Abstract:
The present invention relates to a protection circuit for MOS-technology field-effect transistors. The circuit comprises at least one MOSFET protected by a module for blocking said MOSFET, the module being placed between the gate of the MOSFET and an electrical conductor, the module comprising switched connection means having at least two states: a first state which connects the gate of the MOSFET to the conductor, which is maintained at an electrical potential suitable for blocking the MOSFET, this first state being activated in the presence of an alarm signal; and a second state which disconnects the gate of the MOSFET, this second state being activated in the absence of the alarm signal. The invention applies notably to the protection of the power MOSFETs included in the amplification stages of electronic systems.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a National Stage of International patent application PCT/EP2008/063189, filed on Oct. 1, 2008, which claims priority to foreign French patent application No. FR 07 06898, filed on Oct. 2, 2007, the disclosures of which are hereby incorporated by reference in their entirety. 
     FIELD OF THE INVENTION 
     The present invention relates to a protection circuit for MOS-technology field-effect transistors, these transistors being commonly called MOSFET which is an acronym deriving from the expression “metal-oxide-semiconductor field effect transistor”. The invention applies notably to the protection of the power MOSFETs included in the amplification stages of electronic systems. 
     BACKGROUND OF THE INVENTION 
     The power MOSFETs used in the amplification stages of these systems are sometimes subject to electric shocks that can cause them to be damaged, or even destroyed. As an example, a power amplifier in a radiocommunication system is sometimes subject to abrupt changes of load impedance, for example when an antenna at the output of the amplifier is connected/disconnected or when there is a switch of spectral band in operation. When the system is in use, the accumulation of these electric shocks can degrade the reliability of the amplifier. 
     Moreover, during the optimization phase of the electronic systems, the amplitude of the electric current used may vary strongly because of the tests carried out on the system, thus rendering the components, notably the MOSFETs, vulnerable to destruction. Such destruction during the optimization phases generates significant additional costs, with certain power MOSFETs being particularly costly. 
     Many existing systems rely on the use of MOSFETs whose intrinsic characteristics make it possible to withstand fairly unfavorable operating conditions, without, however, being able to protect the MOSFET against certain incidents such as excessively high excitation currents. Some circuits have been proposed in order to protect the MOSFETs. Notably, it is known to associate with the MOSFET a circuit that behaves like a fuse, cutting the excitation current of the MOSFET in the event of a malfunction. However, the known circuits do not operate at radiofrequencies and/or require a large number of components. 
     The American patent published under the reference U.S. Pat. No. 6,856,200 for the company Marvell International Ltd. provides a protection circuit comprising a switching transistor acting as a switch between the gate of the MOSFET to be protected and the electrical ground, the switching transistor being placed in the closed position when a control signal is emitted to the drain of said switching transistor. However, when this switching transistor is placed in the open position, it acts as an undesirable capacitance between the gate of the MOSFET and the electrical ground, this capacitance disrupting the operation of the circuit comprising the MOSFET, notably at high frequencies. 
     The protection circuit should preferably neither affect the performance of the circuit comprising the MOSFET, nor restrict its operating ranges, in particular temperature-wise. 
     SUMMARY OF THE INVENTION 
     One aim of the invention is to protect a MOSFET from damage or destruction by associating with it a protection module comprising few components and having no or almost no effect on the operation of the circuit comprising said MOSFET. To this end, the subject of the invention is a circuit comprising at least one MOSFET protected by a module for blocking said MOSFET, the module being placed between the gate of the MOSFET and an electrical conductor, the module comprising switched connection means having at least two states:
         a first state which connects the gate of the MOSFET to the conductor, which is maintained at an electrical potential suitable for blocking the MOSFET, this first state being activated in the presence of an alarm signal,   a second state which disconnects the gate of the MOSFET, this second state being activated in the absence of the alarm signal,
 
the circuit being characterized in that the switching means comprise at least one diode and a field-effect switching transistor, the anode of the diode being connected to the gate of the MOSFET, the cathode of the diode being connected to the drain of the switching transistor, the source of the switching transistor being connected to the electrical conductor, and the gate of the switching transistor being controlled by the alarm signal.
       

     The protection circuit thus constitutes a power switch with weak spurious elements, a simple switching transistor not making it possible to both protect the MOSFET and allow it to operate normally unprotected. 
     Unlike the protection circuits of the prior art, the switching means are based on the diode and not on the switching transistor. The switching transistor is used to set the diode to conducting mode and to set it to blocked mode. According to one embodiment, the switching transistor is a field-effect transistor. 
     The diode can be a PIN diode. 
     The circuit according to the invention may comprise a reverse-bias resistor, said resistor being powered on its first terminal by a voltage source and connected by its second terminal to the drain of the switching transistor. 
     According to another embodiment, the diode is a Schottky diode. 
     According to an embodiment in which the MOSFET is enriched, the electrical conductor is connected to the electrical ground. 
     The alarm signal may be generated by a control module comprising at least one comparator which compares a reference signal with a second signal representing a physical quantity to be monitored in order to generate an alert signal when the second signal exceeds a threshold defined by the reference signal, the alarm signal being produced by combining the alert signals obtained from the comparators. 
     According to one embodiment, the MOSFET is a power MOSFET. Moreover, the MOSFET may operate at radiofrequencies. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further features will become apparent from reading the following detailed description, given by way of nonlimiting example in light of the appended drawings which represent: 
         FIG. 1 , a first embodiment of a circuit comprising a protection module according to the invention, 
         FIG. 2 , an equivalent diagram showing the principle of the protection module according to the invention, 
         FIG. 3 , a second embodiment of a circuit comprising a protection module according to the invention, 
         FIG. 4 , one embodiment of a control module producing an alarm signal for the protection module according to the invention, 
         FIG. 5 , one implementation of the protection module according to the invention on symmetrically-mounted MOSFETs. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows one embodiment of a circuit comprising a protection module according to the invention. A MOSFET  102  included in an electronic device  100 , for example a power amplification circuit, is associated with a protection module  101 . 
     A first terminal  101   a  for accessing the protection module  101  is connected to the gate  102   a  of the MOSFET  102  to be protected. A second terminal  101   b  for accessing the protection module  101  is connected to the electrical ground  103 . A third terminal  101   c  for accessing the protection module  101  is connected to a control module  110 . The source  102   b  of the MOSFET  102  is connected to the electrical ground  103 . In the example of  FIG. 1 , the protection module  101  comprises a diode  104 , a resistor  106 , and a transistor  108 . The transistor  108  is, in the example, a field-effect transistor, this type of transistor being appropriate for effecting switching operations. The anode  104   a  of the diode  104  is connected to the first terminal  101   a  for accessing the protection module  101 . The cathode  104   b  of the diode  104  is connected to a first terminal  106   a  of the resistor  106  and to the drain  108   c  of the transistor  108 . The source  108   b  of the transistor  108  is connected to the second terminal  101   b  for accessing the protection module  101 , that is, to the electrical ground  103 , whereas the gate  108   a  of the transistor  108  is connected to the third terminal  101   c  for accessing the protection module  101 , in other words to the control module  110 . The second terminal  106   b  of the resistor  106  is connected to a voltage source  114 . 
     The control module  110  receives at the input  110   a  indicator signals  112  obtained, for example, from sensors arranged in the electronic device  100 . These sensors are used, for example, to monitor the temperature, the standing wave ratio in the circuit or even the current received at the gate of the MOSFET  102 . A sensor indicating an abnormal value makes it possible to anticipate hostile operating conditions for the MOSFET  102 , an abnormal value triggering an alarm signal and/or a control signal in order to prevent damage to the MOSFET  102 . For example, an abrupt change of load at the output of the electronic device  100  suddenly increases the standing wave ratio in the device  100 . Also, a threshold, stored for example in the control module  110 , is determined as a function of the minimum standing wave ratio beyond which the protection of the MOSFET  102  must be triggered, this threshold being notably chosen according to the characteristics of the MOSFET  102  used. If the standing wave ratio exceeds the chosen threshold, then an alarm signal  111  is delivered to an output  110   b  of the control module  110 . Other thresholds, each corresponding to a physical quantity to be monitored, can be stored in the control module  110 . 
     In the example of  FIG. 1 , the MOSFET  102  to be protected is enriched, which means that a zero electrical potential at the gate  102   a  of the MOSFET  102  is sufficient to block it. 
     Moreover, the gate  108   a  of the transistor  108  is controlled by a signal obtained from the output  110   b  of the control module  110 . This signal is an electric current received by the gate of the transistor  108   a , thus increasing the voltage VGS between the gate  108   a  and the source  108   b  of the transistor  108 . The increase in the voltage VGS acts as a command that makes it possible, when VGS exceeds the threshold voltage of the transistor  108 , to virtually short-circuit the drain  108   c  of the transistor  108  with its source  108   b , which is connected to the electrical ground  103 . Thus, when a current is produced at the output  110   b  of the control module  110 , there is virtually a short-circuit between the first access terminal  101   a  and the second access terminal  101   b  of the protection module  101 , that is to say, in the example, between the gate  102   a  of the MOSFET  102  and the electrical ground  103 . This conduction makes it possible to divert the gate current  102   a  of the MOSFET  102  to the electrical ground  103 , thus stopping the excitation of the MOSFET  102 . The MOSFET  102  then remains blocked, and therefore protected. 
     According to a variant of the protection module according to the invention, the second access terminal  101   b  of the protection module  101  is connected to a voltage source and not to the electrical ground  103  as illustrated in  FIG. 1 . This variant can notably be employed to protect depleted MOSFETs for which a biasing of the gate  102   a  of the MOSFET  102  with a nonzero voltage is necessary in order to be able to block the MOSFET  102 . As an example, in the case of a MOSFET  102  with depleted N channel, the second access terminal  101   b  of the protection module  101  is connected to a negative voltage source, so as to bias the gate  102   a  of the MOSFET  102  to cancel the conductivity of the conduction channel. 
     As illustrated by  FIG. 2 , the protection module  101  can be modeled as a switch  201  between the gate  102   a  of the MOSFET  102  and an electrical potential which, in the example, is the electrical ground  103 . In practice, the transistor  108  acts as a switch controlled by the control module  110 . In the absence of any anomaly detected by the control module  110 , the switch formed by the transistor  108  and the diode  104  ( FIG. 1 ) remains open, which renders the protection module  101  almost electrically transparent with respect to the MOSFET  102 . When an alarm signal  111  is produced by the module  110 , the voltage VGS increases suddenly and the transistor  108  conducts the current between its drain  108   c  and its source  108   b , in other words the switch  201  is placed in the closed position. 
     The diode  104  masks the spurious capacitance of the transistor  108  when the latter is in open mode, that is to say when it does not conduct the current between its drain  108   c  and its source  108   b . Without the diode  104 , the spurious capacitance of the transistor  108  would be placed in parallel with the MOSFET  102  and would greatly limit the operation of the circuit at high frequencies. Thanks to the presence of the diode  104 , provided with a weak equivalent reverse capacitance, it is possible to cancel the effects of the spurious capacitance of the transistor  108  when it is placed in the open position. As an illustration, for a spurious capacitance C 1  of the transistor  108  equal to 100 pF, an equivalent capacitance C 2  of the diode  104  equal to 2 pF, the resultant capacitance CS due to the transistor  108  and to the diode  104  is equal to 1/(1/C 1 +1/C 2 )≈1.96 pF. In other words, when the spurious capacitance C 1  of the transistor  108  is very large compared to the equivalent capacitance C 2  of the diode  104 , the capacitance CS resulting from the series association of these two capacitances is almost equal to that of the diode  104 , and therefore to a very weak capacitance CS. The damaging influence of this resultant capacitance CS is therefore greatly lessened thanks to the presence of the diode  104 . 
     In order to maintain a positive voltage VDS between the drain  108   c  and the source  108   b  of the transistor  108  when the excitation current of the gate  102   a  of the MOSFET  102  is an alternating current, the diode  104  performs a half-wave rectification of the current. At high frequencies, for example of the order of 1 GHz, a PIN diode (or “positive intrinsic negative diode”) is, preferentially, employed for its short switching times. In this case, the reverse-bias resistor  106  powered by the voltage source  114  becomes necessary. At frequencies that are not so high, for example of the order of a few hundred kilohertz, a Schottky-type diode can be used, in which case the reverse-bias resistor  106  is not necessary, as illustrated by  FIG. 3  which shows a second embodiment of the protection module according to the invention in which no bias resistor is used. Those elements that are identical to the elements already present in the other figures are given the same references. 
     According to another embodiment, the switch  201  between the gate  102   a  of the MOSFET  102  and the electrical ground  103  can be produced with other means, for example by thyristor or by electromechanical microsystems, often called MEMS, “micro-electro-mechanical systems”. 
     In order to better understand the principle of activation of the protection module  101 ,  FIG. 4  shows one embodiment of a control module used for the protection module according to the invention. The control module  110  of  FIG. 4  comprises, for each physical quantity monitored, a threshold voltage comparator  402 ,  404 ,  406 , a flip-flop  408 ,  410 ,  412 , and a light-emitting diode  414 ,  416 ,  418 . In the example of  FIG. 4 , three physical quantities are monitored: the temperature, the standing wave ratio and the excitation current received by the gate  102   a  of the MOSFET  102 . Other physical quantities could be monitored by the device. A first part  421  of the control module  110 , used to monitor temperature, is described in more detail hereinbelow. Each of the other parts  422 ,  423  of the control module  110  is used to monitor another physical quantity and is produced according to the same principle as the first part  421 . 
     The first part  421  comprises a comparator  402  that has two inputs  402   a ,  402   b . A first electrical signal  413 , with characteristics dependent on temperature, is directed to the first input  402   a , while the second input  402   b  receives a reference electrical signal  411 . As an example, the amplitude of the first signal  413  indicates the temperature measured on the electronic device  100  and the reference signal  411  is a 5V DC current. The first signal  413  representing the temperature can be generated by the electronic device  100  using sensors known to those skilled in the art, as can the other signals representing the other physical quantities monitored. The comparator  402  also has an output  402   c  which supplies the result of the comparison between the first signal  413  and the reference electrical signal  411 . In the example, the voltage of the reference signal  411  is chosen as a function of the maximum temperature value accepted by the electronic device  100 . A first signal  413  of voltage greater than that of the reference signal  411  causes an alert signal  415  to be produced at the output  402   c  of the comparator. When the voltage of the first signal  413  does not exceed the voltage of the reference signal  411 , no signal is emitted at the output of the comparator  402  of the example. For the other parts  421 ,  422 ,  423  of the control module  110 , one and the same voltage value or different reference voltage values can be used. The output  402   c  of the comparator  402  is connected to a flip-flop  408  mounted in series with a light-emitting diode  414 . In the absence of an alert signal  415  at the output  402   c  of the comparator  402 , the flip-flop  408  remains in the open position. As soon as an alert signal  415  is received by the flip-flop  408 , said flip-flop is placed in the closed position, so as to switch on the light-emitting diode  414 . This light-emitting diode  414  is used to warn the user of the electronic device  100  that an abnormal temperature has been reached. According to a simplified embodiment, the control module  110  includes neither flip-flop nor light-emitting diode. 
     Each part  421 ,  422 ,  423  is therefore able to supply an alert signal at the output  402   c ,  404   c ,  406   c  of the comparator  402 ,  404 ,  406 . To produce the alarm signal  111  used to activate the protection module  101  ( FIG. 1 ), these alert signals are combined in a logic function, which in the example is an “inclusive or” module  420 . Thus, the output  402   c ,  404   c ,  406   c  of each comparator  402 ,  404 ,  406  is connected to an input  420   a ,  420   b ,  420   c  of the “inclusive or” module  420 , so that, if at least one of the comparators  402 ,  404 ,  406  produces an alert signal, the “inclusive or” module generates an alarm signal  111  on its output  420   d , connected to the output  110   b  of the control module  110 . Consequently, if at least one of the physical quantities monitored changes abnormally, which is the sign of a malfunction of the device  100 , then an alarm signal  111  is generated at the output  110   b  of the control module  110 . 
     One and the same control module  110  can be associated with several protection circuits  101  so as to control  25  several MOSFET gate blockings concurrently. 
     To illustrate this principle,  FIG. 5  shows an implementation of the protection module according to the invention on symmetrically mounted MOSFETs. The gate  502   a  of a first MOSFET  502  is connected to a first protection module  501  and the gate  502   a ′ of a second MOSFET  502 ′ is connected to a second protection module  501 ′. In the example, each protection module  501 ,  501 ′ comprises, as in the embodiment shown in  FIG. 3 , a transistor  508 ,  508 ′ and a diode  504 ,  504 ′, the anode  504   a ,  504   a ′ of which is connected to the input  501   a ,  501   a ′ of the protection module  501 ,  501 ′ and the cathode  504   b ,  504   b ′ of which is connected to the drain  508   c ,  508   c ′ of the transistor. The source  508   b ,  508   b ′ of the transistor is connected to the electrical ground  503 . A control module  510  receiving indicator signals  512  is connected to the gate  508   a ,  508   a ′ of each of the transistors  508 ,  508 ′. 
     One benefit of the protection module according to the invention is that it requires only a few simple components, which makes it an inexpensive circuit with a small footprint.