Transformers and methods for constructing transformers

Transformers and methods of constructing transformers are disclosed. In one embodiment, a method of constructing a transformer includes wrapping a first primary winding around a core, wrapping a secondary winding around the core, and wrapping a second primary winding around the core. The first primary winding traverses substantially an entire circumference of the core in a first circumferential direction. The secondary winding includes a first half and a second half. The first half traverses substantially the entire circumference of the core in the first circumferential direction, and the second half traverses substantially the entire circumference of the core in a second circumferential direction opposite the first circumferential direction. The second primary winding traverses substantially the entire circumference of the core in the second circumferential direction.

FIELD

This disclosure generally relates to transformers and, more specifically, methods for constructing transformers having a relatively low leakage inductance.

BACKGROUND

Solar panels, also referred to herein as photovoltaic (PV) modules, generally output direct current (DC) electrical power. To properly couple such solar panels to an electrical grid, or otherwise provide alternating current (AC) power, the electrical power received from the solar panels is converted from DC to AC power. At least some known solar power systems use a single stage or a two-stage power converter to convert DC power to AC power. Some systems are controlled by a control system to maximize the power received from the solar panels and to convert the received DC power into AC power that complies with utility grid requirements.

However, at least some known solar power converters are relatively inefficient and/or unreliable. It is desirable for a solar power converter to operate at relatively high efficiency to capture as much energy from a PV module as possible. At least some solar power converters utilize an isolated DC/DC converter including a transformer. One of the loss factors in such converters is the energy loss associated with the leakage inductance of the converter's transformer. In some converters, the losses are proportional to the leakage inductance of the transformer. A greater leakage inductance leads to greater losses and, accordingly, to a lower total conversion efficiency. Some known designs attempt to recover the energy stored in the leakage inductance. These recovery mechanisms, however, are generally not satisfactory.

BRIEF SUMMARY

One aspect of the present disclosure is a method of constructing a transformer. The method includes wrapping a first primary winding around a core, wrapping a secondary winding around the core, and wrapping a second primary winding around the core. The first primary winding traverses substantially an entire circumference of the core in a first circumferential direction. The secondary winding includes a first half and a second half. The first half traverses substantially the entire circumference of the core in the first circumferential direction, and the second half traverses substantially the entire circumference of the core in a second circumferential direction opposite the first circumferential direction. The second primary winding traverses substantially the entire circumference of the core in the second circumferential direction.

Another aspect of the present disclosure is a transformer. The transformer includes a core having an outer circumference, a first primary winding including a plurality of turns wound on the core, a secondary winding including a first portion and a second portion, and a second primary winding including a plurality of turns wound on the core. The plurality of turns of the first primary winding traverse in a first circumferential direction around the core. The first portion of the secondary winding includes a plurality of turns traversing in the first circumferential direction around the core. The second portion of the secondary winding includes a plurality of turns traversing in the second circumferential direction around the core. The plurality of turns of the second primary winding traverse in the second circumferential direction around the core.

Yet another aspect of the present disclosure is a power conversion system. The system includes a power converter configured to convert an input power to an output power. The power converter includes a controller, at least one switch, and a transformer. The transformer includes a core, a first primary winding, a secondary winding, and a second primary winding. The first primary winding includes a plurality of turns wound on the core. The plurality of turns traverses in a first circumferential direction around the core. The secondary winding includes a first portion and a second portion. The first portion includes a plurality of turns traversing in the first circumferential direction around the core. The second portion includes a plurality of turns traversing in a second circumferential direction around the core. The second primary winding includes a plurality of turns wound on the core. The plurality of turns traverses in the second circumferential direction around the core.

DETAILED DESCRIPTION

The embodiments described herein generally relate to transformer. More specifically embodiments described herein relate to methods for constructing transformers having relatively low leakage inductance. Moreover, some embodiments described herein relate to transformers and methods of constructing transformers for power converters for use with a photovoltaic (PV) power source.

Although described herein with reference to power converters in general and for use with a PV source, the teachings of this disclosure may be utilized to construct relatively low leakage inductance transformers for any suitable use.

FIG. 1is a schematic block diagram of this power conversion system100. A power source102is coupled to power conversion system100to supply electrical current to system100. In this embodiment, power source102is a photovoltaic or “solar” array that includes at least one photovoltaic panel. Alternatively or additionally, power source102includes at least one fuel cell, a direct current (DC) generator, and/or any other electric power source that enables power conversion system100to function as described herein.

In this embodiment, power conversion system100includes a power converter104to convert DC power received from power source102, via an input capacitor105, to an alternating current (AC) output. In other embodiments, power converter104may output DC power. This power converter104is a two stage power converter including a first stage106and a second stage108. First stage106is a DC to DC power converter that receives a DC power input from power source102and outputs DC power to second stage108. Second stage108is a DC to AC power converter (sometimes referred to as an inverter) that converts DC power received from first stage106to an AC power output. In other embodiments, power converter104may include more or fewer stages. More particularly, in some embodiments power converter104includes only second stage108.

Power conversion system100also includes a filter110, and a control system112that controls the operation of first stage106and second stage108. An output114of power converter104is coupled to filter110. In this embodiment, filter110is coupled to an electrical distribution network116, such as a power grid of a utility company. Accordingly, power converter104may be referred to as a grid tied inverter. In other embodiments, power converter104may be coupled to any other suitable load.

During operation, power source102generates a substantially direct current (DC), and a DC voltage is generated across input capacitor105. The DC voltage and current are supplied to power converter104. In this embodiment, control system112controls first stage106to convert the DC voltage and current to a substantially rectified DC voltage and current. The DC voltage and current output by first stage106may have different characteristics than the DC voltage and current received by first stage106. For example, the magnitude of the voltage and/or current may be different. Moreover, in this embodiment, first stage106is an isolated converter, which operates, among other things, to isolate power source102from the remainder of power conversion system100and electrical distribution network116. More specifically, in this embodiment, first stage106is a flyback converter. The DC voltage and current output by first stage106are input to second stage108, and control system112controls second stage108to produce AC voltage and current, and to adjust a frequency, a phase, an amplitude, and/or any other characteristic of the AC voltage and current to match the electrical distribution network116characteristics. The adjusted AC voltage and current are transmitted to filter110for removing one or more undesired characteristics from the AC voltage and current, such as undesired frequency components and/or undesired voltage and/or current ripples. The filtered AC voltage and current are then supplied to electrical distribution network116.

FIG. 2is a simplified schematic diagram of this converter200for use as first stage106. Converter200is an isolated converter. More specifically, converter200is a flyback converter. Converter200is operable to receive DC power at an input202and output DC power at an output204. In this embodiment, converter200is operated by control system112to output DC power to electrical second stage108. Generally, the peak output voltage of converter200is greater than the input voltage to converter200.

Converter200includes a transformer206having a first primary winding208, a second primary winding210, and a secondary winding212. Primary windings208and210are magnetically coupled to, but electrically isolated from, secondary winding212. Primary windings208and210are connected to input202in parallel with each other. First primary winding208and second primary winding210are coupled to a switch214. In this embodiment, switch214is a MOSFET. In other embodiments, switch214may be any other suitable switch.

Converter200is generally operated as a flyback converter as known in the art. In general, switch214is switched on and off to store and release energy in transformer206. More specifically, when switch214is closed (also referred to as switched on), current flows through first and second primary windings208and210and energy is stored in the core (not shown inFIG. 2) of transformer206. When switch214is opened (also referred to as switched off), current ceases flowing through first and second primary windings208and210. Current flow is induced in secondary winding212, releasing the energy stored in the core of transformer206to the output204. The output of transformer206, and more specifically the output of secondary winding212, is rectified by a diode218. Thus, DC output power is provided at output204of converter200.

FIG. 3is a schematic diagram of this embodiment of transformer206for use in converter200. Each primary winding208and210includes three windings300connected in parallel. As will be described in more detail below, each primary winding208and210includes three conductors, one for each winding300, connected together in parallel.

In this embodiment, transformer206is a low leakage inductance transformer. As described above, losses in an isolated converter, such as converter200, are proportional to the leakage inductance of the transformer. Accordingly, by reducing the inductance of transformer206over some known designs, the losses in converter200may be reduced.

In one embodiment, a method of constructing a transformer, such as transformer206, includes wrapping a first primary winding around a toroidal core. The first primary winding traverses substantially the entire circumference of the toroidal core in a first circumferential direction. A secondary winding is wrapped around the toroidal core. The secondary winding includes a first half and a second half. The first half of the secondary winding traverses substantially the entire circumference of the toroidal core in the first circumferential direction and the second half of the secondary winding traverses substantially the entire circumference of the toroidal core in a second circumferential direction. The second circumferential direction is opposite the first circumferential direction. The method includes wrapping a second primary winding around the toroidal core. The second primary winding traverses substantially the entire circumference of the toroidal core in the second circumferential direction.

With reference toFIGS. 4-8, construction of an embodiment of transformer206according to the method will be described. The transformer206includes a first primary winding208and a second primary winding210each having twenty-one turns, and a secondary winding having one hundred and sixty turns. The method described above and transformer206are not limited, however, to a transformer as described hereinafter. In other embodiments, transformer206may have a different numbers of turns, a different turns ratios, different numbers of windings, and/or may be constructed of different materials.

FIGS. 4-9illustrate construction of an embodiment of transformer206that may be used in a power converter, such as power converter200. Transformer206includes a magnetic core400visible inFIGS. 4 and 5. In this embodiment, core400is a toroidal core. In other embodiments, core400may have any other suitable shape. The core400is a distributed air gap toroidal core made from powder core material. The powder core material may be any suitable magnetic powder core material or combination of materials including, for example, nickel, iron, molybdenum. Moreover, in other embodiments, core400may be made of any other suitable materials and/or constructions including for example, iron, laminated silicon steel, carbonyl iron, ferrite, tape wound, etc.

InFIG. 4, first primary winding208has been wound onto core400. In this embodiment, three conductors402,404, and406are wrapped around core400in a counterclockwise direction408. Each one of conductors402-406will become one winding300of first primary winding208. In this embodiment, conductors402-406are No. 22 AWG magnet wire. In other embodiments, any other suitable conductive material may be used. Conductors402-406are wrapped around core400twenty-one times beginning at point410to form a twenty-one turn primary winding208. Each turn of conductors402-406is spaced apart from each other turn such that the completed twenty-one turns traverse substantially the entire circumference of core400to form first primary winding208.

Secondary winding212is wound on core400inFIGS. 5-8. A single conductor500is wound around core400beginning at point502to form secondary winding212. Point502is located approximately 180 degrees around the circumference of core400from point410at which first primary winding208began and ended. In this embodiment, conductor500is 0.4 millimeter (mm) triple insulated wire (TIW). In other embodiments, any other suitable conductive material may be used. Conductor500is wrapped around core400forty times beginning at point502and ending at point410inFIG. 5. An additional forty turns are wrapped around core400ending substantially at point502inFIG. 6. InFIG. 6, conductor500has been wrapped a total of eighty turns around core400traversing substantially the entire circumference of core400in counterclockwise direction408beginning and ending at point502. These first eighty turns form a first portion of secondary winding212.

A second portion of secondary winding212, e.g., the remaining eighty turns, is wound on core400in a clockwise direction700inFIGS. 7 and 8using the same conductor500. In the embodiment shown inFIG. 7, approximately forty turns of the second portion of secondary winding212have been wound around core400beginning point502and ending at point410. An additional forty turns are wrapped around core400in clockwise direction700ending substantially at point502inFIG. 8. In the embodiment ofFIG. 8, conductor500has been wrapped eighty turns around core400in counterclockwise direction408and eighty turns around core400in clockwise direction700.

InFIG. 9, second primary winding210has been wound onto core400. In this embodiment, three conductors900,902, and904are wrapped around core400in clockwise direction700. Each one of conductors900-904will become one winding300of second primary winding210. In this embodiment, conductors900-904are No. 22 AWG magnet wire. In other embodiments, any other suitable conductive material may be used. In this embodiment, conductors900-904are wrapped around core400twenty-one times beginning at point410to form a twenty-one turn primary winding210. Each turn of conductors900-904is spaced apart from each other turn such that the completed twenty-one turns traverse substantially the entire circumference of core400to form second primary winding210.

To complete transformer206as schematically shown inFIG. 3, first primary winding208and second primary winding210are connected together in parallel. More specifically, one end of each of conductors402-406is connected to the corresponding end of conductors900-904. Because there is a single secondary winding212, conductor500does not need to be connected to anything to complete construction of transformer206. If desired, the ends of conductor500may be cut to a desired length, insulation may be removed from the cut ends, and the ends may be tinned with solder to prepare transformer206for installation in a circuit. Similarly, the ends of conductors402-406and900-904may be cut to length, stripped and tinned with solder. In some embodiments, additional windings may be added for low power auxiliary circuits, sensing, etc.

When completed, transformer206includes a first primary winding208and a second primary winding210, each of which includes three windings, having a same number of turns. Moreover, first primary winding208and secondary winding210are wound in opposite directions around core400. Similarly, secondary winding212includes a first portion and a second portion, having a same number of turns. The first portion and the second portion are wound in opposite directions around core400. As will be explained in more detail below, these winding techniques may produce transformers having improved characteristics, including reduced leakage inductance, over some known transformers.

A prototype transformer was built as described herein. Specifically, the prototype included a powdered material toroidal core. The primary windings were wound from No. 22 AWG magnet wire, and the secondary winding was wound with 0.4 mm TIW in the manner shown inFIGS. 4-9. The two primary windings were connected in parallel, as also described above. The resulting transformer exhibited a primary magnetizing inductance (Lm) of 26.1 microhenries (uH), a primary self resonant frequency of 471 kilohertz (kHz), a leakage inductance (Le) of 0.112 uH, and a DC resistance of 0.010 ohms. The secondary winding of the prototype transformer had a DC resistance of 0.872 ohms. The turns ratio of the prototype was 7.62 and the leakage ratio, i.e. Le/Lm, was 0.0043.

A second transformer was constructed using techniques other than those described herein. In the second transformer, the first primary windings were wound around the core, the secondary winding was wound over the first primary winding, and the second primary windings was wound over the secondary winding. All of the windings were wound with the same direction around the circumference of the toroidal core, e.g., all clockwise or all counterclockwise. Furthermore, the second transformer did not match the number of turns on the first and second primary windings that were then paralleled. The second transformer had a leakage ratio three times the leakage ratio of the prototype transformer constructed using the methods described herein.

A third transformer was constructed exactly the same as the second transformer, but matching the number of turns in the first and second primary windings. The primary windings and the secondary winding were all wound in the same direction around the circumference of the toroidal core, e.g., all clockwise or all counterclockwise. The third transformer had a leakage ratio two times the leakage ratio of the prototype transformer constructed using the methods described herein.

The reduced leakage inductance, and hence the reduced leakage ratio, of the prototype transformer constructed according to an embodiment of this disclosure as compared to, for example, the third prototype may be explained with reference toFIG. 10.FIG. 10shows a toroidal core1000with a single conductor1002wrapped in fourteen turns around core1000in a counterclockwise direction. Each turn of conductor1002forms a loop around core1000. As will be understood by one skilled in the art, current flowing through conductor1002generates a magnetic flux through core1000. However, conductor1002also describes a turn around the circumference of core1000, indicated by path1004, sometimes referred to as a turn in air or air turn. Because of this turn in air, current through conductor1002also creates a magnetic flux that is not coupled to core1000. This magnetic flux coupled through the air results in an increased leakage inductance. By winding first primary winding208and second primary winding210in opposite directions around the circumference of core400according to this disclosure, the turn in air from each winding208and210is canceled out, resulting in a zero net turn in air. Similarly, by winding a first portion of secondary winding212in a first direction around the core400and the second portion of the secondary winding212in a second direction, the turn in air of each portion is canceled out by the other, also resulting in a zero net turn in air. The leakage inductance may be further reduced by matching the number of turns in the opposite direction of the windings, e.g. matching the number of turns of first primary winding208and second primary winding210.

Transformers constructed in accordance with this disclosure have a lower leakage inductance, and when such transformers are used in power converters, the lower leakage inductance results in reduced losses and higher efficiency.