Latent fault detection in a redundant power supply

Abstract: A power switching circuit module includes two power rails coupling independent power supplies to the input of a DC controller and test circuits to detect latent faults in power mixing devices included in the circuit.

Description: The present invention relates generally to testing devices for faults. 2. Description of the related art. Many digital systems often utilize redundancy as one mechanism of achieving fault tolerance. As an example, a DC controller (DCC) can be coupled to two independent power sources to operate in a fault tolerant manner. The coupling of the independent power sources to the DCC is accomplished utilizing power mixing component such as transistors, fuses, and isolation diodes. It is possible for the DCC to operate properly even if one or more of the power mixing components has a latent open circuit of short circuit fault. However, in the event that one of the power supplies fails to produce power or experiences a short circuit to ground a latent fault in a power mixing device could cause failure of the system. Thus, typically such latent faults are not discovered until the actual failure of the system. The present invention provides a power circuit as defined in claim 1, including a system for testing for latent faults in power mixing devices coupling two independent power sources to a DC controller. According to one aspect of the invention, the circuit includes first and second power rails and power mixing devices for coupling, respectively, first and second power sources to the inputs of a DC controller. According to a further aspect of the invention, a latent short circuit fault in a power mixing device may be detected by momentarily turning off power on a first rail and monitoring a test node on the first rail between the power supply and the power mixing device to detect a momentary return to ground at the test node on the first rail. According to a still further aspect of the invention, an open circuit in a power mixing device in a first rail may be detected by turning of a second rail and detecting a power failure in the circuit module. Other features and advantages of the invention will become apparent in view of the following detailed description and appended drawings. Fig. 1 is a schematic diagram of a preferred embodiment of the invention; and Fig. 2 is a circuit diagram illustrating a possible short circuit fault. Fig. 1 is a schematic diagram of a preferred embodiment of the invention. In Fig. 1 a power circuit 10 includes two power rails, -48VA and -48VB, which are coupled to independent power sources (not shown). In a preferred embodiment the two power rails share a common return path called -48V return. Accordingly, all potentials are negative with respect to -48V return. The circuit 10 includes a first power rail 30 which is coupled to the input of a DC controller (DCC) 32 through a first fuse 34, a first power switching transistor 36, and a first isolation diode 38 coupled in series. Similarly, a second power rail 40 is coupled to the input of the DCC 32 through a second fuse 44, a second power switching transistor 46, and a second isolation diode 48 coupled in series. The diodes 38 and 48 combine the rails to provide power to the input 32i of the DCC 32. The return terminal 32r of the DCC 32 is coupled to -48V return 50. A first power switching circuit 60 is controlled by ENABLE A1 and ENABLE A2 signals provided by a control unit (CU) 62 via first and second ENABLE A lines 18 and 20. The ENABLE signals are logic level signals. The ENABLE signals are coupled to the control diode input of a first switching opto-isolator 64. Resistors R1 and R2, capacitor C1, and diode CR1 form a gate bias circuit for the gate of N-channel MOS transistor 36. This gate bias circuit is coupled between the first power rail 30 and the signal transistor input of the first switching opto-isolator 64. A second switching control circuit 70 is controlled by ENABLE B1 and ENABLE B2 signals supplied by the control unit 62 via first and second ENABLE B lines 18' and 20' and includes a second control opto-isolator 64 and gate bias circuit formed by resistors R3 and R4, capacitor C2, and diode CR2. A first test circuit 80 includes a first test opto-isolator 82 having its control input coupled to a first test node 84 located between the drain of the first power switching N-channel MOS transistor 36 and the cathode of the first isolation diode 38. The first test node 84 is coupled to the -48V return by a first pull-up resistor 86. The signal transistor input of the first test opto-isolator 82 provides a logic level RAILAOK to the CU 62 via a RAILAOK line 88. A second test circuit 90 includes a second test opto-isolator 92 having its control diode input coupled to a second test node 94 located between the drain of the N-channel MOS transistor 46 and the cathode of the second isolation diode 48. The second test node 94 is coupled to the -48V return by a second pull-up resistor 96. The signal transistor input of the second test opto-isolator 92 provides a logic level RAILBOK to the CU 62 via a RAILBOK line 98. The operation of the power switching circuits 60 and 70 will now be described. If either ENABLE Al or ENABLE A2 are asserted then current flows through the first gate bias generating circuit via the signal transistor input of the first switching opto-isolator 64 to generate a positive V GS to turn on the first power switching transistor 38 and switch on the first rail 30. The assertion of either ENABLE B1 or ENABLE B2 similarly switches on the second rail 40. The function of the isolation diodes 36 and 46 will now be described with reference to Fig. 2. In Fig. 2 dark bars 102 and 104 represent short circuits faults. The short circuit fault 104 in the first power supply (PS1) is a short to ground. If the first isolation diode 38 is not shorted then it will prevent current from flowing from PS2 through the PS1 short to ground and prevent damage due to an overcurrent condition. However, if the first isolation diode has a short circuit fault then current flows through both short circuits 102 and 104 and a system wide power outage would occur. The function of the test circuits 80 and 90 to uncover latent faults in the power mixing elements will now be described with reference to Fig. 1. The CU 62 monitors first and second test nodes 84 and 94 through the RAILAOK and RAILBOK signals, respectively, under all allowable conditions of the four ENABLE signals. When both rails are switched on both RAILOK signals are held low (TRUE) by test opto-isolators 82 and 84, i.e., when the first power switching transistor 36 is on the first test node 84 is energized and current flows through the control diode input of the first test opto-isolator 82 to turn on the transistor part of the opto-isolator and couple the RAILAOK signal to logic ground. If the first fuse 34 or power pass transistor 36 are open due to an open circuit fault then the test node 84 will not be energized when either ENABLE A1 or ENABLE A2 is asserted so that the RAILAOK signal will go high (FALSE). Thus, the control circuit 80 detects a latent open circuit when either of the ENABLE A signals is asserted and the monitored RAILAOK signal is FALSE. The first power switching transistor 36 and first isolation diode 38 can be scrubbed for short circuit faults by momentarily negating both ENABLE A signals and asserting either of the ENABLE B signals. In this case, if no devices have faults the first test node 84 is isolated from both power sources because the first power switching transistor 36 is off and the first isolation diode 38 is reverse biased. The first test node 84 is pulled up to the return voltage by the pull-up resistor 86 which typically has a high value such as 20K. Accordingly, no current flows through the control diode input of the first test opto-isolator 82 and the RAILOK signal changes momentarily to FALSE. If the CU 62 fails to detect the momentary transition of the RAILAOK signal from TRUE to FALSE then either the first power switching transistor 34 or first isolation diode 36 has a short circuit fault. A latent open circuit fault in the first isolation diode 36 can be detected by negating both ENABLE B signals and asserting either ENABLE A signal. If the first isolation diode 38 is open due to an open circuit fault then the DCC 32 stops operation because the second rail is switched off and no power can reach its input 32i because the first isolation diode 38 is open. Accordingly, it is possible to scrub each of the power mixing devices, i.e., fuses, the power switching transistors, and isolation diodes for latent open circuit and short circuit faults. Further, appropriate isolation of two nominally independent power sources is assured while allowing the DCC 32 to operate off of either/both of the power sources in a fault-tolerant fashion due to the normal isolating characteristics of (scrubbed good) mixing/isolation diodes 38 and 48 and the normal energy limiting characteristics of (scrubbed good) fuses 34 and 44. The invention has now been described with reference to the preferred embodiments. Alternatives and substitutions will now be apparent to persons of ordinary skill in the art. For example, isolation/mixing devices other than diodes, such as diode connected MOS transistors could be utilized. Additionally, the polarities and voltage levels described with reference to the preferred embodiments are not required to practice the invention. Accordingly, it is not intended to limit the invention except as provided by the appended claims.

Claims: A power circuit (10) for supplying power from multiple power sources to a device (32) having a power input port, (32i), the power circuit including a system for detecting latent faults in power mixing devices comprising: a first power rail (30) for coupling a fist power source (-48VA) to the power input port (32i), said first power rail (30) including a first power switching transistor (36) and a first isolation diode (38) coupled in series, and having a first test node (84) between said first power switching transistor (36) and said first isolation diode (38), with said first isolation diode (38) coupling said first test node (84) to the power input port (32i) of the device (32) and isolating the first test node (84) from other power sources (-48VB) unless said first isolation diode (38) has a short circuit fault (102); a second power rail (40) coupling a second power source (-48VB) to the power input port (32i), said second power rail including a second power switching transistor (46) and a second isolation diode (48) coupled in series, and having a second test node (94) between said second power switching transistor (46) and said second isolation diode (48), with said second isolation diode (48) coupling said second test node (94) to the power input port (32i) of the device (32) and isolating the second test node (94) from other power sources (-48VA) unless said second isolation diode (48) has a short circuit fault; a first switching circuit (60), coupled to receive a first enable signal (ENABLE A1 or ENABLE A2) and coupled to said first power switching transistor (36), for turning on said first power switching transistor (36), when said first enable signal is asserted, to couple said first power source (-48VA) to said first test node (84) to energize said first test node (84) unless said first power switching transistor (36) has an open circuit fault; a second switching circuit (70), coupled to receive a second enable signal (ENABLE B1 or ENABLE B2) and coupled to said second power switching transistor (46), for turning on said second power switching transistor (46), when said second enable signal is asserted, to couple said second power source (-48VB) to said second test node (94) to energize said second test node (94) unless said second power switching transistor (46) has an open circuit fault; a first test circuit (80), coupled to said first test node (84), for asserting a first OK signal (RAILAOK) if said first test node (84) is energized; a second test circuit (90), coupled to said second test node (94), for asserting a second OK signal (RAILBOK) if said second test node (94) is energized; whereby said first OK signal (RAILAOK) is negated if said first enable signal (ENABLE A1 OR ENABLE A2) is asserted but said first power switching transistor (36) has an open circuit fault; and whereby said first OK signal (RAILAOK) is asserted if said first enable signal (ENABLE A1 or ENABLE A2) is negated, but said second enable signal (ENABLE B1 or ENABLE B2) is asserted and said first isolation diode (38) has a short circuit fault (102). The power circuit of claim 1 wherein: said first test circuit (80) includes an opto-isolator (82), having a diode control input coupled to said first test node (84), for asserting said first test signal (RAILAOK) when said first test node (84) is energized. The power circuit of claim 1 wherein: said first power rail further comprises an energy-limiting fuse (34) connected in series between the first power source (-48VA) and said first test node (84), whereby said first OK signal is negated if said first enable signal is asserted but said fuse (34) has an open circuit fault. The power circuit of claim 1 further comprising: a control unit (62), coupled to said first and second switching circuits (60,70) and said first and second test circuits (80,90), for monitoring said OK signals (RAILAOK, RAILBOK) when said enable signals are asserted and negated to scrub said diodes (38,48) for faults.

Inventors: ALDRIDGE DAVID L, BISSELL STEPHEN R, BROWN DAVID, BUNTON WILLIAM P, ALDRIDGE, DAVID L., BISSELL, STEPHEN R., BROWN, DAVID, BUNTON, WILLIAM P.
Applicants: TANDEM COMPUTERS INC, TANDEM COMPUTERS INCORPORATED
Application year: 1995
Main classification: H02J9
IPC codes: G06F11, H02J9, G01R31, H02J1, G05F1