Abstract:
The present invention discloses a cascade EMP protection circuit, which comprises an LEMP protection circuit and a fast-response protection circuit, wherein a capacitive reactance varactor element is cascaded to the path of signal transmission. Thereby, the present invention can protect electronic devices against LEMP or EMP released by an electronic weapon (NEMP, HEMP, or PEMP).

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a cascade electromagnetic pulse protection circuit, which can inhibit the fast or slow overvoltage surge induced by EMP (Electromagnetic Pulse). 
         [0003]    2. Description of the Related Art 
         [0004]    Electromagnetic weapons may be classified into the NEMP (Nuclear Electromagnetic Pulse) bomb, the HEMP (High-altitude Electromagnetic Pulse) bomb, the PEMP (Portable Electromagnetic Pulse) weapon. The NEMP bomb has an affected area with a radius of hundreds of kilometers. As a nuclear explosion can damage so vast an area and will cause so intense an international reaction, it is very unlikely to be directly used in the future war. Besides, the NEMP bomb may also damage our own communication systems and monitoring systems. The EMP warhead carried by a long-range missile has a medium affected area with a radius of tens of kilometers. The PEMP weapon, which is a small-size bomb carried by a human being or a small-size carrier, and has an affected area with a radius of less than one kilometer. In the long run, EMP bombs must be one of the weapons used in war. 
         [0005]    The explosion of an EMP weapon will generate a pulse of a very high strength electric field characterized in that the electric field strength rises so quickly that the maximum strength is reached within 10 nanoseconds, and that the electric field strength generated by the EMP weapon explosion is higher that that of an LEMP (Lightning Electromagnetic Pulse) and is between 50 and 100 KV/m. The fast rising of so intense an electric field strength will induce a current as great as thousands of amperes in an antenna or a longer cable. Because of so fast a strength rising rate, so high a voltage and so great a current, general lightning arresters are unlikely to protect electronic devices against the damage caused by an EMP weapon explosion. 
         [0006]    When an EMP rushes into a communication/information device, it may induce a voltage as high as from 10 to 150 kV. If there is none protection circuit, the front end electronic elements are unlikely to tolerate such a high voltage. Currently, communication electronic devices generally have so-called lightning arresting devices, which are usually low-speed LEMP protection elements formed of spark gap switches or zinc oxide elements. As the LEMP protection element operates slowly, it has a long response time and a higher clamping voltage. 
         [0007]    For current LEMP protection elements, the EMP inhibition effect is determined by the response time. As the spark gap switches or zinc oxide elements have a longer response time, they will not inhibit EMP until the surge voltage has been very high. Therefore, the current LEMP protection elements cannot protect an electronic device against a fast-rising EMP electric field. 
         [0008]    IC (Integrated Circuit) generally has a high-speed ESD (Electrostatic Discharge) protection element (usually TVS (Transient Voltage Suppressor), DIAC (Diode for Alternating Current), MOV (Metal Oxide Varistor), etc.) at the front end. The high-speed ESD protection element can respond very fast and can tolerate a voltage as high as 8000V. However, the high-speed ESD protection element is not a high current tolerant element. The electromagnetic pulse of a lightning or an EMP bomb has a greater pulse width. A longtime overvoltage induces a current effect and generates a large amount of heat, which will firstly burns out the high-speed ESD protection element and then burns out the internal structures of IC. Therefore, a high-speed ESD protection element can only withstand electrostatic discharge but cannot tolerate high-energy EMP attacks. 
         [0009]    Generally, a communication device has a lightning arresting device (LEMP protection element), and an IC has an ESD protection element at the front end thereof. However, even parallel connecting an LEMP protection element and an ESD protection element cannot yet solve the abovementioned problem. Current will firstly flow through the high-speed ESD protection element, which turns on fast but cannot tolerate high current, until it burns out. Then, the voltage rises again. The front end IC maybe has burned out before the LEMP protection element starts to operate. Besides, as all high-speed protection elements are capacitive elements, they will result in the insertion loss of communication/information devices and affect communication distance. 
         [0010]    A Taiwan patent No. 588888, which was filed by the Inventors on Sep. 24, 2002, disclosed a method to solve the above-mentioned problem, wherein a rear stage ESD protection element is triggered firstly, and the potential is established on a cascaded resistor, and then a front stage high-power LEMP protection element is triggered reversely. Thus, the charge following can be drained out, and the entire system is protected. Besides, the insertion loss of input signal is also compensated. However, the conventional technology still has the problem of insertion capacitance, which is likely to cause high-frequency loss. In applications that the frequency is higher than 1 GHz, small-capacitance MOV or TVS elements are adopted. Thus, the current draining capacity is smaller, and the EMP impact on the performance of electronic devices cannot be effectively reduced. For low-frequency applications, the conventional technology still has problems to be solved. For an electric field strength higher than 300 KV/m, the conventional technology also has room to improve. 
       SUMMARY OF THE INVENTION 
       [0011]    The primary objective of the present invention is to provide a novel cascade EMP (Electromagnetic Pulse) protection circuit to solve the abovementioned problems, wherein the insertion loss of a fast-response protection element is further compensated, or the structure of a fast-response protection element is modified; varactors, etc., are cascaded to signal paths to effectively protect electronic devices against the damage caused by EMP. 
         [0012]    The present invention discloses a cascade EMP protection circuit, which integrates an LEMP protection circuit with a new type fast-response protection circuit, wherein in the fast-response protection circuit, EDS elements are self-cascaded to reduce insertion loss; alternatively, inductive compensation elements are cascaded to save EDS elements at a higher frequency and form a high-pass circuit. A capacitive-reactance element may also be cascaded to the circuit, wherein the capacitive-reactance element is a varactor having a capacitance varying with the input voltage. The varactor normally has a high capacitance and a low impedance. When an intense overvoltage EMP appears, the varactor has a low capacitance and a high impedance, which impedes signal coupling and thus protect electronic elements from the damage of a surge pulse. 
         [0013]    Alternatively, in the cascade EMP protection circuit disclosed by the present, an impedance element can be used to integrate an EMP protection circuit and a fast-response protection circuit, wherein the fast-response protection circuit can be self-cascaded or cascaded to a compensation element; further, the fast-response protection circuit together with the EMP protection circuit is cascaded to a capacitive-reactance element, which may be the varactor mentioned above. Thereby, the present invention can protect electronic devices against EMP damage. 
         [0014]    Below, several embodiments are described in detail to further demonstrate the present invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  is a block diagram schematically showing a cascade EMP protection circuit according to a first embodiment of the present invention; 
           [0016]      FIG. 2  and  FIG. 3  are block diagrams schematically showing a cascade EMP protection circuit according to a second embodiment of the present invention, wherein impedance elements are respectively arranged at different positions; 
           [0017]      FIG. 4  is a block diagram schematically showing a cascade EMP protection circuit according to a third embodiment of the present invention; and 
           [0018]      FIG. 5  is a block diagram schematically showing a conventional EMP protection circuit. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0019]    The present invention proposes a cascade EMP protection circuit, wherein a fast-response protection circuit is integrated with an LEMP protection circuit, and a new architecture is applied to a communication system, whereby the present invention not only can prevent from a common lightning damage but also can protect against various sudden EMP attacks. Besides, insertion loss is also compensated. Thus, the present invention can apply to higher frequency cases and can tolerate/inhibit a higher energy surge. 
         [0020]    Refer to  FIG. 5  for a conventional EMP protection circuit, wherein a fast-response protection element  43  and an LEMP protection element  41  are integrated by an impedance element  42 , and an insertion-loss compensation element  44  is cascaded to the fast-response protection element  43 . A fast-response protection element generally has capacitance; therefore, a fast-response protection element usually causes an insertion loss when it applies to a high-frequency communication device. Further, insertion loss rises with the increase of frequency. In conventional technologies, an inductive compensation element  44  is cascaded to the fast-response element  43  to compensate frequency. For example, in the Taiwan patent No. 588888 the Inventors filed on Sep. 24, 2002, the fast-response element may be a semiconductor element (TVS or DIAC) or a zinc oxide element (MOV); therefore, the fast-response protection element itself is a capacitive element and may function as a low-pass filter and can drain surge energy when a voltage surge of thousands of volts attacks. However, the conventional technologies have an inferior draining of sudden surges at a medium frequency because there is an inductor existing. At a higher frequency, the combination of the fast-response protection element  43  and the inductive compensation element  44  will create a retard effect and impair broadband frequency-hopping applications. 
         [0021]    Refer to  FIG. 1  for an EMP protection circuit according to a first embodiment of the present invention. In this embodiment, a fast-response protection element  13  and an LEMP protection element  11  are integrated by an impedance element  12 . In medium-frequency applications, a fast-response protection  14  is cascaded to the fast-response protection element  13  to perform compensation. As the capacitance is decreased by the cascade of the fast-response protection elements  13  and  14 , the insertion loss is also reduced. A fuse  15  may be added to prevent from an induction event caused by that the antenna contacts a naked power cable. 
         [0022]    Refer to  FIG. 2  for an EMP protection circuit according to a second embodiment of the present invention. In this embodiment, a high-pass protection inductor  24  and an LEMP protection element  21  are integrated by an impedance element  22 . At high-frequency applications, the architecture of the original low-pass protection circuit is directly modified into a high-pass protection circuit. The high-pass protection inductor  24  of a high-frequency communication/information device directly adopts an inductor having an inductance less than 10 nF. As the spectrum of NEMP or LEMP is less than 300 MHz, a high-frequency communication/information device may directly adopt a small inductor, which can prevent from insertion loss. Then, the impedance element can be displaced to the front end, such as the impedance element  25  shown in  FIG. 3 . Alternatively, the impedance elements can be appropriately distributed on both sides of the high-pass protection inductor  24 . When a surge appears, V L =L×dI/dt. The voltage generated by the inductive reactance will trigger the front-end slow-response LEMP protection element, and the impedance element  25  protects the rear-stage circuit. 
         [0023]    Refer to  FIG. 4  for an EMP protection circuit according to a third embodiment of the present invention. In this embodiment, at least one capacitive reactance element  36  is arranged between and cascaded to a surge protection circuit  30  and a protected circuit  39 . The capacitive reactance element  36  is a varactor having a capacitance varying with the input voltage. In a normal state, the varactor has a high capacitance and a low impedance, and signals can reach the communication system. When an abnormal high voltage appears in the system (EMP appears), the varactor can automatically decrease the capacitance and increase the impedance lest the system be damaged. In other words, the cascade EMP protection circuit of the present invention will fast trigger the protection circuit  30  firstly and utilizes the capacitive reactance element  36  to block a surge; thereby, the communication system can be free from the influence of EMP. The abovementioned varactor may have an MSM (Metal/Semiconductor/Metal) structure or an MISIM (Metal/Insulator/Semiconductor/Insulator/Metal) structure which can with or without a two dimension electron gas (2 DEG) structure. 
         [0024]    Those described above are only the embodiments to exemplify the present invention but not to limit the scope of the present invention. Any equivalent modification or variation according to the spirit of the present invention is to be also included within the scope of the claims stated below.