Abstract:
A device for protecting a voltage source and a load supplied with power by the voltage source, comprises a switching element interposed between the voltage source and the load and is associated with a current limiting circuit including a measuring unit for measuring the current provided by the source and a control unit for controlling the switching element so as to prevent the current from exceeding a predetermined current threshold, and a voltage limiting circuit adapted to control the switching element so as to prevent the voltage supplied to the load from exceeding a predetermined voltage threshold.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a device for protecting a voltage source and a load supplied with power by said source. 
   In particular, although not exclusively, it may be applied to power distribution systems in which it is difficult, or even impossible to control the bus and load impedance characteristics. More generally, it applies to all systems that use a direct current, such as in automobiles (42V), telecommunications (48 V), spacecraft, in particular, the international space station ISS. 
   2. Description of the Prior Art 
   Protection circuits have already been proposed, but generally such circuits perform a specific function. For instance, current limiting circuits or surge protection circuits, power limiting circuits, current peak suppressing circuits, and overvoltage protection circuits are known. 
   Surge protection circuits are generally comprised of a switching element such as a MOSFET (Metal Oxide Semiconductor Field-Effect Transistor) combined with a current measuring device. When the current intensity reaches a certain threshold value, the switching element is controlled so as to maintain the current intensity at or below this threshold value, thereby inducing a potential difference across the switching element and therefore, a reduction in the voltage and thus, in the current applied to the load. For that purpose, the switching element must be placed in a substantially linear mode of operation (as opposed to a saturated mode when it is used as a switch-only element). As a result, it has to dissipate power proportionately to the product of the current and the voltage applied thereto. However, the switching element&#39;s capacity to dissipate power is limited, and the switching element has to be quickly turned-off to avoid being damaged. 
   Thus, the current limiting capacity of current limiters is always associated with the maximum time during which they can operate in the linear mode, which is a short duration on the order of a few milliseconds. 
   In a complex power distribution system, the loads may be of any kind and the load controlling switch may be remote from both the source and the load (this is the case in the international space station). If the switch is used as a relay in the off or on state, such a control poses no stability problem. On the other hand, if the switch is used in a linear mode of operation for controlling the maximum current (as is the case of a current limiter), serious stability problems may occur. Generally, such stability problems are solved by limiting the bandwidth of the current loop, which increases the response time, or by inserting a known and controlled impedance upstream and downstream the current limiter, which requires capacitors and damping networks, thus increasing the required size of such a device. 
   Finally, such a surge protection circuit needs to be protected against overvoltages which may damage it because the circuit has been triggered in the off-state to protect the load (off-state circuit protection). 
   The most efficient surge protection circuits (adapted to loads with no overvoltage allowance) are those which short-circuit the voltage source by means of a thyristor, and thus transform the overvoltage into an overcurrent. Such circuits therefore require an overcurrent protection device which has the above-mentioned drawbacks. 
   Furthermore, this protection is well-suited to fault-induced overvoltages. On the other hand, in complex power distribution systems, these overvoltages may occur in a normal situation and therefore, may not result in the protection circuit being triggered. 
   Circuits for suppressing voltage peaks generally comprise an RC damping network or Zener diodes or also so-called “transorb” diodes that can absorb an amount of energy by an avalanche effect, and thus, restrict the voltage with a certain accuracy, on the order of +/−10% of their Zener voltage. 
   These circuits must in any case absorb the peak energy and reduce the voltage down to a safety level, but should also be compatible with overvoltages and transient phenomena liable to occur in the system to be protected. It has been found that in complex electrical power distribution systems, it is nearly impossible to ensure both of these functions through conventional means in a reliable manner. 
   Power limiting circuits are designed for measuring the voltage and current applied to a load, and control a power supply switching element (such as for current limiters) so as to maintain the product of voltage and current constant. Carrying out this regulation as a function of the product of voltage and current proves to be a complex operation. 
   Circuits for damping the quality factor (Q factor) have also been suggested. When the impedance of the circuit comprising the power supply bus and the load has a high Q factor, oscillations that occur upon transient overvoltages at the source or transient overcurrents at the load, may be observed. In order to suppress such oscillations, it is known to use a large damping capacitance which is unavoidably bulky and heavy. Such a capacitance is therefore not suited to spacecraft. 
   Generally, the prior art circuits may not withstand certain overvoltages smaller than those which trigger them upon shut-off. In addition, their operation depends on the source characteristics and the load impedance. 
   SUMMARY OF THE INVENTION 
   The invention is aimed at overcoming these drawbacks and, in particular, at providing a protection device which is adapted to any type of load, while having a compact size and limited losses, both on the load side and on the power supply side. This goal is achieved by providing a device for protecting a voltage source and a load supplied with power by said voltage source, comprising a switching element interposed between the voltage source and the load and combined with a current limiting circuit including a current measuring unit for measuring the current supplied by the source and a control unit for controlling the switching element so as to prevent the current from exceeding a predetermined current threshold. 
   According to the present invention, said device further comprises a voltage limiting circuit adapted to control the switching element so as to prevent the voltage supplied to the load from exceeding a predefined voltage threshold. 
   The inventive protection device may be inserted at any location between the source and the load, and acts transparently without influencing the rated load capabilities. 
   Advantageously, the voltage limiting circuit comprises means for detecting voltage changes at the device output which are fed-back to the current limiting circuit control unit so as to also provide functions of impedance stabilization, quality-factor damping and impedance matching between the voltage source and the load. 
   According to a preferred embodiment of the invention, the switching element comprises a MOSFET transistor which is mounted in series on the positive line of the power supply bus coupling the voltage source to the load, and has its gate supplied by the control member which acts upon the transistor as if it were a current source. 
   Preferably, the transistor is maintained in a linear mode of operation for preventing the current and voltage applied to the load from exceeding predetermined thresholds, the device further comprising a trigger circuit for turning the transistor off after a certain time of operation in the linear mode. 
   According to a preferred embodiment of the present invention, the control unit in the current limiting circuit comprises an amplifier for amplifying the current measurement supplied by the source and controlling the switching element, and the voltage limiting circuit is coupled to the amplifier for controlling the switching element in case of an overvoltage. 
   According to a further preferred embodiment of the present invention, the voltage limiting circuit comprises a Zener diode mounted so as to clamp the voltage applied to the load to a predetermined value at the onset of an overvoltage, wherein the voltage limiting circuit controls the switching element so as to take over on the Zener diode and limit the voltage at the end of the overvoltage. 
   According to another preferred embodiment of the present invention, the voltage limiting circuit comprises a Zener diode mounted so as to absorb most of the overvoltages applied by the source, whereas the switching element is controlled by the voltage limiting circuit so as to be placed in its linear mode of operation. 
   According to still another preferred embodiment of the present invention, the transistor is chosen in order to have sufficient gate-source and gate-drain stray capacitances for the transistor to be controlled, in case of an energy surge, so as to be placed in its linear mode of operation by means of the current injected in the stray capacitances. 
   According to yet another preferred embodiment of the present invention, the device further comprises a power limiting circuit adapted to control the switching element so as to prevent the power supplied by the source from exceeding a predefined power threshold during a time period exceeding a given value. 

   
     BRIEF DECRIPTION OF THE DRAWINGS 
     A preferred embodiment of the invention will be described below by way of non-limiting example with references to the accompanying drawings, in which: 
       FIG. 1  shows a power supply circuit incorporating a protection device according to the present invention; 
       FIG. 2  shows a more detailed view of the protection device shown in  FIG. 1 ; 
       FIGS. 3 and 4  show a detailed view of exemplary implementations of two portions of the device shown in  FIG. 2 ; 
       FIGS. 5 to 8  show, in the form of curves the operation of the protection device of the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  shows a protection device designed according to the present invention based on a single switching element, and intended to be interposed on a DC power supply bus which couples the voltage source  2  to a load  3 . The load  3  may be modeled as an inductor L L  which is mounted in series with a resistor R L  in parallel with a capacitor C L . The power supply bus comprises a bus section  4 ,  5  which couples the voltage source  2  to device  1  and a section  4 ′,  5 ′, which couples device  1  to load  3 , each section being comprised of a positive supply line  4 ,  4 ′ and a negative supply line  5 ,  5 ′. 
   The power supply bus may naturally have a resistive and inductive series impedance and a small capacitance. 
   There may be provided an input Zener diode Z i  between voltage source  2  and protection device  1 , and an output Zener diode Z o  between the protection device and load  3 . The voltage-transient suppressing diode Z i  enables, by dissipating the power stored within the circuit, the line inductance to be set to the maximum voltage allowed by the switching element in protection device  1 . On the other hand, this diode enables due to its small impedance the current of the energy surges to be converted into voltage, so that the protection device  1  can regulate the voltage applied to load  3 . 
   Zener diode Z o  allows the voltage to be set, if required, to a value allowed by the load until the switching element in the protection device responds to limit the voltage. Said Zener diode may be replaced by a freewheeling diode. 
   In  FIG. 2 , protection device  1  comprises, successively, in the order from the voltage source: 
   a power limiting circuit  11 , 
   a current limiting or surge protection circuit  12 , 
   a cut-off triggering circuit  13 , 
   a switching circuit  14 , and 
   a voltage limiting or overvoltage protection circuit  15  also providing an impedance stabilization function. 
   According to the present invention, the switching function of protection circuits  11 ,  12  and  15  is provided by the switching circuit  14  alone, in association with the triggering circuit  13 . 
   Switching circuit  14  is designed around a P-channel MOSFET transistor, T M , having its drain and source arranged in series on the positive supply line  4 ,  4 ′, and its gate supplied through a Zener diode Z 3 , with its reverse terminal coupled to supply line  4 . In addition, a gate of transistor T M  is coupled to the supply line  5 ′ through a forward-mounted diode D 1 , in series with a resistor R 4 . The resistance of resistor R 4  is chosen to be sufficiently high for the transistor T M  to be placed in its saturated mode as soon as a sufficient voltage is applied across lines  4  and  5 . A high value of resistance R 4  also allows the power consumption of the device to be limited. 
   The power limiting circuit  11  comprises a Zener diode Z 1  in series with a resistor R 2 , which are mounted together in parallel between lines  4  and  5 . This circuit is arranged at the input of the device in order to control the input power, this power monitoring alone being required in a limited power distribution system. In fact, this circuit allows the power hyperbolic law to be approximated, as a function of voltage and current, by line segments and one curve portion defined by Zener diode Z 1 . 
   If the input power is too high and applied for too long, the circuit triggers a cut-off by means of the switching circuit  13 , in the same manner as the current limiting circuit or surge protection circuit  12 . 
   The current limiting or surge protection circuit  12  is designed around the switching circuit  13  and comprises a current measuring device to control the latter. Circuit  12  relies upon the voltage characteristics of Zener diode Z 3  associated with transistor T M , so that the current limiting function can be combined simply with other protection functions (voltage limitation and impedance stabilization). This circuit comprises an amplifier A 1  provided at its first input with a voltage supplied by a voltage source S 1 , to which is added the voltage at the junction point between Zener diode Z 1  and resistor R 2 . The second input to amplifier A 1  is provided with a current measurement value of the current flowing through a resistor R 1  mounted in series on line  4 ,  4 ′. Amplifier A 1  acts to amplify the current measurement across resistor R 1  and to control the switch  14  in order to place transistor T M  in a linear mode of operation beyond a given threshold so as to maintain the current in resistor R 1  below said threshold. 
   When the current flowing within resistor R 1  increases, the voltage across Zener diode Z 3  decreases up to a point where this voltage becomes insufficient for diode Z 3  to be conducting. Transistor T M  then switches to its linear mode of operation, which reduces the current through resistor R 1 . 
   The current limit depends on the resistance of resistor R 1  which advantageously is adjustable. 
   In relation to prior art solutions, this circuit has a lesser accuracy as far as the exact value of the current limitation is concerned, but is substantially simpler and faster. 
   In the above-described circuit, all of the control electronics are placed either on the return line (voltage control), or on the line from the voltage source (current control), and acts on the switching element (transistor T M ) as a current source. These provisions allow the common-mode transients that might be generated by the source to be avoided. 
   Circuit  15  both provides an overvoltage protection and peak suppression function, an impedance stabilization function and a Q-factor damping and impedance matching function between the voltage source and the load. 
   The two latter functions, in particular, are provided by a capacitor C 2  and a resistor R 7 , connected in series between lines  4  and  5 , the junction point between capacitor C 2  and resistor R 7  being connected to the input of an amplifier A 2  having its other input connected to a voltage source S 2  and its output connected to the gate of transistor T M . This circuit responds to the positive-going voltage changes between lines  4 ,  4 ′ and  5 ,  5 ′ to damp oscillations, which tend to occur downstream transistor T M  over line  4 ′. If a negative-going voltage change occurs, it is seen as an overload, which is handled by the current limiting circuit  12 . Besides, the oscillations are eliminated since transistor T M  goes into its linear mode of operation as soon as they appear, which causes the Q-factor to be damped. 
   The voltage limiting and peak suppression function is mainly ensured by Zener diode Z 4 , which is mounted in parallel with capacitor C 2 . This Zener diode operates in conjunction with transistor T M  in order to block the transient overvoltages and to restrict the voltage applied to the load. The limiting voltage is determined by the voltage characteristics of Zener diode Z 4 . In case of a power peak, Zener diode Z 4  is conducting. As a result, the output of amplifier A 2  places transistor T M  in its linear mode of operation, whereas Zener diode Z 4  absorbs a major portion of the power during the overvoltage. In this manner, transistor T M  only undergoes the power peak during a very short time period, which is much smaller than 10 μs. 
   Peak suppression is carried out by combining the circuit that performs the voltage limiting function and Zener diode Z i , also known as a “transorb” diode, and comprises a pn junction having a small impedance in case of avalanche breakdown. The transorb diode absorbs the power in the peak by converting the peak current into a voltage (but does not clamp the voltage to a level acceptable for the load). The voltage clamped by the diode is selected as a function of the power to be dissipated (and not as a function of the maximum voltage applicable to the load, as in the prior art). The protection device  1  therefore adjusts the power supply bus voltage to a value acceptable for those components of device  1  which are located downstream on bus  4 ′,  5 ′, during surge suppression. In the worst case, protection device  1  only needs to absorb the voltage clamped by the transorb diode. 
   The combination of transorb diode Z i  and voltage limiting circuit  15  thus allows the problem of power surge absorption, and separately, the problem of regulating the voltage applied to the load to be solved. In practice, the transorb diode Z i  comprises several series-mounted transorb diodes so as to avoid sensitiveness to single-component failure. 
   The cut-off trigger circuit  13  allows to make sure that if transistor T M  is placed in its linear mode of operation after an overvoltage or overcurrent, this situation does not last more than a few milliseconds so as to avoid any possible damage to transistor T M , which dissipates the excess power by heating-up. Therefore, this circuit acts to turn transistor T M  off after a few milliseconds of operation in its linear mode. In the example shown in  FIG. 2 , this circuit comprises a pnp transistor T 1  arranged between line  4  and the output of current limiting circuit  12 , the base of this transistor being connected to the forward terminal of a diode D 2 . The other terminal of diode D 2  is connected, on the one hand, to line  4  through a capacitor C 1  arranged in parallel between the collector and the emitter of another pnp transistor T 2 , and on the other hand, to line  5  through a reverse-mounted Zener diode Z 2 , in series with a resistor R 3 . The base of transistor T 2  is coupled through a resistor R 4  to line  4 , and through a resistor R 5  to the junction between diode D 1  and resistor R 6  of a switching circuit  14 . 
   Capacitor C 1  is charged through the Zener diode Z 2  and resistor R 3 . In normal operation, that is in the absence of any overvoltage or overcurrent, capacitor C 1  is short-circuited by transistor T 2  being controlled through a divider bridge comprised of resistors R 4  and R 5 , by Zener diode R 3 , which biases the base-emitter junction to more than 1 volt, thus ensuring its saturation. When transistor T M  is placed in its linear mode of operation because of an overcurrent, transistor T 2  switches to the linear mode and capacitor C 1  charges (to more than 1 V). Transistor T 1  then changes state and acts as a latching flip-flop by assuming the state of the output of the surge protection circuit  12  or overvoltage protection circuit  15 , which causes capacitor C 1  to discharge. The time constant of the discharge circuit is defined by the product of the capacitor&#39;s capacitance C 1  and resistance R 3 . When the input voltage (over line  4 ) increases, the time constant should be reduced in proportion so as to maintain the power dissipation in transistor T M  constant. Thus, circuit  13  allows the power dissipated by transistor T M  to be controlled. 
     FIG. 3  shows an implementation example of current limiting circuit  12 . In this figure, circuit  12  comprises two pnp transistors, T 3 , T 4 , arranged as a dual transistor (the bases of both transistors being connected to one another), having their collectors connected to the two terminals of resistor R 1 , respectively, through two respective resistors R 9 , R 10 . The collector of transistor T 3  is further coupled to the input of circuit  12  through a resistor R 8 , this input being connected to the junction point between Zener diode Z 1  and resistor R 2 . The emitter of transistor T 3  is connected to the output B 1  of circuit  12 . The emitter of transistor T 4  is connected to its base, and coupled to line  5  through a resistor R 11  for biasing both transistors. The emitter of transistor T 4  is also coupled to an input B 2  of circuit  12  through a resistor R 12  mounted in parallel with a decoupling capacitor C 3 . Output B 1  is intended to be connected to the gate of transistor T M , whereas output B 2  is to be connected to an input of overvoltage protection circuit  15 . 
   As a result, resistor R 6 , which is grounded and coupled to the gate of transistor T M  behaves as a biasing resistor for transistor T M . 
   When the current increases within the current measuring resistor R 1 , the base-emitter voltage of transistor T 4  increases. The current flowing through resistor R 10  therefore increases, which decreases by the same amount the current flowing through Zener diode Z 3  and therefore, its voltage. When the voltage across the Zener diode becomes smaller than the Zener voltage, transistor T M  goes into its linear mode of operation, thereby limiting the current in resistor R 1 . 
   If higher accuracy is desired, it is preferable to use a dual transistor (implemented as a single component) for transistors T 3  and T 4 , rather than having two separate transistors. 
     FIG. 4  shows an implementation example of an overvoltage protection circuit  15 , and in particular, the impedance stabilization and Q-factor damping functions of said circuit. These functions are simplified in that they rely upon detecting positive-going voltage changes (which are the most dangerous ones). 
   This circuit comprises an npn transistor T 5  having its collector connected to an output E 1  of circuit  15  (connected to input B 2  of circuit  12 ), and its emitter coupled to biasing resistor R 13 . The base of said transistor, on the one hand, is connected to the junction point between capacitor C 2  and Zener diode Z 4 , and on the other hand, is coupled to line  5  through a resistor R 7  and a reverse-mounted diode D 3 . 
   This circuit acts as a large capacitance connected between lines  4  and  5 , the voltage changes generating a current which is amplified by the circuit comprised of transistors T 5 , T 3 , T 4  and series resistors. 
   Actually, transistor T 5  is mounted as a voltage follower for amplifying the current by a factor equal to the ratio of resistances R 7  to R 13 . The current through the base of transistor T 5  is reflected in its collector, which generates an additional base current within transistor T 4 . Due to a mirror effect, this base current flows through transistor T 3  and is amplified by an amplification factor equal to the ratio of the resistances of resistors R 9  and R 10 , so that the amplified current controls the gate of transistor T M . 
   It should be noted that resistances R 6  and R 10  should be matched with the voltage across lines  4  and  5 , so as to optimize their residual consumption. 
   In the device shown in  FIG. 2 , wherein circuits  12  and  15  are those shown in  FIGS. 3 and 4 , the functions of amplifiers A 1  and A 2  are partially integrated in that the control function of transistor T M  is unique (output B 1 ), since circuit  15  shown in  FIG. 4  only performs an amplification of the voltage measurement, which measurement is supplied to input B 2  of circuit  12  shown in  FIG. 3 . Actually, the amplified voltage measurement at output E 1  acts as a biasing voltage for the current amplifier and therefore influences the way switch  14  is controlled. 
   In addition, if Zener diode Z o  is replaced by a diode arranged in the same direction, the overvoltages and power surge protection function is carried out in a slightly different way. Indeed, in this case, Zener diode Z 4  simply clamps the voltage level of a first portion of the overvoltage, and triggers transistor T M , which, after the voltage loop response time, regulates the voltage at a smaller value than the Zener voltage of diode Z 4 . Thus, the Zener diode clamps the voltage during a few microseconds, and then allows the transistor T M  to take over for limiting the voltage. 
   Thanks to these provisions, the protection device described so far by way of example performs a large number of protective functions with a reduced number of discrete electronic components (31 components) in a substantial manner relative to prior art devices. 
   It may be noted that the inventive device has, in addition to a power limiting function performed by circuit  11 , a squared power limiting characteristic, since it has separate functions of current limitation and voltage limitation and therefore has independently defined voltage and current limits. In order to implement the hyperbolic power-limiting characteristic, the current limit only needs to be reduced when the voltage exceeds its rated value. 
   The law obeyed by the change in the current limit as a function of the input voltage is defined by two line segments having characteristics depending on diode Z 1 , resistor R 2  and the resistance between the junction point of Z 1  and R 2  and the gate of transistor T M , that is, resistor R 8  in the example shown in  FIG. 3 . 
   Also, it may be noted that the inventive device may be triggered following power surges, thus allowing it to be protected in the same way as the load, even if the input transorb diode Z i  fails in the off state. If a fault occurs on the load side, the device continues to isolate the load even after a power surge. 
   The chosen transistor T M  preferably has significant gate-source and gate-drain capacitances. Thus, in case of power surge on the power supply bus  4 - 5 , transistor T M  is controlled by the voltage change thanks to the current injected into said stray capacitances. The switch does not need to continue being controlled at a low impedance in order to maintain the latter in the off-state after the voltage peak. On the contrary, this control is purposely designed in order to allow the circuit to be turned on again immediately after the transient, while avoiding destruction of the switch. The transorb diode Z i  is provided in order to restrict the transient voltages applied to the load. 
   If the switch is in the off state before the transient, this means that the load is faulty since the switch is always in the on state in the absence of fault when the bus voltage is present. In this case, as long as the protection is effective on the supply bus side, the power surge may be injected without any damage risk. In any case, an immediate restart after a power surge will damp out the oscillations that may result from this surge. 
   The device described so far is insensitive to any component failure, since lines  4  and  5  are never short-circuited in case a component fails. 
   The curves shown in  FIGS. 5 to 8  illustrate the performance of the inventive device  1 , when connected to various kinds of load  3 , in response to an inductive overvoltage of about 200 mJ. Advantageously, this energy is absorbed by the transorb diode Z i  at 240 V. 
   These curves were obtained with a device designed for operation with a rated voltage supply of 120 V for load powers of less than 200 W, with an input current limit of 1.5 A, for a voltage ranging from 90 to 165 V, and for eliminating a maximum inductive energy of 500 mJ, with a voltage limit on the load side of 170 V, and a trigger time of 1 to 2 ms in case of overvoltage or overcurrent. 
   In order to obtain such performance, the components in device  1  may have, for example, the following parameters: 
   
     
       
             
             
             
             
           
         
             
                 
             
           
           
             
               Z3: Zener voltage of 
               R2: 100 kΩ 
               R6: 60 kΩ 
               R10: 225 kΩ 
             
             
               11 V 
             
             
               Cl: 1 μF 
               R3: 400 kΩ 
               R7: 500 kΩ 
               R11: 120 kΩ 
             
             
               C2: 0.1 nF 
               R4: 30 kΩ 
               R8: 15 kΩ 
               R12: 30 kΩ 
             
             
               C3: 100 pF 
               R5: 250 kΩ 
               R9: 105 kΩ 
               R13: 100 kΩ 
             
             
                 
             
           
        
       
     
   
   The curves shown in  FIGS. 5   a  to  5   c  show, with different time scales, the voltage variations as a function of time at the input (curve  21 ) and the output (curve  22 ) of device  1  with a resistive and inductive load (R L =108Ω, L L =5 μH et C L =0). These curves show that device  1  responds in about 8 μs to limit the overvoltage of 240 V to about 170 V for the whole duration of the overvoltage. 
   The curves shown in  FIGS. 6   a  to  6   b  show, with different time scales, the voltage variations as a function of time at the input (curve  23 ) and output (curve  24 ) of device  1  with a purely resistive load (R L =108 Ω, L L =0 et C L =0). These curves show that the inventive device on the one hand prevents the output voltage from exceeding 170 V and on the other hand, responds in about 10Ωs to limit the overvoltage to this value. In particular,  FIG. 6   b  shows that the voltage is clamped at a lower level by diode Z o  during the first 10 μs, and then the input voltage is clamped by diode Z i  which suppresses the energy stored within the line, whereas the output voltage is regulated by transistor T M  placed in its linear mode, diode Z o  being blocked. 
   As is apparent from  FIGS. 7   a  and  7   b , device  1  prevents the voltage applied to a resistive and capacitive load  3  (R L =108 □, L L =0 et C L =1 μF) from exceeding the output Zener diode voltage, or about 180 V (curve  26 ). Then, this voltage is reduced to 170 V nearly 40 μs after the onset of the overvoltage applied as input (curve  25 ). 
     FIGS. 8   a  and  8   b  show the voltage variations at the input (curve  27 ) and the output (curve  28 ) of device  1  with a resistive, inductive and capacitive load (R L =180 □, L L =5 μH and C L =1 μF).