Patent Application
20060139832
Common mode high voltage transients are typically protected by bulk capacitance, Y-capacitors, gas discharge tubes (GDT), or a combination of metal oxide varistors (MOVs) and GDTs. A problem with usage of bulk capacitance, such as Y-capacitors, for common mode protection is susceptibility to failing United Laboratories, Inc. (UL) current leakage testing. The higher the capacitance level that is sufficient to absorb the higher energies, the greater the difficulty of current leakage becomes.
The problem with gas discharge tubes (GDT) is limited life during high energy transients. GDT implementations eventually fail leaving a device unprotected.
Metal oxide varistors (MOVs) alone are not used for common mode protection due to regulatory high potential (Hi-pot) specifications. Thus, MOVs are commonly used in combination with GDTs or used in differential mode alone.
In accordance with a surge protection device, an embodiment of a filter circuit comprises a Y-capacitor, a first metal oxide varistor (MOV) coupled between a first line and the Y-capacitor, and a second metal oxide varistor (MOV) coupled between a second line and the Y-capacitor.
Embodiments of the invention relating to both structure and method of operation, may best be understood by referring to the following description and accompanying drawings whereby:
Referring to
A resistor 112 is coupled in parallel with the Y-capacitor 102 and the resulting resistor-capacitor (RC) parallel circuit 114 is connected to ground 116. The Y-capacitor 102 has a resistor R 112 in parallel to assist in discharging the Y-capacitor 102 upon the occurrence of any transient voltage spike or surge that charges the Y-capacitor 102. Both the Y-capacitor 102 and the resistor R 112 terminate to chassis ground 116.
The Y-capacitor 102 protects the metal oxide varistors (MOVs) 104, 108 from large direct current (DC) voltages such as voltages occurring during high potential testing, and permits the metal oxide varistors to discharge during high frequency, high energy transients such as transients occurring during electrical storms and inductive motor start-ups.
The first 106 and second 110 lines are typically first and second voltage inlet lines, respectively. The voltage inlet lines are otherwise termed a line voltage or active line, and a neutral line.
Referring to
In a further additional embodiment, the metal oxide varistors 204 and 208 may be thermally-protected metal oxide varistors (TMOVs). In some embodiments, internally thermally protected MOVs include a thermal fuse element inside an epoxy coating of a metal oxide varistor, in close contact with an internal ceramic disk.
Conventional surge protection circuits sometimes use bulk Y-capacitors but in a configuration that may lead to higher leakage currents, thereby risking failure of current leakage testing specifications. Generally, the higher the capacitance of the bulk capacitor, the more leakage current measured. Other conventional designs use gas discharge tubes (GDT), an implementation with limited reliability due to weakening of the device with each incurred transient. Some conventional designs use metal oxide varistors (MOVs) in series with GDTs to protect the GDTs from catastrophic failure, an implementation that adds cost and bulk.
Metal oxide varistors (MOVs) are a common type of varistor that typically contain zinc oxide grains in a matrix of other metal oxides. The zinc and metal oxides are sandwiched between two electrodes. The boundary between adjacent metal oxide grains forms a diode junction that constrains current to flow in only one direction. The randomly-oriented metal oxide grains form a mass functionally analogous to a network of back-to-back diode pairs with the pairs in parallel with many other pairs. Application of a small to moderate voltage across the electrodes causes only a small current to flow, causing a reverse leakage through the diode junctions. Application of a large voltage causes the diode junctions to break down due to avalanche effect and large current flows. A highly nonlinear current-voltage characteristic results with the MOV having a high resistance at low voltages and a low resistance at high voltages.
The metal oxide varistor (MOV), Y-capacitor, and resistor configurations depicted in
The illustrative common mode surge protection filter circuits 100, 200 facilitate passage of various United Laboratories (UL) tests including high potential (Hi-pot) testing, current leakage testing, Bi-wave testing, and Ring wave testing. The Hi-pot test involves application of a high voltage to a device to determine status of electrical insulation and may otherwise be termed voltage withstand testing, dielectric strength testing, and insulation breakdown testing.
Current leakage testing involves measurement of continuous leakage currents from an enclosure back to the system. Typically tests are performed on various combinations of open and closed earth conductors, with normal and reversed polarity conditions, and with the neutral conductor open and closed.
Bi-wave testing involves measurement of surge voltage in response to a bi-wave or two waves combined including a voltage in one time frame and current in a second time frame. In one example, an applied impulse is 6000 volts at a rise time of 1.2 microseconds (μs) and 50 μs to decay to half power followed by 3000 amperes at a rise time of 8 μs and 20 μs to decay to half power.
The ring wave surge test simulates surges resulting from switching events. For example, a 100 kHz sinusoidal wave test voltage with amplitude between 2 and 6 kV and current capabilities of 200 and 500 amperes may be applied.
Referring to
The electronic equipment unit 300 may further include at least one outlet 324 coupled to the electrical wire 306 and formed into the chassis 304. In various embodiments, the electronic equipment unit 300 can be a surge suppression device. Several examples of surge suppression devices 300 are surge suppression receptacles, entertainment power centers, rack mount surge suppressors, whole-house surge suppressors, programmable power control systems, plug-in noise filters, uninterruptible power supplies (UPS), and power strips.
Referring to
The electronic systems 400 may further include a differential transformer 404 coupled between the common mode surge protection filter circuit 302 and the at least one electronic device 402.
In some embodiments of the surge suppression devices 300 and electronic systems 400, the first 320 and second 322 MOVs may be thermally-protected metal oxide varistors (TMOVs). Similarly, first and second thermal cut-offs may be coupled between the line voltage and neutral lines and the first and second MOVs, respectively, as is shown in
Referring to
While the present disclosure describes various embodiments, these embodiments are to be understood as illustrative and do not limit the claim scope. Many variations, modifications, additions and improvements of the described embodiments are possible. For example, those having ordinary skill in the art will readily implement the steps necessary to provide the structures and methods disclosed herein, and will understand that the process parameters, materials, and dimensions are given by way of example only. The parameters, materials, and dimensions can be varied to achieve the desired structure as well as modifications, which are within the scope of the claims. For example, although the filter is described for usage with particular examples of electronic equipment units and electronic systems, the filter may be implemented in any suitable electronic equipment units a and electronic systems.
