WO1989004562A1

WO1989004562A1 - Power supply system for segmented loads

Info

Publication number: WO1989004562A1
Application number: PCT/US1988/003803
Authority: WO
Inventors: Patrice Raynaud Albert Lethellier
Original assignee: Unisys Corporation
Priority date: 1987-11-09
Filing date: 1988-10-31
Publication date: 1989-05-18
Also published as: JPH02502332A; DE3887321T2; KR920007374B1; JPH0650937B2; EP0340292B1; US4760276A; KR890702310A; EP0340292A1; DE3887321D1

Abstract

A power supply system (Fig. 2) which is comprised of multiple sets of power supplies (set 1, 2 and 3); each set having an output terminal (OT1, 2, and 3) for furnishing load current at a certain voltage; and a respective load (L1, L2 and L3) connected to the output terminal of each set; wherein a variable resistor means (20a, 20b, 20c) is provided which selectively interconnects the respective output terminals (OT1, 2 and 3) of said sets to each other; said variable resistor means (20a, 20b, 20c) including a means (e.g., Fig. 5) for varying the conductance between any two of said output terminals such that the conductance increases from zero mhos by a predetermined small step and thereafter gets progressively larger, and vice versa.

Description

POWER SUPPLY SYSTEM FOR SEGMENTED LOADS

BACKGROUND

This disclosure relates to power supply system which furnish DC current at a certain voltage to electrica loads; and more particularly, it relates to such powe supply systems in which the loads are segmented and th power supply system has redundancy.

Typical electrical loads for such power supplie are integrated circuits. They are usually packaged o printed circuit boards which have power and ground pins a well as multiple input/output pins for receiving an sending signals. Hundreds of these boards are ofte included in a single electronic system, such as a larg data processing system or a large communications system Multiple backplanes are commonly provided to hold th boards in groups of 10 to 20 and send signals between them and each backplane has its own power and ground buses. One way to provide power to such a multiple backplane system is to cable the power buses of all the backplanes together, and to connect them to a set of power supplies which operate in parallel to share in furnishing the total load current. Also, a redundant supply (i.e., an extra supply) can be included in such a system so that if any one power supply fails, the electronic system can still operate. This type of power supply system, including a redundant supply, is described in United States Patent 4,698,738 by J. Miller and J. Walker which is assigne_d to the present assignee.

However, in a multiple backplane electronic system, it is often desirable to have the backplanes an_d their supplies segmented (electrically isolated) from each other. Such segmenting enables the supplies for just one backplane to be turned off while the supplies and the circuitry of the remaining backplanes continue to operate. For example, large data processing systems often contain multiple digital computers, each of which is housed on a different backplane. When the circuitry in one of those computers fails or needs to be upgraded, it is desirable to be able to power down just the backplane of that one computer so that the repair or upgrade can be made while at the same time, the remaining computers continue to operate. However, with the above referenced power supply system, this cannot be done because there, power cannot be independently applied to and removed from the individual backplanes.

One way to solve the above problem is to not connect the power buses of the backplanes together, and to provide a separate set of power supplies (such as those of the referenced power supply system) for each backplane. But in that case, a separate redundant supply would also have to be provided for each backplane; and that woul_d substantially increase the cost of the system. For example, consider an electronic system of FIG. 1 which has three loads L_]_, L2 r and L3. Suppose further that load h-_ requires two power supplies 1A and IB to furnish its load current; load L₂ requires just one power supply 2A to furnish its load current; and load L3 requires three power supplies 3A, 3B, and 3C to furnish its load current. In that case, a total of six power supplies are required to furnish the needed load current, but an additional three redundant supplies R (one for each load) are also required to provide redundancy. Thus, redundancy increases the cost of the system by 50%.

INTENTIONALLY LEFT BLANK

BRIEF DESCRIPTION OF THE DRAWINGS; Various features and advantages of the invention are described herein in detail in conjunction with the accompanying drawings wherein:

FIG. 1 shows a power supply system which has segmented loads and redundancy; FIG. 2 shows a preferred embodiment of a power supply system which is constructed according to the invention;

FIG. 3 is a more specific version of the FIG. 2 embodiment in which the current requirements for the loads and the current-furnishing capacities for the supplies are all specified;

FIG. 4 is a set of equations which explain the time sequence by which the FIG. 3 system operates;

FIG. 5 shows a preferred physical makeup for some of the components in the system of FIGs. 2 and 3;

FIG. 6 illustrates the operation of the FIG. 5 component; and

FIG. 7 shows the operation of the FIG. 3 syste under the conditions where one power supply fails. DETAILED DESCRIPTION

Referring now to FIG. 2, one preferred embodiment of a power supply system which is constructed in accordance with the invention will be described in detail. This powe supply system includes three sets of power supplies which are hereinafter referred to as SET1, SET2, and SET3. In SET1 there are two power supplies PS_]__a and SIJ-_J; in SET2 there is one power supply PS_2a; and in SET3 there are three power supplies PS3_a, PS3b, and PS3_C. Each set of power supplies has an output terminal which furnishes load current at a predetermined voltage. In FIG. 2, the output^' terminal of the SET1 power supplies is indicated as OT ; the output terminal of the SET2 power supply is indicated as OT₂ ; and the output terminal of the SET3 power supplies is indicated as OT3. A load _]_ is connected directly to output terminal OT ; a load L₂ is connected directly to output terminal OT₂; and a load L₃ is connected directly to output terminal OT3.

Each set of power supplies further includes a current-sharing circuit which operates to equalize the load current that each power supply in a set furnishes to its respective output terminal. In FIG. 1, the current-sharing circuit for the SET1 power supplies is schematically indi¬ cated by a dashed line CS]_; the current-sharing circuit for the SET2 power supply is indicated by a dashed line CS₂; and the current-sharing circuit for the SET3 power supply is indicated by a dashed line CS3. All of the details of one suitable current-sharing circuit are given in United States patent 4,698,738; and they are herein incorporated by reference.

Now in accordance with a preferred enftodirrerrt of the invention, a 20 i^^ ^ovi ed which selectively connects and disconnects the output terminals OT]_, OT₂, and OT3 of the power supply sets as they are running. In FIG. 2, connector 20 is illustrated as being comprised of three variable resistors 20a, 20b, and 20c which respectively connect output terminals OT_l7 OT₂, and OT₃ to a node 20d. Each of these resistors has a conductance that increases from zero by a predetermined small step and thereafter progressively increases to a highly conductive value.

Also in accordance with the preferred βi odimeπt, a switch 30 is provided which selectively interconnects, and discon¬ nects, the respective current-sharing circuits of each of the power supply sets. In FIG. 2, switch 30 is shown as consisting of three single pole single throw switches 30a, 30b, and 30c which respectively interconnect the current-sharing circuits CS]_, CS₂, and CS3 to a node 30d. When the current-sharing circuits of any two power suppl sets are connected via switch 30, and the output terminals of . those same two sets are connected via connector 20 through a high conductance, then the supplies of both sets equalize their output currents.

Further in accordance with the preferred embodiment, th total number of power supplies is selected such that all of the supplies together, minus any one supply, have a tota current-furnishing capacity which meets the curren requirements of all of the loads; and at the same time, n set of supplies is made so large as to include a redundan supply. One particular example of this is shown in FIG. 3. There, the loads L_, L₂, and L3 are respectively shown as requiring 550 amps, 450 amps, and 1400 amps; the SET1 powe supplies have a capacity of 500+500 amps; the SET2 suppl has a capacity of 500 amps; and the SET3 supplies have capacity of 500+500+500 amps. Now, to understand how the power supply system of FIG. 3 operates, reference should be made to FIG. 4 wherein the state of that power supply system is shown at various time instants tø through t- . Initially, beginning at time tn, all of the variable resistances 20a-20c are in their most conductive position, and all of the current-sharing switches 30a-30c are closed. Suitably, the resistance of each varia_ble resistance at this time is less than 20 micro ohms.

Due to the above switch and resistance positions, the power supplies in all of the sets furnish equal amounts of loa_d current. In the specific example of FIG. 3, current I_T from each of the SETl supplies equals 400 amps as given by equation 1. Current I₂ from the SET2 supply and I3 from each of the SET3 supplies equals current I]_ as is given by equations 2 and 3. Switch 20a carries a

'current I4 which is +250 amps as is given by equation 4.

_Suppose now that the FIG. 3 system continues to operate in the above state until some sort of fault occurs in loa_d i or a feature needs to be added to ^. Then the need arises to remove power from load ]_ so that the problem can be worked on. To that end, at time t]_, switch

30a is opened thereby stopping any current-sharing between the power supplies of SETl and the power supplies of SET2 and SET3. As a result, the current furnished by the supplies of SETl will stop tracking the current of the other supply sets, and a small voltage difference will occur _between the output terminal of SETl and the other supply sets.

_Assume now that the voltage on output terminal OT]_ is slightly less than the voltage on output terminals 0T₂ and _O -₃. This is stated by equation 5. Due to the slight voltage im_balance, the SET2 and SET3 power supplies will try to furnish all of the current to all of the loads ^, L₂, and 3. This is stated by equation 6. However, the power supplies of SET2 and SET3 do not have enough current-furnishing capacity to furnish all of the load current. This is shown by equations 7 and 8. Equation 7 shows that the four supplies of SET2 and SET3 each need to furnish 600 amps in order to supply all of the load current; and equation 8 says that each supply can only furnish 500 amps. Thus, currents I₂ and I3 will be limited to 500 amps as is given by equation 9; and the excess of that current over the requirements of loads L₂ and L3 will equal the current 14 through switch 20a. Thus, current I4, as is stated by equation 10, becomes -150 amps. Load I_ι requires 550 amps; the difference between that amount and the 150 amps through switch 20a is furnished by the SETl power supplies; and so the supplies PS__a and PS__]-, each furnish 200 amps.

Next, at time t , the resistance of the variable resistor 20a is slightly increased from its minimum value. For example, as stated by equation 12, the resistance increases to 50 micro ohms. Thus, if the current 14 through resistor 20a remains unchanged, the voltage across resistor 20a will rise to 7.5 millivolts. Any rise in voltage across resistor 20a has, however, an important constraint. Specifically, the voltage across resistor 20a cannot exceed the tolerances with which the voltages can vary between the output terminals of any two power supplies.

Constant voltage power supplies have a tolerance of +ΔV on their output terminal voltage which is caused by the accuracy with which the constant voltage can be preset. For low voltage supplies (i.e., zero to ten volts), that voltage tolerance _%^~7 of any one supply typically will be +10 millivolts. Thus the maximum voltage differenc between any two supplies is 2 Δv which occurs when one supply voltage is preset at nominal minus ^v and the other supply voltage is preset at nominal plus .V . Typi¬ cally, 2 j ^~7 has a maximum of 20 millivolts, and this is stated by equation 14. Since the 7.5 millivolt drop across resistor 20a is less than this largest preset voltage difference between the output terminals of two supplies, the state of the system as shown at time t₂ is stable and can occur. Next, at time instant t3, the resistance of resistor 20a is increased further to 200 micro ohms. This is stated by equation 16. Thus, if the 150 amps continues to flow through resistor 20a, the voltage drop across resistor 20a will be 30 millivolts. This is stated by equation 17. But such a 30 millivolts drop cannot occur because it is greater than the maximum tolerance that is possible between the output terminal voltage of any two supplies. Consequently, the drop across resistor 20a will be limited to that maximum or 20 millivolts. This is stated by equation 18.

Now, given a 20 millivolt maximum drop across resistor 20a while its resistance is 200 micro ohms, then the current 14 must equal -100 amps. This is stated^' by equation 19, and it is less than the -150 amps that occur at times to _r \ , and t . Consequently, the power supplies of SETl will increase the amount of current which they furnish to load L_ as is shown by equation 20; and the power supplies of sets 2 and 3 will decrease the amount of current which they furnish as is shown by equation 21. Next, at time t4 , the resistance of resistor 20a is further increased to 4000 micro ohms. This is stated by equation 22. As a result, the current I drops even further to just 5 amps as is stated by equation 23. Thus the load current from the supplies of SETl increase further as is given by equation 24; and the current from th supplies of SET2 and SET3 decrease further as is given b equation 25.

Finally, at time tζ , the variable resistor 20 open circuits, and so its conductance goes to zero. Thi is stated by equation 26. As a result, current I₄ goe from 5 amps to zero. This change in current which occur when the resistance 20a is opened is, however, very small Thus no hazardous sparking occurs, no data damaging RF radiation occurs, and no data damaging sag or spike i output terminal voltage occurs. This is stated by equatio 27.

Once resistor 20a is opened, the power supplies o SETl furnish all of the current to load I_ι; and the powe supplies of SET2 and SET3 furnish all of the current t loads L₂ and L₃. This is stated by equations 28 and 29 In other words, load L_ι and the power supplies of SETl hav been isolated from the rest of the system.

At this point, the power supplies of SETl can b turned off so that load Li can be worked on (i.e., problem repaired or a feature added). And at the sam time, the loads L₂ and L3 can continue to operate Allowing loads L and L3 to operate in this fashion is very important feature since, for example, it can bring i thousands of dollars of extra billings in the case wher each load is a computer in a multiprocessor data processin system.

After the work on load L is complete, the syste is reconnected by performing all of the above describe operations in reverse order. Thus, the initial state o the system will be as given at time t5; the next state o the system will be as is given at time t4; etc. And during the reconnection sequence, no hazardous sparkin occurs, no data damaging RFI occurs, and no data damaging sag or spike in output terminal voltage occurs since the step in conductance is small and thus the step in load current is small, Turning now to FIG. 5, a preferred embodiment of each of the variable resistors 20a, 20b, and 20c will be described. This embodiment includes one member 40 which has an elongated passageway 41; and it includes anothe member 42 which is shaped to slide into the passageway 41 and engage the passageway surfaces. Preferably there is some elasticity between the members 40 and 42 as they engage so they make intimate contact but do not wear out. Symbol "d" in FIG. 5 indicates the distance by which membe 42 is inserted into the passageway 41; and the conductance between the members 40 and 42 increases as the distance "d" increases.

- As was previously explained, it is critical that the step in conductance between .the members 40 and 42 is very small as those members initially engage. In the FIG. 5 embodiment, this small step is achieved by providing a _beveled tip 43 on member 42. By beveling the tip 43, the surface area between the members 40 and 42 at their point of initial engagement can be made as small as desired, and thus the initial conductance between those members is reduced by a like amount.

Conductance between the members 40 and 42 increases in a nonlinear fashion until all of the tip 43 is in the passageway 41. Thereafter, the conductance between the members 40 and 42 rapidly increases in a linear fashion with the distance "d". All of this is shown in FIG. 6 by curve 44.

Preferably, the entrance to the passageway 41 is covered with an insulator 45. This insulator 45 prevents the end of the tip 43 from being accidentally pushed flus against that portion of member 40 which the insulator 4 covers. Such accidental contact must be prevented since i would produce a large step in conductance between th members 40 and 42. Also, the insulator 45 within th passageway serves as a guide which prevents "contac bounce" from occurring between the members 40 and 42 a they initially engage.

Consider now the sequence which occurs when an one of the power supplies fails. This situation is illus trated in FIG. 7 wherein power supply PS3_C is shown to hav failed. When a supply fails, it automatically turns itsel off and acts like an open circuit. Current to the load Li, L₂, and L₃ is then furnished by all of the remainin power supplies which are still operating.

For the system which is illustrated in FIG. 7 each of the remaining power supplies will furnish 480 amp after supply PS_3a fails. This is calculated a 550+450+1400 divided by 5. Current I will equal 480X2-50 current I5 will equal 480-450; and current Ig will be th sum of current I4 and current I5.

One primary feature of the above system is that i the event of a power supply failure, all of the loads wil continue to operate even though no one load has an extra o redundant supply. This is very important because th alternative of adding an extra or redundant supply in eac set would significantly increase the system's cost an thereby make it less competitive. Further, the addition o an extra supply in each set would decrease the system' MTBF (mean time between failure) simply because there woul be more parts that might fail.

To replace the failed power supply S3_C, repairman merely needs to pull that supply out of a socke which attaches it to the output terminal an current-sharing circuit, and to insert a new supply. Whe that occurs, the power supply system will then revert bac to the state which it had before supply PS_3a failed (i.e. the state which is described at time t₀ in FIG. 4).

A preferred embodiment of the invention has no been described in detail. In addition., however, man changes and modifications can be made to these detail without departing from the nature and spirit of th invention.

For example, in the embodiment of FIG. 3, th power supplies of all of the sets have the sam current-furnishing capacity. But as an alternative, th current-furnishing capacity of the supplies can vary fro set to set and they can also vary within a set.

To illustrate this point, suppose that load L3 i FIG. 3 only requires 1100 amps. Then, power supply PS₃ can be changed to have a current-carrying capacity of onl 200 amps. When all of the loads and all of the supplie are operating, each supply will share in furnishing loa current in proportion to its current-furnishing capacity _An_d when any one power supply fails, the remaining powe supplies will likewise share in picking up the added load.

As another alternative, the physical makeup of th variable resistors 20a, 20b, and 20c can be modified fro that which is shown in FIG. 5; however, it is critical tha the initial step in conductance from zero remains small For example, instead of having a beveled tip 43 on membe 42, just a small narrow portion of the surface o passageway 41 can be made conductive at the passagewa entrance; and that conductive portion can then be mad progressively wider with the distance "d". Some latitude is allowable on the maximum size of the step in conductance which occurs in the variable resistors 20a, 20b, and 20c. However, as the size of the step increases, a corresponding step in load current also occurs in each of the power supply sets; and that in turn causes a voltage sag or spike to occur at the loads. Eventually, a point is reached at which the step in current is so large that the resulting sag or spike in voltage causes the circuitry in a load to operate improperly. So, to avoid this problem, the product 2/ VAC pre¬ ferably is limited to that which causes a voltage sag or spike at the loads which is less than 3% of the nominal supply voltage. Alternatively, the product 2 Ac is limited to be less than 20% of the respective current- furnishing capacity of each power supply set. Here as before, ± ΔV is the permissible preset tolerance of the output voltage. In many cases, these constraints can be met by limiting A^c to be less than one thousand mhos

(i.e., - limiting 1 Δ C to be more than 1000 micro ohms). Note that, as was described above in conjunctio with FIG. 4, a large change in load current also can occu when one of the current-sharing switches is opened o closed. In FIG. 4 at time ti, current I4 changed from +250 amps to a -150 amps. But that change in current doe not cause the voltage on the output terminals to go out o the regulation band; nor does it generate RFI radiation. This is because the speed with which the current chang occurs is limited to the reaction time of th current-sharing circuitry of the power supplies Typically, the total change in current would occur over 1 to 50 milliseconds. By comparison, when one of th variable resistors 20a, 20b, or 20c is opened, the curren through that resistor drops to zero instantaneously. Note also that certain other problems will occu if the current-sharing switch of a set of supplies is no opened before the variable resistor for that set o supplies is opened, and vice versa. Suppose, for example, that in FIG. 3, the variable resistor 20a is opened whil the current-sharing switches 30a, 30b, and 30c are closed. When that occurs, all of the supplies will continue to tr to share in furnishing the total load current, but suc sharing will be impossible since resistor 20a is open. I their effort to share the load current, the power supplie of SETl will increase their output terminal voltage and th power supplies of SET2 and SET3 will decrease their outpu terminal voltage. And this will continue until th terminal output voltages of all the supplies go out o regulation. Conversely, the power supply system of FIG. will work, although in an unevenly stressed fashion, if th current-sharing switches 30a, 30b, and 30c always remai open. In fact, the current-sharing between the supplies o each set can be disabled, and the system will still work.

Note further that in order to maintain its prese output voltage, power supplies sense their actual outpu voltage and adjust it up or down until it equals the prese value. But that sensing can occur at the power suppl output terminal, or at the load. So preferably, th terminals of the variable resistors 20a, 20b, and 20c whic interconnect the power supply sets are placed at the point where the power supply sets sense their output voltage. _Otherwise, the voltage across a variable resistor coul exceed 2 Λv, and that in turn would increase the instan taneous change in current which occurs through tha component when its step in conductance Λ c occurs.

In view, therefore, of all of the above, it is t be understood that the invention is not limited to th details of the illustrated embodiments, but is defined b the appended claims.

Claims

WHAT IS CLAIMED IS:

1. A power supply system which is comprised of: multiple sets of power supplies; each set having an output terminal for furnishing load current at a certain voltage; a respective load connected to the output terminal of each set; and each set further having a current-sharing circuit by which the load currents furnished from the supplies within a set are shared; wherein all of said supplies together, minus any one supply, have a total current-furnishing capacity which meets the current requirements of all of said loads, and yet no set of supplies includes a redundant supply; a switching means is provided which selectively interconnects, and disconnects, the respective current- sharing circuits of said sets such that the supplies of all connected sets furnish equalized load currents; and variable resistors are provided which selectively interconnect the respective output terminals of said sets through a conductance that increases from zero by a predetermined small step and thereafter progressively increases to a short circuit, and which selectively _disconnects the respective output terminals through an oppositely varying conductance.

_2< A power supply system according to claim 1 wherei said predetermined small step in conductance is limited t that which produces a voltage sag or spike at said load which is less than 3% of said certain voltage.

3_# p_ power supply system according to claim 2 wherei each of said variable resistors includes first and secon members with respective surfaces that slideably engage i selectable amounts, with the degree of said conductanc increasing with the amount of engagement.

4. A power supply system according to claim 3 wherei said first member includes an elongated passageway, an said second member includes an elongated- portion whic slides into said passageway and engages its surface i selectable amounts to vary said conductance.

5. A power supply system according to claim 4 wherei said elongated portion has a beveled tip which minimize the amount of initial engagement with said passageway.

g_^ p_ power supply system according to claim 5 wherei each respective load is a backplane in a computer system.

7 . A power supply system which is comprised of : multiple sets of power supplies ; each set having an output terminal f or furnishing load current at a certain voltage ; a respective load connected to the output terminal of each set ; and each set further having a current-sharing circu it by which the load currents furnished f rom the supplies within a set are shared ; wherein all of said supplies together, minus any one supply, have a total current-furnishing capacity which meets the current requirements of all of said loads , and yet no set of supplies includes a redundant supply; a f i rst means is provided which selectively enables the respective current-sharing circuits of any of said sets of power supplies to operate together and share in furnishing their load currents ; and a second means - is provided which selectively interconnects the output terminals of said sets through respective conductances which increase f rom zero by a predetermined small step and thereafter progress ive ly increase to a short circu it , and vice versa.

g_ _ power supply system according to claim 7 wherei said predetermined small step in conductance is limited t that which confines any voltage sag or spike at said load to less than 3% of said certain voltage.

_9# A power supply system according to claim 7 wherei said predetermined small step in conductance is limited t that which produces a current step of less than 20% of th respective current-furnishing capacity of each set o supplies.

ø_β _ power supply system according to claim 7 wherei said predetermined small step in conductance is limited t one thousand mhos.

_ power supply system according to claim 7 wherei said second means includes first and second members wit respective surfaces that slideably engage in selectabl amounts, with the degree of said conductance increasin with the amount of engagement.

12. A power supply system according to claim 7 wherei each set of power supplies maintains said certain voltag by sensing the voltage at a node between its outpu terminal and its load, and wherein said variabl conductances of said second means are connected at suc nodes.

13. A power supply system which is comprised of: multiple sets of power supplies; each set having an output terminal for furnishing load current at a certain voltage; and a respective load connected to the output terminal of each set; wherein

^" all of said supplies together, minus any one supply, have a total current-furnishing capacity which meets the current requirements of all of said loads, and yet no set of supplies includes a redundant supply; and a means is provided which interconnects the respective output terminals of said sets and selectively varies the conductance between them such that the conductance increases from zero by a predetermined small step and thereafter gets progressively larger, and vice versa.