Abstract:
Described is a technology by which magnetic flux is used to provide backup power. A transformer has a line power source controllably coupled to a first input winding, and secondary power source controllably coupled to a second input winding. A controller monitors the line power and switches to the secondary power source if the line power voltage drops too low, or uses the secondary power source to augment the line power source if the line power current gets too high. Also described is incrementally transitioning from the secondary power source back to the line power source.

Description:
BACKGROUND 
       [0001]    Uninterruptible power supplies (UPS) designs typically work by inputting AC, rectifying the AC to DC that is coupled to a battery, and then inverting the battery output back to AC power. In the event the input AC goes away, the battery continues supplying the inverter with power until AC power returns or the battery is drained. 
         [0002]    However, even high efficiency uninterruptible power supplies lose a lot of energy, primarily to heat, and at best may be on the order of ninety-three percent efficient in contemporary designs. This loss is highly undesirable in large data center scenarios where megawatts of power are typically needed. One solution provides a complex, customized design that bypasses the battery when AC is available. However, this is also expensive. 
         [0003]    AC/DC rack level or in server battery backup/UPS designs create problems for high impedance power sources such as diesel generators (which are run when the regular source of AC power is lost, before the batteries are drained). One problem is that when the input voltage (VAC) drops below a specified (Under Voltage Protection, or UVP) level, the server powers supplies shut off and transfer load the local energy storage. When the generator is ready VAC input voltage increases to the point of operation, the power supply automatically turns on, the server load is removed from the local energy storage, and picked up by the power supply and then the generator. At this time, generator experiences an abrupt load increase from zero to one-hundred percent, e.g., a 2.5MW generator needs to transition from 0 W to 2.5MW with 10 msec from 10,000 250 W servers. Due the high output impedance characteristic of the generator, a high rate transient load increase causes an output voltage droop. The voltage droop is substantial enough to trip the UVP in the power supplies which summarily shut off and remove the load from the generator (one-hundred percent to zero percent transition). A continuous “on/off/on/off” or “motor boating” of energy dump is created. This behavior continues until local energy storage is depleted and cannot sustain the servers during the loss of VAC, or a component failure in the generator, distribution, or server power supply occurs. 
         [0004]    Methods to correct this problem at the server include varying the UVP threshold, VAC good threshold, and soft start circuit delay circuits. For a server application on the order of ten thousand of servers this introduces several thousand power supply designs, each with its own turn off-and-on signature. Even then, this method is not fail proof. Further, the addition of power supply designs/part numbers increases the cost of unit production as well as warranty repair management. 
         [0005]    A second tier control of reloading the generator is to delay the static transfer switchover. This still creates a block loading effect. Combined with variable turn off/turn on signatures, this is solution is complicated to deploy, expense to manufacture, and support. 
       SUMMARY 
       [0006]    This Summary is provided to introduce a selection of representative concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used in any way that would limit the scope of the claimed subject matter. 
         [0007]    Briefly, various aspects of the subject matter described herein are directed towards a technology by which a first power source and a second power source are controllable coupled through magnetic flux to provide a power supply. Switching mechanisms are automatically controlled to input power from the first power source to a first transformer winding, or to input power from the second power source to a second transformer winding, or to both. Voltage and/or current monitoring may be used to control the switching mechanisms. 
         [0008]    In one aspect, a load transfer mechanism for a power supply includes a transformer having a first input winding, a second input winding, and an output winding. A controller switches load between the first and second sources of power, or both, to achieve a desired output voltage corresponding to the output winding. The controller monitors power supply state to determine the switching of the load, including by selectively coupling the first source of power to the first input winding, or the second source of power to the second input winding, or both, based upon the power supply state. The controller may incrementally transitions at least some of the load from one source of power to the other source of power based upon the monitored state. 
         [0009]    One aspect is directed towards monitoring input voltage corresponding to power from a first power source coupled to a first input winding of a transformer. If the input voltage drops below an under voltage protection threshold, described is switching to couple power from a second power source to a second input winding of the transformer. Also descried is monitoring current corresponding to power from the first power source, and if the current exceeds an over current state, switching to couple at least some power from the second power source to the second input winding of the transformer. 
         [0010]    Other advantages may become apparent from the following detailed description when taken in conjunction with the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The present invention is illustrated by way of example and not limited in the accompanying figures in which like reference numerals indicate similar elements and in which: 
           [0012]      FIG. 1  is a block diagram showing example components of a transformer coupled current capping power supply topology according to one example implementation. 
           [0013]      FIG. 2  is a flow diagram showing example steps that may be taken by a controller to operate a transformer coupled current capping power supply. 
           [0014]      FIG. 3  is a block diagram representing how a battery may be selectively coupled to a transformer coupled current capping power supply. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    Various aspects of the technology described herein are generally directed towards an integrated load transfer mechanism for an isolated power supply (e.g., AC/DC) with active power factor correction. The technology operates via the introduction of magnetic flux at the primary isolation transformer fed from local energy storage devices, typically batteries. As will be understood, the technology is able to limit line cord current magnitude and rate of increase for the purpose of providing local energy backup/ride through, graceful controlled reloading (zero to one hundred percent) of high impedance AC sources and line cord power capping. 
         [0016]    It should be understood that any of the examples herein are non-limiting. For example, the technology is exemplified with respect to AC power in and DC power out, however the input may be AC power or DC power, of any suitable voltage, and the output may be DC or AC. Further, while the technology is generally applicable to IT power systems that support and operate through AC loss, AC brownout or sag, which are sourced from high impedance local power generation such as diesel generators, the technology applies to other applications, such as relatively small backup systems. Still further, while a voltage-based power supply that outputs a desired voltage is described, a current-based power supply that outputs a desired current is equivalent. As such, the present invention is not limited to any particular embodiments, aspects, concepts, structures, functionalities or examples described herein. Rather, any of the embodiments, aspects, concepts, structures, functionalities or examples described herein are non-limiting, and the present invention may be used various ways that provide benefits and advantages in power supply technology in general. 
         [0017]      FIG. 1  shows components of one example implementation of a power supply configured in accordance with the technology described herein. Input AC voltage, referred to as Vin, is converted to DC power via conventional power factor correction and rectification (block  102 ). In normal operation, a switch mechanism  104  (e.g., comprising one or more FETs) is pulsed by a controller  106  (e.g., a pulse width modulation controller) to produce rectified AC that is fed through a primary side transformer  107 , e.g., a ferrite core isolation transformer  108 . This in turn causes the secondary side transformer  110  to produce AC, which is rectified to DC power (block  112 ), which may be used for any suitable purpose, including powering a computer system. 
         [0018]    As described herein, the controller  106  may control another switch mechanism  114  (e.g., FET-based) to provide local energy storage (e.g., power from battery  116 ) AC-rectified power to another transformer winding  118 , thereby introducing magnetic flux instead or in addition to the magnetic flux from the primary (line cord-based) winding, basically creating a dual input, single output transformer design. The local energy storage may be an integral component of the power supply (e.g., built into a computer system) or may be a pluggable device. 
         [0019]    In one implementation, the controller includes sensor interfaces  120  for monitoring various power supply state data including voltages and currents in the power supply, and performing various operations based upon the power supply&#39;s operating state, as described herein. As shown in the example controller  106  of  FIG. 1 , a controller processor  122  runs logic  124  in a memory  126  to perform such operations. The controller  106  may be implemented in any suitable hardware and/or software device. One operation that may be performed by the controller  106  includes controlling charging circuitry  128  via a suitable switch mechanism  130 . Ways to charge the battery (in normal operation) include off the DC power, or off the transformer T 1 . 
         [0020]    In one implementation, the controller  106  monitors the input line current (I line ), input voltage (V line ), isolation transformer current (I T1 ), primary side transformer FET switching duty cycle (Q P ), secondary side transformer FET switching duty cycle (Q s ), battery discharge FET duty cycle (Q B ), battery charge FET duty cycle and output voltage (V out ). The controller  106  acts on the monitored points, adjusting Q P , Q s , Q B  and Q c  such that the output voltage is within a desired range, including while encountering AC loss and return, AC brownout or sag and return, and during high rate load transients. Note that the controller may be powered from the primary side and have its ground relative thereto, with a separate battery backup (e.g., a small watch/medical-type batter) or battery backup from the secondary side via an isolation transformer so operation continues while the primary side is not providing voltage. Because the transformer is an isolation transformer, optical coupling or other mechanisms may be used to allow the controller to control the switches/monitor the secondary side. Alternatively, the controller may run on the secondary side, with chassis ground and similar isolation to allow signaling/monitoring on with the primary side. 
         [0021]      FIG. 2  is a flow diagram showing various example operations of the exemplified controller  106 . For purposes of description, the following nomenclature may be used:
       Period of time for loading the generator from 0% to 100% (P)   Steps required for loading the generator from 0% to 100% (S)   Time between steps of reloading increase (t Step )       
 
         [0000]        t   Step =( P )/( S )       Magnitude of load step between steps of reloading (L Step )         
         [0000]        L   Step =(generator capacity)/(number of steps in reloading)       VAC input voltage (V ln )   Input current (I Line )   Battery switching duty cycle (Q B )   Primary transformer switching duty cycle (Q P )   Primary transformer current (I T1 )   DC output voltage (V out )   DC output switching duty cycle (Q S )   Battery charge switching duty cycle (Q C )         
         [0034]    The example steps of  FIG. 2  include how the controller operates during AC Fault and severe brown out/sag conditions, in which the voltage input magnitude droops below acceptable levels below under voltage protection threshold and returns to acceptable voltage levels after a period of time. In this state, the input line current does not exceed acceptable thresholds.  FIG. 2  also includes steps for handling nominal brown out/sag conditions, in which the voltage input magnitude droops below acceptable levels but remains above the under voltage protection threshold. This results in increased input line current exceeding acceptable levels. The input line current is reduced upon input voltage returning to acceptable levels after a period of time. 
         [0035]    Step  202  represents evaluating the input voltage Vin against the under voltage protection threshold. If not in an under voltage state, step  202  branches to steps  204  and  206  where the current is checked against an over current threshold. If the input voltage and the current are acceptable, the controller continues to operate the power supply operates normally. If the input voltage is acceptable but the current is too high (e.g., a nominal brownout condition), then step  208  represents controlling the switch duty cycles to augment the line input voltage power with battery power. This continues until the line voltage and current are acceptable, at which time the controller transitions away from battery power, e.g., in incremental steps, as represented via step  210  and  212 . 
         [0036]    If instead at step  202  the input voltage is in an under voltage state, step  202  branches to step  214  and  216  where the switches are controlled to switch the power supply to supply magnetic flux to the transformer from the battery. This continues until the under voltage condition ends. However, as described herein, the controller  106  does not switch directly back to V in  primary transformer side (from the AC line in or a generator), but instead controllably transitions the power supply from the battery back to the V in  primary transformer side power. 
         [0037]    Summarizing using the above nomenclature, for nominal brownout/sag conditions, if the VAC Input Voltage (V ln )&gt;=the under voltage protection threshold and input current (I Line )&lt;over current protection threshold power supply operates normally, as represented via steps  202 ,  204  and  206 . If the VAC input voltage (V ln )&gt;the under voltage protection threshold but the input current (I Line )&gt;over current protection threshold, power supply conversion decreases energy taken from the line cord by a reduction of primary transformer switching duty cycle (Q P ). Simultaneously, additional energy is pulled from the LES into the primary transformer such that such that DC Output Switching Duty Cycle (Q S ) is able maintain desired DC Output Voltage (V out ). These concepts are represented by step  208 . 
         [0038]    As represented via steps  210  and  212 ), when the VAC input voltage (V ln ) increases to (or remains at) acceptable levels and input current (I Line ) falls below over current protection threshold the controller  106  begins to increase the duty cycle to (Q P ), introducing additional energy into T 1  such that the current input (I Line ) is less than the calculated L Step . The controller  106  may increase the duty cycle to (Q P ) at a t Step  rate. During this process, the battery switching duty cycle (Q B ) is reduced accordingly such that the DC output switching duty cycle (Q S ) is able to maintain the desired DC Output Voltage (V out ). Once the battery switching duty cycle (Q B ) is reduced to zero, the power supply is in normal operation. 
         [0039]    For AC Fault and severe brownout/sag conditions, this may be stated as:
       If VAC Input Voltage (V ln )&gt;=under voltage protection threshold, the power supply operates normally.   If the VAC Input Voltage (V ln )&lt;=the under voltage protection threshold (step  202 ), power supply conversion stops with the primary transformer switching duty cycle (Q P ) controlled to zero. At this time, the battery switching duty cycle (Q B ) increases (step  214 ), pulling energy from the local energy storage (e.g., battery or batteries) into the primary transformer such that V out  is regulated.   When the VAC input voltage (V ln )&gt;the under voltage protection threshold (at step  218 ), the controller begins to increase the duty cycle to (Q P ), introducing additional energy into T 1  such that the current input (I Line ) is less than calculated L Step . The controller  106  may increase the duty cycle to (Q P ) at the t Step  rate. During this transition process (step  220 ) the battery switching duty cycle (Q B ) is reduced accordingly such that the DC output switching duty cycle (Q S ) is able to maintain the desired DC Output Voltage (V out ). Once the battery switching duty cycle (Q B ) is reduced to zero, the power supply is in normal operation.       
 
         [0043]    As can be seen with this topology, integrating the battery energy into the T 1  transformer allows for increased low voltage operational range as the energy pulled from the line cord is proportional to voltage and can work down to much lower voltage values than presently acceptable. A byproduct is that the LES holdup/ride through time can be extended. The under voltage threshold point no longer need be derived from the line cord fuse/breaker, but by functional duty cycle and operating voltage at the primary transformer. 
         [0044]    Turning to additional aspects, the technology may be used to facilitate power smoothing. For example, consider a set of ten thousand servers that generally run at eight amperes each, but occasionally spike to thirteen amperes during occasional rare periods of high demand. Providing the capacity to run ten thousand servers at up to thirteen amps each is very expensive, and thus in many situations the number of available servers heretofore needed to be reduced because sufficient power was not available. The technology described herein allows the extra current needed on only rare occasions to be drawn from the battery, whereby all suitably-equipped servers may remain available. 
         [0045]    Further, to implement the technology in many scenarios, smaller, longer lasting (e.g., lithium ion) batteries may be used relative to larger batteries (e.g., lead acid) used in other technologies. A larger transformer is used, however the corresponding cost is relatively inexpensive. 
         [0046]    In home and small office scenarios, a power supply may be provided in a computer system that allows battery backup to be obtaining by plugging in a suitable battery. For example, in  FIG. 3 , having a rechargeable battery  330  coupled to the power supply via a coupling  332  such as a jack or other suitable connector allows an optionally attached battery backup. The coupling  332  may be external to the computer system, or inside the computing device&#39;s cover. Any appliance with a power supply may benefit from a similar pluggable battery backup system; an inverter may be used for AC appliances. 
         [0047]    While the invention is susceptible to various modifications and alternative constructions, certain illustrated embodiments thereof are shown in the drawings and have been described above in detail. It should be understood, however, that there is no intention to limit the invention to the specific forms disclosed, but on the contrary, the intention is to cover all modifications, alternative constructions, and equivalents falling within the spirit and scope of the invention.