Transformer with split primary winding

The present application claims priority to U.S. Provisional Patent Application 61/083,717, filed Jul. 25, 2008, which is incorporated herein by reference in its entirety.

1. Technical Field

The present invention relates in general to electrical circuits, and in particular, to transformers and related powered systems.

2. Description of the Related Art

Power conditioning circuits have evolved over the years from linear regulators to low frequency pulse-width modulated systems and then to high frequency square wave converters. More recently, resonant converters have been adopted in order to achieve improved performance in a more compact, lightweight and inexpensive package.

A resonant converter is a power conditioning system that employs a resonant L-C circuit in performing power conversion. In general, a resonant converter controls the operation of one or more switches to apply a square wave of voltage (or current) to the resonant circuit. Energy within the resonant circuit is then transferred to a load coupled to the converter output.

As will be appreciated by those skilled in the art, numerous topologies of resonant converters are possible, including boost mode, buck mode and buck-boost (flyback) and boost-buck. Many common resonant converter topologies employ one or more transformers to achieve design objectives, such as impedance matching, isolation between the input and output, reduction of electromagnetic interference (EMI), etc. The present disclosure describes a transformer design that may be employed, without limitation, as a transformer in a resonant converter.

In at least one embodiment, a transformer includes a core having a first leg, a second leg and a third leg, a split primary winding including first turns about the first leg electrically coupled with second turns about the third leg, and a secondary winding about the second leg. Magnetic flux linking the first turns of the split primary winding and magnetic flux linking the second turns of the primary winding link the secondary winding.

The present invention, as well as a preferred mode of use, will best be understood by reference to the following detailed description of one or more illustrative embodiments when read in conjunction with the accompanying drawings, wherein:

FIG. 1 is high level schematic diagram of a resonant converter in accordance with one embodiment;

FIG. 2A is a first section view of an exemplary embodiment of a transformer that may be employed, for example, in a converter, such as the resonant converter depicted in FIG. 1 ;

FIG. 2B is a second section view of the exemplary embodiment of a transformer along the line 2 - 2 depicted in FIG. 2A ;

FIG. 2C is a schematic diagram depicting the turns of a split primary winding connected in series;

FIG. 2D is a schematic diagram depicting the turns of a split primary winding connected in parallel;

FIG. 3 is a high level logical flowchart of an exemplary process of making a transformer in accordance with one embodiment;

FIG. 4A is a high level schematic diagram of an external AC-to-DC converter coupled to a powered system in accordance with one embodiment; and

FIG. 4B is a high level schematic diagram of an internal AC-to-DC converter coupled to a powered system in accordance with one embodiment.

With reference now to the figures and in particular with reference to FIG. 1 , there is illustrated a high level schematic diagram of an exemplary resonant converter 100 including a transformer 120 in accordance with one embodiment. As shown, on the primary side of transformer 120 , resonant converter 100 includes a DC voltage source V_link 102 , which can be provided by any suitable voltage source. In many common applications, such as home appliances, computers and other consumer electronics, and fluorescent light ballasts, V_link 102 may have a voltage of between approximately 150 VDC and 400 VDC, and may be implemented, for example, by a rectifier of AC line (mains) power. In such applications, the power provided by V_link 102 may optionally further have power factor correction applied. For ease of reference, the positive terminal of V_link 102 is referred to herein as node A, and the negative terminal of V_link 102 is referred to as node B.

A pair of switching devices, such as field effect transistors (FETs) 104 a , 104 b , are coupled in a half bridge across V_link 102 in order to provide a desired voltage waveform, such as a square wave. Switching of FETs 104 a and 104 b and thus the voltage waveform (and the associated current waveform) are controlled by gate drive circuit 106 . In many applications, gate drive circuit 106 operates FETs 104 a and 104 b in complementary manner, meaning that while FET 104 a is on, FET 104 b is off and vice versa.

Coupled in parallel with each of FETs 104 a , 104 b is a respective parasitic capacitor C_par 108 a or C_par 108 b shunting a corresponding one of the switching devices. Thus, C_par 108 a is coupled between node A and an intermediate node D between FETs 104 a and 104 b , and C_par 108 b is coupled between node D and node B.

A pair of resonant capacitors C_res 110 a and C_res 110 b is further coupled across V_link 102 between nodes A and B. The values of C_res 110 a and C_res 110 b are selected to determine the desired resonant frequency of resonant converter 100 . C_res 110 a and C_res 110 b are coupled together at node E.

As is known in the art, transformer 120 includes a primary winding and a secondary winding whose turn ratio is one factor governing the voltage induced across the secondary winding. In the equivalent circuit shown in FIG. 1 , transformer 120 is characterized by a coupled inductance L_cpld 122 , as well as a leakage inductance L_lkg 124 , which together with C_res 110 a and C_res 110 b , forms the resonant circuit of resonant converter 100 . As described further below with respect to FIGS. 2A-2B , coupled inductance L_cpld 124 and leakage inductance L_lkg 122 are preferably not implemented as separate inductor components, but are instead simply characteristics of transformer 120 determined by its construction. In such embodiments, the primary winding of transformer 120 is coupled between nodes D and E.

The secondary winding of transformer 120 , which has terminals defined by nodes F and G, is coupled to a filtered full wave rectifier. The filtered full wave rectifier includes four diodes 130 a , 130 b , 130 c and 130 d , where diode 130 a has an input coupled to node F and an output coupled to a first output node H, diode 130 b has an input coupled to node G and an output coupled to first output node C, diode 130 c has an input coupled to a second output node J and an output coupled to node F, and diode 130 d has an input coupled to second output node J and an output coupled to node G. The filtered full wave rectifier further includes an output capacitor C_out 134 coupled across output nodes H and J to filter or smooth the ripple in the pulsating DC power (e.g., 20 VDC) created by the full wave rectifier. The magnitude of the ripple depends, for example, on the input voltage magnitude and frequency, the filter capacitance, and the load resistance R_load 136 coupled across output nodes H and J.

Referring now to FIG. 2A , there is depicted a section view of an exemplary embodiment of a transformer 120 that may be employed, for example, within a converter, such as resonant converter 100 of FIG. 1 . In the exemplary embodiment, transformer 120 has a high permeability core (e.g., a ferrite core) having at least outer first and third legs 202 a and 202 c and an inner second leg 202 b disposed between first and third legs 202 a and 202 c . The core may be, for example, a commercially available EE core formed of a first E core section 200 a and a second E core section 200 b . Alternatively, the core may be a commercially available EI core formed of an E core section and an I core section (not shown). As shown in FIG. 2B , which is a section view along line 2 - 2 of FIG. 2A , first E core section 200 a (and second E core section 200 b ) and each leg 202 thereof commonly have (but are not required to have) a square or rectangular cross-section.

As further shown in FIG. 2A , when the core of transformer 120 is assembled, second leg 202 b has an air gap 204 b formed therein between the core sections, which as discussed below defines the so-called leakage inductance of transformer 120 . Additionally, first and third legs 202 a and 202 c may optionally have respective air gaps 204 a and 204 c formed therein between the core sections. In embodiments in which first and second legs 202 a and 202 c include air gaps 204 a and 204 c , air gaps 204 a and 204 c are typically smaller than air gap 204 b in second leg 202 b.

Each of first, second and third legs 202 a , 202 b and 202 c carries a respective one of bobbins 206 a , 206 b and 206 c . Bobbins 206 a , 206 b and 206 c may be formed, for example, of a thermoplastic (e.g., Polybutylene Terephthalate (PBT)) or thermoset plastic (e.g., a phenolic). Each bobbin 206 a , 206 b , or 206 c has a core housing portion 208 for carrying an electrically conductive wire coil forming a primary or secondary winding and a core spacing portion 210 that maintains physical separation of the wire coil from the core and physical separation of the wire coil and the core from an external transformer housing, if any.

In the illustrated embodiment, second bobbin 206 b carries the secondary winding 212 b , which is preferably but not necessarily wound on second bobbin 206 b in a single layer. In many common applications in which transformer 120 is a step-down transformer in which secondary winding 212 b has a lower voltage and higher current than the primary winding, secondary winding 212 b is formed of larger wire than the primary winding. It is further preferable if secondary winding 212 b is formed of multi-strand Litz wire in order to minimize skin effect losses and proximity effect losses.

In the depicted embodiment, first bobbin 206 a and third bobbin 206 c carry respective portions 212 a and 212 c of a split primary winding, which are electrically connected, either in series as shown in FIG. 2C or in parallel as shown in FIG. 2D . As noted above, the primary winding usually has a higher number of turns and a lower current than the secondary winding, permitting use of a smaller diameter wire in the primary winding than in the secondary winding. The wire diameter requirement can be further reduced by the connection of the two primary winding portions 212 a and 212 c in parallel as shown in FIG. 2D , meaning that each of portions 212 a and 212 c carries only a portion of the total primary winding current. The primary winding may benefit from the use of Litz wire, but there is no requirement that the wire be stranded in view of the higher number of turns and (if applicable) its parallel construction.

In contrast to the construction of conventional split primary windings, portions 212 a and 212 c of the primary winding have unequal numbers of turns. In many implementations, the numbers of turns are fairly close and differ by, for example, between approximately 1 and 20 percent, and more particularly, about 10 percent. FIG. 2A illustrates portion 212 a having a larger number of turns than portion 212 c ; however, it should be appreciated that it is immaterial which of portions 212 a and 212 c has a higher turn count.

The choice between series and parallel construction for the split primary winding is application-dependent. In a parallel construction, portions 212 a and 212 c of the split primary winding have the same voltage across them, but produce different fluxes. In this case, portion 212 c , which has fewer turns, produces more flux and carries more current than portion 212 a ; however, as the current through portions 212 a and 212 c is summed, the current through each portion of the split primary winding is less than if a series construction is utilized and thus a smaller gauge wire can be employed. In a series construction, portions 212 a and 212 c of the split primary winding carry the same current, but have different voltages across them. In this case, portion 212 a , which has more turns, produces more flux and sustains a larger voltage than portion 212 c . Thus, an implementation with higher current (and hence a need for larger diameter wire) and lower voltage would favor a parallel connection, and an implementation with lower current and higher voltage (and hence a need for thicker insulation) would favor a series connection. It should also be noted that specific dimensional constraints and construction considerations, such as wire type, ease of termination and soldering, and bobbin shape and material, can all contribute to a selection of one construction over the other.

As installed on the core, the primary and secondary windings are physically separated, facilitating the placement of insulation barriers 214 a and 214 b between first and second legs 202 a and 202 b and between second and third legs 202 b and 202 c , respectively. The physical separation of the primary and secondary windings also reduces the capacitance between the primary and secondary windings, desirably reducing the coupling of electromagnetic interference (EMI) across transformer 120 .

When energized by the application of current to the split primary winding, the magnitudes of the resulting magnetic fluxes 220 a and 220 b respectively linking portions 212 a and 212 c of the primary winding are inversely proportional to the number of winding turns. Thus, magnetic flux 220 b of portion 212 c is larger than magnetic flux 220 a of portion 212 a . As indicated, portions 212 a and 212 c of the primary winding are phased so that magnetic fluxes 220 a and 220 b add into second leg 202 b , linking with secondary winding 212 b . The additional magnetic flux 222 generated by the lower turn secondary winding 212 b circulates in the outer ring of the core in opposition to magnetic flux 220 b of portion 212 c in order to offset the shortage of magnetic flux supplied by portion 212 a of the primary winding. This differential magnetic flux 222 does not flow through second leg 202 b and is not linked with secondary winding 212 b , giving rise to a well-controlled leakage inductance 124 .

As utilized herein, the term “leakage inductance” is employed to conform to industry tradition. However, it should be noted that in the depicted embodiment the major fraction of this uncoupled “leakage” inductance is not magnetic flux leaking outside the transformer core into the surrounding space, but rather is flux rerouted from secondary winding 212 b as the result of a deliberate design choice in order to reduce the magnetic coupling between the primary and secondary windings. The magnitude of leakage inductance 124 depends only on the core geometry, the difference in turns between the two portions 212 a , 212 c of the split primary winding, and the (usually small) air gap(s) 204 a , 204 c in first and third legs 202 a and 202 c , but does not depend appreciably on the winding relative position and shape, as is usually the case for a true leakage inductance. Consequently, the magnitude of leakage inductance 124 and thus the current in secondary winding 212 b can be easily controlled during the manufacturing process.

The coupled (or magnetizing) inductance 122 is determined primarily by the gap in the center leg, which is usually held to tight tolerances during the manufacture of transformer 120 .

Referring now to FIG. 3 , there is illustrated a high level logical flowchart of an exemplary process of making a transformer 120 in accordance with one embodiment. As a logical flowchart, at least some of the illustrated steps may be performed concurrently or in an order other than that shown.

The process begins at block 300 and then proceeds to block 302 , which depicts providing a core having at least three legs. As noted above, the core may be, for example, an EE or EI ferrite core. At block 304 , portions 212 a and 212 c of the primary winding are wound about first and third bobbins 206 a and 206 c , respectively. The numbers of turns in portions 212 a and 212 c is unequal, and is selected to determine the desired value of the leakage inductance L_lkg 124 of transformer 120 . In addition, at block 306 secondary winding 212 b is wound about second bobbin 206 b . The primary and secondary windings may then optionally be subjected to a varnish impregnation treatment in order to fix the windings in place relative to bobbins 206 a , 206 b and 206 c.

As indicated at block 308 , first, second and third bobbins 206 a , 206 b and 206 c are placed on first, second and third legs 202 a , 202 b and 202 c , respectively. Bobbins 206 a , 206 b and 206 c are installed in relative orientation to ensure that magnetic fluxes 220 a and 220 b are additive in second leg 202 b as shown in FIG. 2A . The primary and secondary windings are electrically insulated from one another by the installation of interstitial insulation barriers 214 a and 214 b , as depicted at block 310 . In addition, portions 212 a and 212 c of the primary winding are electrically coupled in parallel, as illustrated at block 312 . Transformer 120 may optionally be packaged in an external housing. The process shown in FIG. 3 thereafter terminates at block 314 .

With reference now to FIGS. 4A-4B , there are illustrated high level schematic diagrams of alternative embodiments of a powered system, which may be any type of system that utilizes electrical power. In each of the depicted embodiments, a powered system 400 is electrically connected to and receives power from a converter (or power supply) 402 , which in many common implementations is an AC-to-DC converter, such as the resonant converter depicted in FIG. 1 or another type of converter. Converter 402 includes a transformer having a split primary winding as described herein. As indicated in FIG. 4A , converter 402 may be external to powered system 400 , as is currently common for portable electronic devices, such as laptop computers, mobile telephones, portable music players, etc. Alternatively, as depicted in FIG. 4B , converter 402 may be implemented internal to a housing of the powered system 400 , for example, as is currently common for desktop personal computer systems.

A transformer design has been described, which is suitable for use, for example, in a converter. The transformer design provides, inter alia:

1. isolation between the primary and secondary windings that satisfies various safety standards; 2. a precise voltage ratio, which is not necessarily related to the primary-to-secondary turn ratio given the low coupling coefficient; 3. a high leakage inductance of controlled value; 4. a compact design that can be sized to conform to dimensional constraints.

While one or more preferred embodiments have been described, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the claims set forth below.