Abstract:
A transient voltage surge suppressor (TVSS) device, suitable for commercial/industrial applications, incorporates diagnostic circuitry. The TVSS provides seven modes of protection for three phases of a typical electrical service. A surge panel can protect against large current transients. The large current-handling capability stems from passing a large amount of surge current from one layer of a multilayer printed circuit to another through the use of arrays of plated through holes.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims the benefit of U.S. Provisional Application No. 60/658,069, filed Mar. 3, 2005. 
     
    
     FIELD OF THE INVENTION  
       [0002]     This invention relates to transient voltage surge suppression (TVSS) devices, and more particularly to TVSS devices for commercial and industrial applications where the devices incorporate diagnostic circuitry.  
       BACKGROUND OF THE INVENTION  
       [0003]     Transient voltage surge suppression (TVSS) devices, referred to interchangeably herein as surge suppressors and voltage-clamping devices, are commonly known for use in suppressing such over-voltage transients to protect voltage-surge intolerant circuitry. TVSS devices include nonlinear, voltage-dependent resistive elements which display electrical behavior similar to that displayed by a pair of series-connected, back-to-back Zener diodes. At normal voltages, below the TVSS clamping voltage level, TVSS devices display a high resistance with a small leakage current. When subjected to a large transient voltage (a voltage above the clamping voltage of the TVSS device), the TVSS device may operate in a low resistance region which increases current flow through the device. When the voltage is increased, the TVSS, due to its characteristics, presents a lower resistance path to a current from a power source, thereby diverting most of the current away from connected circuitry. The potentially destructive surge energy can be dissipated or passed through the voltage-clamping (TVSS) device and its operating current returns to its normal range after the surge.  
         [0004]     Metal oxide varistors (MOVs) may be utilized as TVSS devices. One technique for protecting metal oxide varistors (MOVs) requires adding a current fuse in series with the MOV, which trips to an open state to protect the MOV when particular transient over-voltages are detected. Transients with I 2 t ratings that are greater than the fuse rating but just below the MOV rating will blow the fuse, electrically removing the MOV from the over-voltage condition. Under circumstances where the fuse displays an I 2 t rating such that commonly occurring transients are insufficient to blow the fuse (that is, from a few to 10,000 amperes) but of insufficient magnitude to force the MOV to its low impedance state, the MOV may be subjected to overheating, possibly leading to thermal runaway. Steady state, abnormal over-voltage conditions below those at which the fuse will blow may also generate sufficiently high currents through the MOV leading to dangerous overheating.  
         [0005]     A second common technique for protecting MOVs from overheating due to abnormal steady state or transient over-voltage conditions utilizes a thermal cutoff device (TCO) provided electrically in series with the MOV. A TCO is an electrical device that senses the temperature of a surface of an object such as an electrical circuit and trips to a high impedance state (open circuit) at a particular maximum rated temperature. When a TCO is connected in series with an MOV, the TCO senses the surface temperature of the MOV and trips to an open circuit at a particular maximum rated temperature, cutting off voltage to the MOV.  
         [0006]     In order to dissipate large surge currents, the TVSS device is typically connected to a bus bar. However, the clamping performance of the surge protector may be degraded by the wide spacing of the bus bars and the additional mechanical connections required to build a bus bar type panel. The mechanical connections lead to greater resistance and thus reduced performance in conducting the surge current.  
         [0007]     Another method of dissipating surge currents is described in U.S. Pat. No. 5,303,116 (Grotz). This patent describes a surge protector whose components are mounted on a printed circuit board (PCB), with the circuit leads configured to present a large surface area, so that the surge current runs along the surface of the board.  
         [0008]     It is also desirable to gather and store information relating to voltage surge events, particularly the time and magnitude of the surge. The TVSS device therefore preferably has diagnostic circuitry with a surge sensing circuit including a peak detector. A typical peak detector uses an op-amp to amplify the transient signal to a more usable level. The output of the op-amp first goes through a forward biased diode and then to a capacitor. The diode acts as a one-way gate. The capacitor in effect holds the charge of the amplified transient signal even after the transient subsides. The diagnostic circuitry measures the voltage on the capacitor to determine the level of the transient. The capacitor requires a finite amount of time, characterized by the RC time constant, to charge up to the transient level. The longer the capacitor takes to charge, the likelier it is to not reach the peak of the transient before the transient subsides. A fast charging capacitor would be preferable, but the capacitor would then be quicker to discharge once it is measured by the diagnostic circuitry. For the signal to be usable (e.g. readable by a microprocessor), the capacitor must be capable of holding the charge long enough for the signal to be read.  
         [0009]     Accordingly, there is a need for a TVSS device which provides improved conduction and dissipation of surge currents, fast response in the diagnostic circuitry, and a surge event signal readable by a microprocessor.  
       SUMMARY OF THE INVENTION  
       [0010]     The present invention addresses the above-described need by providing a commercial/industrial TVSS which incorporates diagnostic circuitry. According to one aspect of the invention, seven modes of protection are provided for three phases of a typical electrical service, with a surge protection module for each mode. Large current transients may be handled by the device by dissipating the current in a multilayer PCB having the surge protection modules mounted thereon. The surge current passes from one layer of the PCB to another through arrays of plated through holes extending through the PCB.. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  is a schematic diagram of a surge module incorporated in a TVSS according to an embodiment of the invention.  
         [0012]      FIGS. 2A and 2B  are top and side views, respectively, of a surge module diagnostics printed circuit board (PCB) assembly.  
         [0013]      FIG. 3  is a schematic diagram of a membrane switch used in an embodiment of the invention.  
         [0014]      FIGS. 4A and 4B  are side and top views, respectively, of a surge module printed circuit board assembly.  
         [0015]      FIG. 5  is an illustration of an assembly of a surge module.  
         [0016]      FIG. 6  is an illustration of a surge module printed circuit board.  
         [0017]      FIG. 7  is a schematic diagram of a surge module diagnostics circuit, in accordance with an embodiment of the invention.  
         [0018]      FIG. 8 . is an illustration of a surge module diagnostics printed circuit board.  
         [0019]      FIG. 9A  is a schematic block diagram of a surge sensing circuit in accordance with an embodiment of the invention.  
         [0020]      FIG. 9B  is a schematic diagram of a peak detector circuit including an emitter follower, in accordance with an embodiment of the invention.  
         [0021]      FIGS. 10A and 10B  illustrate a sub-panel including a membrane switch and a display, in accordance with an embodiment of the invention.  
         [0022]      FIG. 11  is an illustration of a liquid crystal display and driver assembly incorporated in an embodiment of the invention.  
         [0023]      FIG. 12  illustrates an implementation of a liquid crystal display in an embodiment of the invention.  
         [0024]      FIG. 13  is an illustration of a main surge module assembly including seven surge modules, in accordance with an embodiment of the invention.  
         [0025]      FIG. 14  is a schematic diagram of a main surge module assembly.  
         [0026]      FIGS. 15A and 15B  are edge and plan views, respectively, of a main surge module assembly printed circuit board (PCB).  
         [0027]      FIG. 16  is an assembly drawing showing mounting of modules and components on the main surge module assembly PCB.  
         [0028]      FIGS. 17A-17E  respectively illustrate five printed circuit board layers for the main surge module assembly PCB, in accordance with an embodiment of the invention.  
     
    
     DETAILED DESCRIPTION  
       [0029]     A preferred embodiment of the invention will be described in which voltage surge protection is provided for a typical 3-phase commercial/industrial electrical service. There are seven distinct modes of surge protection for the three phases: line  1  to ground, line  1  to neutral, line  2  to ground, line  2  to neutral, line  3  to ground, line  3  to neutral and neutral to ground. A surge protection module is provided for each of these modes. As shown in  FIG. 1 , an individual surge protection module  1  (referred to herein simply as a surge module) has input/output terminals  2 ,  3  connected according to one of the seven above-noted combinations (e.g. the first module has its input connected to line  1  and its output connected to ground, etc.). Module  1  has four parallel branches, each of which has a metal oxide varistor (MOV)  4  in series with a thermal cut-off device (TCO)  5  protecting the MOV. Each module includes a double-sided printed circuit board (PCB) having the MOVs and TCOs mounted thereon. Surge protection can be provided by redirecting a surge from line  1 , line  2  and/or line  3  to ground or neutral. The number of MOVs in each module (four in this embodiment) is chosen so as to match the desired surge current rating for the overall device with the surge current rating of an individual MOV. As discussed in more detail below, the PCB is provided with a large number of plated through holes to conduct current between layers of the PCB and thus to efficiently dissipate surge currents.  
         [0030]     Each of the seven surge modules includes diagnostic circuitry mounted on a PCB  20 , as shown in  FIGS. 2A and 2B . The diagnostic circuits for the modules interface with a main board of the device through a ten pin connector. Each module has a two-color light-emitting diode (LED)  21  which indicates whether a TCO has failed; a green LED indicates that all TCOs in the module are closed, while a red LED indicates that one or more of the TCOs in the module have opened. A signal also may be sent to a main diagnostic board (described in more detail below) to provide an audible alarm and a display of the current level of surge protection (e.g. “module # 1  75% ”after one TCO has failed). The main diagnostic board preferably is connected to a panel including a membrane switch (shown schematically in  FIG. 3 ) for disabling the audio alarm, scrolling through displays relating to different modules, etc.  
         [0031]     The four MOVs  4  and TCOs  5  are mounted on a PCB  41  (as shown in  FIGS. 4A and 4B ), along with the diagnostic board  20  including the LED  21 . An assembly  10  for each of the surge modules is shown in  FIG. 5 . The module has a housing (typically of polycarbonate) including a cover  42  and a base plate  43 . The PCB  41  (see  FIG. 6 ) is plated on both sides with copper (the plating being typically 3 ounces of copper per square foot) with plated through holes providing connections from one side of the board to the other.  
         [0032]     The diagnostic circuitry for each of the seven surge modules is shown schematically in  FIG. 7 . A connector  71  connects to the four TCO/MOV combinations in the module  1  (see  FIG. 1 ), so that the status of each TCO is monitored.. In a surge event, the signal arriving through connector  71  will generally be high-voltage AC; optocouplers  77  are used to isolate the high-voltage AC from the low-voltage DC of the diagnostic circuit. The output signal from the optocouplers is buffered using NOT gates  78 , thereby providing a clean digital logic signal as to whether an individual TCO is closed or open. The four signals  72 - 1 ,  72 - 2 ,  72 - 3 ,  72 - 4  (one for each TCO of the module  1  in this embodiment) are input to an AND gate  73 . The output state of the AND gate will change when any of the four TCOs in the surge module becomes non-operational. The logical output of the AND gate  73  controls the red/green LED  74 ; the LED output is green when all of the TCOs are operational, and red when one or more are not operational. The four logic signals are also coupled to the connector  75  and thereby led to the main diagnostic board. The diagnostic circuitry shown schematically in  FIG. 7  is physically realized on the PCB  20  shown in  FIG. 8 . As noted above, this PCB is part of the assembly  10  of the surge module.  
         [0033]     The seven surge modules are mounted on a main PC board; in this embodiment, the main PC board is a six (6) layer PCB with copper plating (3 ounces/square foot) with high current carrying capability, details of which are given below. The main diagnostic board has the main diagnostic circuitry mounted thereon; this circuitry includes an open loop fast transient peak detector for measuring the phase and amplitude of a surge event, and also provides time and date stamping of the event.  
         [0034]     The surge sensing circuit in this embodiment of the invention is illustrated in the block diagram of  FIG. 9A . The surge sensing circuit has a wire  90  passing through a coil  91  to generate a current signal which is representative of the surge voltage. This signal is connected to an input terminal  97  of a peak detector circuit  92 , which in turn is coupled to a microprocessor  93 . A very fast peak detector circuit is required to detect surges having a duration in the high nanosecond (ns to low microsecond (μs) (e.g.,. 800 ns to 10 μs) range. On the other hand, the peak detector must maintain the signal level representing the surge long enough for the microprocessor to read. As noted above, a conventional open loop peak detector coupled to a capacitor has a response speed limited by the charging rate of the capacitor. In this embodiment, the problem of response speed is addressed by coupling an emitter-follower to the capacitor. As shown in  FIG. 9B , the emitter-follower  95  is a transistor where the input signal (the amplified transient from the op-amp of peak detector  92 ) is connected to the base of the transistor. The emitter is then connected to the capacitor  94 . The transistor&#39;s collector is connected to a voltage source node  96 . In this configuration, the transistor is used as a buffer to maintain the peak value of the capacitor voltage for the necessary period of time for the microprocessor to read.  
         [0035]     The microprocessor (controlled by software typically written in C) is adapted to record the date, time and magnitude of a surge event, and store the surge information on a RAM chip. The microprocessor reads the digital value of the surge and converts it to a human-readable format to be displayed on the LCD (described below). The microprocessor also monitors the TCOs in the surge modules and causes warning messages to be displayed on the LCD when surge protection has been degraded.  
         [0036]     Diagnostic information is conveyed from the main diagnostic board to the user via a sub-panel which includes a liquid crystal display (LCD) and the membrane switch ( FIGS. 10A and 10B ). In this embodiment, the main diagnostic board contains twelve LEDs  110 , visible on the sub-panel: 4 red, 4 yellow and 4 green for each line phase and neutral-to-ground. When all the TCOs for a given line are closed, the green LED is on. For example, if no faults are present between line  1  and neutral or between line  1  and ground, the green LED next to “Line  1 ” on the sub-panel will be lit (see  FIG. 10A ). When one TCO on a surge module opens, the green LED corresponding to that module will turn off and the yellow LED will turn on. When two or more TCOs on any of the surge modules open, the yellow and red LEDs corresponding to that module will turn on. When one or more TCOs open, an audible alarm will sound; the user will be able to disable the alarm by pushing a button (such as the “Reset” button  112  in  FIG. 10A ) or moving a switch to the “off” position.  
         [0037]     As shown in  FIG. 10A , and more particularly in  FIGS. 11 and 12 , the LCD is used to display the surge information and the amount of protection a surge module is providing on a specific phase. During normal operation, the LCD will display the magnitude, date and time of a surge event. When a TCO opens, the LCD will toggle between the surge information and the amount of surge protection being provided. Additional information for the user&#39;s convenience (e.g. the manufacturer&#39;s telephone number) may also be displayed.  
         [0038]     In this embodiment, the membrane switch buttons may be used to perform a number of functions: enable/disable the audio alarm; scroll through the stored surge data (using “up”and “down”buttons  120 ); download surge information; delete some or all of the stored surge information; perform diagnostics on the panel; and clear the LCD display. In addition, the user may download surge information to a computer using a standard RS-232 communication port.  
         [0039]     The surge modules on the main PC board, the main diagnostic board, and the diagnostic display sub-panel may all be conveniently located inside a standard NEMA 4X enclosure, with the user interface (including a touch pad such as the above-described membrane switch) and the LCD on the exterior of a door thereof.  
         [0040]     The main surge module assembly  130 , illustrated in  FIG. 13 , includes a sheet metal mounting plate  131 , a phenolic insulator  132 , a PCB (the main surge module board)  133 , and a thermal form cover  134 . An electrical schematic diagram of the main surge module assembly is shown in  FIG. 14 . As noted above, each of the seven surge modules  1  has a connector  75 , through which connections  141  are led to the main diagnostic board.  
         [0041]     In a preferred embodiment, the PCB  133  has six layers of plated copper (3 ounces/square foot) interspersed with insulating material (see  FIG. 15A ). These six layers, beginning with the top (component) layer  151 , are used respectively for component connections; diagnostics (low voltage); a neutral connection; a line voltage connection; chassis ground; and ground.  FIG. 15B  is a plan view of the PCB, showing how the seven surge modules, together with various other components, may be placed on the board.  FIG. 16  is a plan view of the PCB populated with those components.  
         [0042]     It is noteworthy that the main surge module PCB  133  has a large number of through holes having conductive material therein, which permit transfer of surge current to transfer from one layer of the PCB to another without degradation to the PCB. Using a PCB in high current surge applications is beneficial because of the relative short length of the traces and the close proximity of the traces which both assist in improving the clamping performance. This is an advantage over conventional arrangements using bus bars to transfer surge currents.  
         [0043]     Different plated layers of PCB  133  are shown in  FIGS. 17A-17E . Where connections are to be made from one layer to another, an array of holes is drilled completely through the PC Board. As shown in  FIG. 17A , arrays  171  of  10  holes each are drilled in several locations; array  172  has  50  holes. In other locations, arrays may have up to 70 holes. The internal cylindrical surface of each one of these holes is plated with a conductive material (copper in this embodiment) and connections are made with the six conductive layers as needed. Therefore, although the plating operation leaves a relatively thin conductive layer, the use of arrays of many through-hole connections allows a large overall current capacity. Accordingly, a large surge current may be conducted from one layer of the PCB to another, thus avoiding damage to the PCB and other parts of the device.  
         [0044]     Another benefit of this PCB arrangement derives from the parasitic capacitance of the short, closely spaced conductors in the through-hole arrays. The parasitic capacitance effectively provides an additional current path between the two closely spaced conductors. When a surge event occurs, some of the current will be shunted across the two conductors through this capacitance, thereby improving the clamping performance of the device.  
         [0045]     While the invention has been described in terms of specific embodiments, it is evident in view of the foregoing description that numerous alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, the invention is intended to -encompass all such alternatives, modifications and variations which fall within the scope and spirit of the invention and the following claims.