Abstract:
An information handling system (IHS) is provided which is powered by a zero voltage switching power supply. The power supply is capable of maintaining regulation even under very light loads. The supply includes first and second switches arranged in complementary configuration to deliver current to a load dependent inductor. This load dependent inductor acts as an energy source which supplies energy to parasitic elements in the first and second switches to aid in switching of the first and second switches during power supply operation.

Description:
BACKGROUND  
       [0001]     The disclosures herein relate generally to information handling systems (IHS&#39;s) and more particularly to switching power supplies for IHS&#39;s.  
         [0002]     As the value and use of information continue to increase, individuals and businesses seek additional ways to process and store information. One option available to users is information handling systems. An information handling system (IHS) generally processes, compiles, stores, and/or communicates information or data for business, personal, or other purposes thereby allowing users to take advantage of the value of the information. Because technology and information handling needs and requirements vary between different users or applications, information handling systems may also vary regarding what information is handled, how the information is handled, how much information is processed, stored, or communicated, and how quickly and efficiently the information may be processed, stored, or communicated. The variations in information handling systems allow for information handling systems to be general or configured for a specific user or specific use such as financial transaction processing, airline reservations, enterprise data storage, or global communications. In addition, information handling systems may include a variety of hardware and software components that may be configured to process, store, and communicate information and may include one or more computer systems, data storage systems, and networking systems.  
         [0003]     IHS&#39;s often employ switching power supplies to provide voltage and current to the various circuits within IHS&#39;s. One typical type of switching power supply includes a pair of switching transistors which are alternately switched on and off to provide energy to an inductor, capacitor and resistive load. Early switching power supplies employed driver circuitry to provide drive signals to turn the switching transistors on and off as needed. Unfortunately the drive signals required significant energy to perform the switching function and this contributed to relatively low operational efficiency of these supplies. Zero voltage switching power supplies were developed to minimize the voltage and energy needed to turn the switching transistors on and off. In this approach, the energy stored in the inductor is used to assist the switching of the switching transistors. One zero voltage switching power supply approach is described in the publication, Zero-Voltage Switching Quasi Square Wave Converters, by Igor Goryanskey, NIFKI, Moscow, Russia, the disclosure of which is incorporated herein by reference.  
         [0004]     Unfortunately, while the zero voltage switching power supply approach increases overall efficiency, another problem is encountered when zero voltage switching is employed. Under very low loads, namely high impedance loads, the size of the inductor must be very large in order for zero voltage switching to occur. This is so because there must be sufficient current flowing in the inductor so that the inductor has enough energy to provide zero voltage switching. If the load is light, namely high impedance, the inductor current can be so small that the field around the inductor is insufficient to provide the energy needed for zero voltage switching.  
         [0005]     What is needed is a way to achieve zero voltage switching in a switching power supply even under low load conditions.  
       SUMMARY  
       [0006]     Accordingly, in one embodiment, a method is disclosed for operating an information handling system (IHS) including a switching power supply. The method includes storing energy in a load dependent inductor exhibiting an inductance which increases as current through the inductor decreases. The method also includes supplying energy from the load dependent inductor to switches in the switching power supply to achieve zero voltage switching of the switches. The method further includes providing energy from the switching power supply to power the IHS.  
         [0007]     In another embodiment, an information handling system (IHS) is disclosed which includes a processor and a memory coupled to the processor. The IHS also includes a power input coupled to the processor and the memory. The IHS further includes a switching power supply coupled to the power input. The switching power supply includes a load dependent inductor for storing energy, the load dependent inductor exhibiting an inductance which increases as current through the inductor decreases. The switching power supply also includes first and second switches arranged in complementary configuration, the load dependent inductor supplying energy to the first and second switches to achieve zero voltage switching of the first and second switches. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]      FIG. 1  is a block diagram of the disclosed information handling system (IHS).  
         [0009]      FIG. 2  is a representation of the zero voltage switching power supply in the IHS of  FIG. 1 .  
         [0010]      FIG. 3A  is a representation of a conventional inductor with a core having a constant gap distance.  
         [0011]      FIG. 3B-3D  are representations of inductors with a cores having non-constant gap distances.  
         [0012]      FIG. 4A  is a representation of a conventional inductor with a core having a constant gap distance.  
         [0013]      FIG. 4B-4G  are representations of inductors with cores having non-constant gap distances.  
         [0014]      FIG. 5A-5E  are representations of inductors with cores having non-constant gap distances. 
     
    
     DETAILED DESCRIPTION  
       [0015]      FIG. 1  is a block diagram of the disclosed information handling system (IHS)  100  employing a zero voltage switching power supply  200 . For purposes of this disclosure, an information handling system (IHS) 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.  
         [0016]     In one embodiment, IHS  100  includes a processor  105  such as an Intel Pentium series processor or one of many other processors currently available. An Intel Hub Architecture (IHA) chipset  110  provides IHS  100  with glue-logic that connects processor  105  to other components of IHS  100 . Chipset  110  carries out graphics/memory controller hub functions in its memory controller hub or MCH  111 . Chipset  110  carries out I/O controller functions in its I/O controller hub or ICH  112 . More specifically, the MCH  111  of chipset  110  acts as a host controller which communicates with a graphics controller  115  coupled thereto. Graphics controller  115  is coupled to a display  120 . The MCH of chipset  110  also acts as a controller for main memory  125  which is coupled thereto.  
         [0017]     Input devices  130  such as a mouse, keyboard, and tablet, are coupled to the ICH  112  of chipset  110 . An expansion bus  135 , such as a Peripheral Component Interconnect (PCI) bus, PCI Express bus, SATA bus or other bus is coupled to chipset  110  as shown to enable IHS  100  to be connected to other devices which provide IHS  100  with additional functionality. A peripheral device bus  140  such as a universal serial bus (USB) is coupled to the ICH of chipset  110  as shown. System basic input-output system (BIOS)  145  is coupled to chipset  110  as shown. A nonvolatile memory such as CMOS or FLASH memory is used to store BIOS software  145 . A network interface controller (NIC)  150  is coupled to ICH of chipset  110  to facilitate connection of system  100  to other information handling systems. A media drive controller  155  is coupled to the ICH of chipset  110  so that devices such as media drive  160  can be connected to chipset  110  and processor  105 . Devices that can be coupled to media drive controller  155  include CD-ROM drives, DVD drives, hard disk drives and other fixed or removable media drives. IHS  100  includes an operating system which is stored on media drive  160 . Typical operating systems which can be stored on media drive  160  include Microsoft Windows XP, Microsoft Windows 2000 and the Linux operating systems. (Microsoft and Windows are trademarks of Microsoft Corporation.)  
         [0018]     IHS  100  includes a main power button switch  165  coupled to chipset  110 . When main power button switch  165  is pressed, chipset  110  generates a power on/wake signal which is supplied to a power supply  200  that is coupled to chipset  110 . Power supply  200  includes an output VO which is coupled to one or more power planes in IHS  100 . When power button  165  is pressed the power on/wake signal instructs power supply  200  to turn on and supply an output voltage, VO.  
         [0019]      FIG. 2  is a schematic diagram of one embodiment of power supply  200 . Power supply  200  includes an input  207 A,  207 B which is connected to AC mains  205 . Power supply  200  includes a rectifier  210  which is coupled across input  207 A,  207 B to rectify AC mains power from AC to pulsating DC. Capacitors  215 ,  220  are coupled together at node  225 . The split capacitor structure  215 ,  220  thus formed is coupled across rectifier  210  as shown. Switches  230 ,  235  are coupled to a common node  240  as shown. The switching circuit formed by switches  230 ,  235  is coupled in parallel with split capacitor  215 ,  220  and rectifier  210 . Switches  230 ,  235  can be virtually any electronic switch, for example, FETs, bipolar transistors, SCRs, triacs and so forth.  
         [0020]     Power supply  200  includes a transformer  245  having a primary winding  245 A and a secondary winding  245 B. An inductor  250  is coupled in series with primary winding  245 A. The structure thus formed by inductor  250  and primary winding  245 A is coupled across nodes  225  and  240  as shown. The ends of secondary winding  245 B are coupled by respective diodes  255  and  260  to a node  265 . An output capacitor  270  is coupled between node  265  and ground. A load  275 , such as the power plane or planes of an IHS, is coupled between node  265  and ground.  
         [0021]     Switches  230  and  235  are alternately opened and closed in complementary fashion while supply  200  operates. Switching signals from driver  280  are provided to switches  230  and  235  as part of the switching process. The output voltage VO is compared with a desired output reference voltage, VREF, by error comparator  285 . An error signal is generated at the output of error comparator  285  which is coupled to a voltage controlled oscillator (VCO)  290 . The error signal is an indication of how far off the actual output voltage, VO, is from the desired output voltage, VREF. Accordingly, the frequency of VCO  290  is varied to control the frequency of the driver signal pulses used in switching switches  230  and  235  on and off. The frequency of the driver signal is varied until VO equals VREF.  
         [0022]     In more detail, this particular embodiment of power supply  200  operates as follows. The AC mains voltage at inputs  207 A,  207 B is rectified into a pulsating DC current by rectifier  210 . This pulsating DC current is filtered by capacitors  215  and  220  and results in a DC voltage across these capacitors. Half of this voltage appears at node  225  between capacitors  215 ,  220 . Assuming that switch  230  is closed and switch  235  is open, a current I 1  flows in the direction indicated in  FIG. 2 . The current I 1  causes inductor  250  to build a magnetic field. The inductor current rises linearly as it follows V=L di/dt. The current I 1  flows through the primary winding  245 A of transformer  245  and returns via node  225  and capacitor  220  to ground as shown. The current I 1  flowing in primary winding  245 A causes a secondary current IS to be induced in the secondary winding  245 B of transformer  245 . This current will flow through either diode  255  or diode  260 . For purposes of this example, it is assumed that secondary current IS flows through diode  255  in the direction indicated in  FIG. 2 . The secondary current IS then passes through node  265  to the parallel combination of output capacitor  20  and load RL  275 .  
         [0023]     Thus far, circuit operation has been described during the time that switch  230  is on. Switch  230  is now turned off by the switching signal from driver  280 . However, switch  235 B is not immediately turned on. At this point it is noted that switches  230  and  235  include parasitic body diodes  230 A and  230 B, respectively. These body diodes are parasitics which are inherent in the fabrication of MOS FET switches. If bipolar transistors are used as switches  230  and  235 , then discrete diodes are added to these switches since bipolar transistors do not have intrinsic parasitic body diodes. When switch  230  is turned off while current was flowing in inductor  250  in the direction indicated for current I 1 , the field of the inductor is built up and, due to Lenz&#39;s law, inductor current will continue to flow in the same direction. Inductor  250  becomes a current source. The energy from inductor  250  assists transistor  230  in turning off and also assists transistor  235 &#39;s intrinsic body diode  235 B in turning on. After switch  230  has transitioned losslessly as described above and switch  235 &#39;s body diode  235 B is turned on, switch  235  is now turned on by the switching signal from driver  280 . Turning switch  235  on causes inductor  250  to discharge with its current eventually changing direction and flowing toward node  240  as shown by current I 1 . Switch  235  then turns off under the instruction of the driver signal from drive  280 . This transition again occurs losslessly due to body diode action. The driver signal from driver  280  then turns switch  230  back on and the process repeats. Zero voltage switching saves a substantial amount of energy by conserving energy that would otherwise be consumed during transistor switching.  
         [0024]     While the zero voltage switching technology described above results in a more efficient power supply, unfortunately another problem is created, namely the problem of load dependency. For zero voltage switching to occur, there must be a substantial current flowing in inductor  250  to maintain the field therein. From the discussion above it will be recalled that the energy stored in the field of inductor  250  is what makes zero voltage switching possible. With very low loading, i.e. large impedance values for load  275 , it is possible that the power drawn by the load may go lower than 1 watt. Under such very light loading conditions it is possible that the current drawn through inductor  250  will become so small that a critical point is passed where the field becomes so small that zero voltage switching is not sustained. It is possible to offset this effect to some degree by making inductor  250  very large to increase the field with lower currents. However this runs counter to the design goal of making the power supply smaller. To summarize, load dependency is the problem wherein the impedance value of the load must be sufficiently low to sustain a minimum current flow through the inductor to maintain the field of stored energy needed to provide zero voltage switching.  
         [0025]     Past zero voltage switching power supplies have used a constant gap inductor, for example an inductor  300 , the C-shaped core  302  of which is shown in  FIG. 3A . In  FIG. 3A  the gap distance is shown as DG. The gap distance DG is constant as you move from the inner diameter (ID) to the outer diameter (OD) of inductor  300 . In other words, the gap at the ID is the same as the gap at the OD. In contrast to the constant gap inductor of  FIG. 3A , power supply  200  of  FIG. 2  employs a non-constant gap inductor, for example inductor  310  of  FIG. 3B , as inductor  250 . Non-constant gap inductor  310  exhibits a C-shaped or toroid-shaped core  312  which is interrupted by a gap  314  that forms arms  316  and  318  on the opposed sides of the gap. A winding  319  is wound around inductor  310  as shown. In a non-constant gap inductor, the gap distance DG varies from a distance DG 1  at the ID to a distance DG 2  at the OD, or from the inner surface  312 A to the outer surface  312 B of the core. Such an inductor may also be referred to as a swinging choke herein and the gap may also be referred to as a load dependent gap. By varying the gap distance from ID to OD, inductor  310  is made to be load dependent because the inductance that it exhibits varies with the impedance of the load  275  which determines how much current is pulled through inductor  310 .  
         [0026]     In load dependent inductor  310 , the inductance increases as the amount of current pulled through the inductor decreases. This compensates for the tendency of the zero voltage switching power supply  200  of  FIG. 2  to lose regulation under light loads, i.e. high impedance loads. This compensation effect whereby the inductance increases as the current through the inductor decreases is due to the change in gap distance, DG, as you move from inner surface  312 A to outer surface  312 B of core  312  of inductor  310  of  FIG. 3B . This phenomenon increases the operating range of a zero voltage switching power supply to operate with significantly lighter loads and still maintain regulation. For convenience in showing the geometries of the cores in  FIG. 3A-3D , the inductors are shown without windings. Windings can be wrapped around the cores in the conventional fashion.  
         [0027]      FIGS. 3C and 3D  respectively show other non-constant gap inductors  320  and  330  which can be used in switching power supply  200  of  FIG. 2 . The gap geometries depicted in  FIGS. 3C and 3D  are variations of the gap geometry depicted in  FIG. 3B .  
         [0028]      FIG. 4A  depicts a conventional EI-shaped core for an inductor.  FIG. 4B-4G  depict EI-shaped core configurations that can be used to form load dependent inductor  250  in switching power supply  200 . The depicted cores include E portions and I portions. The letters E and I refer to the geometries of the E and I portions. For example, as shown in  FIG. 4B , core  405  of inductor  400  includes an E portion  410  and an I portion  415 . I portion  415  includes an inner surface  415 A and an outer surface  415 B. A gap  420  is formed between an arm  410 A of E portion  410  and an arm  415 C of I portion  415 . The gap distance DG of non-constant gap inductor  400  varies from a distance DG 1  at outer surface  415 B to a larger gap distance DG 2  at inner surface  415 A. The remaining inductors illustrated in  FIG. 4C-4G  also exhibit a varying gap distance or non-constant gap distance from the inner surface to the outer surface thereof.  FIG. 5A-5E  depict embodiments similar to those shown in  FIG. 4B-4G  except the center leg of the E portion is omitted.  
         [0029]     A zero voltage switching power supply is thus disclosed which employs a load dependent non-constant gap inductor that allows the power supply to maintain zero voltage switching even when the power supply is operated with a very light load.  
         [0030]     Although illustrative embodiments have been shown and described, a wide range of modification, change and substitution is contemplated in the foregoing disclosure and in some instances, some features of an embodiment may be employed without a corresponding use of other features. Accordingly, it is appropriate that the appended claims be construed broadly and in manner consistent with the scope of the embodiments disclosed herein.