Abstract:
The present invention relates to a power supply device having an electromagnetic interference (“EMI”) suppression circuit connected to the input terminals. The EMI suppression circuit is mounted within the power supply device to ensure a low-impedance path to ground. Furthermore, the EMI suppression circuit can be quickly, easily and safely disconnected from ground without opening the power supply device.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to an electric power supply device having an internal electromagnetic interference filter. 
     BACKGROUND OF THE INVENTION 
     Electromagnetic interference (“EMI”) is an electrical noise current which is usually present in the radio-wave frequency range. This current originates from within a system of electrical devices rather than an outside source such as a radio signal transmitter. Electric power supplies are known to be significant generators of EMI. In connection with electric power supplies, EMI is undesirable because, for example, it can disturb the operation or degrade the performance of other equipment connected to the same source of AC power. 
     EMI filters frequently incorporate “Y” capacitors (“Y-caps”) as part of their noise suppression circuitry. These circuit elements typically connect near a power supply&#39;s input terminals and terminate at an electrical ground. By diverting common-mode current to an electrical ground, undesirable electrical noise current can be suppressed thereby preventing EMI from leaving the power supply via its input terminals and disturbing other equipment electrically connected to the same AC source. 
     In most known power supplies, EMI filtering incorporates a Y-cap circuit which is permanently connected to the power supply ground within the power supply&#39;s housing. This configuration typically allows the capacitor to be located very close to the electrical ground, thereby optimizing EMI suppression. However, the internally mounted Y-caps of conventional power supply devices are very difficult to disconnect from the power supply ground. 
     For a variety of reasons, some power supply end-users do not want a Y-cap circuit permanently connected to the power supply ground. One drawback in using a Y-cap circuit for EMI suppression is the attendant “leakage current” which flows into the electrical ground. For instance, whenever an alternating current (AC) voltage is applied across a Y-cap circuit, some amount of current will “leak” through it. 
     Some power supply users prefer a supply with minimal or no leakage current, even at the expense of reduced EMI suppression. For example, electric power supplies are frequently used to drive medical equipment. In this application of a power supply, leakage current must be reduced in order to ensure the safety of medical patients and equipment operators. 
     It is therefore an object of the present invention to provide a power supply device which has externally detachable “Y” capacitors located within the power supply. 
     It is a further object of the present invention to provide a power supply with an EMI suppression circuit which has a low-impedance path to an electrical ground. 
     SUMMARY OF THE INVENTION 
     The disadvantages of conventional power supplies have been overcome by the power supply device of the present invention. This power supply features an externally detachable, internally mounted Y-cap circuit having a low-impedance path for optimal EMI suppression. 
     One embodiment of the present invention features a power supply device including a chassis, a printed circuit board attached to the chassis, and a housing sheltering the printed circuit board. This embodiment further includes input terminals for connecting the power supply to its power source, wherein the input terminals are electrically connected to the printed circuit board, and an electromagnetic interference suppression circuit is connected to the input terminals. The electromagnetic interference suppression circuit may include capacitors electrically connected to the printed circuit board and to the input terminals. This circuit may also include a grounding mechanism of electrically conducting material detachably and electrically connected to both an electrical ground and the printed circuit board for electrically grounding the capacitors, wherein the grounding mechanism is accessible from outside the power supply device. 
     The power supply device of the present invention includes several advantages over conventional power supply devices. For example, the technique employed is advantageous because EMI can be suppressed without the need for an extra power supply terminal. A second advantageous feature of the power supply device of the present invention is the short impedance path between the capacitors in the Y-cap circuit and their corresponding connection to ground. 
     A further advantage of the power supply of the present invention is the ease with which an operator can safely and quickly detach the Y-cap circuit—which is located within the power supply&#39;s housing—without opening the housing. Typically, manufacturers construct power supplies such that it is not convenient for the customer or operator to open the housing. The power supply of the present invention allows the customer or operator to disconnect the internally mounted Y-cap circuit by simply removing a fastener accessible from outside the power supply. Thus, the Y-cap circuit can be detached easily, safely and without damaging the power supply housing, chassis or internal components. 
     A power supply device featuring externally detachable Y capacitors is particularly advantageous in two situations. In some applications, a power supply is connected to a source that has its own EMI filter. Here, the internal Y-cap circuit would be detached so that the resulting combination would not pay the leakage-current penalty twice. Disconnecting the Y-cap circuit located within the power supply causes a significant reduction in leakage current. In other applications, the device powered by the supply cannot perform optimally, or sometimes even adequately, if leakage current exists. In this second scenario, the power supply operator avoids leakage current by detaching the internal Y-cap circuit for some uses while retaining the ability to reconnect the Y-cap circuit when the power supply is put to alternative uses. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing features and other aspects of the invention are explained in the following description taken in connection with the accompanying drawings, wherein: 
     FIG. 1 is a simplified cross-sectional view of the power supply device according to one embodiment of the present invention; 
     FIG. 2 is a perspective view of the power supply device according to the embodiment of the present invention shown in FIG. 1; and 
     FIG. 3 is a simplified cross-sectional view of the power supply device according to a second embodiment of the present invention; 
     FIG. 4 is a perspective view of the power supply device according to the embodiment of the present invention shown in FIG. 3; and 
     FIG. 5 is a perspective view of the power supply device according to a third embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Various embodiments of the power supply device of the present invention will now be described with reference to the drawings. 
     With reference to FIGS. 1 and 2, the primary components of the power supply device  10  are displayed in their respective positions relative to one another. The power supply device  10  of the embodiment depicted in FIGS. 1 and 2 includes a chassis  20 , a housing  30 , input terminals  40  and a planar printed circuit board (“PCB”)  50 , each of which will now be described in greater detail below. 
     The chassis  20  functions as a base upon which various components of the power supply device are attached or affixed. The chassis  20  can be manufactured from a number of conductive materials such as a metal material. In addition, the chassis can be comprised of a single member, or multiple members. In the embodiment shown in FIGS. 1 and 2, the chassis  20  contains a dimple  60  and at least one hole  160  in the dimple  60 . 
     The housing  30  is permanently or semi-permanently attached to the chassis  20  in order to shelter the PCB  50  from the surrounding environment. This housing  30  can be manufactured from a number of materials such as metals, plastics and composite fibers, among others. It can be made of a single member, or multiple members. Furthermore, it is to be understood that the size and shape of the housing can be determined by considerations such as aesthetic appeal, arrangement and size of the other components of the power supply, and the particular application of the power supply device. In the embodiments shown in FIGS. 3-5, the housing  30  contains a dimple  65  and at least one hole  165  in the dimple  65 . 
     The PCB  50  is typically comprised of an epoxy/fiberglass laminate substrate clad with an etched sheet of copper to delineate conductive paths. The PCB  50  has a top surface  70  and a bottom surface  80 . Typically, electronic components used to generate electrical power are interconnected on the top surface  70  of the PCB  50 . Other electronic components may also be interconnected on the PCB  50 , including components used to suppress EMI. 
     In order to suppress EMI according to one embodiment of the present invention, capacitors  90  are mounted on and electrically connected to the PCB  50 . An EMI suppression circuit is created by electrically interconnecting the capacitors  90  between the input terminals  40  and an electrical ground. In the embodiment depicted in FIGS. 1 and 2, the chassis  20  serves as the electrical ground. In alternative embodiments as shown in FIGS. 3-5, the housing is used to ground the EMI suppression circuit. However, it is to be understood that the housing  30  cannot be used as an electrical ground unless it is made of electrically conductive material. 
     In order to facilitate electrical grounding of the PCB  50 , a nut  100  is physically and electrically connected to the PCB  50 . This nut  100  can be manufactured from a number of conductive materials including, for example, metal material. A number  4 - 40  broaching nut is used in the embodiment shown in FIG.  1 . 
     With reference to FIGS. 1 and 3, the capacitors  90  are electrically grounded to the chassis  20  as shown in FIG. 1 (or the housing  30  as shown in FIG. 3) by threading or otherwise inserting a fastener  110  through the hole  160  in the chassis  20  (or the hole  165  in the housing  30 ) and into engagement with the nut  100 . This fastener  110  can be manufactured from a number of conductive materials including, for example, metal material. A number  4 - 40  flat-head screw is used in the embodiment shown in FIG.  1 . In an alternative embodiment (not shown), a bolt is used. However, it is to be understood that other fasteners may be used, so that the type of fastener  110  used in the present invention need not be limited to threaded fasteners. 
     With reference to FIG.1, if no hole initially exists in the chassis  20 , the hole  160  can be created when the fastener  110  is threaded or otherwise inserted through the chassis  20  for the purpose of electrically grounding the capacitors  90 . Similarly, with reference to FIG. 3, if no hole initially exists in the housing  30 , the hole  165  can be created when the fastener  110  is threaded or otherwise inserted through the housing  30 . 
     It is to be understood that the fastener  110  can be inserted or removed with hardware equipment including without limitation: a screwdriver or wrench. However, in alternative embodiments of the present invention (not shown), the fastener  110  can also be inserted or removed without hardware equipment. 
     According to various embodiments of the present invention, the head  115  of the fastener  110  is approximately flush with the outer wall  120  of the chassis  20  (as shown in FIG. 2) or the outside wall  125  of the housing  30  (as shown in FIGS. 4 and 5) when the fastener  110  is fully engaged. However, in alternative embodiments (not shown), the head  115  of the fastener need not be flush with either the chassis  20  or the housing  30 . 
     As shown in the embodiment depicted in FIG. 1, a lockwasher  130  is disposed between the head  115  of the fastener  110  and the chassis wall  120 . This lockwasher  130  performs a number of functions. First, the teeth  140  of the lockwasher  130  scrape the paint away from the outer wall  120  of the chassis  20  (or, in another embodiment, the outside wall  125  of the housing  30 ) as the fastener  110  is being engaged with the nut  100 . This ensures a solid electrical connection between the fastener  110  and the chassis  20  (or the housing  30 ), thereby providing an electrical ground for the Y-cap circuit. Second, the lockwasher  130  firmly secures the fastener  110  in place to prevent inadvertent disengagement of the fastener  110  from the nut  100 . This lockwasher  130  can be manufactured from a number of conductive materials including, for example, metal material. A conical number  4  external tooth lockwasher is used in one embodiment. 
     In the embodiment shown in FIG. 1, a bumper  150  made of rubber, nylon or other electrically insulating material is located near the dimple  60  between the chassis  20  and the PCB  50 . This bumper prevents damage to the PCB  50  if the fastener  110  is over-tightened. 
     Although specific embodiments of the present invention have been shown and described, it is to be understood that there are other embodiments which are equivalent to the described embodiments. Accordingly, it is to be understood that the invention is not to be limited by the specific illustrated embodiments, but only by the scope of the appended claims.