Abstract:
A power supply system includes a power supply; a load coupled to the power supply via a power supply line to receive a voltage therefrom; and a circuit protection device. The circuit protection device includes at least one switch device coupled between the power supply and the load on the power supply line; a first controller coupled to the at least one switch for: A. monitoring current flow through the at least one switch; B. maintaining the at least one switch in an ON state while current flows through the at least one switch in a first direction; and C. causing the at least one switch to toggle to an OFF state if current flowing through the at least one switch flows in a second direction and a second controller coupled to the power supply line between the power supply and the at least one switch and coupled to the at least one switch for sensing an amount of current flowing between the power supply and the at least one switch and causing the at least one switch to toggle to the OFF state when the current sensed by the second controller exceeds a reference value.

Description:
FIELD OF THE INVENTION 
   The present invention is directed generally to power supply systems and more particularly to a method and system for providing fault protection in a power supply system. 
   BACKGROUND OF THE INVENTION 
   High-availability data storage systems require multiple power supplies which are used to supply redundant power to certain components of the data storage system via, in particular the disk drive array, via a common power bus. Each power supply may also supply power to other subsystem components in a non-redundant manner. In order to prevent faults in one of the power supplies or subsystem components from adversely affecting the performance of the components in the rest of the data storage system, these systems typically include fault protection devices which prevent faults in the common load from affecting the operation of the power supplies and which prevent faults in a power supply from affecting the operation of the other subsystem components. 
     FIG. 1  is a schematic diagram of a prior art power supply system  100  including a plurality of power supplies  102 A,  102 B,  102 N for supplying redundant power to a common load  104  via common power bus V 100 . Each of power supplies  102 A,  102 B,  102 N also supply power in a non-redundant manner to loads  106 A,  106 B and  106 N, respectively. Each branch  108 A- 108 N, which delivers power from each power supply  102 A- 102 N to the common power bus V 100  includes fault protection devices  110 A- 110 N and  116 A- 116 N, respectively, which are connected to their respective power supplies  102 A- 102 N via power supply lines V 100 A-V 100 N. 
   Fault protection devices  110 A- 110 N provide over-current protection which prevents a fault in the common load from adversely affecting the operation of the associated power supply and the subsystem component loads  106  connected to the associated power supply. For example, a short in the common load would cause an excessive forward current in the power supply lines V 100 A-V 100 N. This excessive forward current in supply lines V 100 A-V 100 N could cause the associated power lines (V 200 A-V 200 N and/or V 300 A-V 300 N and/or Vn_A-Vn_N) to either droop or cease to operate altogether if the power supply has internal overcurrent protection. Over-current protection devices  110 A- 110 N may include a passive device, such as a fuse  112 A- 112 N, which trips when the current in power supply line V 100 A-V 100 N exceeds a reference value. Alternatively, over-current protection devices  110 A- 110 N may include an active device  114 A- 114 N. Since each active device  114 A- 114 N is identical in configuration and operation, only active device  114 A will be described. Active device  114 A includes a resistor  130 A connected in series with a semiconductor switch  132 A, such as a MOSFET. The voltage drop across the resistor  132 A, which corresponds to the amount of current flowing in power supply line V 100 A, is processed in an amplifier/comparator  134 A, which has its output connected to the control terminal of the semiconductor switch  132 A. When the current in power supply line V 100 A exceeds a reference value (corresponding to the maximum acceptable current flow), the semiconductor switch  132 A is shut off by the output of the amplifier/comparator  134 A, thus preventing the excessive forward current from continuing to flow. 
   Fault protection devices  116 A- 116 N provide an ORing function which prevents a fault, such as a short, in the associated power supply  102 A- 102 N, which would cause a reverse current in the common power bus V 100 , from affecting the operation of the common load  104 , by dragging down the common power bus V 100 . ORing devices  116 A- 116 N may include a passive device, such as a diode  118 A- 118 N, which would block any reverse current in the common power bus V 100 . However, such a diode may have a forward voltage drop across it that may be unacceptable for the power supply system. 
   Alternatively, a single N-channel MOSFET ORing device  120 A- 120 N may be connected in series between the over-current protection device  110 A- 110 N and the common power bus V 100  to provide the ORing function. By connecting the source terminal of the MOSFET to the power supply side and the drain terminal to the common power bus V 100 , current cannot travel in a reverse direction, from the common power bus V 100  to the power supply, unless the gate terminal is driven to the ON state by the ORing controller. As is known in the art, the ORing controller monitors the direction of current flow through the MOSFET, creates a sufficient gate voltage via a charge pump circuit to keep the MOSFET in the ON state when current is flowing in the forward direction, from the power supply to the common load and toggles the MOSFET to the OFF state if current begins to flow in the reverse direction. Such an ORing controller can be built using many different discrete components or an “off-the-shelf” controller, such as the MAX8535 controller from Maxim Integrated Products of Sunnyvale, Calif., may be used, with minimal external components, to provide the ORing function to the MOSFET. 
   Another alternative design of the ORing controller is shown at  122 A- 122 N. This design includes a pair of MOSFETs connected in series, but in opposing directions to each other, between the over-current protection device  110 A- 110 N and the common power bus V 100 . This design, in addition to providing the ORing function, provides a “soft start” feature, which limits the current demanded from the load at power-up. The soft start feature allows the power-up current to increase gradually by transitioning the MOSFETs at a predetermined rate from the OFF state to the ON state. Since the internal body diode in the MOSFET provides a forward current path through the MOSFET, even when the MOSFET is in the OFF state, the single MOSFET device  120 A- 120 N cannot be used to provide the soft-start feature, which is achieved by connecting the second MOSFET in series with the first MOSFET, but in the opposite direction as the first MOSFET. An ORing controller, such as that described above, can be configured to ramp up the gate voltages for both MOSFETs gradually, thus allowing the pair to operate in the linear region during power-up. 
   While the above configurations of the over-current protection devices and ORing devices provide the desired protection features, the number of components needed to provide the over-current and ORing functions increases the cost of the system and degrades the electrical and thermal efficiency of the system. 
   SUMMARY OF THE INVENTION 
   The present invention is directed to a power supply system which provides fault protection including over-current protection and an ORing function with the soft-start feature. The system operates to prevent excessive forward currents caused by a fault in a load commonly powered by a number of power supplies from affecting the operation of the power supplies. The system also prevents reverse currents, caused by faults on the power supplies from affecting the commonly-powered load. The system includes a reduced number of components compared to the prior art, resulting in a lower-cost system having improved electrical and thermal efficiency. 
   According to one aspect, a power supply system includes a power supply; a load coupled to the power supply via a power supply line to receive a voltage therefrom; and a circuit protection device. The circuit protection device includes at least one switch device coupled between the power supply and the load on the power supply line; a first controller coupled to the at least one switch for: A. monitoring current flow through the at least one switch; B. maintaining the at least one switch in an ON state while current flows through the at least one switch in a first direction; and C. causing the at least one switch to toggle to an OFF state if current flowing through the at least one switch flows in a second direction and a second controller coupled to the power supply line between the power supply and the at least one switch and coupled to the at least one switch for sensing an amount of current flowing between the power supply and the at least one switch and causing the at least one switch to toggle to the OFF state when the current sensed by the second controller exceeds a reference value 
   The at least one switch may include a pair of MOSFETs. The first controller may include a first input coupled to the power supply line between the pair of MOSFETs and the power supply, a second input coupled to the power supply line between the pair of MOSFETs and the load, and an output coupled to gate terminals of the pair of MOSFETs, wherein, when the output is in a first state, the pair of MOSFETs is in the ON state and when the output is in a second state, the pair of MOSFETs is in the OFF state. The second controller may include a current sensing device coupled to the power supply line between the power supply and the pair of MOSFETs for sensing the current in the power supply line and outputting a sensed voltage corresponding to the sensed current, a comparing device for comparing the sensed voltage to a reference voltage and outputting a first output when the sensed voltage exceeds the reference voltage and a switch coupled between the comparing device and the gate terminals of the pair of MOSFETs, wherein the switch, upon receiving the first output of the comparing device, operates to toggle the pair of MOSFETs to the OFF state. The first controller may include a timer device and, upon receiving the first output from the comparing device, the second controller switch operates to disable the timer device, thus driving the output of the first controller from the first state to the second state, causing the pair of MOSFETs to toggle to the OFF state. The first controller may include an undervoltage protection device and, upon receiving the first output from the comparing device, the second controller switch operates to enable the undervoltage protection device, thus driving the output of the first controller from the first state to the second state, causing the pair of MOSFETs to toggle to the OFF state. Upon receiving the first output from the comparing device, the second controller switch may operate to pull the control terminals from the first state to the second state, causing the pair of MOSFETs to toggle to the OFF state. 
   According to another aspect, a power supply system includes a power supply; a load coupled to the power supply via a power supply line to receive a voltage therefrom; at least one switch device coupled between the power supply and the load on the power supply line; a first controller coupled to the at least one switch for causing the at least one switch to toggle to an OFF state if current flowing through the at least one switch flows in a direction opposite a normal operating current direction; and a second controller coupled to the power supply line between the power supply and the at least one switch and coupled to the at least one switch for sensing an amount of current flowing between the power supply and the at least one switch and causing the at least one switch to toggle to the OFF state when the current sensed by the second controller exceeds a reference value. 
   According to another aspect, a method of providing fault protection in a power supply system, the method includes: 
   A. monitoring a current flowing from a power supply to a load via a power supply line; 
   B. toggling a switch device coupled between the power supply and the load in the power supply line from an ON state to an OFF state when the current begins to flow from the load to the power supply; 
   C. monitoring the amplitude of the current flowing in the power supply line; and 
   D. toggling the switch device from the ON state to the OFF state when the amplitude of the current in the power supply line exceeds a reference value. 
   According to yet another aspect, a fault protection system, includes means for monitoring a current flowing from a power supply to a load via a power supply line; means for toggling a switch device coupled between the power supply and the load in the power supply line from an ON state to an OFF state when the current begins to flow from the load to the power supply; means for monitoring the amplitude of the current flowing in the power supply line; and means for toggling the switch device from the ON state to the OFF state when the amplitude of the current in the power supply line exceeds a reference value. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features of the invention will become more readily apparent from the following detailed description when read together with the accompanying drawings, in which: 
       FIG. 1  is a schematic diagram of a Prior Art power supply system having discrete over-current protection and ORing devices; 
       FIG. 2  is a schematic diagram of the power supply system according to the present invention having integrated over-current protection and ORing devices; 
       FIG. 3  is a schematic circuit diagram of one embodiment of the fault protection device of the present invention; 
       FIG. 4  is a schematic circuit diagram of a second embodiment of the fault protection device of the present invention; 
       FIG. 5  is a schematic circuit diagram of a third embodiment of the fault protection device of the present invention; and 
       FIG. 6  is a schematic circuit diagram of a fourth embodiment of the fault protection device of the present invention. 
   

   DETAILED DESCRIPTION 
     FIG. 2  is a schematic diagram of a power supply system  10  including a plurality of power supplies  12 A,  12 B,  12 N for supplying redundant power to a common load  14  via common power bus V 1 . Each of power supplies  12 A,  12 B,  12 N also supply power in a non-redundant manner to loads  16 A,  16 B and  16 N, respectively. Each branch  18 A- 18 N, which delivers power from each power supply  12 A- 12 N to the common power bus V 1 , includes fault protection devices  20 A- 20 N, respectively, which are connected to their respective power supplies  12 A- 12 N via power supply lines V 1 A-V 1 N. 
   Since fault protection devices  20 A- 20 N are identical in configuration and operation, only fault protection device  20 A will be specifically described. Fault protection device  20 A includes an over-current detection device  22  coupled in series with switches Q 1  and Q 2 , which are preferably MOSFETs, between the power supply  12 A and the common power bus V 1 . MOSFETs Q 1  and Q 2  are connected in opposing directions to each other and have their gate terminals commonly connected. As shown in  FIG. 2 , MOSFET Q 1  has its source terminal coupled to the over-current detection device  22  and its drain terminal coupled to the drain terminal of MOSFET Q 2 . The source terminal of MOSFET Q 2  is coupled to the common power bus V 1 . It will be understood that the direction in which each of the MOSFETs Q 1  and Q 2  are connected does not matter, as long as they are connected in opposite directions with respect to each other. A gate control device  24  is coupled to the gate terminals of MOSFETs Q 1  and Q 2 , the upstream side (supply side) of MOSFETs Q 1  and Q 2 , the downstream side (V 1  side) of MOSFETs Q 1  and Q 2 , and to an output of the over-current detection device  22 . 
     FIGS. 3-6  are schematic diagrams showing different embodiments of the fault protection device  20 A. In each of the embodiments, the configuration and operation of the gate control device  24 , absent the influence of the output of the over-current detection device  22 , is identical. The gate control device  24  includes an ORing controller  26 , such as the MAX8535 controller from Maxim Integrated Products of Sunnyvale, Calif. The Gate output of the controller  26  is coupled to the gate terminals of the MOSFETs Q 1  and Q 2  through gate resistor Rg, for providing a gate voltage V G  thereto. The Gate output is also coupled to ground through capacitor Cg. The input current pin CS of the controller  26  is connected to the common power bus V 1  and the Vcc input is connected to the V 1 A power supply line. Under voltage protection pin UVP is coupled to the V 1 A power supply line through a voltage divider including resistors Rx and Ry and the Timer pin is coupled to ground through resistor Rt. 
   In operation, the gate control device  24  determines when MOSFETs Q 1  and Q 2  should be toggled to the ON state and the OFF state, based on current conditions present in the common power bus V 1  and the power supply line V 1 A. The voltage on power supply line V 1 A is divided down by resistors Rx and Ry. This voltage is then input to the UVP pin of the controller  26  and compared to a UVP reference voltage level. If the divided voltage input to the UVP pin of the controller  26  drops below the UVP reference voltage level, indicating that no or low voltage is present on the power supply line V 1 A, the controller  26  switches the MOSFETs Q 1  and Q 2  to the OFF state by disabling the Gate output. When the divided voltage input to the UVP pin of the controller  26  is above the reference voltage level, the Gate output is enabled to provide the gate voltage V G  to the gate terminals of MOSFETs Q 1  and Q 2  to toggle the MOSFETs Q 1  and Q 2  to the ON state. 
   The rate at which the MOSFETs Q 1  and Q 2  are turned on is a function of the rate at which the gate voltage V G  is ramped up. The values of resistor Rg and capacitor Cg, as well as resistor Rt, which controls the internal charge pump frequency of the controller  26 , are manipulated to determine how quickly the gate voltage V G  is ramped up. As described above, controlling the rate at which the gate voltage V G  increases allows the power-up current to increase gradually by transitioning the MOSFETs at a predetermined rate from the OFF state to the ON state. This is the “soft-start” feature which limits the current demanded from the load at power-up. This “soft-start” feature is particularly useful when there is system capacitance on the Common Load Bus (V 1 ) that would demand a surge current at power-up if it were not limited by the “soft-start” feature. 
   The controller  26  also measures the voltage drop across the MOSFETs Q 1  and Q 2  by comparing the voltages received at its CS and VCC pins. Through the measurement of the voltage drop, the controller  26  determines the direction of current flow through the MOSFETs Q 1  and Q 2 . When the controller  26  determines that current is flowing through the MOSFETs Q 1  and Q 2  in the direction indicated by arrow  30 , the MOSFETs Q 1  and Q 2  are maintained in the ON state. However, if the controller  26  determines that current is flowing through the MOSFETs Q 1  and Q 2  in the direction opposite that indicated by arrow  30 , the Gate output goes low and the MOSFETs Q 1  and Q 2  are toggled to the OFF state, thus preventing the reverse current flow from dragging down the Load Bus (V 1 ). 
   Shown in  FIG. 3  is one embodiment of the over-current detection device  22 A. Over-current detection device  22 A includes a sense resistor Rs connected in series with the MOSFETs Q 1  and Q 2  between the power supply  12 A and the MOSFETs Q 1  and Q 2 . Resistor Rs senses the current flowing in power supply line V 1 A and the resulting voltage drop Vs across resistor Rs is input to an amplifier  32 . The amplified voltage is then input to a comparator  34  where it is compared to an over-current detection threshold voltage, Vref. Preferably, the comparator  34  operates in a latch-and-compare manner. As long as the voltage input to the comparator  34  is less than the over-current detection threshold voltage Vref, meaning that the current flowing through resistor Rs is below a threshold value, the output of the comparator  34  remains low and the gate control device  24  operates as described above. 
   If the voltage input to the comparator  34  is greater than the over-current detection threshold voltage Vref, meaning that the current flowing through resistor Rs has exceeded the threshold value, the output of the comparator  34  goes open-collector, causing transistor Q 3  to be turned on by the voltage supplied to transistor Q 3  through pull-up resistor Rp. In the configuration of over-current detection device  22 A of  FIG. 3 , transistor Q 3  has its collector terminal coupled to the Timer pin of controller  26  and its emitter coupled to ground. Therefore, when transistor Q 3  is turned on, the Timer pin is shorted to ground, causing the charge pump (not shown) internal to the controller  26  to shut down, which causes the Gate output of the controller to go low. This causes the gate voltage V G  to also go low, which toggles the MOSFETs Q 1  and Q 2  to the OFF state, which blocks the excessive current detected in the resistor Rs from continuing to flow. 
   In the configuration of over-current detection device  22 B shown in  FIG. 4 , the operation of the resistor Rs, amplifier  32  and comparator  34  is the same as in over-current detection device  22 A, described above. However, in over-current detection device  22 B, transistor Q 3  has its collector terminal coupled to the UVP pin of controller  26  and its emitter coupled to ground. Therefore, when the output of comparator  34  goes high, based on the detection of a current that exceeds the over-current detection threshold Vref, transistor Q 3  is turned on through resistor Rp, causing the UVP pin to be shorted to ground. This causes the UVP reference voltage level to be compared to the ground voltage. Since the ground voltage input to the UVP pin of the controller  26  is below the UVP reference voltage level, the controller  26  determines that no or low voltage is present on the power supply line V 1 A, and the controller  26  drives the Gate output low. This causes the gate voltage V G  to also go low, which toggles the MOSFETs Q 1  and Q 2  to the OFF state, which blocks the excessive current detected in the resistor Rs from continuing to flow. 
   In the configuration of over-current detection device  22 C shown in  FIG. 5 , the operation of the resistor Rs, amplifier  32  and comparator  34  is the same as in over-current detection device  22 A, described above. However, in over-current detection device  22 C, transistor Q 3  has its collector terminal coupled to the common gate terminals of MOSFETs Q 1  and Q 2  and its emitter coupled to ground. Therefore, when the output of comparator  34  goes high, based on the detection of a current that exceeds the over-current detection threshold Vref, transistor Q 3  is turned on through resistor Rp. This causes the common gate terminals of MOSFETs Q 1  and Q 2  to be shorted to ground, which causes the MOSFETs Q 1  and Q 2  to be toggled to the OFF state, which blocks the excessive current detected in the resistor Rs from continuing to flow. Since the Gate output of the controller  26  remains high, the difference in voltage between the Gate output and the common gate terminals of the MOSFETs is dropped across resistor Rg. 
   In the configuration of over-current detection device  22 D shown in  FIG. 6 , the operation of the resistor Rs, amplifier  32 , comparator  34  and transistor Q 3  is the same as in over-current detection device  22 B, described above with reference to  FIG. 4 . However, over-current detection device  22 D further includes a reset device  40  coupled between the output of the comparator  34  and a reset input of the comparator  34 . Reset device  40  may include any type of timing circuitry, which outputs a reset signal to the comparator a predetermined period of time after the output of the comparator goes high. The reset signal causes the comparator to unlatch the currently latched voltage and to latch on the next available voltage. While the comparator is unlatched, its output is low and the controller  26  is enabled to drive its Gate output high, resulting in MOSFETs Q 1  and Q 2  being toggled to the ON state. If the next voltage latched by the comparator is below the over-current detection threshold Vref, the controller  26  maintains this mode of operation. However, if the next voltage latched by the comparator exceeds the over-current detection threshold Vref, the output of the comparator goes high, causing the controller  26  to drive its Gate output low, causing the MOSFETs Q 1  and Q 2  to be toggled to the OFF state. After the predetermined period of time dictated by the reset device  40 , the comparator will be reset by the reset device  40  and the process will repeat. In the event of a continued over-current condition, the MOSFETs Q 1  and Q 2  will be toggled between the ON and OFF states at a rate determined by the duty cycle of the reset device  40 . This configuration enables the system to be reset after the MOSFETs are shut down without requiring external intervention should the fault be caused by a temporary situation in the system. 
   It will be understood that the reset device  40  may also be used in conjunction with the over-current detection devices  22 A and  22 C shown in  FIGS. 3 and 5 , to provide a reset signal to the comparator  34 . This would require that the reset device  40  have an input coupled to the output of the comparator  34  and an output coupled to the reset input of comparator  34 . The operation of such configurations is identical to that described above. 
   Accordingly, the present invention provides a fault protection for a power supply system which minimizes the number of components used to carry out the over-current detection, ORing and soft start functions. All of these functions are able to be carried out using a pair of MOSFETs connected in series between the common load and the power supply. In an alternative embodiment, the over-current detection and soft-start functions may be carried out using a single MOSFET connected in series between the common load and the power supply, having its gate coupled to the Gate output of the controller. The minimal number of components connected in series between the common power bus and the power supplies increases the electrical and thermal efficiency of the power supply system. The invention may be utilized to supply fault protection in any type of redundant power supply system, in which a number of power supplies provide power to a commonly connected load. 
   The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. For example, the MOSFETs Q 1  and Q 2  and the transistor Q 3  may be any type of switches that are capable of carrying out the functions described herein. The present embodiments are therefore to be considered in respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of the equivalency of the claims are therefore intended to be embraced therein.