Abstract:
The invention relates to a multi-staged overvoltage protection circuit, in particular for information-technology systems, comprising at least one coarse protection element and at least one fine protection element, wherein the at least one fine protection element can be activated by a triggering device depending on a threshold. According to the invention, the applied operating voltage is led to an evaluation device, which generates a reference voltage. Also provided is an evaluation unit, firstly for checking to see whether the current operating voltage is above the reference voltage, secondly for checking to see whether the voltage excess exceeds a previously determined level, and thirdly for establishing whether the rate of change of the operating voltage is greater than another previously determined value so that a transient overvoltage exists, wherein the triggering device then receives an activation signal from the evaluation device.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable 
     THE NAMES OF THE PARTIES TO A JOINT RESEARCH AGREEMENT 
     Not Applicable 
     INCORPORATION-BY-REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISC 
     Not Applicable 
     BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The invention relates to a multi-stage overvoltage protection circuit, in particular for information technology systems, comprising at least one coarse-protection element and at least one fine-protection element, wherein the at least one fine-protection element can be activated by a triggering device depending on a threshold. 
     (2) Description of the Related Art 
     Overvoltage protection circuits consisting of multiple stages are described in the known prior art. Usually, a first stage is formed by a component capable of arresting high currents whilst having a comparatively high operating voltage. Such a component may be a gas discharge arrester. Although such a protection circuit can be used in a great nominal value range, it does not provide sufficient protection for information technology equipment such as computers, telephone systems or the like due to the high protection level. Therefore, there is usually provided an additional stage, in which a fine-protection element, namely a Z-diode, a TVS-diode or the like is provided, which is rated for the signal voltage so that a limitation as near as possible to the nominal voltage is guaranteed. At least one decoupling element is provided between the coarse-protection element and the fine-protection element, which serves to coordinate the behavior of the aforementioned stages. 
     An amplifier circuit comprising an overvoltage protection device is disclosed in DE 198 45 281 A1. The fine-protection element is a transistor short-circuiting the provided amplifier signal wires if a fixed threshold is exceeded. Specifically, a transistor protection circuit is connected downstream of the actual overvoltage arrester in the overvoltage interference signal direction, whose transistors are blocked during normal operation and are switched when an overvoltage occurs, by limiting the maximum acceptable voltage. 
     In the voltage-limiting device according to DE 10 2004 036 164 A1 at least one voltage input and one voltage output are provided. A first voltage-limiting unit is provided with at least one input and one output such that at least a first switch is provided in the first voltage-limiting unit between the input and the output. In addition, a voltage sensor is provided in the first voltage-limiting unit, which is configured to open the first switch when a voltage is present at the input of the first voltage-limiting unit that is greater than a predeterminable maximum voltage value. 
     EP 1 278 283 A2 describes a device for the protection of electronic components against overvoltages, comprising at least one transistor which is switchable by a trigger circuit upon reaching predetermined overvoltage values, thereby converting disturbing electrical energy to thermal energy. The fine-protection element used is a Z-diode. 
     PCT/WO 2004/006408 A1 discloses a protection circuit having an electronic decoupling element TBU which goes into a high-impedance state if a current value is exceeded, thereby limiting the flow of current to a terminal. The disadvantage of this teaching is that a voltage-limiting component has to be provided in the terminal in order to generate a sufficiently high flow of current in the event of an overvoltage. 
     In the protection unit for a modem according to PCT/WO 02/50973 A1 a fine-protection element is switched in by a thryristor. The thyristor is driven by the current flow to a terminal. The disadvantage of this circuit configuration is that this terminal, too, has to include a voltage-limiting component in order to generate a sufficient flow of current in the event of an overvoltage. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     Based on the solutions according to the prior art it is the object of the invention to provide a further developed multi-stage overvoltage protection circuit, in particular for information technology systems, which can be used for a greatest possible voltage range and with the aid of which an overvoltage limitation as near as possible to the operating voltage can be accomplished. 
     The solution to the object of the invention is achieved with the combination of the features according to the teaching of patent claim  1 . The dependent claims define at least useful embodiments and advancements. 
     The multi-stage overvoltage protection circuit according to the invention is capable of automatically adjusting itself to present operating voltages. If the value of the detected operating voltage is intolerably exceeded by an overvoltage, the voltage level is limited and the resulting disturbance current is arrested. 
     In contrast to known overvoltage protection circuits it is not necessary to set up a specific type of overvoltage protection device for each operating voltage level. 
     According to the invention the present operating voltage is passed to an analyzer which generates a reference voltage. 
     In addition, an evaluation unit is provided, firstly for checking whether the current operating voltage exceeds the reference voltage. Secondly, it is checked whether the value exceedance exceeds a predetermined level in order to find out, in the third place, whether the rate of change of the operating voltage is greater than another predetermined value. Should this be the case, a transient overvoltage is concluded and the triggering device receives an activation signal from the analyzer. 
     In the event of a value exceedance of the predetermined level of the operating voltage, which is associated with low rates of change, the analyzer provides a new, current reference voltage which is adapted to the current operating conditions. 
     In one embodiment of the invention the operating voltage in the analyzer is passed via a diode D 2  to a capacitor C 1 , which capacitor C 1  is connected to an operating voltage potential via a resistor R 3  connected in series. The voltage present at capacitor C 1  is the reference voltage. 
     The time constant for the rate of change of the operating voltage is defined by the resistor R 3  and the capacitor C 1 , wherein an evaluable voltage is present at R 3  if the time constant is exceeded. 
     At the connection point between the capacitor C 1  and the resistor R 3  a Zener diode D 3  is provided, which defines the additional range of the change of the operating voltage. 
     At the output end of the Zener diode D 3  another capacitor C 2  is provided, which is connected towards the operating voltage potential, in order to limit the slew rate of the trigger voltage of the fine-protection element. 
     The fine-protection element is triggered by a transistor the base of which is connected to the aforementioned capacitor C 2 . 
     Another Zener diode D 4  for adjusting the maximum limit voltage is connected between the operating voltage potential and the base of the transistor. 
     Moreover, a decoupling element in the form of a resistor, an inductance, a capacity and/or a TBU unit is provided between the protection stages in a manner known per se. 
     The invention shall be explained in more detail below by means of an embodiment and with the aid of figures: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings: 
         FIG. 1  shows a known overvoltage protection circuit according to the prior art, comprised of two stages A 1  and V 1 , with a decoupling resistor R 1  being connected between the input IN and the output OUT; 
         FIG. 2  shows a block diagram of the device according to the invention; 
         FIG. 3  shows a program flow chart with respect to the evaluation of a voltage change by the analyzer; 
         FIG. 4  shows the voltage-time behavior in the event of a change in an acceptable range near the reference voltage, e.g. a signal voltage superimposed on a supply voltage, without limiting this system-induced change of the operating voltage; 
         FIG. 5  shows a voltage-time behavior in the case where the acceptable range near the reference voltage is exceeded, while the slew rate of the voltage change does not yet exceed a fixed value, so that the change is likewise an acceptable change, with the consequence that the voltage level is not limited, but a new reference voltage is generated; 
         FIG. 6  shows voltage-time behaviors in which the acceptable range near the reference voltage is exceeded, the slew rate of the voltage change likewise exceeds a fixed value, the overvoltage being a transient one, with the consequence that a limitation of the present voltage is initiated; 
         FIG. 7  shows a first example of a circuit of the embodiment of the overvoltage protection circuit; 
         FIG. 8  shows a second embodiment of the overvoltage protection circuit with a bipolar circuit configuration for limiting positive and negative operating voltages; and 
         FIG. 9  shows a third embodiment of an overvoltage protection circuit with electronic decoupling elements (TBU). 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As is shown in the block diagram of  FIG. 2  the overvoltage protection circuit comprises a coarse-protection element and a controllable fine-protection device between input IN and output OUT. Both aforementioned assemblies are separated from each other by a decoupling unit. 
     In addition, an analyzing electronics is provided, which is connected to a control electronics which, again, acts on the controllable protection device at the output end. As can be seen in  FIG. 2 , the present operating voltage is supplied to the analyzing electronics, and this electronic device continuously monitors the operating voltage. 
     If the present voltage increases a decision is made in consideration of predefined criteria whether the voltage increase is an unacceptable overvoltage. In the event of an overvoltage the (fine) protection device is activated by a control electronics or triggering device, respectively. 
     Depending on the capacity of the controllable (fine) protection device a coarse protection may be connected upstream thereof. 
     The analyzing electronics has the function to distinguish between a voltage change in the form of an overvoltage or an additional change of the operating voltage. 
     The evaluation of the voltage changes by the analyzing electronics is shown in the flow depicted in  FIG. 3 . 
     Initially, a reference voltage is generated from the supplied operating voltage, which is used for adjusting the nominal voltage and limit voltage for the overvoltage protection circuit. 
     If the current operating voltage varies relative to the reference voltage a decision is made whether an acceptable change of the operating voltage or an overvoltage is present. 
     For the decision the following criteria are evaluated:
     a) If the change is within an acceptable range near the reference voltage a signal voltage is concerned which is superimposed on a supply voltage. In this case, this system-induced change of the operating voltage will not be limited. A corresponding behavior is shown in  FIG. 4 .   b) In case the acceptable range of the reference voltage is exceeded, while the slew rate of the voltage change does not exceed a fixed value, however, an unacceptable change of the operating voltage is detected. However, in this case, too, the voltage level is not yet limited. Rather, a new reference voltage is generated, thereby fixing a new nominal voltage and limit voltage for the overvoltage protection circuit. This behavior is shown in  FIG. 5 .   c) If the acceptable range near the reference voltage is exceeded, and if the slew rate of the voltage change exceeds a fixed value, the overvoltage is a transient one. In this case, a limitation of the voltage is initiated, namely as shown in  FIG. 6 .   

       FIG. 7  shows a first embodiment of a practically realized overvoltage protection circuit for limiting positive voltage impulses for signals in information technology systems. 
     A thyristor T 1  is used as fine-protection element, which is triggered by a transistor T 2 . 
     By means of the transistor T 2  the control current for the thyristor T 1  is reduced by the factor of the current amplification in order to reduce the load on the signal circuit. 
     The voltage at the capacitor C 1  is proportional to the DC component of the signal voltage. The time constant defined by resistor R 3  and capacitor C 1  determines the adjustment rate of this reference voltage. 
     If the slew rate of the present signal exceeds this time constant a voltage at resistor R 3  decreases. 
     The acceptable range for the change of the operating voltage is defined by the selection of the Zener voltage of the Zener diode D 3 . In the circuit shown, which includes diode D 1  and transistor T 2 , this value is obtained by U z +2.3 V. If the voltage at resistor R 3  exceeds this value, thyristor T 1  is triggered. 
     The slew rate of the trigger voltage for thyristor T 1  is limited by means of capacitor C 2 . If the signal voltages have high slew rates (square-wave signals) this is advantageous in order to avoid an undesired activation. 
     The Zener diode D 4  defines the maximum limit voltage of the introduced circuit. 
       FIG. 8  shows another embodiment of the invention, which is based on the explanations of the circuit according to  FIG. 7 , but which is bipolar so as to allow a limitation of positive and negative operating voltages. 
     The circuit configuration for bipolar operating voltages as shown in  FIG. 8  comprises two thyristors T 1 , T 3  for the respective polarity. Alternatively, a triac could be used as well. 
     In the example shown in  FIG. 8  the analyzing electronics is configured to be redundant. Thus, the reference voltage for positive direct voltage components is generated by C 1 . If the DC voltages are negative, the reference voltage is generated by C 3 . An additional advantage of this embodiment is that the connections of the overvoltage protection circuit may be confused, meaning that a reverse polarity protection is obtained and errors during the installation are precluded. 
     In the example according to  FIG. 8  the thyristors are triggered separately depending on the respective polarity of the provided thyristor. The use of a triac or of thyristors with two gates, which are triggerable on the anode side and the cathode side, also allows an embodiment with a separate analyzing electronics and control. 
     In the embodiment of the circuit configuration according to  FIG. 9  the advantages of electronic decoupling elements (TBU) are combined with the introduced inventive solution of a triggerable fine-protection element in a bipolar configuration. 
     With respect to the function of the analyzing electronics and control of the thyristors the circuit configuration shown in  FIG. 9  largely corresponds to the example according to  FIG. 8 . A difference is the use of a TBU (Transient Blocking Unit) instead of a conventional resistor or an inductance as decoupling element. 
     The function of the TBU can be compared with that of an electronic switch. The TBU monitors the current flow between its input and output. If the current exceeds a fixed value, the TBU is activated and very quickly switches off the connection in-output. This function allows the realization of an ideal decoupling element by means of which a very small current load on the fine protection and, along with that, a very low protection level for the overvoltage protection circuit can be obtained. 
     TBU components have the disadvantage that the resetting from the triggered state, i.e. the high-impedance state, to the original state, the low-impedance state, the voltage at the TBU has to be lower than an assigned value, in the specific case 14V. However, during the operation with a direct voltage &gt;14V this cannot be guaranteed without additional measures. Due to the parallel connection of a resistor relative to the TBU the invention provides for a simple possibility to achieve this with a predefined maximum operating voltage. 
     With respect to the example shown this resistance can be determined as follows: 
     Maximum operating current: 100 mA 
     Voltage at TBU: ≦10V 
     
       
         
           
             R 
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                   10 
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                   ⁢ 
                   V 
                 
                 
                   0.1 
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                   A 
                 
               
               = 
               
                 100 
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                 ⁢ 
                 Ω 
               
             
           
         
       
     
     Of course, also a combination in the form of a series or parallel connection of TBU and resistor as coupling element is possible. 
     In order for the TBU, after its activation, to return to its initial low-impedance state after the interference has faded, it has been ensured that the voltage drop at the TBU falls below an assigned value. By suitably dimensioning the combination TBU and resistor a necessary resetting may be ensured. 
     The introduced solution provides for an overvoltage protection circuit which automatically adapts itself to the respective operating voltage, whereby an analyzing electronics ensures an activation of the protection device if a relevant overvoltage is detected. 
     The value for the limit voltage is continuously adapted to the operating voltage so that an optimum voltage limitation is obtained for the respective current operating voltage value if an overvoltage occurs. 
     The analyzing electronics is capable of carrying out an evaluation on the basis of a signal analysis (rate of change, voltage swing, maximum level) of the operating voltage in order to clearly determine whether an acceptable operating voltage or an overvoltage is present. The control circuit for the direct voltage component of the operating voltage can be realized with a low capacitance, and a bipolar configuration of the circuit for positive and negative signals and operating voltages is possible. 
     The limitation of the operating voltage can be adjusted to a fixed upper limit value by an additional Z-diode wherein, in this case, the voltage-limiting circuit can be activated directly, by bypassing the analyzing electronics. Due to the series connection of a diode and a thyristor, as provided in the embodiments, it is possible to reduce the reverse current and the capacity of the entire circuit configuration. 
     Additionally, it is pointed out that rates of change occurring when the operating voltage is switched on or when operating conditions change, are, according to the invention, not meant to result in a reaction of the overvoltage protection circuit, for example, if an operating voltage of 24 V DC is switched on. This switch-on process can be assumed to take a typical time of 1 ms. This would result in a rate of change of 24V/ms. 
     Contrary to this, overvoltage impulses caused by the effect of lightning or switching activities have a substantially higher slew rate. For example, in the directives for telecommunication systems the lowest influence to be assumed is specified with a pulse shape 10/700 μs and an amplitude of 1 kV. This results in a rate of change of 100V/μs.