuit 54. We find there a vertical wave front 71 reflecting a sudden establishment of the current in the secondary winding 35 during the transition from the storage phase to the restitution phase, then a plateau 72 in which the voltage representative of the current measurement V _info stays at clipping level. The decreasing part 73 which follows the plateau 72 corresponds to the drop in current at the end of the restitution phase, without the clipping action of the measurement adaptation circuit 54. This decreasing part 73 continues up to a zero value. However, at the end of the restitution phase, there is a risk of short-circuiting if the rectifier transistor 40 is on while the voltage across the terminals of the secondary winding 35 is reversed. It is therefore preferable for the gate command V _GATE to command the blocking state of the rectification transistor 40 ahead of the current measurement V _info .

To this end, the secondary controller 42 proceeds to a comparison between the current measurement V _info and a threshold voltage 74, the gate command V _GATE controlling the on state only when the current measurement V _info is greater than the threshold voltage 74. The value of the threshold voltage 74 essentially depends on time, and therefore on the slope of the decreasing part 73. Preferably, the threshold voltage value 74 is chosen in order to ensure an advance of the interruption of the command (compared to the zero crossing of the current measurement V _info ) corresponding to at least five times, and preferably ten times, the characteristic response time of the secondary controller 42, ie typically between 150 ns and 350 ns. Thus, the converter is configured so that the threshold voltage 74 is reached by measuring the current V _info at a point 75 anticipating by at least 150 ms a zero crossing of the decreasing part 73 of the current measurement profile.

If the threshold voltage 74 is too low, the safety margin is small, and the risk of short circuit cannot be ruled out. Furthermore, too low a value could assimilate a disturbance to a wavefront 71, resulting in a damaging erroneous conduction command. If threshold voltage 74 is too high, then performance may be degraded. The threshold voltage 74 can for example be between 25% and 75% of the clipping value, and preferably between 40% and 65% of this clipping value. For example, with a clipping value of 5 V, a threshold voltage 74 of 2.7 V presents a good compromise.

The lower curve 80 shows the gate control V _GATE as a function of time, which here takes the form of a step. Preferably, the gate command V _GATE presents voltage values at least 1.5 times greater than those of the current measurement V _info , and preferably at least 2 times greater, at least when the values of the current measurement V _info are higher than the threshold voltage 74. The lower curve 80 is presented synchronously with the middle curve 70, and any offset related to the responsiveness of the secondary controller 42 is not taken into account due to its low impact. The wavefront 81 of the lower curve 80 is synchronous with the wavefront 71 of the middle curve 70. The plateau 82 of the lower curve 80 corresponds to a control of the on state of the rectifying transistor 40. The plateau 82 extends beyond the inflection between the plateau 72 and the decreasing part 73 of the median curve 70, as long as the voltage measurement V _info takes values greater than the threshold voltage 74. As soon as this threshold voltage 74 is reached at point 75 of the middle curve 70, the gate command V _GATE is interrupted by the interruption 83 of the lower curve. The gate command V _GATE then commands the blocking state of the rectifying transistor 40. This results in a safety margin between the interruption of the on state and the passage to zero of the voltage representative of the measurement of the current V _info .

The invention is not limited to the embodiment described and shown in the appended figures. Modifications remain possible, in particular from the point of view of the constitution of the various technical characteristics or by substitution of technical equivalents, without thereby departing from the scope of protection of the invention.

Claims (10)

Synchronous rectification voltage converter comprising a primary circuit (31) and a secondary circuit (32), the primary circuit (31) comprising a primary winding (33), the secondary circuit (32) comprising a secondary winding (35) facing to the primary winding (33) to form a transformer, the secondary circuit (32) comprising a rectifying transistor (40) connected to a terminal (36) of the secondary winding (35) and of which a gate control (V _GATE ) defines the on or off state of the rectifying transistor (40),
characterized in that the secondary circuit (32) comprises a current sensor (50) connected in series to a terminal (37) of the secondary winding (35), the current sensor (50) being configured to deliver a measurement of the current (V _info ) circulating in the secondary winding (35), the gate control (V _GATE ) of the rectifying transistor (40) being a function of the current measurement. Voltage converter according to claim 1, in which the current sensor (50) comprises a first winding (51) and a second winding (52) which form two coupled inductors, the first winding (51) being connected in series with the second terminal (37) of the secondary winding (37) and to the output (64) of the converter, the second winding (52) of the current sensor (50) being connected to a measurement matching circuit (54) configured to deliver the current measurement. A voltage converter according to claim 2, wherein the second winding (52) of the current sensor (50) comprises at least fifty times as many turns as the first winding (51) of the current sensor (50). Voltage converter according to one of Claims 2 or 3, in which the measurement matching circuit (54) comprises a voltage limiter (60) comprising at least one Zener diode (D1) in a forward direction between ground and an output node (62) of the current sensor (50). Converter according to the preceding claim, in which the voltage limiter (60) comprises another diode (D4) in a forward direction between the output node (62) of the current sensor (50) and ground. Voltage converter according to one of Claims 4 to 5, in which the measurement adaptation circuit (54) comprises two branches (56, 58) connected to ground, in parallel but separated by a diode (D3), each branch (56, 58) comprising a resistor (R3, R4). Converter according to any one of the preceding claims, in which the secondary circuit (32) comprises a secondary controller (42), the output node (62) of the current sensor (50) being connected to the secondary controller (42), a gate control output (V _GATE ) of the secondary controller (42) being connected to the gate of the rectifying transistor (40). Converter according to the preceding claim, in which the secondary controller (42) is configured to control, by the gate control (V _GATE ), the on state of the rectifying transistor (40) when the current measurement is greater than a voltage threshold (74), and the blocking state of the rectifying transistor (40) when the current measurement is lower than a threshold voltage (74). Converter according to any one of the preceding claims, in which the current measurement (V _info ) supplied by the current sensor (50) to the secondary controller (42) has a profile with a plateau (72) followed by a decreasing part ( 73). Converter according to one of Claims 8 or 9, configured so that the threshold voltage (74) is reached by measuring the current (V _info ) at a point (75) anticipating by at least 150 ns a zero crossing of the decreasing part (73) of the current measurement profile.