Patent Publication Number: US-2004051383-A1

Title: Switching mode current limiting power controller circuit

Description:
TECHNICAL FIELD  
       [0001] The present invention relates generally to remote power controllers for direct current power distribution systems, and more particularly, to a method and apparatus for efficiently limiting current through a remote power controller channel.  
       BACKGROUND OF THE INVENTION  
       [0002] Multi-channel remote power controller (RPC) assemblies are utilized in various direct current power distribution systems. The assemblies may include multiple RPCs corresponding to multiple load channels. The RPCs function as remote switches and circuit breakers activating and deactivating current from a single power source to various sets of loads.  
       [0003] The RPC channels may be modular and packaged together as a single unit having multiple channels paralleled to provide required power and protection levels for the various sets of loads. The single unit design allows the RPCs to be flexible and to be applied throughout a power system. The single unit design also allows for several RPC units having different total power capacities to be fabricated using similar building blocks.  
       [0004] Referring now to FIG. 1, a schematic view of a prior art RPC  10 , is shown. The RPC  10  supplies power to a load. A main switch  12  receives power and an input signal, in the form of an “ON/OFF” command, through an input terminal  16 . When the RPC  10  is in an “ON” state, as determined by state of input signal  14 , through terminal  15 , current  17  passes from the input terminal  16  through the main switch  12  to a current sensor  18 . The current  17  is monitored by the current sensor  18  and passes to an output terminal  20  before being received by a load, not shown. The main switch  12  is operated by a feedback loop  22 . The feedback loop  22  includes the current sensor  18 , a current amplifier  24 , and a driver  26 .  
       [0005] The RPC  10  has a current limiting feature in that current  17  passing through the main switch  12  is measured via the current sensor  18 , the output of which is then compared to a current reference signal  28 . When the measured current level is less than a level of the current reference signal  28 , the driver  26  fully drives the main switch  12 , assuring minimal voltage drop across the switch. When the measured current level is greater than or equal to a current level of the current reference signal  28  the driver  26  reduces drive to the main switch  12 , to control current through the switch  12  to a level nominally equal to the level set by the current reference signal  28 , preventing damage to various componentry.  
       [0006] Output of the current amplifier  24  is thresholded by a threshold comparator  25  having a bi-valued output level, which is integrated by an integrator  30 . Output of the integrator  30  is then compared to a reference time duration signal  32  by a trip comparator  34 . When the measured current level is above a predetermined value and remains above that value for a length of time that is greater than a reference time duration signal  32  a fault exists. The trip comparator  34  upon determining that a fault exists signals the driver  26  to deactivate the main switch  12 . A snubber circuit  36  is incorporated at the input terminal  16  to limit voltage transients when main switch  12  is opened under load. A freewheeling diode  38  is electrically coupled to the output terminal  20  to provide a recirculation path for the current flowing to external cable and loads, both of which having various inductance levels.  
       [0007] To prevent a fault on one load from pulling down or browning out the voltage level on other loads that are fed from the same power source, current limiting is desirable prior to tripping, referring to deactivation of a channel, and is commonly used in RPCs. Current limiting requires a feedback loop of finite bandwidth, which has a finite transport delay.  
       [0008] One disadvantage associated with existing RPC assemblies is that peak “let-through” current passing through a RPC can cause power quality degradation, dips and transients, on other channels that are operating from the same power source. Transport delay and bandwidth limitation of the feedback loop prevents instant activation of the current limiting feature. The resulting let-through current causes the dips and voltage transients on corresponding loads in turn causing the loads not to function or to malfunction. The only existing factor that limits this current let-through effect is the inductances of cables and other parasitic impedances that limit rate of rise of current during a hard fault until the current limiting loop overrides and assumes control.  
       [0009] Another disadvantage with existing RPC assemblies is resulting interactions between feedback loops. Paralleled RPCs that are supplying the same load are not only interconnected at the output terminals but also between current amplifiers and drivers, which is represented by break  40  in FIG. 1. Bandwidth of the current limiting loop is usually set as wide as practically possible to minimize let-through currents and the degradation of power quality they produce. Paralleling of two or more channels, each channel having an associated feedback loop, is likely to result in interactions between the loops due to shared impedances, including feeder and load cable inductances and load/fault impedances, that couple the loops. The interactions of the loops can result in high current ripple during current limiting, inaccurate current limiting levels, as well as inaccurate trip delay time.  
       [0010] Additionally, most current limiting loops put a RPC switch element into a “linear” mode while controlling current. Therefore, under certain fault conditions, such as a short circuit on a RPC output, the switch functions with a full source voltage across it while conducting at the current limit level. Resulting apparent power, which is equal to product of the full source voltage and the current limit level, is at a high level. This high power level cannot be sustained for any significant time, usually no longer than 30-100 ms, unless prohibitive amounts of cooling are provided, which are impractical. The stated occurrence is especially true in high voltage DC distribution systems operating with voltages greater than 100V.  
       [0011] Master-Slave topologies have been used to solve the interaction problems between current limiting loops associated with paralleled channels. Unfortunately, to use the topologies a prior knowledge of which channels are to be paralleled is required. Control interconnection requirements between the Master and Slave channels may also be required. Also using these topologies eliminates the ability to freely parallel channels.  
       [0012] Mechanical switches have been used in RPCs in DC distribution systems for many years, but are not practical without current limiting. The mechanical switches cannot avoid snubbing, absorbing transient energy as not to reach a high voltage level, and power quality degradation. Current limiting must be provided upstream from the mechanical switches, since the switches are slow to open and tend to weld closed when used to interrupt fault currents at bus voltages above 28V. Also, significant snubbing must be used to limit voltage transients when switch contacts open under load, even when there is no fault present. In order to perform current limiting upstream considerable circuitry complexity must be added to designs, which may still suffer from interactions of the current limit loops.  
       [0013] It would therefore be desirable to develop a technique for limiting current through a RPC channel without having current interaction problems, complicated snubbing circuitry, and power quality degradation.  
       SUMMARY OF THE INVENTION  
       [0014] The present invention provides a method and apparatus for controlling level of current through a remote power controller channel. A remote power controller circuit is provided including a main switch and a current limiting circuit. The main switch has a switching duty factor and is operative between a first state and a second state. The main switch when in the first state allows current to pass to an output terminal and when in the second state preventing current from passing to the output terminal. The current limiting circuit is electrically coupled to the main switch and includes a limiting inductor that controls rate of change of current flow to the output terminal. A flyback diode is electrically coupled to the limiting inductor and provides a current path for the limiting inductor to discharge when the main switch is in the second state. A method of performing the same is also provided.  
       [0015] One of several advantages of the present invention is the ability to control the turn on rate and the turn off rate of the main switch during normal, non-fault operation, thereby providing soft powering and depowering of a load. Soft powering of a load minimizes interactions and ripple effects on other channels.  
       [0016] Another advantage of the present invention is the ability to also operate the main switch quickly to achieve duty factor controlled current limiting. The ability to control the switching duty factor of the main switch increases versatility and modularity, such that the present invention may be used for an increased number of applications and aids in the prevention of damage to related circuitry and loads.  
       [0017] Furthermore, the present invention provides averaged current limiting via the limiting inductor as well as the ability to achieve stable operation of paralleled channels while current limiting. The current limiting features of the present invention eliminate the need for interconnections between parallel channels except for at output terminals.  
       [0018] Moreover, the present invention by controlling the rate of change of current passing through the output terminal eliminates the need for snubbing when the remote power controller channel trips as a result of a load fault. Thereby, reducing manufacturing costs involved in production of a remote power controller.  
       [0019] Other advantages and features of the present invention will become apparent when viewed in light of the detailed description of the preferred embodiment when taken in conjunction with the attached drawings and appended claims. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0020]FIG. 1 is a schematic view of a prior art remote power controller;  
     [0021]FIG. 2 is a schematic view of a multi-channel remote power controller assembly in accordance with an embodiment of the present invention;  
     [0022]FIG. 3 is a schematic view of a remote power controller in accordance with an embodiment of the present; and  
     [0023]FIG. 4 is a logic flow diagram illustrating a method of limiting current through the remote power controller in accordance with an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
     [0024] In each of the following figures, the same reference numerals are used to refer to the same components. While the present invention is described with respect to a method and apparatus for controlling the rate of change of current through a remote power controller channel, the present invention may be adapted to be used in various applications including: spacestations, spacecraft, satellites, aircraft, ground vehicles, ground installations, watercraft, or other applications requiring the use of remote power controllers. Although, the present invention is suitable for 100-300 volt direct current power bus applications, the present invention may also be applied in applications operating in other voltage ranges.  
     [0025] In the following description, various operating parameters and components are described for one constructed embodiment. These specific parameters and components are included as examples and are not meant to be limiting.  
     [0026] Referring now to FIG. 2, a block schematic view of a multi-channel remote power controller assembly  50  in accordance with an embodiment of the present invention, is shown. The assembly  50 , as an example, is within aircraft  52  and includes loads  54  drawing current from a common power supply  56 . A series of remote power controllers  58  are electrically coupled to the common power supply  56 , and supply current to respective loads  54 , in the form of input signals  60 . Each power controller  58  corresponds to a specific channel of N channels. The loads that require more current than a single channel is capable of providing may be supplied by multiple paralleled channels, as illustrated by load 1 . The loads  54  are electrically coupled to the power controllers  58  via an output power connector  64 . The loads  54  may be any electrical component associated with the aircraft  52 . A main controller  66  monitors telemetry signals  68  and trip latch signals  70  from the power controllers  58  and generates,command signals  72  in response to a communication signal  73 . The communication signal  73  may be generated within the controller  66  or may be received externally from another electrical component within or external to the aircraft  52 . The power controllers  58  in response to the command signals  72  supply power to the loads  54 , in the form of output signals  74 .  
     [0027] The controller  66  is preferably microprocessor-based such as a computer having a central processing unit, memory (RAM and/or ROM), and associated input and output buses. The controller  66  may be a portion of a central control unit or a stand-alone controller. The controller  66  in addition to monitoring the telemetry signals  68  and the trip latch signals  70  may also be in communication with other aircraft components. The communication may be onboard or offboard communication.  
     [0028] The controller  66  and the power controllers  58  communicate through command signals containing “ON” and “OFF” commands, command acknowledgments, and health monitoring data or status data. This is further explained in detail below.  
     [0029] Referring now to FIG. 3, a schematic view of a remote power controller circuit  58  in accordance with an embodiment of the present, is shown. The power controller  58  includes a soft switching circuit  80  and a current limiting circuit  82 . The power controller  58  may also include a tripping circuit  84  and a current limiting loop  86  as described in further detail below. The switching circuit  80  controls rate that the current limiting circuit  82  receives power. The input signal  60  is received through an input terminal  88  through an input filter  90  to the switching circuit  80 . Discontinuities of current at the input terminal  88  are smoothed by the input filter  90  to prevent other channels, operating from the same power source  56 , from suffering from out of specification ripple voltage. The current limiting circuit  82  upon receiving the power from the switching circuit  80  controls the rate of change of current flow to an output terminal  92 . The power controller  58  may also include telemetry terminals  94  and a trip status terminal  96 .  
     [0030] The switching circuit  80  includes a main switch  98 , in general having a first state and a second state that are associated with On and OFF commands within the command signals  72  and which are determinative of whether the input signal  60  passes to the output terminal  92 . The main switch  98  has a variable switching duty factor. The first state and the second state, hereinafter, are referred to as an ON state and an OFF state. The first comparator  100  has a first comparator input terminal  102 , a first reference terminal  104 , and a first comparator output terminal  106 . The first comparator  100  compares a current signal  108 , received from a current sensor  110  within the current limiting circuit  82 , with a first reference signal  112  and generates a first comparator signal  114 . A first driver  116  is electrically coupled to the first comparator output terminal  106  and switches the main switch  98  to the ON state in response to the first comparator signal  114 . A soft driver input  118 , of a soft driver  120 , is electrically coupled to a command terminal  122 , by which the switching circuit  80  receives the command signal  72 . A first soft driver output  124  is coupled to a soft-on timer  125 , which is coupled to a gate  128  and a second soft driver output  126  is electrically coupled to the main switch  98 . The soft driver  120  controls switching rate that the main switch  98  is switched between the ON state and the OFF state. The soft driver  120  overrides the first driver  116  via the gate  128 .  
     [0031] Although in a preferred embodiment of the present invention the main switch  98  is an enhancement mode n-channel power field effect transistor (FET), which uses external stimulus to generate free carriers in its conduction path to allow the main switch  98  to switch to the ON state. The main switch  98  may also be an insulated gate bipolar transistor, a junction FET, or other solid state switching device known in the art.  
     [0032] The first comparator  100  has a hysteresis characteristic. The hysteresis characteristic aids in controlling channel current by switching between stated of the main switch  12  for a time period determined by a rate of change of current and the amount of hysteresis. The hysteresis is not separate from the first comparator  100 , but is built into the comparator  100  by using a small amount of positive feedback.  
     [0033] The current limiting circuit  82  includes a flyback diode  130  and a limiting inductor  134 . The current limiting circuit  82  may also include the current sensor  110  electrically coupled to the main switch  98 , a first cathode side  129  of the flyback diode  130 , the first comparator input terminal  102 , and a limiting inductor input  132  of the limiting inductor  134 . The flyback diode  130  provides a current path for the limiting inductor  134  to discharge when the main switch  98  is in the OFF state. The limiting inductor  134  controls the rate of change of current flow to the output terminal  92 . The limiting circuit  82  may also include a free wheeling diode  136  having a second cathode side  138  electrically coupled to a limiting inductor output  140 . The free wheeling diode  136  aids in handling high frequency inductive loads. The anode sides  142  and  144  of the flyback diode  130  and the free wheeling diode  136 , respectively, are electrically coupled to ground  146 .  
     [0034] The tripping circuit  84  includes a second comparator  148  having a second comparator input terminal  150  that is electrically coupled to the current sensor  110 . The second comparator  148  also has a second reference terminal  152  and a second comparator output terminal  154 . The second comparator  148  compares the current signal  108  to a second reference signal  156  and generates a second comparator signal  158 . An integrator  160  having an integrator input  162  that is electrically coupled to the second comparator output terminal  154 , integrates the second comparator signal  158  and generates an overload time signal  164 . A third comparator  166  having a third comparator input terminal  168  that is electrically coupled to the first reference terminal  104  and the integrator output  170 , compares the overload time signal  164  to a third reference signal  172 . The third comparator  166  also has a third reference terminal  174  and a third comparator output terminal  176 . The third comparator output terminal  176  is electrically coupled to the first reference terminal  104 . A trip latch input  178  of a trip latch  180  is electrically coupled to the third comparator output terminal  176  and causes the main switch  98  to switch to the OFF state in response to the third reference signal  172 . The trip latch  180  provides a trip status signal  182  to the controller  66 , via trip status terminal  96 . A first trip latch output terminal  184  is electrically coupled to the third comparator input terminal  168 . A second trip latch output terminal  186  is electrically coupled to the soft driver input  118  and the command terminal  122 .  
     [0035] The telemetry terminals  94  include a voltage telemetry terminal  190  and a current telemetry terminal  188 . A voltage telemetry signal  192  is generated by amplifying and voltage dividing the output signal via a voltage divider  194  and a first amplifier  196 . A current telemetry signal  198  is generated by amplifying the current signal  108  via a second amplifier  200 . The voltage telemetry signal  192  and the current telemetry signal  198  are monitored by the controller  66 . The telemetry signals  68  and the trip status signals  70  may be isolated from the other circuitry using optocouplers or other isolation devices known in the art.  
     [0036] The controller  66  accounts for multiple channels supplying power to a single load in communicating with the power controllers  58 . The controller  66  sums the current telemetry signals to provide a total current signal that represents total current that a load is drawing. The voltage telemetry signals are also monitored to determine a voltage signal associated with the particular load. The voltage signal may be determined by averaging voltages at each voltage telemetry terminal or by using some other algorithm known in the art.  
     [0037] Referring now also to FIG. 4, a logic flow diagram illustrating a method of limiting current through the remote power controller  58  in accordance with an embodiment of the present invention, is shown. Operation of the remote power controller  58  may be illustrated by a combination of three state diagrams.  
     [0038] A first state diagram  207  illustrates state of the power controller  58  in response to command signal  72  and switching circuit  80 . A second state diagram  208  illustrates state of the tripping circuit  84 . The first state diagram  207  and the second state diagram  208  also illustrate control of the state of the gate  128  and the first reference signal  112 . A third state diagram  209  illustrates state of the current limiting circuit  82 . Interactions between the state diagrams  207 ,  208 , and  209  are through control of the gate  128  and in response to the first reference signal  112 .  
     [0039] Referring now to the first state diagram  207 , which illustrates normal activation and deactivation of the power controller  58 .  
     [0040] In step  210 , when the switching circuit  80  receives the command signal  72  to operate in an “ON” state, the soft driver  120  slowly increases voltage level of driver output  126 , increasing conduction of the switch  98 . While increasing conduction of the switch  98  a time reference of the soft-on timer  125  is increased, as generally indicated by step  211 . When the soft-on timer  125  reaches a pre-set timing threshold the soft-on timer  125  activates gate  128 . Following step  211 , step  214  is performed.  
     [0041] In step  214 , when the soft-on timer  125  is operated in a “Full ON” state step  215  is performed, since the gate  128  is not enabled until the switch  98  reaches full conduction, otherwise step  210  is performed.  
     [0042] In step  215 , the soft-on timer  125  activates the gate  128  at approximately the same time the switch  98  is operating at full conduction for a field effect transistor (FET).  
     [0043] In steps  212  and  213 , when the command signal  72  is no longer in the ON state the gate  128  is disabled and the switch  98  is slowly driven out of conduction by the soft driver  120 . The gate  128  allows the first driver  116  to force the switch  98  into an “OFF” state in the event of an over current condition. The gate  128  also prevents the driver  116  from forcing the switch  98  into the ON state when the switch  98  is deactivated, such as during soft ON/OFF operating conditions or when an overload trip has occurred.  
     [0044] Referring now to the second state diagram  208 , which describes the operation of the tripping circuit  84 .  
     [0045] In step  220 , when the power controller  58  is not in an overload condition, which is monitored by the second comparator  148  via the second reference signal  156 , the overload timing integrator  160  is slowly discharged. When an overload condition is detected by the second comparator  148 , overload time value of the overload integrator  160  is increased.  
     [0046] In step  223 , when overload time value of the overload integrator  160  is greater than value of a third reference signal  172  the trip latch  180  is set to a “HI” state. The third reference signal  172  corresponds to a preset limit, as detected by the third comparator  166 .  
     [0047] In step  225 , after the trip latch  180  is set, value of the first reference signal  112  is decreased to ramp down current  60  and the switch  98  is quickly switch to the OFF state, as generally indicated in step  217 .  
     [0048] In step  226 , value of the first reference signal is approximately equal to zero the gate  128  is disabled and the switch  98  is switched into the OFF state.  
     [0049] In step  227 , the tripping circuit  84  remains in the above-described state until the trip latch  180  is reset. When the trip latch  180  is reset the power controller  58  is reactivated as in a normal operating condition and step  220  is performed.  
     [0050] Referring now to the third state diagram  209 , which illustrates the operation of the current limiting circuit  82  and describes control of current limit level of the current  60 .  
     [0051] In step  216 , when the gate  128  is active and level of the current  60  is less than value of the first reference signal  112  plus one half of the hysteresis level of the first comparator  100 , the switch  98  is driven into ohmic operation. When level of the current  60  exceeds the value of the first reference signal  112  plus one half of the hysteresis level of the first comparator  100  the switch  98  is driven into the OFF state.  
     [0052] In step  217 , when the switch  98  is quickly switched to the OFF state by the driver  116  the inherent nature of the inductor  134  maintains current flow through terminal  92 . The flyback diode  130  voltage potential at cathode  129  is slightly less than voltage potential of ground  146 , resulting in a negative voltage across the inductor  134  since the voltage potential at the inductor output  140  is set by load on power controller  58 . The negative voltage across the inductor  134  results in a decrease in the inductor current.  
     [0053] In step  218 , the switch  98  remains in the OFF state until level of the current  60  decays to approximately equal one half of the hysteresis level of the first comparator  100  below level of the first reference signal  112 , at which time the switch  98  is driven into the ON state when the gate  128  is active.  
     [0054] In step  219 , when the switch  98  is switched to the ON state, the flyback diode  130  is reverse biased to deactivate the diode. With the switch  98  in the ON state, the voltage across the inductor  134  is positive, causing an increase in the inductor current. When level of the current  60  in the inductor  134  increases to one half of the hysteresis level of the comparator  100  above level of the first reference signal  112 , the power controller  58  returns to step  216 . When an overload condition on the output  92  has been cleared, level of the current  60  remains less than level of the first reference signal  112  and the switch  98  remains in the ON state until another overload condition exists or the power controller  58  is disabled. The switch  98  is unable to switch to the ON state when the gate  128  is disabled, as in steps  212  and  226 .  
     [0055] The above-described steps are meant to be a general illustrative description of the operation of the power controllers  58 , the steps may be performed synchronously or in a different order.  
     [0056] The present invention provides a single remote power controller unit design that addresses a large diverse quantity of loads without requiring physical changes to the remote power controller. The present invention also provides modular remote power controllers that reduce the volume of various different remote power controllers required for a particular application. Therefore, reducing initial design and fabrication time and costs.  
     [0057] The above-described apparatus, to one skilled in the art, is capable of being adapted for various purposes and is not limited to the following systems: space stations, spacecraft, satellites, aircrafts, ground vehicles, ships or other applications requiring the use of remote power controllers. The above-described invention may also be varied without deviating from the spirit and scope of the invention as contemplated by the following claims.