Abstract:
A circuit for connecting lower AC voltage-rated AC-DC power supplies with higher voltage power sources. A power line matching transformer connecting the source to the power supplies needs only to support the self-dissipation and output current mismatch between supplies. The circuit can also protect the line matching transformer from overheating in various fault scenarios.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The invention pertains to the field of power supply circuits. More particularly, the invention pertains to voltage adaptors for power supply modules which allow multiple modules with a lower AC voltage input requirement to be used with a higher AC voltage supply. 
         [0003]    2. Description of Related Art 
         [0004]    Very often it is required to deploy switch mode AC-DC power conversion assemblies powered from a 380V or 440V or 480V, three phase power system where no neutral connection is available. As typical switch mode power supply modules are designed to operate from a 208V or 220V or 230V or 240V power line, this is typically achieved by utilizing a power line frequency step down transformer that has to carry the entire rated power of the system. This entails uneconomical usage of space, excessive weight and increased cost. 
         [0005]    While it is possible to specifically design AC-DC power supplies to work from a 380-440-480VAC power line, this generally requires a significant design effort that many times is beyond the economic scope of the program. A design technology that enables the use of preexisting power supply modules rated to 208-220-230-240VAC input without an excessively large, heavy and expensive step down transformer is warranted. 
       SUMMARY OF THE INVENTION 
       [0006]    The invention provides a circuit for connecting lower AC voltage-rated (for example, 208-220-230-240VAC) AC-DC power supplies with higher voltage (for example 380-440-480VAC) power lines. By using the invention, the power line matching transformer no longer needs to supply the rated input power of the supplies, but rather need only support the self-dissipation and output current mismatch between supplies. The circuit technology also protects the line matching transformer from overheating in various fault scenarios. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0007]      FIG. 1  shows a block diagram of a first embodiment of the invention. 
           [0008]      FIG. 2  shows a block diagram of a second embodiment of the invention. 
           [0009]      FIG. 3  shows a block diagram of a third embodiment of the invention. 
           [0010]      FIG. 4  shows a block diagram of multiple examples of the first embodiment of the invention, used to service a three phase power line. 
           [0011]      FIG. 5  shows a block diagram of a fourth embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0012]      FIGS. 1-5  show various embodiments of the invention. In each of the figures, an AC power source  1  supplies two AC-DC power supplies  4   a  and  4   b,  each of which has a an AC power input  5   a  and  5   b  and a DC power output  6   a  and  6   b.  The DC outputs  6   a  and  6   b  are connected together to provide a DC output  8  to a common load  3 . 
         [0013]    A controller  7  ( FIG. 1 ),  27  ( FIG. 2 ) or  37  ( FIG. 3 ) can assert a signal on a shut-down line  9   a  coupled to a shutdown input of power supply  4   a,  and also on a shut-down line  9   b  coupled to a shutdown input of power supply  4   b.  This allows the controller  7  or  27  or  37  to selectively shut down either or both of the power supplies  4   a  or  4   b..    
         [0014]    In one common requirement, AC source  1  may be, for example, a power line at 440 VAC, and the AC-DC power supplies  4   a  and  4   b  have an input voltage requirement of 220 VAC. It will be recognized that other voltage combinations are possible within the teachings of the invention. 
         [0015]    Switch mode AC-DC power supplies have, as their nature, a negative resistance input impedance characteristic. If two supplies  4   a    4   b  have their input circuits  5   a    5   b  connected in series across an input power source, they will tend to share this voltage evenly if their input power is identical. As there may be variations in internal power losses from converter to converter (although these are typically small for a given converter type), and delivered output current to a common load  3  may vary (although modern converters are designed to share current to a high degree when powering a common load), a balancing mechanism is warranted to force the voltage split between the converters to be relatively equal. Otherwise, the converters may divide the input voltage in a non-uniform way to the extent where either one converter shuts down due to low voltage, or is damaged due to excessively high voltage. 
         [0016]    In  FIG. 1 , each AC-DC power supply  4   a    4   b  is configured to have the same circuit and components. These power supplies  4   a    4   b  also feature output current sharing circuits that force their delivered output current to be within a specific tolerance of each other when powering a common load  3 . This configuration then dictates the maximum input current or power imbalance to be bounded by the difference in internal dissipation within the supplies (equal to the energy conversion difference between the two units) and the difference in output current delivered by the units. 
         [0017]    A line frequency transformer  2  is implemented in an auto-transformer configuration to force input voltage balance between the AC inputs  5   a  and  5   b  of the two power supplies  4   a  and  4   b.  The transformer has a first winding  2   a  and and a second winding  2   b,  of equal length, connected in series at a center tap  2   c.  The AC source  1  is connected to opposite ends of windings  2   a  and  2   b.  The AC input  5   a  of power supply  4   a  is connected across winding  2   a,  and the AC input  5   b  of power supply  4   b  is connected across winding  2   b.    
         [0018]    This transformer  2  need only be sized to carry the mismatch in input power between the two units  4   a  and  4   b,  not the total rated power of the system. For example, if each of the power supplies  4   a  and  4   b  are rated at 2.7 kW per unit, the mismatch might be typically on the order of 100-200 watts. In a conventional step-down transformer design, this would require a transformer capable of supplying the full 5.4 kW for the two supplies. In the design of the invention, however, transformer  2  need only be rated at 200 W, versus 5.4 kW total system power. 
         [0019]    As long as both units are working normally, transformer  2  will operate within its rated capability. However, if one of the power converters, say  4   a,  suffers a fault, the remaining power converter  4   b  will continue to try to deliver power on output  8  connected to the load  3 . This condition could result in over loading the balance transformer  2 , resulting in additional failures. 
         [0020]    In order to prevent this event, in the embodiment of  FIG. 1 , a monitor circuit is implemented in controller  7  that monitors an output  10   a  and  10   b  from power supplies  4   a  and  4   b  which has a signal representing the delivered output power from the supply. If the controller  7  determines there is a mismatch between  10   a  and  10   b  greater than a predetermined safe value, it electronically asserts the shutdown signal on  9   a  and  9   b  which shuts both power supplies  4   a  and  4   b  down to prevent transformer  2  from over-heating. 
         [0021]      FIG. 2  presents an alternate method to detect and protect against balance transformer over-dissipation. 
         [0022]    In  FIG. 2 , the current on center tap  2   c  of balance transformer  2  is monitored by a current sensor  20 . The signal output  21  from the current sensor  20  is input to controller  27 . 
         [0023]    Under normal circumstances the current on center-tap  2   c  is below a well-defined threshold. If for some reason signal  21  representing the current on center tap  2   c  indicates that the current exceeds this threshold, a potential transformer over-current situation is indicated. The controller  27  would then assert the shutdown signal on  9   a  and  9   b,  which would cause both power supplies  4   a  and  4   b  to shut down so as to protect the transformer  2 . 
         [0024]      FIGS. 3 and 5  present another alternate method to detect and protect against balance transformer over-dissipation. 
         [0025]    In  FIG. 3 , the temperature of balance transformer  2  is monitored by a temperature sensor  30 , which sends a signal on line  31  to controller  37 . If the signal on  31  indicates that the sensor  30  has detected a temperature in excess of a predetermined safe level, then controller  37  would assert the shutdown signal on  9   a  and  9   b,  which would cause both power supplies  4   a  and  4   b  to shut down so as to protect the transformer  2 . 
         [0026]    Other protection schemes are also possible, such as monitoring the difference between the input voltages  5   a    5   b  on each supply  4   a    4   b  and asserting a shut-down signal on  9   a    9   b  if this difference in voltage is determined to be above a predetermined level. 
         [0027]      FIG. 5  differs from  FIG. 3  in that  FIG. 5  shows the outputs  6   a  and  6   b  of power supplies  4   a  and  4   b  connected in series, rather than in parallel as in the other figures. 
         [0028]    It will be understood by one skilled in the art that using this arrangement, the voltages at the outputs  6   a  and  6   b  are summed at the power output  8 , as opposed to the currents supplied by the two power supplies  4   a  and  4   b  being summed at output  8  in the parallel arrangements used  FIGS. 1-4 . 
         [0029]      FIG. 4  shows how three single-phase units  40   a,    40   b  and  40   c  can be used to service a 380-480VAC, 3-phase “Delta” (i.e., 3-wire) power line  50 . Each single-phase unit  40   a - 40   c  is, in this example, the circuit of the embodiment of  FIG. 1 . It will be understood that this is provided as an example, and any of the embodiments of the invention could be used. 
         [0030]    Each of the single-phase units  40   a    40   b    40   c  has a power input connected to one of the phases of the power line  50 . So, phase  101   a  is connected to input  41   a  of unit  40   a,  phase Φ b  is connected to input  41   b  of unit  40   c,  and phase Φ c  is connected to input  41   c  of unit  40   c.  Each single-phase power unit  40   a - 40   c  has an output  48   a - 48   c  connected to a load  43   a - 43   c,  as discussed above with respect to  FIGS. 1-3 . 
         [0031]    Accordingly, it is to be understood that the embodiments of the invention herein described are merely illustrative of the application of the principles of the invention. Reference herein to details of the illustrated embodiments is not intended to limit the scope of the claims, which themselves recite those features regarded as essential to the invention.