TRANSFORMER PROVIDING LOW OUTPUT VOLTAGE

Abstract: A transformer providing low output voltage. A transformer core has two outer leg portions and a center portion. A primary winding has a first portion looped around one of the legs so that a current passed through the first winding will produce a magnetic flux in that leg that circulates in either the right hand or left hand sense. A second portion of the input signal winding is looped around the other leg in the opposite sense. This provides for a magnetic flux circulating through the two outer leg portions in the same sense, and provides that the magnetic flux circulating through the center portion is zero. The secondary winding is preferably provided as a fractional loop around one of the outer legs.

Description: TRANSFORMER PROVIDING LOW OUTPUT VOLTAGE Background of the invention The present invention relates to a planar transformer providing low output voltage, particularly a planar transformer for use in DC-DC power converters. As microprocessors and other integrated semiconductor devices become denser, it is desirable to lower their supply voltage requirements. Accordingly, supply voltages for digital electronics have been reduced from 5 volts to 3.3 volts, then to 2.5 volts, and are now being reduced to 1.6 volts. The trend is expected to continue. A power converter for supplying the integrated circuits typically employs a transformer to accept an input at a relatively high voltage and produce a lower output voltage. As is well known, standard transformers typically employ a ferrite core around which wires forming primary (input) and secondary (output) circuits are wound. These standard transformers are notoriously expensive and bulky however, and alternative transformer embodiments have been used in computer power supplies to lower cost and decrease size. One such embodiment is the "planar" transformer, wherein the wires are replaced by traces in one or more layers of a circuit board. The prior art transformer has a limitation in the output voltage that it is capable of producing. Particularly, magnetic technology has typically been limited to the output voltage that is produced by one turn of the secondary. To address this limitation, fractional turns have been employed. Referring to Figure 1 as an example, a core 2 is shown having a center leg 3 around which a primary winding 4 is looped. The core 2 has two secondary "legs" 6a and 6b, and a secondary winding 8 is looped around one of the legs 6a one half- turn. A problem with this transformer is that magnetic flux circulating from the center leg through the other leg 6b leads to an undesirable leakage inductance. To address this problem, referring to Figure 2, a half-turn of the secondary 8 is looped around the leg 6a and a half-turn is looped around the leg 6b. The two loops contribute to the total output voltage in parallel, and all of the flux in the core links the secondary. However, a problem remains in that the two legs 6a and 6b are not identical, so that the magnetic flux through the respective half-turns is not identical. In response, an additional circulating current flows in the secondary in order to balance the magnetic flux, leading to additional ohmic power loss. Another problem with the prior art as shown in Figure 2 is that the secondary 8 winding is relatively long compared to the secondary winding shown in Figure 1. This also increases ohmic loss in the transformer, and in addition increases stray inductance. Accordingly, there is a need for a transformer providing low output voltage that provides for converting substantially all the magnetic flux circulating in the core of a transformer into an output current, particularly by decreasing ohmic loss and stray inductance. Summary of the Invention The transformer providing low output voltage of the present invention solves the aforementioned problems and meets the aforementioned needs by providing a magnetic core having at least two apertures defining a center portion between the apertures and two outer leg portions. The core has primary and secondary windings. The primary winding receives a first voltage or current and induces a second voltage or current in the secondary winding. The input power is provided at a higher voltage than the output power, the latter which is preferably less than or substantially equal to 3.3 volts. The primary winding has a first portion looped around one of the leg portions so that a current passed through the first winding will produce a magnetic flux in that leg portion that circulates in either the right hand or left hand sense. A second portion of the primary winding is looped around the other leg portion in the opposite sense. This provides for a magnetic flux circulating through the two outer leg portions in the same sense, and provides that the magnetic flux circulating through the center portion is zero. The secondary winding is preferably provided as a fractional loop around one of the outer leg portions. Therefore, it is a principal object of the present invention to provide a novel and improved transformer providing low output voltage. It is another object of the present invention to provide a transformer providing low output voltage that provides for converting substantially all the magnetic flux circulating in the core of a transformer into an output current. It is yet another object of the present invention to provide a transformer providing low output voltage that provides for high efficiency. It is still another object of the invention to provide a transformer providing low output voltage that provides for minimal leakage inductance. It is a further object of the present invention to provide a transformer providing low output voltage that provides for minimal ohmic loss. It is still a further object of the present invention to provide such a transformer at lower cost. The foregoing and other objects, features and advantages of the present invention will be more readily understood upon consideration of the following detailed description of the invention, taken in conjunction with the following drawings. Brief Description of the Drawings 1 - 10 Figure 1 is a pictorial schematic of a prior art planar transformer shown in horizontal cross- section. Figure 2 is a pictorial schematic of another prior art planar transformer shown in horizontal cross-section. Figure 3 is a pictorial schematic of a prior art transformer shown in side elevation. Figure 4 is a schematic of a circuit equivalent to the prior art transformer of Figure 1. Figure 5A is a pictorial schematic of a transformer providing a low output voltage according to the present invention shown in side elevation. Figure 5B is a pictorial schematic of the transformer of Figure 5A with the center portion removed. Figure 6 is a pictorial schematic of a center tap embodiment of a transformer providing a low output voltage according to the present invention shown in horizontal cross- section. Figure 7 is a pictorial schematic of the center tap embodiment of a transformer providing a low output voltage of Figure 6 in a power converter circuit employing an integrated choke. Figure 8 is a pictorial schematic of a multiple core embodiment of the transformer of Figure 5. Figure 9 is a pictorial schematic of a multiple core embodiment of the transformer of Figure 6. Figure 10 is a pictorial schematic of a multiple core embodiment of the transformer of Figure 7. Detailed Description of a Preferred Embodiment As discussed above, Figures 1 and 2 illustrate prior art transformers providing low output voltage as a result of employing fractional turns or loops in the secondary winding. Shown in Figures 1 and 2 are half-turns; however, other fractions of turns may be employed. Turning to Figure 3, the principal of their operation is clarified to further a comparison with the present invention. A core 2 has a center portion 4 and two leg portions 6a and 6b. A primary winding 8 is coupled to a source of voltage or current P in . The primary winding is looped around the leg portions so that current i in flowing through the winding produces a magnetic flux "B," in the leg portion 6a and "B 2 " in the leg portion 6b. The flux "Bι" circulates in one of the right hand or left hand sense, depending on the direction of the current i in , and the flux "B 2 " circulates in the other sense, so that two independent paths of magnetic flux are operative. Twice the magnetic flux that is present in each leg portion 6 circulates through the center portion 4. In an embodiment of the transformer of Figure 3 that corresponds to that of Figure 1, one secondary winding 9a is looped around one of the leg portions 6a. Similarly, in an embodiment of the transformer of Figure 3 corresponding to that of Figure 2, two secondary windings 9a and 9b are looped, respectively, around the leg portions 6a and 6b. In the embodiment corresponding to Figure 1, the winding 9a encircles all of the magnetic flux "B ' but none of the magnetic flux "B 2 ." The magnetic flux "B " is therefore transformed or converted to current flow in the winding 9a wherein the current flows in the winding 9a so as to cancel the magnetic flux "B, " However, due to the lack of a winding 9b, the magnetic flux "B 2 " is not converted to current flow, so that the magnetic flux "B 2 " is not canceled and remains in the core, leading to leakage inductance. Figure 4 shows the equivalent circuit of the embodiment of Figure 1, showing the leakage inductance "L." Alternatively, in the embodiment of the transformer of Figure 3 that corresponds to that of Figure 2, all of the magnetic flux is transformed or converted to current flow only if perfect symmetry is achieved in the windings 9. Since this is not possible, there remains an uncanceled magnetic flux and consequently a remaining-leakage inductance. In addition, connecting the windings 9a and 9b in the manner of the winding 8 in Figure 2 ensures that there will be an increased ohmic loss as well as increased stray inductance. Turning now to Figure 5A, a simplified embodiment of a transformer 10 providing low output voltage according to the present invention is shown to illustrate an outstanding principle of the invention. A core 12 has a center portion 14 and two leg portions 16a and 16b. A primary winding 18 is coupled to a source of voltage or current P in . The primary winding is looped around the leg portions so that current i in flowing through the winding produces a magnetic flux "B" in each leg portion that circulates in one of either the right hand or left hand sense as shown by the arrows. Because of the novel arrangement of the primary winding 18, no magnetic flux circulates through the center portion 14. Particularly, the primary winding is not wound around the center portion 14 as in the prior art, but is instead wound around the leg portions 16a and 16b. A secondary winding 19 may be looped around either or, preferably, both of the leg portions 16. The single turn encloses all of the flux B without the need for creating perfect symmetry in two separate windings. Accordingly, the transformer may be provided with higher efficiency at lower cost, and has a minimal or zero leakage inductance. Turning to Figure 5B, this is particularly so where the center portion 14 has been removed from the core 12. While the center portion may be employed for other purposes, such as described below and such as described in the present inventor's companion application entitled METHOD AND APPARATUS FOR TRANSMITTING A SIGNAL THROUGH A POWER MAGNETIC STRUCTURE, executed on even date herewith and incorporated by reference herein in its entirety, its removal prevents any remaining asymmetry in magnetic flux through the leg portions to lead to leakage inductance by virtue of magnetic flux circulating through the center portion. Referring to Figure 6 a "center-tap" embodiment of the invention is shown. The secondary winding 19 forms a figure 8 pattern that results in looping a fractional turn around the leg portion 16a in one of the right or left hand sense, and continues so as to loop a full turn around the other leg portion 16b in the opposite sense. A node 20 lies on the winding 19 forming the center tap with respect to ends B and C. Figure 7 shows the embodiment of Figure 6 configured as a power converter with an integrated output filtering choke 22 employing the center portion 14 of the core 12. Turning to Figures 8 - 10, multiple core embodiments of the transformers (and circuits) of Figures 5 - 8, respectively, are shown according to the present invention. The multiple core embodiments are based on the principle that, where there are N cores 2 looped by the primary 18, the voltage induced in the secondary 19 is reduced by a factor of 1 /N. For example, employing 3 cores 12a - 12c as shown in Figure 8, each with half-turn secondary loops 19a - 19c, provides the same output voltage Vout as would a single core transformer employing a one-sixth-turn secondary. Similarly, Figure 9 shows three cores 12a - 12c having respective center taps Aa, Ab and Ac, with respect to respective outputs Ba, Ca; Bb, Cb, and Be, Cc. In Figure 10, a respective integrated output filtering chokes 22a - 22c provide outputs Vout(a) - Vout(c), which may be connected in parallel to provided a single output voltage. Figure 8 also shows the use of a secondary winding 19 that is looped around two of the leg portions, as mentioned above. It is to be recognized that, while a particular transformer providing low output voltage has been shown and described as preferred, other configurations and methods could be utilized, in addition to those already mentioned, without departing from the principles of the invention. The terms and expressions which have been employed in the foregoing specification are used therein as terms of description and not of limitation, and there is no intention of the use of such terms and expressions of excluding equivalents of the features shown and described or portions thereof, it being recognized that the scope of the invention is defined and limited only by the claims which follow. Now following a part of the description of the inventor's companion patent application entitled METHOD AND APPARATUS ' FOR TRANSMITTING A SIGNAL THROUGH A POWER MAGNETIC STRUCTURE to which herebefore was referred on page 6. In power circuitry, it is often required to isolate control signals as well as to isolate the output power from the input power. The power transformer that is found in many power circuits accomplishes the latter objective. However, isolating the control signals has generally required an additional isolation device, such as an optocoupler or a signal transformer, adding undesirable expense and complexity to the power circuit. Accordingly, there is a need for a method and apparatus for transmitting a signal through a power magnetic structure that provides for isolating control or other signals in power magnetic circuitry without the expense and complexity of employing an additional isolation device. Brief Description of the Drawings 101 - 107 Figure 101 is a pictorial view of a prior art power transformer. Figure 102 is a pictorial schematic of the transformer of Figure 101 showing the direction of magnetic lines of flux induced by current flowing through the primary of the transformer. Figure 103 is a pictorial schematic of the transformer of Figure 101 showing two input signal windings according to the present invention. Figure 104 is a pictorial schematic of the assembly of Figure 103 showing the lines of flux of Figure 102. Figure 105 is a pictorial schematic of the transformer of Figure 101 or Figure 103 showing two output signal windings according to the present invention. Figure 106 is a pictorial schematic of an exemplary circuit employing a method and apparatus for transmitting a signal through a power magnetic structure according to the present invention. Figure 107 is a pictorial block diagram of yet another exemplary circuit employing a method and apparatus for transmitting a signal through a power magnetic structure according to the present invention. Detailed Description of the drawings 101 - 107 Referring to Figure 101, an apparatus 1 10 for transmitting a signal through a power magnetic structure according to the present invention. The power magnetic structure is typically a transformer having a core 113 comprising a center portion 114 and two leg portions 1 16a and 1 16b. The core 1 13 is formed of a material that is magnetically permeable relative to the air, such as ferrite. To transmit power, a primary winding 1 18 is looped around the center portion 1 14 of the core, and a secondary winding 120 is also looped around the center portion of the core, or around either or both of the leg portions 1 16 as is standard practice in the art. A "loop" for purposes herein may encircle the center portion or leg portion once, more than once, or less than once. For example, for low output voltage applications, a loop may be encircle the center portion or leg portion by a fractional turn. Referring to Figure 102, a result of looping the primary winding around the center portion of the core 1 13 is that lines of magnetic flux 121 circulate through one of the leg portions 1 16a in either the right hand or left hand sense (either clockwise or counterclockwise as seen in Figure 102 depending on the polarity of the current "i" flowing through the primary), and circulates through the other leg portion 1 16b in the opposite sense. For purposes herein, a right hand or left hand "sense" in connection with either a winding around or a flux path may be consistently defined as being in accordance with the well known right hand rule. According to the invention, the core 1 13 may also be employed for transmitting a signal wherein it is desired to isolate the output of the signal from the input of the signal. This provides the outstanding advantage of eliminating the expense and circuit complexity of providing a separate optocoupler, transformer or other isolation device. For this purpose, two additional windings are provided. Referring to Figure 103 illustrating two portions of an input signal winding, a first input signal winding portion 124 is looped around one of the leg portions 1 16a so that an input signal current "i in " passed through the first winding portion 124 will produce a magnetic flux 121a in the leg portion 1 16a that circulates in either the right or left hand sense, depending on the polarity of the input signal current. The input signal current passes through the first winding portion from a first terminal 124a of the winding portion to a second terminal 124b of the winding portion. A second input signal winding portion 126 is looped around the other leg portion 1 16b and has a first terminal 126a and a second terminal 126b. The winding portion 126 is looped around the leg portion 1 16b and the terminals 126a, 126b are connected to the terminals 124a, 124b so that the input signal current "i in " passed through the second winding portion 126 will produce a magnetic flux 121 b in the leg portion 1 16b that circulates in the same sense as the magnetic flux produced by the first winding portion. The magnetic flux 121 a is therefore the same as the magnetic flux 121 b. Turning to Figure 104, according to the invention a pair of output signal winding portions 128, 130 are provided that are electrically isolated from the input winding portions 124, 126. The winding portions 128 and 130 are looped analogously to the winding portions 124 and 126. Thence, the first output signal winding portion 128 is looped around one of the leg portions so that an output current "i out " will flow through the winding portion 128 as a result of the magnetic flux 121 a in the leg portion 1 16a. The output signal current passes through the first output signal winding portion 128 from a first terminal 128a of the winding portion to a second terminal 128b of the winding portion. A second output signal winding portion 130 is looped around the other leg portion 1 16b and has a first terminal 130a and a second terminal 130b. The winding portion 130 is looped around the leg portion 1 16b and the terminals 130a, 130b are connected to the terminals 128a, 128b so that the same output signal current "i out " will flow through the second output signal winding portion 130 as a result of the magnetic flux 121 b in the leg portion 1 16b. Referring to Figure 105, lines of magnetic flux 121 induced by power current "i power " flowing through the primary winding 1 18 links the winding portions 128 and 130 inducing equal and opposite currents "induce " ar| d "in du ce d " therein, so that the voltage "V" across terminals 128b and 130b that is induced by power transmission will be zero. Accordingly, the output signal current is unaffected by the power input. Similarly, currents produced in the input winding portions 124 and 126 by flux linkage with the primary are also cancelled. Turning now to Figure 106, an exemplary circuit employing the apparatus 1 10 as aforedescribed is shown. The circuit is part of a DC-DC converter wherein a switching signal V s is provided as input to a switching circuit 129. It is desired to isolate V s from the input control signal V,, which in this example is derived by differentiating a square wave voltage source V. Shown are the primary winding 1 18, the input signal winding portions 124 and 126, and the output signal winding portions 128 and 130. Referring to Figure 107, an input signal transmitted through the appararatus 1 10 may be modulated and the output signal demodulated to provide additional signal integrity. As mentioned, an outstanding advantage provided by the invention is the capability of electrically isolating an output signal from an input signal by utilizing an existing power transformer. Moreover, as has been shown, the invention provides for magnetically isolating the input and output signals from the input and output power that is transmitted by the transformer as well, so that the signals and the power can be transmitted without affecting each other. Further, the primary winding may be used to transmit one of the signal input or output currents with a secondary winding (which has not been shown) being used to transmit the other signal current, while the signal input and output windings can be used as primary and secondary power train windings. Although the preferred embodiment of the invention employs the core 1 13 for transmitting a signal through the input and output signal windings, these may be employed as another power train (primary and secondary) for transmitting power independently of the main power train as well. In this regard, it may be noted that the magnetic flux through the leg portions of the core 1 13 is the superposition of the magnetic flux due to the primary and that due to the input signal winding. However, the total magnetic flux resulting from this superposition at any instant leads to magnetic core loss and, therefore, increased heat dissipation in the transformer which is generally undesirable. Where the input signal winding transmits is used to transmit relatively low power compared to the primary winding, the additional core loss may be negligible. On the other hand, where it is desired to transmit appreciable power through the input signal winding, this additional loss may be unacceptable. Then, the frequency and phase of either or both the currents flowing through the primary and input signal windings may be adjusted to minimize this loss as will now be readily apparent to those having ordinary skill. It is to be recognized that, while a particular method and apparatus for transmitting a signal through a power magnetic structure has been shown and described as preferred, other configurations and methods could be utilized, in addition to those already mentioned, without departing from the principles of the invention. The terms and expressions which have been employed in the foregoing specification are used therein as terms of description and not of limitation, and there is no intention of the use of such terms and expressions of excluding equivalents of the features shown and described or portions thereof, it being recognized that the scope of the invention is defined and limited only by the claims which follow.

Claims: Claims: 1. A transformer for providing a low output voltage, comprising: - a magnetic core having at least two apertures defining a center portion therebetween and two leg portions; - a primary winding having a first portion looped around one of said leg portions in either the right or left hand sense, and a second portion looped around the other leg portion in the opposite sense forming a power input; and - a secondary winding having a first portion looped around one of said leg portions, said secondary winding having two ends forming a first power output. 2. The transformer of claim 1 , wherein said secondary winding has said first portion looped a fractional turn. 3. The transformer of claim 1 , wherein said first portion of said secondary winding is looped around one of the leg portions in either the right or left hand sense and around the remaining leg portion in the opposite sense, wherein a first node disposed on said secondary winding between said two ends forms a center tap with respect thereto. 4. The transformer of claim 3, further comprising a third winding looped around said center portion, said third winding having two ends, wherein said two ends of said secondary winding are coupled together to form a second node, wherein one of said ends of said third winding is coupled to said second node, and wherein the other end of said third winding forms a second power output with respect to said first node. 5. The transformer of claim 4, wherein said third winding is looped round said center portion a fractional turn. 6. A method for providing a low output voltage, comprising the steps of: - providing a magnetic core having at least two apertures defining a center portion therebetween and two leg portions; - providing a first portion of a primary winding as being looped around one of said leg portions in one of the right or left hand sense; - providing a second portion of said primary winding as being looped around the other leg portion in the opposite sense; - applying a power input to said primary winding; - providing a secondary winding having two ends; - providing a first portion of said secondary winding as being looped around one or both of said leg portions; and - thereby providing a power output across said two ends of said secondary winding. 7. The method of claim 6, wherein said step of providing said first portion of said secondary winding as being looped around one of said leg portions provides said first portion of said secondary winding as being looped around said one of said leg portions a fractional turn. 8. The method of claim 6, further comprising providing said power input to be substantially greater than 3.3 volts, and providing said power output to be less than or substantially equal to 3.3 volts. 9. The method of claim 6, further comprising providing said first portion of said secondary winding to be looped around one of the leg portions in either the right or left hand sense and around the remaining leg portion in the opposite sense, and forming a center tap along said secondary winding between said two ends with respect thereto. 10. The method of claim 9, further comprising providing a third winding having two ends and being looped around said center portion, coupling said two ends of said secondary winding together to form a second node, coupling one of said ends of said third winding to said second node, and forming a second power output from the other end of said third winding, along with said first node. 1 1.The method of claim 9, further comprising providing a third winding having two ends and being looped around said center portion, coupling said two ends of said secondary winding together to form a second node, coupling one of said ends of said third winding to said second node, and taking a second power output with respect to said first node at the other end of said third winding. 12. The method of claim 6, further comprising providing at least one additional magnetic core having at least two apertures defining a center portion therebetween and two leg portions, providing said first portion of said primary winding as being looped around one of said leg portions of said at least one additional magnetic core in said one of the right or left hand sense, and providing said second portion of said primary winding as being looped around the other leg portion of said at least one additional magnetic core in the opposite sense. 13. The method of claim 12, further comprising providing said first portion of said secondary winding as being looped around said one or both of said leg portions in either the right hand or left hand sense, and providing a first portion of another secondary winding as being looped around one of said leg portions of said at least one additional magnetic core in the same said sense as said first portion of said secondary winding, said other secondary winding having two ends coupled in parallel to said two ends of said secondary winding.

Inventors: JITARU IONEL DAN, JITARU, IONEL, DAN
Applicants: ASCOM ENERGY SYSTEMS AG, JITARU IONEL DAN, JITARU, IONEL, DAN
Application year: 2001
Main classification: H01F30
IPC codes: H01F30