Operation of a redundant power supply without isolation devices

Methods and apparatuses are disclosed that may allow elimination of isolation devices for redundant power supplies while mitigating the undesirable effects of their elimination.

BACKGROUND

Computers and other electronic systems are ubiquitous in society, and as a result, their reliability has become increasingly important. One method of providing reliability for computers and other electrical systems is to implement redundant power supplies. As the name implies, redundant power supplies offer an alternative power supply if a first power supply becomes unavailable. Each individual power supply is usually capable of providing all the required power of a computer system, and the multiple power supplies usually provide redundancy by being coupled to a common voltage bus. Although these multiple devices may couple to a common voltage bus, they often include isolation devices so that failure of one of the power supplies does not affect the other power supplies or the voltage bus.

NOTATION AND NOMENCLATURE

DETAILED DESCRIPTION

In an effort to reduce power consumption, improve efficiency and reliability, as well as decrease system cost, power supply designers are actively engaged in reducing or eliminating the number of subcomponents contained in these redundant power supplies. Eliminating subcomponents, however, can have detrimental effects. Specifically, the elimination of isolation devices may cause the redundant power supplies to undesirably affect the power delivery of the other power supplies that are coupled to the voltage bus. Further, eliminating isolation devices may make it difficult to distinguish which power supply, among the many that may be part of a redundant power supply, has failed.

Methods and apparatuses are disclosed that may allow elimination of isolation devices for redundant power supplies while mitigating the undesirable effects of their elimination. While the thrust of this discussion pertains to redundant power supplies for computer systems, one of ordinary skill in the art will appreciate that any electrical system capable of implementing redundant power is also capable of implementing the disclosed embodiments. For example, the disclosed embodiments may find application in medical instrumentation, navigation equipment, and telecommunication applications.

Referring now toFIG. 1a block diagram of an exemplary computer system2is illustrated. Computer system2includes a central processing unit (CPU)10that couples to a bridge logic device12via a system bus (S-BUS). Bridge logic device12may be referred to as a “North bridge.” In some embodiments, bridge12couples to a memory14by a memory bus (M-BUS). In other embodiments, however, CPU10includes an integrated memory controller, and memory14connects directly to CPU10.

Bridge12also couples to PCI-Express® slots18A-B using the PCI-Express® bus standard as disclosed in “PCI-Express Base Specification 1.0a,” available from the Peripheral Component Interconnect (PCI) Special Interest Group (PCI-SIG) and incorporated herein by reference. Slots18A-B may physically reside on the same printed circuit board (also referred to as a “system board” or “mother board”) as CPU10. Regardless of the actual implementation of computer system2, a redundant power supply21also may be provided in order to maintain an uninterrupted source of power during operation.

FIG. 2represents an exemplary block diagram of redundant power supply21including several individual power supplies25and30. Although the embodiment depicted inFIG. 2shows two individual power supplies25and30, one of ordinary skill in the art would appreciate that many individual power supplies are possible. As is illustrated, power supplies25and30couple to a system board35, which may house one or more of the components of computer system2(shown inFIG. 1). While computer system2is operating, power supplies25and30may be physically inserted into or removed from system board35. This capability is often referred to as “hot plugging” or “hot swapping.” For example, if power supply25fails, it may be hot swapped with a replacement power supply while power supply30handles the power requirements for computer system2. In this manner, each swappable power supply is capable of handling the power requirements for the entire computer system.

Each power supply provides positive and negative voltage outputs (indicated by the “+” and “−” signs respectively), which couple to positive and negative voltage busses40and45respectively. For ease of discussion, negative bus45will hereinafter be referred to as a ground bus, because the ground bus is usually the most negative bus in the system. In some embodiments, however, the outputs of each power supply provides a voltage that is greater than ground on the positive output on a voltage that is less than ground on its negative output.

Voltage bus40and ground bus45provide power for system board35. Since each power supply is coupled to common busses40and45the condition and presence of each power supply25and30may impact the delivery of power to system board35. For example, if power supply25fails and its positive and ground outputs are shorted together, this may short the positive and ground busses40and45together, which may cause interruption of power delivery to system board35and affect other devices in computer system2.

Additionally, if a failed power supply is being replaced with a new power supply, the new power supply will initially be uncharged (i.e., 0 volts between its positive and ground output terminals) and may charge itself by draining current from other power supplies that are coupled to busses40and45. Hence, redundant power supplies, such as power supply21, traditionally require isolation devices.

Isolation devices often include diodes and transistors that logically “OR” the positive and ground busses40and45together. Regardless of whether a diode OR-ing arrangement is used for isolation or a transistor isolation arrangement is used for isolation, both the diodes and the transistors consume a portion of the power that is delivered to busses40and45. More particularly, in the case of diodes, each diode has a fixed forward bias voltage drop across it so that the amount of power consumed by the diode is based on the amount of current flowing through it. Therefore, the power consumption of each isolation diode increases as the power delivered increases, which decreases the overall efficiency of the power supplies. Isolation transistors also consume a portion of the power they are delivering, albeit to a lesser extent.

While isolation transistors offer the advantage of consuming less power than isolation diodes, detecting which power supply from among the many that may be coupled to voltage bus is the faulty power supply becomes more difficult as the voltage across the isolation devices decreases.

Ideally, reducing this voltage to zero would provide minimal power consumption. This situation is depicted inFIG. 2where no isolation devices are present. As mentioned above, in this arrangement, if either power supply25or30fails and shorts its positive and ground terminals together, then busses40and45may be shorted together. Regardless of whether power supply25is faulty or power supply30is faulty, with the isolation devices eliminated, monitoring circuitry (not shown inFIG. 2) that monitors the positive and ground terminals of a power supply may be unable to determine if the fault occurred in power supply25or power supply30.

Also, in the complete elimination arrangement shown inFIG. 2, one of the power supplies may experience an “over voltage” condition where the power supply generates an output voltage that is too high. Such an over voltage situation may make it difficult to determine which power supply25or30is generating excessive voltage because they are commonly coupled to busses40and45. For example, if power supply25is generating an output voltage that is too high, it may be difficult for the internal detection circuitry of power supply30(not shown inFIG. 2) to distinguish if the over voltage condition is because of power supply30or power supply25.

FIGS. 3-5illustrate methods and apparatuses that may allow elimination of isolation devices (such as diodes or transistors) in redundant power supply arrangements while mitigating the undesirable effects of their elimination.

Referring toFIG. 3A, a removable power supply70according to one embodiment is illustrated. In some embodiments, power supply70may mitigate undesirable effects that result from eliminating the isolation devices in a redundant power system (such as excessive current draw during hot swapping). Power supply70may function as one of many power supplies within a redundant power supply system20(shown inFIG. 2). In this manner, power supply70may be hot swapped into and out of a computer system such as computer system2(shown inFIG. 1). Thus, power supply70is capable of handling the entire power requirements for computer system2.

Power supply70includes front end circuitry75that is coupled to an AC source80. As would be appreciated by one of ordinary skill in the art, front end circuitry75may include electromagnetic interference filters, diode rectifiers, and power factor correction, filtering circuitry and primary switching circuitry (none of which is shown inFIG. 3A). Front end circuitry75may be coupled to a high frequency transformer85that isolates high voltage from the power delivered at the positive and ground terminals. Transformer85is coupled to the anode connection of diodes90. One of ordinary skill in the art will appreciate that transformer85and rectifying diodes90may provide further rectification of the output voltage to an inductor95that is coupled to the cathode connection of rectifying diodes90.

An internal capacitance100is coupled between the output terminals of power supply70and the inductor95. In addition, an external bus capacitance105is coupled to busses40and45respectively. Although no isolation device (such as diodes or transistors) is necessary to couple power supply70to busses40and45, capacitances100and105may assist in mitigating undesired effects resulting from removing the isolation devices. The total value of capacitances100and105is determined from the specification for the maximum voltage ripple on busses40and45. For example, if busses40and45deliver 12 volts, and the current delivered by power supply70is around80amps, then the maximum ripple specification may require about 2 mF of total capacitance. With this total value known, capacitance100is preferably selected such that its value is less than around 10% of the total value and capacitance105is selected such that its value is around 90% or less of the total value. Thus, in our example, capacitance100may be about 50 μF and capacitance105may be around 3600 μF.

During a hot insertion event, power supply70may prevent drawing a large amount of current from other power supplies that also are coupled to busses40and45. Prior to being inserted into the system, power supply70will be off and capacitance100initially will be uncharged. Upon being inserted into the system, capacitance100will draw current from the busses40and45. By choosing the capacitance100on power supply70to be small compared to capacitance105on busses40and45, the rate of current draw for capacitance100may be controlled such that the voltage on busses40and45does not dip below a predetermined value, say 10%. For example, in a 12 volt bus voltage scenario, the maximum allowable swing may be +/− 10% or 1.2V. In some embodiments, the value of capacitance100is less than about 10% of the value of capacitance105.

FIGS. 3B and 3Crepresent experimental results derived from implementing the embodiment shown inFIG. 3Awith a 50 μF ceramic capacitor implemented for capacitance100and an 3600 μF capacitor implemented for capacitance105.FIG. 3Bdepicts a hot insertion of power supply70with a load current of about30A. Referring toFIG. 3B, channel1represents the voltage on a bus, such as busses40or45. The vertical scale for channel1inFIG. 3Bis 500 mV/div with offset 10.0V, while the horizontal scale is 1 μS/div for all of the waveforms inFIG. 3B. Channel2represents a measurement of the current into capacitance100(measured as a voltage drop across a 0.833 mΩ measurement resistor). The vertical scale for channel2is 40 mV/div. Channel3represents the output voltage of a power supply, such as power supply70, which is being inserted into the redundant power system. The vertical scale for channel3is 5 V/div. As can be appreciated from an inspection ofFIG. 3B, a peak current of around100A conducts through capacitance100for less than 2 μS, resulting in a dip in the output voltage of around 400 mV, which is within the 10% limit. Thus, the embodiment shown inFIG. 3Aallows the isolation device to be eliminated from power supply70while mitigating the current draw on system voltage bus upon hot insertion.

The embodiment shown inFIG. 3Aalso may mitigate the effects of a hot extraction that result from elimination of the isolation device, as illustrated inFIG. 3C. One particular risk of hot extraction is that there will be a momentary arcing as the power supply is extracted, especially when the physical air gap between the power supply and the voltage bus is relatively small. By implementing the embodiment depicted inFIG. 3A, the current stored in inductor95will flow through capacitance100as power supply70is extracted, instead of flowing through the physical air gap between power supply70and busses40and45. This current path is indicated by the dashed arrow inFIG. 3A. In some embodiments in which arcing is eliminated in this manner, the value of the inductor is between about 1-10 μH and the value of capacitance is between about 50-100 μF.

FIG. 3Cdepicts hot extraction of power supply70with a load current of about15A. Referring toFIG. 3C, channel1represents the voltage on a bus, such as bus40. The vertical scale for channel1inFIG. 3Cis 500 mV/div with offset 10V, while the horizontal scale is 200 μS/div for all of the waveforms inFIG. 3C. Channel2represents a measurement of the current out of power supply70as measured by a “clamp on” meter available from Tektronics Corporation. The vertical scale for channel2is 10 A/Div. Channel3represents the output voltage of a power supply, such as power supply70, which is being extracted from the redundant power system. (Note that the time scale shown inFIG. 3Cis small enough that the output voltage of channel3appears to be maintained, however, in practice this voltage will decay to zero as capacitance100discharges). The vertical scale for channel3is 5V/div. As can be appreciated from an inspection ofFIG. 3C, the output voltage on channel3does not over-shoot and the output current on channel2is also falling because the power supply is being extracted. Importantly, the bus voltage on channel1remains substantially the same with a dip of less than 200 mV. Thus, the embodiment shown inFIG. 3Aallows the isolation device to be eliminated from power supply70while reducing arcing between the power supply and the system voltage bus upon hot extraction.

As mentioned previously, removing isolation devices (according to the various embodiments) may make it difficult to detect which power supply, among the many that may be coupled to the voltage bus, is causing an over voltage condition.FIG. 4depicts another embodiment of a removable power supply150that may be used to detect and mitigate an over voltage condition.

Power supply150includes a sampling diode155, with its anode connected to the cathode terminals of rectifying diodes90. The cathode terminal of sampling diode155is coupled to gain stage160through a resistor165. A second resistor170forms a resistor divider circuit with resistor165, and a capacitance175is coupled in parallel with resistor170. The output of gain stage160is coupled to a power supply control circuit180. Although not shown inFIG. 4, a buffer may be implemented before gain stage160to isolate noise from power supply150.

During operation, sampling diode155may sample the voltage at the cathode terminals of rectifying diodes90, which is labeled as node Z inFIG. 4. Note that the sampling performed by sampling diode155at node Z comes before inductor95and the output stage so that the sampled value at node Z is isolated from the output terminals of power supply150. In this manner, sampling diode155may sample the condition of power supply150independent of the other power supplies that may be present in the system.

Further, inductor95couples directly to busses40and45and limits the amount of current that enters or exits power supply150, where the precise amount of current limiting varies based on the value of inductor95. By limiting the current, the voltage at node Z may begin to build up and cause control circuit180to trip and turn power supply150off. Since numerous power sources (such as power supply150) may be coupled to busses40and45, inductor95may provide isolation between these numerous power sources.

Additionally, sampling diode155may provide further isolation in that noise from control circuitry180or gain stage160is prohibited from traversing from the cathode to the anode of sampling diode155. In other words, by virtue of the fact that sampling diode155should be forward biased to pass signals, when node Z is sufficiently above zero by this forward bias amount (e.g., 0.7 volts) signals traverse from node Z to control circuit180.

The sampled voltage value from sampling diode155is resistor divided by resistors165and170. In some embodiments, resistor165is ten times the size of resistor170, for example, 100 kΩ and 10 kΩ respectively. Depending on the value of capacitor175, it may either detect the peak or average sampled value from node Z. For example, if capacitor175is 4.7 μF, then the average value may be detected, whereas if capacitor175is 1000 pF, then the peak value of the signal from node Z will be detected.

In some embodiments, the peak value may be used instead of the average value. For example, if the tolerances of the overall system are such that a quick response to power supply variations are desired, then peak values may be used. On the other hand, if tolerances are configured that quick response to power supply variations are not necessary, then average values may be used.

In addition to gain stage160being coupled to node Z through resistor165, gain stage160is coupled to a reference voltage Vref. Based on comparing Vrefto the voltage across capacitor175(i.e., the average or peak value sampled at node Z) gain stage160may actuate control circuit180to turn power supply150on and off based on this comparison. Thus, in the event of an over voltage condition at the outputs of power supply150, control circuitry180may turn power supply150off.

Note that the monitoring and disabling of power supply150does not involve isolation devices in the same path that the supply current is flowing. As a result, circuitry180has the ability to provide isolation capability to power supply150without consuming as much power as the traditional isolation schemes shown inFIGS. 3A and 3B. Additionally, signal diodes (such as the MMBD7000LT1 available from On Semiconductor) may be used instead of the high power diodes (such as diode50) that are traditionally used for rectifying the power signals. Generally, these signal diodes have a much lower current rating, for example, 1 A compared to 80-100 A of the traditional high power diodes.

In the event that sampling diode155(shown inFIG. 4) is eliminated altogether, the value of resistor165may be chosen with a high enough value such that resistor165, along with inductor95, isolate noise from control circuitry180from trickling back into the output terminals of power supply150and also isolate noise from front end75and transformer85from polluting sampling circuitry. For example, resistor165may be 100 kΩ in these embodiments.

FIG. 5represents an alternative embodiment of power supply150that couples the anode terminals of two sampling diodes190(such as the MMBD7000LT1 available from On Semiconductor) to transformer85. Again, because sampling diodes190are implemented outside of the current path, the power consumption is less than the traditional isolation schemes shown inFIGS. 3A and 3B.

The above discussion is meant to be illustrative of the principles and various embodiments of the present invention. Numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. For example, although resistors and capacitances may be described and represented as single devices, one of ordinary skill in the art would appreciate that these resistors and capacitances actually may be implemented with multiple devices that are arranged in a binary-weighted arrangement. It is intended that the following claims be interpreted to embrace all such variations and modifications.