Transformer with interleaved shielding windings

A transformer includes first and second primary windings serially electrically connected in a primary-side series combination. The transformer further includes a secondary winding disposed between the first primary winding and the second primary winding. The transformer further includes first and second shielding windings serially electrically connected in a shielding series combination. The first shielding winding is disposed between the first primary winding and the secondary winding, and the second shielding winding is disposed between the second primary winding and the secondary winding.

BACKGROUND

Switch-mode power supplies (“power converters”) are power management components in modern electronic devices. They provide, among other things, efficient and galvanically isolated power to multiple loads. To achieve high power processing efficiency and/or galvanic isolation, conventionally one or more magnetically coupled elements, semiconductor switches and associated gate driver circuits are required. Some power converters, such as fly-back converters, include a transformer that couples a primary-side of the power converter to a secondary-side of the power converter. An input voltage is received at the primary-side of the power converter and an output voltage is produced at the secondary-side of the power converter.

Inter-winding parasitic capacitive coupling occurs between primary windings of the transformer and secondary windings of the transformer. Such inter-winding capacitance allows a common mode noise current to flow from the primary-side of the power converter to the secondary-side of the power converter. The common mode noise current typically returns from the secondary-side of the power converter to the primary-side of the power converter via a ground path or parasitic capacitance, thereby producing undesirable or impermissible electromagnetic interference (EMI) at a voltage input of the power converter.

Some applications, such as USB power delivery devices (USB-PD), conventionally require a very low value Y Capacitor (typically less than 470 pF) for EMI noise filtering. For such application, an effective and consistent noise shielding structure is conventionally essential for the system to meet EMI standard requirements.

SUMMARY

In some embodiments, a transformer includes a first primary winding and a second primary winding serially electrically connected in a primary-side series combination, the primary-side series combination having a first primary-side terminal and a second primary-side terminal. The transformer further includes a secondary winding having a first secondary-side terminal and a second secondary-side terminal and disposed between the first primary winding and the second primary winding. The transformer further includes a first shielding winding and a second shielding winding serially electrically connected in a shielding series combination at an intermediate shielding terminal, the shielding series combination having a first shielding terminal and a second shielding terminal. The first shielding winding is disposed between the first primary winding and the secondary winding, and the second shielding winding is disposed between the second primary winding and the secondary winding.

In some embodiments, the first primary-side terminal is configured to be electrically connected to a first node at a primary-side of a power converter; the second primary-side terminal is configured to be electrically connected to a second node at the primary-side of the power converter; the first secondary-side terminal is configured to be electrically connected to a first node at a secondary-side of the power converter; the second secondary-side terminal is configured to be electrically connected to a second node at the secondary-side of the power converter; the intermediate shielding terminal is configured to be electrically connected to a third node at the primary-side of the power converter; and the first shielding terminal and the second shielding terminal are configured to be electrically floating.

In some embodiments, the third node at the primary-side of the power converter is a primary-side ground node. In some embodiments, the first node at the primary-side of the power converter is a drain node of a main switch of the power converter; the second node at the primary-side of the power converter is an input voltage node for an input voltage of the power converter; the first node at the secondary-side of the power converter is a drain node of a secondary-side switch of the power converter; and the second node at the secondary-side of the power converter is an output voltage node for an output voltage of the power converter.

In some embodiments, the transformer also includes a cylindrical bobbin having a central core. The first primary winding is wound around the central core of the cylindrical bobbin. The first shielding winding is wound around the first primary winding. The secondary winding is wound around the first shielding winding. The second shielding winding is wound around the secondary winding. The second primary winding is wound around the second shielding winding.

In some embodiments, the first shielding winding and the second shielding winding are aligned symmetrically on opposite sides of the secondary winding.

In some embodiments, a power converter includes the above-described transformer.

In some embodiments, a method of forming the above transformer is disclosed.

DETAILED DESCRIPTION

In applications like USB-PD which requires a very low value Y Capacitor (typically less than 470 pF) for EMI noise filtering, an effective and consistent noise shielding structure is essential for the system to meet EMI standard requirements. As disclosed herein, a transformer having interleaved shielding windings advantageously blocks common mode noise current from flowing between the primary and secondary windings of the transformer. As such, power converters implemented using the transformer as disclosed herein can advantageously use a low capacitance Y Capacitor, or even no Y Capacitor, and still comply with the EMI standard requirements. The transformer disclosed herein is advantageously manufacturable using a straightforward approach with good repeatability and consistency regardless of the particular transformer manufacturing process used.

FIG.1is a simplified circuit schematic of a conventional power converter100under EMI test. Some elements of the power converter100have been omitted fromFIG.1to simplify the description of power converter100but are understood to be present. A voltage source ACinis received at a Line Impedance Stabilization Network (LISN)101. The LISN101is used to perform conducted and radiated radio-frequency emission and susceptibility (EMI) tests of the power converter100. A voltage source Vin′, based on the voltage source ACin, is provided by the LISN101to an input side of the power converter100. The input side of the power converter100generally includes an input voltage filter block122, a rectifier block116, an input voltage buffer capacitor C1, a main switch M1driven by a pulse-width-modulation (PWM) signal PWMM1, and a primary-side controller circuit (“Ctrl.”)118. The input voltage filter block122, the rectifier block116and the input buffer capacitor C1provide a filtered, buffered, rectified, or otherwise conditioned input voltage Vin(i.e., a DC input voltage) to a transformer102.

The transformer102transfers power from the primary-side of the power converter100to a secondary-side of the power converter100and generally includes primary windings104with a first terminal130(‘A’) and a second terminal131(‘B). The secondary-side of the power converter100generally includes secondary windings106of the transformer102with a first terminal132(‘C’) and a second terminal133(‘D’), an output buffer circuit112, a synchronous rectifier switch M2, a synchronous rectifier switch controller circuit (“Ctrl.”)120, and is configurable to be connected to a load RL.

The second terminal131of the primary windings104receives the DC input voltage Vin. The first terminal130of the primary windings104is coupled to a drain node of the main switch M1. The main switch M1controls a current through the primary windings104to charge a magnetizing inductance of the transformer102during a first portion of a switching cycle of the power converter100. The synchronous rectifier switch M2controls a current flow through the secondary windings106to discharge the transformer102into the output buffer circuit112and the load RLduring a subsequent portion of the switching cycle.

When the main switch M1is enabled by the primary-side controller circuit118during the first portion of a switching cycle, current flows through the primary windings104to a voltage bias node such as earth ground, illustrated inFIG.1as a triangle coupled to a source node of the main switch M1. The current flow through the primary windings104causes energy to be stored in the magnetizing inductance and a leakage inductance of the transformer102. When the main switch M1is disabled in a subsequent portion of the switching cycle, an output voltage Voutis generated at the output buffer circuit112and is provided to the load RL.

FIG.2is a simplified schematic representation of a modeled power converter200that models a common mode noise current propagation path through the power converter100ofFIG.1for conducted noise analysis. The modeled power converter200generally includes the transformer102ofFIG.1that includes the terminals130-133. Additionally the modeled power converter200includes a primary-side common mode noise source242, a secondary-side common mode noise source244, a common-mode noise filter circuit246, a resistor RLISNof the LISN101, a Y Capacitor CY, a representation of a parasitic capacitance CSE, and representations of primary-side to secondary-side inter-winding parasitic capacitance Cpsof the transformer102. Also shown is a representation of a common mode noise current ipswhich flows through the transformer102due to the inter-winding capacitance Cps.

The inter-winding capacitance Cpsis a significant path for common mode noise current between the primary-side of the modeled power converter200(i.e., the portion connected to the terminals130and131of the transformer102) and the secondary-side of the modeled power converter200(i.e., the portion connected to the terminals132and133of the transformer102). The switches M1, M2ofFIG.1are modeled as respective voltage sources, embodied as the primary-side common mode noise source242and the secondary-side common mode noise source244. Performance of the switches M1, M2ofFIG.1can introduce severe voltage pulses,

d⁢VAd⁢t,and⁢d⁢VCd⁢t
generate the common mode noise current ipswhich flows through the inter-winding capacitance Cpsand then returns to the primary-side of the modeled power converter200either through the parasitic capacitance CSEwhen the secondary-side is not connected to earth ground, or directly through a ground-loop when the secondary-side is connected to earth ground. The Y Capacitor CYis used to mitigate noise on the ground loop by providing an alternate path for the common mode noise current ips.

Nulling the flow of the common mode noise current ipsfrom the primary windings104of the transformer102to the secondary windings106of the transformer102can dramatically reduce measured conducted noise at the resistor RLISNof the LISN101. The common mode noise current ipsgenerated from the voltage pulses

d⁢VAd⁢t,and⁢d⁢VCd⁢t
acting on the inter-winding capacitance Cpscan be generally expressed as

ip⁢s=Cp⁢s×d⁢Vd⁢t
Thus, to reduce or eliminate the common mode noise current ips, either the inter-winding capacitance Cpscan be reduced or eliminated, or voltage balancing techniques can be used to minimize the voltage pulses

FIG.3Ais another simplified schematic representation of a modeled power converter300that models the power converter100ofFIG.1for conducted noise analysis and illustrates an idealized transformer configuration for reducing the common mode noise current ips. The modeled power converter300generally includes a transformer302that includes terminals330(‘A’),331(‘B’),332(‘C’),333(‘D’),334(‘S1’), and335(‘S′1’). As indicated by the legend350, the transformer302also includes primary windings304, secondary windings306, shielding windings308, and inter-winding parasitic capacitance Cps. Additionally, the modeled power converter300includes a primary-side common mode noise source342, a secondary-side common mode noise source344, a common-mode noise filter circuit346, and a resistor RLISNof the LISN101. The primary-side common mode noise source342, secondary-side common mode noise source344, and common-mode noise filter circuit346are the same or similar to the elements242,244, and246, respectively, described with respect toFIG.2. Also shown is a representation of the common mode noise current ipswhich has been reduced to 0 Amps due to the idealized configuration of the transformer302.

Similar to that as described with regard toFIG.2, the switches M1, M2ofFIG.1are modeled as respective voltage sources, embodied as the primary-side common mode noise source342and the secondary-side common mode noise source344which introduce voltage pulses,

d⁢VAd⁢t,and⁢d⁢VCd⁢t
generate the common mode noise current ipswhich flows through the inter-winding capacitance Cpsof the transformer302. However, as shown, the shielding windings308are identical in number and alignment as compared to the secondary windings306. Thus, assuming that voltage developed across the windings306,308is distributed linearly along the respective windings306,308, then for a specific point on the shielding windings308, that point shares the same voltage potential as compared to a corresponding point on the secondary windings306. Consequently, there is no voltage difference between the shielding windings308and the secondary windings306. Because there is no voltage difference between the windings306,308, no displacement current flows between the windings306,308. Thus, the common mode noise current ipsflowing through the primary windings304to the secondary windings306is blocked.

FIG.3Bis a schematic representation360of the transformer302shown inFIG.3A. As described with reference toFIG.3A, the transformer302includes terminals330-335, the primary windings304, the secondary windings306, and the shielding windings308. Also shown is the inter-winding capacitance Cpsand a representation of a common-mode current ipswhich has been reduced to 0 Amps due to the idealized configuration of the transformer302.

In an ideal physical implementation of the transformer302, a bobbin, such as a bobbin470shown inFIG.4, is wound such that strands of the shielding windings308and strands of the secondary windings306are exactly aligned. For reference, the bobbin470has a first dimension H that is parallel to a first extent of the bobbin470, and a second dimension W that is perpendicular to the first dimension H.

Given the idealized implementation of the transformer302using the bobbin470, simplified graphs502and504ofFIG.5, with reference to legend550, illustrate a voltage potential V developed at each point of the windings306,308along the first dimension H of the bobbin470. The graph502illustrates the voltage V developed across shielding windings308of the transformer302along the first dimension H of the bobbin470, and the graph504illustrates the voltage V developed across secondary windings306of the transformer302along the first dimension H of the bobbin470. As shown by the graphs502,504, for each position along the first dimension H of the bobbin470, a voltage potential of the shielding windings308is the same as a voltage potential of the secondary windings306. Thus, because there is no voltage difference between the shielding windings308and the secondary windings306, common mode noise current ipscannot flow from the shielding windings308to the secondary windings306.

In practice, however, the idealized configuration of the transformer302, which assumes strict alignment between the shielding windings308and the secondary windings306as the bobbin470is wound cannot be repeatably implemented.FIG.6includes a simplified representation of a non-ideal transformer602having a bobbin670(shown as a cross-section) that is similar to the bobbin470. The transformer602includes primary windings (not shown), shielding windings608, secondary windings606, and inter-winding parasitic capacitance Cps, as indicated by a legend650. The shielding windings608include terminals S1and S′1, and the secondary windings606include terminals C and D. The bobbin670has a first dimension H, and a second dimension W that is perpendicular to the first dimension H. As shown inFIG.6, practically, the shielding windings608and secondary windings606of a transformer602that is similar to the transformer302ofFIG.3Acannot be perfectly aligned, and as such, a voltage difference between the shielding windings608and the secondary windings606will cause common mode noise current ipsto flow from the shielding winding608to the secondary windings606.

The non-idealities of the transformer602are further illustrated inFIG.7. A simplified graph702ofFIG.7, with reference to legend750, illustrate a voltage potential V developed at various points of the windings606,608along the first dimension H of the bobbin670for the non-ideal transformer602. As shown, because of misalignment between the windings606,608, the voltage V developed across shielding windings608of the transformer602and the secondary windings606of the transformer602is not identical for each point along the first dimension H of the bobbin670. Because the shielding windings608and the secondary windings606are not sufficiently aligned along the first dimension H, the voltage V developed across the windings606,608develops a displacement current between the windings606,608. Thus, common mode noise current ipsflows from the secondary windings to the shielding windings.

FIG.8Aillustrates a simplified drawing of a portion of a transformer802having a generally cylindrical bobbin870, in accordance with some embodiments. Only a portion of a cross-section of the bobbin870and the windings is shown to simplify the description, so it is understood that a second portion is generally a mirrored reflection of the portion shown reflected about the bottom horizontal axis. In some embodiments, the bobbin870provides a central ferrite core for the transformer802. The bobbin870has a first dimension H and a second dimension W that is perpendicular to the first dimension H, similar to that as described with reference to the bobbin470ofFIG.4. The transformer802includes first primary windings1810having a terminal A, first shielding windings1812having terminals S1and S′1, secondary windings814having terminals C and D, second shielding windings2816having terminals S2and S′2, second primary windings2818having a terminal B, and isolation tapes820that are illustrated as thick black bars. The first primary windings1810and the second primary windings2818are electrically serially connected with each other, as indicated by a white line connecting the windings810,818. As shown, the first shielding windings1812are disposed along the first dimension H between the first primary windings1810and the secondary windings814. The second shielding windings2816are disposed along the first dimension H between the second primary windings2818and the secondary windings814. Thus, the shielding windings812,816are interleaved between the primary windings810,818and the secondary windings814of the transformer802, thereby advantageously eliminating common mode noise current flow between the shielding windings812,816and the secondary windings814of the transformer802, as described below. In this configuration, the first primary windings1810are wound around the core of the bobbin870, the first shielding windings1812are wound around the first primary windings1810, the secondary windings814are wound around the first shielding windings1812, the second shielding windings2816are wound around the secondary windings814, and the second primary windings2818are wound around the second shielding windings2816.

In some embodiments, the two separate shielding windings812,816start at a terminal shared by S′1and S′2at a first side of the bobbin870and end (at S1and S2) at the same position at the opposite (second) side of the bobbin870. Therefore, the shielding windings812,816are advantageously aligned symmetrical about the secondary windings814. Because the shielding windings812,816are aligned symmetrically on either side (i.e., both of two opposite sides—inside and outside) of the secondary windings814, for each point along the first dimension H of the bobbin870, a voltage of one shielding winding (e.g.812) is the same as the voltage of the other shielding winding (e.g.,816). Consequently, there is no voltage difference between the two shielding windings812,816. Thus, displacement current which flows from the first shielding windings812to the secondary windings814is canceled by displacement current which flows from the second shielding windings816to the secondary windings814. As a result, the common mode noise current ipsis advantageously blocked from flowing from a primary side of the transformer802to a secondary side of the transformer802. Advantageously, because the shielding windings812,816are arranged symmetrically on either side of the secondary windings814, the transformer802is easily manufacturable because the shielding windings812,816do not have to be individually exactly aligned with the secondary windings814.

FIG.8Bis a simplified schematic representation860of the transformer802shown inFIG.8A, in accordance with some embodiments. As described with reference toFIG.8A, the transformer802includes primary windings810,818, the secondary windings814, the shielding windings812,816, and the terminals A (830), B (831), C (832), D (833), S1(834), S′1(835), S2(836), and S′2(837). The terminals835and837are configured to be electrically connected to each other. The terminals834,836are configured to be floating (e.g., the terminals834,836may be surrounded by tape or another electrical insulating material). Also shown is the inter-winding capacitance Cpsand a representation of a common mode noise current ipswhich has advantageously been reduced to 0 Amps due to the interleaved shielding windings812,816which are aligned symmetrically on either side of the secondary windings814.

A simplified graph880ofFIG.8C, with reference to legend850, illustrates a voltage potential V developed at each point along the windings812,814,816and along the first dimension H of the bobbin870for the transformer802, in accordance with some embodiments. Also shown is the second dimension W of the bobbin870. As shown, because the shielding windings812,816are aligned symmetrically on either side of the secondary windings814, displacement current from the first shielding windings812to the secondary windings814is canceled by displacement current from the second shielding windings816to the secondary windings814. As a result, common mode noise current ipsis advantageously blocked from flowing from a primary side of the transformer802to a secondary side of the transformer802.

FIG.9is a simplified schematic of a portion of a power converter900that implements the power converter100ofFIG.1using the transformer802, in accordance with some embodiments. Some portions of the power converter100have been omitted fromFIG.9to simplify the description but are understood to be present. The power converter900includes the input voltage capacitor C1(electrically connected across an input voltage node for the input voltage Vinand a primary-side ground node) that receives the input voltage Vin, the main switch M1, the primary-side controller circuit118, the output buffer circuit112(electrically connected across an output voltage node for the output voltage Voutand a secondary-side ground node) across which the output voltage Voutis developed, the secondary-side switch M2, and the synchronous rectifier switch controller circuit120. The power converter900is configurable to be connected to the load RL.

As shown, the transformer802includes the primary windings810,818, the secondary windings814, the shielding windings812,816, and the terminals A (830), B (831), C (832), D (833), S1(834), S′1(835), S2(836), and S′2(837). The first primary-side terminal A (830) is configured to be electrically connected to the drain node of the main switch M1(i.e., a first node at the primary-side of the power converter900). The source node of the main switch M1is electrically connected to the primary-side ground node. The gate node of the main switch M1is electrically connected to and controlled by the primary-side controller circuit118. The second primary-side terminal B (831) is configured to be electrically connected to the input voltage node for the input voltage Vin(i.e., a second node at the primary-side of the power converter900). The terminals835and837are electrically connected to each other as the intermediate shielding terminal and are configured to be electrically connected to the primary-side of the power converter900at the voltage bias node, such as the primary-side ground node (i.e., a third node at the primary-side of the power converter). The first shielding terminal834and the second shielding terminal836of the shielding windings812,816, respectively, are configured to be electrically floating. The first secondary-side terminal C (832) is configured to be electrically connected to a drain node of the secondary-side switch M2(i.e., a first node at a secondary-side of the power converter900). The source node of the secondary-side switch M2is electrically connected to the secondary-side ground node. The gate node of the secondary-side switch M2is electrically connected to and controlled by the synchronous rectifier switch controller circuit120. The second secondary-side terminal D (833) is configured to be electrically connected to the output voltage node for the output voltage Vout(i.e., a second node at the secondary-side of the power converter900). Also shown is the inter-winding capacitance Cpsand a representation of a common mode noise current ipswhich has advantageously been reduced to 0 Amps due the interleaved shielding windings812,816which are aligned symmetrically on either side of the secondary windings814. As shown, because the common mode noise current ipsis blocked from flowing from the primary side of the power converter900to the secondary side of the power converter900, no Y Capacitor (or a very small Y Capacitor) is required to meet EMI standards.

FIG.10shows example transformer specification details1000for the transformer802, in accordance with some embodiments. In practice, for a transformer, such as the transformer802, having two shielding layers, the number of turns of shielding windings is based on the number of secondary windings of the transformer. In some embodiments, if the start terminals of the shielding windings and secondary windings are on the opposite side of the bobbin, the number of shielding winding turns equals the number of secondary winding turns plus 0.5. In other embodiments, if the start terminals of the shielding windings and the secondary windings are on the same side of the bobbin, the number of shielding winding turns equals the number of secondary winding turns. In some embodiments, the AWG of the shielding windings should be selected to be small enough such that there is minimum skin effect loss. In some embodiments, the shielding layer should completely fill the bobbin winding window. In such embodiments, the number of parallel wire strands of the shielding windings can be calculated as,

where K is the bobbin winding window filling factor, in the range of 0.5 to 1, depending on the winding process.

Reference has been made in detail to embodiments of the disclosed invention, one or more examples of which have been illustrated in the accompanying figures. Each example has been provided by way of explanation of the present technology, not as a limitation of the present technology. In fact, while the specification has been described in detail with respect to specific embodiments of the invention, it will be appreciated that those skilled in the art, upon attaining an understanding of the foregoing, may readily conceive of alterations to, variations of, and equivalents to these embodiments. For instance, features illustrated or described as part of one embodiment may be used with another embodiment to yield a still further embodiment. Thus, it is intended that the present subject matter covers all such modifications and variations within the scope of the appended claims and their equivalents. These and other modifications and variations to the present invention may be practiced by those of ordinary skill in the art, without departing from the scope of the present invention, which is more particularly set forth in the appended claims. Furthermore, those of ordinary skill in the art will appreciate that the foregoing description is by way of example only, and is not intended to limit the invention.