Abstract:
A method and apparatus for isolating and/or eliminating RFI, EMI, and noise transients in power supply circuits. The circuit isolates DC to DC and AC to DC power coupling by use of dual, parallel current paths that are alternately gated. The gating of the switches is controlled by a control module, which ensures that the input and output of the circuit is electromagnetically blocked during any instant in time, while simultaneously ensuring proper and continuous delivery of current to the circuit output load.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
   This application claims the benefit of U.S. Provisional Application Ser. No. 60/371,614, filed Apr. 10, 2002. 

   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   Not applicable. 
   REFERENCE TO A MICROFICHE APPENDIX 
   Not applicable. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates generally to electrical circuits, and more specifically to an improved method and associated power supply circuit that provides circuit protection from AC line noise, transient electromagnetic interference, EMI, and RFI. 
   2. Discussion of the Related Art Including Information Disclose under 37 CFT 1.97 and 37 CFR 1.98 
   Switched-mode power supplies make use of power transistors or power MOSFETs and typically require the use of a control module. However, the purpose and operation of the semi-conductor switches and of the control module are entirely different from that in the present invention. The primary purpose of a switched-mode circuit is that of voltage regulation, while the primary purpose of the inventive circuit is noise and transient isolation. 
   Switching supplies eliminate the need for a main power transformer and large storage filter capacitors. Instead they rectify voltage directly from the AC input and store DC in a small capacitor. Then, current is released in pulses by a transistor—controlled by a PWM (pulse width modulated) controller. 
   Switched-mode supplies have the advantages of light weight, small size and high efficiency. However, they may not be suitable when the primary requirement for the power supply is low noise. The high frequency switching harmonics generated by such a supply require extensive filters to achieve a low noise ratio. In addition, switched-mode supplies do not block line-noise or transients as in the inventive circuit. When the switched-mode rectifiers are in a conduction state and the switching transistor is on, input line noise may pass through to the output. Additionally, the line-connected rectifiers generate significant power harmonics that are kicked-back onto the power line. 
   Switched-mode supplies do not utilize dual parallel current paths as in the inventive circuit. However, the inventive concepts herein could be added or adapted to various switched-mode designs, which would give them noise-blocking capabilities. 
   “Synchronous rectification” circuits such as those disclosed in U.S. Pat. Nos. 6,188,592, 5,818,704, 6,252,781 and 6,256,214 may bear some cursory resemblance to the inventive circuit. However, they are similar only in that their fundamental operation requires two controlled switch elements, such a power MOSFET. They do not utilize dual parallel current paths as in the inventive circuit. 
   Synchronous rectifiers are usually used in DC-DC voltage conversion where the circuit is designed to convert a DC voltage to a lower voltage. A primary example is the “Buck converter”. However, the purpose of synchronous rectifiers is to convert voltage, while the purpose of the inventive circuit is noise and interference isolation. 
   BRIEF SUMMARY OF THE INVENTION 
   The present invention provides a method and apparatus for isolating and/or eliminating RFI, EMI, and noise transients in power supply circuits. The inventive circuit isolates DC to DC and AC to DC power coupling by “gating” current rather than utilizing conventional filtering methods. The inventive circuit is comprised primarily of “dual parallel” current paths that are alternately gated to ensure continuous current delivery. 
   The dual parallel paths are created with four switches and two storage elements (e.g., capacitors) where each path is defined by two switches and a capacitor. The “switches” may be a relay, solid-state relay, SCR, power MOSFET or other electronic switch whose on/off state may be controlled. The gating of the switches is controlled by a control module, which ensures that the input and output of the inventive circuit is electromagnetically blocked during any instant in time, while simultaneously ensuring proper and continuous delivery of current to the circuit output load. 
   The inventive circuit provides near 100% isolation and immunity to power line transmitted noise, transients, RFI and EMI. The inventive circuit eliminates the need for filter coils and high frequency filter capacitors often required for noise reduction in conventional linear power supply designs. The inventive circuit eliminates both common-mode and differential-mode power line noise from entering the power supply. Additionally, the inventive circuit prevents noise generated by the output load circuitry from being “kicked back” onto the DC input or AC power line. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       FIGS. 1   a  and  1   b  are schematic views of a power supply circuit of this invention in a linear fullwave power supply; 
       FIG. 2  is a schematic view of a power supply circuit of this invention within a linear fullwave rectified power supply; 
       FIGS. 3   a  and  3   b  are schematic views of a power supply circuit of this invention in a buffered DC voltage supply; and 
       FIG. 4  is a schematic view of a power supply circuit of this invention in a linear fullwave bridge rectified power supply. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1   a  and  1   b  are schematic views of a power supply circuit of this invention in a linear fullwave power supply  10 . In operation, the switches  12 ,  14 ,  16 , and  18  are controlled by the control module  20  in such a manner that current path A is charged by the input voltage, while current path B provides current to the circuit output or load L, and vice versa: as the stored power level of path A begins to drop, the control module  20  “gates” the switches allowing current path A to charge, while the fully charged path B provides current to the output. 
   During the charge cycle of path A ( FIG. 1   b ), path A&#39;s second (post-storage) switch  14  is open, blocking transients and noise from the AC input to the output. Simultaneously, path B&#39;s second (post-storage) switch  18  is closed, supplying current to the output load L, while path B&#39;s first switch (pre-storage)  16  is in an off condition, blocking AC input transients and noise to the output. 
   Conversely, during the charge cycle of path B ( FIG. 1   a ), path B&#39;s second switch  18  is open, blocking transients and noise from the AC input to the output. Simultaneously, path A&#39;s second switch  14  is closed, supplying current to the output load L while path A&#39;s first (pre-storage) switch  12  is in open condition, blocking AC input transients and noise to the output. 
   In the inventive circuit, the control module ensures that the following states are true:
         Path A&#39;s first switch  12  and second switch  14  are never on at the same time   Path B&#39;s first switch  16  and second switch  18  are never on at the same time   Path A&#39;s second switch  14  and Path B&#39;s second switch  18  are never on at the same time   Path A&#39;s second switch  14  and Path B&#39;s second switch  18  are alternately gated to supply current to the load   Path A&#39;s first switch  12  and Path B&#39;s first switch  16  are alternately gated to charge Path A storage (capacitor  22 ) and Path B storage (capacitor  24 ) respectively.       

     FIG. 2  is a schematic view of a power supply circuit of this invention within a linear fullwave rectified power supply  30 . The rectifier diodes  32  and  34  serve a dual function, providing AC voltage rectification and acting as path A first switch and path B first switch, respectively. SCRa  36  and SCRb  38  provide the functions of path A second switch and path B second switch, respectively. Diode  32 , capacitor  40 , and SCRa  36  define current path A, while diode  34 , capacitor  42 , and SCRb  38  define current path B. 
   The control module  44  gates the switches synchronized to the AC line frequency. Sense lines SLa  46  and SLb  48  provide a sense signal for the control module by detecting the AC voltage polarity changes. Trigger lines TLa  50  and TLb  52  control the SCR switches. The control module ensures that SCRa  36  and SCRb  38  are never on at the same time. 
   Since diodes  32  and  34  are free running rectifiers, their conduction state is determined by the AC input frequency. Therefore, the control module must simply determine the conduction state of the rectifiers (sense lines SLa  46  and Slb  48 ) and synchronize the opening and closing of SCRa  36  and SCRb  38  to the conduction states of the rectifier diodes  32  and  34 . Essentially, the control module  44  ensures that SCRa  36  and SCRb  38  are never on at the same time. Also the control module ensures that SCRa  36  only turns on after diode  32  goes to an off state. Similarly, the control module ensures that SCRb  38  only turns on after diode  34  goes to an off state. These conditions ensure that AC line input transients and noise are blocked to the circuit output. 
   In this case, the control module  44  is a simple circuit that detects the incoming AC polarity changes and triggers the corresponding SCR. The control module is comprised of resistor  50 , diode  52 , resistor  54 , resistor  56 , diode  58 , and resistor  60 . 
   The size of capacitors  40  and  42  must be selected carefully to ensure that they can provide sufficient worst case current to the load. The size of power supply storage capacitor  62  must be chosen to ensure continuous current delivery due to the fact that there is a transition period (near the AC zero crossing) where both SCRa  36  and SCRb  38  will be in an off condition. 
     FIGS. 3   a  and  3   b  are schematic views of a power supply circuit of this invention in a buffered DC voltage supply  70 . Essentially the circuit splits the DC input into two current paths that are alternately charged and discharged. Each path has an independent storage element, which is represented by capacitors  72  and  74 . Current path A is comprised of path A first switch  76 , path A second switch  78 , and capacitor  72 , while current path B is comprised of path B first switch  80 , path B second switch  82 , and capacitor  74 . 
   The switches  76 ,  78 ,  80 , and  82  may be any power switches that can be gated such as power MOSFETs or other state controlled switches. 
   In operation, the switches are controlled by the control module  84  in such a manner that path A is charged by the input, while path B provides current to the circuit output or load L. As the stored power level of path A begins to drop, the control module “gates” the switches allowing current path A to charge, while the fully charged path B provides current to the output. The switching frequency may be synchronized to the AC line frequency or a higher frequency may be used depending upon other design requirements. 
   Path A Charge/Path B Discharge Cycle: Path A first switch  76  is closed while Path A second switch  78  is open allowing Path A capacitor  72  to be charged by the input voltage. Path A second switch  78  being open prevents source EMI from being coupled through path A to the output. At the same time, Path B first switch  80  is closed while Path B second switch  82  is open allowing Path B capacitor  74  to provide current to the load L. Path B first switch  80  being open prevents source EMI from being coupled through path B to the output. 
   Path B Charge/Path A Discharge Cycle: Path B first switch  80  is closed while Path B second switch  82  is open allowing Path B capacitor  74  to be charged by the input voltage. Path B second switch  82  being open prevents source EMI from being coupled through path B to the output. At the same time, Path A first switch  76  is closed while Path A second switch  78  is open allowing Path A capacitor  72  to provide current to the load. Path A first switch  76  being open prevents source EMI from being coupled through path A to the output. 
   Multiple circuits of this invention may be used to provide isolation between various circuit elements or PCBs. For example, one circuit would provide isolation to a digital circuit while a second circuit would provide isolation to an analog circuit. Due to the inventive circuit&#39;s unique blocking action, the noise generated by one circuit section cannot be kicked-back onto the main power supply. 
     FIG. 4  is a schematic view of a power supply circuit of this invention in a linear fullwave bridge rectified power supply  90 . At the output of the bridge rectifier  92 , the inventive circuit splits the DC voltage into dual parallel current paths. The control module  94  alternately gates the switches Path A first switch  96  and Path B first switch  98  charging capacitors  100  and  102  respectively. The control module  94  supplies current to the output by alternately gating Path A second switch  104  and Path B second switch  106 . 
   The control module ensures that AC line noise and rectifier noise are blocked to the output by ensuring that the following conditions are maintained.
         Path A&#39;s first switch  96  and second switch  104  are never on at the same time   Path B&#39;s first switch  98  and second switch  106  are never on at the same time   Path A&#39;s second switch  104  and Path B&#39;s second switch  106  are never on at the same time   Path A&#39;s second switch  104  and Path B&#39;s second switch  106  are alternately gated to supply current to the load   Path A&#39;s first switch  96  and Path B&#39;s first switch  98  are alternately gated to charge Path A storage (capacitor  100 ) and Path B storage (capacitor  102 ) respectively.       

   The control module may be clocked at the AC line frequency or a higher frequency depending upon the desired output requirements. 
   While this invention has been described in connection with preferred embodiments thereof, it is obvious that modifications and changes therein may be made by those skilled in the art to which it pertains without departing from the spirit and scope of the invention. Accordingly, the scope of this invention is to be limited only by the appended claims.