Patent Publication Number: US-6985063-B2

Title: Transformer core

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates to a transformer core and, in particular, to a transformer core that can realize a fractional number of turns, and a winding structure utilizing such a transformer core. 
   2. Description of the Related Art 
     FIG. 1  shows a conventional transformer core and winding structure, in which the number of turns can only be an integer. In  FIG. 1 , reference number  10  denotes an E-type core, and reference numbers  11  and  12  denote the primary winding and the secondary winding, respectively. 
   One problem with the conventional core is that sometimes the number of turns of the transformer is required to be a fractional number, which the conventional core cannot satisfy. For example, when the number of turns of the primary winding is 10 turns, and the transform ratio is required to be 0.33, the number of turns of the secondary winding must be 3.3 turns. Since the number of turns of the secondary winding of a conventional core can only be an integer, there is no choice but to re-design the transform ratio into 0.3 or 0.4, which results in an error of about 9.1% or 21.2%. 
   Another problem is that in some power switch circuits, the output voltage of the transformer needs to be finely tuned. Since the fractional number of turns cannot be realized in conventional core structures, the output voltage can only be tuned by additional voltage tuning circuits. This results in the increase of both the complexity and the power loss of the power switch circuits. 
   To overcome the above problems, one conventional solution is to vary the winding of the wires on the core structure. As shown in  FIG. 2 , the secondary winding  22  has an additional turn on the side post of the core  20 . This additional turn on the side post can be treated as an additional 0.5 turn of the secondary winding  22 . Using this method, a core structure can realize windings having a turn number of 0.5. 
   However, this solution still has a limitation in that the number of turns can only be a multiple of 0.5. The number of turns of a core structure still cannot be a fraction other than a multiple of 0.5. 
   SUMMARY OF THE INVENTION 
   In view of the above, an objective of the invention is to provide a transformer core that can realize a winding having a fractional number of turns. 
   Another objective of the invention is to provide a transformer winding structure in which the winding has a fractional number of turns. 
   In view of the above-mentioned objectives, the transformer core according to the invention includes a middle post and two side posts. At least one of the two side posts has a trench or a through hole. 
   The invention also provides a winding structure of a transformer. The core of the transformer has a middle post and two side posts, and at least one of the two side posts has a trench or a through hole. The windings on at least one of the two side posts pass through the trench or the through hole. 
   The invention further provides a winding structure of a transformer, in which the core of the transformer includes a bobbin and a middle post. The bobbin has a trench or a through hole, and the winding on the middle post passes through the trench or the through hole of the bobbin. 
   Since the side post or bobbin of the transformer core according to the invention has a trench or through hole, a fractional number of turns can be realized. The turn ratio can be adjusted by adjusting the position of the trench or the through hole. 
   These and other features, aspects, and advantages of the invention will become better understood with regard to the following description and accompanying drawing. 

   
     DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram showing a transformer core in the prior art, in which the number of turns can only be an integer. 
       FIG. 2  is a schematic diagram showing a transformer core in the prior art, in which windings having a turn number multiple of 0.5 can be realized. 
       FIG. 3A  is a schematic diagram showing an EI-type transformer core according to an embodiment of the invention. 
       FIG. 3B  is a top view of the transformer core shown in  FIG. 3A . 
       FIGS. 4A and 4B  are schematic diagrams illustrating the principle of the transformer core of  FIG. 3A  realizing a fractional number of turns. 
       FIGS. 5A to 5J  are schematic diagrams illustrating the transformer core of  FIG. 3A  with different windings having fractional numbers of turns from 1.0 to 1.9. 
       FIG. 6  is a schematic diagram showing a transformer core according to another embodiment of the invention. 
       FIGS. 7A and 7B  are schematic diagrams showing a transformer core according to still another embodiment of the invention. 
       FIGS. 8A to 8J  are schematic diagrams illustrating the transformer core of  FIG. 7A  with different windings having fractional numbers of turns from 1.0 to 1.9. 
       FIGS. 9A and 9B  are schematic diagrams showing a transformer core according to still another embodiment of the invention. 
       FIG. 10  is a schematic diagram showing an EE-type core having trenches. 
       FIG. 11  is a schematic diagram showing an EC-type core having trenches. 
       FIG. 12  is a schematic diagram showing a RM-type core having trenches. 
       FIG. 13  is a schematic diagram showing a Q-type core having trenches. 
       FIGS. 14A and 14B  are schematic diagrams showing a POT-type core according to still another embodiment of the invention. 
       FIGS. 15A and 15B  are schematic diagrams showing another POT-type core according to still another embodiment of the invention. 
       FIG. 16  is a schematic diagram showing still another POT-type core according to still another embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The transformer core according to an embodiment of the invention will be described with reference to the accompanying drawings, wherein the same reference numbers denote the same elements. 
     FIGS. 3A and 3B  show an EI-type transformer structure, which includes an E-type core  32  and an I-type core  31 . The E-type core  32  includes a middle post  34  and two side posts  33 A and  33 B. 
   In a preferred embodiment, trenches  35 A and  35 B are formed on the side posts  33 A and  33 B, respectively. The trench  35 A divides the cross-section of the side post  33 A into two portions, A 1  and A 2  denoted in  FIG. 3B . The trench  35 B also divides the cross-section of the side post  33 B into two portions, A 3  and A 4  denoted in  FIG. 3B . In the present embodiment, the area ratio between A 1  and A 2  is 1:4, and the area ratio between A 3  and A 4  is 2:3. Different turn ratios of a transformer can be executed by adjusting the positions of the trenches  35 A and  35 B. 
   The principle of the transformer core according to the preferred embodiment of the invention will be described hereinbelow. 
   In  FIGS. 4A and 4B , N p  and N s  are the numbers of turns of the primary side and the secondary side of the transformer, respectively. N f  is the number of turns around the cross-section A 4  of the side post  33 B of the EI-type core. u p  and u s  are the voltages of the primary side and secondary side of the transformer, respectively, and i p  and i s  are currents of the primary side and secondary side of the transformer, respectively. Φ 0  and Φ 2  are the magnetic fluxes passing through the middle post  34  and the side post  33 A, respectively, and Φ 11  is the magnetic flux passing through the cross-section A 4  of the side post  33 B. 
   When the current i p  is conducted on the primary side of the transformer, magnetic fluxes Φ 0 , Φ 2  and Φ 11  are generated having directions as shown in  FIG. 4B . From the magnetic flux law: 
             {             Φ   0     =         N   p     ⁢     i   p           R   0     +       R   1     ⁢          R   2                           Φ   2     =           N   p     ⁢     i   p           R   0     +       R   1     ⁢          R   2             ·       R   1         R   1     +     R   2                         Φ   11     =           N   p     ⁢     i   p           R   0     +       R   1     ⁢          R   2             ·   r                 r   =         R   2     ⁢     R   12             R   12     ⁢     R   2       +       R   11     ⁢     R   12       +       R   2     ⁢     R   11                         
 
   wherein R 0 , R 1  and R 2  are magnetic reluctance of the middle post  34  and two side posts  33 A and  33 B. R 11  and R 12  are magnetic reluctance of the cross-sections A 4  and A 3  of the side post  33 B. The magnetic reluctance R 1  is equal to the parallel connection of the reluctance R 11  and R 12, , that is, R 1 =R 11 ∥R 12 . 
   From the electromagnetic induction theory: 
             {             u   p     =       N   p     ⁢       ⅆ     Φ   0         ⅆ   t                       u   s     =           N   s     ⁢       ⅆ     Φ   0         ⅆ   t         ±       N   f     ⁢       ⅆ     Φ   11         ⅆ   t           =       (       N   s     ±       N   f     ·   r       )     ⁢       ⅆ     Φ   0         ⅆ   t                         
 
   (The winding directions of the windings Ns and Nf determine polarity. The polarity is positive if the directions are the same, and negative if the directions are different.) 
   From the above, the transformer ratio of the transformer is 
             u   s       u   p       =           N   s     ±       N   f     ·   r         N   p       =       N   se       N   p           ,       
 
thus the effective turn ratio N se  becomes a fraction. Since reluctance 
         R   =     l     μ   ⁢           ⁢   A         ,       
 
if the length of the magnetic path l of the reluctance R 1  and R 2  are the same, then 
         N   se     =         N   s     ±       N   f     ⁢     A4     A4   +   A3   +   A2   +   A1           =       N   s     ±       N   f     ⁢       A4   Ae     .               
 
In this equation, Ae is the effective cross-section of the EI-type core, which is also the cross-section of the middle post of the core.
 
   If A 1 =( 1/10)*Ae, A 2 =( 4/10)*Ae, A 4 =( 2/10)*Ae, and A 3 =( 3/10)*Ae, that is, A 1 :A 2 :A 3 :A 4 =1:4:3:2, then the fractional number of turns having a precision of 1/10 can be obtained by different winding types.  FIGS. 5A to 5J  illustrate different windings having fractional numbers of turns from 1.0 to 1.9. 
   In the present embodiment, different cross-section ratios are obtained by forming trenches on the side posts. In other embodiments, the trench may be substituted by through holes.  FIG. 6  illustrates an embodiment in which through holes  6 A and  6 B are formed at the positions where the trenches are formed. The fractional number of turns can also be realized by passing the wires through the through holes  6 A and  6 B. The principle is the same as described above. 
   The width and depth of the trenches  35 A and  35 B and the shape of the through holes  6 A and  6 B can be determined according to the diameter of the wires. 
   In the above-described embodiments, a trench or a through hole is formed on each side post. In the embodiment described hereinafter, the trench is formed on one side post only. 
   Referring to  FIGS. 7A and 7B , a trench  75  is formed only on the side post  73 B of an E-type core  72 . No trench is formed on the side post  73 A. The trench  75  divides the cross-section of the side post  73 B into two portions B 1  and B 2 , and the area ratio of the two portions B 1  and B 2  can be determined according to different turn ratio requirements. 
   In the present embodiment, the area ratio of the two portions B 1  and B 2  is 1:4. This core structure can realize different fractions of turn ratios.  FIGS. 8A to 8J  show different winding types realizing turn ratios from 1.0 to 1.9. 
   The embodiment of forming two trenches on each side post will be described hereinafter. 
   Referring to  FIGS. 9A and 9B , two trenches  95 A and  95 B are formed on the side post  93 A of an E-type core  92 , dividing the cross-section of the side post  03 A into three portions C 1 , C 2 , and C 3 . Two trenches  95 C and  95 D are also formed on the side post  93 B, dividing the cross-section of the side post  93 B into three portions C 4 , C 5 , and C 6 . The area ratio of the portions can be determined according to different turn ratio requirements. In the present embodiment, the area ratios are: C 1 :C 2 :C 3 =1:4:1, and C 4 :C 5 :C 6 =1:3:2. The transformer structure of this kind can realize fractions of turn ratios up to a precision of 1/12. The minimum division can be determined according to requirement, and can be infinitely small, theoretically. Turn ratio methods providing multiples of 1/12 are similar to those described previously, and are thus not described herein. 
   In the embodiments shown in  FIGS. 7A to 7B  and  9 A to  9 B, the cross-sections of the side posts are divided by forming trenches. It should be noted that the trenches can be substituted by through holes under the same principle of the invention mentioned above. 
   In the embodiments described previously, the EI-type core is adopted for illustration purpose. However, it should be noted that the invention can also be implemented on other types of cores. For example, The EE-type core shown in  FIG. 10 , the EC-type core shown in  FIG. 11 , the RM type core shown in  FIG. 12 , and the Q-type core shown in  FIG. 13  all include side posts. The methods of trench or through hole formation on the side posts are similar to those of the EI-type core, thus are not repeated herein. 
     FIG. 14A  shows a POT-type core according to another embodiment of the invention. Unlike an EI-type core, a POT-type core includes a middle post  142 , and a bobbin  143  surrounding the middle post  142 . In the present embodiment, four trenches  145  are formed on the bobbin  143 , dividing the cross-section of the bobbin  143  into four portions equally. The transformer core structure of this kind can realize fractions of turn ratios being multiples of ¼. The winding types realizing different turn ratios are shown in  FIG. 14B . 
     FIGS. 15A and 15B  show an embodiment in which three trenches  155  are formed on the bobbin  153  of the POT-type core. The three trenches  155  divide the cross-section of the bobbin  153  into three portions D 1 , D 2 , and D 3 . The ratio of the three portions is: D 1 :D 2 :D 3 =1:2:2. This ratio allows the core to realize a fractional number of turns that is a multiple of ⅕ turns. The winding on the core is shown in  FIG. 15B . 
   It should be noted that as for the embodiments shown in  FIGS. 14A and 15A , the trenches can be substituted by through holes.  FIG. 16  shows a POT-type core having through holes according to another embodiment of the invention. According to this embodiment, eight through holes  165  are formed on the bobbin  163 . The eight through holes  165  divide the cross-section of the bobbin  163  into eight portions equally to realize a fractional number of turns that is a multiple of ⅛. 
   While the invention has been described with reference to preferred embodiments, this description is not intended to be construed in a limiting sense. Various modifications of the embodiment will be apparent to persons skilled in the art upon reference to the description. It is therefore intended that the appended claims encompass any such modifications.