FR2826198A1 - Device and method for thermal protection of a switching component, in particular a power transistor - Google Patents

Abstract

The device acts on a switching component (Q1) which is a power transistor of type MOSFET with the gate (G) controlled by a voltage translation circuit (8) which produces a switching signal (Vgs) derived from a logic control signal (C1) belonging to the range of lower voltage amplitudes. The device comprises an overheat control circuit (14) connected to a detector element (14') which detects the condition of overheating of the switching component (Q1) and generates a logic inhibition signal (SL(T)) which sets the logic control signal (C1) into a state bringing about a cooling of the switching component (Q1). The switching component (Q1) is controlled as a two-position switch, and the range of the switching signal (Vgs) values comprises only two values corresponding to the opening and the closing of the switch, which is determined by the respective logic states of the logic control signal (C1). The device comprises a logic circuit which is an AND gate (16) whose one input and the output transmits the logic control signal (C1) to the voltage translator circuit (8), and the other input receives the logic inhibition signal (SL(T)) from the overheat control circuit (14). The overheat control circuit (14) comprises components for producing a temperature signal obtained by detecting a parameter linked to the functioning of the switching component (Q1), a comparator receiving the temperature signal on one input and a reference signal on the other input, and an inverter. The comparator is of analogue type with hysteresis effect, that is of the Schmitt trigger type. In the first embodiment, the temperature signal is obtained from a voltage on a component such as a diode which is in thermal contact with the switching component (Q1), and the signal is transmitted via an amplifier to the comparator. In the second embodiment, the temperature signal is obtained from the drain-source voltage which is converted into a power value dissipated by the switching organ, and then to the temperature signal as a function of the power value. The conversion includes a mathematical model in the form of analogue or digital circuits. At least a part of the overheat control circuit is implemented as an application-specific integrated circuit (ASIC). The method for the thermal protection of a switching component is implemented by the device.

Description

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THERMAL PROTECTION DEVICE AND METHOD FOR A
SWITCHING BODY
The invention relates to thermal protection of power electronic components, and in particular switching transistors. Power switching transistors are now commonly used in place of conventional electromechanical relays in fields such as that of vehicle equipment. They then intervene to control the power supply to a switched load, such as a convenience (window lift, wipers, seat motorization, etc.) from an electrical control signal.

 FIG. 1 represents an elementary assembly of a power switching transistor QI, in this case of the N-doped MOSFET type, mounted in series between the positive rail + BAT of a vehicle battery (typically at + 12V) and the positive supply input 2a of a switched load 2. More particularly, the source S and the drain D of the transistor QI are respectively connected to the input 2a of the load and to the rail + BAT, the load being itself connected to ground. The current consumed by the load 2 therefore flows through the drain-source channel of the transistor QI and goes to ground, which is connected to the negative terminal of the battery.

 The transistor QI is controlled via its gate G in all or nothing (saturation mode) between a blocked state and a conductive state. Putting the switch transistor QI into the conductive state requires a gate voltage Vgs close to the battery voltage, of the order of 10 volts. However, the switching command is normally produced in the form of low voltage logic signals CI (typically 0-5 volts) coming from a switching logic unit 4.

For example, in the case where the switched load 2 is a window lifter motor, the logic unit 4 will produce, in response to a logic pulse supplied as input by a press button 6 activated by the user, a logic switching signal CI in the form of a voltage of the order of 5 volts. This signal is then maintained over a determined period, for example until the window is completely closed.

 A drive circuit called "translator" 8 is therefore provided between the switching logic 4 and the transistor QI, the role of which is to supply the gate voltage Vgs in accordance with the command expressed by the low voltage signal delivered by the logic of switching 4. In the case of the assembly of FIG. 1, the translator 8

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 has an input for the switching logic signal Cl and an output delivering the voltage Vgs, with the following relationship: CI = 0 V o Vgs = 0 V, Cl = 5 V-> Vgs = 10 V. The translator 8 also provides compliance with the characteristics required for the voltage Vgs (sides, impedance, etc.).

 In practice, it may be necessary to provide thermal protection for the QI transistor, the latter often being placed in an environment of insufficient dissipation for the heat generated under conditions of high stress or malfunction. The approach generally consists in detecting by various means the temperature of the transistor QI so as to force it in the blocked state, or else to reduce its drain-source current, until the return to a normal operating temperature.

QI. Le circuit de détection 10 est conçu de manière à identifier une évolution d'un paramètre de la partie sensible 10'signalant une température de surchauffe du transistor Ql, et à commander la fermeture du transistor en pareil cas. A cette fin, il est prévu un commutateur commandé électriquement 12 entre la grille G et la source S du transistor. Le commutateur dispose d'une entrée de commande reliée au circuit de détection 10, de sorte qu'il reste normalement ouvert, et se ferme sous commande de ce circuit lors d'une surchauffe. Le transistor QI est alors forcé à l'état bloqué jusqu'à ce que la température détectée retourne à une valeur déterminée. FIG. 2 schematically shows a simplified example of a conventional thermal protection assembly for the transistor QI in FIG. 1. This assembly includes a temperature detection circuit 10, a sensitive part 10 ′ of which, for example a diode, is in contact thermal with transistor

IQ. The detection circuit 10 is designed so as to identify a change in a parameter of the sensitive part 10 'signaling an overheating temperature of the transistor Q1, and to control the closing of the transistor in such a case. To this end, an electrically controlled switch 12 is provided between the gate G and the source S of the transistor. The switch has a control input connected to the detection circuit 10, so that it remains normally open, and closes under control of this circuit during overheating. The transistor QI is then forced into the off state until the detected temperature returns to a determined value.

 Examples of thermal protection based on such an assembly or the like can be found in documents US-A-5,187,632, US-A-5,497,285, US-A-5,457,419 and US-A-5,726,481.

 It can be seen that the protection consists in intervening directly on the gate control of the protected transistor QI, and therefore in managing relatively large voltages Vgs. Thus, in the example of FIG. 2, it is necessary to provide for the switch 12 transistors operating at approximately 12 volts, while the detection circuit 10 can be produced by logic elements operating at 3 or 5 volts. As a result, the implementation of the protection is complicated by the fact that it requires at the output components which operate outside the ranges of normal logic voltages.

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 Furthermore, it is necessary to provide decoupling means to isolate at the level of the gate G on the one hand the switching voltage output Vgs (coming from the translator 8 in the example) and on the other hand the electrical environment to which the gate is forced by thermal protection (source potential, load input impedance).

 In view of these problems, the invention proposes thermal protection which does not require providing for intervention directly at the level of the protected power component, and which therefore resolves the aforementioned problems linked to control over relatively high voltages and to decoupling at the level of grid G, for the example considered.

 According to a first aspect, the invention relates to a thermal protection device for a switching member having a switching input controlled via a driving circuit, for example a voltage translator, this circuit producing a switching signal evolving on a first range of values from a switching control logic signal evolving over a second range of values, less than the first, the device comprising control means for detecting an overheating condition of the switching member and forcing the switching member in a state allowing its cooling upon detection of overheating, characterized in that the control means produce a logic inhibition signal, active during detection of overheating, to force the logic control signal from switching to a state causing the cooling of the switching member.

 In the embodiments, the switching member is all-or-nothing controlled, being for example a field effect power transistor of the MOSFET type, the first range of values of the switching signal comprising two values, corresponding respectively upon opening and closing of the switching member, determined by respective logic states of the switching control signal.

 Preferably, the device further comprises a logic circuit, upstream of the drive circuit, having an input and an output for transmitting the switching control signal to the drive circuit, and an input for inhibiting the output. switching command, connected to the logic inhibition signal.

 The control means may include means for

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 producing a temperature signal representative of the temperature of the switching member, obtained from a detection of a parameter linked to the operation of the latter, and a comparator having a comparison input which receives the temperature signal and a input which receives a reference signal, the comparator producing at its output a two-state signal from which the logic inhibition signal is obtained.

 Advantageously, the comparator is of the analog type with hysteresis effect, so as to adopt a first state causing the logic inhibition signal to be put into active state when the temperature of the switching member reaches a first temperature threshold. , and to adopt the second state causing the state inhibition signal to be put into the inactive state when the temperature reaches a second temperature threshold, lower than the first threshold.

 In a first embodiment, the temperature signal is obtained from a measurement of a signal taken from a component in thermal contact with the switching member, this taken signal being transmitted, possibly via an amplifier, to the comparison input of said comparator.

 In a second embodiment, the temperature signal is obtained from a potential difference measurement through the switching member, the device further comprising: - means for detecting the potential difference, - means to convert this potential difference into a power value representative of the power dissipated by the switching member, and - means for supplying the temperature signal as a function of this power value, these means being able to integrate a mathematical model in the form analog or digital circuits.

 At least part of the control means can be implemented in the form of an integrated circuit in a pre-broadcast network (ASIC).

 According to a second aspect, the invention relates to a thermal protection method for a switching member having a switching input controlled via a drive circuit, the latter producing a switching signal evolving over a first range of values from a switching control logic signal evolving over a second range of values, less than the first, the method consisting in detecting an overheating condition of the switching member and forcing the switching member into a state

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 allowing its cooling when an overheating is detected, characterized in that it consists in producing a logic inhibition signal, active during a detection of overheating, to force the switching control logic signal to a state causing cooling of the switching device.

 The invention and the advantages which ensue therefrom will appear more clearly on reading the description which follows of the preferred embodiments, given purely by way of nonlimiting examples with reference to the appended drawings, of which: FIG. 1, already described, is a block diagram of the conventional supply of a switched load by means of a MOSFET power transistor controlled by a logic signal via a translator, - Figure 2, already described, is a block diagram of a switching transistor thermal protection system acting directly at its polarization, in accordance with the prior art, - Figure 3 is a block diagram showing the principle of thermal protection according to the invention, applied to the context of the assembly of the figure 1, - Figure 4 is a block diagram of a first embodiment of the invention, and - Figure 5 is a block diagram of a second embodiment of the invention ntion.

 The principle of the invention will be described with reference to Figure 3, where elements similar to those presented in the introduction (Figures 1 and 2) have the same references.

 As in the case of FIG. 1, we will consider a switched load 2, for example a window regulator motor, controlled on-off by a MOSFET transistor Q 1 of type N by playing on the control voltage Vgs applied to its wire rack. This voltage Vgs is delivered by a translator 8 as a function of logic switching signals CI coming from a switching box 4, for example in response to the activation of the press button 6 for controlling the window regulator.

 A thermal protection device 14, 14 is provided for detecting an overheating condition of the transistor QI and acting accordingly to force it into the blocked (non-conductive) state. This device includes a means 14 ′ for

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 detecting a parameter of the transistor Ql which is a function of its channel temperature, and a control unit 14. At the functional level, the control unit 14 has an input E (T) connected to the detection means 14 ′ and an output SL (T) which produces a binary logic signal whose state 0 or 1 depends on whether the temperature of the transistor QI is respectively above or below a superheating threshold. This threshold is adjustable according to the application envisaged, a typical value being of the order of 175 C.

 The output SL (T) is connected to one of the two inputs of an AND logic gate 16. The other input of the AND gate is connected to the switching signal output CI of the logic unit 4 which controls the switching of the transistor QI via the translator 8.

 In this way, the control unit 14 validates, via the AND gate 16, the transmission of the switching signal CI from the switching logic 4 to the input of the translator 8 as long as the transistor QI remains below overheating threshold (SL (T) in state 1), and inhibits the transmission of this signal when this threshold is crossed (SL (T) in state 0). The blocking control of the transistor QI in the event of overheating is therefore carried out by a logic signal whose levels 0 and 1 are fixed by voltages compatible with those of the switching logic 4, which are typically 0 and 5 volts or less.

 Unlike the prior art, it is therefore unnecessary to provide at the level of the control unit 14 components of sufficiently high power and operating voltage (of the order of Vgs) to intervene directly on the transistor Ql .

 Furthermore, the thermal protection thus obtained does not require any decoupling means to isolate the output of the translator 8 from the input of the grid G in the event of overheating, contrary to the approach described with reference to the figure. 2.

 These remarkable advantages result in particular from the fact that the translator 8 is given the function of developing the control voltages of the gate G of the transistor Ql while also taking into account the logic signals SL (T) coming from the control unit 14 On the hardware level, this double function requires only the addition of a command to inhibit the input of the logic switching signal CI on the translator 8, ie a single AND gate 16 in the example considered.

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 The fact of being able to thus produce the control unit 14 with a low voltage control output SL (T) makes it possible to envisage several embodiments according to the envisaged applications.

According to a first embodiment, represented in FIG. 4, the detection means 14 is a diode in thermal contact with the transistor QI, being for example integrated in the same case. This diode is biased by a constant current source 18, so that the bias voltage V (T) varies in known manner according to an inverse function of the channel temperature of the transistor Q1. As such, we can exploit the fact that the law of voltage variation with temperature is of the same nature for the diode 14 ′ and the transistor Q 1. This voltage V (T) is detected by an operational amplifier in follower circuit 20 , which outputs a temperature signal Tc which is proportional to V (T) and calibrated to represent an identifiable channel temperature level. The signal Tc is supplied to a comparison input of an analog comparator with hysteresis effect 22, produced in the example by means of an operational amplifier mounted to form a "Schmit trigger". In this way, the output S of the comparator 22 has two states corresponding to two respective logic states 0 and 1. The comparator has a reference signal input Vref to establish a comparison threshold. The setpoint signal Vref is fixed to cause the aforementioned signal SL (T) to switch from logic state 1 to logic state 0 when the signal Tc represents an overheating temperature requiring the blocking of the transistor QI. The output S of comparator 22 is then linked to the signal SL (T) according to an inverse logic, ie S = SL (T) bar. This output S is therefore supplied to the input of a logic inverter 24 which delivers the signal SL (T) to the input of the AND gate 16.

In operation, as long as the channel temperature of transistor Q 1 is lower than the superheat threshold (for example 175 C), the signal SL (T) remains in logic state 1, and thus makes it possible to validate by the AND gate 16 the passage of the switching control signal C1 to the translator 8. The switched load 2 is then activated and deactivated by the switching transistor QI according to the principle explained with reference to FIG. 1.

 If the channel temperature reaches the aforementioned threshold, the voltage of the signal Te will then be low enough to cause the output of the comparator 22 to switch to state 1. The inverter 24 then delivers a logic state SL (T) = 0 on the input of AND gate 16, causing the output of the latter to be forced to logic state 0 whatever the state of the switching control signal Cl.

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 so, the translator 8 maintains the gate G of the transistor QI at a blocking voltage Vgs (for example 0 volts). The transistor then being kept in the non-conductive state, its channel temperature may return to a normal level.

 The hysteresis provided by the mounting in trigger Schmit of comparator 22 causes the signal SL (T) to remain in logic state 0 to continue to inhibit the transmission of the switching command signal CI after the voltage increase Tc due to the cooling of the transistor. The switching to logic state 1 of the signal SL (T), again authorizing normal control of the transistor QI by the signal Cl, does not take place until after the voltage Tc has risen above a threshold d hysteresis, which can correspond to a channel temperature of the order of 100-125 C, for example. This avoids any phenomenon of "hunting" which would tend to maintain the transistor QI around its overheating temperature, and it also ensures that the return to normal operation of the transistor QI only takes place when its temperature is well below the overheating threshold.

 According to a second embodiment, the use of a component placed in thermal contact with the transistor QI, such as the aforementioned diode 20, is replaced by the analysis of the drain-source voltage Vds of the transistor. To this end, the detection means 14 is materialized by connection points respectively to the drain and to the source of the transistor QI, between which the voltage Vds exists. These points are connected to the respective inputs (non-inverting and inverting) of an operational amplifier in follower circuit 26 of unit gain, thus reproducing the voltage Vds at its output. This arrangement allows very high impedance detection of Vds, and therefore without interference on the normal operation of the transistor QI. The output of the amplifier 26 is transmitted to a conversion unit 28 whose function is to transform the voltage Vds into a value P representative of the power dissipated by the transistor QI when the latter is driven. The conversion is based on the relationship between the values Vds and P given by: P = Vds2 / Rdson, where Rdson is the drain-source resistance when the transistor QI is conductive. The conversion unit 28 therefore performs a squared rise in the voltage Vds supplied as an input and a calibration of the result by a factor 1 / Rdson. The analog circuits allowing such operations are known in themselves, in particular in the field of television receivers. This value P is closely related to the channel temperature of the transistor, and can be considered to be directly proportional to it in a first approximation.

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est de produire à partir de cette valeur une tension Tc exprimant la température de canal du transistor QI. Dans une réalisation simplifiée, l'unité de modélisation effectue juste un calibrage linéaire de la valeur P, éventuellement avec une conversion courant-tension, selon une règle empirique. L'unité 30 peut ainsi être construite à partir d'un ou de plusieurs amplificateurs opérationnels avec des circuits de paramétrage et de correction de gain. L'étalonnage de l'unité de modélisation 30 peut s'effectuer de manière empirique ou semi-empirique, avec établissement d'une table de correspondance entre des points de mesure de la température de canal du transistor Ql, établie au moyen d'une sonde thermométrique, et des valeurs de P rendues par l'unité de conversion 28 à ces températures de mesure. Ces données expérimentales permettent alors de choisir les composants critiques de l'unité de modélisation et de prévoir des facteurs de correction de gain. La variation de la tension de sortie Tc issue de l'unité de conversion 30 est ainsi identifiable à une variation correspondante de la température de canal du transistor QI. Elle peut donc être exploitée pour la protection thermique de la même manière que dans le premier mode de réalisation (figure 4). The value of P from the conversion unit 28 is transmitted, in the form of voltage or current, to a modeling unit 30 whose function

is to produce from this value a voltage Tc expressing the channel temperature of the transistor QI. In a simplified embodiment, the modeling unit just performs a linear calibration of the value P, possibly with a current-voltage conversion, according to a rule of thumb. The unit 30 can thus be constructed from one or more operational amplifiers with gain setting and correction circuits. The calibration of the modeling unit 30 can be carried out empirically or semi-empirically, with the establishment of a correspondence table between measurement points of the channel temperature of the transistor Q1, established by means of a temperature probe, and P values returned by the conversion unit 28 at these measurement temperatures. These experimental data then make it possible to choose the critical components of the modeling unit and to predict gain correction factors. The variation in the output voltage Tc from the conversion unit 30 can thus be identified with a corresponding variation in the channel temperature of the transistor QI. It can therefore be used for thermal protection in the same way as in the first embodiment (Figure 4).

près que dans ce cas la sortie Tc est proportionnelle à la température du canal du transistor QI, à celui du premier mode de réalisation et ne sera pas décrit à nouveau par souci de concision. On remarque donc que le second mode de réalisation permet aussi d'exploiter les effets avantageux de l'hystéréris grâce au montage en"trigger Schmit"du comparateur 22. In this way, the output Tc of the modeling unit 30 is presented to the comparison input of an analog comparator 22 whose reference input receives the above-mentioned reference voltage Tréf. The operation of this comparator, as well as that of elements 24 and 16, is identical, to the remark

except that in this case the output Tc is proportional to the temperature of the channel of the transistor QI, to that of the first embodiment and will not be described again for the sake of brevity. It is therefore noted that the second embodiment also makes it possible to exploit the advantageous effects of hysteresis thanks to the mounting in "trigger Schmit" of the comparator 22.

 As a variant, it is conceivable to produce the conversion unit 28 and / or the modeling unit 30 according to digital processing techniques. In this case, an analog-digital conversion of the signal Vds is carried out upstream of the conversion unit 28, the obtaining of the value of P then resulting from simple arithmetic operations. The value Tc can also be produced from P by numerical calculation, with reference to tables or maps stored in memory from the aforementioned experimental data.

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 A digital processing can also be envisaged within the framework of the first embodiment, with a digital conversion of the voltage V (T) and calculation of the corresponding value Tc by arithmetic logics and mapped reference points.

 In both embodiments, the digital processing means can be integrated into the switching logic 4 or to another source of intelligence having computing means.

 Preferably, the control unit 14 is produced at least in part as an integrated circuit in the form of a configurable pre-broadcast network, generally known by the Anglo-Saxon term of ASIC (application specific integrated circuit).

One can in particular use pre-broadcast networks having on the same chip of analog elements (operational amplifiers, resistances, capacities, etc.) and logic elements (gates, registers, flip-flops, etc.).

 It is clear that the invention lends itself to numerous variants that can be envisaged by those skilled in the art depending on the applications and constraints to be considered.

 In the examples, the thermal protection proceeds by inhibiting the propagation of the logic switching signal C1 towards the translator, and this by means of the AND gate 16. Other logic configurations can of course be considered with an equivalent technical effect, by example by acting directly on control points of the switching logic 4, or on a possible inhibiting logic input located at the level of the translator 8.

 Finally, although the invention has been described in the context of monitoring a MOSFET transistor, it is apparent that its teaching covers other devices liable to overheat and the control of which is effected by means of a translator or other driver. The invention therefore also relates to any type of power switch (FET, bipolar, relay, ...) or other dissipative load. Thus, in the general case, the translator 8 can be replaced by any other drive circuit allowing switch control from low voltage logic signals.

 Furthermore, the element 14 ′ for detecting a parameter linked to the temperature can be arranged to act elsewhere than on the QI switch, for example by being brought into thermal contact with the switched load 2 or arranged to sample a signal depending on the temperature of this charge.

Claims (12)

1. Thermal protection device for a switching member (Ql) having a switching input (G) controlled via a drive circuit (8), the latter producing a switching signal (Vgs) evolving over a first range of values from a switching control logic signal (CI) evolving over a second range of values, less than the first, the device comprising control means (14 ′, 14) for detecting an overheating condition of the member switch and force the switching member into a state allowing its cooling upon detection of overheating, characterized in that the control means (14 ′, 14) produce a logic inhibition signal (SL (T)) , active during an overheating detection, to force the switching control logic signal (Cl) to a state causing the cooling of the switching member (Ql).  2. Device according to claim 1, characterized in that the switching member (QI) is controlled in all-or-nothing, said first range of values of the switching signal (Vgs) comprising two values, corresponding respectively to opening and closing of the switching member, determined by respective logic states of the switching control signal (C1).  3. Device according to claim 1 or 2, characterized in that the driving circuit is a voltage translator (8).  4. Device according to any one of claims 1 to 3, characterized in that the switching member (QI) is a field effect power transistor, of the MOSFET type.  5. Device according to any one of claims 1 to 4, characterized in that it further comprises a logic circuit (16), upstream of the drive circuit (8), having an input and an output for transmitting the signal <Desc / Clms Page number 12>  switching control (Cl) to the driving circuit, and a switching control output inhibition input, connected to the logic inhibiting signal (SL (T)).  6. Device according to any one of claims 1 to 5, characterized in that the control means (14 ', 14) comprise means (14', 18, 20; 14 ', 26,28, 30) for producing a temperature signal (Tc, Tj) representative of the temperature of the switching member (l), obtained from a detection of a parameter linked to the operation of the latter, and a comparator (22) having an input module which receives the temperature signal and an input which receives a reference signal (Vref), the comparator producing at its output (S) a two-state signal (SL (T) bar) from which the logic signal is obtained inhibition (SL (T)).  7. Device according to claim 6, characterized in that the comparator (22) is of the analog hysteresis effect type, so as to adopt a first state causing the logic inhibition signal (SL (T)) when the temperature of the switching member (QI) reaches a first temperature threshold, and to adopt a second state causing the state inhibition signal to go into the inactive state when the temperature reaches a second threshold of temperature, below the first threshold.  8. Device according to any one of claims 6 or 7, characterized in that said temperature signal (Tc) is obtained from a measurement of a signal (V (T)) taken from a component (14 ') in thermal contact with the switching member (QI), this sampled signal being transmitted, possibly via an amplifier (20), to the comparison input of said comparator (22).  9. Device according to any one of claims 6 or 7, characterized in that said temperature signal (Tj) is obtained from a measurement of potential difference (Vds) through the switching member (Ql) , the device further comprising: - means (20) for detecting said potential difference, - means (28) for converting this potential difference into a power value (P) representative of the power dissipated by the <Desc / Clms Page number 13>  switching, and - means (30) for supplying said temperature signal as a function of this power value.  10. Device according to claim 9, characterized in that the means (30) for supplying said temperature signal (Tj) integrate a mathematical model in the form of analog or digital circuits.  11. Device according to any one of claims 1 to 10, characterized in that at least part of the control means (14 ′, 14) is produced in the form of an integrated circuit in a pre-broadcast network (ASIC).  12. Thermal protection method for a switching device (QI) having a switching input (G) controlled by a drive circuit (8), the latter producing a switching signal (Vgs) evolving over a first range of values from a switching control logic signal (CI) evolving over a second range of values, less than the first, the method consisting in detecting an overheating condition of the switching device and in forcing the switching device in a state allowing its cooling when an overheating is detected, characterized in that it consists in producing a logic inhibition signal (SL (T)), active during a detection of overheating, to force the logic signal switching command (CI) to a state causing the cooling of the switching member (Ql).