Patent Publication Number: US-2006012935-A1

Title: Transient protector circuit for multi-phase energized power supplies

Description:
FIELD OF THE INVENTION  
      This invention relates in general to the field of high energy transient protection. More particularly, this invention relates to the design of circuitry for use in power supplies to protect against high energy transients.  
     BACKGROUND OF THE INVENTION  
      The increasing usage of sensitive solid state devices in modem electrical systems, particularly computers, has given rise to concerns about transients. These concerns stem from the fact that the solid state devices are very susceptible to stray electrical transients which may be present in a distribution system. Transients in an electrical circuit result from the sudden release of previously stored energy. The severity of, and hence the damage caused by, transients depends on their frequency of occurrence, the peak transient currents, the voltages present, and their wave shapes.  
      It is common practice for power supplies to include circuitry that provides protection from high energy transients present at the input. This protection often comprises a metal oxide varistor (“MOV”) connected across the input conductors with an impedance, often a resistor, in series with one of the conductors to limit the current through the MOV.  
      The problem of providing transient protection is compounded for a supply energized from multiple phases. A traditional approach, shown in  FIG. 1 , comprises six MOVs: three between each phase and neutral, and three connected phase to phase. Current limiting resistors are connected in series with each phase. This circuit protects against transients between any of the four inputs.  
      More particularly,  FIG. 1  illustrates a prior art circuit for the prevention of transients in a three phase power supply. There are four inputs into the circuit. The four inputs are A phase, B phase, C phase, and neutral, shown as inputs  101 ,  102 ,  103 , and  104  respectively. While  FIG. 1  is described with reference to a three phase power supply, those skilled in the art will appreciate that the circuit described is extendable for use in power supplies with more than three phases.  
      The circuit employs six MOVs to protect against high transient voltages between the phases. Three MOVs are disposed between each phase and neutral, and three MOVs are connected phase to phase. MOV  151  connects A phase with B phase, MOV  152  connects A phase with C phase, and MOV  153  connects A phase with neutral. MOV  154  connects B phase with C phase, MOV  155  connects B phase with neutral, and MOV  156  connects C phase with neutral. The MOVs are typically of a size sufficient to protect against the largest conceivable transient voltage between any of the phases and between any of the phases and neutral.  
      This circuit is able to protect against transients between any of the four inputs. This circuit effectively reduces high energy transients between any of the phases and any of the phases and neutral using six MOVs to a level benign to successive power supply components.  
       FIG. 2  illustrates a circuit diagram of an additional prior art circuit for the prevention of transients in a three phase power supply. There are four inputs into the circuit: A phase, B phase, C phase, and neutral, shown in  FIG. 2  as inputs  201 ,  202 ,  203 , and  204 , respectively. While  FIG. 2  is described with reference to a three phase power supply, those skilled in the art will appreciate that the circuit described is extendable for use in power supplies with more than three phases.  
      The circuit comprises four MOVs to protect against high energy transient voltages between the four inputs: one MOV between each phase and neutral, and one connected between the positive and negative terminals of the DC output. MOV  251  connects A phase with neutral, MOV  252  connects B phase with neutral, and MOV  253  connects C phase with neutral. MOV  254  connects, through diodes  261 ,  262 ,  263 ,  264 ,  271 ,  272 ,  273 , and  274 , each of the three phases.  
      Current limiting resistors are connected in series with each phase. Resistor  231  is connected in series with the A phase input, resistor  232  is connected in series with the B phase input, and resistor  233  is connected in series with the C phase input.  
      Diodes  261 ,  262 ,  263 ,  264 ,  271 ,  272 ,  273 , and  274 , along with MOV  254 , create a circuit connecting each of the three phases together. This has the effect of routing transients between any of the three phases through the MOV  254 , thus reducing them to a level benign to successive power supply components. The diodes are of the type suited to withstand the high currents that can pass through them as a result of the transients.  
      Diode pairs  261 ,  272  and  262 ,  271  provide a path for transients between inputs  201  and  202 . Transients between inputs  201  and  202  are routed to MOV  254  through either the diode pair  261 ,  272  or  262 ,  271 , depending on the polarity of the transient. MOV  254  lies on the circuit connection between diodes  261 ,  272  and  262 ,  271 , and in conjunction with resistors  231  and  232 , limits the transient voltage to a level benign to the remainder of the circuit.  
      Similarly, diode pairs  261 ,  273  and  263 ,  271  provide a path for transients between inputs  201  and  203 . Transients between inputs  201  and  203  are routed to MOV  254  through either the diode pair  261 ,  273  or  263 ,  271  depending on the polarity of the transient. MOV  254  lies on the circuit connection between diodes  261 ,  273  and  263 ,  271 , and in conjunction with resistors  231  and  233 , limits the transient voltage to a level benign to the remainder of the circuit.  
      Furthermore, diodes  262 ,  273  and  272 ,  263  provide a path for transients between inputs  202  and  203 . Transients between inputs  202  and  203  are routed to MOV  254  through either the diode pair  262 ,  273  or  272 ,  263  depending on the polarity of the transient. MOV  254  lies on the circuit connection between diodes  262 ,  273  and  272 ,  263 , and in conjunction with resistors  232  and  233 , reduces the transient voltage to a level benign to the remainder of the circuit.  
      However, these prior art circuits have significant drawbacks. For example, the cost of MOVs are high. MOVs are significantly more expensive than the other components making up the circuit. Moreover, MOVs of sufficient strength to reduce transients in a power supply are typically very large, especially when compared to the other components in the circuits, such as resistors and capacitors. Reducing the number of MOVs used in a given circuit can dramatically reduce the overall size of the circuit and minimize the requirement for costly circuit board area.  
      What is needed is a circuit that can protect against transients in a multi-phase power supply, while at the same time being significantly less expensive, less complex, and having fewer components than prior art circuits.  
     SUMMARY OF THE INVENTION  
      The present invention is directed to circuits and systems for use in multi-phase power supplies for the control of transient currents. The present invention reduces transient currents to a level benign to succeeding power supply stages, while at the same time being less expensive and smaller than prior art circuits and systems such as those described in  FIGS. 1 and 2 .  
      According to aspects of the invention, phase to phase and phase to neutral MOVs are replaced with a single DC power supply connected MOV. Phase to phase transient currents are limited by resistors that are connected in series to each of the three phase inputs. Comprising a single DC connected MOV, an exemplary circuit provides protection from transients between any of the four inputs. The phase to phase and the phase to neutral transient currents are routed, desirably using diodes, through a single MOV connected between the positive and negative output terminals of the DC power supply. The single MOV is preferably the same size as the MOVs used in prior art circuits. Exemplary circuits and systems according to the present invention offer a net cost savings and can be implemented using significantly less circuit board area than prior methods.  
      Additional features and advantages of the invention will be made apparent from the following detailed description of illustrative embodiments that proceeds with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The foregoing summary, as well as the following detailed description of preferred embodiments, is better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there is shown in the drawings exemplary constructions of the invention; however, the invention is not limited to the specific methods and instrumentalities disclosed. In the drawings:  
       FIG. 1  illustrates a prior art circuit diagram of a circuit for protection against transients in a three phase power supply;  
       FIG. 2  illustrates an additional prior art circuit for protection against transients in a three phase power supply;  
       FIG. 3  illustrates a circuit diagram of an exemplary circuit for protection against transients in a three phase power supply in accordance with the present invention;  
       FIG. 4  is a block diagram of components comprising an exemplary circuit in accordance with the present invention; and  
       FIG. 5  illustrates a flow diagram of an exemplary method of protection in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS  
       FIG. 3  illustrates a circuit diagram of an exemplary circuit for the prevention of transients in a three phase power supply in accordance with the present invention. The circuit utilizes only one MOV  351 . This reduction in the number of MOVs results in a considerable saving of both money and circuit space. There are four inputs into the circuit: A phase, B phase, C phase, and neutral, shown in  FIG. 3  as inputs  301 ,  302 ,  303 , and  304 , respectively. While  FIG. 3  is described with reference to a three phase power supply, those skilled in the art will appreciate that the circuit described is extendable for use in power supplies with more than three phases.  
      The circuit employs one MOV  351  to protect against high energy transient voltages between the phases and between each of the phases and neutral. MOV  351  is connected between the terminals of the DC output. MOV  351  connects, through diodes  361 ,  362 ,  363 ,  364 ,  371 ,  372 ,  373 ,  374 , each of the three phases and each of the three phases and neutral. A considerable cost savings is realized through the elimination of the additional MOVs shown in the prior art. In addition, space is saved on the circuit board allowing for smaller overall circuits.  
      Current limiting resistors are desirably connected in series with each phase. Resistor  331  is connected in series with A phase, resistor  332  is connected in series with B phase, and resistor  334  is connected in series with C phase. While not shown on  FIG. 3 , an impedance may also be connected in series with the neutral input, providing addition reduction in transient energy.  
      Suitable resistors generally have a power rating from 1 to 5 watts and resistances from 20 to 100 ohms. Bulk composition type resistors such as carbon composition or ceramic composition and specially constructed wire wound resistors are preferred.  
      Diodes  361 ,  362 ,  363 ,  364 ,  371 ,  372 ,  373 ,  374 , along with MOV  351  comprise a circuit connecting each of the three phases with one another, and connecting each of the phases and neutral. This exemplary configuration routes transients through the MOV  351 , thus reducing them to a level benign to successive components of the DC power supply. The diodes are preferably of the type suited to withstand the high energy transient voltages that can pass through them.  
      Suitable diodes will have a peak reverse voltage rating equal to or greater than the maximum voltage that can be developed across the MOV and non-repetitive peak forward surge current rating (“Ifsm”) of at least  40 A. An exemplary diode having such characteristics is the General Semiconductor® DGP-15.  
      Diode pairs  361 ,  372  and  371 ,  362  provide a path for transients between inputs  301  and  302 . Transients between inputs  301  and  302  are routed through MOV  351  by either the diode pair  361 ,  372 , or  371 ,  362 , depending on the polarity of the transient. MOV  351  lies on the circuit connection between diode pairs  361 ,  372 , and  371 ,  362  and effectively reduces the transient to a level benign to subsequent DC power supply components.  
      Diode pairs  361 ,  373  and  371 ,  363  provide a path for transients between inputs  301  and  303 . Transients between inputs  301  and  303  are routed through MOV  351  by either the diode pair  361 ,  373 , or  371 ,  363 , depending on the polarity of the transient. MOV  351  lies on the circuit connection between diode pairs  361 ,  373 , and  371 ,  363  and effectively reduces the transient to a level benign to subsequent DC power supply components.  
      Diode pairs  361 ,  374  and  371 ,  364  provide a path for transients between inputs  301  and  304 . Transients between inputs  301  and  304  are routed through MOV  351  by either the diode pair  361 ,  374 , or  371 ,  364 , depending on the polarity of the transient. MOV  351  lies on the circuit connection between diode pairs  361 ,  374 , and  371 ,  364  and effectively reduces the transient to a level benign to subsequent DC power supply components.  
      Diode pairs  362 ,  373  and  372 ,  363  provide a path for transients between inputs  302  and  303 . Transients between inputs  302  and  303  are routed through MOV  351  by either the diode pair  362 ,  373 , or  372 ,  363 , depending on the polarity of the transient. MOV  351  lies on the circuit connection between diode pairs  362 ,  373 , and  372 ,  363  and effectively reduces the transient to a level benign to subsequent DC power supply components.  
      Diode pairs  362 ,  374  and  372 ,  364  provide a path for transients between inputs  302  and  304 . Transients between inputs  302  and  304  are routed through MOV  351  by either the diode pair  362 ,  374 , or  372 ,  364 , depending on the polarity of the transient. MOV  351  lies on the circuit connection between diode pairs  362 ,  373 , and  372 ,  363  and effectively reduces the transient to a level benign to subsequent DC power supply components.  
      Diode pairs  363 ,  374  and  373 ,  364  provide a path for transients between inputs  303  and  304 . Transients between inputs  303  and  304  are routed through MOV  351  by either the diode pair  363 ,  374 , or  373 ,  364 , depending on the polarity of the transient. MOV  351  lies on the circuit connection between diode pairs  363 ,  374 , and  373 ,  364  and effectively reduces the transient to a level benign to subsequent DC power supply components.  
      The MOV  351  utilized in the circuit of  FIG. 3  is preferably of the same type as those MOVs in the prior art circuits (e.g.,  FIGS. 1 and 2 ). The exemplary circuit of a  FIG. 3  replaces all of the MOVs of prior art  FIGS. 1 and 2  with a single MOV  351 . Thus, MOV  351  is preferably of a size sufficient to withstand multiple simultaneous transients between each the three phases. Suitable MOV&#39;s will typically have an energy rating in the 50 to 350 joule range and a clamping voltage rating of 900 to 1000 volts.  
       FIG. 4  is a block diagram of exemplary components comprising another exemplary circuit in accordance with the present invention. Such an exemplary circuit, which may be similar to that described with respect to  FIG. 3 , comprises several components including an input component  404 , an impedance component  423 , a diode network component  434 , a transient protection component  448 , and a DC output component  455 .  
      The input component  404  receives the multi-phase electrical input, and desirably comprises inputs for the A phase, B phase, C phase, and a neutral input. Any method, technique, or system known in the art for receiving multi-phased input into a power supply can be used. While the exemplary embodiment is described with reference an A phase, a B phase, and a C phase, the present invention is applicable for use in power supplies with greater than three phases. An exemplary input component is illustrated in  FIG. 3 , for example, as phase inputs  301 - 303  and neutral input  304 .  
      The impedance component  423  creates an impedance in series with each of the phase inputs to limit the size of transient voltages. An impedance may also be connected in series with the neutral input, providing additional reduction in transient energy. Any method, system, or technique known in the art for creating impedance in a circuit, such as a resistor may be used. An exemplary impedance component  423  is illustrated in  FIG. 3 , for example, as resistors  331 - 333 .  
      The diode network component  434  directs transient currents between any of the three phase inputs and any of the three phase inputs and neutral. In addition, the diode network component is arranged to produce full wave rectification from the AC current to produce the DC output. The diode network component  434  can comprise eight diodes in total, for example, with two diodes connected in series between each of the phase inputs, and two diodes connected in series between each of the phase inputs and neutral. An exemplary diode network component  434  is illustrated in  FIG. 3 , for example, as diodes  361 - 364  and diodes  371 - 374 .  
      The transient protection component  448  desirably reduces transient currents flowing between any of the three phase inputs and any of the three phase inputs and neutral, to a level benign to subsequent components of the DC power supply. The transient protection component  448  can comprise a single MOV connected in series, through the diode network  434 , between each of the phase inputs, and each of the phase inputs and neutral, for example. Preferably, any transient in the circuit will be directed through the single MOV and reduced. An exemplary transient protection component  448  is illustrated in  FIG. 3  as MOV  351 , for example.  
      The DC output component  455  provides DC current to an attached device.  
       FIG. 5  illustrates a flow diagram of an exemplary method of protection in accordance with the present invention. An input current is received in a multi-phased power supply at  504 , and may include transient voltages between any of the phases and any of the phases and neutral. An impedance connected in series with each of the phase inputs limits the transient current at  517 . Transient currents between any of the phases and any of the phases and neutral pass through a diode network at  536 . The transient currents pass through the MOV at  546 , where the MOV effectively reduces the transient energy to benign levels suitable for the remaining power supply components. At  558 , the benign transient leaves the circuit as output from the DC power supply.  
      More particularly, the input current is received at  504  and desirably comprises three phase inputs and a neutral input, as shown, for example, at inputs  301 - 304 , in  FIG. 3 . A transient current can arise between any of the three inputs and any of the three inputs and neutral, and are desirably reduced at  517  through application of an impedance. The applied impedance may comprise current limiting resistors connected in series with each of the phase inputs, to reduce any transient currents that may have entered the circuit. An additional resistor may also be connected in series with the neutral input. An exemplary resistor set is shown in  FIG. 3 , at  331 ,  332 , and  334 .  
      At  536 , the received current and any transient currents are routed through a diode network (e.g., the network shown in  FIG. 3  and comprising diodes  361 - 364  and  371 - 374 ). The diodes are desirably connected in series with each of the phase input and neutral. This arrangement, necessary to produce full wave rectification from the AC inputs, forces any transient currents to travel through an MOV (e.g., MOV  351  in  FIG. 3 ) connected between the terminals of the DC output, where, at  546 , they are desirably reduced to an acceptable level. As a result, the power supply DC output, at  558 , contains transients at a level that are benign to other circuits connected to the output.  
      It should be understood that the inventive principles described in this application are not limited to the components or configurations described in this application. It should be understood that the principles, concepts, systems, and methods shown in this application may be practiced with different equipment than is described in this application without departing from the principles of the invention.  
      Although illustrated and described herein with reference to certain specific embodiments, the present invention is nevertheless not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the invention.