Abstract:
A module in accordance with the present invention includes at least one electromagnetic pump and a power supply circuit for the electromagnetic pump. The tight coupling between the pump and its power supply afford more easily driving the pump with a low voltage, high current output of the power supply, and ease of integration in system design.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S)  
       [0001]     The present application claims the benefit of the following U.S. Provisional Applications, each of which is hereby incorporated by reference in its entirety: 
        U.S. Provisional Application No. 60/610,815 entitled “Magnetofluiddynamic Pumps Technology,” filed on Sep. 17, 2004;     U.S. Provisional Application No. 60/611,115 entitled “Magnetofluiddynamic Pump Configuration Utilizing Conductive Fluid Electrode Channel,” filed on Sep. 17, 2004; and     U.S. Provisional Application No. 60/611,651 entitled “Integrated Electromagnetic Pump and Power Supply Module,” filed on Sep. 20, 2004.        
 
         [0005]     The present application is related to co-pending U.S. application No. xx/xxx,xxx (Attorney Docket 089-0013), entitled “Series-Gated Secondary Loop Power Supply Configuration for Electromagnetic Pump and Integral Combination Thereof,” by Uttam Ghoshal, et al., filed on even date herewith, which application is hereby incorporated by reference in its entirety. 
     
    
     BACKGROUND  
       [0006]     1. Field of the Invention  
         [0007]     The present invention relates to electromagnetic pumps, power supply circuits for electromagnetic pumps, and modules including both an electromagnetic pump and a power supply circuit for the electromagnetic pump.  
         [0008]     2. Description of the Related Art  
         [0009]     Electromagnetic pumps (EMPs) are used for pumping of conductive fluids such as liquid metals. Such pumps, also known by some as magnetofluiddynamic (MFD) pumps or even magnetohydrodynamic (MHD) pumps (even though fluids other than water may actually be employed), find use in systems such as electricity generators, propulsion systems and micro-electromechanical systems. Exemplary applications of MFD pumps include pumping mercury in electrolyte baths in the production of chlorine and caustic soda, the controllable feeding of smelt, the mixing and pumping of molten aluminum, and in magnetofluiddynamic stirrers. MFD pumps are generally more reliable and safe compared to other kinds of pumps, as MFD pumps do not have any moving parts (except, of course, the conductive fluid itself).  
         [0010]     The conductive fluid in a MFD pump is pumped by taking advantage of the phenomenon wherein a charge carrier moving in a magnetic field experiences a force perpendicular to both its direction of movement and the magnetic field. The force (F) of many moving charge carriers, i.e., a current (I), moving a distance (L) in a magnetic field having a flux density (B) is expressed as F=B·I·L (assuming a resultant force perpendicular to both the magnetic field and current flow).  
         [0011]     The simplest implementation of such a pump may be accomplished by applying a DC bias across a pair of electrodes placed on either side of a flow channel of the pump containing the conductive fluid. A DC voltage is applied across the electrodes to produce an electric current from one electrode, through the conductive fluid, to the other electrode. A pair of permanent magnets may be placed above and below, respectively, the flow channel to create a magnetic field within the flow channel perpendicular to the direction of the current flow across the flow channel. A resulting electromagnetic force acts upon the conductive fluid in a direction perpendicular to the plane defined by the electric current and magnetic field, causing the conductive fluid to flow through the flow channel and thus through the pump. Exemplary MFD pumps are described in U.S. Pat. No. 6,658,861, and in U.S. Pat. No. 6,708,501.  
       SUMMARY  
       [0012]     To improve the pumping capability of a MFD pump, the net electromagnetic force on the conductive fluid in the pump should be increased. There are several methods by which the net force on the conductive fluid may be increased. For example, the net force may be increased by increasing the magnitude of the current flowing through the conductive fluid, by increasing the magnetic flux density, or by increasing the path length traveled by the charge carriers (the current).  
         [0013]     Increasing the current is attractive, so long as overall power dissipation does not rise unacceptably. But since the electrical conductivity of most conductive fluids is very high, the impedance of an MFD pump may be extremely low (e.g., 1 mOhm), and thus the voltage drop across the electrodes within an MFD pump may be extremely low (e.g., 10-30 mV) and the current through the MFD pump may be extremely high (e.g., 10-20 A). Generating such a high current output at such a low voltage presents difficulties in efficient power supply design, and delivering such an output can lead to routing and conductor sizing difficulties, both of which can detract from the advantages otherwise provided by use of an MFD pump.  
         [0014]     A module in accordance with the present invention includes an electromagnetic pump and a power supply circuit for the electromagnetic pump. In some embodiments the power supply circuit is responsive to a DC input voltage, while in certain other embodiments the power supply circuit is responsive to an AC input voltage.  
         [0015]     In some embodiments, the electromagnetic pump includes a chamber through which a conductive fluid may flow in a fluid flow direction, means for creating within the chamber a magnetic field oriented in a direction generally perpendicular to the fluid flow direction, and a pair of electrodes on opposing sides of the chamber. The electrodes may be oriented such that a current flowing between the electrodes flows in a direction that is perpendicular to both the magnetic field and to the fluid flow direction. The magnetic field direction may have a significant vector component which is perpendicular to the fluid flow direction, and the current flow direction may have a significant vector component which is perpendicular to both the magnetic field direction and the fluid flow direction. The means for creating a magnetic field within the chamber may include an electromagnet coupled to the chamber, or alternatively may include at least one permanent magnet coupled to the chamber.  
         [0016]     The power supply circuit may include any of a variety of circuit configurations, including without limitation a flyback circuit configuration, a forward converter circuit configuration, a full bridge circuit coupled to drive a magnetic primary, and a half-bridge circuit coupled to drive a magnetic primary. In some embodiments the power supply circuit includes a secondary winding in series with the electromagnetic pump but with no rectifying device in series therewith, while in other embodiments such a secondary circuit includes a rectifying device. The electromagnetic pump may include a pair of permanent magnets respectively coupled to opposite sides of the chamber to create the magnetic field within the chamber.  
         [0017]     In certain embodiments the module includes a second electromagnetic pump coupled to the power supply circuit. The power supply circuit may include a wound toroid with a primary winding and two secondary windings, each respective secondary winding coupled in series to a respective switch device controlled to only conduct current therein during a respective half-cycle, and further respectively coupled to a respective one of the electromagnetic pumps. The secondary winding may include no more than 2 turns, and for other embodiments may include no more than 1 turn. In other embodiments each respective secondary winding includes a respective conductor passing through but not looped around the toroid, and then coupled in series to a respective one of two switch devices and to a respective one of the two electromagnetic pump.  
         [0018]     The foregoing is a summary and thus contains, by necessity, simplifications, generalizations and omissions of detail. Consequently, those skilled in the art will appreciate that the foregoing summary is illustrative only and that it is not intended to be in any way limiting of the invention. Other aspects, inventive features, and advantages of the present invention, as defined solely by the claims, may be apparent from the detailed description set forth below.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]     The present invention may be better understood, and its numerous objects, features, and advantages made apparent to those skilled in the art by referencing the accompanying drawings.  
         [0020]      FIG. 1  is a block diagram of a module in accordance with certain embodiments of the invention.  
         [0021]      FIG. 2  is a block diagram of a module in accordance with certain embodiments of the invention.  
         [0022]      FIG. 3A  is a schematic diagram depicting a power supply circuit useful for certain embodiments of the invention.  
         [0023]      FIG. 3B  is a diagram depicting the current flow within a secondary loop circuit relative to current flowing between such secondary loop circuits.  
         [0024]      FIG. 4  is a three-dimensional diagram of a fluidic cooling system incorporating an electromagnetic pump module in accordance with certain embodiments of the present invention.  
         [0025]      FIG. 5  is a side elevation view of an exemplary electromagnetic pump module shown in  FIG. 4 .  
         [0026]      FIG. 6  is a front elevation view of an exemplary electromagnetic pump module shown in  FIG. 4 .  
         [0027]      FIG. 7  is a front view of a first printed wiring board useful for certain embodiments of the invention.  
         [0028]      FIG. 8  is a front view of a second printed wiring board useful for certain embodiments of the invention.  
         [0029]      FIG. 9  is a plan view of an integrated circuit layout useful for implementing a low resistance switching transistor which is useful for certain embodiments of the invention.  
         [0030]      FIG. 10  is a schematic diagram depicting a power supply circuit useful for certain embodiments of the invention.  
         [0031]      FIG. 11  is a schematic diagram depicting another power supply circuit useful for certain embodiments of the invention.  
         [0032]      FIG. 12  is a schematic diagram depicting yet another power supply circuit useful for certain embodiments of the invention.  
         [0033]      FIG. 13  is a schematic diagram depicting still another power supply circuit useful for certain embodiments of the invention.  
         [0034]      FIG. 14  is a cross-sectional diagram representing an electromagnetic pump utilizing an electromagnet that is useful for certain embodiments of the invention.  
         [0035]      FIG. 15  is a waveform diagram illustrating operation of the electromagnetic pump shown in  FIG. 14 .  
         [0036]      FIG. 16  is a schematic diagram depicting still another power supply circuit useful for certain embodiments of the invention.  
         [0037]      FIG. 17  is a table comparing the relative merit of certain embodiments of the invention against a group of possible criteria.  
         [0038]      FIG. 18  is a cross-section diagram of an exemplary magnetofluiddynamic pump in which the electrodes on either side of the chamber, as well as the entire circuit path for the electrical current flowing through the pump chamber, are formed of a conductive fluid channel.  
         [0039]      FIG. 19  is an isometric diagram of an exemplary magnetofluiddynamic pump, such as the embodiment shown in  FIG. 17 , in which the electrodes on either side of the chamber and the entire secondary loop circuit are formed of a conductive fluid channel.  
         [0040]      FIG. 20  depicts a generalized block diagram of a MFD pump having a conductive fluid electrode.  
     
    
       [0041]     The use of the same reference symbols in different drawings indicates similar or identical items.  
       DETAILED DESCRIPTION  
       [0042]     Referring now to  FIG. 1 , a module  100  in accordance with some embodiments of the present invention includes at least one electromagnetic pump  102  and a power supply circuit  104  for the electromagnetic pump  102 . The electromagnetic pump  102  includes a chamber through which a conductive fluid may flow (not shown), a fluid inlet  108 , and a fluid outlet  110 . A magnetic field is created within the chamber, preferably oriented in a direction generally perpendicular to the fluid flow direction. A pair of electrodes is disposed on opposing sides of the chamber and oriented such that a current flowing between the electrodes flows in a direction that is generally perpendicular to both the magnetic field and to the fluid flow direction. In certain embodiments, the magnetic field direction has a significant vector component which is perpendicular to the fluid flow direction, and the current flow direction has a significant vector component which is perpendicular to both the magnetic field direction and the fluid flow direction. Additional details of useful electromagnetic pumps are described in co-pending U.S. application Ser. No. 10/443,190 entitled “Direct Current Magnetohydrodynamic Pump Configurations” by Andrew Carl Miner, et al., filed May 22, 2003, the disclosure of which is hereby incorporated by reference in its entirety.  
         [0043]     The power supply circuit  104  receives a source of power conveyed on power terminals  105 , and may receive one or more control signals conveyed on input terminals  107 . Such control signals may include signals for modulating the amount of fluid flow, for turning on and off the fluid flow, and/or other useful capabilities. The source of power may be an AC voltage such as, for example, a 120 VAC line voltage or a lower magnitude AC voltage, or may be a DC voltage such as, for example, a 4-8 VDC voltage, or even a 4-12 VDC voltage. Such a DC voltage may be any convenient voltage in a system within which the module  100  may reside (e.g., 5 VDC; 12 VDC), or may be specifically generated for use with the power supply circuit  104 . The power supply circuit  104  generates one or more output signals conveyed on bus  106  coupled to the electromagnetic pump  102 . Such output signals may be high-current, very low voltage outputs, as described below.  
         [0044]     Referring now to  FIG. 2 , a module  120  in accordance with some embodiments of the present invention includes two electromagnetic pumps  122 ,  123  and a power supply circuit  124  for the electromagnetic pumps  122 ,  123 . The electromagnetic pump  122  includes, as before, a chamber through which a conductive fluid may flow (not shown), a fluid inlet  128 , and a fluid outlet  129 . Similarly, electromagnetic pump  123  includes a chamber through which the conductive fluid may flow (not shown), a fluid inlet  131 , and a fluid outlet  130 . The configuration of each of the two electromagnetic pumps  122 ,  123  may be similar or identical to that described above.  
         [0045]     The fluid outlet  129  of electromagnetic pump  122  is connected to the fluid input  131  of electromagnetic pump  123  to create a fluid path which passes from a module fluid input  140 , through both electromagnetic pumps  122 ,  123 , and out through a module fluid output  142 . A power supply circuit  124  receives a source of power conveyed on power terminals  132 ,  134  which are here shown, for example, as DC power terminal  132  and ground reference terminal  134 . The power supply circuit  124  may optionally receive one or more control signals conveyed on control input terminals  137 . The source of DC power may be a voltage such as, for example, a 4-8 VDC voltage. In the embodiment depicted, the power supply circuit  124  generates a first output signal DRIVE 1  conveyed by way of a pair of conductors  126  to electromagnetic pump  122 , and generates a second output signal DRIVE 2  conveyed by way of a pair of conductors  127  to electromagnetic pump  123 .  
         [0046]     The DRIVE 1  and DRIVE 2  output signals are each generated to provide a high current through the respective electromagnetic pump with a very low voltage present across the respective electromagnetic pump. In certain embodiments, these two output signals  126 ,  127  may be continuous (i.e., DC) currents or pulsed (i.e., AC) currents, and such pulsed currents may be in-phase, overlapping in phase, or out-of-phase signals.  
         [0047]     An exemplary power supply circuit  124  is depicted as power supply circuit  150  in  FIG. 3A , which is shown connected to electromagnetic pumps  122 ,  123 . Generally, the power supply circuit  150  functions as a switching DC-DC converter type of circuit, but having an extremely high output current through the electromagnetic pumps and an extremely low output voltage across the electromagnetic pumps.  
         [0048]     An oscillating signal having a frequency of, for example, 20 kHz is generated on node  162 , which is coupled to one end of a primary winding  167  of transformer  163 . The other end (node  166 ) of the primary winding  167  is AC-coupled to ground by capacitor  164 , which allows node  166  to also oscillate at the same excitation frequency as node  162 , and with a similar amplitude (but with a different phase) as node  162 . During a first one of the two half-cycles of the oscillation period, switch device  170  is turned on by a sufficiently high voltage on node  162  (i.e., above the threshold voltage of device  170 ) and causes current to flow in the “upper” loop formed by secondary winding  168 , switch device  170 , and electromagnetic pump  122 . During the second half-cycle, the voltage on node  162  is driven low, the switch device  170  is turned off, and no current flows through secondary winding  168 . Resistor  180  functions to provide a ground reference for the secondary circuit. Relative to the very low impedance of the secondary loop itself (i.e., device  170 , wire  126 A,  126 B, electromagnetic pump  122 , and secondary coil  168 ), the exemplary 1 ohm value of this resistor  180  is actually quite large, and substantially most of the current flows within the secondary loop circuit rather than through the resistor  180 . For example, the device  170  may have a nominal impedance of approximately 1 milliOhm, and may be implemented as a single device (as drawn in the figure) or as multiple parallel devices to help achieve the desired low impedance. For example, three parallel-connected Si7868DP devices from Vishay Siliconix may be used to implement device  170 . The impedance of the electromagnetic pump  122  may have an approximate value of only 1 milliOhm.  
         [0049]     During this second half-cycle, node  166  is high enough in voltage to turn on switch device  171 , and causes current to flow in the “lower” loop formed by secondary winding  169 , switch device  171 , and electromagnetic pump  123 . Substantially all the flux created in the transformer  163  by the primary winding  167  during the second half-cycle is coupled to the secondary winding  169  because switch device  170  is off and ensures that no current can flow through the other secondary winding  168 . Resistor  181  functions to provide a ground reference for the secondary circuit. Relative to the very low impedance of the secondary loop itself (i.e., device  171 , electromagnetic pump  123 , secondary coil  169 , and the interconnecting wiring), the exemplary 1 ohm value of this resistor  181  is actually quite large, and substantially most of the current flows within the secondary loop circuit rather than through the resistor  181 .  
         [0050]     The oscillating signal conveyed on node  162  may be adequately generated in many different ways, including using discrete transistors, LC oscillators, RC oscillators, integrated circuits providing oscillator functions, integrated driver or buffer circuits, single integrated circuits providing both oscillator and driver functions, and others. One such way is shown as part of the power supply circuit  150  depicted in  FIG. 3A . An integrated circuit  152  functions as an oscillator, providing a square-wave output signal on output node  155  having, for example, a frequency of 20 kHz. The integrated circuit  152  is coupled to the DC power terminal  132  (through resistor  154 ) and further coupled to the ground reference power terminal  134  (i.e., “ground”). A bypass capacitor  153  provides filtering for the voltage operably conveyed on the DC power terminal  132 . The integrated circuit  152  may be implemented using, for example, the LTC6900 available from Linear Technology, Inc.  
         [0051]     The square-wave output signal  155  is coupled to a pair  156  of buffers  158 ,  159  to generate complementary signals, which are then coupled to drive a pair of N-channel (NMOS) transistors (arranged here in a totem-pole configuration) to provide a higher drive capability output signal  162  for driving the winding  167 , as described above. As depicted in the figure, the pair  156  of buffers may be implemented within a single integrated circuit, such as the LTC1693-2 available from Linear Technology, Inc. The pair  157  of NMOS driver transistors  160 ,  161  may be implemented, for example, using the Si6946DQ available from Vishay Siliconix. Many other circuit configurations for generating such a buffered signal  162  may alternatively be used. For example, bipolar transistors may be employed as the driver pair  157 , either as a complementary pair (i.e., NPN and PNP) or as a pair of like polarity transistors (e.g., both NPN). One of ordinary skill will appreciate many equivalent circuits and structures for generating a low frequency oscillating signal with high drive capability.  
         [0052]     Referring now to  FIG. 3B , the two pumps  122  and  123  are shown in a three-dimensional schematic diagram to help illustrate the role of resistors  180  and  181 . Assume the top secondary loop circuit is “on” and the bottom secondary loop circuit is off. If current traversing the top secondary loop conducts along the fluid path to the bottom loop, rather than traversing around just the top loop, the pumping efficiency of the pump  122  will be diminished. The resistors  180 ,  181 , although sized in this exemplary embodiment as 1 ohm resistors, are actually quite large relative to the desired impedance of each secondary loop, and so the stray current, I STRAY , is kept small. Yet the resistors  180 ,  181  are still low enough in impedance to effectively provide a voltage reference (i.e., “ground” reference) for the secondary loop circuits of the power supply circuit transformer.  
         [0053]     A system incorporating the exemplary module  100  thus far described is depicted in  FIG. 4 , such as for dissipating heat from a high power density device. The system includes a source exchanger  202  (e.g., a “thermal collector”), the pump/power supply module  100 , and a thermal dissipater  204  coupled in series by a conductive fluid path  210 , such as a conduit, pipe, tubing, or other structure. In some systems, a second source exchanger (not shown) may be coupled in fluidic series with the source exchanger  202  by a continuous conductive fluid path  210 .  
         [0054]     In certain particularly desirable embodiments, the source exchanger  202  may be implemented to draw heat away from an integrated circuit or other packaged electronic device, such as within a notebook computer or other electronic enclosure, and transfer the heat to the conductive fluid flowing within the conductive fluid path  210  (propelled by the electromagnetic pump within the module  100 ). The thermal dissipater  204  may be implemented to dissipate such heat conveyed by the conductive fluid to a larger heat sink, to ambient air, or to some other thermal sink. Other configurations may be configured so that heat flow is reversed, thereby heating a device rather than cooling it.  
         [0055]     Multiple electromagnetic pumps may be provided in series configuration (e.g., such as in the dual pump module  100  as shown, or by two single pump modules, as described below) where fluid power supplied by one pump is not sufficient to circulate the conductive fluid in the form of a closed loop. This may be the case when the thermal dissipater  204  is placed at a relatively large distance away from the source exchanger  202 . Two electromagnetic pumps in fluidic series may also be useful where there is sudden loss in the pressure head, such as in a configuration where the fluid pipes  210  take sharp turns (like in case of laptop joints) where a significant drop in the pressure may be observed.  
         [0056]     The system  200  includes a solid-fluid heat exchanger (e.g., the source exchanger  202 ) placed adjacent to a high power density device to be cooled. The solid-fluid heat exchanger  202  is filled, in certain exemplary embodiments, with a liquid metal or other conductive fluid that absorbs the heat from the high power density device. The conductive fluid path  210  passes through solid-fluid heat exchanger  201  and circulates the conductive fluid through the heat dissipater  204 , which releases the heat to the atmosphere, and circulates the cooled conductive fluid back to the source exchanger  202 . The module  100  provides the fluid power for circulating the conductive fluid in the form of a closed loop. In this manner, the system  200  provides for the transport and dissipation of heat at a predefined distance away from a high power density device coupled to the source exchanger  202 . This distance is determined based on the form factor (the configuration and physical arrangement of the various components in and around the high power density device). Thus system  200  provides for heat dissipation in the cases where dissipating heat in the proximity of the high power density device  202  is not desirable. For example, in a computer, the heat dissipated by components such as the microprocessor or the power unit may be in proximity of components like memory, and this heat may lead to permanent loss of data from the memory or shortened component lifetimes of various devices within the computer. Thus it is desirable that the heat generated by the microprocessor/power unit is dissipated at a location some distance away from components that may get damaged.  
         [0057]     The thermal dissipater  204  may be constructed of a low thermal resistance material (e.g., copper and aluminum) and has a large surface area for effectively dissipating heat to the atmosphere. The thermal dissipater  204  may dissipate heat by natural convection or by forced convection with the use of a fan. A finned structure (as shown in the figure) is sometimes advantageously used as a heat sink. In some embodiments, the conductive fluid may also circulate through its fins. It should be apparent to one of ordinary skill in the art that other heat sink structures may alternatively be used.  
         [0058]     Referring now to  FIG. 5 , a side view is depicted of an exemplary embodiment  220  of a module in accordance with the present invention. This particular module includes two electromagnetic pumps  226 ,  228  which are connected so that fluid flowing into a fluid inlet  222  flows sequentially through electromagnetic pump  226 , electromagnetic pump  228 , and out of the module from fluid outlet  224 . Each of the two electromagnetic pumps includes a pair of permanent magnets housed on either side of the internal pump chamber. The electromagnetic pump  228  includes a pair of housings  230 ,  232  which hold the pair of permanent magnets for electromagnetic pump  228 .  
         [0059]     In some embodiments, a printed wiring board  236  includes portions of the power supply circuit for the module  220 , and particularly includes circuitry coupled to the secondary windings of the transformer core  238 . The primary winding and additional circuitry for excitation of the primary winding is not shown in  FIG. 5 . A fluid inlet  222  is provided for receiving conductive fluid, which is pumped by the two series pumps and conveyed out a fluid outlet  224 . Permanent magnets  230 ,  232  are illustrated for the second of the two series pumps along with one of its electrodes  228 . An electrode for the first pump is labeled as  226 . Most conductors forming each secondary circuit are formed by bus bar structures, such as  234  and  240  to reduce electrical resistance as well for structural stability. The switch devices for the secondary loop circuits are disposed on printed wiring board  236 . Another side view of the exemplary module  220  is shown in  FIG. 6 .  
         [0060]     In embodiments of the power supply circuit which utilize a switch device in the secondary circuit, it is advantageous to limit the voltage drop across such a switch device in order to achieve a high current through the electromagnetic pump having a very low voltage across the pump. Referring now to  FIG. 9 , a desirable layout  300  is shown for a switch device useful for the power supply circuit. The layout  300  corresponds to an insulated gate field effect transistor (i.e., IGFET), which frequently are also called MOSFETS (literally “Metal-Oxide-Semiconductor Field Effect Transistor”) or even just FET. Such a FET is typically a three terminal device having drain, gate, and source terminals, although other variations are known. In  FIG. 9 , a three-terminal FET is shown having a drain terminal  302 , a gate terminal  304 , and a source terminal  306 . If such a FET is an N-channel FET (i.e., as depicted by switch device  170  in  FIG. 3A ), boundary  308  corresponds to an active area region formed within a p-type substrate or well. The gate terminal  304  is implemented as a patterned polysilicon layer having multiple horizontal and multiple vertical stripes, thereby forming closed regions of active area surrounded by gate polysilicon. Alternating ones of these closed regions are connected to the drain terminal  302  and to the source terminal  306 . This provides a transistor with a large effective “width” for a given amount of area consumed on the integrated circuit, and also provides a very low resistance in both the source and drain regions of the FET.  
         [0061]     Another power supply circuit which includes a switched secondary circuit (i.e., a switch device interrupting at times current flow in a secondary loop) and which is useful for the present invention, is shown in  FIG. 10 . The power supply circuit  320  has many structural similarities to the power supply circuit  150  shown in  FIG. 3A , but this power supply circuit  320  may be viewed as a “full-bridge” circuit whereas the power supply circuit  150  may be viewed as a “single-bridge” or “half-bridge” circuit. The power supply circuit  320  includes a second high-drive capability driver circuit for generating a second square-wave signal  326  which is generally out of phase with the first square-wave signal  162 . A second pair of buffers  322  is responsive to the signal  155 , but are reversed in polarity such that the second pair  324  of driver transistors generates a signal on node  326  which is complementary to that conveyed on node  162 .  
         [0062]     By having a pair of high drive outputs  162 ,  326 , both ends of the primary winding  167  may be driven. One end of the primary winding  167  (node  328 ) is driven through a core balancing capacitor  332  by node  162 , and the other end (node  326 ) is driven directly. The core balancing capacitor  332  ensures that misbalances between the signals  162 ,  326  do not result in a DC signal across the primary coil  167 . The series combination of capacitor  330  and resistor  329  functions as a “snubber” circuit to reduce instantaneous voltage spikes which might otherwise result across the primary coil  167 .  
         [0063]     Relative to the half-bridge circuit depicted in  FIG. 3A  and described above, the turns ratio of the transformer  331  (i.e., the ratio of turns between the primary coil  333  and each secondary coil) is depicted as being 100:1. In the half-bridge circuit depicted in  FIG. 3A  and described above, the turns ratio of the transformer  163  is depicted as being 50:1. Because the complementary signals  162 ,  326  are each a ground-to-V DD  signal and are out-of-phase with each other, a total bias of 2·V DD  is impressed across the primary coil  333  (compared with only a total bias of V DD  across primary coil  167 ), and the resultant voltage induced in the respective secondary coils for both circuits is substantially similar.  
         [0064]     Yet another power supply circuit useful for the present invention is shown in  FIG. 11 . Here the power supply circuit  350  again has many structural similarities to the power supply circuit  150  shown in  FIG. 3A , but this power supply circuit  350 , which is configured for driving a single electromagnetic pump, omits the switch device in the secondary circuit coupled to the electromagnetic pump, and utilizes different turns ratios for the two secondary windings. This power supply circuit  350 , like that shown in  FIG. 3A , is a single-bridge circuit which actively drives only one end of the primary winding, but also includes a current limiting transistor  352  in the grounding path for the driver for output node  356 . By adjusting the reference voltage V R  (labeled  354 ) coupled to the gate of transistor  352 , the current through the primary winding may be controlled.  
         [0065]     In operation, during one of the half-cycles current flows through secondary circuit  358  (i.e., through the secondary winding  360  and the electromagnetic pump  368 ), but no current flows though the other secondary circuit  364  because the switch device  366  is turned off. In this way all the flux generated by the primary winding is coupled to just one of the two secondary windings, in this case secondary winding  360 . During the other half-cycle, a current flows through secondary circuit  358  in the reverse direction than before, but in this half-cycle device  366  is turned on and current also flows through secondary circuit  364 . If, for example, the secondary winding  360  has one turn, the secondary winding  362  has five turns, and the primary winding  370  has fifty turns, then in the case when both secondary circuits are conducting, flux in the transformer core is coupled into all six turns of the two secondary windings, and the total induced current is significantly lower than if coupled into just one secondary winding having just one turn.  
         [0066]     For the secondary circuit which includes the electromagnetic pump, during one half-cycle a high magnitude current (e.g., 25 A) flows in one direction, but during the other half-cycle, a much lower current (e.g., 5 A) flows in the opposite direction. Although the conductive fluid within the electromagnetic pump is “pushed” in one direction during the one half-cycle, and pushed in the opposite direction during the other half-cycle, the relative magnitude of these two forces are different (because the current through the electromagnetic pump is different each half-cycle), and the net effect of the electromagnetic pump is to force the conductive fluid in only one direction. Colloquially, this may be viewed as a “5 steps forward, 1 step back” manner of operation. The flow of conductive fluid through the pump(s) may be further rectified by using Tesla valves, which are constructed to preferentially favor fluid flow in one direction through the valve over the other direction. Advantageously, this power supply circuit  350  is relatively simple, being a single bridge circuit and, although still utilizes two secondary windings, is configured to relatively efficiently drive only one electromagnetic pump.  
         [0067]     Another power supply configuration well suited for use with a single electromagnetic pump is shown in  FIG. 12 . Here, the power supply circuit  400  is arranged in a flyback configuration. As described in earlier embodiments, node  155  conveys a low-frequency square-wave signal generated by, for example, integrated circuit oscillator  152 . This signal is buffered by buffer  159  and driver FET  161  to generate a high-drive capability signal on node  402  having the same frequency as node  155 . In this embodiment, a full totem pole driver is not used because the flyback transformer  404  may be adequately driven by just a “pull-down” only driver stage (i.e., buffer  159  and FET  161 ). During one-half of the cycle, node  402  is essentially grounded by FET  161 , thus allowing current to build up through primary winding  406 , thus storing magnetic energy in the transformer. During the other half-cycle, the FET  161  shuts off and the voltage of node  402  shoots above the V DD  voltage conveyed on power supply node  132 , causing the secondary circuit switch device  410  to turn on, and thus causing current to flow through the secondary winding  408  and through the electromagnetic pump  412 . The magnetic energy stored during the first half-cycle is discharged during the second half-cycle.  
         [0068]     Referring now to  FIG. 13 , yet another configuration is shown of a power supply circuit useful for the present invention. Here, a forward converter configuration  420  is depicted. Complementary signals  162  and  326  (e.g., as might be generated in the manner shown in  FIG. 10 , or by some other suitable technique) conveyed to a group of switches  162 ,  428 , and  430  to pump current into a choke  432  during one half-cycle (e.g., when switch  428  is turned on), and then to provide a path for such choke current to recirculate (labeled as  434 ) during the other half-cycle (e.g., when switch  430  is turned on), thereby providing a continuous load current, in this case through the electromagnetic pump  122 . A transformer includes primary winding  424 , which is energized when switch transistor  422  is turned on by signal  162 , and further includes secondary winding  426 .  
         [0069]     The present invention need not incorporate power supply circuits which are or are similar to DC-DC converter circuits, nor which necessarily incorporate permanent magnets in the electromagnetic pump portions. For example, an electromagnetic pump  440  utilizing a first AC signal to excite an electromagnet, and utilizing a second AC signal to generate current flow through the conductive fluid within the pump chamber, is depicted in  FIG. 14 . An AC magnetic field is created in the pump chamber  454  by magnetic core  442  and coil  444 , when an AC signal is provided across terminals  446  and  448 . The polarity of the magnetic field created within the chamber  454  reverses each half-cycle of the exciting signal coupled to the coil  444 . This alone might suggest that the conductive fluid is forced in one direction (i.e., into the page, as drawn) during one half-cycle, but forced in the other direction (i.e., out of the page) during the other half-cycle, resulting in no net movement of the fluid. However, if the electrical current flowing across the pump chamber  454  and through the conductive fluid is also an AC signal, in accordance with the right-hand rule, the net force applied to the conductive fluid is in the same direction during both half-cycles.  FIG. 15  depicts exemplary waveforms of the coil current, I COIL , labeled as  462 , and of the AC fluid current, I e , labeled as  464 . The resultant force imparted to the conductive fluid is labeled as  466 , is a pulsed signal having the shape of a half-sinusoid.  
         [0070]     Another power supply configuration well suited for use with a single electromagnetic (i.e., MFD) pump is shown in  FIG. 16 . Here, an exemplary “buck” converter configuration  480  is depicted. A clock and driver circuit  482  generates two complementary signals on respective nodes  485  and  487 . Such a clock and driver circuit  482  may be implemented in any of a wide variety of configurations, as described above, and may be configured to generate its complementary output signals  485 ,  487  having a frequency of around 20 KHz. Such a frequency is a desirable frequency as it is higher than the usual audio band (and thus does not readily generate audible noise) and yet is well below other frequencies of interest within the system, and thus it not as likely to interfere with the remainder of the system. The complementary signals  485 ,  487  are conveyed respectively to driver devices  486 ,  488  to pump current into a choke  490  and through the MFD pump  492  during one half-cycle when driver device  486  is turned on, and then to provide a path for such choke current to recirculate during the other half-cycle when driver device  488  is turned on, thereby providing a non-uniform unipolar load current through the electromagnetic pump  492 . This unipolar current varies in magnitude, slowly rising in magnitude when device  486  is on, and slowly decreasing when device  488  is on. This circuit  480  is particularly simple and may be inexpensively implemented. However, the current drawn from the VDD supply coupled to node  132  is relatively high in average magnitude and is also non-uniform since the operation of the driver devices  486 ,  488  contributes to current spikes in the operating current. The bypass capacitor  484  is included and sized appropriately to help reduce power supply noise as a result of these current spikes.  
         [0071]     Referring now to  FIG. 17 , a chart is shown which compares the relative size, power efficiency, simplicity, and cost of various ones of the power supply circuits described above. The “optimal” choice, of course, may depend upon the relative importance of the various factors listed, and possibly other factors, for a given application. For example, if cost is the paramount concern, then a Single Pump Flyback configuration (e.g., an exemplary embodiment of which is depicted in  FIG. 12 ) may be more desirable. Alternatively, if power efficiency is paramount, then an Improved Single Pump configuration (e.g., an exemplary embodiment of which is depicted in  FIG. 11 ) may be more desirable.  
         [0072]     Referring now to  FIG. 18 , a MFD pump  500  is depicted in which the electrodes on either side of the chamber, as well as the entire circuit path for the electrical current flowing through the pump chamber, are formed of a conductive fluid channel. In the figure, the pump is depicted in a cross-sectional view, showing a fluid chamber  502  with a pair of permanent magnets  504 ,  505  respectively above and below the chamber  502 . (The conductive fluid flow direction would be either into or out of the page.) A conductive fluid channel  506  forms both electrodes on either side wall of the chamber  506 , and also forms the circuit path carrying the current which flows through the chamber  506 . While the fluid which fills the conductive fluid channel  506  may be (as is shown here) the same fluid which flows through the fluid chamber  502  of the pump (which flows either into or out of the page), there is no fluid flow through the conductive fluid channel  506  (i.e., the conductive fluid “electrode”) because the openings on either side of the fluid chamber  502  into the conductive fluid channel  506  are preferably symmetrically located within the chamber and are thus equipressure points. The conductive fluid is present within the conductive fluid channel  506  to support the flow of electrical current, particularly from one side of the fluid chamber  502  to the other to propel the conductive fluid in a direction normal to the page.  
         [0073]     In the exemplary structure shown, the conductive fluid channel  506  is routed through a magnetic toroid  508 , thus forming one “turn” of a secondary winding. A primary winding  510  is also wound around the toroid  508  (here shown, for clarity, as having many “turns”). In exemplary embodiments, the turns ratio for such a transformer formed by toroid  508 , primary winding  510 , and secondary winding formed by conductive fluid channel  506  may advantageously be 50:1, or 100:1, or some other useful value, to achieve a very high current output through the conductive fluid channel  506  and through the pump chamber  502 . In other embodiments, the conductive fluid channel  506  may be formed to include an additional turn around the toroid  508 , giving rise to a secondary winding having 2 turns, or may include additional turns.  
         [0074]     Referring now to  FIG. 19 , a MFD pump  550  is depicted in which the electrodes on either side of the chamber and the entire secondary loop circuit are formed of a conductive fluid channel. In the figure, the pump  550  is depicted in a three-dimensional view, showing a fluid inlet  560  and a fluid outlet  562 . A pump chamber (not shown explicitly) is the region within the fluid path between the fluid inlet  560  and fluid outlet  562  which is located between a pair of permanent magnets  554 ,  556  respectively above and below the pump chamber. Exemplary magnets  554 ,  556  may be small NdFeB permanent magnets placed approximately 2.4 mm apart. A conductive fluid channel  558  forms both electrodes on either side wall of the chamber (one of which is labeled  570 ), and also forms the circuit path carrying the current which flows through the chamber. The conductive fluid channel  558  is routed through a magnetic “toroid”  564 , thus forming one “turn” of a secondary winding. The toroid  564  is actually depicted as a more rectilinear closed magnetic core structure, although any of a variety of similar shapes may be utilized, including a literal toroidal shape. A primary winding  566  is also wound around the toroid  564 . The turns ratio may be selected based upon the power supply circuit utilized, the desired output current level, the details of the magnetic core structure, and other factors. In other embodiments, the conductive fluid channel  558  may be formed to include one or more additional turns around the toroid  564 , giving rise to a secondary winding having 2 or more turns.  
         [0075]     A useful MFD pump having a conductive fluid electrode may be generalized as shown in  FIG. 20 . Such an MFD pump  600  includes a flow chamber  602  through which the conductive fluid is caused to flow (either into or out of the page) by the electromagnetic force exerted upon the fluid. A magnetic field is created in the flow chamber  602  by a magnetic structure  604  above the flow chamber  602  (and optionally by a second magnetic structure  606  below the flow chamber  602 ). The electrodes on either side of the flow chamber  602  and the closed circuit path through which the current through the flow chamber  602  flows is formed by a conductive fluid channel  608 . The conductive fluid channel  608  may open directly into the flow chamber  602  (as depicted) in which case the conductive fluid channel  608  is operably filled with the same conductive fluid that flows through the pump  600  (even though the conductive fluid with the conductive fluid channel  608  is not cause to move), which eliminates any contact resistance between the “electrodes” and the conductive fluid within the pump. Alternatively, the conductive fluid channel  608  may be filled with the same or another conductive fluid, and the ends of the conductive fluid channel  608  sealed with a conductive barrier.  
         [0076]     The current which flows through the conductive fluid channel  608  and thus across the flow chamber  602  may be generated by an inductive circuit  610 , such as a transformer as shown in previous embodiments. Alternatively, a current may be induced in the conductive fluid channel  608  by an inductive coil formed around the conductive fluid channel  608 , or by other inductive means.  
         [0077]     In the various described embodiments, the various fluid paths, such as conductive fluid path  210 , and portions of the electromagnetic pumps themselves may be constructed of polymer materials such as Teflon® or polyurethane. Alternatively, refractory metals such as tungsten, vanadium or molybdenum may also be used as the material of construction. Polymers like Teflon® prove to be good conduit materials as they are inert to most chemicals, provide low resistance to flow of liquids and are resistant to high temperature corrosion, and can be easily machined. Certain metallic structures, such as nickel-coated copper, can also be used. Useful configurations and construction details of the source exchanger  202  and thermal dissipater  204  are described in the above-referenced U.S. Pat. No. 6,658,861, the disclosure of which is hereby incorporated by reference in its entirety.  
         [0078]     In certain applications, the system may need to be provided with electromagnetic interference (EMI) shielding to shield other devices in the system from electromagnetic radiations generated by the MFD pump(s). These electromagnetic radiations, if not shielded, might adversely affect the performance of other devices. Accordingly, the electromagnetic pump of the module  100  may be enclosed within a housing that provides EMI shielding. This EMI shielding may be provided using standard methods such as magnetic shields and EMI shielding tapes, and which shielding may be made using high magnetic permeability materials such as steel, nickel, alnico, or permandur or other specially processed materials.  
         [0079]     In some embodiments, the conductive fluid may be a liquid metal, and further may be an alloy of gallium (Ga) and indium (In). Preferred compositions comprise 65 to 75% by mass gallium and 20 to 25% indium. Materials such as tin, copper, zinc and bismuth may also be present in small percentages. One such preferred composition comprises 66% gallium, 20% indium, 11% tin, 1% copper, 1% zinc and 1% bismuth. Some examples of the commercially available GaIn alloys include Galistan, which is popular as a substitute for mercury (Hg) in medical applications, and Newmerc. The various properties of a GaIn alloy make it a desirable liquid metal for use in closed circulation heat dissipation systems, such as depicted in  FIG. 4 . The GaIn alloy can be chosen to span a wide range of temperature with high thermal and electrical conductivities. It has melting points ranging from—15° C. to 30° C. and does not form vapor at least up to 2000° C. It is not toxic and is relatively inexpensive, and easily forms alloys with aluminum and copper. It is inert to polyimides, polycarbonates, glass, alumina, Teflon®, and conducting metals such as tungsten, molybdenum, and nickel, thereby making these materials suitable for construction of tubes, conduits, and/or channels.  
         [0080]     It should be apparent to one of ordinary skill in the art that a number of other liquid metals may be used. For example, liquid metals having high thermal conductivity, high electrical conductivity and high volumetric heat capacity can also be used. Some examples of liquid metals that can be used in an embodiment of the invention include mercury, gallium, sodium potassium eutectic alloy (78% sodium, 22% potassium by mass), bismuth tin alloy (58% bismuth, 42% tin by mass), bismuth lead alloy (55% bismuth, 45% lead) etc. Bismuth based alloys are generally used at high temperatures (40 to 140.degree.C.). Pure indium can be used at temperatures above 156° C. (i.e., the melting point of indium), and mercury, bismuth, and gallium may also be used. Certainly other conductive fluids may be used to advantage, as well.  
         [0081]     One or more of the various embodiments described herein may be used to efficiently provide an output voltage of less than 500 millivolts when coupled to an electromagnetic pump, and in some embodiments an output voltage of less than 250 millivolts, and in still others an output voltage less than 100 millivolts. One or more of the various embodiments described herein may be used to efficiently provide an output current of at least 5 amps when coupled to an electromagnetic pump, and in some embodiments an output current of at least 10 amps. In some embodiments, the output voltage (e.g., across an electromagnetic pump) may be at least 100 times smaller than an operating power supply voltage provided to the power supply circuit. In some embodiments, the output current (e.g., through an electromagnetic pump) may be at least 100 times larger than an operating current drawn from a power supply provided to the power supply circuit. For example, certain embodiments may be configured to provide an output current of 20 A through the electromagnetic pump while only generating a voltage of 20 mV across the electromagnetic pump, and yet the power supply circuit may draw less than 200 mA from a power supply of 2 V or more.  
         [0082]     Several configurations of MFD pumps (also described as magnetohydrodynamic pumps) are described in the above-referenced U.S. application Ser. No. 10/443,190 entitled “Direct Current Magnetohydrodynamic Pump Configurations”. Useful pump configurations, particularly relating to techniques for creating the magnetic flux within the pump chamber, are described in co-pending U.S. Provisional Application No. 60/610,815 entitled “Magnetofluiddynamic Pumps Technology,” filed on Sep. 17, 2004, which application is hereby incorporated by reference in its entirety. Still other useful configurations are described in U.S. Provisional Application No. 60/611,115 entitled “Magnetofluiddynamic Pump Configuration Utilizing Conductive Fluid Electrode Channel,” filed on Sep. 17, 2004, which application is hereby incorporated by reference in its entirety.  
         [0083]     As used herein, coupled may mean coupled indirectly or directly. A periodic signal need not be sinusoidal. An asymmetric current through a device conducts in one direction more than in an opposite direction, including the case that it conducts only in one direction (e.g., a unipolar current). A pulsed unipolar current includes a non-uniform unipolar current, including (but not requiring) the case when the value of the current between “pulses” is substantially zero. A first direction that is generally perpendicular to a second direction may include angles therebetween in the range of approximately 60° to 120° (i.e., a significant vector component which is perpendicular). A first direction that is substantially perpendicular to a second direction may include angles therebetween in the range of approximately 80° to 100°.  
         [0084]     While certain embodiments of the invention have been illustrated and described, it should be clear that the invention is not to be limited to these embodiments only. The inventive concepts described herein may be used alone or in various combinations. Numerous modifications, changes, variations, substitutions, and equivalents will be apparent to those of ordinary skill in the art without departing from the spirit and scope of the invention, which is defined in the following appended claims.