Abstract:
An apparatus and method for reducing common mode noise capacitive coupling from a primary winding to a secondary winding in a transformer. In an embodiment, the primary winding has two terminals and a plurality of coil turns therebetween formed by a plurality of PCB layers sandwiched together, each having at least one of the coil turns formed thereon. The coils turns are connected in a predetermined way to form the primary winding. One terminal of the primary winding is connected to a coil turn on a first one of the PCB layers, and the other terminal is connected to a coil turn on a second one of the PCB layers. The PCB layers are stacked to form the primary winding. The secondary winding or windings are positioned adjacent to a selected one of the stacked PCB layers that is in a position between the first and second PCB layers.

Description:
FIELD OF INVENTION 
   The present invention relates to transformers, and more particularly a low noise planar transformer and a method of construction thereof. 
   BACKGROUND OF THE INVENTION 
   Electromagnetic components such as transformers have traditionally been constructed by winding one or more conductors about a cylindrical or toroidal core. This method of construction requires that a conductor, such as a wire, be wrapped around the outer surface of the core. The resulting components are expensive and time consuming to manufacture, and do not readily lend themselves to miniaturization or automated assembly. 
   More recently, electromagnetic components have been constructed using printed circuit board (PCB) manufacturing techniques, where windings and individual winding turns are formed from a stack of PCB layers wherein each layer includes one or more conducting traces patterned on the surface of the PCB layer, or formed from a multi-layer PCB having such conducting traces on each layer. The use of PCB conductive traces as windings has several advantages over conventional, wound windings. First, the assembled PCB winding has a smaller mounting footprint than conventional windings, since it does not need extra leads or soldering pads. Second, the PCB winding assembly is much simpler than conventional windings, since the winding and other components in the winding circuit of a multilayer PCB can be board mounted using the same reflow and automation processes used to mount other components. Third, a multi-layer PCB winding has improved reliability since the likelihood of shorting across adjacent turns of the winding is greatly reduced or substantially eliminated. 
   In a multi-layer PCB, a PCB winding is formed from a plurality of patterned conductive traces, typically of copper, each formed on a separate insulating layer of the multi-layer PCB. Each trace forms a nearly closed typically circular pattern, so as to create the electromagnetic equivalent of one turn or loop of a prior art wire formed winding. Terminal points are formed at the ends of each trace for making connections to other traces on other layers, so as to form the individual turns of the winding. For example, the pattern can be a “C” shape with a terminal point at each of the two extreme points of the C. The PCB winding is formed by connecting the traces from different layers of the PCB through the intervening insulating PCB layers. These connections are typically plated through holes or vias in the PCB insulating layers. The traces can be connected in various ways. The traces can all be connected in series to form a winding where each trace is a separate turn of the winding. In this example, the terminal ends of each trace are offset from the traces on the adjacent levels, so that the plated through holes in each level do not intersect. Two or more traces can also be connected in parallel to decrease the impedance of a particular turn of the winding. In yet another alternate embodiment, one or more of the traces can be formed as separate windings. In each case, the resultant winding (or windings) is a function of the way in which the conductive traces on each layer of the multi-layer PCB are connected together and coupled to external circuits, to thereby create a planar transformer. 
   The inductance of a winding formed using a multi-layer PCB can be increased by introducing a core of a magnetic material through an aperture formed in the PCB layers that extends through a central non-conducting region of each layer. Alternatively, the core can be configured to surround the PCB. The core is typically included as part of a housing for the multi-layer PCB winding. Conductive leads or vias are included on one or more of the PCB layers to enable the efficient electrical connection of the PCB winding to an external circuit, for example, by surface mounting and reflow soldering of the PCB winding to another PCB having other circuit components. This use of a multi-layer PCB to fabricate electromagnetic components results in smaller, more easily manufactured, and more reproducible components than is possible using a winding formed from a wire wrapped about a core. 
   In order to achieve better coupling and to reduce the leakage inductance of the transformer, the primary and secondary windings of the transformer are typically placed in close proximity to one another. One drawback of this arrangement is that it increases the capacitive coupling between the primary and secondary windings, which results in the generation of increased electromagnetic interference (EMI). That is, due to the inter-winding capacitance of the transformer, common mode noise will be injected into the secondary. In a planar, low profile transformer required for low profile packaging, this inter-winding capacitance is larger and, as a result, the common mode noise injection via this parasitic capacitance is larger. 
   This drawback is especially significant for a two switch forward converter. Unlike in a single switch forward converter, the primary winding in a two switch forward converter is not connected to either the positive or the return side of the converter&#39;s input voltage. The switches in the two switch forward converter are typically MOSFETs. The converter having MOSFET switches is also referred to herein as a two FET forward converter. 
     FIG. 1  shows a prior art two FET forward converter  10 . The converter  10  has an input terminal  14  to which an input DC voltage, V in , is coupled, relative to a ground potential at an input terminal  16 , and an output terminal  32  where the output DC voltage, V OUT , is provided relative to ground. Converter  10  includes a transformer  42  having primary winding  2  and a secondary winding  6 . Each winding has a first and second end. A first power switch  34  is coupled between the first end of primary winding  2  and input terminal  14 . A second power switch  36  is connected between the second end of primary winding  2  and input terminal  16 . Power switch  34  is connected in series with primary winding  2  and power switch  36  across the input DC voltage terminals. A diode  18  is connected between the second end of primary winding  2  and input terminal  14 . The diode  22  is connected between the first end of primary winding  2  and input terminal  16 . Each of the power switches  34 ,  36  is preferably a MOSFET having a source, a drain, and a gate. A controller (not shown) preferably provides a control signal, e.g. a pulse width modulated (PWM) signal, coupled to each control input of power switches  34  and  36 . 
   On the secondary side of the forward converter  10 , transformer  42  has a secondary winding  6  having a second end connected to output terminal,  38 . Converter  10  includes an inductor  24  connected in series with a diode  26  between output terminal  32  and the first end of secondary winding  6 . A capacitor  28  is connected across the output terminals  32 ,  38 . A diode  44  is connected between the junction of the cathode of diode  26  and inductor  24  and output terminal  38 . 
   As shown in  FIG. 1 , converter  10  has a primary winding  2  having two terminals  7  and  9 . Primary winding terminal  7  is connected to the source terminal of switch  34 . Primary winding terminal  9  is connected to the drain terminal of switch  36 . For the two switch forward converter  10 , the voltage swing at primary winding terminals  7  and  9  is at a maximum during normal operation. If primary winding terminals  7  and  9  are located near the secondary winding  6  of transformer  42 , a significant amount of common mode noise is coupled from the primary side to the secondary side of the transformer  42  due to the capacitance between primary winding  2  and secondary winding  6 . This coupled common mode noise increases EMI for converter  10 . 
   U.S. Pat. No. 5,990,776 (“the &#39;776 patent”) discloses a single ended switch forward converter that includes one FET switch for switching the primary winding. The &#39;776 patent discloses a primary-secondary-primary (“pri-sec-pri”) type transformer construction. The &#39;776 patent discloses a transformer wherein all of the primary and secondary windings are integrated in a PCB. 
   The &#39;776 patent teaches that the top winding  72  connected to the input voltage source is the quiet area of the primary winding since it exhibits a lower voltage swing, and that therefore it is logical to locate the secondary in the vicinity of winding  72 . However, due to reasons of symmetry, the secondary winding  80  in &#39;776 is positioned between primary windings  74  and  76 . 
   Unlike in the single switch forward converter for which the &#39;776 patent teachings were directed, the primary winding in a two switch forward converter is not connected to either the positive or the return side of the converter&#39;s input voltage. One drawback of the &#39;776 patent, therefore, is that it does not address the unique problems in reducing common mode noise for a two switch forward converter. The &#39;776 patent does not disclose, for instance, the optimum location for the secondary winding in a two switch forward converter. 
   U.S. Pat. No. 6,211,767 discloses a transformer having a secondary copper strip mounted and fixed on the primary winding PCB by means of solderable via holes, but does not disclose a design to significantly reduce common node noise. 
   A need therefore exists to reduce common mode noise for a planar transformer. The need especially exists to reduce common mode noise for a planar transformer designed for use in two FET forward converters and which can also be used in single ended, half bridge converters and push pull converters. 
   SUMMARY OF THE INVENTION 
   The present invention solves the problems of prior art devices by providing a method of construction of a planar transformer that minimizes capacitively coupled common mode noise from the primary winding to the secondary winding of the transformer. Broadly stated, the present invention comprises a method for reducing common mode noise coupling from the primary winding to the secondary winding in a transformer, wherein the primary winding includes first and second terminals and a plurality of coil turns therebetween formed by a plurality of printed circuit board (PCB) layers sandwiched together, each having at least one of the coil turns formed thereon, wherein the coils on each of the PCB layers are connected in a predetermined way to form the primary winding, and wherein the first terminal is connected to a coil on a first PCB layer and the second terminal is connected to a coil on a second PCB layer, comprising the steps of stacking the PCB layers to form the primary winding and positioning the secondary winding adjacent to a selected one of the PCB layers that is in a position in the stack that is substantially midway between the first and second PCB layers such that said secondary winding is positioned at a quiet point that exhibits the lowest voltage swing. 
   Broadly stated, according to another embodiment, in a transformer including a primary winding and first and second secondary windings, the primary winding having first and second terminals and a plurality of coil turns therebetween formed by a plurality of printed circuit board (PCB) layers sandwiched together, each having at least one of the coil turns formed thereon, wherein the coils turns on each the PCB layer are connected in a predetermined way to form the primary winding and wherein the first terminal is connected to a coil turn on a first PCB layer and the second terminal is connected to a coil turn on a second PCB layer, the present invention provides a method for reducing common mode noise coupling from the primary winding to the secondary winding comprising the steps of stacking a first half of the PCB layers including the first PCB layer to form a first half of the primary winding; stacking a second half of the PCB layers including the second PCB layer to form a second half of the primary winding; stacking the first and second halves to form the primary winding; positioning the first secondary winding adjacent to a selected one of the PCB layers in the first half of the primary winding in a position in the stack that is farthest from the first PCB layer; and positioning the second secondary winding adjacent to a selected one of the PCB layers in the second half of the primary winding in a position in the stack that is farthest from the second PCB layer. 
   Broadly stated, according to another embodiment, in a matrix transformer comprising first and second transformers, the first transformer including a first primary winding and first and second secondary windings, the first primary winding comprising a first series combination of windings connected in parallel with a second series combination of windings, the second transformer including a second primary winding and third and fourth secondary windings, the second primary winding comprising a third series combination of windings connected in parallel with a fourth series combination of windings, the first primary winding is connected in series with the second primary winding to form a third primary winding between first and second terminals, a parallel combination of the first and second secondary windings is connected in parallel with a parallel combination of the third and fourth secondary windings to form a fifth secondary winding; the third primary winding having a plurality of coil turns formed by a plurality of printed circuit board (PCB) layers sandwiched together, each having at least one of the coil turns formed thereon, wherein the coils turns on each the PCB layer are connected in a predetermined way to form the third primary winding and wherein the first terminal is connected to a coil turn on a first PCB layer and the second terminal is connected to a coil turn on a second PCB layer, the present invention provides a method for reducing common mode noise coupling from the third primary winding to the fifth secondary winding comprising the steps of stacking the PCB layers to form the third primary winding; and positioning each the parallel combination of secondary windings adjacent to a selected one of the PCB layers that is in a position in the stack that is substantially midway between the first and second terminals. 
   Broadly stated, the present invention also provides a planar transformer for reducing common mode noise comprising a plurality of printed circuit board (PCB) layers; a primary winding having first and second terminals and a plurality of coil turns therebetween formed by said plurality of printed circuit board (PCB) layers sandwiched together; each having at least one of said coil turns formed thereon, wherein the coils turns on each said PCB layer are connected in a predetermined way to form said primary winding and wherein said first terminal is connected to a coil turn on a first PCB layer and said second terminal is connected to a coil turn on a second PCB layer; wherein a stack of said PCB layers forms said primary winding; and a secondary winding positioned adjacent to a selected one of said PCB layers that is in a position in said stack that is substantially midway between said first and second PCB layers such that said secondary winding is positioned at a quiet point that exhibits the lowest voltage swing. 
   An advantage of the present invention is improved EMI performance by reducing common mode noise coupled from the primary winding to the secondary winding in a power transformer. 
   Another advantage of the present invention is that it reduces the common mode noise coupled to the secondary winding without increasing the leakage inductance. 
   Still another advantage of the present invention is that is readily implemented in a planar transformer using PCB windings, which enables the number of turns of the primary winding in the contact surface between the primary and the secondary winding to be reduced to one turn for minimizing noise coupling. 
   Another advantage of the present invention is that it can be applied for both regular planar transformers and matrix planar transformer. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The forgoing aspects and the attendant advantages of the present invention will become more readily appreciated by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein: 
       FIG. 1  shows a circuit diagram for a prior art two switch forward converter; 
       FIG. 2  shows a circuit diagram for a two switch forward converter of the present invention wherein the contact region between the primary winding and the secondary winding is the mid-portion of the primary winding; 
       FIG. 3  illustrates the arrangement of the windings for a primary-secondary-primary (“pri-sec-pri”) transformer constructed according to an embodiment of the present invention; 
       FIG. 3A  shows a circuit diagram schematic representation of the sandwich pri-sec-pri transformer shown in  FIG. 3 ; 
       FIG. 4  illustrates the arrangement of the windings for a secondary-primary-secondary (“sec-pri-sec”) transformer wherein two halves of the primary winding are combined into one PCB winding according to an embodiment of the present invention; 
       FIG. 4A  shows a circuit diagram schematic representation of the sandwich sec-pri-sec transformer shown in  FIG. 4 ; 
       FIG. 5A  is a partially exploded view of an exemplary layout for construction of a planar transformer according to a preferred embodiment of the present invention; 
       FIG. 5B  illustrates an exemplary arrangement of the primary PCB winding assembly shown in  FIG. 5A ; 
       FIG. 5C  illustrates an exemplary arrangement of the secondary PCB winding of  FIG. 5A ; 
       FIG. 6  illustrates the arrangement of the windings and core for an exemplary planar matrix transformer according to an embodiment of the present invention; 
       FIG. 6A  is a circuit diagram for the matrix transformer shown in  FIG. 6 ; 
       FIG. 7A  is a partially exploded view of an exemplary layout for construction of a planar matrix transformer according to the embodiment of the present invention show in  FIG. 6 ; 
       FIG. 7B  illustrates an exemplary arrangement of the primary PCB winding assembly in  FIG. 7A ; and 
       FIG. 7C  illustrates an exemplary arrangement of the secondary PCB winding in  FIG. 7A . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 2  shows a circuit diagram for a two switch forward converter  100  having an embodiment of the transformer according to the present invention. Converter  100  has an input terminal  14  to which an input DC voltage, V in , is coupled, relative to a ground potential at an input terminal  16 , and an output terminal  32  where the output DC voltage, V OUT , of the converter is provided relative to ground. Converter  100  includes a transformer  142  having a primary winding  112  and a secondary winding  6 . Primary winding  112  comprises a first winding  4  and a second winding  8 . Each winding has a first and second end. The second end of the first winding  4  is connected to the first end of second winding  8 , at a node  5 . A power switch  34  is coupled between the first end of first winding  4  at a node  107  and input terminal  14 . A power switch  36  is connected to the second end of winding  8  at a node  109 . The power switch  34  is connected in series with first winding  4 , second winding  8 , and power switch  36  across the input DC voltage terminals. A diode  18  is connected in series between the second end of winding  8  and the input terminal  14 . A diode  22  is connected in series between the first end of winding  4  and the input terminal  16 . Each of the power switches  34 ,  36  is preferably a MOSFET having a source, a drain, and a gate. A controller (not shown) provides a control signal, e.g. a pulse width modulated (PWM) signal, that is coupled to each control input of power switches  34  and  36 . 
   For converter  100 , the turns ratio of first winding  4  and second winding  8  are equal. During normal operation, the mid portion at node  5  between the first winding  4  and the second winding  8 , i.e. the middle of the primary winding  112 , exhibits the lowest voltage swing. The voltage level at node  5  is limited to about half of the input voltage. As a result, node  5  is the quiet point of primary winding  112 , and therefore is the optimum contact region for the secondary winding  6 . As seen in  FIG. 2 , the primary winding contact region for the secondary winding  6  is the middle of the primary winding  112 . 
     FIG. 3  illustrates the arrangement of the windings for a primary-secondary-primary sandwich transformer  200 . This primary-secondary-primary sandwich transformer construction shown in  FIG. 3  is also referred to as a pri-sec-pri transformer construction. The sandwich transformer  200  has a primary winding  204 , a secondary winding  206 , and a core  202 . The corresponding circuit diagram representation for transformer  200  is shown in  FIG. 3A . Primary winding  204  comprises windings  210 ,  212 ,  214 ,  216 ,  218 , and  220 . Primary winding  204  has terminals P 1  and P 2 . For the sandwich transformer  200 , secondary winding  206  is sandwiched between the primary windings  210 ,  212 ,  214 ,  216 ,  218 , and  220 . Secondary winding  206  has terminals S 1  and S 2 . 
   During normal operation, the mid-portion of primary winding  204 , between windings  214  and  216 , exhibits the lowest voltage swing. The voltage level between windings  214  and  216  is limited to about half of the input voltage. As a result, the point between windings  214  and  216  is the quiet point of primary winding  204  and thus the optimum contact region for the secondary winding  206 . As seen in  FIG. 3 , the contact area (or region) for the secondary winding  206  is the quiet point between primary windings  214  and  216  in the middle of the primary winding  204 . 
   When used in a two switch forward converter, e.g., as shown in  FIGS. 1 and 2 , the voltage swing in the primary winding  204  becomes larger for the winding turns that are closer to the drain of the MOSFETs. Conventionally, the sandwich transformer  200  could be constructed as a wire wound transformer, wherein each winding comprising a plurality of turns concentrically wound about a common axis. A drawback of such a wire wound sandwich transformer is that, if the number of turns in winding  214  and  216 , shown in  FIG. 3  is large, the common mode noise coupled to the secondary winding  204  is still large due to the large voltage swing in the windings  214  and  216 . What is also needed is to reduce this additional source of common mode noise. 
   For a planar transformer, the windings and individual winding turns are formed from one or more conducting layers patterned on the surface of an insulating PCB layer, or on each layer of a multilayer PCB. Thus, for a planar transformer, the number of turns of the primary winding at the contact layer close to secondary winding can be as small as one turn. Thus, according to one embodiment of the present invention, the sandwich transformer  200  is constructed as a planar transformer wherein windings  214  and  216  are each preferably one turn. Since windings  214  and  216  each preferably comprise only one turn, the voltage swing at windings  214  and  216  at the contact layer close to the secondary winding  206  is reduced, thereby further reducing the common mode noise. 
     FIG. 4  illustrates the arrangement of the windings for a sec-pri-sec transformer  300  wherein two halves of the primary windings are combined into one PCB winding according to an alternative embodiment of the present invention.  FIG. 4A  shows a circuit diagram schematic representation of the sec-pri-sec sandwich transformer in  FIG. 4 . Transformer  300  includes a primary winding  308  and secondary windings  322  and  324 . Primary winding  308  has terminals  330  and  332 . Secondary winding  322  has terminals  334  and  336 . Secondary winding  324  has terminals  338  and  340 . Primary winding  308  comprises a set of four windings  326 ,  310 ,  312 , and  314  connected in series and another set of four windings  316 ,  318 ,  320 , and  324  connected in series. The two sets of windings each comprise half of the primary winding  308 . These two sets are connected in series to form primary winding  308 . 
   As can be seen in  FIG. 4A , for a planar embodiment comprised of sandwiched PCB layers, secondary winding  322  is adjacent to the layer on which primary winding  314  is formed and secondary winding  324  is adjacent to the layer on which primary winding  320  is formed. 
   The secondary windings  322 ,  324  are thus located at the furthest point from the terminals  330 ,  332  of primary winding  308 , at a position that is farthest away from the largest source of common mode noise. Preferably, primary windings  314  and  320  each comprise one coil turn so as to further reduce the source of common mode noise for the corresponding adjacent secondary winding. Preferably, windings  314  and  320  are mounted on the same PCB so as to simplify construction. 
     FIG. 5A  is a partially exploded view of an exemplary layout for construction of a planar transformer  400  according to a preferred embodiment of the present invention. Planar transformer  400  has a core  402 , a primary winding assembly  408 , and a secondary winding assembly  406 .  FIG. 5B  illustrates an exemplary arrangement of the primary PCB winding assembly  408 .  FIG. 5C  illustrates a preferred arrangement of the secondary PCB winding  406  of planar transformer  400  constructed according to the present invention. 
   A matrix transformer is a planar transformer wherein two halves of the primary winding of the transformer are split and put into two different transformers. An alternative embodiment of the present invention is a transformer and corresponding PCB winding construction method for a low noise planar matrix transformer.  FIG. 6  illustrates the arrangement of the windings and core for an exemplary planar matrix transformer  500  according to the present invention.  FIG. 6A  is a circuit diagram for the matrix transformer  500  shown in  FIG. 6 . The planar matrix transformer comprises a transformer  510  and a transformer  520 . The AP′ 3 , and AP′ 4 , connected in parallel with a series combination of windings AP 1 , AP 2 , AP′ 3 , and AP′ 4 . The number of windings should be selected as required for a particular application. The primary winding  528  of transformer  520  comprises a series combination of windings BP′ 1 , BP′ 2 , BP′ 3 , and BP′ 4  connected in parallel with a series combination of windings BP 1 , BP 2 , BP 3 , and BP 4 . 
   As seen in  FIGS. 6 and 6A , for the planar matrix transformer  500 , the primary winding  508  of transformer  510  is connected in series with the primary winding  528  of transformer  520 . The series connected primary windings have terminals  530  and  532 . The parallel combination of the secondary windings  512 ,  514  of transformer  510 , also labeled as AS 1  and AS 2 , is connected in parallel with a parallel combination of the secondary windings  516 ,  518  of transformer  520 , also labeled as BS 1  and BS 2 . 
     FIG. 7A  is a partially exploded view of an exemplary layout for construction of a planar matrix transformer  600  according to the embodiment of the present invention shown in  FIG. 6 . Planar matrix transformer  600  has a core  602 , a primary winding assembly  608 , and a secondary winding assembly  606 .  FIG. 7B  illustrates an exemplary arrangement of the primary PCB winding assembly  608 .  FIG. 7C  illustrates an exemplary arrangement of the secondary PCB winding  606 . As can be seen, multiple coil turns are preferably formed on each layer of the primary winding  608 . 
   The foregoing detailed description of the invention has been provided for the purposes of illustration and description. Although exemplary embodiments of the present invention have been described in detail herein with reference to the accompanying drawings, it is to be understood that the invention is not limited to the precise embodiments disclosed, and that various changes and modifications to the present invention are possible in light of the above teaching.