Abstract:
A shield that protects high-value input resistors in a power meter against unwanted effects due to electromagnetic interference from a nearby power supply and/or due to crosstalk from adjacent phases. The shield includes multiple printed circuit board shields that are arranged between each of the input resistors on a main printed circuit board in the power meter. Each PCB shield has a conductive layer that provides the shielding against unwanted energy. The resistors are arranged in a diagonal or parallel manner between each pair of PCB shields to prevent the resistor from movement, which prevents pin fatigue and fixes the value of the parasitic capacitance that is produced in the resistor-PCB-shield combination. In another configuration, the PCB shield is made of a flexible material, and snakes between and over the top or around the side ends of each resistor in a serpentine fashion, protecting the resistors from unwanted energies from both the top and the sides. The PCB shields disclosed herein eliminate variations in the percent error of the measurement phases, which contributes to achieving a highly accurate meter with an overall accuracy of less than 0.1%.

Description:
FIELD OF THE INVENTION 
       [0001]    The present disclosure relates generally to electronic component shielding, and, more particularly, to a printed circuit board shield assembly to shield resistors against crosstalk and interference due to electromagnetic energy produced by nearby electronic components. 
       BACKGROUND 
       [0002]    In a compact power meter, the power supply components include large capacitors, transformers, and other electronic components for converting the high-power inputs to the power meter into smaller voltages sufficient to power the sensitive electronics within the power meter. These power supply components can generate a significant amount of interference in the form of electromagnetic energy. This electromagnetic interference or EMI can adversely affect the performance of other circuits located near the power supply. Moreover, in power meters that receive multiple phases of voltage or current, the inputs are typically located right next to each other and are susceptible to crosstalk interference, where current flowing through one conductor carrying one phase of electricity can create electric and/or magnetic fields that interfere with signals passing through an adjacent conductor carrying a different phase. The overall effect of EMI interference and crosstalk interference is a degradation in the quality of the signals that are converted into corresponding digital values, resulting in a less accurate power meter. The more these original signals are degraded by interference, the less accurate the meter readings will be. The overall accuracy of a meter is expressed in terms of percent error, which is the minimum acceptable deviation by a measured voltage from the original voltage. Existing meters are typically designed to meet or exceed a percent error of 0.2% or less, but there is a need for a meter having a percent error of 0.1% or less. Aspects of the present disclosure are intended to satisfy this and other needs. 
       BRIEF SUMMARY 
       [0003]    A highly accurate power meter is achieved by reducing the effects of external influences such as EMI due to high-power components in the meter&#39;s power supply and the effects of crosstalk from adjacent phase inputs to the power meter. To do so, aspects of the present disclosure propose to insert a shield composed of one or more shielded printed circuit boards (PCBs) having a conductive material inside the rigid or flexible printed circuit boards such that the shield exists in a path of the electromagnetic energy produced by the power supply and energy from crosstalk signals in adjacent phases. In an exemplary configuration, four resistors are disposed on a main printed circuit board inside a housing of the power meter. A PCB shield is placed between each of the four resistors and one PCB shield is placed on either side of the outermost resistors. The power supply is disposed on a circuit board that is placed near (such as above) the main PCB, such that electromagnetic energy produced by high-power components of the power supply will create field lines, the strongest of which will tend to run generally across the surface of the main PCB and couple with the exposed voltage input resistors. Without a PCB shield, these fields would couple directly with the voltage signals passing through the input resistors, interfering with these signals and causing variability in the measurements. By inserting a PCB shield between each resistor and opposite the outermost resistors, a barrier is created to the electromagnetic fields produced by the power supply or other nearby electronic components, shielding the resistors from their effects. Though some fields may couple over the tops of the resistors, these fields are much weaker and can be ignored. However, an optional cover can be placed over the PCB shields and corresponding resistors to protect the resistors. The cover can also include a conductive material to provide further shielding over the tops of the resistors. 
         [0004]    A resistor sandwiched between two grounded PCB shields can look and behave like a capacitor, creating further unwanted effects on the input signals being measured by the power meter. Aspects of the present disclosure propose to angle the resistors so that they are diagonally spaced between adjacent pairs of PCB shields, forming a N-shape via each resistor and pair of PCB shields. This locks the resistor in place, which serves two purposes: First, it prevents the resistor pins from being flexed and avoids weakening of the resistor pins. Secondly, it fixes the distance between the PCB shield and the resistor, so that any parasitic capacitance created between the resistor and PCB shields will be of a fixed value, which can then be compensated for. Alternately, the resistors can be oriented so that they are parallel with adjacent sides of the PCB shields providing the components are fixed and supported to maintain consistent spacing. 
         [0005]    In another configuration, the PCB shield is composed of flexible materials, sometimes called a flex-PCB, with a flexible conductive material inside the dielectric material of the flex-PCB. In this configuration, the PCB shield is snaked over and between each resistor in a serpentine fashion to provide a shield both over and surrounding both sides of each resistor. This configuration protects each resistor from electromagnetic energy and energy due to crosstalk from both sides and from the tops of each resistor. The main PCB itself forms a barrier to any unwanted energy (e.g., EMI or crosstalk) passing through the main PCB, so it is not necessary to shield the bottoms of each resistor, as they should be generally well-shielded against this unwanted energy. No cover is proposed in this configuration, though one is not precluded either. 
         [0006]    The present disclosure without any further modifications to an existing power meter reduces the variability in the voltage input measurements from within 0.05% to less than 0.005%. Power meters using the aspects of the present disclosure will be poised to not merely satisfy but far exceed any applicable regulations, codes, or standards. 
         [0007]    The foregoing and additional aspects and implementations of the present disclosure will be apparent to those of ordinary skill in the art in view of the detailed description of various embodiments and/or aspects, which is made with reference to the drawings, a brief description of which is provided next. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    The foregoing and other advantages of the present disclosure will become apparent upon reading the following detailed description and upon reference to the drawings. 
           [0009]      FIG. 1  is a cut-away perspective view of a power meter with its housing removed to reveal a printed circuit board (PCB) assembly according to aspects of the present disclosure on a main PCB in a stacked relationship with a second PCB that includes the power meter&#39;s power supply; 
           [0010]      FIG. 2  is a schematic of example circuitry involved in converting the input voltages to corresponding digital values; 
           [0011]      FIG. 3A  is a perspective view of a top surface of the main printed circuit board shown in  FIG. 1 ; 
           [0012]      FIG. 3A-1  is a cross-sectional representation (not to scale) of one of the PCB shields shown in  FIG. 3A ; 
           [0013]      FIG. 3A-2  is a perspective representation (not to scale) of one of the resistors shown in  FIG. 3A ; 
           [0014]      FIG. 3B  is a top view of the main PCB shown in  FIG. 3A ; 
           [0015]      FIG. 4  is a perspective view of a top surface of a main printed circuit board having a flexible PCB shield according to an aspect of the present disclosure; 
           [0016]      FIG. 5A  is a chart showing the percent error when a constant voltage is applied to the inputs while a range of phase currents are applied to a power meter without the PCB assembly according to the present disclosure; and 
           [0017]      FIG. 5B  is a chart showing the percent error when a constant voltage is applied to the inputs while a range of phase currents are applied to a power meter having a PCB assembly according to the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]      FIG. 1  is an illustration of a power meter  100  with part of its housing  102  removed to reveal electronic components within the housing  102 . A printed circuit board assembly  104  is shown within the housing  102 . The printed circuit board assembly  104  includes a main printed circuit board (PCB)  106  and a PCB shield assembly  108 . A second circuit board  110  within the housing  102  includes a power supply  112 , which powers electronic components  114  on the main PCB  106 . The power supply  112  is arranged within the housing  102  in a stacked relationship relative to the main PCB  106 . A major flat surface  118  of the second PCB  110  is parallel to a major flat surface  116  (see  FIG. 3A ) of the main PCB  106 , as can be seen in  FIG. 1 . In this stacked configuration, the field lines of electromagnetic energy produced by high-power components in the power supply  112 , which conventionally includes capacitors, transformers, and rectifiers, will tend to emanate away from the power supply  112  and then curve back toward the main PCB  106 , creating field lines that run across the major surface  116  of the main PCB  106 . Similarly, energy due to crosstalk interference will emanate from one resistor to another due to their proximity to one another on the main PCB  106 . The PCB shield assembly  108  blocks these unwanted energies from affecting the voltages (or currents) as they travel from the inputs of the power meter  100  to electronic components on the main PCB  106 . Although the illustrated example shows the second PCB  110  in a stacked, parallel relationship with the main PCB  106 , in other configurations the power supply  112  and the main PCB  106  can be located relative to one another in other relationships, such as co-planar or in a staggered relationship. The PCB shield assembly  108  should be arranged on the main PCB  106  to block the strongest field lines in unwanted electromagnetic energy produced by the power supply  112  and/or in unwanted crosstalk interference between adjacent inputs to the power meter  100 . 
         [0019]    The printed circuit board assembly  104  includes at least one high-value precision resistor  300   a  (see  FIG. 3A ) disposed on the main printed circuit board  106  and electrically coupled to a corresponding input  200   a  (see  FIG. 2 ) to the power meter  100 . The first input  200   a  carries a current or a voltage measured by the power meter  100 . In the illustrated examples, the power meter  100  measures voltages, but in other configurations contemplated by the present disclosure, the power meter  100  can measure current or both current and voltage. In  FIG. 3A , four high-value precision resistors  300   a - d  are shown, but the present disclosure is intended to cover at least one high-value precision resistor, even though four are shown in the illustrated examples. The high-value precision resistors  300   a - d  have a very high value, for example on the order of mega-ohms, and in the illustrated example shown in  FIG. 3A , can have a rectangular shape, though the present disclosure is not limited to any particular form factor for the resistors. The resistors  300   a - d  are capable of ranging the line input voltage, such as on the order of 240V or 480V nominal, to a level that is acceptable for input into an analog-to-digital (A/D) converter (ADC)  202  (shown in  FIG. 2 ), such as on the order of millivolts or less than 5V. 
         [0020]    The PCB shield assembly  108  includes at least two PCB shields  302   a - b , or, as shown in  FIG. 3A , five PCB shields  302   a - e , depending on the number of resistors  300  used. For example, in a configuration in which two resistors  300   a,b  are used, three PCB shields  302   a - c  are used. As shown in  FIG. 3A , because there are four voltage input resistors  300   a - d , there are five PCB shields  302   a - e , one on either end of the resistors  300   a - d , and one in between each pair of resistors as can be seen from the top view of the main PCB  106  shown in  FIG. 3B . Each PCB shield  302   a,b,c,d,e  includes a conductive layer  305  disposed within an electrically insulating dielectric substrate  306  as can be seen from the cross-sectional view of a PCB shield  302  in  FIG. 3A-1 . A height dimension, h 1 , of the PCB shield  302  is at least equal to a height dimension, h 2 , of the resistor  300  relative to the major surface  116  of the main PCB  106 , such that h 1 &gt;h 2 . The major surface of each of the PCB shields  302   a - e  has a length L 1  (see  FIG. 3B ) that is at least as long as the length L 2  of the major surface  308   a - d  of each of the resistors  300   a - d , such that L 1 &gt;L 2 . 
         [0021]    The PCB shield  302  is secured to the main PCB  106  such that the resistor  300   a  is arranged between two major surfaces  304   a,b  of the PCB shield assembly  108 . A distance between the resistor  300  and each of the two surfaces is less than a longest dimension of the resistor. The term “major surface,” as used herein, refers to the largest contiguous surface relative to all of the surfaces of a particular component to which the major surface belongs. Each PCB shield  302  includes two major surfaces, one on either side of the PCB shield  302 . A major surface  308   a,b,c,d  along the longest dimension, L 2  (see  FIG. 3A-2 ), of each of the resistors  300   a,b,c,d  is oriented relative to adjacent pairs of the PCB shields  302  in a non-parallel manner. In the illustration shown in  FIG. 3B , the resistors  300   a,b,c,d  are oriented in a diagonal manner between adjacent pairs of the PCB shields  302   a,b,c,d,e , such that each resistor-PCB-pair combination forms an N-shape (or a backwards N-shape depending on perspective). Orienting the resistors  300  relative to the PCB shield pairs  302  in this manner prevents the resistor  300  from physically moving. This orientation keeps the resistor pins from flexing and breaking. Also, since the capacitance is dependent on the distance between two conductive materials, fixing the distance between the resistor and PCB pair will produce a non-varying parasitic capacitance which can then be compensated for. 
         [0022]    The length, L 1  (see  FIG. 3B ), of the PCB shield  302  is at least as long as the length, L 2 , of the major surface of the resistor  300 . Each of the PCB shields  302   a,b,c,d,e  includes a set of pins or one or more tabs that are soldered to a ground plane of the main PCB  106 . They are intended to keep the PCB shields  302  fixed to the main PCB  106 , in addition to connecting the conductive layers  305  inside each of the PCB shields  302   a - e.    
         [0023]    In the configuration illustrated in  FIG. 3A , the outermost PCB shields  302   a,e  block unwanted electromagnetic energy produced by the power supply  112  located above the resistors  300 , which will tend to emanate away from the power supply  112  and then circle back toward the main PCB  106 , running along its major flat surface  116 . Likewise, the internal PCB shields  302   b,c,d  located between each of the resistors  300   a,b,c,d  will block unwanted crosstalk energy between adjacent resistors. The field lines will be weaker at the exposed tops of each of the resistors  300   a,b,c,d , so in some configurations, no further shielded is needed to protect the exposed tops of the resistors  300   a - d.    
         [0024]    A cover  310  ( FIG. 3A ) is disposed over the PCB shields  302   a - e  and the resistors  300   a - d . The cover  310  can be made of any electrically insulating dielectric material. Optionally, a conductive layer can be incorporated within the cover  310  to provide further shielding against EMI from coupling over the tops of the resistors  300   a - d . The cover  310  includes a first set of indentations  312   a - e  opposing a second set of indentations  314   a - e  offset from the first set of indentations  312   a - e  such that each of the indentations  312 ,  314  corresponds to a space between the resistor and respective ones of the PCB shields. The offset is necessary because the resistors  300   a - d  are arranged diagonally relative to each of the PCB shields  302   a - e . The cover  310  is used to keep the resistors  300  and the PCB shields  302  securely in place on the main PCB  106 , and can also be used to further shield the resistors  300  against EMI produced by the power supply  112  or other EMI-producing components within the power meter  100 . The cover  310  also protects any protruding pins from the nearby second PCB  110  from contacting any part of the resistors  300 , providing an additional level of protection to the voltage input resistors  300 . 
         [0025]      FIG. 2  is a schematic illustration of a digital conversion circuit  200  on the main PCB  106 . The digital conversion circuit  200  converts voltages being monitored by the power meter  100  into corresponding digital values in the A/D converter  202 . The circuit  200  receives four voltage inputs, labeled A, B, C, and REF in  FIG. 2 , and numbered  200   a,b,c,d , respectively. The first three voltage inputs A, B, and C, correspond to different phases of the input voltage being monitored by the power meter, and these phases are typically labeled as A, B, and C, each one lagging or leading the other by 120 degrees. The resistors  300   a,b,c,d  are physically housed in a package, which in the example shown in  FIG. 3A  has a generally rectangular shape, with at least one resistor in the package. For example, the resistor  300   a  includes a voltage input resistor RN 10 A, having a value of 5MΩ, and a feedback resistor RN 10 B, having a value of 13 kΩ. Similarly, the resistor  300   b  includes a voltage input resistor RN 9 A, having a value of 5MΩ, and a feedback resistor RN 9 B, having a value of 13KΩ. The resistor  300   c  includes a voltage input resistor RN 8 A, having a value of 5MΩ, and a feedback resistor RN 8 B, having a value of 13KΩ. The resistor  300   d  includes a voltage input resistor RN 7 A, having a value of 5MΩ, and a voltage divider resistor RN 7 B, having a value of 13KΩ. These values are exemplary only, and as mentioned above, the values of the voltage input resistors should be set to a value sufficient to range the input voltage from the line(s) to which the power meter  100  is connected to values acceptable to be input into the A/D converter  202 . By incorporating the feedback resistors RN 10 B, RN 9 B, RN 8 B, and RN 7 B into the same package as the voltage input resistors RN 10 A, RN 9 A, RN 8 A, and RN 7 A, the amplifier outputs are less susceptible to relative changes in value due to temperature or time. 
         [0026]    The voltages present at the inputs  200   a,b,c,d  are reduced, commensurate with the value of the resistors  300   a,b,c,d , to corresponding input voltages V 1 _IN, V 2 _IN, V 3 _IN, and VN_IN, which are received at respective amplifiers  204   a,b,c,d . The amplifiers  204   a,b,c,d  amplify the corresponding input voltages to produce amplified input voltages, labeled V 1 _SIG, V 2 _SIG, V 3 _SIG, and VN_SIG. The amplified input voltages are received by corresponding low-pass filter blocks  206   a,b,c,d  to produce filtered input voltages V 1 _FILT, V 2 _FILT, V 3 _FILT, and VN_FILT. These filtered input voltages are received by the A/D converter  202 , which conventionally convert the input voltages to corresponding digital values indicative of the original voltages received on inputs  200   a,b,c,d.    
         [0027]    As mentioned above, even though four resistors  300   a - d  are shown in the drawings, the present disclosure is not limited to four-resistor configurations. For example, in a three-resistor configuration, three high-value precision resistors, such as resistors  300   a - c , are disposed on the main PCB  106  and electrically coupled to corresponding inputs, such as inputs  200   a - c , to the power meter  100 . Each of the inputs  200   a,b,c  carry different phases of a current or a voltage being measured by the power meter  100 . The PCB shield assembly  108  includes four (instead of five used in the four-resistor combination) PCB shields, such as the PCB shields  302   a,b,c,d  each having a conductive layer  305  disposed within an electrically insulating dielectric substrate  306  and arranged on the main PCB  106  such that each of the at three resistors  300   a,b,c  is arranged between at least a pair of the PCB shields  302  to minimize crosstalk between adjacent resistors. A major surface  308   a,b,c  of each of the three resistors  300   a,b,c  is oriented in a non-parallel manner relative to a major surface of adjacent pairs of the PCB shields  302   a,b,c,d , which face the respective resistors  300   a,b,c . For example, as shown in  FIG. 3A , the resistors  300   a,b,c  are arranged in a diagonal manner between adjacent pairs of the PCB shields  302   a,b,c,d  such that each resistor-PCB-shield-pair combination forms a generally N shape (or backwards N shape). 
         [0028]    Turning now to  FIG. 4 , a single, flexible PCB shield  402  is shown instead of five separate PCB shields  302   a - e  shown in  FIG. 3A . Like each of the PCB shields  302   a - e , shown in  FIG. 3A-1 , the flexible PCB shield  402  includes a flexible conductive material, such as copper, sandwiched between an insulating dielectric material or substrate so that the conductive material acts as a shield to protect the resistors  400   a - d  against unwanted energies due to electromagnetic interference produced by the power supply  112  or due to crosstalk from adjacent phase(s). The resistors  400   a - d  are just like the resistors  300   a - d  shown and described in connection with  FIGS. 3A-3B , except that the resistors  400   a - d  are arranged parallel to one another so that each resistor  400   a,b,c,d  can be received within corresponding U-shaped spaces  412   a,b,c,d  formed in the serpentine-shaped, flexible PCB shield  402 . Each U-shaped portion  412   a,b,c,d  of the flexible PCB shield  402  extends over the tops of each of the resistors  400   a,b,c,d  and in between each adjacent pair of resistors  400 . By snaking over and in between each of the resistors  400 , the flexible PCB shield  402  operates to protect the resistors  400  like the cover  310  shown in  FIG. 3A  against protruding pins from the second circuit board  110  located above the flexible PCB shield  402  and to shield the resistors  400  on all exposed sides against unwanted energies, such as EMI energy from other electronic components within the power meter  100  or crosstalk energy from adjacent phases. Alternately, instead of snaking the flexible PCT shield  402  over and in between each of the resistors  400 , the flexible PCT shield  402  can snake around the sides of and in between each of the resistors  400 . 
         [0029]    Turning now to  FIGS. 5A and 5B , two plots are illustrated comparing the variability in percent error seen by each voltage phase input to a power meter without the PCB shield assembly  108  described in the present disclosure ( FIG. 5A ), against the variability in percent error seen by each voltage phase input to the power meter  100  with the PCB shield assembly  108  described herein ( FIG. 5B ). In  FIGS. 5A and 5B  three voltage phases, A, B, C were measured downstream of the input resistors to the power meter and being monitored by the power meter and being compared against an external reference. The plot shows current on the x-axis because these voltage measurements were taken while various currents were being applied on each of the three current phases (not shown on the plots) over the course of approximately 10 minutes. This effectively shows a plot of voltage phase error over time. In  FIG. 5A , the percent error on the y-axis is caused by EMI and resistor crosstalk, resulting in the percent error for one of the phases always exceeding 0.01%, and the other two phases changing in magnitude of error over the course of the test. This test was conducted where the input voltage was held at 120V at 60 Hz, 25 degrees C., using a 0.5 power factor (PF). 
         [0030]    The second plot shown in  FIG. 5B  was taken under the same test conditions as those for the plot shown in  FIG. 5A , except that now the PCB shield assembly  108  shown in  FIG. 3A  is installed. With the PCB shield assembly  108  installed, a dramatic reduction in the percent error and the variations in error during the test can be seen. All three phases consistently exhibit a percent error of less than 0.005%. 
         [0031]    Although the illustrated examples above have been described in connection with a power meter  100 , aspects of the present disclosure can be applied to any electronic device having electronic components susceptible to interference due to electromagnetic energy produced by other electronic components within the device and/or to crosstalk interference due to nearby electronic components. 
         [0032]    While particular implementations and applications of the present disclosure have been illustrated and described, it is to be understood that the present disclosure is not limited to the precise construction and compositions disclosed herein and that various modifications, changes, and variations can be apparent from the foregoing descriptions without departing from the spirit and scope of the invention as defined in the appended claims.