Abstract:
A switched power rail monitor compares voltages provided by a switched power rail and a power source to detect a predetermined differential indicative of a fault at the switched power rail. A switched power rail fault is communicated to a power manager to take corrective action, such as cutting off power to information handling system components. In one embodiment, a pair of bipolar junction transistors monitor the voltage differential to send an enable signal if the differential is within limits and a disable signal if the differential exceeds limits.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates in general to the field of information handling system power supplies, and more particularly to a system and method for power good monitor for multiple information handling system power supplies. 
         [0003]    2. Description of the Related Art 
         [0004]    As the value and use of information continues to increase, individuals and businesses seek additional ways to process and store information. One option available to users is information handling systems. An information handling system generally processes, compiles, stores, and/or communicates information or data for business, personal, or other purposes thereby allowing users to take advantage of the value of the information. Because technology and information handling needs and requirements vary between different users or applications, information handling systems may also vary regarding what information is handled, how the information is handled, how much information is processed, stored, or communicated, and how quickly and efficiently the information may be processed, stored, or communicated. The variations in information handling systems allow for information handling systems to be general or configured for a specific user or specific use such as financial transaction processing, airline reservations, enterprise data storage, or global communications. In addition, information handling systems may include a variety of hardware and software components that may be configured to process, store, and communicate information and may include one or more computer systems, data storage systems, and networking systems. 
         [0005]    One way that information handling systems provide flexibility to accomplish a variety of tasks is that they are built from a variety of processing components. Selection of appropriate processing components, such as CPUs, hard disk drives, embedded controllers, etc . . . , allows an end user to achieve a desired performance level for a given cost. The result is that an information handling system can have a wide variety of components, each with its own power supply requirements. To meet the power needs of processing components, an information handling system often includes power supplies with multiple voltage levels and varied power capabilities. In some instances, information handling systems, such as portable system, use power rail switches to further partition power supply wells. Using power rail switches provides a “subset” power source that aids portable information handling system power management and provides increased flexibility in available power supply sources within a system with reduced cost and footprint. 
         [0006]    One difficulty with the use of power rail switches to partition power supply is that each switched power rail should be monitored to ensure that it remains within operating limits. Typically, an information handling system monitors power rails to generate a combined “Power Good” feedback signal that indicates the power rails are working properly. If a failure of any power rail is detected, the Power Good feedback signal is terminated so that a power manager of the information handling system, typically found in an embedded controller, knows that one or more power rails needed for operation of the information handling system are not operational and can take appropriate corrective steps. Failure to detect defective power rails may violate power sequencing specifications, leading to an extreme overheating of integrated circuits that receive power from a failed power rail. Without a failure indication, control logic in the embedded controller, such as a PMC, would continue to enable power delivery to partially-powered target integrated circuits causing severe backdrive and overheating of the integrated circuit. Extreme heat from partial power applied to an integrated circuit may result in catastrophic failure to the information handling system. Although the application of partial power from a switched power rail failure can result in catastrophic damage, switched power rails often remain unmonitored due to the expense of “power good” solutions. Generally, monitoring of non-switched rails by DC-DC regulators or comparator integrated circuits is relied upon to detect and correct power system failures. 
       SUMMARY OF THE INVENTION 
       [0007]    Therefore, a need has arisen for a system and method that detects failures at switched power rails to provide Power Good feedback to an information handling system power manager. 
         [0008]    In accordance with the present invention, a system and method are provided which substantially reduce the disadvantages and problems associated with previous methods and systems for detecting information handling system power system failures. Switched power rails of an information handling system are monitored to detect faults so that power can be shut off to information handling system components in the event of a partial power failure due to a switched power rail failure. 
         [0009]    More specifically, an information handling system&#39;s components are powered by plural switched power rails that interface with a power source by selectively engaging or disengaging a switch. A switched power rail monitor associated with each switched power rail compares the voltage of the switched power rail with the source voltage to detect a fault at a predetermined differential. For example, first and second bipolar junction transistors monitor the switched power rail and source voltages to send an enable signal if the voltages are within limits and a disable signal if the voltages exceed the predetermined differential. In one embodiment, plural switched power rail monitors communicate the enable or disable signal to a cascade monitor that performs an OR function to issue a disable signal to a power manager if any one switched power rail monitor outputs a disable signal. 
         [0010]    The present invention provides a number of important technical advantages. One example of an important technical advantage is that switched power rails are monitored to detect failures and to report the failures by disabling a Power Good feedback signal. Monitoring of switched power rails prevents application of partial power to integrated circuits by cutting off power when the Power Good feedback signal indicates any failure. The use of bipolar junction transistors provides a low cost comparator-based power good circuit solution that is practical for commercial use in an information handling system having multiple switch power rail circuits. Monitoring for a Power Good feedback signal that includes switched power rails helps to prevent partial power failures that might otherwise lead to overheating of integrated circuits in an information handling system. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The present invention may be better understood, and its numerous objects, features and advantages made apparent to those skilled in the art by referencing the accompanying drawings. The use of the same reference number throughout the several figures designates a like or similar element. 
           [0012]      FIG. 1  depicts a block diagram of an information handling system having a power supply with plural switched power rails; 
           [0013]      FIG. 2  depicts a circuit diagram of the interaction of a switched power rail with a power supply and embedded controller; 
           [0014]      FIG. 3  depicts a circuit diagram of an example embodiment of a switched power rail monitor; and 
           [0015]      FIG. 4  depicts a circuit diagram of cascaded switched power rail monitors input to a cascade monitor to provide a single Power Good signal from plural switched power rail monitors. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    Monitoring information handling system switched power rails for failures helps to prevent damage to processing components by partial powering of the components. For purposes of this disclosure, an information handling system may include any instrumentality or aggregate of instrumentalities operable to compute, classify, process, transmit, receive, retrieve, originate, switch, store, display, manifest, detect, record, reproduce, handle, or utilize any form of information, intelligence, or data for business, scientific, control, or other purposes. For example, an information handling system may be a personal computer, a network storage device, or any other suitable device and may vary in size, shape, performance, functionality, and price. The information handling system may include random access memory (RAM), one or more processing resources such as a central processing unit (CPU) or hardware or software control logic, ROM, and/or other types of nonvolatile memory. Additional components of the information handling system may include one or more disk drives, one or more network ports for communicating with external devices as well as various input and output (I/O) devices, such as a keyboard, a mouse, and a video display. The information handling system may also include one or more buses operable to transmit communications between the various hardware components. 
         [0017]    Referring now to  FIG. 1 , a block diagram depicts an information handling system  10  having a power supply  12  with plural switched power rails  14 . Information handling system  10  has plural processing components that cooperate to process information, such as a CPU  16 , a hard disk drive (HDD)  18 , RAM  20 , a modem  22 , a network interface card (NIC)  24 , chipset  26  and an embedded controller  28 . Power supply  12  provides power to the processing components with switched power rails  14  and other types of power rails that are connected with the processing components through a motherboard  30 . A power manager  32  running on embedded controller  28  manages the power output by power supply  12 . A power good monitor  34  associated with power supply  12  monitors switched power rails  14  for failures and reports the failures to power manager  32  for appropriate corrective action, such as shutting down information handling system  10 . 
         [0018]    Referring now to  FIG. 2 , a circuit diagram depicts the interaction of a switched power rail  14  with a power supply  12  and embedded controller  28 . An unregulated power source enters a DC-DC regulator  36  at pin  1  and outputs a non-switched power source Vcc from pin  3 . Application of power to DC-DC regulator  36  results in a Power Good signal from pin  4  to a Vcc Power Good pin  3  of embedded controller  28 . As long as a Power Good signal enters embedded controller  28  at pin  4 , such as from a switched power monitoring  38 , embedded controller  28  returns an enable signal from pin to DC-DC regulator  36  pin  5  to continue to apply power to the information handling system. However, if switched power monitor circuit  38  fails to provide Power Good signal, embedded controller  28  ceases the Power Good enable signal from pin  1  to pin  5  of DC-DC regulator  36  so that power is turned off to the information handling system. Power output as Vcc proceeds to a power switch  40  that controls the application of power to a switched power rail  14 . When power switch  40  is closed to provide power to switched power rail  14 , power also enters switched power rail monitor  38  for a comparison with VCC. If the difference between the voltage Vcc at the non-switched power rail differs by more than a predetermined amount from the voltage of switched power rail  14 , switched power rail monitor  38  ceases a Power Good signal to pin  4  of embedded controller  28  so that power is turned off from DC-DC regulator  36 . 
         [0019]    Referring now to  FIG. 3 , a circuit diagram depicts an example embodiment of a switched power rail monitor  38 . Switch power rail monitor  38  provides a low cost comparator-based power good monitor using a low base emitter voltage (Vbe) saturation voltage of PNP bipolar junction transistors (BJT), which are normally used as an electronic switch. The BJTs monitor the voltage difference between the reference voltage Vcc and the switched power rail voltage to generate a power good signal for predetermined voltage parameters. If the switched power rail voltage falls below Vcc by more than a predetermined circuit threshold voltage, the power good signal is deasserted. The circuit threshold voltage is determined by the difference of a forward BJT Vbe saturation voltage, such as 0.6Volts, and a diode forward voltage, such as 0.3Volts. Thus, the combined threshold voltage using these examples is about 0.3Volts. 
         [0020]    As depicted in  FIG. 3 , switched power rail monitor  38  5V_RUN is a switched power rail  14  that derives power from 5V_ALW through a power switch  40 , as depicted in  FIG. 2 . A base emitter junction of a BJT Q 27  is connected across the switch for the monitored switched power rail from 5V_ALW to 5V_RUN. Switched power rail monitor circuit  38  operates based upon the voltage differential of deltaV=V — 5RUN−V — 5ALW. When the deltaV is greater than Vbe_bjt_Q 27  minus Vdiode_forward_d 14 , such as a value of 0.3V, current flows from the base node of the transistor to ground through resistors R 261  and R 240 . This allows current to flow from the source BJT collector node of Q 27  to the base node of the NPN BJT Q 24 , thus turning on transistor Q 24  to pull the collector node to ground. Thus, in the example depicted in  FIG. 3 , the power good signal output from transistor Q 24  is low to indicate a fault when V — 5RUN&lt;4.7V where V — 5ALW is 5V. When the source power rail and switched power rail are within the predetermined limits, such as 0.3V, the voltage at the base node of PNP BJT Q 27  will be V — 5VALW minus Vdiode_d 14  and have a value of less 0.6V for Vbe. This prevents the flow of base-emitter current so that PNP BJT Q 27  is off, resulting in NPN BJT Q 24  turning off to float the collector node. The collector node of Q 24  is pulled up with a pull-up resistor so that a logic high-level output from Q 24  indicates a positive power good signal. A series resistor R 261  disposed between the reference voltage 5V_ALW and the switched rail 5V_RUN to limit the base-emitter current of BJT Q 27 . A diode D 14  prevents backdrive between the reference and switched rails when the switched rail is off. 
         [0021]    Referring now to  FIG. 4 , a circuit diagram depicts cascaded switched power rail monitors  38  input to a cascade monitor  42  to provide a single Power Good signal from plural monitors  38 . A switched power rail monitor is associated with each of plural switched power rails. The output of each switched power rail monitor forms a wired OR function to output a single Power Good signal from cascade monitor  42 . In addition to cascading of the switched power rails, other open-drain or open-collector power good signals are cascaded from other source power supplies. Thus, a combined Power Good signal is obtained to indicate that both switched rails and non-switched rails are within the predetermined Vf-Vbe limit of the circuit. Control logic within the embedded controller can utilize the power good signal for power sequencing and diagnostics. 
         [0022]    Although the present invention has been described in detail, it should be understood that various changes, substitutions and alterations could be made hereto without departing from the spirit and scope of the invention as defined by the appended claims.