Patent Publication Number: US-6987333-B2

Title: Active circuit protection for switched power supply system

Description:
This application is a Continuation of Ser. No. 09/814,525 filed Mar. 22, 2001, now U.S. Pat. No. 6,600,239. 

   FIELD OF THE INVENTION 
   The present invention pertains generally to switched power supplies connected in parallel to a common load, and more particularly to protection logic that protects the switching circuitry and prevents the currently active power supply of a switched power supply system from either reaching a current limit condition or causing a large voltage deviation at the load. 
   BACKGROUND OF THE INVENTION 
   In many electronic circuit applications, multiple power supplies are connected in parallel to drive a common load during different times of operation. One application example is a device that implements a standby or “sleep” mode. During standby mode such a device might use a low power DC supply such as a battery or DC-DC converter to power the minimal circuitry required to “awaken” the device, and upon awakening switch to a higher power DC supply that supports the current requirements of the functional circuitry. 
   In switched power supply systems, switching devices are used to switch different power supplies to actively provide power to a common load. These switching devices are controlled using dedicated control logic that only allows one voltage source to supply power to the common load. In many applications, the load is sensitive to large voltage deviations. Accordingly, it is important to limit the voltage deviation seen at the load even when the source of power is being switched from one power supply to another. 
   In voltage deviation sensitive loads, the implementation choice of the switching devices becomes important. Switching relays switch too slowly to meet strict voltage deviation limitation requirements when used alone. The switching performance can be improved with the use of very large capacitors; however, this increase the expense and size of the overall system. 
   Analog switches are also a poor choice for voltage deviation sensitive loads. Analog switches are characterized by a high internal resistance, which can create a voltage drop at the load greater than the allowed voltage deviation during normal operation. 
   Recently, N-Channel MOSFETs are being used to switch between multiple different power supplies to actively power a common load. In such a switching arrangement, the MOSFETs are connected with their drains tied together at the load and their respective sources connected to the output of their respective power supplies. 
   As termed herein, when a MOSFET switch associated with a particular power supply is turned OFF to isolate its respective power supply from the load, the respective power supply is referred to as an “isolated power supply”. When the MOSFET switch is turned ON to connect its respective power supply to the load, the respective power supply is referred to herein as an “active power supply”. As will be appreciated by those skilled in the art, in a switched power supply system, all power supplies switchably connected to the load may remain powered ON; accordingly, although an isolated power supply is isolated from the load, it may still supply power at its output. 
   Due to its construction, an N-Channel MOSFET is characterized by an intrinsic body diode across the source and drain. In particular, the anode of the intrinsic body diode is connected at the source node and the cathode is connected at the drain node. In the MOSFET arrangement just described, wherein the drains of each switching MOSFET are tied together, the cathodes of the intrinsic body diodes in the MOSFETs are tied together. This design configuration creates the appearance of using OR-ing diodes. The voltage source outputs must be within a diode drop (approximately 0.6 volts) of each other because if the output voltage of an isolated power supply is greater than a diode drop of an active power supply, it will forward bias the intrinsic body diode in the isolated power supply&#39;s associated MOSFET switch and will also supply power to the load. Accordingly, unless the output voltages of each of the power supplies are within a diode drop of each other, their associated MOSFET switches will not provide isolation even if one MOSFET switch is on and the others are off. In particular, the power supply with an output voltage greater than a diode drop of another power supply will source current to the load even though its MOSFET switch is turned off by the forward bias created by the voltage differential across the intrinsic body diode of its switch. 
   Even if the output voltages of each switched power supply are within a diode drop of one another, a failure in the active power supply will cause a forward bias of the intrinsic body diode of the isolation switch of the isolated power supply, causing the isolated power supply to supply power directly into the failed power supply. The active power supply may then go into current limit. If the active power supply is allowed to continue to operate in current limit, it may eventually damage the MOSFET switch of the isolated power supply due to excessive power dissipation in its intrinsic body diode. 
   A need therefore exists for protecting the MOSFET isolation switches in a MOSFET switched power supply system when a failure occurs in one of the power supplies. A need also exists for protecting the remaining non faulty power supplies to ensure that the remaining power supplies, and therefore the load, remains within specified tolerance limits. 
   SUMMARY OF THE INVENTION 
   The present invention solves the problems of the prior art by preventing the active power supply of a switched power supply system from either reaching a current limit condition or causing a large voltage deviation at its output and at the load. The invention protects the switching circuit components from being damaged. The invention also ensures that the system will continue to run without interruption even if a failure occurs in the active power supply that is currently supplying power to the load. 
   In accordance with the invention, an active protection circuit operates to control the switching of the MOSFET isolation switches. A monitoring circuit operates to sense and turn off the isolation switch of the currently active power supply if it senses reverse current flowing through the switch. Simultaneously, a controller receives indication that the active power supply is out of specification, and actively switches the system voltage source to the other power supply. The controller actively ensures that the isolation switch of the faulty power supply remains off until it determines otherwise. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be better understood from a reading of the following detailed description taken in conjunction with the drawing in which like reference designators are used to designate like elements, and in which: 
       FIG. 1  is schematic block diagram of a switched power supply system incorporating an active protection circuit in accordance with the invention; 
       FIG. 2  is an operational flowchart of an exemplary embodiment of the method of the invention; and 
       FIG. 3  is a schematic block diagram illustrating an alternative embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  is a schematic block diagram of a dual power supply system  100  comprising switching control logic implemented in accordance with the invention. System  100  includes a first and second power supply  112  and  114  operatively connected in parallel to a common load  110  comprising electronic components. First power supply  112  supplies power at an output  120 ; similarly second power supply  114  supplies power at an output  170 . A pair of isolation switches  124  and  174  are operatively connected between respective power supplies  112  and  114 , both with outputs connected to the common load  110  at node  130 . As described in greater detail below, the outputs of the first power supply  112  and the second power supply  114  are operatively connected together in parallel, yet may be isolated from each other by operation of isolation switches  124  and  174 . 
   In particular, the first isolation switch  124  (sometimes referred to as the first isolation MOSFET) has an input (source S) connected to the output  120  of first power supply  112 , an output (drain D) connected to the common load  110  at node  130 , and a control (gate G) which allows or disallows operative connection of the first power supply  112  to node  130 . Similarly, the second isolation switch  174  (sometimes referred to the second isolation MOSFET) has an input (source S) connected to the output  170  of second power supply  114 , an output (drain D) connected to the common load  110  at node  130 , and a control (gate G) which allows or disallows operative connection of the second power supply  114  to node  130 . 
   In the preferred embodiment, first and second isolation switches  124 ,  174  are each implemented with an N-channel MOSFET which exhibits an intrinsic body diode. By connecting the drains of the first and second isolation MOSFETs  124 ,  174 , the intrinsic diode in each MOSFET gives the functionality of a diode OR-ing arrangement to provide isolation to the outputs, as described in detail hereinafter. In particular, the source S of the first isolation MOSFET  124  is electrically connected to the output  120  of first power supply  112 , and its drain D is connected to the load at node  130 . The drain-to-source current I DS  in the first isolation MOSFET  124  is switchable between an ON mode and an OFF mode by application of a bias voltage on the gate. When in the ON mode, current flows from the source to the drain, and a voltage corresponding to the current flow is generated between the source S and drain D. As will be described in greater detail below, this voltage is used to determine the current flow, including the direction of current flow, through the first isolation MOSFET  124 . As described in the background section, an N-channel MOSFET has an intrinsic body diode acting between the source and the drain wherein the anode of the diode is connected to the source and the cathode is connected to the drain. The intrinsic body diode of the first isolation MOSFET  124  serves to isolate the first power supply  112  from node  130  when the voltage at output  120  is less than a diode drop greater than the voltage present on node  130 . 
   The operation of the second isolation switch  174  is similar to that of the first isolation switch  124 , but isolates the second power supply  114  from node  130  when the voltage at output  170  is less than a diode drop greater than the voltage present on node  130 . 
   A first monitoring circuit  116  is operatively connected between the input S and output D of the first isolation switch  124  to provide control of the first isolation switch  124 , and therefore the operative connection of the first power supply  112  to node  130 . 
   The first monitoring circuit  116  includes a first differential amplifier  140  and first voltage comparator  150 . The non-inverting input  142  of the first differential amplifier  140  is electrically connected to the source S of the first isolation MOSFET  124  and the inverting input  144  is electrically connected to the drain D of the first isolation MOSFET  124 . It should be noted that other components, not shown in  FIG. 1 , may be associated with the first differential amplifier  140 . The first differential amplifier  140  serves to measure the voltage drop between the source S and drain D of the first isolation MOSFET  124 . It is to be understood that the use of a differential amplifier to measure a voltage is for illustration purposes only and that other voltage measuring devices may be used to measure the voltage drop between the source and drain of the first isolation MOSFET  124 . The output of the first differential amplifier  140  is electrically connected to the non-inverting input  151  of a first voltage comparator  150  by way of a line  152 . A preselected voltage V REF  is input to the inverting input  153  of the first voltage comparator  150 . The first voltage comparator  150  compares the output of the first differential amplifier  140  to the preselected voltage V REF . The output of the first voltage comparator  150  is electrically connected to the gate of the first isolation MOSFET  124  by way of a line  154 . It should be noted that other electronic components, not shown in  FIG. 1 , may be associated with the first voltage comparator  150 . 
   Describing now the connections to the second power supply  114 , a second monitoring circuit  118  is operatively connected between the input S and output D of the second isolation switch  174  to provide control of the second isolation switch  174 , and therefore the operative connection of the second power supply  114  to node  130 . 
   The second monitoring circuit  118  includes a second differential amplifier  180  and second voltage comparator  190 . The non-inverting input  182  of the second differential amplifier  180  is electrically connected to the source S of the second isolation MOSFET  174  and the inverting input  184  is electrically connected to the drain D of the second isolation MOSFET  174 . The second differential amplifier  180  serves to measure the voltage drop between the source and drain of second isolation MOSFET  174 . It is to be understood that the use of the second differential amplifier  180  is for illustration purposes only and that other voltage measuring devices may be used to measure the voltage drop between the source and drain of the second isolation MOSFET  174 . It is also to be understood that other components, not shown, may be associated with the second differential amplifier  180 . The output of the second differential amplifier  180  is electrically connected to the non-inverting input  191  of a second voltage comparator  190  by way of a line  192 . The preselected voltage V REF  is input to the inverting input  193  of the second voltage comparator  190 . The second voltage comparator  190  compares the output of the second differential amplifier  180  to the preselected voltage V REF . The output of the first voltage comparator  190  is electrically connected to the gate of the second isolation MOSFET  174  by way of a line  194 . It is to be understood that other electronic components, not shown in  FIG. 1 , may be associated with the second voltage comparator  190 . 
   In a switched power supply system, it is typically desired that in normal operation only one or the other of the power supplies  112 ,  114  supply power to the load at any given time. For example, suppose that the load  110  is a device that includes a standby or “sleep” mode that utilizes a low power battery or DC-DC converter as the first power supply  112  to supply +3.3 volts to the circuitry (in the load  110  but not shown) that monitors when to wake up the device and that performs the wakeup functions. In this example, one of the functions performed by the wakeup function is to switch the active power source from the low-power first power supply  112  to a high-power second power supply  114  in order to meet the power requirements of the fully functioning load  110 . Accordingly, during proper normal operation, it is desirable that only one or the other of the power supplies  112 ,  114  supply power to the load  110  at any given time. However, if a fault occurs in the currently active power supply, then the active protection circuit of the invention, described hereinafter, will switch the current active supply from the faulty power supply to the remaining good power supply. 
     FIG. 2  is a flow diagram illustrating an exemplary embodiment of the method of the invention. As illustrated, at power up, as shown in step  202 , one of the power supplies is operatively connected to the load to actively supply power to the load  110 , and the other power supplies are isolated from the load or off. The system status is then monitored  204 , including monitoring the output voltage of the activated power supply (step  204 A), monitoring the reverse current in the isolation switch of the active power supply (step  204 B), and monitoring other system status such as the system mode (sleep vs. wakeup) (step  204 C). If a status change is detected in the system mode (for example, the system is to wake up), as detected in step  206 C, the currently active power supply is isolated from the load by turning OFF  208  the isolation switch of the active power supply, and one of the remaining good power supplies is operatively connected to the load to actively supply power to the load by turning ON  210  the isolation switch of a selected remaining good power supply. 
   If a failure occurs in the output voltage of the active power supply, as detected at step ( 206 A), or if reverse current is sensed in the isolation switch  174  of the active power supply  114 , as detected at step ( 206 B), the isolation switch of the active power supply is turned OFF to block reverse current from flowing to the active power supply, and the isolation switch of a selected remaining good power supply is turned ON to allow the selected power supply to actively supply power to the load. 
   Turning back to  FIG. 1 , the active protection circuit of the invention, shown at  160 , will now be described in detail. In particular, in the illustrative embodiment, the active protection circuit  160  connects to the control lines  154  and  194  and hence to the gates G of first and second isolation MOSFETs  124  and  174 . In the illustrative embodiment, active protection circuit  160  comprises a controller  162  implemented as a state machine (not shown) in a Field Programmable Gate Array (FPGA). A power supply monitoring circuit  161  monitors the voltages at outputs  120  and  170  and outputs status signal(s)  163  indicating whether one or the other of the voltages at the outputs  120 ,  170  of the power supplies  112 ,  114  fall out of specification. In the preferred embodiment, specification means +3.3 volts plus or minus a preselected tolerance amount. The controller  162  receives the status signals  163 . Controller  162  may also receive other system state information signals such as a wakeup signal  164  which may be used to determine when to switch power supply sources during normal operation. It should be noted that other system status signals, not shown in  FIG. 1 , may be input to the controller  162  for use thereby to control the isolation switches  124 ,  174 . 
   In the illustrative embodiment, controller  162  generates an output signal on line  165 , which is used to control a first control switching device  156 . The first control switching device  156  has an input (source S) connected to a low voltage source (e.g., ground), an output (drain D) connected to the output of the comparator  150  and control input G of first isolation MOSFET  124 , and a control (gate G) controlled by controller  162  on line  165 . As described in detail hereinafter, the first control switching device  156  provides a mechanism for the controller  162  to turn the isolation switches  124 ,  174  ON or OFF from the controller  162 . 
   Active protection circuit  160  may further comprise a second control switching device  196 . Second control switching device  196  has its source S connected to a low voltage source (e.g., ground), and its drain D connected to line  194  (and therefore the gate G of second isolation MOSFET  174 ). The gate G of second control switching device  196  is connected to the drain D of first control switching device  156 , which is in turn controlled by the controller  162  on line  165 . The first control switching device  156  and second control switching device  196  control first and second isolation MOSFETs  124  and  174 , such that both switching isolation MOSFETs  124 ,  174  will not be turned ON at the same time. 
   In discussing the operation of the active protection circuit, the following assumptions are made: (1) at system startup both power supplies  112 ,  114  are initially powered on, (2) during normal operation only one power supply  112 ,  114  actively supplies power to the load at a time, and (3) at startup the system is first placed in a standby mode which uses the first power supply  112  and later switches to using the second power supply  114  when full functionality is required. In operation, at system startup, controller  162  outputs a low voltage on line  165 . During the initial moments of the power up process, pull-down resistor  155  ensures that the line  165  will be pulled low. Accordingly, the first control switching device  156  is turned OFF, which isolates line  154  from the source S (ground) of first control switching device  154 . Line  154  is pulled high by pull-up resistor  164 , which turns ON the first isolation MOSFET  124 . Also at system startup, the second control switching device  196  is turned ON since line  154  is pulled to V cc  by pull-up resistor  164 , thereby pulling line  194  low and ensuring that the second MOSFET isolation switch  174  is turned OFF. 
   Accordingly, at system startup, the first power supply  112  actively supplies power to the load  110 , while the second power supply  114  is isolated from the load. During this state, the source of the first isolation MOSFET  124  will have a voltage of +3.3 volts, which will be present on the noninverting input  142  to the first differential amplifier  140 . The voltage at the source S is slightly higher than the voltage at the drain D, meaning that current is flowing from the first power supply  112  to the load  110 . Accordingly, approximately +3.3 volts will be present on the non-inverting input  142  of the differential amplifier  140  and a slightly lower voltage will be present on the inverting input  144  of the differential amplifier  140 . The gain of the first differential amplifier  140  is preselected so that it is able to measure the small voltage difference between the source S and the drain D of the first isolation MOSFET  124 . The gain of the first differential amplifier  140  is also high enough to cause the output of the first differential amplifier  140  to be greater than the voltage V REF  when a slight voltage difference between the source S and the drain D of the first isolation MOSFET  124  is measured. Accordingly, the first voltage comparator  150  will maintain a high voltage on line  154  and therefore at the gate G of the first isolation MOSFET  124 , which keeps the first isolation MOSFET  124  ON. 
   During the time that the first power supply  112  actively supplies power to the system, second power supply  114  is isolated from the load. However, because the second power supply  114  is powered on, approximately +3.3 volts will be present on the non-inverting input  182  of the differential amplifier  180 . As discussed above, a slightly lower voltage will be present on node  130  as supplied by the first power supply  112  and this slightly lower voltage will be present on the inverting input  184  of the differential amplifier  180 . As a result, the second differential amplifier  180  will output a high voltage on line  192 . The high voltage output by the differential amplifier  180  is compared to the positive reference voltage VR by second voltage comparator  190 , which causes a high voltage output onto line  194 . However, because second control switching device  196  is in the ON state, the line  194  is pulled to a low voltage. The low voltage on line  194  is present on the gate G of second isolation switch  174 , ensuring that it remains in the OFF state and that the second power supply  114  is isolated from the load  110 . The intrinsic body diode of the second isolation MOSFET  174  blocks current from the first power supply  112  from flowing into the second power supply  114 , to provide isolation. 
   When, for whatever reason (e.g., the device is switching out of a standby mode into a full functionality mode), the source of power is to be switched from the first power supply  112  to the second power supply  114 , the controller  162  places a high voltage on line  165 , which is present at the gate G of first control switching device  156 . Accordingly, the first control switching device  156  is turned ON, allowing current to flow therethrough. Since the source S is at a low voltage level (e.g., ground), line  154  is pulled low, turning OFF the first isolation MOSFET  124 . 
   When line  154  is pulled low by turning on first control switching device  156 , the low voltage is present at the gate G of second control switching device  196 , causing the device  196  to turn OFF. Pull-up resistor  166  pulls line  194  high, which turns ON the second isolation MOSFET  174  to allow the second power supply  114  to actively supply power to the load  110 . 
   When the second isolation MOSFET  174  is ON, the voltage at the source S will be slightly higher than the voltage at the drain D if current is flowing from the second power supply  114 . The gain of the second differential amplifier  180  is preselected so that it is able to measure the small voltage difference between the source and the drain of the second isolation MOSFET  174 . The gain of the second differential amplifier  180  is also high enough to cause the output of the second differential amplifier  180  to be greater than the voltage V REF  when the slight voltage difference between the source and the drain of the second isolation MOSFET  174  is measured. Accordingly, the second voltage comparator  190  outputs a high voltage to the gate of the second isolation MOSFET  174 , which keeps the second isolation MOSFET  174  ON whenever the second power supply  114  is selected as the active power supply by the controller (by turning the second control switching device  196  OFF). 
   Due in part to the low output resistances of conventional power supplies, when one power supply fails in a switched power supply system, its output voltage can drop below specification, causing it to sink current from the remaining power supplies. If during normal operation a fault occurs in the second power supply  114  such that the voltage at the source S of second isolation MOSFET  174  drops low enough that the first power supply  112  starts to source current into the second power supply  114 , the voltage at the drain D of the second isolation MOSFET  174  will be greater than the voltage at the source S of the second isolation MOSFET  174 . Accordingly, current from first power supply  112  will flow through the intrinsic body diode of first isolation MOSFET  124 , through second isolation MOSFET  174  and into the second power supply  114 . The voltage at the source S of the second isolation MOSFET  174  will fall below the voltage at the drain D and the differential amplifier  180  will detect the negative difference and output a low voltage level on line  192 . The low voltage level output on line  192  will be below the reference voltage V REF , which will cause the second voltage comparator  190  to output a low voltage level on line  194 , thereby turning OFF the second isolation switch  174 . When the isolation switch  174  is off, the intrinsic body diode blocks reverse current from flowing through the switch to the failed second power supply  114 . This will protect the remaining first power supply  112  from going into current limit and prevent a large enough voltage deviation at its output  120  that could cause a failure in the load  110 . 
   However, at the time power supply  114  failed, first isolation MOSFET  124  was still OFF. If first isolation MOSFET  124  is allowed to remain OFF after the second isolation MOSFET  174  has been turned OFF in response to a failure in the second power supply  114 , all the current in the load  110  will be sourced by the first power supply  112  and will flow through the intrinsic body diode of the first isolation MOSFET  124  and generate heat, which may damage the first isolation MOSFET  124 . Accordingly, the active protection circuit operates to turn ON the first isolation MOSFET  124  upon detection of a failure in the second power supply  114 . In particular, a supply monitoring circuit  161 , preferably implemented by a voltage comparator (not shown), monitors the output voltages  120 ,  170  of the first and second power supplies  112 ,  114 , detects when the output voltages  120 ,  170  are out of specification (e.g., +3.3 volts +/− a predetermined tolerance amount), and outputs status signal(s)  163 . The controller  162  receives the status signal(s)  163  and determines whether or not and when to activate the active protection circuit  160  (by placing a low voltage level on line  165  presented at the gate G of the first control switching device  156 ). When a failure condition in the second power supply  114  is detected, the controller  162  outputs a low voltage level on line  165 , which is present at the gate G of first control switching device  156  to turn OFF the switch  156 . Accordingly, line  154  connected to both the gate G of the first isolation MOSFET  124  and the gate G of the second control switching device  196  is pulled to a high voltage level via the pull-up resistor  164 . The first isolation MOSFET  124  will then turn ON to allow the first power supply  112  to actively supply power to the load  110 . Turning OFF the first control switching device  156  also causes the second control switching device  196  to turn ON, which will pull line  194  to a low voltage level (e.g., ground) and thereby ensure that the second isolation MOSFET  174  will remain OFF regardless of voltage difference sensed by the monitoring circuit  118 . The second isolation MOSFET  174  will remain OFF until the controller  162  allows it to turn on (for example, if it detects that the second power supply  114  has come back within specification (e.g., +3.3 volts +/− tolerance). 
   It will be appreciated from the above description that the active protection circuit  160  actively protects the MOSFET isolation switches  124  and  174  from becoming damaged due to a failure in one of the power supplies  112 ,  114 , and also ensures that the power supplied to the load  110  is uninterrupted. 
   The active protection circuit  160  provides another protection. In the event that the load  110  draws excessive current and the second power supply  114  is switched to the common load  110 , and the second power supply  114  is current limited, the first power supply  112  will begin to source current through the intrinsic body diode of the first isolation MOSFET  124  such that both power supplies  112  and  114  will source current to the load  110 . Since the second power supply  114  is selected as the active power supply, its isolation MOSFET  174  will be ON, and therefore the drain-to-source resistance R DS  of the intrinsic body diode of the second isolation MOSFET  174  will be lower than the drain-to-source resistance R DS  of the intrinsic body diode of the first isolation MOSFET  124 . Accordingly, the second power supply  114  will reach current limit and go out of specification prior to the first power supply  112 . When the second power supply  114  goes out of specification, the supply monitoring circuit  161  detects this condition and informs the controller  162  via the status line(s)  163 . The controller  162  then turns OFF the first control switching device  156  by placing a low voltage at the gate G of the device  156 , which allows line  154  to be pulled high by pull-up resistor  164  to turn ON the first isolation MOSFET  124  to allow current to flow through the MOSFET  124  itself and not through the intrinsic body diode. This will ensure that the MOSFET  124  does not get damaged due to excessive power dissipation. 
   It is to be understood that any number of power supplies may be connected in parallel with associated monitoring circuitry and switching control circuitry. It is also to be understood that only the power supplies that are required to be isolated from the other components of the power supply circuit  100  need to have monitoring circuits and switching control logic associated with them. 
     FIG. 3  is an alternative embodiment of the first isolation switch  124 . As shown, a pair of back-to-back N-channel MOSFETs  124   a,    124   b  replaces the first isolation MOSFET  124  of  FIG. 1 . As illustrated, the N-channel MOSFETs  124   a  and  124   b  are connected with their drains D tied together. The source of MOSFET  124   a  is electrically connected to the output  120  of the first power supply  112 , and the source of MOSFET  124   b  is electrically connected to node  130 . The gates G of both MOSFETs  124   a  and  124   b  are tied together and electrically connected to line  154 . In operation, when second power supply  114  has been selected to actively supply power to the load  110 , the second isolation MOSFET  174  is ON and line  154  is at a low voltage to turn off both MOSFETs  124   a  and  124   b  in order to isolate the first power supply  112  from the load  110 . If a failure occurs in the load  110 , current cannot through MOSFET  124   a  because of the reverse bias on the intrinsic diode of MOSFET  124   b.  Meanwhile, as described above, switching isolation MOSFET  174  will remain on until the controller  162  detects that the second power supply  114  has gone out of specification, and subsequently turns OFF the second isolation MOSFET  174  and simultaneously turns ON both MOSFETs  124   a  and  124   b,  allowing current to flow from power supply  112 . 
   It will be appreciated from the above detailed description that the present invention affords several advantages over the prior art. With the active protection control circuit of the invention, failure of the active power supply or the detection of a short within the load is immediately detected, which allows the active protection control circuit to switch the currently active power supplies. This technique protects the isolation MOSFETs and prevents the remaining good power supply from either reaching a current limit condition or causing a large voltage deviation on its output. 
   Although the invention has been described in terms of the illustrative embodiments, it will be appreciated by those skilled in the art that various changes and modifications may be made to the illustrative embodiments without departing from the spirit or scope of the invention. It is intended that the scope of the invention not be limited in any way to the illustrative embodiment shown and described but that the invention be limited only by the claims appended hereto.