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 . referring now to fig1 , 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 . referring now to fig2 , 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 . referring now to fig3 , 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 . as depicted in fig3 , 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 fig2 . 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 fig3 , 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 . referring now to fig4 , 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 . 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 .