Patent Publication Number: US-2010127811-A1

Title: Transformer with Center Tap Encompassing Primary Winding

Description:
FIELD 
     This invention relates to devices for efficiently down converting high DC supply voltages to relatively lower AC or DC voltages. 
     BACKGROUND 
     Switch-mode DC-to-DC converters convert one DC voltage level to another. Such converters typically perform the conversion by applying AC voltage with a specific frequency and duty across the primary winding of a transformer, thereby coupling AC voltage to the secondary winding of the transformer. The AC voltage on the secondary winding can then be rectified to produce a DC output voltage. The turns ratio of the primary and secondary windings of the transformer determines, in part, the voltage step-up or step-down ratio provided by the converter. The output voltage can also be finely regulated using pulse-width-modulation (PWM) drive techniques. 
     Emerging applications for DC-to-DC converters require high efficiency conversion of relatively high input voltages. For example, a high-energy storage device described in U.S. Pat. No. 7,033,406 claims to safely store charge at 3,500 volts. This voltage will have to be down converted efficiently and regulated for use with equipment that requires relatively lower supply voltages. For example, conventional battery powered motor vehicles might benefit from a high-energy storage device, but the electric motors employed to drive them typically require input voltages of less than 100 volts. Voltage converters suitable for this task should be robust, inexpensive, and compact to ensure commercial viability. There is therefore a need for robust, compact, and efficient voltage converters that handle relatively high input voltages. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The subject matter disclosed is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings and in which like reference numerals refer to similar elements and in which: 
         FIG. 1  depicts a voltage converter  100  in accordance with one embodiment. 
         FIG. 2  depicts a voltage converter  200  in accordance with another embodiment. 
         FIG. 3  depicts an output-regulated DC-to-DC converter system  300  in accordance with another embodiment. 
         FIG. 4A  schematically depicts an output transformer  400 , in accordance with one embodiment, coupled to a conventional rectifier  405 . 
         FIG. 4B  is a cross-sectional view of transformer  400 , in accordance with one embodiment, mounted to an optional heat sink  410 . 
         FIG. 5A  includes plan, side, and cross-sectional views of a component  500  for use in transformer  400  of  FIG. 4B . 
         FIG. 5B  is an exploded view of a transformer body  535 , which includes an opposing pair of components  500  of  FIG. 5A , and a cylindrical core C. 
         FIG. 5C  is an assembled view of transformer body  535  of  FIG. 5B . 
         FIG. 5D  shows an exploded view of assembly  535  similar to that of  FIG. 5B  but in cross-section. 
         FIG. 5E  shows the elements of  FIG. 5D  assembled, in cross-section, and includes a plan view of the resulting assembly  535  to identify the cross-section of  FIG. 5D  as along line B-B. 
         FIG. 5F  is the same view of transformer  400  provided in  FIG. 4B  but amended to include labels for the physical features of the same transformer illustrated schematically in  FIG. 4A . 
         FIG. 6A  depicts a housing portion  600 , in accordance with another embodiment, that can be used with another similar juxtaposed housing portion (not shown) to form a transformer housing similar to that of  FIG. 5A-5D . 
         FIG. 6B  depicts a housing portion  615  in accordance with yet another embodiment. 
         FIG. 6C  depicts a housing portion  630  in accordance with an embodiment in which a single-turn winding is formed of three conductors  635  attached to a housing portion  640 . 
         FIG. 6D  depicts a transformer  645  that employs two juxtaposed housing portions  630  of  FIG. 6C  to form a second of windings and a center tap. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  depicts a voltage converter  100  in accordance with one embodiment. A relatively high supply voltage is divided across a number of components such that none of the components receives the full supply voltage. Accordingly, voltage converter  100  can be assembled using relatively small and inexpensive components. 
     Converter  100  includes a PWM controller  105 , a first transformer T 1 , a second transformer T 2 , a pair of bridge circuits  110  and  115  disposed between supply terminals HV and ground, a third transformer T 3 , and a rectifier  120 . PWM controller  105 , via transformers T 1  and T 2 , stimulates bridge circuits  110  and  115  to drive current in alternate directions through respective primary windings P 1  and P 2  of transformer T 3 , and thereby develop an alternating voltage across the secondary winding S 1 . Transformer T 3  steps down the high DC supply voltage HV to create a relatively lower voltage signal across secondary S 1 . Converter  100  is a DC-to-DC converter in this embodiment, so rectifier  120  is included to covert the alternating signal across secondary S 1  into a relatively low DC output voltage LV to a load  125 . 
     Bridge circuit  110  includes series-connected transistor switches Q 1  and Q 2 , a series pair of resistors R 1  and R 2 , and a series pair of capacitors C 1  and C 2 . The first primary winding P 1  of transformer T 3  is coupled between a first node N 1  common to transistors Q 1  and Q 2  and a second node N 2  common to resistors R 1  and R 2  and capacitors C 1  and C 2 . Bridge circuit  115  is essentially identical, and includes series connected transistor switches Q 3  and Q 4 , a pair of resistors R 3  and R 4 , and a series pair of capacitors C 3  and C 4 . The second primary winding P 2  of transformer T 3  is coupled between a node common to transistors Q 3  and Q 4  and a node common to all four of resistors R 3  and R 4  and capacitors C 3  and C 4 . Resistors R 1  and R 2  ensure the voltage across respective capacitors C 1  and C 2  remains below the breakdown voltage of the capacitors. Resistors R 1  and R 2  likewise, via primary P 1 , divide the voltage across transistors Q 1  and Q 2 , which are 800-volt MOSFETs in an embodiment in which voltage HV is about 1,400 volts. In general, the transistors should be rated to withstand more than HV/N volts, where N is the number of bridge circuits stacked between the high-voltage supply terminals. Other embodiments can employ different types of switches, such as insulated-gate bipolar transistors. 
     PWM controller  105  produces a pair of drive signals D 1  and D 2 , one on the primary winding of transformer T 1  and the other on the primary winding of transformer T 2 . Drive signals D 1  and D 2  may be square waves timed to a common clock pulse (not shown), and can be pulse-width modulated to change the power delivered to load  125 . Controller  105  may be set to define a dead time when switching between transistors to prevent shorting the high-voltage supply terminals HV to ground. PWM controllers are commercially available and are well-known to those of skill in the art. A detailed discussion of PWM controller  105  is therefore omitted for brevity. 
     Converter  100  is off, which means voltage level LV is zero, when input signals IN and IN\ are held equal. Resistors R 1 -R 4  divide the high voltage between the supply terminals equally among capacitors C 1 -C 4  to prevent potentially damaging voltages from developing across the capacitors and transistors. Furthermore, the RMS current is provided to transformer T 3  is divided between to capacitors, which further reduces the stress on capacitors C 1 -C 4 . 
     To turn on converter  100 , PWM controller  105  introduces complementary square waves on terminals IN and IN\ such that difference signal IN-IN\ is presented across the primary winding of transformer T 1 . Signal IN-IN\ periodically reverses polarity, and consequently reverses the direction of current flow through the primary and secondary windings of transformer T 1 . Transistors Q 1  and Q 3  turn on and transistors Q 2  and Q 4  turn off when current flows through the secondary winding of transformer T 1  in a first direction, and transistors Q 1  and Q 3  turn off and Q 2  and Q 4  turn on when current flows in the opposite direction. Signal IN-IN\ thus causes converter  100  to alternately turn on transistor pairs Q 1 /Q 3  and Q 2 /Q 4 . 
     When PWM controller  105  turns transistors Q 1  and Q 3  on, current flows from capacitors C 1  and C 2  through primary winding P 1  to the node common to capacitors C 1  and C 2 ; and from capacitors C 3  and C 4  through primary winding P 2  to the node common to capacitors C 3  and C 4 . Because pairs of capacitors provide current through each primary winding, each of capacitors is required to accommodate half of the total RMS current through one primary. Capacitors C 1 -C 4  can therefore be smaller, less expensive, or both. 
     PWM controller  105  then turns transistors Q 1  and Q 3  off briefly before turning transistors Q 2  and Q 4  on to prevent a direct short between the supply terminals and across each bridge circuit. With transistors Q 2  and Q 4  on, the charge on the node common to capacitors C 1  and C 2  discharges through primary winding P 1  and transistor Q 2 , and the charge on the node common to capacitors C 3  and C 4  discharges through primary winding P 2  and transistor Q 4 . Turning on transistors Q 1  and Q 3  and turning off transistors Q 2  and Q 4  begins the cycle anew. PWM controller  105  thus stimulates bridge circuits  110  and  115  to pass high-voltage alternating current through primary windings P 1  and P 2 , and consequently through secondary winding S 1 . Rectifier  120  rectifies the resulting signal across secondary winding S 1  to provide the relatively lower DC output voltage LV. 
     In an embodiment in which the voltage across bridge circuits  110  and  115  is 1,200 volts, the alternating DC signal developed on the node common to transistors Q 1  and Q 2  alternates between approximately 600 volts and approximately 1,200 volts, and the node common to transistors Q 3  and Q 4  alternates between zero and 600 volts. None of the components experience the full 1,200 volts from the power supply, which allows for selection of smaller, less expensive components, a longer mean time between failures, or both. 
       FIG. 2  depicts a voltage converter  200  in accordance with another embodiment. Converter  200  includes four transformers T 1 , T 2 , T 3 , and T 4 ; three bridge circuits  220 ,  225 , and  230 ; and a current monitor  270 . Bridge circuits  220 ,  225 , and  230  extend between supply terminals ST 1  and ST 2 , which provide DC supply voltage levels that differ by about 1,800 volts in one embodiment. The bridge circuits are identical in this example, so the following discussion is limited to bridge circuit  220  for brevity. 
     Bridge circuit  220  is similar to bridge circuit  110  of  FIG. 1 , like-labeled elements being the same or similar. The gates of transistors Q 1  and Q 2  are coupled to respective secondary windings of transformers T 1  and T 2  via an optional parallel connection  250  of a resistor and a diode-resistor series combination, which may be included to increase the turn-off time relative to the turn-on time, and thereby provide some degree of protection against cross-conduction between the transistors within each bridge circuit. The sources of transistors Q 1  and Q 2  are coupled to their respective gates via transorbs  260 , which prevent over-voltage conditions from damaging the gate/source junction. A snubber circuit SN 1  extends between the input terminals of primary P 1  to suppress (“snub”) electrical transients, and thereby protects the components of bridge circuit  220 . The snubber circuits additionally improve the stability between bridge circuits  220 ,  225 , and  230 . Other forms of snubber circuits might also be used, or snubber circuits may be omitted in other embodiments. 
     Bridge circuits  220 ,  225 , and  230  provide outputs on respective primary windings P 1 , P 2 , and P 3  of transformer T 3 . The output voltage is taken across terminals OUT 1  and OUT 2  from the secondary S of transformer T 3 . Transformer T 4  is coupled between current-sense circuit  270  and the output of bridge circuit  220 . Circuit  270  issues an over-current alarm OC when the output current from bridge  220  exceeds a predefined threshold. Alarm OC can be used to shut down or otherwise limit the output power of converter  200 . 
       FIG. 3  depicts an output-regulated DC-to-DC converter system  300  in accordance with another embodiment. System  300  combines a pair of voltage converters  200  of the type detailed in connection with  FIG. 2  to down-convert 3,600 volts DC (VDC) to about 35 VDC between a low-voltage output node LV and ground GND. A conventional PWM controller  305 , via a driver  310 , provides pulse-width-modulated input stimuli to ports GD 1  and GD 2  of both voltage converters  200 , the outputs of which are serially connected across a rectifier  315 . Controller  305  senses and regulates output voltage LV by controlling the duty cycles of the stimulus signals to converters  200 . 
       FIG. 4A  schematically depicts an output transformer  400 , in accordance with one embodiment, coupled to a conventional rectifier  405 . Transformer  400  has six primary windings P 1 -P 6 , a core C, and two secondary windings S 1  and S 2  divided by a center tap CT. Transformer  400  is coupled to rectifier  405  via a pair of output lines TL 1  and TL 2  and a center-tap line TCT. An embodiment of transformer  400  with three primary windings can be used in place of output transformer T 3  of  FIG. 2 , while the depicted embodiment can be used with the stacked configuration of  FIG. 3  to receive six input signals, three from each of the two stacks of bridge circuits. 
       FIG. 4B  is a cross-sectional view of transformer  400 , in accordance with one embodiment, mounted to an optional heatsink  410 . The labels of  FIG. 4A  are reproduced in  FIG. 4B  to identify the physical structures of transformer  400  that correspond to the like-identified circuit nodes and features of  FIG. 4A . Primary windings P 3 -P 6  are omitted in  FIG. 4B  for ease of illustration. The following discussion details the physical components identified in the cross section of  4 B and shows how they are combined to form a robust, compact, and efficient transformer. 
       FIG. 5A  includes plan, side, and cross-sectional views of a component  500  for use in transformer  400  of  FIG. 4B . Component  500  includes a projection  505 , a housing portion  510 , an aperture  515 , assembly holes  520 , ports  525 , and a connection hole  530 . The functions of these elements will be discussed below. The lowermost view is a cross-section taken along line A-A of the plan view. 
       FIG. 5B  is an exploded view of a transformer body  535 , which includes an opposing pair of components  500  of  FIG. 5A , and a cylindrical core C. The two components  500  mate together such that their respective projections  505  extend through core C and housing portions  510  encompass core C.  FIG. 5C  depicts the resulting assembly  535 . 
       FIG. 5D  shows an exploded view of assembly  535  similar to that of  FIG. 5B  but in cross-section. 
       FIG. 5E  shows the elements of  FIG. 5D  assembled, in cross-section, and includes a plan view of the resulting assembly  535  to identify the cross-section of  FIG. 5D  as along line B-B. 
       FIG. 5F  is essentially the same view of transformer  400  provided in  FIG. 4B  but amended to include labels for the physical features of the same transformer illustrated schematically in  FIG. 4A . Winding s P 3 -P 6  are omitted for ease of illustration, and heatsink  410 , primary P 1 , and secondary line TL 2  are positioned differently to provide access to all the connections from one side of the transformer. The leads to primary windings P 1  and P 2  enter the transformer via ports  525  ( FIG. 5A ); primary windings P 1  and P 2  wrap around core C some number of times. The number of turns each primary winding takes around core C will depend upon the desired voltage step to be provided by the transformer. Projections  505  of the two components  500  brought together to form the body of transformer  400  extend through core C to become the two secondary windings S 1  and S 2 . The two housing portions  510  together form both the transformer housing and center tap CT. In this way, both the primary and secondary windings are adjacent and in close proximity to the core. 
     Housing portions  510  can be formed of conductive materials, such as aluminum or copper, and can be connected together by extending fasteners through assembly holes  520  ( FIG. 5A ), though different methods of fastening housing portions  510  might also be used. Whatever the mechanism, the resulting connection should be robust and provide low electrical resistance. Cavities within the assembly can be filled with a suitable potting compound. 
     The embodiment of  FIG. 5F  is compact, efficient, and easily manufactured. Further, the resulting package can easily include or otherwise accommodate a heatsink. The invention can easily be extended to other shapes, materials, and configurations, as will be understood to those of skill in the art. Some examples are detailed in connection with  FIGS. 6A-6D . 
       FIG. 6A  depicts a housing portion  600 , in accordance with another embodiment, that can be used with another similar juxtaposed housing portion (not shown) to form a transformer housing similar to that of  FIG. 5A-5D . Housing portion  600  is similar to housing portion  510  of  FIG. 5A  except that portion  600  includes a bifurcated primary (two furcations  605 ) in lieu of a single protrusion  505 , and includes two apertures  610  through which to admit conductors to connect to furcations  605  on the juxtaposed housing portion. 
       FIG. 6B  depicts a housing portion  615  in accordance with yet another embodiment. A single secondary winding  620  is formed using a conductor connected (e.g., soldered) to housing portion  615  at a bond  625 . Winding  620  functions like protrusion  505  of  FIGS. 5A-5D , may include one or a plurality of striations, and may be insulated. Additional striations increase the effective surface area of winding  620 , which may in turn improve performance at relatively high frequencies. 
       FIG. 6C  depicts a housing portion  630  in accordance with an embodiment in which a single-turn winding is formed of three conductors  635  attached to a housing portion  640 . 
       FIG. 6D  depicts a transformer  645  that employs two juxtaposed housing portions  630  of  FIG. 6C  to form a second of windings and a center tap. Transformer  645  additionally includes a core  650  and a second pair of windings P 1  and P 2 . As in earlier examples, windings P 1  and P 2  wrap around the core one time, but this is just one example. The ends of the three uppermost conductors  635  may be tied together to form a single winding node WN 1 , while the lowermost conductor  635  may be tied together to form a single winding node WN 2 . As before, conductors  635  may be single or multi-conductor, and may be insulated. 
     While the present invention has been described in connection with specific embodiments, variations of these embodiments will be obvious to those of ordinary skill in the art. For example, the sense of the transformers disclosed above can be reversed so that those windings described as “primary” would be “secondary” windings, and vice versa. Moreover, some components are shown directly connected to one another while others are shown connected via intermediate components. In each instance the method of interconnection, or “coupling,” establishes some desired electrical communication between two or more circuit nodes, or terminals. Such coupling may often be accomplished using a number of circuit configurations, as will be understood by those of skill in the art. Therefore, the spirit and scope of the appended claims should not be limited to the foregoing description. Only those claims specifically reciting “means for” or “step for” should be construed in the manner required under the sixth paragraph of 35 U.S.C. §112.