Abstract:
Methods and arrangements for reducing partial discharges on a printed circuit board are provided. A method of reducing partial discharge in a printed circuit board includes providing a conducting surface coupled to a component under at least one of electrical and thermal stress, wherein the conducting surface is a metallic plate.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to methods and arrangements for reducing partial discharges on printed circuit boards, and more particularly, to methods and arrangements for reducing partial discharges on printed circuit boards used in high voltage generators. 
     In high voltage generators used in medical imaging systems, such as, for example, an X-ray generator, high voltage DC is normally generated using a multiplier/doubler circuit operating at high frequency. The output voltage of the transformer is typically in tens of kilovolts (kV) and the operating frequency is in several tens of kilohertz (kHz). The rectifier units may include cascaded configurations to achieve a higher output voltage, as a higher output power is often needed for high quality X-ray generation. The operation of the rectifier units at these very high voltage levels and frequencies may result in high electrical and thermal stresses around the components on printed circuit boards (PCB). 
     Multiplier circuits are known to include components such as transformer coils, diodes, and capacitors mounted on a PCB forming a rectifier assembly and encased for example, within a polypropylene casing filled with oil. The oil around the multiplier PCB inside the polypropylene casing acts as a coolant and insulation. 
     Components and PCBs used in high and low voltage applications are likely to operate at very high stress levels. A configuration to reduce these stress zones requires adequate clearance and creepage distance between components mounted on the PCB. The amount of clearance depends on the breakdown strength of the surrounding medium such as air or oil. The creepage distance depends on the electric stress at the PCB surface and its interface with the ambient medium. 
     Further, and for example, in a multiplier PCB, the connections, for example, the transformer secondary coil connection to the diode through solder on the PCB (triple junction formed with solder, PCB and oil) and other diode solder points form high stress zones having electric stress. These high stress zones may cause partial discharges (PD) on the surface of the PCB that may be further enhanced at high temperature. Partial discharge and high temperature stresses together deteriorate transformer oil and PCB surface. Partial discharges may cause flashover between various components on the PCB if creepage distance is not adequate or cause puncture in the PCB if there is a significant accumulation of charges due to partial discharge on the surface of the PCB. 
     Thus, known methods of component mounting and connection may not provide compact assembly of components in a PCB in high power applications. Further, these may not provide reduced partial discharge conditions on the PCB. 
     BRIEF DESCRIPTION OF THE INVENTION 
     In one embodiment, a method for reducing partial discharge in a printed circuit board, is provided. The method includes providing a conducting surface coupled to a component under at least one of electrical and thermal stress in the printed circuit board, wherein the conducting surface is a metallic plate. 
     In yet another embodiment, a method for reducing partial discharge in a printed circuit board is provided. The method includes coupling a conductive element to a high voltage connection wherein the conductive element is a corona suppressor. 
     In another embodiment, a method for reducing partial discharge in a multiplier printed circuit board (PCB) is provided, the multiple PCB having components forming graded voltage levels at different locations in the PCB. The method includes providing multiple conducting surfaces coupled to at least one component under electrical and thermal stress on the PCB within an X-ray system wherein the conducting surfaces are metal plates. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side cross-sectional view of a PCB in an X-ray system according to an embodiment of the present invention. 
         FIG. 2  is a block diagram of a metallic plate and solid dielectric arrangement according to an embodiment of the present invention. 
         FIG. 3  is a side cross-sectional view of a PCB in an X-ray system with a corona suppressor according to an embodiment of the present invention. 
         FIG. 4  is a diagram of corona suppressor elements for use with a PCB according to various embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Various embodiments of the present invention provide a method of reducing partial discharges on printed circuit boards (PCBs) used especially in high voltage generators used in an X-ray system. However, the various embodiments are not so limited, and may be implemented in connection with other systems, such as, for example, diagnostic medical imaging systems, industrial inspection systems, security scanners, particle accelerators, etc. 
       FIG. 1  is a side cross-sectional view of a PCB in an X-ray system (not shown) according to one embodiment of the present invention. The PCB  400  includes a board member  40  having a first surface  401  on a first side and second surface  402  on a second side. In the embodiment shown, the first surface  401  is the component side of the PCB  400  and the second surface  402  is the solder side of the PCB  400 . The board member  40  may have other components, for example, a diode  42  if configured as a rectifier, and soldered on the PCB  400 . It should be noted that the rectifier PCB may include serial but zig-zag mounting of diodes (not shown) and thereby have graded voltage levels along its length. 
     A connection point  46  (e.g., solder joint) at the first surface  401  may be at a high voltage and susceptible to partial discharges due to high electric field concentration. The field concentration at the connection point  46  on the first surface  401  of the PCB  400  is substantially reduced by providing a conducting surface, for example, a metallic plate  45  having the same potential as the connection point  46  below the board member  40 . In the embodiment shown in  FIG. 1 , the diode  42  also having similar voltage as the connection point  46  is electrically connected to the metallic plate  45  through leads  47  extending from the diode  42  through a dielectric  44  and connected (e.g., soldered) onto the metallic plate  45 . The metallic plate  45  is insulated from the solders at the second surface  402 . The dielectric  44  (e.g., epoxy) may be provided to the second surface  402  of the board member  40  to insulate the metallic plate  45  from the solder at the second surface  402 . The dielectric at the second surface of the board member also reduces or eliminates field concentration at various solder points on the solder side of the PCB  400 . 
     The electric field concentration at diode lead and PCB interface on the first surface may be reduced with use of plurality of metallic plates  51  as described in  FIG. 2 . The metallic plates  51  may be connected with the last diode lead in a row and to a first diode  42  in the next row (not shown in the figure). These diode leads have similar potential and are at opposite sides of the PCB  400  due to serial, but zig-zag arrangement of the diodes  42 . The metallic plates  51  are thereby maintained at an electric potential substantially equal to the potential of diodes  42  on the first surface  401 . Note that the potential of the metal plates vary depending on the voltage gradient on the top surface of the PCB  400 . The dielectric  44  is used as insulation between the plates. The thickness of the insulation between the metallic plates and solder points to reduce partial discharges on the connection point and at diode leads is less and thereby results in a compact package for high power rectifier PCB. 
     The heat generated at the connection point  46  and at the diode  42  are distributed by conduction to the metallic plates  51  in the PCB  400 . It should be noted that in one embodiment the metallic plates  45 ,  51  are configured having rounded edges to minimize the field concentration at the edges. 
     Further, in one embodiment, the dielectric  44  may be constructed or formed of an epoxy material and the metallic plates  51  may be constructed of copper. However, other materials may be used as the dielectric  44  and/or metallic plate  51  as desired or needed. For example, the dielectric may be insulating oil or any solidified or solid insulating material. 
       FIG. 3  is a side elevation view of a PCB  410  in an X-ray system according to another embodiment of the present invention. In this embodiment, the PCB  410  includes a board member  60  having a first surface  61  on a first side and a second surface  62  on a second side. The board member  60  includes a connection point, such as, for example, a solder joint  65 . A corona suppressor  68  is connected in the vicinity adjacent to or at the location of the solder joint  65 . The corona suppressor  68  includes a head  70  and may be configured as, for example, drawing board pins, paper clips, paper pins with round heads, screws, spirally wound single strand wires or other designs, and a combination thereof, as shown in  FIG. 4 . By this connection, the corona suppressor  68  shields the solder joint  65 , thereby reducing the electric field intensity at the connection point (e.g., triple junction) on the PCB  410 . More specifically. the head  70  is positioned a distance from the first surface  61  and/or the second surface  62  of the board member  60  that includes the solder joint  65  such that the head  70  shields the solder joint  65 . The corona suppressor  68  may be coupled from either the first surface  61  or the second surface  62  or both the surfaces  61  and  62 . 
     It should be noted that various forms and configurations of corona suppressors may be constructed and provided as a component on the PCB  410  as desired or needed. Corona suppressors for PCB  410  may be standardized based on voltage rating and shield effectiveness. 
     Modifications to the embodiment shown in  FIG. 3  are contemplated. For example, a corona suppressor  68  may be coupled to both the first surface  61  and second surface  62  of the board member  60 . Further, and for example, a plurality of corona suppressors  68  (shown in  FIG. 4 ) may be connected at the solder joint  65 . 
     Thus, various embodiments of the present invention provide a PCB packaging that is constructed to have more uniform electrical stress distribution for reduced partial discharge during high power applications. Additionally, various embodiments include corona suppressors for PCBs to reduce partial discharges at connection points on the PCB. 
     While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.