Patent Publication Number: US-2021184581-A1

Title: Low-voltage dc-dc converter including zero voltage switching and method of driving same

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a Continuation of U.S. application Ser. No. 16/513,195, filed Jul. 16, 2019, which claims priority to and the benefit of Korean Patent Application No. 2018-0083575, filed on Jul. 18, 2018, which is incorporated by reference for all purposes as if set forth herein. 
    
    
     BACKGROUND 
     Field 
     Exemplary embodiments relate to a low-voltage DC-DC converter and a method of driving the same. 
     Discussion of the Background 
     Low-voltage direct current (DC)-DC converters (LDCs) are electric power supplies which are necessarily required to take charge of supplying electric power to electric loads and charging low-voltage auxiliary batteries installed in eco-friendly vehicles. 
     Such low-voltage DC-DC converters are classified into various kinds according to driving methods, and high power is converted in a full-bridge manner to perform an isolated switching operation. 
     However, switching power loss occurs in a conventional LDC during a control switching operation of a power switching element. 
     Meanwhile, in order to solve this problem, Korean Unexamined Patent Publication No. 1998-040074 (Title: A ZERO VOLTAGE SWITCHING DC-DC STEP DOWN CONVERTER) discloses a zero voltage switching technology of a power switching element. 
     That is, the related art uses a zero voltage switching operation to reduce switching power loss. 
     However, in the related art, a zero voltage switching inductor should be additionally installed so as to secure the zero voltage switching operation. 
     However, since the zero voltage switching inductor generates heat during a high speed switching operation of the power switching element, a heat dissipation structure using a heat sink plate is needed and a large space is needed from a viewpoint of a system. 
     The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and, therefore, it may contain information that does not constitute prior art. 
     SUMMARY 
     Exemplary embodiments of the present invention is directed to providing a low-voltage DC-DC converter capable of reducing a size and costs of a product and increasing a power density by adding an air gap in a core region of a transformer and adjusting a turns ratio of the transformer, and a method of driving the same. 
     However, technical target objectives of the present embodiments are not limited to the above-described technical objectives, and other technical objectives may be present. 
     According to an aspect of the present invention, there is provided a low-voltage DC-DC converter including a switch configured to convert a high voltage supplied from a high-voltage battery to an alternating current (AC) voltage, a transformer including an air gap in a core region and configured to lower the AC voltage to a low voltage using an adjusted turns ratio, and a power supply configured to rectify the low voltage and supply the rectified low voltage to a load. 
     Power switching elements may be disposed in the switch in a full-bridge manner. 
     The turns ratio may be set on the basis of a primary input current, a secondary input current, and a magnetizing current of the transformer. 
     The air gap in the core region and the turns ratio may be adjusted by comparing a minimum inductance value of power switching elements for a zero voltage switching operation and a leakage inductance value of the transformer. 
     The minimum inductance value may be set on the basis of a parasitic capacitance of power switching elements and energy stored in a zero voltage switching inductor. 
     The leakage inductance value may be calculated on the basis of a primary leakage inductance value, the number of primary turns, and a primary circuit resistance of the transformer. 
     In addition, according to another aspect of the present invention, there is provided a method of driving a low-voltage DC-DC converter, the method including converting a high voltage supplied from a high-voltage battery to an AC voltage, lowering the AC voltage to a low voltage using a transformer in which an air gap is included in a core region and a turns ratio is adjusted, and rectifying the low voltage and supplying the rectified low voltage to a load. 
     The high voltage may be converted to the AC voltage through power switching elements disposed in a full-bridge manner. 
     The turns ratio may be set on the basis of a primary input current, a secondary input current, and a magnetizing current of the transformer. 
     The air gap in the core region and the turns ratio may be adjusted by comparing a minimum inductance value of power switching elements for a zero voltage switching operation and a leakage inductance value of the transformer. 
     The minimum inductance value may be set on the basis of a parasitic capacitance of the power switching elements and energy stored in a zero voltage switching inductor. 
     The leakage inductance value may be calculated on the basis of a primary leakage inductance value, the number of primary turns, and a primary circuit resistance of the transformer. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention, and together with the description serve to explain the principles of the invention. 
         FIG. 1  is a view illustrating a low-voltage direct current (DC)-DC converter operated in a full-bridge manner according to the related art; 
         FIG. 2  is a configuration diagram illustrating a low-voltage DC-DC converter according to one embodiment of the present invention; 
         FIG. 3  is a view for describing the low-voltage DC-DC converter according to one embodiment of the present invention; 
         FIG. 4  is a view illustrating a transformer of the low-voltage DC-DC converter according to one embodiment of the present invention; 
         FIG. 5  is a flowchart of a method of driving the low-voltage DC-DC converter according to one embodiment of the present invention; 
         FIG. 6  is a flowchart of a method of setting the transformer of the low-voltage DC-DC converter according to one embodiment of the present invention; 
         FIG. 7  is a view illustrating an internal structure of a transformer of a low-voltage DC-DC converter according to each of the related art and one embodiment of the present invention; 
         FIG. 8A  is a view showing output waveforms of the low-voltage DC-DC converter according to each of the related art,  FIG. 8B  is a view showing output waveforms of the low-voltage DC-DC converter according to one embodiment of the present invention; and 
         FIG. 9  is a table showing output values of the low-voltage DC-DC converter according to each of the related art and one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS 
     The invention is described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure is thorough, and will fully convey the scope of the invention to those skilled in the art. 
     Hereinafter, embodiments that are easily performed by those skilled in the art will be described in detail with reference to the accompanying drawings. However, the embodiments of the present invention may be implemented in several different forms and are not limited to the embodiments described herein. In addition, parts irrelevant to the description are omitted in the drawings in order to clearly explain the embodiments of the present invention. 
     Meanwhile, the terms used herein are provided only to describe the embodiments of the present invention and not for purposes of limitation. Unless the context clearly indicates otherwise, the singular forms include the plural forms. It will be understood that the terms “comprise” and “comprising,” when used herein, specify some stated components, steps, operations and/or elements, but do not preclude the presence or addition of one or more other components, steps, operations and/or elements. 
     The present invention relates to a low-voltage direct current (DC)-DC converter  100  and a method of driving the same. 
     In an eco-friendly vehicle, a low-voltage DC-DC converter is a necessary device for charging a low-voltage auxiliary battery with a high voltage output from a high voltage main battery and supplying power to various electric loads installed in the vehicle. In addition, high power conversion uses a full-bridge manner to perform an isolated switching operation. 
     Hereinafter, an operation of a conventional low-voltage DC-DC converter will be described with reference to  FIG. 1 . 
       FIG. 1  is a view illustrating a low-voltage DC-DC converter operated in a full-bridge manner according to the related art. 
       FIG. 1  is a view illustrating a low-voltage DC-DC converter operated in a full-bridge manner according to the related art. 
     In this case, since power loss occurs during the operation of the power switching elements Q 1 , Q 2 , Q 3 , and Q 4 , a zero voltage switching operation is used to minimize the switching power loss. 
     The zero voltage switching operation is a high speed switching technique of a power element and an important factor contributing to achievement of high efficiency in the converter field, but requires a zero voltage switching inductor Lzvs. 
     However, since the zero voltage switching inductor Lzvs generates heat during the switching operation of the power switching elements Q 1 , Q 2 , Q 3 , and Q 4 , a heat dissipation structure using a heat sink or a cooling system therefor is needed, and thus a large space is needed from a viewpoint of a system and a size of a product is increased. 
     In one embodiment of the present invention, an air gap is added in a core region of a transformer  120  and a turns ratio of the transformer  120  is adjusted to substitute for the conventional zero voltage switching inductor. 
     Hereinafter, a low-voltage DC-DC converter  100  according to one embodiment of the present invention will be described with reference to  FIGS. 2 to 4 . 
       FIG. 2  is a configuration diagram illustrating a low-voltage DC-DC converter  100  according to one embodiment of the present invention,  FIG. 3  is a view for describing the low-voltage DC-DC converter  100  according to one embodiment of the present invention, and  FIG. 4  is a view illustrating a transformer  120  of the low-voltage DC-DC converter  100  according to one embodiment of the present invention. 
     As illustrated in  FIG. 2 , the low-voltage DC-DC converter  100  according to one embodiment of the present invention includes a switch  110 , the transformer  120 , and a power supply  130 . 
     The switch  110  converts a high voltage supplied from a high-voltage battery of a vehicle to an AC voltage. 
     Referring to  FIG. 3 , power switching elements Q 1 , Q 2 , Q 3 , and Q 4  are disposed in the switch  110  in a full-bridge manner and may perform a zero voltage switching operation using a leakage inductance Llkl which will be described below. 
     In addition, an inductor Lzvs, which is installed for a zero voltage switching operation according to the related art, is removed from the switch  110 , an air gap is added in a core region of the above-described transformer  120 , and a turns ratio thereof is adjusted to substitute for a primary leakage inductance Llkl of the transformer  120 , thereby reducing material costs and a size of a product. In addition, since the size of the product is reduced, a power density of the product can be increased. 
     The transformer  120  lowers an AC voltage output from the switch  110  to a low voltage and transmits the lowered voltage to the power supply  130 . 
     Referring to  FIG. 4 , since the air gap is added in the core region of the transformer  120 , a magnetizing inductance Lm is decreased according to Equation 1. 
     
       
         
           
             
               
                 
                   Lm 
                   = 
                   
                     
                       N 
                        
                       
                           
                       
                        
                       
                         1 
                         2 
                       
                     
                     
                       Rc 
                       + 
                       Rg 
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     1 
                   
                   ] 
                 
               
             
           
         
       
     
     Here, Lm is a magnetizing inductance of the transformer  120 , N1 is the number of primary turns of the transformer  120 , Rc is a magnetic resistance of the core region, and Rg is a magnetic resistance of the air gap. 
     Since a core of the transformer  120  according to one embodiment of the present invention is formed of a magnetic material and may not perform a normal function when a current, which exceeds a saturation property inherent to the magnetic material, is applied thereto, a turns ratio of the transformer  120  is considered to decrease a magnetizing current. 
     Accordingly, by adjusting the turns ratio of the transformer  120 , effects can be achieved in that a magnetizing current im of the transformer  120  is decreased according to Equation 2 and a primary leakage inductance Llkl of the transformer  120  is increased according to Equation 3. 
     
       
         
           
             
               
                 
                   im 
                   = 
                   
                     
                       i 
                        
                       
                           
                       
                        
                       1 
                     
                     - 
                     
                       
                         
                           N 
                            
                           
                               
                           
                            
                           2 
                         
                         
                           N 
                            
                           
                               
                           
                            
                           1 
                         
                       
                        
                       i 
                        
                       
                           
                       
                        
                       2 
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     2 
                   
                   ] 
                 
               
             
           
         
       
     
     Here, im is a magnetizing current of the transformer  120 , i1 is a primary input current of the transformer  120   1 , N1 is the number of primary turns of the transformer  120 , N2 is the number of secondary turns of the transformer  120 , and i2 is a secondary output current of the transformer  120 . 
     
       
         
           
             
               
                 
                   
                     Llk 
                      
                     
                         
                     
                      
                     1 
                   
                   = 
                   
                     
                       N 
                        
                       
                           
                       
                        
                       
                         1 
                         2 
                       
                     
                     
                       R 
                        
                       
                           
                       
                        
                       1 
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     3 
                   
                   ] 
                 
               
             
           
         
       
     
     Here, Llkl is a primary leakage inductance value of the transformer  120 , N1 is the number of primary turns of the transformer  120 , and R1 is a primary circuit resistance of the transformer  120 . 
     The power supply  130  rectifies a low voltage output from the transformer  120  and supplies the rectified low voltage to a load. 
     Hereinafter, a method of driving the low-voltage DC-DC converter  100  will be described with reference to  FIG. 5 . 
       FIG. 5  is a flowchart of a method of driving the low-voltage DC-DC converter  100  according to one embodiment of the present invention. 
     Referring to  FIG. 5 , first, in the method of driving the low-voltage DC-DC converter  100 , a high voltage supplied from the high-voltage battery of the vehicle is converted to an AC voltage (S 110 ). 
     At this moment, according to one embodiment of the present invention, the power switching elements may be disposed in the full-bridge manner to convert the high voltage of the vehicle to the AC voltage. 
     Then, the AC voltage is lowered to a low voltage using the transformer  120  in which the air gap is added in the core region and a turns ratio is adjusted. 
     At this moment, since the specific description in which the air gap is added in the core region of the transformer  120  and the turns ratio is adjusted has been described with reference to  FIG. 4  and Equations 1 to 3 which are described above, the specific description will be omitted below. 
     Finally, the low voltage is rectified and supplied to a low-voltage auxiliary battery and an electric load of the vehicle (S 130 ). 
     In the above description, the operations of S 110  to S 130  may be further divided into additional operations or combined into a decreased number of operations. In addition, some operations may be omitted as necessary, or the order of the operations may also be changed. In addition, even other omitted operations, which are already described with reference to  FIG. 4 , may also be applied to the method of driving the low-voltage DC-DC converter  100  of  FIG. 5 . 
     Meanwhile, the transformer  120  of the low-voltage DC-DC converter  100  according to one embodiment of the present invention may be set through a method of  FIG. 6 . 
       FIG. 6  is a flowchart of a method of setting the transformer  120  of the low-voltage DC-DC converter  100  according to one embodiment of the present invention. 
     Referring to  FIG. 6 , first, a power switching element is set by considering a voltage/current capacity and a parasitic capacitance (S 210 ). 
     Then, a value of a minimum inductance Lmin is set to secure a zero voltage switching operation (S 220 ). 
     Here, the minimum inductance value is a minimum value through which transformer  120  secures the zero voltage switching operation of the power switching element and is set such that the parasitic capacitance of the power switching element is less than energy stored in the transformer  120 . 
     Then, a turns ratio of the transformer  120  is set and an air gap is adjusted according to one embodiment of the present invention (S 230 ). 
     At this moment, an input voltage range is considered to set the turns ratio of the transformer  120  and adjust the air gap of the core region. 
     Then, a value of the leakage inductance Llkl of the transformer  120  is calculated using above-described Equation 3 (S 240 ). 
     Then, the set value of the minimum inductance Lmin and the value of the calculated leakage inductance Llkl are compared (S 250 ). 
     In a case in which the value of the minimum inductance Lmin is less than the value of the leakage inductance Llkl in the comparison result, the transformer  120  formed under conditions of the decreased magnetizing current and magnetizing inductance and the increased value of the leakage inductance is applied to the product (S 260 ). 
     Conversely, in a case in which the value of the minimum inductance Lmin is greater than the value of the leakage inductance Llkl, an operation S 230  in which the turns ratio is set and the air gap is adjusted is performed again. 
     Hereinafter, an internal structure of a transformer  120  according to the related art, an internal structure of the transformer  120  according to one embodiment of the present invention, and changes in output values according to addition of the air gap in the core region of the transformer  120  and the adjustment of the turns ratio will be described with reference to  FIGS. 7 to 9 . 
       FIG. 7  is a view illustrating the internal structure of the transformer  120  of the low-voltage DC-DC converter  100  according to each of the related art and one embodiment of the present invention. 
     Referring to  FIG. 7 , in one embodiment of the present invention, a zero voltage switching inductor, which is additionally installed in the transformer  120  according to the related art, is removed from the transformer  120 , the air gap is added in the core region of the transformer  120 , and the turns ratio of the transformer  120  is adjusted to substitute for the zero voltage switching inductor. Accordingly, the number of components can be decreased in comparison with the conventional transformer, thereby reducing material costs and a size of the product. In addition, since the size of the product is decreased, a power density of the product can be increased. 
       FIG. 8A  is a view showing output waveforms of the low-voltage DC-DC converter according to each of the related art and  FIG. 8B  is a view showing output waveforms of the low-voltage DC-DC converter  100  according to one embodiment of the present invention, and the result which can be easily compared among a plurality of performing results is shown.  FIG. 9  is a table showing output values of the low-voltage DC-DC converter  100  according to each of the related art and one embodiment of the present invention. 
       FIG. 8A  shows waveforms formed using output values according to the related art and  FIG. 8B  shows waveforms formed using output values after the air gap is added in the core region of the transformer  120  and the turns ratio is adjusted. 
     According to one embodiment of the present invention, the turns ratio may be adjusted to be 11:1:1. 
     Referring to  FIG. 9 , the input voltage and the input current according to one embodiment of the present invention is the same as those according to the related art. A magnetizing inductance of 800 pH according to the related art is decreased to 210 pH after the air gap is added in the core region of the transformer  120  and the turns ratio is adjusted according to one embodiment of the present invention. In addition, the leakage inductance of the transformer  120  is increased from 2.8 pH to 4.3 pH. 
     In the simulation result, it can be seen that, as the magnetizing inductance is decreased, the magnetizing current is decreased, and operation performance of the low-voltage DC-DC converter  100  is higher than or equal to that of a conventional low-voltage DC-DC converter, and since the magnetizing current is decreased, the input current is decreased in comparison with the conventional low-voltage DC-DC converter so that an effect of an increase in system efficiency can be expected. In addition, it was seen that a margin for a duty ratio of the transformer  120  was also able to be secured. 
     According to one embodiment of the present invention, costs and a size of a product can be significantly reduced by adding an air gap in a core region of a transformer and adjusting a turns ratio. 
     A power density of the low-voltage DC-DC converter can also be increased according to the size reduction of the product. 
     The above description is only exemplary, and it will be understood by those skilled in the art that the invention may be performed in other concrete forms without changing the technological scope and essential features. Therefore, the above-described embodiments should be considered as only examples in all aspects and not for purposes of limitation. For example, each component described as a single type may be realized in a distributed manner, and similarly, components that are described as being distributed may be realized in a coupled manner. 
     The scope of the present invention is defined by the appended claims, and encompasses all modifications or alterations derived from meanings, the scope and equivalents of the appended claims.