Planar transformer having fractional windings

A planar transformer is fabricated on a multilayer printed circuit board having more than two layers. A magnetic core includes a common leg and a first and a second return leg that form a first and second core window, respectively. A first coil includes a first coil winding formed on the circuit board. The first coil winding passes through each of the first and second core windows. A second coil includes a plurality of coil windings formed on the circuit board. Two or more of the plurality of coil windings include fractional turn windings. Each of the plurality of coil windings passes through at least one of the first and the second core windows and is interconnected such that the sum of ampere turn products from all of the coil windings passing through each of the first and the second core windows is substantially equal to zero.

BACKGROUND OF THE INVENTION

Fractional turns used in switching power supply transformers can significantly increase the voltage resolution between a primary and a secondary winding. For example, it may be desirable in certain applications to have particular ratios of input voltage to one or more output voltages. This ratio is usually determined by the relative number of turns, or “turns ratio” of the various windings of the transformer.

SUMMARY OF THE INVENTION

In one embodiment, a planar transformer is fabricated on a multilayer printed circuit board having more than two layers. The planar transformer includes a magnetic core that is coupled to the multilayer printed circuit board. The magnetic core includes a common leg and at least a first and a second return leg. The common leg and the first return leg form a first core window. The common leg and the second return leg form a second core window. A first coil includes a first coil winding formed on one or more layers of the multilayer printed circuit board. The first coil winding passes through each of the first and second core windows. A second coil includes a plurality of coil windings formed on one or more layers of the multilayer printed circuit board. Two or more of the plurality of coil windings are fractional turn windings. Each of the plurality of coil windings pass through at least one of the first and the second core windows and are interconnected such that the sum of ampere turn products from all of the coil windings passing through each of the first and the second core windows is substantially equal to zero.

The magnetic core can also include a third return leg that forms a third core window. In one embodiment, at least two of the fractional windings are half turn windings. In one embodiment, the common leg and a plurality of return legs correspond to a plurality of core windows. In one embodiment, a magnetic flux generated in the common leg is substantially equally distributed in the plurality of return legs.

In one embodiment, the absolute value of the difference between an ampere turn product from the first coil winding passing through the first core window and the sum of ampere turn products of the plurality of coil windings passing through the first core window is less than ten percent of the ampere turn product from the first coil winding passing through the first core window.

In one embodiment, the absolute value of the difference between an ampere turn product from the first coil winding passing through the second core window and the sum of ampere turn products of the plurality of coil windings passing through the second core window is less than ten percent of the ampere turn product from the first coil winding passing through the second core window.

In some embodiments, one or more of the common leg, the first return leg, and the second return leg passes through the multilayer printed circuit board. The magnetic core can include multiple parts. The multiple parts can be coupled together from opposite sides of the printed circuit board.

In one embodiment, the first coil is the primary coil and the second coil is the secondary coil. In another embodiment, the second coil is the primary winding and the first coil is the secondary coil. In one embodiment, the magnetic core includes a pre-fabricated magnetic material. In one embodiment, the planar transformer is a component in an audio amplifier.

In another embodiment, a power supply includes a voltage input terminal. The power supply also includes a planar transformer electrically coupled to the voltage input terminal. The planar transformer is fabricated on a multilayer printed circuit board having more than two layers. The planar transformer includes a magnetic core that is coupled to the multilayer printed circuit board. The magnetic core includes a common leg and at least a first and a second return leg. The common leg and the first return leg form a first core window. The common leg and the second return leg form a second core window. A first coil includes a first coil winding formed on one or more layers of the multilayer printed circuit board. The first coil winding passes through each of the first and second core windows. A second coil includes a plurality of coil windings formed on one or more layers of the multilayer printed circuit board. Two or more of the plurality of coil windings are fractional turn windings. Each of the plurality of coil windings pass through at least one of the first and the second core windows and are interconnected such that the sum of ampere turn products from all of the coil windings passing through each of the first and the second core windows is substantially equal to zero. An output terminal is coupled to the planar transformer.

In one embodiment, the output terminal supplies voltage to an audio amplifier. The magnetic core can include the common leg and a plurality of return legs that correspond to a plurality of core windows. In one embodiment, a magnetic flux generated in the common leg is substantially equally distributed in the plurality of return legs.

In one embodiment, the absolute value of the difference between an ampere turn product from the first coil winding passing through the first core window and the sum of ampere turn products of the plurality of coil windings passing through the first core window is less than ten percent of the ampere turn product from the first coil winding passing through the first core window.

In one embodiment, the absolute value of the difference between an ampere turn product from the first coil winding passing through the second core window and the sum of ampere turn products of the plurality of coil windings passing through the second core window is less than ten percent of the ampere turn product from the first coil winding passing through the second core window.

In one embodiment, two or more of the fractional windings comprise half turn windings. The magnetic core can be fabricated from a pre-fabricated magnetic material. In one embodiment, one or more of the common leg, the first return leg, and the second return leg passes through the multilayer printed circuit board. The magnetic core can include multiple parts. The multiple parts are coupled together from opposite sides of the printed circuit board.

A method for transforming an electrical current, according to one embodiment, includes forming a magnetic core comprising a first core window and a second core window. The magnetic core is coupled to a multilayer printed circuit board including more than two layers. A first coil having a first coil winding is formed on one or more layers of a multilayer printed circuit board. The first coil winding passes through each of the first and second core windows. A second coil having a plurality of coil windings is formed on one or more layers of the multilayer printed circuit board. Two or more of the plurality of coil windings include fractional turn windings. Each of the plurality of coil windings pass through at least one of the first and the second core windows and are interconnected such that the sum of ampere turn products from all of the coil windings passing through each of the first and the second core windows is substantially equal to zero.

In one embodiment, two or more of the fractional windings are half turn windings. In one embodiment, the magnetic core includes a common leg and a plurality of return legs that correspond to a plurality of core windows. The method can further include generating a magnetic flux in the common leg and equally distributing the magnetic flux in the plurality of return legs. The method can also include passing at least one of the common leg, the first return leg, and the second return leg through the multilayer printed circuit board.

In one embodiment, a planar transformer includes a multilayer printed circuit board having more than two layers. A first coil includes at least one full turn winding formed on one or more layers of the multilayer printed circuit board. A second coil includes a plurality of windings formed on one or more layers of the multilayer printed circuit board. Two or more of the plurality of windings are fractional turn windings that are connected in a parallel configuration. A magnetic core inductively couples the plurality of windings to the at least one full turn winding. The magnetic core includes two or more core windows corresponding to the at least two fractional turn windings.

In one embodiment, each of the at least two fractional windings passes through one of the at least two core windows. The magnetic core can include a common leg and a plurality of legs that correspond to a plurality of core windows. In one embodiment, two or more of the fractional windings are half turn windings.

In one embodiment, the absolute value of the difference between an ampere turn product from the full turn winding passing through one of the two core windows and the sum of ampere turn products of the plurality of coil windings passing through the one of the two core windows is less than ten percent of the ampere turn product from the full turn winding passing through the one of the two core windows. The magnetic core can be fabricated from a pre-fabricated magnetic material. In one embodiment, the transformer is a component of an audio amplifier.

A method for transforming an electrical current, according to one embodiment, includes forming a first coil having at least one full turn winding on one or more layers of a multilayer printed circuit board having more than two layers. A second coil having a plurality of windings is formed on one or more layers of the multilayer printed circuit board. Two or more of the plurality of windings are fractional turn windings that are connected in a parallel configuration. A magnetic core having two or more core windows that correspond to the two or more fractional turn windings inductively couples the plurality of windings to the at least one full turn winding.

In one embodiment, two or more of the fractional windings are half turn windings. In one embodiment, the absolute value of the difference between an ampere turn product from the full turn winding passing through one of the two core windows and the sum of ampere turn products of the plurality of coil windings passing through the one of the two core windows is less than ten percent of the ampere turn product from the full turn winding passing through the one of the two core windows.

DETAILED DESCRIPTION

Fractional turns used in switching power supply transformers can significantly increase the voltage resolution between a primary and a secondary winding. As switching frequencies increase and the required primary turns count decreases, it is more and more difficult to get the desired turns ratio between windings using integer turns counts. For example, megahertz (MHz) switching power converters operating from an automotive 14.4 Volt bus only require a single turn primary and fractional turns can be used to step down, or to get any significant resolution in available step-up ratios.

A planar transformer for an audio amplifier according to one embodiment is fabricated on a multilayer printed circuit board. The multilayer printed circuit can include more than two layers. A first coil including one or more coil windings is formed on one or more layers of the multilayer printed circuit board. A second coil including a plurality of coil windings is formed on one or more layers of the multilayer printed circuit board. A number of the plurality of second coil windings include fractional windings. The first coil can be the primary coil or the secondary coil. The second coil can be the primary coil or the secondary coil.

A magnetic core inductively couples the first coil to the second coil. The core can include a common leg, a first return leg and a second return leg. The common leg and the first return leg create a first core window. The common leg and the second return leg create a second core window. The common leg and any plurality of return legs correspond to a plurality of core windows. Each fractional winding passes through a core window. By a “fractional winding” we mean a partial turn winding that passes through less than all of the core windows. The fractional value of the partial turn winding cannot be smaller than the reciprocal of the number of core windows. For example, in a transformer having two core windows, the fractional value of the partial turn winding cannot be smaller than one-half. In a transformer having three core windows, the fractional value of the partial turn winding cannot be smaller than one-third. In a transformer having four core windows, the fractional value of the partial turn winding cannot be smaller than one-quarter. However, the fractional value of a partial turn winding in a transformer having four windows can be one-half or three-quarters, for example.

As will be described in more detail herein, the sum of ampere-turn products from all of the coil windings passing through each core window is substantially equal to zero. In general, this condition requires that the number of fractional turn windings be constrained by symmetry in the ampere-turn products through each core window. One way to satisfy the symmetrical ampere-turn products is to have one fractional turn winding in each core window and to connect these fractional turn windings in parallel so they have an equal current. For example, a transformer having two core windows requires an integer multiple of two half-turn windings. A transformer having three core windows requires an integer multiple of three one-third turn windings, for example.

FIG. 1illustrates a transformer100fabricated on a multiple layer printed circuit board101according to one embodiment of the invention. In one embodiment, the transformer100is an autotransformer. The term “autotransformer” as used herein denotes a transformer that includes a single, continuous winding that is tapped to provide either a step-up or step-down function. In this configuration, the transformer100has at least part of the windings common to both primary and secondary circuits. The voltage across the secondary winding has the same relationship to the voltage across the primary that it would have if they were two distinct windings. The techniques and principles taught by embodiments of the present invention are not limited to autotransformer configurations and can also be applied to transformers with electrically isolated winding configurations.

The multiple layer printed circuit board101includes six layers. The layers are positioned on top of each other in a layered configuration, but are shown adjacent to each other for illustrative purposes. The multiple layer printed circuit board101can include apertures103for receiving a ferrite core (not shown). The ferrite core (not shown) can include a top section and a bottom section. The top section and the bottom section are assembled together such that a portion of the top and/or bottom section is positioned inside the aperture103. The ferrite core can include an E-shaped core or can be a core having any suitable shape. In one embodiment, the ferrite core (not shown) can include two symmetric E-shaped cores that are coupled together from opposite sides of the multiple layer printed circuit board101. The ferrite core can be pre-fabricated material. For example, the ferrite core can be formed through pressing and sintering.

There are several techniques that can be used to assemble the ferrite core. For example, a mechanical clip (not shown) can be used to hold the top section and the bottom section together. The top section and the bottom section can sometimes include slots to receive the mechanical clip. The slots prevent the mechanical clip from adding additional height to the assembly and prevent the top section and the bottom section from moving laterally. Alternatively, tape can be used to assemble the ferrite core. In one embodiment, a high temperature adhesive is used to assemble the ferrite core.

In one embodiment, the transformer100includes a first layer102having a first terminal104that is electrically coupled to a first coil winding106. The first coil winding106is a one and one-half turn winding that is terminated at a second terminal108. In this embodiment, the first coil winding106is tapped at terminal110. The term “tap” as used herein denotes a connection point along a transformer winding that allows the number of turns to be selected. In this case, terminal110selects a half turn of first coil winding106.

A first fractional turn winding114is a half turn winding having a third terminal112and a fourth terminal116. The term “fractional turn winding” as used herein denotes a winding that is less than a full turn. For example, although in this embodiment, the first fractional turn winding114is a half-turn winding, the fractional turn winding can be a third-turn winding. Using known techniques not described in detail herein, the first coil winding106as well as the first fractional turn winding114can be formed either by chemically etching a layer of electrically conducting material, such as copper, deposited on the face of a circuit board, or by depositing electrically conducting material on the face of the circuit board. The first coil winding106as well as the first fractional turn winding114can be circular, helical, rectangular, or any other suitable shape.

A second layer120includes a second coil winding122. The second coil winding122is a full turn winding having a fifth124and sixth terminal126. A third layer130includes a third coil winding132. The third coil winding132includes one and one-half turn windings having a seventh134and eighth terminal136. The third layer130also includes a second fractional turn winding138having ninth140and tenth terminals142.

A fourth layer143includes a fourth coil winding144. The fourth coil winding144includes one and one-half turn windings having a eleventh146and twelfth terminal148. The twelfth terminal148is coupled to the eighth terminal136of the third layer130through a via149. The term “via” as used herein denotes a metalized through hole that couples one layer of a printed circuit to another layer. A via can also be used to make an electrical connection from one winding to other circuit components (not shown). The fourth layer143also includes a third fractional winding150having thirteenth152and fourteenth terminals154. A fifth layer156includes a fifth coil winding158. The fifth coil winding158is a full turn winding having a fifteenth160and sixteenth terminal162.

A sixth layer164can include a seventeenth terminal166that is electrically coupled to a sixth coil winding168. The seventeenth terminal166is electrically coupled to the first terminal104of the first layer102through via169. The sixth coil winding168is a one and one-half turn winding that is terminated at a eighteenth terminal170. Terminal172is used to tap the sixth coil winding168, selecting a half turn of sixth coil winding168. A fourth fractional winding176includes a nineteenth terminal174and a twentieth terminal178.

Although the coil windings are substantially spiral in shape, various discontinuities are designed into the windings. These discontinuities can be used to optimize the layout of the transformer100. For example, jumpers180,182,184,186,188, and190can be used to complete a current path through the various coils. The jumpers can slightly modify the shape of each spiral coil, but these small irregularities in the shapes of the coils do not substantially impact the performance of the transformer100.

FIG. 2illustrates a cross-sectional view of the transformer100ofFIG. 1. The first layer102and the sixth layer164are mirror images of one another. The second layer120and the fifth layer156are also mirror images of one another. The third layer130and the fourth layer143are also mirror images of one another. A core200having a top section202and a bottom section204is assembled through the aperture103(FIG. 1) of the multi-layer circuit board101. The top section202and the bottom section204can embody an E-shaped core. The core200can be any other suitably shaped core. For example, one or more cup-shaped cores can be used.

The core200includes a common leg206, a first return leg208and a second return leg210. The common leg206and the first return leg208create a first core window212. The common leg206and the second return leg210create a second core window214. The common leg206and any plurality of return legs correspond to a plurality of core windows.

The first layer102includes the first coil winding106and the first fractional turn winding114. The first coil winding106is a one and one-half turn winding that twice passes through the first core window212and once passes through the second core window214. The first fractional turn winding114passes though the second core window214once.

The second layer120includes the second coil winding122. The second coil winding122is a full turn winding that passes through the first core window212and the second core window214.

The third layer130includes the third coil winding132and the second fractional turn winding138. The third coil winding132is a one and one-half turn winding that once passes through the first core window212and twice passes through the second core window214. The second fractional turn winding138passes though the first core window212once.

The fourth layer143includes the fourth coil winding144and the third fractional turn winding150. The fourth coil winding144is a one and one-half turn winding that twice passes through the first core window212and once passes through the second core window214. The third fractional turn winding150passes though the second core window214once.

The fifth layer156includes the fifth coil winding158. The fifth coil winding158is a full turn winding that passes through the first core window212and the second core window214.

The sixth layer164includes the sixth coil winding168and the fourth fractional turn winding176. The sixth coil winding168is a one and one-half turn winding that once passes through the first core window212and twice passes through the second core window214. The fourth fractional turn winding176passes though the first core window212once.

The various coil windings on the various layers can be fabricated with different widths and different thicknesses. For example, the second coil winding122is significantly wider than both the first coil winding106and the first fractional turn winding114. The shape, width, and thickness of each coil winding are designed to optimize the performance of the transformer100. Various other shapes and sizes of the coil windings can also be used. For example, thicker coils can generally conduct higher currents than thinner coils. Additionally, wider coils can generally conduct higher currents than narrow coils.

The transformer100ofFIG. 2includes a first coil having a coil winding. The coil winding can include one or more turns and can support a current. The current in the coil winding multiplied by the number of turns of the coil winding is referred to as an ampere turn product. Each coil in a plurality of coils can include an ampere turn product and the total of the ampere turn products of the plurality of coils is referred to as the sum of ampere turn products.

Each core window212,214can include two or more coil windings. In one embodiment, the sum of the ampere turn products from all of the coil windings in each core window212,214is substantially equal to zero. By substantially equal to zero, we mean (in a transformer having a primary coil winding and a secondary coil winding that both pass through a core window) that the absolute value of the difference between the ampere turn product from the primary coil winding passing through the core window and the ampere turn product from the secondary coil winding passing through the core window is less than ten percent of the ampere turn product from the primary coil winding passing through the core window.

The current in a transformer can be divided into a magnetizing current and a load current. In the disclosure herein, the load currents and their reflection in the primary winding sum to substantially zero assuming that the magnetizing current is ignored. There will always be a magnetizing current component to the primary current. This magnetizing current is substantially independent of the load current, and is typically less than ten percent of the maximum primary reflected load current. The values of the magnetizing current for different loads can be established by using standard transformer design techniques which will not be described herein. The magnetizing current will essentially be ignored in the following description.

The embodiment ofFIG. 2can include an additional constraint on the sum of ampere turn products in each core window212,214. Each primary coil passes once through each core window212,214such that the sum of ampere turn products from the primary coils in each core window212,214is substantially equal. Thus, the magnetic flux through each core window212,214is also substantially equal and results in a balanced configuration.

In a magnetic core having multiple windows, the sum of ampere turn products from the total number of coil windings passing through each core window can be equal in a balanced configuration. For example, in a magnetic core having two core windows, the sum of ampere turn products from the total number of coil windings passing through the first core window and the sum of ampere turn products from the total number of coil windings passing through the second core window are equal and result in a balanced magnetic flux in the magnetic core.

The core can be divided into any number of sections or core windows, each core window can have an equal magnetic cross section. In one embodiment, each core window produces a balanced magnetic load.

In one embodiment, a fractional turn winding passes through each core window. Since each core window includes a fractional turn, these fractional turns can have essentially equal load currents. One way to achieve equal load currents is to configure the fractional turns in parallel. In one embodiment, currents induced in the fractional windings generate a balanced magnetic flux through the magnetic core.

FIG. 3is a schematic illustration of the transformer100ofFIG. 1. The schematic illustration shows a first core window212and a second core window214. The first layer102includes the first coil winding106. The first coil winding106includes one and one-half turns. One half-turn of the first coil winding106passes through the first core window212and another half-turn of the first coil winding106passes through the second core window214. The other half-turn of the first coil winding106also passes through the first core window212.

A tap terminal110is provided for first coil winding106. The black dot at one terminal or the other of each winding is called a phase or polarity mark. Currents entering the marked terminals create magnetic flux in the same direction in the core.

A positive voltage applied across a marked terminal of a winding will result in a positive voltage at the marked terminal of a magnetically coupled winding. If an unmarked terminal of a winding is connected to a marked terminal of a magnetically coupled winding, the two windings will be in phase and their ampere-turns will add. If they are connected in the opposite sense, their ampere-turns will cancel.

The first terminal104of the first coil winding106is electrically coupled to the seventeenth terminal166of the sixth coil winding168. This electrical coupling is achieved through via169(FIG. 1). The first layer102also includes the first fractional winding114. The first fractional winding114passes through the second core window214.

The second layer120includes the second coil winding122. The second coil winding122includes one full turn. One-half turn of the second coil winding122passes through the first core window212. The other one-half turn of the second coil winding122passes through the second core window214.

The third layer130includes the third coil winding132and the second fractional winding138. The third coil winding132includes one and one-half turns. One half-turn of the third coil winding132passes through the second core window214. Another half-turn of the third coil winding132passes through the first core window212and the other half-turn of the third coil winding132passes through the second core window214. The second fractional winding138passes through the first core window212.

The eighth terminal136of the third coil winding132is electrically coupled to the twelfth terminal148of the fourth coil winding144. This electrical coupling is achieved through via149(FIG. 1).

The fourth layer143includes the fourth coil winding144. The fourth layer143also includes the third fractional winding150. The fourth coil winding144includes one and one-half turns. One half-turn of the fourth coil winding144passes through the first core window212and another half-turn of the fourth coil winding144passes through the second core window214. The other half-turn of the fourth coil winding144also passes through the first core window212. The third fractional winding150passes through the second core window214.

The fifth layer156includes the fifth coil winding158. The fifth coil winding158includes one full turn. One-half turn of the fifth coil winding158passes through the first core window212. The other one-half turn of the fifth coil winding158passes through the second core window214.

The sixth layer164includes the sixth coil winding168. The sixth layer164also includes the fourth fractional winding176. The sixth coil winding168includes one and one-half turns. One half-turn of the sixth coil winding168passes through the second core window214. Another half-turn of the sixth coil winding168passes through the first core window212and the other half-turn of the sixth coil winding168passes through the second core window214. The fourth fractional winding176passes through the first core window212.

A terminal tap172is provided for sixth coil winding168. The first terminal104of the first coil winding106is electrically coupled to the seventeenth terminal166of the sixth coil winding168. This electrical coupling is achieved through via169.

In one embodiment, the sum of the ampere turn products from all of the coil windings in each core window212,214is substantially equal to zero. The following nomenclature will be used while referring toFIG. 3andFIG. 4. A current “IYYY” represents the current flow at a terminal “YYY”. A winding turn “TXXX” represents the winding turn “XXX” through a core window. For example, the sum of ampere-turn products of windings passing through the first window212with the transistor Q3(FIG. 4) in the on-state and the transistor Q4(FIG. 4) in the off-state can be expressed by the following:
−I1082T106−I110T106−I126T122−I142T138−I134T132−I1462T144+I162T158−I174T176−I170T168=0
and I108=I126=I142=I134=I172=0, since there is essentially no current flow through these terminals when Q3(FIG. 4) is in the on-state and Q4(FIG. 4) is in the off-state. Rearranging the previous equation yields the following:
I162T158=I110T106+2I146T144+I174T176+I170T168.

Since Txxxrepresents one winding pass through the first window212, we can set Txxxequal to 1, which yields:
I162=I110+2I146+I174+I170.

The sum of ampere-turn products of windings passing through the second window214with the transistor Q3(FIG. 4) in the on-state and the transistor Q4(FIG. 4) in the off-state can be expressed by the following:
−I108T106−I112T114−I126T122−I1342T132−I146T144−I152T150+I162T158−I174T168−I1702T168=0
and I108=I112=I126=I134=I172=0. Rearranging the previous equation yields the following:
I162T158=I146T144+I152T150+2I170T168.

Since Txxxrepresents one winding pass through the second window214, we can set Txxxequal to 1, which yields:
I162=I146+I152+2I170.

The current I162flowing through the first window212and the current I162flowing through the second window214must be equal. Thus,
I162(through window212)=I162(through window214)
and
I110+2I146+I174+I170=I146+I152+2I170
and rearranging the previous equation yields,
I110+I146+I174=I152+I170.

Since the current I174and the current I152both feed the voltage +(1.5*VLL), they are essentially equal in value. Additionally, since the current I170and the current I146both feed the voltage −(1.5*VLL), they are also essentially equal in value. This leads to the conclusion that I110must be equal to zero, since all ampere-turn products through each window212,214sum to zero, ignoring magnetizing current.

Thus, all ampere-turn products sum to zero except for I110. It should be noted that I110feeds the voltage +(0.5*VLL). However, the current I110is a small current compared with the current I162. In one embodiment, the value of the current I110is less than ten percent of the value of the current I162.

The sum of ampere-turn products of windings passing through the first window212with the transistor Q3(FIG. 4) in the off-state and the transistor Q4(FIG. 4) in the on-state can be expressed by the following:
+I1082T106+I110T106−I126T122+I142T138+I134T132+I1462T144+I162T158+I174T176+I170T168=0
and I110=I146=I162=I174=I170=0, since there is essentially no current flow through these terminals when Q3(FIG. 4) is in the off-state and Q4(FIG. 4) is in the on-state. Rearranging the previous equation yields the following:
I126T122=I1082T106+I142T138+I134T132.

Since Txxxrepresents one winding pass through the first window212, we can set Txxxequal to 1, which yields:
I126=2I108+I142+I134.

The sum of ampere-turn products of windings passing through the second window214with the transistor Q3(FIG. 4) in the off-state and the transistor Q4(FIG. 4) in the on-state can be expressed by the following:
+I108T106+I112T114−I126T122+I1342T132+I146T144+I152T150+I162T158+I172T168+I1702T168=0
and I146=I152=I162=I170=0. Rearranging the previous equation yields the following:
I126T122=I108T106+I112T114+I1342T132+I172T168.
Since Txxxrepresents one winding pass through the second window214, we can set Txxxequal to 1, which yields:
I126=I108+I112+2I134+I172.
The current I126flowing through the first window212and the current I126flowing through the second window214must be equal. Thus,
I126(through window212)=I126(through window214)
and
2I108+I142+I134=I108+I112+2I134+I172
and rearranging the previous equation yields,
I108+I142=I112+I134+I172.

Since the current I142and the current I112both feed the voltage +(1.5*VLL), they are essentially equal in value. Additionally, since the current I108and the current I134both feed the voltage −(1.5*VLL), they are also essentially equal in value. This leads to the conclusion that I172must be equal to zero, since all ampere-turn products through each window212,214sum to zero.

Thus, all ampere-turn products sum to zero except for I172. It should be noted that I172feeds the voltage +(0.5*VLL). However, the current I172is a small current compared with the current I126. In one embodiment, the value of the current I172is less than ten percent of the value of the current I126.

FIG. 4is a schematic illustration of a power supply circuit300including the transformer100ofFIG. 1. The transformer100includes two step-up autotransformer windings, two step-up isolation transformer windings, and two other step-up isolated transformer windings with tapped windings for a step down output.

The first terminal104, the eighth terminal136, the twelfth terminal148, and the seventeenth terminal166of the transformer100are coupled to ground302. The fourth terminal116, the fifth terminal124, the ninth terminal140, the fourteenth terminal154, the fifteenth terminal160, and the twentieth terminal178are all coupled to the voltage source VLL304.

The sixth terminal126of the transformer100is coupled to the drain terminal306of a transistor Q4(MOSFET)308. The source terminal310of the transistor Q4308is coupled to ground302.

The sixteenth terminal162of the transformer100is coupled to the drain312of a transistor Q3314. The source terminal316of the transistor Q3314is coupled to ground302.

In operation, during the first half of the cycle, the transistor Q4308is activated. A load connected to the output terminal322causes a current to flow through the second coil winding122as well as the first114and the second fractional windings138. The first114and the second fractional windings138are connected in a parallel configuration. By parallel configuration, we mean that the two windings, including their output diodes, are connected to common points at their beginning and end. By properly designing this parallel connection, the currents through the two windings will be substantially equal. This first segment of the autotransformer includes one and one-half turns thereby forming a step-up transformer. Thus, the output322is equivalent to +(1.5*VLL).

During the second half of the cycle, the transistor Q3314is activated and the transistor Q4308is deactivated. The load connected to the output terminal322causes a current to flow through the fifth coil winding158as well as the third150and the fourth fractional windings176. The third150and the fourth fractional windings176are connected in a parallel configuration. This second segment of the autotransformer includes one and one-half turns and is symmetrical to the first segment. The output322is again equivalent to +(1.5*VLL).

The transformer100also includes a first pair of isolation transformer windings144,132, and a second pair of isolation transformer windings168,106that are symmetric to the first pair. Each winding144,132,168,106includes one and one-half turns thereby forming step-up transformers. By properly configuring the phasing of the windings144,132,168,106(as indicating in theFIG. 4), the output320can be designed to be equivalent to −(1.5*VLL).

Additionally, the two step-up isolated transformer windings106and168include taps110and172, respectively. The tapped windings106,168each include one-half winding to create a step down transformer output324of +(0.5*VLL).

Other power supply configurations (not shown) can also be used including configurations using planar transformers having discrete primary and secondary windings.

Additionally, the foregoing description is intended to be merely illustrative of the present invention and should not be construed as limiting the appended claims to any particular embodiment or group of embodiments. Thus, while the present invention has been described with reference to exemplary embodiments, it should also be appreciated that numerous modifications and alternative embodiments may be devised by those having ordinary skill in the art without departing from the broader and intended spirit and scope of the present invention as set forth in the claims that follow. In addition, the section headings included herein are intended to facilitate a review but are not intended to limit the scope of the present invention. Accordingly, the specification and drawings are to be regarded in an illustrative manner and are not intended to limit the scope of the appended claims.

In interpreting the appended claims, it should be understood that:

d) several “means” may be represented by the same item or hardware or software implemented structure or function;

e) any of the disclosed elements may be comprised of hardware portions (e.g., including discrete and integrated electronic circuitry), software portions (e.g., computer programming), and any combination thereof;

f) hardware portions may be comprised of one or both of analog and digital portions;

h) no specific sequence of acts or steps is intended to be required unless specifically indicated.