Patent Publication Number: US-2018041120-A1

Title: High step down dc/dc converter

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The present invention relates generally to DC-DC converters. 
     2. Discussion of Related Art 
     DC-DC converters are commonly used in a variety of applications to convert input DC power at a first voltage level to output DC power at a second voltage level. Such DC-DC converters can step-up voltage (i.e., boost voltage), step-down voltage (i.e., buck voltage), and/or provide isolation between the input and output power. 
     SUMMARY 
     At least one aspect of the invention is directed to a DC-DC converter system comprising a positive input configured to be coupled to a positive terminal of a DC voltage source, a negative input configured to be coupled to a negative terminal of the DC voltage source, an output configured to be coupled to a load and to provide output DC power to the load, a plurality of switches coupled to the positive input and the negative input and configured to receive input DC power having a DC voltage level from the DC voltage source, a first capacitor coupled to the plurality of switches and the output, a second capacitor coupled to the plurality of switches and the output, and a controller coupled to the plurality of switches and configured to operate the plurality of switches such that in a first mode of operation of the DC-DC converter system, voltage across the first capacitor is at a level equal to substantially half of the DC voltage level of the input DC power, in a second mode of operation of the DC-DC converter system, voltage across the second capacitor is at a level equal to substantially half of the DC voltage level of the input DC power, and an output voltage level of the output DC power is stepped down, by a voltage step down ratio, in relation to the DC voltage level of the input DC power. 
     According to one embodiment, the DC-DC converter system further comprises a first inductor coupled between the plurality of switches and the output, and a second inductor coupled between the first capacitor and the output. In one embodiment, the plurality of switches includes a first switch coupled between the positive input and the first capacitor, a second switch coupled between the second capacitor and the output, and a third switch coupled between the first inductor and the output. In another embodiment, in the first mode of operation the controller is further configured to provide control signals to the first switch to close the first switch, thereby coupling the positive input to the first capacitor, provide control signals to the second switch to close the second switch, thereby coupling the output to the second capacitor, and provide control signals to the third switch to close the third switch, thereby coupling the output to the first inductor. 
     According to another embodiment, the plurality of switches further includes a fourth switch coupled between the negative input and the second capacitor, a fifth switch coupled between the first capacitor and the first inductor, and a sixth switch coupled between the second inductor and the output. In one embodiment, in the second mode of operation the controller is further configured to provide control signals to the fourth switch to close the fourth switch, thereby coupling the negative input to the second capacitor, provide control signals to the fifth switch to close the fifth switch, thereby coupling the second capacitor to the output via the first inductor, and provide control signals to the sixth switch to close the sixth switch, thereby coupling the output to the second inductor. 
     According to one embodiment, in a third mode of operation of the DC-DC converter system, the controller is further configured to provide control signals to the third switch to close the third switch, thereby coupling the output to the first inductor, and provide control signals to the sixth switch to close the sixth switch, thereby coupling the output to the second inductor, wherein the controller is further configured to operate the DC-DC converter system in the third mode of operation when the DC-DC converter system is transitioning between the first and second modes of operation. In one embodiment, a positive terminal of the first capacitor is coupled to the first switch and a negative terminal of the first capacitor is coupled to the second inductor. In another embodiment, a positive terminal of the second capacitor is coupled to the negative terminal of the first capacitor and a negative terminal of the second capacitor is coupled to the fourth switch. 
     Another aspect of the invention is directed to a method for operating a DC-DC converter system, the method comprising receiving, with a plurality of switches, input DC power having a DC voltage level, from a DC voltage source, operating, with a controller coupled to the plurality of switches, the plurality of switches in a first mode of operation such that voltage across a first capacitor coupled to the plurality of switches is at a level equal to substantially half of the DC voltage level of the input DC power, operating, with the controller, the plurality of switches in a second mode of operation such that voltage across a second capacitor coupled to the plurality of switches is at a level equal to substantially half of the DC voltage level of the input DC power, providing, output DC power having an output voltage level, to a load coupled to the plurality of switches, and operating, with the controller, the plurality of switches such that the output voltage level of the output DC power is stepped down, by a voltage step down ratio, in relation to the DC voltage level of the input DC power. 
     According to one embodiment, the DC-DC converter system includes a positive input configured to be coupled to a positive terminal of the DC voltage source, a negative input configured to be coupled to a negative terminal of the DC voltage source, an output configured to be coupled to a load and to provide output DC power to the load, a first inductor coupled between the plurality of switches and the output, and a second inductor coupled between the first capacitor and the output, and operating the plurality of switches in the first mode of operation includes coupling the positive input to the output via a first power path including the first capacitor and the second inductor, coupling the second capacitor to the output via a second power path including the second inductor, and coupling the first inductor to the output via a third power path. 
     According to another embodiment, operating the plurality of switches in the second mode of operation includes coupling the negative input to the output via a fourth power path including the second capacitor and the first inductor, coupling the first capacitor to the output via a fifth power path including the first inductor, and coupling the second inductor to the output via a sixth power path. In one embodiment, the method further comprises alternating operation of the plurality of switches in the first mode of operation and the second mode of operation. In another embodiment, the method further comprises operating the plurality of switches in a third mode of operation when operation of the plurality of switches is transitioning between the first and second modes of operation. In one embodiment, operating the plurality of switches in the third mode of operation includes coupling the first inductor to the output via the third power path, and coupling the second inductor to the output via the sixth power path. 
     According to one embodiment, receiving the input DC power from the DC voltage source includes receiving the input DC power from a front end Power Factor Correction (PFC) pre-regulator that utilizes simple diode rectifiers to generate the input DC power. 
     At least one aspect of the invention is directed to a DC-DC converter system comprising a positive input configured to be coupled to a positive terminal of a DC voltage source, a negative input configured to be coupled to a negative terminal of the DC voltage source, an output configured to be coupled to a load and to provide output DC power to the load, and means for receiving DC input power having a DC voltage level from the DC voltage source, splitting the DC input power into two voltage sources, each of the two voltage sources having a voltage level equal to substantially half of the DC voltage level of the DC input power, and for stepping down, by a voltage step down ratio, the DC voltage level of the input DC power to a stepped down voltage level at the output. 
     According to one embodiment, the means for splitting the DC input power into two voltage sources includes means for generating, in a first mode of operation, a first voltage source having a level equal to substantially half of the DC voltage level of the DC input power. In another embodiment, the means for splitting the DC input power into two voltage sources further includes means for generating, in a second mode of operation, a second voltage source having a level equal to substantially half of the DC voltage level of the DC input power. 
     According to another embodiment, the means for stepping down the DC voltage level of the input DC power includes means for generating, in the first mode of operation, a first plurality of power paths in the DC-DC converter system to step down, by the voltage step down ratio, the DC voltage level of the input DC power to the stepped down voltage level. In one embodiment, the means for stepping down the DC voltage level of the input DC power further includes means for generating, in the second mode of operation, a second plurality of power paths in the DC-DC converter system to step down, by the voltage step down ratio, the DC voltage level of the input DC power to the stepped down voltage level. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various aspects of at least one embodiment are discussed below with reference to the accompanying figures, which are not intended to be drawn to scale. The figures are included to provide an illustration and a further understanding of the various aspects and embodiments, and are incorporated in and constitute a part of this specification, but are not intended as a definition of the limits of any particular embodiment. The drawings, together with the remainder of the specification, serve to explain principles and operations of the described and claimed aspects and embodiments. In the figures, each identical or nearly identical component that is illustrated in various figures is represented by a like numeral. For purposes of clarity, not every component may be labeled in every figure. In the figures: 
         FIG. 1  is a circuit diagram of a DC-DC converter system according to aspects described herein; 
         FIG. 2  is a timing diagram illustrating operation of a DC-DC converter system during different modes of operation according to aspects described herein; 
         FIG. 3  is a circuit diagram illustrating operation of a DC-DC converter in a first mode of operation according to aspects described herein; 
         FIG. 4  is a circuit diagram illustrating operation of DC-DC converter in a second mode of operation according to aspects described herein; and 
         FIG. 5  is a circuit diagram illustrating operation of a DC-DC converter in a third mode of operation according to aspects described herein. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Examples of the methods and systems discussed herein are not limited in application to the details of construction and the arrangement of components set forth in the following description or illustrated in the accompanying drawings. The methods and systems are capable of embodiment in other embodiments and of being practiced or of being carried out in various ways. Examples of specific embodiments are provided herein for illustrative purposes only and are not intended to be limiting. In particular, acts, components, elements and features discussed in connection with any one or more examples are not intended to be excluded from a similar role in any other examples. 
     Also, the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. Any references to examples, embodiments, components, elements or acts of the systems and methods herein referred to in the singular may also embrace embodiments including a plurality, and any references in plural to any embodiment, component, element or act herein may also embrace embodiments including only a singularity. References in the singular or plural form are not intended to limit the presently disclosed systems or methods, their components, acts, or elements. The use herein of “including,” “comprising,” “having,” “containing,” “involving,” and variations thereof is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. References to “or” may be construed as inclusive so that any terms described using “or” may indicate any of a single, more than one, and all of the described terms. In addition, in the event of inconsistent usages of terms between this document and documents incorporated herein by reference, the term usage in the incorporated references is supplementary to that of this document; for irreconcilable inconsistencies, the term usage in this document controls. 
     As discussed above, DC-DC converters are commonly used in a variety of applications to convert input DC power at a first voltage level to output DC power at a second voltage level. For example, common rectifiers utilized in telecommunications systems or in server power supplies employ a front end Power Factor Correction (PFC) pre-regulator that outputs HVDC (e.g., 380 Vdc or above) and an LLC resonant converter with synchronous rectification to generate low voltage outputs (e.g., 48V or 12V). The synchronous rectification provided by the LLC resonant converter may allow for high efficiency and power density requirements. However, such LLC resonant converters are typically complex and require both hardware and firmware solutions to account for phase shifts in the sinusoidal current through the synchronous rectifier attributable to changing load requirements and changing input voltage. 
     A non-isolated high step-down DC-DC converter system is provided herein that provides high efficiency and high power density. In addition, as the DC-DC converter system described herein has a relatively high voltage transfer ratio, it is suitable to receive an input dc voltage source from a front end PFC pre-regulator that utilizes simple diode rectifiers rather than synchronous rectifiers. 
       FIG. 1  is a circuit diagram of a DC-DC converter system  100  according to aspects described herein. The system  100  includes a positive input  103 , a negative input  105 , capacitor C 1   104 , capacitor C 2   106 , switch S 1   108 , switch S 2   110 , switch S 3   112 , switch S 4   114 , capacitor C 3   116 , capacitor C 4   118 , switch S 5   120 , switch S 6   122 , inductor L 1   124 , inductor L 2   126 , capacitor Co  128 , and output  125 . According to one embodiment, the switches S 1 -S 6  are Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFET); however, in other embodiments, any one of the switches S 1 -S 6  can be a different type of transistor or switch. 
     The positive input  103  is configured to be coupled to a positive terminal of a DC voltage source  102  and the negative input  105  is configured to be coupled to a negative terminal of the DC voltage source  102 . A first terminal of capacitor C 1   104  is coupled to the positive input  103  and a first terminal of capacitor C 2   106  is configured to be coupled to the negative input  105 . According to one embodiment, the DC source  102  is provided by a rectifier (e.g., a rectifier of a PFC pre-regulator); however, in other embodiments, the DC source  102  can be provided by another appropriate system. A second terminal of the capacitor C 1   104  is coupled to a second terminal of the capacitor C 2   106 . The source of switch S 1   108  is coupled to the positive input  103  and the source of switch S 3   112  is coupled to the negative input  105 . 
     The drain of switch S 1   108  is coupled to the source of switch S 2   110 . The drain of switch S 2   110  is coupled to a first terminal of the inductor L 1   124 . A second terminal of the inductor L 1   124  is coupled to a positive terminal of capacitor Co  128 . The second terminal of the inductor L 1   124  is also configured to be coupled to a load  130  via the output  125 . The drain of switch S 3   112  is coupled to the source of switch S 4   114 . The drain of switch S 4   114  is coupled to the source of switch S 5   120 , the source of switch S 6   122 , and a negative terminal of capacitor Co  128 . The drain of switch S 4   114  is also configured to be coupled to the load  130 . A positive terminal  117  of capacitor C 3   116  is coupled to the drain of switch S 1   108 . A negative terminal  119  of capacitor C 3   116  is coupled to a first terminal of inductor L 2   126 . A second terminal of inductor L 2   126  is coupled to the positive terminal of capacitor Co  128 . 
     A negative terminal  121  of capacitor C 4   118  is coupled to the drain of switch S 3   112 . A positive terminal  123  of capacitor C 4   118  is coupled to the drain of switch S 5   120 . The drain of switch S 5   120  is also coupled to the negative terminal  119  of capacitor C 3   116 . The drain of switch S 6   122  is coupled to the drain of switch S 2   110 , to the second terminal of capacitor C 1   104 , and to the second terminal of capacitor C 2   106 . The gate of each switch S 1 -S 6  is coupled to a controller  101  and configured to receive control signals from the controller  101 . 
     Operation of the converter system  100  is described below with respect to  FIGS. 2-5 .  FIG. 2  is a timing diagram illustrating operation of the converter during different modes of operation (T 1 -T 4 ).  FIG. 2  includes a first trace  202  illustrating control signals provided by the controller  101  to the gates of switches S 1   108  and S 4   114  during the different modes of operation (T 1 -T 4 ), a second trace  204  illustrating control signals provided by the controller  101  to the gates of switches S 2   110  and S 3   112  during the different modes of operation (T 1 -T 4 ), a third trace  206  illustrating control signals provided by the controller  101  to the gate of switch S 5   120  during the different modes of operation (T 1 -T 4 ), and a fourth trace  208  illustrating control signals provided by the controller  101  to the gate of switch S 6   122  during the different modes of operation (T 1 -T 4 ).  FIG. 3  is a circuit diagram illustrating operation of the converter in a first mode of operation (T 1 ).  FIG. 4  is a circuit diagram illustrating operation of the converter in a second mode of operation (T 2 ) and a fourth mode of operation (T 4 ).  FIG. 5  is a circuit diagram illustrating operation of the converter in a third mode of operation (T 3 ). 
     As shown in  FIGS. 2-3 , during the first mode of operation (T 1 ), the controller  101  sends a high control signal  202  to the gates of switch S 1   108  and switch S 4   114  to close switch S 1   108  and switch S 4   14 , a low control signal  204  to the gates of switch S 2   110  and switch S 3   112  to maintain switch S 2   110  and switch S 3   112  in an open state, a low control signal  206  to the gate of switch S 5   120  to maintain switch S 5   120  in an open state, and a high control signal  208  to the gate of switch S 6   122  to close switch S 6   122 . 
     In the first mode of operation (T 1 ), once switch S 1   108 , switch S 4   114 , and switch S 6   122  are closed and the system  100  is receiving DC power having a DC voltage level (Vdc) from the DC voltage source  102  (e.g., from a PFC preregulator), the system  100  operates with multiple power paths to provide a stepped down DC voltage to the load  130 . As shown in  FIG. 3 , a first power path  302  includes the capacitor C 1   104 , switch S 1   108 , capacitor C 3   116 , inductor L 2   126 , capacitor Co  128 , and switch S 6   122 ; a second power path  304  includes capacitor C 4   118 , inductor L 2   126 , capacitor Co  128 , and switch S 4   114 ; and a third power path  306  includes inductor L 1   124 , load  130 , and switch S 6   122 . In the first mode of operation (T 1 ), the power paths  302 - 306  function such that the system  100  operates in accordance with the following equations: 
         Vc 4* D=Vo    
     (where Vc4 is the voltage across capacitor C 4   118 , D is a step-down ratio, and Vo is the output voltage at the output  125 ); 
         Vc 1= Vc 3+ Vc 4 
     (where Vc1 is the input voltage across capacitor C 1   104  and Vc3 is the voltage across capacitor C 3   116 ); and 
         Vc 4=½ Vc 1.
 
     As shown in  FIGS. 2 and 4 , during the second mode of operation (T 2 ), the controller  101  sends a low control signal  202  to the gates of switch S 1   108  and switch S 4   114  to maintain switch S 1   108  and switch S 4   14  in an open state, a low control signal  204  to the gates of switch S 2   110  and switch S 3   112  to maintain switch S 2   110  and switch S 3   112  in an open state, a high control signal  206  to the gate of switch S 5   120  to close switch S 5   120 , and a high control signal  208  to the gate of switch S 6   122  to close switch S 6   122 . In the second mode of operation (T 2 ), once switch S 5   120  and S 6   122  are closed, the system  100  operates with multiple power paths. As shown in  FIG. 4 , a first power path  402  includes the inductor L 1   124 , the load  130  and the switch S 6   122 ; and a second power path  404  includes the inductor L 2   124 , the capacitor Co  128 , and the switch S 5   120 . 
     As shown in  FIGS. 2 and 5 , during the third mode of operation (T 3 ), the controller  101  sends a low control signal  202  to the gates of switch S 1   108  and switch S 4   114  to maintain switch S 1   108  and switch S 4   14  in an open state, a high control signal  204  to the gates of switch S 2   110  and switch S 3   112  to close switch S 2   110  and switch S 3   112 , a high control signal  206  to the gate of switch S 5   120  to close switch S 5   120 , and a low control signal  208  to the gate of switch S 6   122  to maintain switch S 6   122  in an open state. 
     In the third mode of operation (T 3 ), once switch S 2   110 , switch S 3   112 , and switch S 5   120  are closed and the system  100  is receiving DC power having the DC voltage level (Vdc) from the DC voltage source  102  (e.g., from a PFC preregulator), the system  100  operates with multiple power paths to provide a stepped down DC voltage to the load  130 . As shown in  FIG. 5 , a first power path  502  includes capacitor C 3   116 , switch S 2   110 , inductor L 1   124 , load  130 , and switch S 5   120 ; a second power path  504  includes capacitor C 2   106 , inductor L 1   124 , load  130 , switch S 5   120 , capacitor C 4 ,  118 , and switch S 3   112 ; and a third power path  506  includes inductor L 2   126 , capacitor Co  128 , and switch S 5   120 . In the third mode of operation (T 3 ), the power paths  402 - 406  function such that the system  100  operates in accordance with the following equations: 
         Vc 3* D=Vo;    
         Vc 2= Vc 3+ Vc 4; 
         Vc 3=½ Vc ; and
 
         Vc 1= Vc 2=½ Vdc.  
 
     As shown in  FIGS. 2 and 4 , during the fourth mode of operation (T 4 ), the controller  101  sends a low control signal  202  to the gates of switch S 1   108  and switch S 4   114  to maintain switch S 1   108  and switch S 4   14  in an open state, a low control signal  204  to the gates of switch S 2   110  and switch S 3   112  to maintain switch S 2   110  and switch S 3   112  in an open state, a high control signal  206  to the gate of switch S 5   120  to close switch S 5   120 , and a high control signal  208  to the gate of switch S 6   122  to close switch S 6   122 . In the fourth mode of operation (T 4 ), once switch S 5   120  and S 6   122  are closed, the system  100  operates with multiple power paths. As shown in  FIG. 4 , a first power path  402  includes the inductor L 1   124 , the load  130  and the switch S 6   122 ; and a second power path  404  includes the inductor L 2   124 , the capacitor Co  128 , and the switch S 5   120 . Operation of the system  100  in the fourth mode of operation (T 4 ) is the same as the operation of the system  100  in the second mode of operation (T 2 ). After the fourth mode of operation (T 4 ), the system can transition back to the first mode of operation (T 1 ). 
     By operating the system as described above (over modes of operation T 1 -T 4 ), the DC voltage source  102  is effectively split into two ½ voltage sources (e.g., Vc1=Vc2=½ Vdc) and the system  100  can operate with a relatively high voltage transfer ratio defined by the following: 
         Vo= ¼ Vdc*D  
 
     (Vo is the voltage across the load  130 ).
 
In at least one embodiment, operation of the system  100  as described above results in a voltage transfer ratio that is at least two times larger than the voltage transfer ratio of a typical DC-DC converter. For example, in one embodiment, operation of the system  100  as described above results in a voltage transfer ratio of 0.5; however, in other embodiments, the system  100  can be operated to result in a voltage transfer ratio of another appropriate value.
 
     In addition to a larger voltage transfer ratio, the system  100  described above achieves a relatively high voltage transfer ratio with fewer components than a typical DC-DC converter. For example, during operation of the system  100  as described above, the current waveforms through the system  100  are square waves with substantially unchanging phases. As such, the system  100  may not need to implement the same hardware and firmware solutions utilized by typical synchronous rectifiers to account for phase shifts in the current waveforms due to changing load requirements or input voltage (as discussed above). 
     A non-isolated high step-down DC-DC converter system is provided herein that is able to provide high efficiency and high power density. In addition, as the DC-DC converter system described herein has a relatively high voltage transfer ratio, it is suitable to receive an input dc voltage source from a front end PFC pre-regulator that utilizes simple diode rectifiers rather than synchronous rectifiers. 
     Having thus described several aspects of at least one embodiment of this invention, it is to be appreciated various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description and drawings are by way of example only.