Patent Publication Number: US-2009237195-A1

Title: Center-tapped transformer

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority of Chinese application no. 200810026969.0, filed on Mar. 21, 2008. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a transformer, more particularly to a center-tapped transformer. 
     2. Description of the Related Art 
     Most electronic apparatus include a transformer as a core component to satisfy power transformation requirements. A transformer has an inherent leakage inductance. In particular, some magnetic lines of force generated when electricity is supplied to a primary winding do not pass through a secondary winding and thus do not generate corresponding electric current in the secondary winding. The leakage inductance is a measure of inductance of such magnetic lines of force (also called leakage flux). 
     In general, the leakage inductance of a transformer should be kept as small as possible. However, in some applications, the transformer is required to have a certain level of leakage inductance, such as when the leakage inductance is employed as a resonance inductance, or when the leakage inductance of a common-mode inductor is employed as a differential-mode inductance, etc. 
       FIG. 1  is a sectional diagram of a conventional center-tapped transformer  100 , which includes a tubular spool  102 , a primary winding  104 , a first secondary winding  106 , a second secondary winding  108 , a first isolating unit  110 , a second isolating unit  112 , and an iron core (not shown). The spool  102  is formed with a hollow portion  114  for extension of the iron core therethrough. The primary winding  104  is wound on the spool  102 . The first secondary winding  106  is wound around the primary winding  104  and is spaced apart there from by the ring-shaped first isolating unit  110 . The second secondary winding  108  is wound around the first secondary winding  106  and is spaced apart therefrom by the ring-shaped second isolating unit  112 . 
       FIG. 2   a  is a schematic diagram of an asymmetric half-bridge LLC circuit including the transformer  100 , wherein (Lm) is the excitation inductance of the transformer  100  and (L 1 ) is the leakage inductance of the primary winding  104 . When a sinusoidal current (Ii) (such as the waveform  101  in  FIG. 2d ) is inputted into the transformer  100  at anode  15 of the circuit, the circuit will output a rectified current (Io) (such as the waveform  103  in  FIG. 2   d ).  FIGS. 2   b  and  2   c  show two different working states of the asymmetric half-bridge LLC circuit, respectively. During a positive half-cycle of the waveform of the input current (Ii), a diode (D 1 ) conducts, a diode (D 2 ) is cutoff, and the primary winding induces a leakage inductance (Ls 1 ). On the other hand, during a negative half-cycle of the waveform of the input current (Ii), the diode (D 2 ) conducts, the diode (D 1 ) is cutoff, and the primary winding induces a leakage inductance (Ls 2 ). In theory, the values of the leakage inductances (Ls 1 ) and (Ls 2 ) should be close to each other in order for the circuit to work more efficiently and to reduce power loss. 
     Since the leakage inductance (L 1 ) of the primary winding  104  of the transformer  100  will vary with the change in the input current (Ii), there is a relatively large difference between the values of the leakage inductances (Ls 1 ) and (Ls 2 ), which in turn results in non-uniform amplitude of the output current (Io), as evident from the waveform  103  in  FIG. 2   d.  Due to the high and low peak values of the output current (Io), the circuit experiences larger power loss, thereby restricting applications of the transformer  100  and circuits employing the same. 
       FIG. 3  is a sectional diagram of another conventional center-tapped transformer  200 , which includes a tubular spool  202 , a primary winding  204 , a first secondary winding  206 , a second secondary winding  208 , a first isolating unit  212 , a second isolating unit  210 , and an iron core (not shown). The spool  202  is formed with a hollow portion  214  for extension of the iron core therethrough. The primary winding  204  is wound on an upper section of the spool  202 . The first secondary winding  206  is wound on a lower section of the spool  202  and is spaced apart from the primary winding  204  by the ring-shaped first isolating unit  212 . The second secondary winding  208  is wound around the first secondary winding  206  and is spaced apart therefrom by the ring-shaped second isolating unit  210 . 
     Compared to the transformer  100  of  FIG. 1 , the leakage inductance of the primary winding  204  of the transformer  200  is maintained at a certain level for different circuit working states, and the insulation distance between the primary winding  204  and the first and second secondary winding units  206 ,  208  has a positive effect on safety specifications. Nevertheless, the leakage inductance of the transformer  200  and circuits employing the same is relatively large, which restricts applications of the same. 
     It is apparent from the foregoing that the conventional center-tapped transformers  100 ,  200  either have non-uniform leakage inductance or a rather large leakage inductance, which results in large circuit power loss and restricts applications of the same. 
     SUMMARY OF THE INVENTION 
     Therefore, the object of the present invention is to provide a center-tapped transformer that can overcome at least one of the abovementioned drawbacks of the prior art. 
     Accordingly, a center-tapped transformer of this invention comprises: 
     a first spool defining a spool axis and having an axially extending first spool part and a second spool part that extends coaxially from the first spool part; 
     a first primary winding unit surrounding the first and second spool parts; 
     first and second secondary winding units disposed on one side of the first primary winding unit and surrounding the first and second spool parts, respectively; and 
     a first isolating unit disposed between the first and second secondary winding units to separate the first and second secondary winding units from each other. 
     Preferably, the center-tapped transformer further comprises a second isolating unit disposed between said one side of the first primary winding unit and the first and second secondary winding units to separate the first primary winding unit from the first and second secondary winding units. 
     According to one embodiment, the first primary winding unit is disposed between the first spool and the first and second secondary winding units. 
     According to another embodiment, the first and second secondary winding units are disposed between the first spool and the first primary winding unit. 
     In view of the arrangement of the first and second secondary winding units, the first and second secondary winding units have the same positional relationship relative to the first primary winding unit. Accordingly, when the center-tapped transformer of this invention is applied to an asymmetric half-bridge LLC circuit, the leakage inductance of the first primary winding unit is maintained for different working states. In addition, through adjustment of the thickness of the first isolating unit, the leakage inductance of the first primary winding unit can be adjusted, thereby rendering the center-tapped transformer of this invention suitable for a wide range of applications. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other features and advantages of the present invention will become apparent in the following detailed description of the preferred embodiments with reference to the accompanying drawings, of which: 
         FIG. 1  is a sectional diagram of a conventional center-tapped transformer; 
         FIG. 2   a  is a circuit diagram of an asymmetric half-bridge LLC circuit including the transformer of  FIG. 1 ; 
         FIGS. 2   b  and  2   c  show two different working states of the asymmetric half-bridge LLC circuit of  FIG. 2   a;    
         FIG. 2   d  illustrates input and output current waveforms in the asymmetric half-bridge LLC circuit of  FIG. 2   a;    
         FIG. 3  is a sectional diagram of another conventional center-tapped transformer; 
         FIG. 4   a  is a sectional diagram of the first preferred embodiment of a center-tapped transformer according to the present invention; 
         FIG. 4   b  is a circuit diagram of an asymmetric half-bridge LLC circuit including the first preferred embodiment; 
         FIG. 4   c  illustrates input and output current waveforms in the asymmetric half-bridge LLC circuit of  FIG. 4   b;    
         FIG. 5  is a sectional diagram of the second preferred embodiment of a center-tapped transformer according to the present invention; 
         FIG. 6  is a sectional diagram of the third preferred embodiment of a center-tapped transformer according to the present invention; 
         FIG. 7   a  is a partly exploded sectional diagram of the fourth preferred embodiment of a center-tapped transformer according to the present invention; 
         FIG. 7   b  is an assembled sectional diagram of the fourth preferred embodiment; 
         FIG. 8  is a partly exploded sectional diagram of the fifth preferred embodiment of a center-tapped transformer according to the present invention; and 
         FIGS. 9   a  and  9   b  are perspective views of two spools suitable for use in the sixth preferred embodiment of a center-tapped transformer according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to  FIG. 4   a,  the first preferred embodiment of a center-tapped transformer  3  according to the present invention is shown to include a first spool  30 , a first primary winding unit  31 , a first secondary winding unit  33 , a second secondary winding unit  34 , a first isolating unit  35 , a second isolating unit  36 , and an iron core (not shown). 
     The first spool  30  has a surrounding wall  301  that defines a spool axis (A), and that has an axially extending first spool part  302  and a second spool part  303  extending coaxially from the first spool part  302 . 
     The first primary winding unit  31  surrounds the first and second spool parts  302 ,  303  of the surrounding wall  301  of the first spool  30 . 
     In this embodiment, the first and second secondary winding units  33 ,  34  are disposed on an outer side of the first primary winding unit  31  and surround the first and second spool parts  302 ,  303  of the surrounding wall  301  of the first spool  30 , respectively. 
     The first isolating unit  35  is disposed between the first and second secondary winding units  33 ,  34  to separate the first and second secondary winding units  33 ,  34  from each other. 
     The second isolating unit  36  is disposed between the outer side of the first primary winding unit  31  and the first and second secondary winding units  33 ,  34  to separate the first primary winding unit  31  from the first and second secondary winding units  33 ,  34 . 
     The iron core is to be extended into the surrounding wall  301  of the first spool  30 , and can be any one of the following: EE type iron core, EC type iron core, EF type iron core, ER type iron core, PQ type iron core, EER type iron core, EFD type iron core, ERL type iron core and PM type iron core. Since the feature of this invention does not reside in the iron core, further details of the same are omitted herein for the sake of brevity. 
     In this embodiment, the second isolating unit  36  is tubular. The first isolating unit  35  surrounds a middle part of the second isolating unit  36  and is disposed transverse to an outer peripheral surface of the second isolating unit  36 . 
     The first primary winding unit  31  is wound on an entire length of the surrounding wall  301  of the first spool  30 . The tubular second isolating unit  36  is sleeved on the outer side of the first primary winding unit  31  such that the first primary winding unit  31  is confined between the surrounding wall  301  of the first spool  30  and the second isolating unit  36 . The first secondary winding unit  33  and the second secondary winding unit  34  are coaxially disposed, are disposed side-by-side along the spool axis (A), and are wound on an outer side of the second isolating unit  36 . In this embodiment, in order to obtain a more stable output current, the first and second secondary winding units  33 ,  34  are evenly and respectively wound on upper and lower portions of the second isolating unit  36  and are separated from each other by the first isolating unit  35 . The first isolating unit  35  and the second isolating unit  36  can be an insulating tape or an insulating material such as plastic. 
       FIG. 4   b  is a circuit diagram of an asymmetric half-bridge LLC circuit including the first preferred embodiment.  FIG. 4   c  illustrates waveforms of input and output currents in the asymmetric half-bridge LLC circuit of  FIG. 4   b.  When a sinusoidal current (Ii) (such as the waveform  38  in  FIG. 4   c ) is inputted into the transformer  3 , compared to the waveform  101  in  FIG. 2   d,  since the leakage inductance (L 1 ) of the first primary winding unit  31  does not vary with changes in the input current (Ii), the waveform  38  of the resonant current (Ii) closely resembles a pure sinusoidal wave. In addition, peak values of the output current (Io) are uniform, and the waveform of the output current (Io) is continuous (see the waveform  39  in  FIG. 4   c ). As such, power loss of the circuit including the transformer  3  is less, and efficiency is higher. 
     In the first preferred embodiment of this invention, apart from ensuring that the leakage inductance (L 1 ) of the first primary winding unit  31  is maintained at a certain level under different working states, the thickness of the first isolating unit  35  can be adjusted according to a requirement of the circuit application, such that the distance between the first and second secondary winding units  33 ,  34  is adjusted so as to obtain the requisite leakage inductance. The center-tapped transformer  3  therefore has a wide range of applications. 
     Referring to  FIG. 5 , the second preferred embodiment of a center-tapped transformer  4  according to this invention is shown to include a first spool  40 , a first primary winding unit  41 , a first secondary winding unit  43 , a second secondary winding unit  44 , a first isolating unit  45 , a second isolating unit  46 , and an iron core (not shown). In this embodiment, the first secondary winding unit  43  and the second secondary winding unit  44  are disposed between the first primary winding unit  41  and the surrounding wall  401  of the first spool  40 , and are separated from each other by the first isolating unit  45 , which surrounds a middle part of the surrounding wall  401  of the first spool  40  and is disposed transverse to an outer peripheral surface of the surrounding wall  401 . The first secondary winding unit  43  and the second secondary winding unit  44  are coaxially disposed, are disposed side-by-side along the spool axis (A) defined by the surrounding wall  401 , and are wound on the surrounding wall  401 . The tubular second isolating unit  46  is sleeved on outer sides of the first and second secondary winding units  43 ,  44  such that the first and second secondary winding units  43 ,  44  are confined between the surrounding wall  401  of the first spool  40  and the second isolating unit  46 . The first primary winding unit  41  is wound on an entire length of the second isolating unit  46 . 
     Like the first preferred embodiment, the leakage inductance of the first primary winding unit  41  is maintained at a certain level under different working states, and the thickness of the first isolating unit  45  can be adjusted according to the leakage inductance required by a circuit application. 
     Referring to  FIG. 6 , the third preferred embodiment of a center-tapped transformer  5  according to this invention is shown to include a first spool  50 , a first primary winding unit  51 , a first secondary winding unit  53 , a second secondary winding unit  54 , a first isolating unit  55 , a second isolating unit  56 , and an iron core (not shown). Compared to the first preferred embodiment, this embodiment further includes a second primary winding unit  52  surrounding the first and second secondary winding units  53 ,  54 , and a tubular third isolating unit  57  disposed between the first and second secondary winding units  53 ,  54  and the second primary winding unit  52  to separate the second primary winding unit  52  from the first and second secondary winding units  53 ,  54 . The second primary winding unit  52  is wound on the third isolating unit  57 . Like the first and second preferred embodiments, the leakage inductance of the first primary winding unit  51  is maintained at a certain level under different working states, and the thickness of the first isolating unit  55  can be adjusted according to the leakage inductance required by a circuit application. 
     Referring to  FIG. 7   a,  the fourth preferred embodiment of a center-tapped transformer  6  according to this invention is shown to include a first spool  603  having a surrounding wall  601 , and a second spool  604  having a tubular second isolating unit  602  that permits insertion of the first spool  603  therein. A first isolating unit  65  surrounds a middle part of the second isolating unit  602  and is disposed transverse to an outer peripheral surface of the second isolating unit  602 . A first primary winding unit  61  is wound on an entire length of the surrounding wall  601  of the first spool  603 . A first secondary winding unit  63  and a second secondary winding unit  64  are wound evenly and respectively on upper and lower portions of the second isolating unit  602 , and are separated from each other by the first isolating unit  65 . 
     In this embodiment, after winding the first and second secondary winding units  63 ,  64  on the second spool  604 , and winding the first primary winding unit  61  on the first spool  603 , the assembly of the first primary winding unit  61  and the first spool  603  is inserted into the second spool  604 , as best shown in  FIG. 7   b.  At this time, the first primary winding unit  61  is confined between the second isolating unit  602  and the surrounding wall  601  of the first spool  603 , and the first and second secondary winding units  63 ,  64  are respectively wound on upper and lower portions of the second isolating unit  602 . In this manner, the manufacturing process of the center-tapped transformer  6  is simplified, and the manufacturing time is shortened. 
     Referring to  FIG. 8 , the fifth preferred embodiment of a center-tapped transformer  7  according to this invention is shown to include a first spool  703  having a surrounding wall  701 , and a second spool  704  having a tubular second isolating unit  702  that permits insertion of the first spool  703  therein. Unlike the fourth preferred embodiment, a first isolating unit  75  surrounds a middle part of the surrounding wall  701  of the first spool  703  and is disposed transverse to an outer peripheral surface of the surrounding wall  701 . A first primary winding unit  71  is wound on an entire length of the second isolating unit  702 . A first secondary winding unit  73  and a second secondary winding unit  74  are wound evenly and respectively on upper and lower portions of the surrounding wall  701  of the first spool  703 , and are separated from each other by the first isolating unit  75 . Since the first and second spools  703 ,  704  can be coupled together in a manner similar to that in the fourth preferred embodiment, the advantages of the fourth preferred embodiment are likewise achieved by the fifth preferred embodiment. 
     Referring to  FIGS. 9   a  and  9   b,  the sixth preferred embodiment of a center-tapped transformer  8  according to this invention is shown to differ from the second preferred embodiment in that the first isolating unit includes first and second plates  851 ,  852  that lie on a plane transverse to a spool axis defined by a surrounding wall  801  of the first spool  80 ,  80 ′. The first and second plates  851 ,  852  cooperate with the surrounding wall  801  of the first spool  80 ,  80 ′ to define a pair of notches  853  adapted for passage of conductive wires (not shown) of the first primary winding unit so that the latter is able to wind around the entire length of the surrounding wall  801  without being restricted by the first isolating unit.  FIG. 9   a  illustrates an upright spool  80 , whereas  FIG. 9   b  illustrates a horizontal spool  80 ′. Depending on requirements, either one of the spools  80 ,  80 ′ may be selected for use in the center-tapped transformer  8 . 
     Some of the advantages of this invention are summarized below: 
     1. Taking the embodiment of  FIG. 4   a  as an example, since the first and second secondary winding units  33 ,  34  have the same positional relationship relative to the first primary winding unit  31  and the iron core (not shown) in the first spool  30 , when a sinusoidal current is inputted into the first primary winding unit  31 , the leakage inductance is maintained at a certain level under different circuit working states. 
     2. Taking the embodiment of  FIG. 5  as an example, through adjustment of the thickness of the first isolating unit  45  that separates the first and second secondary winding units  43 ,  44  from each other, the leakage inductance of the first primary winding unit  41  is adjusted, thereby rendering the center-tapped transformer of this invention suitable for a wide range of applications. 
     3. By using the leakage inductance of the center-tapped transformer of this invention for resonance inductance of transformers of LLC type, LC type, etc., additional inductors to serve as resonance inductance are not needed, thereby reducing transformer power loss and saving space. 
     While the present invention has been described in connection with what are considered the most practical and preferred embodiments, it is understood that this invention is not limited to the disclosed embodiments but is intended to cover various arrangements included within the spirit and scope of the broadest interpretation so as to encompass all such modifications and equivalent arrangements.