Abstract:
A power supply is filtered while the amount of in-rush current required by the capacitive charging of the filter is limited. A resistor may be placed in parallel with a relay where the parallel combination is in series with the filtering capacitor. The relay is open upon applying power to the filter and remains open while the capacitor at least partially charges to cause charging current to pass through the resistor and thereby limit in-rush current. The relay is closed thereafter to provide a short circuit around the resistor and thereby unrestrict filtering by the capacitor. Alternatively, a common mode choke is disposed between a capacitor across input nodes and a capacitor across output nodes, whereby the size of the capacitors may be reduced to limit the in-rush current due to the common mode choke. The power supply filter may be included as part of a power distribution panel.

Description:
TECHNICAL FIELD  
         [0001]    The present invention relates to filtering of power supplies. More particularly, the present invention relates to limiting the amount of in-rush current that occurs upon applying voltage to the power supply filter.  
         BACKGROUND  
         [0002]    Power supplies are filtered to prevent noise from being transferred from the power supply to the load. Noise may emanate from various sources, including ripple from an alternating current (AC) power source that is rectified to provide direct current (DC) voltage from the power supply. Furthermore, loads that receive energy from the power supplies may emanate noise that returns to the power supply and is potentially distributed to additional loads being powered.  
           [0003]    To prevent noise from being distributed by a power supply to its associated loads, a power supply filter is used to suppress the noise. The power supply filter generally contains capacitive and/or inductive components that obstruct frequencies that contribute to the noise. To provide a steady level of DC voltage, large capacitors may be connected in parallel with the power supply to suppress any voltage ripple or other noise. When the power supply is first energized, the large capacitor may contain little or no charge and as a result a large in-rush current may occur to instantaneously charge the capacitor.  
           [0004]    For safety, power supplies employ over-current protection such as a fuse or circuit breaker. The large in-rush current that may occur may be much greater than the normal operating current being drawn by the load of the power supply. Therefore, the large inrush current blows the fuse or trips the circuit breaker, and proper operation of the power supply is inhibited.  
           [0005]    Attempts to prevent the in-rush current from aggravating the over-current protection have included operators manually connecting a resistor in series between the charge capacitor and power supply for a period of time and then removing the resistor to directly reconnect the capacitor across the power supply output. For relatively high-voltage power supplies, the charged capacitor presents a dangerous potential for electrical shock when the operator is manually handling leads from the capacitor.  
           [0006]    Thus, power supply filters are a necessary feature of power supplies, but they result in additional problems and hazards due to the in-rush current problem.  
         SUMMARY  
         [0007]    Embodiments of the present invention provide systems and methods that address the shortcomings of power supply filters resulting from the in-rush current problem. The embodiments provide various circuit elements for limiting the amount of in-rush current that exists upon energizing the power supply. The various circuit elements also allow the filtering to be unrestricted even though the in-rush current is limited to avoid tripping any over-current protection that may be provided for the power supply. The various embodiments include power supply filters positioned within power distribution panels.  
           [0008]    One embodiment involves placing a resistor in series with the filtering capacitor. A relay is in parallel with the resistor and is left in an open circuit condition upon energizing the power supply so that current passes through the resistor when charging the capacitor. Once the filtering capacitor is at least partially charged, the relay is switched to a closed circuit condition to short circuit the resistor, and thereby directly connect the filtering capacitor across the voltage nodes of the power supply that the load may be connected across. The relay may be controlled in various ways. In one embodiment, the relay is controlled based on a comparison of the voltage across the filtering capacitor to the power supply voltage. In another embodiment, the relay is controlled based on expiration of a set amount of time.  
           [0009]    Another embodiment involves placing a first filtering capacitor across first and second input nodes, a second filter capacitor across first and second output nodes, and a common mode choke between the input and output nodes. The common mode choke is disposed such that current flows between the first input node and first output node by passing in one direction through the common mode choke and current flows between the second output node and the second input node by passing through the common mode choke in a second direction opposite the first. The placement and operation of the common mode choke allows the first and second capacitors to have a relatively small capacitance. 
       
    
    
     DESCRIPTION OF THE DRAWINGS  
       [0010]    [0010]FIG. 1 is a block diagram of one embodiment where a comparison of power supply voltage to filtering capacitor voltage are compared to determine when to short circuit a relay in parallel with a charging resistor.  
         [0011]    [0011]FIG. 2 is a block diagram of one embodiment where a timer determines when to short circuit a relay in parallel with a charging resistor.  
         [0012]    [0012]FIG. 3 is a block diagram of one embodiment where a common mode choke in disposed between a filtering capacitor across input nodes and a filtering capacitor across output nodes to permit relatively small capacitance to be used.  
         [0013]    [0013]FIG. 4 is a top view of a power distribution panel employing one or more of the embodiments of FIGS.  1 - 3 .  
         [0014]    [0014]FIG. 5 is a front view of the power distribution panel of FIG. 4. 
     
    
     DETAILED DESCRIPTION  
       [0015]    [0015]FIG. 1 shows a block diagram of one embodiment where in-rush current is automatically limited. This exemplary power supply has a DC-DC converter  118  that outputs a power supply voltage across input voltage nodes  102 ,  126 . As shown in each of the embodiments of FIGS.  1 - 3 , the power supply may be configured to provide negative voltage to a load such that input node  102  is negative relative to an input node  126  and output node  110  is negative relative to an output node  112 . Alternatively, the power supply of these embodiments may be configured to provide positive voltage to the load.  
         [0016]    To provide second order filtering in this embodiment, an inductor  104  is placed in series with the parallel combination of the load connected across output nodes  110 ,  112  and a capacitor  122 . Alternatively, the inductor  104  may be omitted to provide first order filtering. To prevent in-rush current necessary for charging the capacitor  122 , a resistor  124  is placed permanently in series with the capacitor  122  so that the current to the capacitor  122  is limited upon applying power from the DC-DC converter  118 .  
         [0017]    To allow the capacitor  122  to provide unrestricted filtering during operation, a first set of contacts of a relay  120  are connected in parallel with the resistor  124 . While the capacitor is charging, the relay  120  is maintained in an open circuit state so that current is forced through the resistor  124 . However, once the capacitor  122  is charged to a satisfactory level, the relay  120  is moved to a short circuit state so that current bypasses the resistor  124 .  
         [0018]    To properly control the operation of the relay  120 , a comparator  116  and a relay control circuit  114  are included. The comparator  116  receives voltage from the output of the DC-DC converter  118 , and may receive logic power from the DC-DC converter  118  or other DC source. The comparator  116  also receives the voltage across the capacitor  122 . The comparator  116  compares the power supply voltage output from the DC-DC converter  118  to the voltage across the capacitor  116 , and outputs a signal representative of the difference between the two. When the two voltages are approximately equal, the comparator&#39;s output changes to approximately zero.  
         [0019]    The relay control circuit  114  receives logic power and the output from the comparator  116 . When the comparator output indicates a difference between the two voltages being compared, the relay control circuit  114  holds the relay  120  in the open circuit state. When the comparator output is approximately zero, the relay control circuit  114  changes states of the relay  120  to create a short circuit around the resistor  124 . The contact rating of the relay  120  is preferred to be the same as the maximum ripple current rating for the capacitor  122 .  
         [0020]    A visual indication may be provided to the operator of the power supply to indicate when charging of the capacitor  122  has completed. The visual indication includes a light emitting diode (LED)  108  in series with a biasing resistor  106  and a second set of contacts of the relay  120 . These components are connected between the output node  110  and ground  128 . When the relay  120  is in the open circuit state to force current through the resistor  124 , current does not pass through and illuminate the LED  108 . When the relay  120  switches to the short circuit state to by-pass the resistor  124 , current flows through the LED  108  to illuminate it.  
         [0021]    [0021]FIG. 2 shows a block diagram of an alternative embodiment where in-rush current is automatically limited. This embodiment employs timing of the charging rather than monitoring of the charge voltage. This exemplary power supply has a DC-DC converter  218  that outputs a power supply voltage across input voltage nodes  202 ,  226 .  
         [0022]    To provide second order filtering in this embodiment, an inductor  204  is placed in series with the parallel combination of the load connected across output nodes  210 ,  212  and a capacitor  222 . Alternatively, the inductor  204  may be omitted to provide first order filtering. To prevent in-rush current necessary for charging the capacitor  222 , a resistor  224  may be placed permanently in series with the capacitor  222  so that the current to the capacitor  222  is limited upon applying power from the DC-DC converter  218 .  
         [0023]    To allow the capacitor  222  to provide unrestricted filtering during operation, a first set of contacts of a relay  220  are connected in parallel with the resistor  224 . While the capacitor  222  is charging, the relay  220  is maintained in an open circuit state so that current is forced through the resistor  224 . However, once the capacitor  222  has been charging for a satisfactory amount of time, the relay  220  is moved to a short circuit state so that current by-passes the resistor  224 .  
         [0024]    To properly control the operation of the relay  220 , a timer  216  and a relay control circuit  214  are included. The timer  216  may receive voltage from the output of the DC-DC converter  218  to start the timer  216 , and may receive logic power from the DC-DC converter  218  or other DC source. If the DC-DC converter  218  energizes the logic power output simultaneously with energizing the normal power supply output, then the timer  216  may be started by the application of logic power so that connection to the normal power supply output is not necessary. The timer  216  outputs a first value prior to the set amount of time elapsing, and then changes to a second value thereafter.  
         [0025]    The relay control circuit  214  receives the logic power and receives the output from the timer  216 . When the timer output indicates that the set amount of time has not yet expired, the relay control circuit  214  holds the relay  220  in the open circuit state. When the timer output changes to indicate that the set amount of time has elapsed, the relay control circuit  214  changes states of the relay  220  to create a short circuit around the resistor  224 . As with the embodiment of FIG. 1, the contact rating of the relay  220  is preferred to be the same as the maximum ripple current rating for the capacitor  222 .  
         [0026]    This embodiment may also provide a visual indication to the operator of the power supply to indicate when charging of the capacitor  122  has completed. The visual indication includes a light emitting diode (LED)  208  in series with a biasing resistor  206  and a second set of contacts of the relay  220 . These components are connected between the output node  210  and ground  228 . When the relay  220  is in the open circuit state to force current through the resistor  224 , current does not pass through and illuminate the LED  208 . When the relay  220  switches to the short circuit state to by-pass the resistor  224 , current flows through the LED  208  to illuminate it.  
         [0027]    Exemplary values for the components of the embodiments of FIG. 1 and FIG. 2 are given. One skilled in the art will recognize that the values are only examples and that many other values may be used depending upon the particular application. A typical value for the resistor  124 ,  224  is 1000 ohms. A typical value for the capacitor  122 ,  222  is 12,000 micro-farads. This creates a time constant of 12 seconds, so using a 30 second timer  216  allows the capacitor  222  to charge to 95% of the power supply voltage. Longer intervals for the timer  216  are equally suitable.  
         [0028]    Due to the resistor  124 ,  224 , for a 52.2 Volt power supply, the maximum in-rush current is limited to 0.0522 Amps. This in-rush current is sufficiently low relative to a typical operating current of the power supply so that over-current protection is not tripped by the in-rush current. If no resistor  124 ,  224  is in place to limit the in-rush current for a 52.2 Volt power supply and a 12,000 micro-farad capacitor, in-rush current would reach a maximum of 626 Amps assuming a 1 millisecond reaction time for the over-current protection. The power through the resistor  124 ,  224  is 2.73 Watts with a 52.2 Volt power supply, so a 5 Watt resistor  124 ,  224  is suitable in this example.  
         [0029]    [0029]FIG. 3 shows another alternative embodiment of a power supply filter that limits in-rush current. A first capacitor  312  is connected across power supply input nodes  302 ,  314  where the power supply voltage from a DC-DC converter (not shown) is applied. A second capacitor  308  is connected across power supply output nodes  304 ,  306  that the load is also connected across.  
         [0030]    A common mode choke  310  is connected within the filter so that current flows through one winding of the common mode choke  310  in a first direction when flowing between the input node  302  and the output node  304 . Likewise, the common mode choke  310  is also connected within the filter so that current flows through a second winding of the common mode choke  310  in a second direction, opposite the first direction, as current flows between the output node  306  and input node  314 .  
         [0031]    The opposing currents within the common mode choke result in additional filtering so that the capacitors  312  and  308  may have a much smaller capacitance that results in a very small in-rush current. For example, two 1 micro-farad capacitors  312 ,  308  may be used in place of the 12,000 micro-farad capacitor  122 ,  222  of the previous embodiments. Because there is only a very minimal delay before the capacitors  308 ,  312  are charged, a visual indication is not as useful as for the previous embodiments.  
         [0032]    However, this embodiment may also provide a visual indication to the operator of the power supply to indicate when power in on. The visual indication includes a light emitting diode (LED)  318  in series with a biasing resistor  316  that is connected across the positive and negative leads. This visual indication circuit can be located anywhere in the filter assembly of FIG. 3.  
         [0033]    The various filter assembly embodiments of FIGS.  1 - 3  for limiting the in-rush current may be included in a power distribution panel. FIGS. 4 and 5 show an exemplary power distribution panel  422  contained in a housing  426  that may be mounted within a frame, rack, cabinet, chassis, or other structure (not shown). This example of a power distribution panel  422  has two power supply filters  410 ,  412  that power two different output loads. The power distribution panel  422  receives an input voltage through input connectors  402 ,  408 . The input voltage is supplied across the input nodes of each filter assembly  410 ,  412 .  
         [0034]    The filter assemblies  410 ,  412  output the filtered voltage to over-current protection devices  424 , such as fuse blocks  416 ,  418  or circuit breakers (not shown). Fuse blocks  416 ,  418  contain one or more fuses that provide over-current protection to the individual downstream devices (not shown) being powered through the filter assemblies  410 ,  412 . The filtered voltage is supplied through the fuse blocks  416 ,  418  to output voltage connectors  404 ,  406  that supply the voltage to the downstream devices.  
         [0035]    The power distribution panel  422  includes mounting brackets  414 ,  420  that secure the power distribution panel  422  within the chassis. The power distribution panel  422  also includes a faceplate  510  that supports the fuse blocks  416 ,  418  as well as LEDs. Two LEDs  502 ,  508  are connected to the respective fuse panels  416 ,  418  such that a blown fuse triggers the LED  502 ,  508  to indicate to the operator that a fuse has blown. Two LEDs  504 ,  506  are connected to the filter assemblies  410 ,  412  to illuminate when the filtering capacitors have sufficiently charged, as was discussed above.  
         [0036]    The various embodiments described above are provided by way of illustration only and should not be construed to limit the invention. Those skilled in the art will readily recognize various modifications and changes that may be made to the present invention without following the example embodiments and applications illustrated and described herein, and without departing from the true spirit and scope of the present invention, which is set forth in the following claims.