Power electronic device with improved thermal time constant

Abstract

The present invention relates to a power electronic device (1) comprising: - power electronic components (12, 14); - a cooling plate (8); - a circuit (2) comprising the power electronic components (12, 14) and electrically connecting them, the circuit (2) being rigidly attached to the cooling plate (8); - input (3) and output (4) conductors of the circuit (2), the power electronic device (1) being characterised in that it comprises a phase-change material (6) in which the circuit (2) is embedded.

Description

Description Title of the invention: Power electronic device with improved thermal time constant

[0001] The present invention relates to the fields of electrotechnics and thermodynamics, and more specifically concerns a power electronics device such as an inverter, in particular for a motor vehicle.

[0002] Electric or hybrid electric vehicles include power electronics devices linked to the operation of their powertrains, their electric chargers or their on-board networks. Indeed, such a vehicle includes a so-called high-voltage traction battery, in the order of 200 to 800V (Volt), to power an electric motor providing sufficient torque to move the vehicle, the electric motor generally being an alternating current motor. The vehicle must therefore integrate a power inverter converting the direct voltage supplied by the traction battery into alternating voltage.The vehicle also generally incorporates a direct current - direct current power converter to supply an on-board network of the vehicle from the traction battery, in addition to or instead of a much lower voltage service battery, and also generally incorporates a power charger to recharge the traction battery from an external charging station.

[0003] These power electronic devices include power electronic components, which are subjected to high currents of several hundred amperes during operation, causing a rise in temperature of these components, which can damage them if this temperature is not maintained below a certain threshold. For example, excessively high temperatures can melt power transistors, or damage stator or rotor inductances.

[0004] Therefore, such an electric or hybrid vehicle implements at least one mechanism for controlling the temperature of its power electronic devices. Such a mechanism limits, for example, the power supplied to the motor as soon as the temperature of the components of the power inverter or the electric motor exceeds a high temperature threshold, set for example at around one hundred degrees Celsius. Such a control mechanism therefore limits the operating power peaks of the vehicle, so that the temperature of the power electronic components does not exceed a critical temperature threshold higher than the high temperature threshold. The critical temperature threshold is, for example, the junction temperature of a silicon transistor, i.e. approximately 150°C (degrees Celsius). The power peaks are generally limited to a duration of around ten to twenty seconds. Such a peak power corresponds for example to a sudden restart of the vehicle.

[0005] Alternatively or additionally, power modules are generally oversized to provide the torque requested by the driver, without having to activate a temperature control mechanism.

[0006] To effectively stop the rise in temperature of the power electronic components, particularly during these power peaks, the electric or hybrid vehicle includes a cooling system for the power electronic components. This system may include a circulation of heat transfer fluid, passing through a cooling plate on which is fixed at least one power electronic device such as the inverter or the direct current - direct current converter. Alternatively or in addition, the cooling plate is cooled by an air flow coming from outside the vehicle, and includes cooling fins facilitating heat exchange with the air flow.

[0007] In such a cooling system, the power electronic components of the power electronic device, such as transistors, are fixed to the cooling plate by means of one or more electrically insulating layers, but whose thermal conductivity is often not very efficient. As a result, the thermal resistance of the power electronic device is high. In addition, since the surface area of ​​the transistors is very small, the heat exchange surface between the transistors and the cooling plate is very small. Finally, the power conductive bars or inductances of the power electronic device are not cooled by the cooling plate. All this means that the thermal time constant of the power electronic device cooled by the cooling plate is approximately one second, which is ten to twenty times smaller than the duration of a power peak.This thermal time constant is representative of the time required for the temperature of the power electronics device, and in particular its electronic components, to follow an increase in temperature linked to the electric current flowing through it, despite the cooling plate.

[0008] To avoid overheating during power peaks, the vehicle's power electronic components are therefore sized to withstand these power peaks, with a value of two to three times that of the vehicle's nominal operating power. The size of the power electronic components is therefore oversized, worth two to three times the size of these components if they were only used at their nominal power. This oversizing involves a cost, and does not contribute to efforts to reduce vehicle weight and reduce their energy consumption. In addition, since vehicle engine compartments are very cluttered, it is desirable to optimize the size of the devices that these compartments house.

[0009] There is therefore a need to optimize the size of power electronics devices within electric or hybrid electric vehicles.

[0010] The invention aims to remedy at least in part the drawbacks of the prior art, by providing a power electronics device whose components can be sized for maximum use of their capacity at nominal power, thanks to better management of their temperature, and a powertrain comprising this power electronics device.

[0011] To this end, the present invention proposes a power electronics device comprising: - power electronic components, - a cooling plate, - a circuit comprising the electronic power components and connecting them electrically, the circuit being secured to the cooling plate, - input and output conductors of the circuit, the power electronics device being characterized in that it comprises a phase change material embedding the circuit. The input and output conductors emerge from the phase change material, or are not covered with the phase change material.

[0012] Thanks to the invention, the power electronic components are, for example, sized to operate at their maximum capacity at nominal power only. Indeed, during nominal operation, the cooling plate provides the cooling necessary to maintain the temperature of the power electronic components below their critical temperature, as in the prior art. On the other hand, during power peaks, the phase change material absorbs the excess heat not dissipated by the cooling plate, by passing from the solid state to the liquid state, which allows the power electronic components to remain below their critical temperature. Once the power peak is exceeded, the cooling plate manages to effectively cool the power electronic components, and the phase change material solidifies where it had become liquid.

[0013] According to a preferred feature of the invention, the phase change material is sized in volume so as not to liquefy completely during power peaks. Preferably, the phase change material occupies a volume whose contours extend around the circuit from the cooling plate to at least one plane parallel to the cooling plate, such that the circuit is between the cooling plate and the plane. When the cooling plate is not flat or in a single piece, the phase change material occupies a volume such that it submerges the power electronic components of the power electronic device outside the contact surface between the components and the cooling plate(s).

[0014] The phase change material is preferably chosen from materials capable of changing from a solid state to a liquid state at a temperature lower than the maximum junction temperature of a transistor, therefore lower than 150°C if the components of the power electronics device are silicon transistors or lower than 200°C if the components of the power electronics device are gallium nitride or silicon carbide transistors.

[0015] The phase change material is, for example, a paraffin composed of an alkane with more than 17 carbon atoms. This could be hexacontane (60 carbon atoms) with a melting point of 100°C.

[0016] According to an optional feature of the invention, the power electronics device comprises a housing housing the phase change material, the housing comprising side walls extending laterally to the circuit from the cooling plate to an upper wall of the housing, a space being left free between the upper wall of the housing and the phase change material. The housing allows the phase change material to be held in shape. The space provided between the phase change material and the upper wall of the housing allows the material to expand under heat without deforming the housing.

[0017] The input and output conductors extend outside the housing. For this purpose, the top wall of the housing has, for example, holes for the input and output conductors.

[0018] In one embodiment of the invention, the circuit is arranged and fixed on an electrically insulating and thermally conductive substrate layer, itself secured to the cooling plate. This substrate layer makes it possible to electrically insulate the cooling plate from the circuit, while allowing the latter to be cooled.

[0019] The substrate layer is notably gripped between at least a first metal layer of the circuit and a second metal layer attached to the cooling plate. This characteristic provides good mechanical strength for the circuit and its attachment to the cooling plate, allowing identical expansion under heat, with the metal layers gripped against the substrate layer. The substrate layer is, for example, made of ceramic. It therefore does not necessarily have the same coefficient of expansion as the metal layers, which are generally made of copper.

[0020] In one embodiment of the invention, the cooling plate comprises a heat transfer fluid circulation channel. Alternatively, the cooling plate- The heat sink is connected to a heat sink via thermal paste.

[0021] The invention also relates to an electric powertrain comprising a power electronics device according to the invention. The electric powertrain has advantages similar to those of the power electronics device according to the invention.

[0022] Other characteristics and advantages of the invention will become apparent from the following description on the one hand, and from several examples of embodiment given for informational and non-limiting purposes with reference to the single attached schematic drawing on the other hand, in which:

[0023] [Fig-1] schematically represents a power electronics device according to the invention.

[0024] A power electronics device 1 according to the invention, shown [Fig.l], is in one embodiment of the invention, a power inverter of an electric or hybrid electric vehicle. It comprises in particular at least one diode 12 and one transistor 14 (not all the components of the power electronics device 1 are shown). Of course, these components are given solely by way of example, their nature varying depending on the power electronics device according to the invention.

[0025] The diode 12 and the transistor 14 are connected by a circuit 2 comprising copper wires 11 and first metal layers 28, 24 made of flat copper. In particular, the diode 12 and the transistor 14 are soldered respectively by means of a solder 34 and 36 on the first metal layer 28. An output conductor 4, in the form of a copper bar, is soldered by a solder 30 on the first metal layer 28, and is arranged orthogonally thereto. Similarly, an input conductor 3 also in the form of a copper bar, is soldered by a solder 32 to the first metal layer 24 and is arranged orthogonally thereto. It should be noted that other input and output conductors of the inverter are not shown but are arranged in a similar manner to the input 3 and output 4 conductors.

[0026] The first metal layers 24, 28 of the circuit 2 are sintered on a flat ceramic layer 22. A second metal layer 26 is sintered on the side opposite the first metal layers 24, 28 on the ceramic layer 22. The ceramic layer 22 is therefore gripped between the first metal layers 28, 24 and the second metal layer 26.

[0027] The second metal layer 26 is furthermore brazed, via a brazing 38, to a cooling plate 8, comprising a heat transfer fluid circulation channel 9, for example glycolated water. The heat transfer fluid circulation channel 9 is connected to a cooling circuit of the vehicle, not shown.

[0028] Components 12, 14 are power electronic components di- designed to operate at the nominal electrical power of a powertrain of the vehicle powered by the power electronics device 1, i.e. at a power such that the cooling plate 8 is sufficient for the temperature of these power electronic components 12, 14 not to exceed their junction temperatures, over an indefinite period of operation of the powertrain operating at this nominal power. On the other hand, the power electronic components 12, 14 are not designed to operate at a power peak equal to two or three times the nominal power of the vehicle, for an indefinite period.

[0029] In order to withstand such power peaks of operation of the powertrain which can last from 10 to 20 seconds, the power electronics device 1 comprises a phase change material 6 embedding a part of the input 3 and output 4 conductors, the circuit 2, the ceramic layer 22 and the second metallic layer 26 of copper which is brazed on the cooling plate 8. This phase change material 6 is for example a block of paraffin with more than 17 carbon atoms, this block being housed in a housing 5 comprising side walls 52, 54 extending laterally to the circuit 2 from the cooling plate 8 to an upper wall 56 of the housing, parallel to the cooling plate 8.

[0030] The block of phase-change material 6 occupies the entire internal volume of the housing left free between the cooling plate 8 and a plane 62 substantially parallel to the upper wall 56 of the housing 5. The space 7 between this parallel plane 62 and the upper wall 56 of the housing 5 is empty. It allows the phase-change material 6 to expand under the action of heat without damaging the housing 5.

[0031] The input 3 and output 4 conductors exit the housing 5 through holes arranged in the upper wall 56 of the housing 5.

[0032] During a power peak, the phase-change material 6 becomes liquid around the components 12, 14 to be cooled, and this for the entire duration of the power peak. There is therefore a liquid phase q>2 of the phase-change material 6 around the components 12, 14 for the duration of the power peak, this liquid phase q>2 being contained in a solid phase<pl du matériau 6 à changement de phase, séparée de la phase liquide q> 2 by a separation surface 60 shown in dotted lines on [Eig.l]. The volume of phase change material 6 is in fact calculated so that during a peak in operating power of the vehicle's powertrain, the paraffin contained in the housing 5 does not melt entirely.

[0033] When the power peak ends, this separation surface 60 decreases until disappearing under the action of the cooling plate 8 which evacuates the calories generated during the power peak and which were absorbed in the phase change material 6 by its change of state. Between two power peaks, the phase change material 6 therefore only has a single solid phase q> 1.

[0034] The presence of the phase-change material 6 therefore makes it possible to increase the thermal time constant of the power electronics device 1 to a value greater than the duration of the operating power peaks of the vehicle's powertrain. In other words, during power peaks, the temperature of the electronic components of the device will not increase or will increase very little, by not following the dynamics of temperature increase normally linked to the increase in power during these peaks.

[0035] Of course, the invention is not limited to the examples which have just been described and numerous adjustments can be made to these examples without departing from the scope of the invention. In particular as a variant and depending on the type of power electronic components to be cooled, the cooling plate may comprise cooling fins and not a cooling channel. The method of fixing the components may also present various techniques present in the prior art, for example the second metal layer 26 may be sintered on the cooling plate 8 instead of being brazed there. Finally the circuit 2 may be arranged other than on a ceramic plate, in particular if it comprises inductances, these may be secured to the cooling plate 8 directly or only by means of an electrically insulating layer made of polymer for example.

Claims

Claims

1. Power electronics device (1) comprising: - power electronic components (12, 14), - a cooling plate (8), - a circuit (2) comprising the electronic power components (12, 14) and electrically connecting them, the circuit (2) being secured to the cooling plate (8), - input (3) and output (4) conductors of the circuit (2), the power electronics device (1) being characterized in that it comprises a phase change material (6) embedding the circuit (2).

2. Power electronics device (1) according to the preceding claim, in which the phase change material (6) occupies a volume whose contours extend around the circuit (2) from the cooling plate (8) to at least one plane parallel to the cooling plate (8), such that the circuit (2) is between the cooling plate (8) and the plane.

3. A power electronics device (1) according to claim 1 or 2, wherein the phase change material (6) is a paraffin composed of an alkane having more than 17 carbon atoms.

4. Power electronics device (1) according to any one of the preceding claims, comprising a housing (5) housing the phase change material (6), the housing (5) comprising side walls (52, 54) extending laterally to the circuit (2) from the cooling plate (8) to an upper wall (56) of the housing, a space (7) being left free between the upper wall (56) of the housing (5) and the phase change material (6).

5. Power electronics device (1) according to any one of the preceding claims, in which the circuit (2) is arranged and fixed on an electrically insulating and thermally conductive substrate layer (22), itself secured to the cooling plate (8).

6. Power electronics device (1) according to the preceding claim, in which the substrate layer (22) is gripped between on the one hand at least a first metal layer (24, 28) of the circuit (2) and on the other hand a second metal layer (26), fixed to the cooling plate (8).

7. Power electronics device (1) according to claim 5 or 6, wherein the substrate layer (22) is ceramic.

8. Power electronics device (1) according to any one of claims 1 to 7, in which the cooling plate (8) comprises a heat transfer fluid circulation channel (9).

9. A power electronics device (1) according to any preceding claim, wherein the cooling plate (8) is connected to a heat sink via a thermal paste (9).

10. Electric powertrain comprising a power electronics device (1) according to any one of the preceding claims.

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