Method for regulating the switching speed of a power transistor in an electrical system

Abstract

The invention relates to a method for regulating the switching speed of a transistor in an electrical system, the electrical system comprising the transistor and a control circuit configured to supply the transistor with two distinct voltage levels, namely a first voltage level for switching the transistor on and a second voltage level for switching the transistor off, characterized in that it comprises a step of modifying a supply voltage to the control circuit, including the modification of the first voltage level and / or the second voltage level, as a function of at least one operational parameter specific to the electrical system or its surrounding environment. (See Figure 3 for the abstract.)

Description

Title of the invention: Method for regulating the switching speed of a power transistor in an electrical system. TECHNICAL FIELD OF THE INVENTION

[0001] The invention falls within the field of optimizing the operation of electrical systems, such as inverters, in particular to adapt to variable operating conditions.

[0002] The invention aims in particular to refine the control of power transistors within such an electrical system, thus allowing the switching speed to be modulated according to variations in operating conditions, such as transistor temperature, switched current or voltage.

[0003] Although the invention finds particular application in the electric traction systems of electric or hybrid motor vehicles, its application extends to all fields using inverters, as well as to other electrical systems such as on-board chargers (also called OBCs for "On Board Charger" in Anglo-Saxon terminology), DC / DC converters and, in general, to various power converters. STATE OF THE ART

[0004] An inverter is an electrical system that allows the generation of alternating current from a direct current from an electrical source such as a battery.

[0005] It includes a power stage comprising, for example, power modules, and more generally the power electronics of the inverter.

[0006] The power stage includes a set of electronic switches, which are typically power transistors.

[0007] Several power transistor technologies can be used in an inverter, including: - Insulated gate bipolar transistors, also known as "IGBTs", from the Anglo-Saxon terminology "Insulated Gate Bipolar Transistor", and - Insulated gate field-effect transistors also called "MOSFET", an Anglo-Saxon acronym for "Metal Oxide Semiconductor Field Effect Transistor", which can be translated as "metal-oxide-semiconductor field-effect transistor".

[0008] Each power transistor is driven by a dedicated control circuit, often called a "driver" in Anglo-Saxon terminology, which is configured to provide adequate voltage levels to turn the power transistor on or off.

[0009] The control circuit serves as an interface between a microcontroller or logic circuit operating at low voltage, and the power transistor which generally requires a higher voltage level to operate correctly.

[0010] It is sometimes necessary to be able to adjust the switching speed of a power transistor.

[0011] For example, a decrease in switching speed may be sought to mitigate overvoltages resulting from current variations in parasitic inductances or when the temperature in the electrical system is relatively high, while an increase in switching speed may be sought to minimize switching losses.

[0012] To vary the switching speed of a power transistor, it is known to integrate several sets of resistors on a control circuit board, along with a switching system for the control circuit output to the resistor set corresponding to the desired switching speed. However, the control circuit is rather bulky and expensive. Furthermore, once defined, the choice of resistors is fixed and does not allow for dynamic adjustments based on changes in the inverter's operating conditions.

[0013] Another approach consists of developing a custom control circuit, also called an "ASIC" (Application-Specific Integrated Circuit) in Anglo-Saxon terminology, integrating the function of regulating the switching speed of the power transistor by controlling the gate current. Despite the high efficiency of such a circuit, this approach requires significant investments in terms of development time and costs.

[0014] Finally, there are commercially available control circuits that incorporate a switching speed adjustment function. However, these control circuits are much more expensive than standard circuits, thus increasing the overall cost of the electrical system. The choice of these circuits is also limited, restricting the designer's flexibility in selecting a suitable control circuit in relation to the other electrical system specifications. Description of the invention

[0015] The present invention aims to remedy all or part of the drawbacks of the prior art mentioned above. In particular, the invention aims to provide a method for regulating the switching speed of a power transistor in an electrical system in a simple, convenient, and economical manner.

[0016] The invention thus relates, according to a first aspect, to a method for regulating the switching speed of a transistor in an electrical system, the electrical system comprising the transistor and a control circuit configured to provide the transistor two distinct voltage levels, namely a first voltage level for the activation of the transistor and a second voltage level for the deactivation of the transistor, characterized in that it includes a step of modifying a supply voltage of the control circuit comprising the modification of the first voltage level and / or the second voltage level, as a function of at least one operational parameter specific to the electrical system or its surrounding environment.

[0017] In the method according to the invention, action is taken directly on the supply voltage of the control circuit, rather than on the control circuit itself, to modify either the first voltage level, or the second voltage level, or both, and thus regulate the switching speed of the transistor.

[0018] Thanks to the method according to the invention, a wide choice of control circuits can be considered for regulating the switching speed, including control circuits that do not incorporate a switching speed adaptation function. Indeed, a multitude of control circuits can operate within a certain voltage range to provide the transistor with the first and second voltage levels, thus offering great flexibility for the fabrication of an electrical system. It is then possible to use relatively basic control circuits, in order to minimize the manufacturing costs of the electrical system.

[0019] Furthermore, the method according to the invention makes it possible to simultaneously regulate the switching speed of several transistors, simply by modifying a common supply voltage to the control circuits associated with the transistors, which is very simple to implement and does not require any additional components.

[0020] Preferred, simple, convenient and economical features of the method according to the invention are presented below.

[0021] The power transistor can be selected from one of the following transistors: insulated gate bipolar transistor, silicon insulated gate field-effect transistor, silicon carbide field-effect transistor and gallium nitride transistor.

[0022] At least one operational parameter includes a representative data point of the transistor temperature, switched current or switched voltage.

[0023] At least one representative data point of the transistor temperature may include a temperature measured directly in an area of ​​the electrical system where the transistor is located, in particular by means of a temperature sensor.

[0024] At least one representative data point for the transistor temperature may include the switched current and / or the switched voltage and / or the total thermal resistance between the transistor junction and ambient air or a heat transfer fluid, such as water, in the case where the transistor is cooled by such a fluid. The transistor temperature may be estimated from representative data. of the transistor's operation, such as the switched current and / or the switched voltage.

[0025] By switched current, the present description refers to the current that the transistor allows to pass or stop depending on its state.

[0026] By switched voltage, the present description refers to the voltage which is applied across the terminals of the transistor and which is controlled by the latter during switching.

[0027] The control circuit being configured to be powered by a first supply voltage so as to provide the power transistor with the first voltage level and by a second supply voltage so as to provide the transistor with the second voltage level, the method may include a step of modifying the first supply voltage and / or a step of modifying the second supply voltage comprising modifying respectively the first voltage level and / or the second voltage level.

[0028] The first supply voltage can be between 8V and 20V.

[0029] The second supply voltage is between 0V and -20V.

[0030] The invention also relates, according to a second aspect, to an electrical system comprising a power transistor and a control circuit configured to supply the power transistor with two distinct voltage levels, namely a first voltage level for switching on the power transistor and a second voltage level for switching off the power transistor, characterized in that it comprises a DC / DC converter configured to modify a supply voltage of the control circuit to modify the first voltage level and / or the second voltage level, as a function of at least one operational parameter specific to the electrical system or its surrounding environment.

[0031] By way of example, the DC / DC converter can be a flyback or push-pull type converter. Other types of converters can of course be considered.

[0032] The DC / DC converter can operate in open loop.

[0033] Alternatively, the DC / DC converter can operate in a closed loop.

[0034] The electrical system may include a microcontroller configured to receive the operational parameter and to send a supply voltage setpoint from the control circuit to the DC / DC converter, said supply voltage setpoint being determined as a function of said operational parameter.

[0035] The electrical system may include at least one sensor configured to measure a representative data point of the transistor temperature, the switched current or the switched voltage, and at least one operational parameter including a data point representative of the transistor temperature, the switched current or the switched voltage.

[0036] The control circuit may include a first power supply terminal configured to receive a first supply voltage so as to provide the power transistor with the first voltage level and a second power supply terminal configured to receive a second supply voltage so as to provide the power transistor with the second voltage level.

[0037] The electrical system can be an electrical inverter.

[0038] The invention also relates, according to a third aspect, to using the regulation method according to the first aspect of the invention to optimize the performance of an inverter in an electric vehicle or a power conversion device. BRIEF DESCRIPTION OF THE FIGURES

[0039] The invention, according to an exemplary embodiment, will be well understood and its advantages will become more apparent upon reading the following detailed description, given by way of example and not limiting in any way, with reference to the attached drawings.

[0040] Fig. 1 is a block diagram showing steps of a regulation process according to the invention.

[0041] Fig. 2 schematically and partially represents an electrical system configured to implement at least one step of the regulation process of Fig. 1.

[0042] Fig. 3 is a graph representing the voltage at terminal Vc of the control circuit as a function of a command received by the control circuit.

[0043] Fig. 4 is a graph illustrating different switching phases during the activation of the transistor in Fig. 2.

[0044] Fig. 5 is a graph illustrating different switching phases during the deactivation of the transistor in Fig. 2. DETAILED DESCRIPTION OF THE INVENTION

[0045] Fig. 1 represents a method for regulating the switching speed of a power transistor in an electrical system.

[0046] The electrical system includes the power transistor and a control circuit, or driver, configured to supply the power transistor with two distinct voltage levels, namely a first voltage level for the activation of the power transistor, i.e. its transition from a non-conducting state to a conducting state, and a second voltage level for the deactivation of the power transistor, i.e. its transition from the conducting state to the non-conducting state.

[0047] The electrical system is for example an inverter, an on-board charger or OBC, a DC / DC voltage converter and is generally applicable to power converters.

[0048] The regulation method 100 includes a step of modifying a supply voltage of the control circuit comprising modifying at least one of the first voltage level and the second voltage level, as a function of at least one operational parameter specific to the electrical system or its surrounding environment.

[0049] At least one operational parameter includes, for example, a representative data point of the temperature, the switched current or the switched voltage.

[0050] For example, the supply voltage of the control circuit can be decreased or increased when the representative temperature data indicates that the transistor temperature is too high or too low.

[0051] The modification step 101 here includes a substep of modification of a first supply voltage 1011 and a substep of modification of the second supply voltage 1012 comprising the modification respectively of the first voltage level and the second voltage level.

[0052] Modifying the first voltage level and the second voltage level results in a variation in the switching speed of the power transistor, respectively during its activation and deactivation.

[0053] It should be noted that the first supply voltage can be modified independently of the second supply voltage, and vice versa.

[0054] The step of modifying the first supply voltage and the second supply voltage therefore makes it possible to regulate the switching speed of the power transistor, either when it is activated, or when it is deactivated, or both.

[0055] The first supply voltage can vary within a range from 8V to 20V. Of course, this range may vary depending on the supply voltages allowed by the control circuit and the voltages required by the power transistor for its activation.

[0056] The second supply voltage can, for example, vary within a range from 0V to -20V, for example from 0V to 15V. Of course, this range can vary depending on the supply voltages permissible by the control circuit and the voltages required by the power transistor for its deactivation.

[0057] These values ​​are understood here with respect to a reference terminal of a control circuit of the electrical system, described with reference to [Fig.2].

[0058] The [Fig.2] partially represents an electrical system 1 configured to implement the regulation process of the [Fig.1].

[0059] The electrical system 1 comprises a power transistor 2, a gate resistor 3 and a control circuit 4 or driver, configured to supply the transistor with power 2 two distinct voltage levels, namely a first voltage level for the activation of power transistor 2 and a second voltage level for the deactivation of power transistor 2.

[0060] The electrical system 1 also includes a DC / DC converter and a microcontroller which are not shown here.

[0061] The microcontroller is configured to receive the operational parameter and to send a supply voltage setpoint from the control circuit to the DC / DC converter, the supply voltage setpoint being determined according to the operational parameter.

[0062] The electrical system 1 includes a sensor configured to measure the operational parameter which is not shown here.

[0063] The power transistor 2 is here an insulated-gate bipolar transistor, or IGBT. Other types of power transistors, such as gallium nitride (GaN) transistors or insulated-gate field-effect transistors or MOSFETs, and in particular Silicon (Si) or Silicon Carbide (SiC) power transistors, can of course be used.

[0064] The power transistor 2 comprises a collector 5, an emitter 6, a gate 7 and a freewheeling diode 8 or anti-parallel diode, connected between the collector 5 and the emitter 6.

[0065] The control circuit 4 here includes an input terminal 9 configured to receive a logic level control signal Cde, a first supply terminal 10 configured to receive a first supply Ud+, a second supply terminal 11 configured to receive a second supply voltage Ud-, a reference terminal 12 serving as a reference point for the first and second supply voltages Ud+ and Ud-, and an output terminal 13 configured to output a gate control signal Vc.

[0066] Alternatively, the control circuit may include a single power supply terminal.

[0067] In an embodiment not shown, the power supply terminal 11 is electrically connected to the reference terminal 12.

[0068] The reference terminal 12 is electrically connected to the emitter 6 of the power transistor 2.

[0069] The gate resistor 3 comprises two terminals, one of which is electrically connected to the output terminal 13 of the control circuit 4 and the other is electrically connected to the gate 7 of the power transistor 2.

[0070] Figure 3 is a graph showing the relationship between the gate control signal Vc and the logic level control signal Cde. The horizontal axis represents time and the vertical axis represents voltage.

[0071] The logic level control signal Cde alternates between two states, namely a high state and a low state.

[0072] The logic level control signal Cde is for example generated by a microcontroller, a logic circuit, or any other type of electrical system control circuit, not shown here.

[0073] When the logic level signal Cde is in the high state, the gate control signal Vc has a voltage equal to the first supply voltage Ud +, corresponding to a first voltage level for the activation of the power transistor.

[0074] When the logic level signal Cde is in the low state, the gate control signal Vd has a voltage equal to the second supply voltage Ud-, corresponding to a second voltage level for the deactivation of the power transistor.

[0075] The logic level control signal Cde therefore determines here in a binary manner the voltage level of the gate control signal Vc generated by the control circuit, and thus the activation and deactivation of the power transistor.

[0076] Figures 4 and 5 illustrate in more detail the operation of the power transistor of [Fig.2] respectively when the logic level control signal Cde goes from the low state to the high state and when the logic level control signal Cde goes from the high state to the low state.

[0077] It should be noted that the operation described below also applies to other types of power transistors, such as insulated-gate field-effect transistors or MOSFETs.

[0078] When the logic level control signal Cde is low, the gate-emitter voltage Vge, which corresponds to the second supply voltage Ud-, is below the threshold required to activate the power transistor. The power transistor is then in its non-conducting state.

[0079] In this state, the collector-emitter voltage Vce is maintained at a high level, indicating that the power transistor is preventing current flow. The collector current is, therefore, zero.

[0080] When the logic level control signal Cde goes from the low state to the high state, four successive phases are observed, numbered from 1 to 4 at the bottom of the graph in [Fig.5].

[0081] In a first phase, numbered 1, the voltage between the gate and the emitter Vge gradually increases until it reaches a threshold voltage Vth, and the input capacitance Ciss or gate capacitance of the power transistor charges.

[0082] The duration of this first phase can be determined by the expression:

[0083] H = C / ^gJn / ' Ud •

[0084] where Rg is the grid resistance and Ud is the amplitude between the first supply voltage Ud+ and the second supply voltage Ud-.

[0085] The second phase, numbered 2, begins when the gate-emitter voltage Vge reaches the threshold voltage Vth. In the second phase, the power transistor begins to conduct, with the collector current increasing until it reaches a high level while the collector-emitter voltage Vce remains constant. This phase thus corresponds to the current switching of the power transistor. The gate-emitter voltage Vge gradually increases until it reaches a pinch-off voltage Vp.

[0086] The duration of this second phase can be determined by the expression: / 2 = Cissjigln ( )

[0087] It can be observed here that the duration of the current switching phase of the power transistor depends on the amplitude Ud between the first supply voltage Ud+ and the second supply voltage Ud-. Consequently, it is possible to vary the switching speed by modifying one of the first and second supply voltages, or by modifying them simultaneously, so as to modify the amplitude Ud.

[0088] The third phase, numbered 3, begins when the collector current reaches its maximum value, and therefore when the gate-emitter voltage Vge reaches the pinch-off voltage Vp. In this third phase, the collector-emitter voltage Vce decreases until it stabilizes at a low level corresponding to the transistor's conducting state. This phase thus corresponds to the voltage switching of the transistor.

[0089] The duration of this third phase can be determined by the expression: [00'01 tî=Cgd£g.^p

[0091] where Cgd is the capacitance between the gate and the emitter, and Udc is the switched voltage.

[0092] It is observed here that the duration of the voltage switching phase of the power transistor depends on the switched voltage and the first supply voltage Ud+. Consequently, a change in the first supply voltage Ud+ leads to a variation in the switching speed of the power transistor during the activation phase.

[0093] The fourth phase, numbered 4, begins when the collector-emitter voltage Vce reaches its low level. In this fourth phase, the input capacitance Ciss finishes charging, with the gate-emitter voltage Vge characterized by an exponentially increasing curve starting from the pinch-off voltage Vp and tending towards the first supply voltage Ud+.

[0094] When the logic level control signal Cde changes from the high state to the low state, it is possible to distinguish four successive phases, numbered from 5 to 8 at the bottom of the graph in [Fig.5].

[0095] In a first phase, numbered 5, the gate-emitter voltage Vge gradually decreases until it reaches the pinch-off voltage Vp, while the collector-emitter voltage Vce and the collector current remain constant. The transistor is then in its conducting state while the transistor's input capacitance Ciss discharges.

[0096] The duration of this first phase can be determined by the expression:

[0097] ,5 = a.rfg.ln(^)

[0098] The second phase, numbered 6, begins when the gate-emitter voltage Vge reaches the pinch-off voltage Vp. In the second phase, the transistor is switching to its non-conducting state, with the collector-emitter voltage Vce increasing while the gate-emitter voltage Vge and the collector current remain constant. This phase thus corresponds to the voltage switching of the transistor.

[0099] The duration of this second phase can be determined by the expression 76=

[0100] It is observed here that the duration of the voltage switching phase of the power transistor depends on the switched voltage and the second supply voltage Ud-. Consequently, the modification of the second supply voltage Ud- leads to a variation in the switching speed of the power transistor, in the deactivation phase.

[0101] The third phase, numbered 7, begins when the collector-emitter voltage Vce has reached its high level. In this third phase, the power transistor begins to stop conducting, with the collector current gradually decreasing while the collector-emitter voltage Vce exhibits a peak.

[0102] This peak is attributable to the opposition to the rapid variation of the collector current induced by the inductive elements of the circuit. This phenomenon is typically described by the expression:

[0103] L.di / dt

[0104] where L represents the inductance and di / dt the rate of change of the current.

[0105] This phase thus corresponds to the current switching of the power transistor. The voltage between the gate and the emitter Vge also decreases until the threshold voltage Vth.

[0106] The duration of this third phase can be determined by the expression: 101071

[0108] It is observed here that the duration of the current switching phase of the power transistor depends on the second supply voltage Ud-. Consequently, a change in the second supply voltage Ud- leads to a variation in the switching speed of the power transistor during the switch-off phase.

[0109] The fourth phase, numbered 8, begins when the gate-emitter voltage Vge reaches the threshold voltage Vth. In this fourth phase, the collector current decreases further to a very low value or even zero, and the collector-emitter voltage Vce stabilizes at a high level, corresponding to the non-conducting state of the power transistor. The input capacitance of the power transistor finishes discharging, with the gate-emitter voltage Vge characterized by a decreasing exponential curve starting from the threshold voltage Vth and tending towards the second supply voltage Ud-.

[0110] It is thus understood that the variation of the first supply voltage Ud+ and the second supply voltage Ud- influences the rate at which the input capacitance Ciss of the power transistor charges and discharges, and therefore the switching rate of the power transistor.

[0111] Indeed, the gate of a power transistor can be conceptually represented by a simplified RC circuit model due to the intrinsic presence of gate resistance and input capacitance in the gate structure.

[0112] In an RC circuit, which is a circuit containing a resistor and a capacitor, the rate of change of the voltage across the capacitor during charging or discharging is mainly determined by the time constant of the circuit, which is the product of the resistance and the capacitance of the capacitor.

[0113] However, the rate of change of the voltage across the capacitor also depends on the voltage applied to this RC circuit.

[0114] In the method according to the invention, the control circuit supply voltage is acted upon directly, rather than upon the control circuit itself, to regulate the switching speed of the transistor.

[0115] In this way, a wide choice of control circuits can be considered for regulating the switching speed, including control circuits that do not incorporate a switching speed adaptation function. Indeed, a multitude of control circuits can operate within a certain voltage range to provide the transistor with the first and second voltage levels, thus offering considerable freedom in the fabrication of an electrical system. It is then possible to use relatively basic control circuits in order to minimize the manufacturing costs of the electrical system.

[0116] Furthermore, the method according to the invention makes it possible to regulate simultaneously the switching speed of several transistors simply by modifying the supply voltage, provided that the latter is common to several control circuits, which is very simple to implement and does not require any additional components.

Claims

Demands

1. A method for regulating (100) the switching speed of a transistor in an electrical system, the electrical system comprising the transistor and a control circuit configured to supply the transistor with two distinct voltage levels, namely a first voltage level for switching the transistor on and a second voltage level for switching the transistor off, characterized in that it comprises a step of modifying a supply voltage of the control circuit comprising modifying the first voltage level and / or the second voltage level, as a function of at least one operational parameter specific to the electrical system or its surrounding environment.

2. A control method according to claim 1, characterized in that the power transistor is selected from one of the following transistors: insulated gate bipolar transistor, silicon insulated gate field-effect transistor, silicon carbide field-effect transistor and gallium nitride transistor.

3. A control method according to claim 1 or 2, characterized in that at least one operational parameter includes a data point representative of the transistor temperature, the switched current or the switched voltage.

4. A control method according to any one of claims 1 to 3, characterized in that the control circuit is configured to be powered by a first supply voltage so as to provide the power transistor with the first voltage level and by a second supply voltage so as to provide the transistor with the second voltage level, the method includes a step of modifying the first supply voltage (1011) and / or a step of modifying the second supply voltage (1012) comprising modifying respectively the first voltage level and / or the second voltage level.

5. Regulation method according to claim 4, characterized in that the first supply voltage is between 8V and 20V.

6. Regulation method according to claim 4 or 5, characterized in that the second supply voltage is between 0V and -20V.

7. Electrical system comprising a power transistor (2) and a control circuit (4) configured to supply the power transistor (2) with two distinct voltage levels, namely a first voltage level for turning on the power transistor (2) and a second voltage level for turning off the power transistor (2), characterized in that it comprises a DC / DC converter configured to modify a supply voltage of the control circuit (4) to modify the first voltage level and / or the second voltage level, as a function of at least one operational parameter specific to the electrical system (1) or its surrounding environment.

8. Electrical system according to claim 7, characterized in that it comprises a microcontroller configured to receive at least one operational parameter and to send a supply voltage setpoint from the control circuit to the DC / DC converter, said supply voltage setpoint being determined as a function of said at least one operational parameter.

9. Electrical system according to claim 7 or 8, characterized in that it comprises at least one sensor configured to measure a data representative of the transistor temperature, the switched current or the switched voltage, and at least one operational parameter comprising a data representative of the transistor temperature, the switched current or the switched voltage.

10. Electrical system according to any one of claims 7 to 9, characterized in that the control circuit (4) comprises a first supply terminal (10) configured to receive a first supply voltage so as to provide the power transistor (2) with the first voltage level and a second supply terminal (11) configured to receive a second supply voltage so as to provide the power transistor (2) with the second voltage level.

11. Electrical system according to any one of claims 7 to 10, characterized in that the electrical system (1) is an electrical inverter.

12. Use of the control method (100) according to any one of claims 1 to 6 to optimize the performance of an inverter in an electric vehicle or power conversion device.

Images