Abstract:
A noise suppression circuit board design to provide a power supply to an inductive heating element is disclosed. The noise suppression circuit provides a configuration for a multi-layer circuit board that has radiation noise emissions below the CISPR 11 (limits and methods of measurements of radio disturbance characteristics of industrial, scientific and medical (ISM) radio-frequency equipment) thresholds in the 30 MHz to 1 GHz frequency range.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    It is well known that electrical equipment will generate electromagnetic emissions, including radio frequency emissions of radio frequency interference (“RFI”). RFI is an electromagnetic disturbance which is generated by electrical apparatus and can be received by and effect other equipment. In power supplies, the RFI is most commonly generated by the switching devices. Steep voltage steps and high switching frequencies increase the high frequency content of RFI disturbances. The RFI disturbances can couple to other components in the power supply, producing noise radiation over a broad frequency spectrum. 
         [0002]    International basic emissions standards have been prepared by the “Comité´ international special des perturbations radioélectriques—International special committee of radio interferences” or “CISPR” and adopted by national and international authorities. CISPR 11 applies to industrial, scientific and medical electrical equipment operating in the frequency range 0 Hz to 400 GHz and to domestic and similar appliances designed to generate and/or use locally radio-frequency energy. CISPR 11 also cover emission requirements related to radio-frequency (RF) disturbances in the frequency range of 9 kHz to 400 GHz. The graph of  FIG. 10  generally depicts the CISPR 11 disturbance field strength limits. 
         [0003]    The CISPR 11 limits are applicable to inductive heating devices, such as inductive heated soldering stations that fall under the category of industrial equipment. Similar standards apply to inductive cooking system. RFI emissions from these types of inductive heating systems is a concern as the inductive heating systems include a power supply that provides a high frequency alternating (AC) current to a heater coil assembly. Also, in order to control temperature, the high frequency AC current may be pulsed on and off, providing very steep voltage steps. Accordingly, the circuit design for inductive heating devices disclosed herein contemplates isolating the causes of RFI emissions so as to attempt to minimize the RFI disturbances. 
       SUMMARY OF THE INVENTION 
       [0004]    The present invention is directed to design features that may be implemented in a multi-layer circuit board so as to reduce the radiation noise to levels compatible with the CISPER 11 requirements in the 30 MHz to 1 GHz frequency range. The circuit design features include laying out the circuit components onto sections of the circuit board and segregating the ground patterns of the copper foil of the circuit board with cut lines etched through the copper foil of the ground pattern to define respective sections or zones in the circuit board for mounting discrete circuit components. The circuit design features also include incorporating a comparatively thick heat dissipation sheet of insulation material between transistor components and a heat sink. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0005]    The following drawings are not necessarily to scale, emphasis instead being placed generally upon illustrating the principles of the invention. The foregoing and other features and advantages of the present invention, as well as the invention itself, will be more fully understood from the following description of preferred and exemplary embodiments, when read together with the accompanying drawings, in which: 
           [0006]      FIG. 1  is a perspective view of an inductive solder station, handle and an inductive heater cartridge assembly; 
           [0007]      FIG. 2  is a plan view of the top layer of a four layer circuit board suitable for providing the power supply for the inductive solder station of  FIG. 1 ; 
           [0008]      FIG. 3  is a plan view of the first mid layer of a four layer circuit board suitable for providing the power supply for the inductive solder station of  FIG. 1 ; 
           [0009]      FIG. 4  is a plan view of the second mid layer of a four layer circuit board suitable for providing the power supply for the inductive solder station of  FIG. 1 ; 
           [0010]      FIG. 5  is a plan view of the bottom layer of a four layer circuit board suitable for providing the power supply for the inductive solder station of  FIG. 1 ; 
           [0011]      FIG. 6  is a plan view of the top layer of the four layer circuit board of  FIG. 2 , showing the circuit mounting zones generally defined by the configurations of the respective ground cut lines; 
           [0012]      FIG. 7  is a plan view of the bottom layer of the four layer circuit board of  FIG. 2 , showing the circuit mounting zones generally defined by the configurations of the respective ground cut lines; 
           [0013]      FIG. 8  is a side view schematic diagram of an assembly of a transistor, thermal interface and heat sink attached to the circuit board; and 
           [0014]      FIG. 9  is a Radiation Field Strength v. Frequency chart for the inductive soldering station using the circuit board design features described herein. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]      FIG. 1  illustrates an exemplary inductive heating assembly incorporating an embodiment of the present invention. As shown in  FIG. 1 , the inductive heating assembly is depicted as an inductive soldering station  10  having a removable cabled handle  12  that may receive and power an inductive heater cartridge assembly  14 . The solder station  10  provides a high frequency AC current to the heater cartridge assembly  14  to heat and control the temperature of the tip of the heater cartridge assembly  14 . The solder station  10  includes power supply and control circuitry mounted on a four layer circuit board  20 , having circuit layout structures as shown in the plan views of  FIGS. 2 through 5 . 
         [0016]      FIG. 2  is a top side plan view of the top layer  22  of the four layer circuit board  20  suitable for mounting the electrical components providing the power supply and control circuitry for the inductive heating assembly, depicted as the inductive solder station of  FIG. 1 . As depicted in  FIG. 2 , the top layer  22  includes core  24  that may be formed from a laminated glass epoxy, and including an etched copper foil having a plurality of holes, or as commonly known in the field, a plurality of vias  26 , and circuit trace lines  28  defined by etching the copper foil to allow for the mounting and appropriate interconnection of circuit components. As also depicted in  FIG. 2 , the top layer has four ground cut lines  30 ,  32 ,  34  and  36 , respectively, that separate and isolate the ground pattern into different portions or zones of the four layer circuit board  20  and define the layout of the locations of the various circuit subsystems, as described below. The ground cut lines  30 ,  32 ,  34  and  36  extend through the copper foil to expose the underlying glass epoxy core. 
         [0017]      FIG. 3  is a top side plan view showing the features of the first mid layer  42  of the four layer circuit board  20 . As illustrated, the mid layer  42  also includes the plurality of vias  26  positioned so as to match the vias  26  of the top layer  22 . The view of the first mid layer  42  also shows the locations of the ground cut lines  30 ,  32 ,  34  and  36  that are etched through the copper foil of the top layer  22  and first mid layer  42 . While as depicted the first mid layer does not include circuit trace lines, they may be included if appropriate for the circuit design. 
         [0018]      FIG. 4  is a top side plan view of the second mid layer  52  of the four layer circuit board  20 . As illustrated, the second mid layer  52  also includes a laminate  54  and the plurality of vias  26  positioned so as to match the vias  26  of the top layer  22  and first mid layer  42  for the mounting of the circuit elements. As depicted, the second mid layer  52  also includes circuit trace lines  58 , interconnecting select vias  26 . The view of the second mid layer  52  also shows the positions of a second set of ground cut lines  60 ,  62 ,  64 ,  66  and  68 , respectively. These ground cut lines are etched through the copper foil of the second mid layer to match the ground cut lines  60 ,  62 ,  64 ,  66  and  68 , respectively etched through the copper foil of the laminated core of the bottom layer as depicted in  FIG. 5  and described below. 
         [0019]      FIG. 5  is a top side plan view of the bottom layer  70  of the four layer circuit board  20 . As illustrated, the bottom layer  70  also shows the laminated core  74  that may be formed from a glass epoxy, and the plurality of vias  26  positioned so as to match the vias  26  of the top layer  22 , first mid layer  42 , and second mid layer  52 , allowing for the mounting of the circuit components. As depicted, the bottom layer  70  also includes a plurality of circuit trace lines  78  interconnecting select vias  26 . The view of the bottom layer  70  also shows the second set of ground cut lines  60 ,  62 ,  64 ,  66  and  68 , respectively, extending through the copper foil on the laminated core  74 . 
         [0020]    As known in the art of multi-layer circuit board fabrication, the first mid layer  42  and the second mid layer  52  would be separated by a core or laminate material (not shown) to isolate the respective mid layers. The core would include the vias  26  to allow mounting of the circuit components. As may be appreciated from a comparison of the locations of the ground cut lines ground cut lines  30 ,  32 ,  34  and  36  of  FIGS. 2 and 3  as against the locations of the ground cut lines  60 ,  62 ,  64 ,  66  and  68  of  FIGS. 4 and 5 , the respective ground cut lines are not necessarily identically positioned. As depicted, the respective ground cut lines may or may not be not be identically shaped or positioned to accommodate the circuit trace lines on the respective top layer  22  and bottom layer  70  surfaces that interconnect various circuit elements. In the configuration of  FIGS. 2-5 , only ground cut line  34  on the top layer  22  and ground cut line  64  on the bottom layer  70  are identically shaped and positioned. 
         [0021]      FIG. 6  is another plan view of the top layer  22  of the four layer circuit board  20  of  FIG. 2 , showing four primary circuit mounting zones generally defined by the configurations of the respective ground cut lines. The ground cut lines  30 ,  32 ,  34  and  36  segregate the top layer  22  into four primary zones for cluster mounting the circuit components that provide various functions of the induction heating device circuitry mounted on the four layer circuit board  20 . Segregating the ground pattern of the top layer  22  into the four primary zones, separated by the ground cut lines, and strategically placing the circuit components providing the various functions required of the induction heating device decreases the RFI emissions. 
         [0022]    The first primary zone  80  comprises most of the left side (as viewed in  FIG. 6 ) of the top layer  22  of the four layer circuit board  20 . The first primary zone is roughly defined on its right side by the left-most vertical portions of ground cut lines  30 ,  32  and  36  as well as the upper horizontal portion of ground cut line  36 . Generally, the circuit components mounted in the first primary zone  80  include the power supply components for the inductive heating assembly. Accordingly, the top portion of primary zone  80  may include the DC power rectification circuitry. The middle portion of primary zone  80  may include the mode switchover circuitry and the bottom left portion of primary zone  80  may include the power source for the driver circuit. 
         [0023]    The second primary zone  82  generally comprises the section of the top layer  22  within the generally box shaped portion of ground cut line  36 , at the lower center section of top layer  22 . The circuit components mounted in the second primary zone  82  are limited to the oscillator circuitry, including a crystal oscillator that may preferably provide a 13.56 MHz output. 
         [0024]    The third primary zone  84  generally comprises the upper right corner of the top layer  22  including the section confined within the box shaped portion of ground cut line  30 , and the right corner of the top layer  22  to the right of ground cut line  30 . The circuit components mounted in the third primary zone  84  include the transistor for the inductive heater driver circuits to amplify the signal from the oscillator circuit and transistor amplifier circuits to amplify the signal derived from the driver circuit. 
         [0025]    The fourth primary zone  86  generally comprises the remainder of the top layer  22 , which physically consists of the right side of the top layer below the horizontal portion of ground cut line  30 . The fourth primary zone  86  is further segregated into sub-zones, including sub-zone  90 , bounded by the almost closed box shaped section of ground cut line  32 , sub-zone  92  positioned to the right of sub-zone  90  and ground cut line  32 , sub-zone  94  positioned below ground cut line  32  and above ground cut line  34 , and sub-zone  96  generally including the lower right corner of the top layer  22  below the ground cut line  34 . The circuit components mounted within the sub-zone  90  of the fourth primary zone  86  include an coil-capacitor circuit portion of the driver circuitry. The circuit components mounted within the sub-zones  92 ,  94  and  96  of the fourth primary zone  86  generally include a coil capacitor circuit portion of the amplifier circuit and filter circuitry in sub-zone  92  and sub-zone  94 . The circuit components mounted within the sub-zone  96  include a second filter circuit and control circuitry for the induction heating device. Each of these circuitry subsystems may include a coil-capacitor circuit component mounted on respective paired vias. The respective ground cut lines defining the sub-zones isolate the ground pattern of the coil-capacitor circuit components from one another. 
         [0026]      FIG. 7  is a plan view of the bottom layer  70  of the four layer circuit board  20  of  FIG. 5 , showing the four primary circuit mounting zones generally defined by the configurations of the respective ground cut lines. As depicted in  FIG. 7 , the first primary zone  80  and second primary zone  82  occupy the entire left side of the bottom layer  70 , bounded at approximately the center line by ground cut lines  60  and  62  and the bottom portion of ground cut line  66 . As illustrated best in  FIG. 7 , the bottom layer  70  has circuit trace  102  extending from the second primary zone  82  up to the third primary zone  84  at the top right side of the bottom layer  70 . Circuit trace  102  runs along ground cut line  62 , and in an upper portion circuit trace  102  is positioned between ground cut line  62  and ground cut line  60 . Similarly, as also depicted in  FIG. 7 , the bottom layer  70  includes circuit trace  104  extending from the second primary zone  82  up along the left side of the fourth primary zone  86  to the inductive coil-capacitor circuit portion of the driver circuitry mounted in sub-zone  90 . The circuit trace  104  is bounded between ground cut lines  62  and  66  on the bottom layer  70 . The circuit traces  102  and  104  interconnect the oscillator circuits mounted in the second primary zone  82  to the driver circuits mounted in primary zone  84  and sub-zone  90 , conducting the high frequency AC signals from the oscillator to the driver circuits. As the circuit traces  102  and  104  conducting the high frequency AC signal may be the source of RFI emissions, bounding the circuit traces  102  and  104  between the ground cut lines is believed to reduce RFI emissions. 
         [0027]    The fourth primary zone  86 , as also depicted in  FIG. 7 , includes the sub-zones  90 ,  92 ,  94  and  96 . Sub-zone  90  is generally defined by a portion of ground cut line  62  and an “L” shaped section of ground cut line  66 . Sub-zone  92  is generally defined on its left side by the portion of ground cut line  62  bounding sub-zone  90  and on the right side by the edge of the circuit board. Sub-zone  94  is generally defined by the space between ground cut line  64  and ground cut line  66 . Sub-zone  96  is generally defined as the lower right corner of the bottom layer  70 , below the ground cut line  66 . As may be appreciated from a comparison between the positions of the primary zones  80 ,  82 ,  84  and  86 , as well as the positions of the sub-zones  90 ,  92 ,  94  and  96 , the ground cut lines  60 ,  62 ,  64 ,  66  and  68  of the bottom layer  70  isolate the ground pattern of the inductive coil-capacitors mounted on the top layer  22  from one another. 
         [0028]    The electrical components providing the power supply and control circuitry for the inductive heater assembly include a plurality of transistors mounted in several locations on the top layer  22  of the circuit board  20 . The locations of the transistors have been found to contribute to the RFI emissions in certain frequencies that may be generated by the inductive heater assembly. The  FIG. 8  depicts a side view schematic diagram of an assembly of a transistor  110 , thermal interface  112  and heat sink  114  attached to the circuit board  20  that has been found to reduce or eliminate these RFI emissions. The transistor  110  includes posts  116  to allow mounting on, and electrical connections to, the circuit board  20 . Similarly, the heat sink has posts (not shown) that allow attachment to the circuit board  20 . As depicted, the transistor  110  is separated from the thermal heat sink  114  by the thermal interface  112 . The thermal interface  112  is made from a high hardness silicon rubber material suitable for acting as a thermal interface. The thick thermal interface depicted in  FIG. 8  was found to reduce the capacitive coupling as between field effect transistors  110  and the heat sink  114 , reducing the RFI emissions. The preferred thickness of the thermal interface depends upon the amount of the RFI emission level that may need to be limited. Increasing the thickness of the interface reduces the capacitive coupling, but increasing the thickness decreases the heat dissipation through the heat sink. The most suitable thickness for a particular application is thus dependent upon the desired emission suppression and required heat dissipation. In an exemplary embodiment, the thermal interface  112  has a thickness of about 0.8 mm. Preferably, the thermal interface  112  has a thickness of at least 0.7 mm and up to 1.0 mm for most applications. 
         [0029]      FIG. 9  is a chart depicting Frequency on the X-axis and RFI field strength on the Y-axis for the inductive heater assembly incorporating the features of the four-layer circuit board and circuitry described above. The chart of  FIG. 9  includes the frequency range from 30 MHz to 1 GHz. The field strength is measured from 0 dBμV/m to 60 dBμV/m, according to the CISPR 11 standard. The RFI emissions of the inductive solder station are less than the maximum field strength levels for the entire frequency range from 30 MHz to 1 GHz. 
         [0030]    Those skilled in the art will readily appreciate that the disclosure herein is meant to be exemplary and actual parameters and materials depend upon the specific application for which the process and materials of the present invention are used. For example, the detailed description herein describes a four layer circuit board, but the concepts of the disclosure can be applied to other multi-layer circuit board configurations, particularly such configurations having two or more copper foil ground patterns. In addition, one or more middle layers of the multi-layer circuit board may include a plurality of ground cut lines etched through a copper ground foil to segregate the ground portions of said copper foil into zones that may or may not match the zones of the top or bottom layers. It is, therefore, to be understood that the foregoing embodiments are presented by way of example only and that, within the scope of the appended claims and equivalents thereto; the invention may be practiced otherwise than as specifically described.