Abstract:
A planar transformer arrangement and method provide isolation between an input signal and an output signal. The planar transformer arrangement includes a planar medium having a first layer, a second layer, and a dielectric interlayer arranged between the first and second layers; at least one meandering primary winding arranged on the first layer of the planar medium, a current flow being induced within the primary winding in accordance with the input signal; at least one meandering secondary winding arranged on the second layer of the planar medium, the primary and secondary windings forming a planar transformer, whereby a voltage is induced across the secondary winding in accordance with the current flow within the primary winding; and a mode elimination arrangement configured to produce a compensated voltage by compensating for a common mode interference on the voltage induced across the secondary winding, the mode elimination arrangement being further configured to generate the output signal in accordance with the compensated voltage; wherein the dielectric interlayer of the planar medium provides a voltage isolation between the primary and secondary windings.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   The present application is a divisional of U.S. patent application Ser. No. 10/452,679 filed May 30, 2003, entitled PLANAR TRANSFORMER ARRANGEMENT and is based on and claims the benefit of U.S. Provisional Application No. 60/384,724, filed on May 31, 2002, entitled “PLANAR TRANSFORMER AND DIFFERENTIAL STRUCTURE,” and U.S. Provisional Application No. 60/420,914, filed on Oct. 23, 2002, entitled “SWITCHING VOLTAGE REGULATOR FOR SWITCH MODE POWER SUPPLY WITH PLANAR TRANSFORMER,” the entire contents of these applications being expressly incorporated herein by reference. 

   FIELD OF THE INVENTION 
   The present invention relates to a planar transformer arrangement and method for isolating driver circuitry and communication circuitry to eliminate magnetic field interference and parasitic capacitance. 
   BACKGROUND INFORMATION 
   Transformers are often used in floating gate driver circuits for driving high power/voltage switches, for example, high voltage IGBTs for motor control and other applications. In such an application, a transformer provides isolation between low voltage driver circuitry and high voltage power switch circuitry. Such transformers may also be employed to communicate data signals between electrically isolated circuits (e.g., to communicate signals via a transceiver). 
   Traditionally, high-voltage isolation has required the use of bulky transformers. However, such transformers may be costly, cumbersome, and all transformers may be negatively affected by unwanted common-mode noise, such as noise generated by parasitic capacitances and/or an external magnetic field. 
   Conventional transformers inherently exhibit two kinds of parasitic capacitances: distributed parasitic capacitances between adjacent windings on a transformer; and interwinding parasitic capacitances between primary and secondary windings of the transformer. These parasitic capacitances result from the close proximity between transformer windings. The magnetic core is generally arranged between the primary and secondary windings of the transformer, so that the magnetic field generated by the transformer may be better conducted. However, operation of the transformer may induce the flow of disadvantageous currents within the magnetic core, if the core, for example, contacts the transformer windings. These currents may result in a degradation of the galvanic insulation between primary and secondary windings. 
   Furthermore, an externally applied magnetic field may result in disadvantageous common mode magnetic interference within conventional transformers. Such a magnetic field may induce the flow of unwanted currents within the primary and/or secondary windings of the transformer. These common-mode currents may cause a magnetic flux to form around the conductors of the primary and/or secondary windings, thereby inducing noise within the windings. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to overcome these disadvantages of conventional transformers. To achieve this object, the present invention provides for a planar transformer arrangement, comprising a plurality of meandering windings (e.g., circular or polygonal printed meandering windings) to be arranged on a planar medium (e.g., a printed circuit board or a general interlayer structure (e.g., metal-oxide-metal) of an integrated circuit), such that at least one primary winding of the planar transformer arrangement is provided on one layer (e.g., one side) of the planar medium (e.g., on one layer of a printed circuit board or on one metal layer of a integrated circuit), and at least one secondary winding of the planar transformer arrangement is provided on another layer (e.g., the other side) of the planar medium, the primary and secondary windings forming a planar transformer. 
   By arranging the planar transformer arrangement in this manner, a dielectric layer of the planar medium (e.g., the printed circuit board or a dielectric oxide layer of the integrated circuit) provides voltage isolation and an open magnetic path between the two primary and secondary windings of the planar transformer arrangement. The voltage isolation provided by the planar medium permits the present invention to be used, for example, in circuits that isolate a gate driver from high voltage IGBT power switches, which may operate at high voltages and at high currents. 
   In accordance with an exemplary embodiment of the present invention, the planar transformer arrangement includes a second planar transformer comprising at least one second primary winding provided on one layer (e.g., on one side) of the planar medium, and at least one second secondary winding provided on another layer (e.g., the other side) of the planar medium. By placing the two planar transformers in close proximity, a differential amplifier arrangement may be used to detect and compensate for common mode electromagnetic interference applied to the two planar transformers (e.g., to compensate for noise caused by an external magnetic field and/or parasitic capacitance between windings). 
   In accordance with still another exemplary embodiment of the present invention, the magnetic mode interference is canceled without using a differential amplifier circuit. For this purpose, each of the windings of the planar transformer includes two windings connected in anti-series. In this manner, magnetic common mode interference may be automatically canceled without need for external compensating circuitry, such as a differential amplifier circuit. 
   In accordance with yet another exemplary embodiment of the present invention, the electromagnetic coupling between the windings of the planar transformer arrangement is improved by providing a magnetic core, for example, a ferrite core, to couple the windings of the two planar transformers. The planar magnetic core may, for example, be applied over the windings of the respective planar transformers on both sides of the planar medium, respectively. 
   In accordance with still another exemplary embodiment of the present invention, two respective metallic shields are provided between the two windings and coupled respectively to primary and secondary ground voltages. In this manner, the shields help prevent interwinding parasitic capacitance from interfering with the planar transformers by operating to magnetically isolate the magnetic flux produced by the interwinding parasitic capacitance. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram of a first exemplary planar transformer arrangement according to the present invention. 
       FIG. 2  is a block diagram of an exemplary mode interference elimination arrangement according to the present invention. 
       FIGS. 3   a  through  3   c  are top, bottom, and cross-sectional views, respectively, of the exemplary planar transformer shown in  FIG. 1 . 
       FIGS. 4   a  and  4   b  are exemplary planar transformer arrangements provided with a magnetic core according to the present invention. 
       FIG. 5  illustrates another exemplary planar transformer arrangement according to the present invention, including a tranceiver circuit to drive planar transformer. 
       FIGS. 6   a  through  6   c  are top, bottom, and cross-sectional views of the exemplary planar transformer arrangement shown in  FIG. 5 . 
       FIGS. 7   a  through  7   c  illustrates yet another exemplary planar transformer arrangement according to the present invention. 
       FIGS. 8   a  and  8   b  illustrate a primary winding connected in anti-series according to the present invention. 
       FIG. 9  illustrates another exemplary planar transformer arrangement provided with metallic shields according to the present invention. 
       FIG. 10  is a top view of a metallic shield illustrated in  FIG. 9 . 
   

   DETAILED DESCRIPTION 
   Referring now to  FIG. 1 , there is seen a first exemplary planar transformer arrangement  100  according to the present invention. Planar transformer arrangement  100  includes a planar transformer  105  having primary and secondary windings  105   a ,  105   b  arranged on respective sides of a planar medium (not shown), e.g., a printed circuit board or an integrated circuit, a single mode detect winding  110  on the same side of the planar medium as the secondary winding  105   b , a mode interference elimination circuit  115  electrically connected to the secondary winding  105   b  of the planar transformer  105  and the single mode detect winding  110 . 
   The exemplary planar transformer arrangement  100  of  FIG. 1  is operable to communicate an input signal  120  applied to the primary winding  105   a  of the planar transformer  105  to an output signal  125 , while providing voltage isolation between the input signal  120  and the output signal  125 . Specifically, an input signal  120  applied to the primary winding  105   a  of the planar transformer  105  induces a current flow within the primary winding  105   a . The magnetic flux caused by the increasing current flow induces a voltage signal (S) across the secondary winding  105   b  of the planar transformer  105 , which is then transmitted by the mode interference elimination circuit  115  as output signal  125 . 
   The mode interference elimination circuit  115  is also configured to prevent common mode magnetic noise interference from corrupting the signal flow between the input and output signals  120 ,  125 . Referring now to  FIG. 2 , there is seen an exemplary mode interference elimination circuit  115  according to the present invention for eliminating a common mode magnetic interference caused by an externally applied magnetic field. Mode interference elimination circuit  115  includes a summation circuit  205  having a high impedance positive input  205   a  electrically connected to the voltage (S) across the secondary winding  105   b , and a high impedance negative input  205   b  electrically connected to the voltage (R) across the mode detect winding  110 . 
   If an external magnetic field is applied to the planar transformer arrangement  100 , a common mode interference voltage will be superimposed on both the voltage (S) across the secondary winding  105   b  and the voltage (R) across the mode detect winding  110 . However, since the interference voltage appears across both windings  105   b ,  110 , the summation circuit  205  operates to cancel the interference voltage effects of the externally applied magnetic field, thereby generating the output signal  125  free of common mode interference. 
   Referring now to  FIGS. 3   a  through  3   c , there is seen top, bottom, and cross-sectional views, respectively, of the exemplary planar transformer  105  and exemplary mode detect winding  110  shown in  FIG. 1 . As shown in  FIGS. 3   a  through  3   c , the windings  105   a ,  105   b ,  110  of the exemplary planar transformer arrangement  100  may be implemented, for example, as meandering traces on a planar medium  300  (e.g., a printed circuit board or an integrated circuit), which forms an open magnetic path between the primary and secondary windings  105   a ,  105   b  of the planar transformer  105 . 
   Referring now to  FIG. 5 , there is seen a second exemplary planar transformer arrangement  500  according to the present invention. The planar transformer arrangement  500  includes primary circuitry  505   a  arranged on one side of a planar medium (not shown) and secondary circuitry  505   b  arranged on the other side of the planar medium (not shown). 
   In applications in which the planar medium is an integrated circuit, the primary and secondary circuitry  505   a ,  505   b  may be arranged on separate silicon dies or, alternatively, may be arranged on the same silicon die. If the primary and secondary circuitry  505   a ,  505   b  are arranged on separate dies, magnetic coupling between the circuitry  505   a ,  505   b  may be effected using two metal interconnection layers separated by a dielectric layer. 
   Planar transformer arrangement  500  is operable as an isolation transceiver to permit input signals (QR′) and (QS′) of primary circuitry  505   a  to be communicated as respective output voltage signals (R″) and (S″) of secondary circuitry  505   b , and to permit input signals (QR″) and (QS″) of the secondary circuitry  505   b  to be communicated as respective output voltage signals (R′) and (S′) of primary circuitry  505   a . In this manner, various signals may be communicated between the primary circuitry  505   a  and the secondary circuitry  505   b , while maintaining electrical isolation. 
   For this purpose, primary circuitry  505   a  includes a primary winding (A) electrically connected to both the negative input terminal of a comparator  530   a  and the positive input terminal of a comparator  530   b  via resistor network  520 , and a primary winding (B) electrically connected to both the positive input terminal of the comparator  530   a  and the negative input terminal of the comparator  530   b  via the resistor network  520 . The first and second primary windings (A), (B) are also electrically connected in parallel to respective diodes  510   b ,  515   b , resistors  510   c ,  515   c , and capacitors  510   d ,  515   d , all of which terminate at source voltage  501 . 
   Secondary circuitry  505   b  includes a secondary winding (C) electrically connected to both the negative input terminal of a comparator  560   a  and the positive input terminal of a comparator  560   b  via resistor network  550 , and a secondary winding (D) electrically connected to both the positive input terminal of the comparator  560   a  and the negative input terminal of the comparator  560   b  via the resistor network  550 . The first and second secondary windings (C), (D) are also electrically connected in parallel to respective diodes  540   b ,  545   b , resistors  540   c ,  545   c , and capacitors  540   d ,  545   d , all of which terminate at source voltage  502 . 
   As shown in  FIGS. 6   a  and  6   c , each of the primary and secondary windings (A), (B), (C), (D) is implemented as a separate meandering trace on a planar medium  300  (e.g., a printed circuit board or integrated circuit), with primary windings (A), (B) being arranged on one layer (e.g., one side) of planar medium  300  and secondary windings (C), (D) being arranged on another layer (e.g., the other side) of planar medium  300 . Specifically, primary winding (A) is arranged over secondary winding (C) to form a first planar transformer  605   a , and primary winding (B) is arranged over secondary winding (D) to form a second planar transformer  605   b , as shown in  FIG. 6   c.    
   In operation, if a pulsed input signal, for example, signal (QR′), is applied to the gate of FET  535   a  of primary circuitry  505   a , a current will be induced within the primary winding (A). The magnetic flux caused by the increasing current flow induces a voltage across the secondary winding (C) of the first planar transformer  605   a , which causes the comparator  560   b  of the secondary circuitry  505   b  to produce a positive output voltage signal (R″). 
   If the primary windings (A), (B) and the secondary windings (C), (D) are arranged adjacent to one another on respective sides of the planar medium, common mode magnetic interference caused by an externally applied magnetic field will induce an interference voltage across both the secondary windings (C), (D). However, since the output stage of the secondary circuitry  505   b  includes two differential comparators  560   a ,  560   b , the interference voltage caused by the common mode magnetic field is effectively eliminated. Specifically, the output stage of the secondary circuitry  505   b  provides the interference voltage to both the positive and negative inputs of the output comparator  560   b , thereby canceling the disadvantageous effects of the interference voltage on the output voltage signal (R″). 
   As described above, the magnetic mode interference may be more effectively canceled by arranging the primary windings (A), (B) and the secondary windings (C), (D) adjacent to one another on respective layers of the planar medium. However, it should be appreciated that the primary windings (A), (B) and the secondary windings (C), (D) may be arranged at a distance from one another, if a particular application of the present invention does not require the compensation of effects caused by common mode magnetic field interference. 
   It should also be appreciated that, although the operation of the exemplary planar transformer arrangement  500  is described only for generating output voltage signal (R″) from input voltage signal (QR′), the exemplary planar transformer arrangement  500  operates similarly to produce output signal (S″) from input signal (QS′), output signal (R′) from input signal (QR″), and output signal (S′) from input signal (QS″). In this manner, the exemplary planar transformer arrangement  500  may operate as a transceiver between the primary and secondary circuits  505   a ,  505   b.    
   Referring now to  FIGS. 4   a  and  4   b , there is seen two variants, respectively, of the exemplary planar transformer arrangement  500  shown in  FIGS. 5 through 6   c . In these exemplary embodiments, the primary windings (A), (B) of planar transformers  605   a ,  605   b  and the secondary windings (C), (D) of planar transformers  605   a ,  605   b  are provided with respective magnetic cores  405   a ,  405   b  (e.g., ferrite) for magnetically coupling the respective windings (A), (B), (C), (D). In this manner, the two windings (A) and (C) of the first planar transformer  605   a  are coupled through both magnetic cores  405   a ,  405   b  and through the open magnetic circuit (e.g., 25 kv/mm) provided by the planar medium  300 . Likewise, the two windings (B) and (D) of the second planar transformer  605   b  are coupled by the same two magnetic cores  405   a ,  405   b  and by the open magnetic circuit provided by the planar medium  300 . 
   Referring now to  FIGS. 7   a  through  7   c , there is seen a third exemplary planar transformer arrangement  700  according to the present invention. In this exemplary embodiment, disadvantageous mode interference is canceled without need for the differential comparators  530   a ,  530   b ,  560   a ,  560   b  of  FIG. 5 . For this purpose, each of the primary windings (A), (B) and secondary windings (C), (D) is formed from two sub-windings connected in anti-series. Specifically, primary winding (A) is formed from two sub-windings (A 1 ), (A 2 ) connected in anti-series, primary winding (B) is formed from two sub-windings (B 1 ), (B 2 ) connected in anti-series, secondary winding (C) is formed from two sub-windings (C 1 ), (C 2 ) connected in anti-series, and secondary winding (D) is formed from two sub-windings (D 1 ), (D 2 ) connected in anti-series. 
   In operation, the third exemplary planar transformer arrangement  700  operates similarly to the exemplary planar transformer arrangement  500  of  FIG. 5 . For example, if a pulsed input signal (QR′) is applied to the gate of FET  535   a  of primary circuitry  505   a , a current will be induced within the sub-windings (A 1 ), (A 2 ) of the primary winding (A), as shown in  FIG. 8   a . The magnetic flux caused by the increasing current flow induces a voltage across the sub-windings (C 1 ), (C 2 ) of the secondary winding (C), which is output as a positive output voltage signal (R″). 
   If a common mode magnetic field (e.g., noise caused by an external magnetic field) is applied, for example, to primary winding (A), the field will cause a current to flow within the primary winding (A). However, unlike the embodiment shown in  FIG. 5 , since the sub-windings (A 1 ), (A 2 ) of the primary winding (A) are connected in anti-series, the externally applied magnetic field will induce the flow of equal currents in opposite directions through each of the sub-windings (A 1 ), (A 2 ), thereby canceling the effects of the common mode interference effects, as shown in  FIG. 7   b . In this manner, no interference voltages are generated and, as such, no additional circuitry is required to compensate for the effects of the common mode magnetic field. 
   To help compensate for a noise interference caused by parasitic capacitance, metallic shields may be provided between the windings and the planar medium  300 . Referring now to  FIG. 9 , there is seen an exemplary planar transformer arrangement  900 , including respective metallic shields  905   a ,  905   b  respectively connected to primary and secondary ground voltages. Transformer arrangement  900  is arranged between the planar medium  300  and respective windings (A), (B) and (C), (D). To electrically isolate the windings (A), (B), (C), (D) from the grounded shields  905   a ,  905   b , respective insulator layers  910   a ,  910   b  are arranged between the shields  905   a ,  905   b  and the respective windings (A), (B) and (C), (D). Furthermore, to prevent current circulation in the metallic shields  905   a ,  905   b , a slit may be cut into the shields  905   a ,  905   b , as shown in  FIG. 10 . 
   By arranging the metallic shields  905   a ,  905   b  in this fashion, the interwinding parasitic capacitance  915  is located between the metallic shields  905   a ,  905   b  and, in this manner, the interwinding parasitic capacitance is better prevented from interfering with the planar transformers  605   a ,  605   b , since the two shields  905   a ,  905   b  operate to magnetically isolate the magnetic flux produced by the interwinding parasitic capacitance  915 .