Abstract:
An adjustable bipolar current source for a load, such as a thermoelectric cooler, includes a voltage-controlled power supply having a unipolar output, and an H-bridge. At least one of the two active elements on a first side and at least one of the two active elements on a second side of the H-bridge comprises an active conductive element responsive to a control signal to set a magnitude of current flow through the active conductive element. Control logic provides the control signals to the active elements on the first and second sides to set the polarity of the current to the load. Logic coupled to the voltage-controlled power supply maintains a supply voltage sufficient to maintain a voltage drop across the active conductive elements within a linear range of operation of the conductive elements. The output of the voltage-controlled power supply is clamped at or near a minimum stable level.

Description:
REFERENCE TO RELATED APPLICATION  
       [0001]    The benefit of U.S. Provisional Patent Application No. 60/338,778, entitled METHOD FOR PROVIDING AN EFFICIENT BIPOLAR CURRENT SOURCE FROM A UNIPOLAR POWER SUPPLY, invented by Anthony J. Alfrey, and filed on Dec. 5, 2001, is hereby claimed. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to controllable current source circuitry, and more particularly to controllable bipolar current sources operable to apply bipolar current across a load, such as a thermoelectric cooler, with a unipolar power supply.  
           [0004]    2. Description of Related Art  
           [0005]    Thermoelectric coolers (TEC) are solid state heat pumps used for a variety of cooling and heating applications, including heat removal and temperature control of thermal loads including laser diodes and laser crystals. The amount of heat transferred is adjusted by controlling the current through the TEC; cooling or heating functions are controlled by changing the direction of current flow through the TEC, so bipolar application of the current is required. As the operating power of said thermal loads rises, thermoelectric cooler assemblies used for these temperature control tasks may require several hundred watts of electrical power, with applied voltages of several tens of volts and currents of tens of amperes.  
           [0006]    A bipolar current supply for driving a TEC may be assembled from an adjustable single-ended or unipolar source by adding electronically-controlled switches or conductive elements  5 ,  6 ,  7 , and  8  in an H-bridge configuration shown in FIG. 1 a.  The magnitude of the voltage applied to the load  4  (or current passing through the load  4 ) is controlled by an input voltage V supply control    3  provided to the controlled power supply  1 , while the polarity of the voltage applied to the load  4  is controlled by the sign of the input voltage V supply control    3  through the use of Control Logic  2 . For example, if the input voltage V supply control    3  is positive with respect to ground, switches  6  and  7  are closed, making terminal  11  positive with respect to terminal  10 . This circuit consisting of switches  5 ,  6 ,  7  and  8  and load  4  may be referred to as a “digital H-bridge” because the conductive elements  5 ,  6 ,  7  and  8  are either fully open or fully closed.  
           [0007]    The action of the circuit in FIG. 1 a  may be graphically described in FIG. 1 b,  wherein the load current through  4  is plotted versus the voltage V supply control    3 . For positive values of V supply control , switches  6  and  7  are closed and terminal  11  is positive with respect to terminal  10 , while for negative values of V supply control , switches  5  and  8  are closed and terminal  10  is positive with respect to terminal  11 . When the load  4  is a thermoelectric cooler, the polarity of terminal  11  with respect to the polarity of terminal  10  determines whether the thermoelectric cooler is used for heating or cooling. An example of this type of circuit utilized for the case in which the load  4  is a thermoelectric cooler is described in U.S. Pat. No. 5,936,987.  
           [0008]    For the voltage-controlled power supply  1 , it is highly desirable to utilize compact, pre-packaged, inexpensive and powerful switching power supplies which are available from commercial vendors. However, due to limitations in the design of said switching power supplies, they are often not adjustable over a full range of voltage output from zero to some maximum voltage, and may exhibit instabilities, or may not function at all when used below some particular minimum voltage V minimum .  
           [0009]    Referring to FIG. 2, an alternative to the above embodiment is to replace the voltage-controlled power supply  1  in FIG. 1 with a fixed-voltage supply  12 , and replace switches  5 ,  6 ,  7  and  8  with voltage-controlled conductive elements  13 ,  15 ,  17  and  19  in which the current flow through said conductive elements depends linearly on a controlling voltage applied to terminals  14 ,  16 ,  18  and  20  of said conductive elements, said controlling voltage derived through the action of linear control circuitry  21  which is in turn further derived from an additional voltage V control    22 . The conductive elements are activated in pairs diagonally across the H-bridge. By activating elements  15 / 17  as a pair, terminal  11  is made positive with respect to terminal  10 . By activating elements  13 / 19  as a pair, terminal  10  is made positive with respect to terminal  11 . One element out of each pair  15 / 17  or  13 / 19  may be a switch-like digital element that is either fully conducting or fully open. The circuitry of FIG. 2 (not including the fixed-voltage supply) may be referred to as a “linear H-bridge”. An example of such a linear bridge is described in the prior art of U.S. Pat. No. 6,023,193.  
           [0010]    This circuit has the disadvantage of being inefficient, which is of special importance in very high current applications. For example, in the worst-case condition for which the voltage drop across the load  4  is one-half of the fixed supply voltage V supply    9 , the power dissipated in the linear element is equal to that delivered to the load  4  and the circuit thereby provides an overall “line-plug” efficiency of no greater than 50%. This forces the use of high-power-dissipation devices for the controlled conductive elements and also necessitates the use of large heat sinking and cooling fan assemblies, increasing both the physical size and cost of the unit.  
         SUMMARY OF THE INVENTION  
         [0011]    It is therefore an objective of the invention to allow the use of inexpensive pre-packaged adjustable-voltage unipolar power supplies, said supplies possibly not providing a full range of output voltages, comprised within a circuit that provides adjustable bipolar output current.  
           [0012]    It is another objective of the invention to minimize the power dissipated in the linear elements of an H-bridge while simultaneously maximizing the transfer of power to a load.  
           [0013]    It is yet another objective of the subject invention to apply H-bridge circuitry to the application of power to a thermoelectric cooler for the purpose of controlling the temperature of a thermal load.  
           [0014]    The present invention provides an inexpensive, compact, programmable, high-current and bipolar current source as a power source for thermoelectric coolers, or any other load requiring a bipolar current source.  
           [0015]    Embodiments of the present invention comprise an apparatus for applying an adjustable bipolar current to a load, such as a thermoelectric cooler. The apparatus includes a voltage-controlled power supply having a unipolar output. An H-bridge has a first side with two active elements connected at a first node in series between the unipolar output of the power supply and ground, and a second side with two active elements connected at a second node in series between the unipolar output of the power supply and ground. The first and second nodes are adapted to be coupled to the load so that current may flow between the first and second nodes through the load, the magnitude and polarity of said current flow through the load resulting from the response of said active elements to control signals. Control logic provides the control signals to the active elements on the first and second sides of the H-bridge such that only one pair of diagonally-opposed active elements is conducting at any time.  
           [0016]    Furthermore, embodiments of the present invention include supply control logic which provides a signal to the voltage-controlled power supply so as to control the output of said voltage-controlled supply, said output being sufficient to maintain the voltage drop across at least one of the active elements comprising said pair of diagonally-opposed elements within a range of voltage drops for which linear operation of said active element is normally obtained, and said output being greater or equal to a threshold voltage above which the voltage-controlled power supply output is sufficient to maintain the operation of said power supply in a stable manner. For purposes of instruction, a load voltage will be defined as the magnitude of the voltage drop across the load connected between said first and second nodes. For a required load voltage greater than said threshold voltage, the supply control logic, in various embodiments, maintains the output of said supply at a level above the load voltage by a headroom value sufficient to maintain a voltage drop across said active element near a lower voltage of the aforesaid range. For a required load voltage less than said threshold voltage, said supply control logic clamps the output of the voltage-controlled supply to a clamp value sufficient to maintain stable operation of the voltage-controlled supply. In this manner, the output of the voltage-controlled power supply is kept at a level that conserves power, while insuring stable, linear operation of the circuit at all values of load voltage.  
           [0017]    Embodiments of the present invention comprise a cooling system including a thermoelectric cooler and a bipolar current source as described above. Such systems include thermal sensors coupled to a thermal load of the thermoelectric cooler and provide feedback for control of the bipolar current source based upon the sensed temperature, to maintain the temperature of the thermal load near a predetermined value.  
           [0018]    Various embodiments of the present invention implement the control logic using analog circuitry, digital circuitry, software control processors, and combinations of analog, digital and software controlled resources.  
           [0019]    The present invention provides therefore a low-cost bipolar current source, with average power consumption substantially less than has been achieved in prior art systems.  
           [0020]    Other aspects and advantages of the present invention can be seen on review of the drawings, the detailed description and the claims, which follow. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]    [0021]FIG. 1 a  is a diagram of a prior art digital H-bridge configured as a bipolar current source.  
         [0022]    [0022]FIG. 1 b  is a graph showing a transfer function for V supply control  versus load current for the circuit of FIG. 1 a.    
         [0023]    [0023]FIG. 2 is a diagram of a prior art linear H-bridge configured as a bipolar current source.  
         [0024]    [0024]FIG. 3 a  is a diagram of a dynamic H-bridge configured as a bipolar current source according to the present invention.  
         [0025]    [0025]FIG. 3 b  is a graph showing V supply  versus V load  for the circuit of FIG. 3 a.    
         [0026]    [0026]FIG. 4 is a graph showing power dissipation for the circuit of the present invention and for the prior art circuit of FIG. 2.  
         [0027]    [0027]FIG. 5 is a circuit diagram for the transfer function logic  100  of the circuit shown in FIG. 3 a.    
         [0028]    [0028]FIG. 6 illustrates cooling system including a thermoelectric cooler with a current source according to the present invention.  
         [0029]    [0029]FIG. 7 is a circuit diagram of one embodiment of an H-bridge configured according to the present invention.  
         [0030]    [0030]FIG. 7 a  is a more detailed diagram of active conductive element  17 , in the circuit of FIG. 7.  
         [0031]    [0031]FIG. 7 b  is a more detailed diagram of active conductive element  15  in the circuit of FIG. 7.  
     
    
     DETAILED DESCRIPTION  
       [0032]    A detailed description of embodiments of the present invention is provided with reference to FIGS. 3 a - 3   b  through  7   a - 7   b.  As described in FIG. 3 a,  a controlled voltage power supply  1 , with a minimum output voltage V minimum , is used in conjunction with a linear H-bridge circuit and additional Transfer Function circuitry  100  to meet the aforementioned objectives. The operation of the complete system may be understood by studying FIG. 3 b  and by considering two modes of operation; the first of said modes being the condition wherein the desired applied load voltage  25  is less than the minimum stable voltage V minimum  of the controlled power supply and the second of said modes being the condition wherein the desired applied load voltage  25  is greater than said minimum stable voltage V minimum .  
         [0033]    In the former case, the output voltage of 1 is held at V minimum  and the current flow through the load is controlled by the linear element pairs  15 / 17  or  13 / 19  depending on the polarity of V control    22 . For example, consider first the condition of the linear pair  13 / 19  disabled and element  15  fully conducting, and element  17  controlling the current through the load  4 . In the preferred embodiment, elements  17  and  19  are voltage-controlled current sinks in which, for example, the current flow from terminal  10  to ground is linearly dependent on a voltage at terminal  18 . Starting at a condition of zero current flow, V supply  is held constant at V minimum . If element  15  is fully conducting, there is no voltage drop across it, for the purpose of this description, and also since V supply =V load +V current sink , as the required load voltage V load    25  increases, the voltage V current sink  across the current sink decreases. At some point, V current sink    43  will reach a minimum value Δ. Said minimum value Δ is at or near a lower limit of a range of voltage drops below which said current sink  17  no longer behaves linearly in response to a control voltage V control  and is determined by the inherent nature of the device used for element  17 . When the required load voltage V load  increases further, the circuit block Transfer Function  100  will sense the resulting drop in (V supply −V load ) below a headroom value, based on Δ for current sink  17  and other active current sinks in the current path through the H-bridge, and said circuitry  100  will work to command an increase in the value of V supply  to keep (V supply −V load ) equal to about the headroom value. Preferably, the headroom value is maintained near a lower limit of the range of linear operation, or at least in a lower part of the range that supports linear operation, in order to conserve power.  
         [0034]    The operation of the Transfer Function  100  may be graphically described as in FIG. 3 b,  in which the headroom value is heuristically represented by minimum value Δ. Its action is to measure the voltage V in    41  and derive a voltage V supply control    42  that, in turn, affects the output of the controlled voltage power supply  1 . For the purposes of description, we will assume that the output voltage V supply    9  of the controlled voltage power supply  1  is equal to the value of the voltage V supply control  and that V load =V in . Referring to FIG. 3 b,  the two operating regions may be observed. If the magnitude of V load  is less than or equal to V minimum −Δ, the output voltage V supply control  of the Transfer Function  100  is fixed to hold V supply =V minimum . As the magnitude of the voltage V load  rises above V minimum −Δ, V supply control  is increased to keep V supply −|V load | near constant and equal to about Δ.  
         [0035]    It is important to note that, in evolving from the prior art described in FIG. 1 to the present art described in FIG. 3 a,  that switches  5  and  6  may also be replaced with voltage-controlled current sinks. It is only necessary to have at least one linear controlled conductive element per each side of the H-bridge if it is to be operable in the manner described in the above teaching.  
         [0036]    Since one objective of the present invention is to reduce the power dissipation of the elements  17  or  19 , it is of benefit to graphically display the power dissipated by the linear element for the circuits described in FIG. 2 and FIG. 3 a.  As discussed above, there are two regions of interest, region I for which the supply voltage is clamped at a threshold voltage at or near V minimum  and region II, for which V supply −|V load | is constant and equal to Δ. For the purposes of discussion, we assume that the linear elements  13  or  15  may be fully conductive and thereby experience no voltage drop. The power dissipated by the element  17  or  19  for region I is:  
           P   I     =         (       V   minimum          V   load       )     -       (     V   load     )     2       R       ,                         
 
         [0037]    reaching a maximum of (V minimum ) 2 /(4R). In region II, the power dissipated is given  
         P   II     =       ΔV   load     R                           
 
         [0038]    Both regions are plotted on the graph in FIG. 4. Additionally, for the circuit design described in FIG. 2 with a power supply operating at a fixed voltage of V maximum , the power dissipated by the linear element is  
       P   =         (       V   maximum          V   load       )     -       (     V   load     )     2       R                           
 
         [0039]    and is also plotted on FIG. 4. As an example, we consider a load  4  consisting of a thermoelectric cooler with a resistance of 4 ohms, and a Vicor model V375A48C600A power supply for  1  that may be operated at a fixed voltage output V maximum  of 48 volts, or as a variable voltage supply with a minimum supply voltage V minimum  of 4 volts and a V maximum  of 48 volts. The maximum power dissipated by the element  17  is then 144 Watts for the circuit design of the prior art (FIG. 2), while the circuit design of the present invention (FIG. 3 a ) reduces the maximum power dissipation to 12 Watts with a minimum voltage drop Δ of 1 volt across the element  17 , demonstrating the advantage offered by the present invention over the prior art.  
         [0040]    Mathematically, we may express the action of the Transfer Function  100  as graphically described in FIG. 3 b  as V supply control =|V in |+Δ or no less than V minimum . While there are several possible embodiments for enabling the Transfer Function  100  described above, one such embodiment is described in FIG. 5. The circuitry may be considered in several sections, labeled  200 ,  300 ,  400  and  500 . The function of the first section  200  is well-known in the art as an Absolute Value circuit, transforming the input load voltage V in    41  which may take on positive or negative values, depending on the polarity of the current flow through the load  4 , into the absolute value V 200 =|V in |. Section  300  performs a Voltage Inversion of the output of section  200 , and further offsets this voltage by an amount corresponding to the minimum voltage drop Δ across the current sink, thus providing an output V 300 =−|V in |−Δ by applying an offset voltage Δ created by voltage reference  305  to the input of the unity gain summing amplifier formed by  301 ,  302 ,  303  and  304 . Section  400 , known in the art as a Clamp circuit, limits the output of the prior section  300  to a value no less negative than −V minimum , said voltage −V minimum  being generated by the action of voltage reference  401  and voltage divider  403  and  402 , so that the voltage at the output of section  400  equals V 400 =−|V in |−Δ or −V minimum , whichever is more negative. Finally, section  500  performs one further Voltage Inversion, resulting in the final transfer function relating V in  and V supply control  to be  
           V   supply control   =|V   in |+Δ or  V   minimum ,  
         [0041]    whichever is greater.  
         [0042]    While the above described circuitry performs the requisite transfer function using analog techniques, the same function may be readily performed using digital software techniques, by using hardwired digital circuitry, a software-controlled processor or a combination of the same.  
         [0043]    While the above described techniques may be used for supplying bipolar current to any load from a unipolar power supply source, one specific application may be the supplying of power to thermoelectric coolers for the purposes of controlling the temperature of a thermal load. FIG. 6 describes the way in which the present invention may be used in a temperature control system, in which the load  4  has been replaced by a thermoelectric cooler  69  and elements  21 ,  13 ,  17 ,  18 ,  19  and  40  are collected into element  50 . Said cooler is used to transfer heat from a thermal load  70  to a heat reservoir  71 , the temperature of said thermal load being detected by a sensor  72 , and the signal representing such temperature being further routed to additional control circuitry  73  and  74  and compared to a signal  75  representing a preselected temperature. Said additional control circuitry then provides a further command voltage V control  to the input of the circuitry described in the present invention, thereby supplying the current needed by the thermoelectric cooler  69  to hold the temperature of the thermal load  70  at the desired value.  
         [0044]    Additionally, a more detailed schematic of one possible embodiment of the elements within the linear H-bridge is provided in FIG. 7. In this embodiment, portions of the logic functions incorporated into the Linear Control Circuitry  21  of FIG. 3 are distributed among linear elements  17  and  19  and inverting amplifier  27 . Signals formerly derived from Linear Control Circuitry  21  and directed to a control input  14  of linear element  13  and a control input  16  of linear element  15  are instead derived from control outputs  18   a  and  20   a  of linear elements  17  and  19  respectively. FIG. 7 a  displays one of two identical linear elements  17  and  19  comprising a current sink based on a high-current MOSFET transistor  601  and an operational amplifier  602 . Current sensing is achieved by measuring the voltage drop across resistor  603 . The resulting voltage is converted into a current when applied across resistor  604  and compared to the current passing through resistor  605  generated by an applied voltage at  18 . The operational amplifier drives the gate of the MOSFET  601  to cause the currents in  604  and  605  to be equal, thereby creating a voltage-controlled current source. Additionally, the current source has an output  18 A which communicates the value of the MOSFET  601  gate voltage to the diagonally opposed elements of the H-bridge. A further inverting amplifier  27  well-known in the art is included as part of the control circuitry of FIG. 7 to insure that only one of the identical elements  17  or  19  is enabled at any time.  
         [0045]    [0045]FIG. 7 b  describes one of two identical controlled elements  13  and  15 . The objective in this embodiment is to apply the same magnitude of voltage between the gate and source terminals of the MOSFET  709  as is applied across the gate and source of the MOSFET  601  so that said MOSFETs become conductive simultaneously. However, since MOSFET  709  is a P-channel device while MOSFET  601  is an N-channel device, and since the source terminal of MOSFET  709  is elevated to the output voltage of the controlled power supply  1  which may vary widely during operation, a level-shifting circuit must be used to reference the voltage to be applied to the gate of MOSFET  709  to said output voltage and also to invert the polarity of said applied voltage as compared to that applied to the MOSFET  601 . Said level-shifting/inverting circuitry consists of resistors  701 ,  702 ,  703 ,  704 ,  706  and  707 , along with amplifier  705  and transistor  708 . Said resistors may be chosen to provide voltage gain so that the magnitude of the voltage applied to the gate of MOSFET  701  is some multiple of that applied to the gate of MOSFET  601 . In addition, MOSFET  709  may be chosen to have a transfer characteristic such that it conducts more fully for a given applied gate voltage than does MOSFET  601 . In this way, MOSFET  709  acts more as a simple switch, exhibiting little or no voltage drop across its drain and source terminals, while MOSFET  601  then acts as the current-controlling element for the diagonally-opposed linear element pair  15 / 17 .  
         [0046]    While the present invention is disclosed by reference to the preferred embodiments and examples detailed above, it is to be understood that these examples are intended in an illustrative rather than in a limiting sense. It is contemplated that modifications and combinations will readily occur to those skilled in the art, which modifications and combinations will be within the spirit of the invention and the scope of the following claims. What is claimed is: