Patent Publication Number: US-2023138281-A1

Title: Symmetric Air-core Planar Transformer with Partial Electromagnetic Interference Shielding

Description:
TECHNICAL FIELD 
     This relates to a low-cost symmetric air-core planar transformer with high coupling and low electromagnetic interference (EMI). 
     BACKGROUND 
     Moving signals and power across an isolation barrier is a common challenge for designers. Isolation might be required for safety, noise immunity or large potential differences between system domains. For example, a cellphone charger is internally isolated to prevent humans from becoming electrically tied to the mains if the connector short-circuits. In other applications like factory robots, sensitive control circuitry sits on a separate ground and is isolated from the motors that draw large DC currents that create noise and ground bounces. Similarly, in electric drive automotive applications, sensitive control circuitry sits on a separate ground and is isolated from the drive motor(s) that draw large DC currents that create noise and ground bounces 
     SUMMARY 
     In described examples, a laminate transformer includes a multilayer substrate having at least first, second, third, and fourth metal layers. The second metal layer and the third metal layer are separated by a voltage barrier having a thickness. A first multiloop coil has at least a first loop on the first metal layer and at least a second loop on the second metal layer. A second multiloop coil has at least a third loop on the third metal layer and at least a fourth loop on the fourth metal layer. A partial EMI shield for the first multiloop coil is on the second metal layer. A partial EMI shield for the second multiloop coil is on the third metal layer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a plot illustrating an example EMI interference standard requirement. 
         FIGS.  2 ,  3    are cross-sectional views of conventional transformers. 
         FIG.  4    is a cross-sectional view of an example symmetric air-core transformer that includes partial EMI shielding. 
         FIG.  5    is an isometric view of example coils for the transformer of  FIG.  4   . 
         FIGS.  6 - 8    are plots of simulated performance of the transformer of  FIG.  4   . 
         FIG.  9 A  is a top view and  FIG.  9 B  is a cross sectional view of an example packaged air-core transformer. 
         FIG.  10    is a cross-sectional view of an example laminate transformer package mounted on a PCB. 
         FIG.  11    is a block diagram of an example isolation device that includes a transformer with partial EMI shielding. 
     
    
    
     DETAILED DESCRIPTION 
     In the drawings, like elements are denoted by like reference numerals for consistency. 
     Galvanic isolation is a principle of isolating functional sections of electrical systems to prevent current flow from one section to another. To prevent current flow, no direct conduction path is permitted. Energy or information can still be exchanged between the sections by other means, such as capacitance, induction, or electromagnetic waves, or by optical, acoustic, or mechanical means. 
     Galvanic isolation may be used where two or more electric circuits must communicate, but their grounds may be at different potentials. It is an effective method of breaking ground loops by preventing unwanted current from flowing between two units sharing a ground conductor. Galvanic isolation is also used for safety, preventing accidental current from reaching ground through a person&#39;s body. 
     The general operation of laminate transformer galvanic isolation devices is known; see, for example, “UCC12050 High-Efficiency, Low-EMI, 5-kVRMS Reinforced Isolation DC-DC Converter,” SNVSB38C, September 2019, revised April 2020, which is incorporated by reference herein. 
     The term “air-core” refers to the permeability of the core in example transformers. In described examples, transformer coils are fabricated on non-magnetic laminate material that has approximately the same permeability as air. Permeability ( 0  is the measure of magnetization that a material obtains in response to an applied magnetic field. The laminate provides mechanical support but does not improve coupling between the transformer coils. 
     Numerous governing bodies regulate the permissible levels of conducted and radiated emissions generated by an end product in order to maintain electromagnetic compatibility (EMC). See, for example: “An overview of conducted EMI specifications for power supplies,” Timothy Hegarty, February 2018, and “An overview of radiated EMI specifications for power supplies, Timothy Hegarty, June 2018 Power converters embedded within products for automotive, communications and industrial application market sectors demand high switching frequencies, advances in circuit topologies and high-speed switching power devices. EMI is an increasingly significant and challenging topic for fast-switching power converters. Moreover, the EMI filter required to achieve regulatory compliance can represent a significant portion of the overall system footprint, volume, and cost. 
       FIG.  1    is a plot illustrating an example radiated EMI interference standard requirement. From an automotive electronic product designer&#39;s perspective, an essential set of conducted and radiated emissions tests are those specified by IEC (International Electrotechnical Commission) CISPR 25 (Comité International Special des Perturbations Radioelectriques), the fourth edition from 2016 being relevant at the time of this writing. This international standard applies to automotive components and modules, with measurements performed using one or two 5μ/50Ωartificial networks (ANs) depending on the grounding configuration. The standard refers to the “protection of onboard receivers,” with conducted noise measured over a frequency range from 150 kHz to 108 MHz in specific frequency bands. These frequency ranges are dispersed across the low wave (LW), medium wave (MW) AM broadcast, short wave (SW), citizen band (CB) mobile service band, very high frequency (VHF), television (TV), and FM broadcast. CISPR 25 specifies conducted emission limits for peak (PK), quasi-peak (QP), and average (AVG) signal detectors. 
       FIG.  1    plots the relevant radiated limit lines for CISPR 25, Class 5, the most stringent requirement from CISPR 25. Even though higher noise spikes are theoretically allowed in the gaps between frequency bands, automotive manufacturers may choose to extend these frequency ranges according to their specific in-house requirements. The limits are quite challenging, particularly the 18 dB μV average (or 38 dB μV peak) limit in the VHF and FM bands spanning 68 MHz to 108 MHz. The filter component&#39;s parasitics degrade EMI filter attenuation at such frequencies. 
     Power-supply products marketed for communications and information technology (IT) end equipment within the EU (European Union) must meet the requirements of EN 55032 (CISPR 32). Equipment intended primarily for use in a residential environment must meet Class B limits, with all other equipment complying with Class A. 
       FIG.  2    is a cross-sectional view of a conventional isolation transformer  200  that does not include inter-coil shielding. Transformer  200  is a laminate transformer in which a planar primary coil of three loops  211 ,  212 ,  213  and a secondary coil of four loops  221 ,  222 ,  223 ,  224  are spaced apart by a barrier space  205  that is sufficient to provide an insulated barrier to withstand a specified breakdown voltage. In this example, barrier space  205  is approximately 100 μm. A differential mode primary voltage VpDM is applied to the primary coil and a secondary voltage VsDM is produced on the secondary coil. VpDM is a functional voltage that is used for the power transfer across the transformer barrier. Transformer  200  does not meet EMI requirements because of differential-mode injection, which is dominant in the part of the coil closer to the driver circuitry. Transformer  200  does meet efficiency requirements because it has a high coupling coefficient of approximately 80% due to the proximity of the secondary coil to the primary coil. Parasitic capacitance  250  is the parasitic capacitance between coils and is the root cause of EMI in isolated converters. It does not contribute to the power transfer but it injects part of the current across the barrier leading to increased EMI. 
       FIG.  3    is a cross-sectional view of a conventional isolation transformer  300  that has full inter-coil shielding. Transformer  300  is a laminate transformer in which a planar primary coil of three loops  311 ,  312 ,  313  and a secondary coil of four loops  321 ,  322 ,  323 ,  324 . In this example, a planar primary shield coil of three loops  331 ,  332 ,  333  and a secondary shield coil of four loops  341 ,  342 ,  343 ,  344  are spaced apart by a barrier space  305  that is sufficient to provide an insulated barrier to withstand a specified breakdown voltage. In this example, barrier space  305  is approximately 100 μm. A primary voltage VpDM is applied to the primary coil and a secondary voltage VsDM is produced on the secondary coil. 
     Transformer  300  does meet EMI requirements because the shield coils reduce differential and common mode EMI. However, transformer  300  does not meet efficiency requirements because the primary and secondary coils are separated further apart by the shield coils and the coupling coefficient is thereby reduced to approximately 62%. Efficiency may be increased by adding magnetic components to increase coupling, but at an increased cost. 
       FIG.  4    is a cross-sectional view of an example symmetric air-core transformer  400  that includes partial EMI shielding. In this example, laminate transformer  400  includes a multilayer laminate substrate  404  that has a top surface and an opposite bottom surface. A primary coil  401  having loops  411 ,  412 ,  413  and a secondary coil  402  having loops  421 ,  422 ,  423 ,  424  are each located on two or more laminate layers of multilayer laminate substrate  404 . 
     In this example, the laminates are copper clad laminates and pre-pregs. Each pre-preg isolation layer has a thickness in the range of 30-70 um. This allows the copper that forms the coils to be much thicker than the metal used in prior digital isolation devices that are formed on a silicon substrate. This allows larger current flows to be handled for power and signal applications. Transformer performance (quality factor, efficiency) may thereby be controlled by using copper thickness of 12 μm-30 μm and multiple metal layers to allow parallel inductor coils and lower coil resistance. In various examples, two to eight, or more metal layers may be used to form a secondary coil and a primary coil. 
     In this example, the primary coil  401  is fabricated using two parallel conductive layers M1, M2 within multilayer laminate substrate  404 . The secondary coil  402  is fabricated using two parallel conductive layers M3, M4 within multilayer laminate substrate  404 . Each conductive layer is patterned and etched to form conductive signal lines that are arranged in a symmetrical loop. Vias are fabricated to connect the separate layers to form a completed coil. A primary voltage VpDM is applied to the primary coil and a secondary voltage VsDM is produced on the secondary coil 
     In this example, a partial primary electromagnetic shield coil of two loops each separated into two portions  431 ,  432 ,  433 ,  434  and a partial secondary electromagnetic shield coil of two loops each separated into two portions  441 ,  442 ,  443 ,  444  are spaced apart by a barrier distance  405  that is sufficient to provide an insulated barrier to withstand a specified breakdown voltage. In this example, barrier distance  405  is approximately 100 μm. Primary shield portions  431 ,  432 ,  433 ,  434  are coupled to a ground reference in the primary voltage domain, while secondary shield loop portions  441 ,  442 ,  443 ,  444  are coupled to a ground reference in the secondary voltage domain. 
     In this example, primary loops  411 ,  412  are on metal layer M1, while primary loop  413  and partial primary shield portions  431 ,  432 ,  433 ,  434  are on metal layer M2. Similarly, secondary loops  421 ,  422  are on metal layer M4, while secondary loops  413 ,  414  and partial secondary shield portions  441 ,  442 ,  443 ,  444  are on metal layer M3. In this manner, primary loop  413  and secondary loops  423 ,  424  are positioned as close together as possible while maintaining the ISO barrier distance of 100 μm. In this example, the isolation barrier is rated to provide an isolation voltage protection of 5 kv. In another example, the isolation barrier may be rated at 3 kv due to a smaller barrier thickness or a lower breakdown voltage rating for the laminate material. In other examples, the isolation rating may be higher or lower than this, depending on the design of the isolation transformer. 
     This configuration meets the CISPR 25 EMI requirement and provides improved coupling of approximately 69% compared to transformer  300  ( FIG.  3   ) at approximately 62%. This configuration also provides a higher Q-factor due to reduced proximity effect which also contributes to improved efficiency. When currents are flowing through two or more nearby conductors the distribution of current is constrained to smaller regions resulting in increased AC resistance 
     In this example, a thermal enhancement feature provides a thermal conductive path  451  with one end coupled to primary shield loop  431  and the other end exposed on the outer surface of multilayer substrate  404 . In this example, thermal pads  454 ,  455 ,  456  are interconnected by vias. Similarly, a thermal path  452  is provided from secondary partial shield portion  444  to an outer surface of multilayer substrate  404  that includes thermal pads interconnected with vias. The various thermal pads of the thermal enhancements may be complete loop portions that extend the length of the respective shield loop portion. In some examples, the thermal pads may be shortened. Multiple vias can be provided to interconnect the thermal pads to improve thermal conductivity. 
       FIG.  5    is an isometric view of example coils for air-core transformer  400 . In this view, primary coil loops  411 ,  412 ,  413  are visible and illustrate the symmetry of the primary coil in the x-y plane. Crossover  561  utilizes vias between metal layer M1 and M2 ( FIG.  4   ) to allow a portion of loops  411 ,  412  to swap places, while crossover  462  utilizes vias between metal layer M1 and M2 to allow a portion of loops  412 ,  413  to swap places. This allows loops  411 ,  412  to be fabricated on metal layer M1 and loop  413  to be fabricated on metal layer M2. In order to provide symmetry around the x-axis  501  of transformer  400 , loop  413  is a middle portion of multiloop coil  401 , while loop portion  411  is one end of multiloop coil  401  and loop portion  414  is an opposite end of multiloop coil  401 . 
     The secondary coil is also symmetrical in the x-y directions and uses crossovers similar to  561 ,  562  to allow loops  421 ,  422  to be fabricated on metal layer M4 and loops  423 ,  424  to be fabricated on metal layer M3 in a symmetrical configuration around the x-axis  501 . 
     Maintaining symmetry in the primary coil  401  and the secondary coil  402  minimizes noise injection across the barrier  405 . 
     In this example, partial primary shield loop portions  431 ,  432 ,  433 ,  434  and partial secondary shield loop portions  441 ,  442 ,  443 ,  444  are approximately a half loop with a space left between adjacent half loop portions to provide a location for crossover  561 ,  562 , etc. 
     In another example, the partial primary and secondary shields may be more of a solid piece of electrically conductive material. For example, loop portions  431 ,  432  may be expanded in width to merge together into a single half loop portion. Similarly, loop portions  433 ,  434  may be expanded and merged, etc. However, larger electrically conductive elements tend to incur higher loses due to eddy currents formed in the shield element. Therefore, dividing the shield into narrower, open-ended elements reduce loses from eddy currents. On the other hand, a more solid shield element will provide increased EMI protection, as long as the higher eddy current loses can be tolerated in a given design. 
     In this example, both the primary coil  401  and the secondary coil  402  can be active, depending on the system design in which it is being used. For that reason, an EMI shield on both the primary coil  401  and the secondary coil  402  is needed. For a design in which only one-way transmission from the primary coil will occur, then the secondary EMI shield could be eliminated. 
       FIGS.  6  and  7    are plots of simulated EMI performance of the transformer of  FIG.  4    in decibels referenced to one microvolt per meter (dB μV/m) vs frequency in megahertz (MHz). In this example simulation, Vin=5V, Vout=5V, operating frequency=33 MHz.  FIG.  6    illustrates CISPR 25, class 5 limits for radiated EMI, while  FIG.  7    illustrates CISPR 32 limits for radiated EMI. CISPR 32 requirement are met with 13 dB margin. For CISPR 25, the simulation indicates an expected pass of class 5 with spread-spectrum modulation (SSM), without additional PCB (printed circuit board) components, such as: ferrite beads, common-mode chokes, integrated stitching capacitor, etc. SSM is an EMI mitigation technique that changes the switching frequency within a defined frequency range (e.g. +/−5%) following a modulation pattern (e.g. triangular pattern) with a modulation frequency (e.g. fm=40 kHz). As a result, the energy is spread across the frequency range (e.g. +/−5%) reducing the magnitude. EMI improvements of 15-20 dB can be achieved. 
     In the example simulation represented by  FIG.  6   , the FM band (70-108 MHz) EMI is less than 10 dB μV/m. 16.8 dB is the max violation of the limit lines and 11.2 dB is the average violation allowed for CISPR 25 for the FM band prior to SSM. 
     In this example, simulation results indicate an efficiency of approximately 70%, which results in a temperature rise of approximately 30 C at a 105 C ambient, assuming a 28-pin package. 
       FIG.  8    includes plots illustrating quality factor (Q) two example simulated configurations  801 ,  802 . Example configuration  801  is the same as transformer  400  ( FIG.  4   ) in which two primary coil loops are on metal layer M1, two primary shield portions and one primary coil loop are on metal layer M2, two secondary shield portions and two secondary coil loops are on metal layer M3, and two secondary coil loops are on metal layer M4. Example configuration  802  has three primary coil loops on metal layer M1, two primary shield portions on metal layer M2, two secondary shield portions on metal layer M3, and four secondary coil loops on metal layer M4. In each configuration, a 100 μm barrier distance is maintained between primary and secondary. 
     Plot lines  811 ,  812 , and  813  represent mutual quality factor Q12, primary quality factor Q11, and secondary quality factor Q22 respectively for configuration  801 . Plot lines  821 ,  822 ,  823  represent Q12, Q11, and Q22 respectively for configuration  802 . Frequency line  830  indicates operation at 33 MHz. As indicated by line  830 , the Q12 for configuration  801  is approximately 35 and for configuration  802  is approximately 29, for an improvement of approximately six. Similarly, Q11 is improved by two and Q22 is improved by four for configuration  801  compared to configuration  802 . This results in an improvement in efficiency from approximately 65% for configuration  802  to approximately 69% for configuration  801 . 
       FIG.  9 A  is a top view and  FIG.  9 B  is a cross-sectional view of an example isolation device  900  that includes a laminate transformer  400  ( FIGS.  4 ,  5   ). In this example, laminate transformer  400  includes a multilayer laminate substrate  404  that has a top surface and an opposite bottom surface. Primary coil  401  and secondary coil  402  ( FIG.  4   ) along with partial primary and secondary shields are each located on layers of multilayer substrate  404 . Primary terminals  981 , secondary terminals  982 , partial primary shield terminals  983 , and partial secondary terminals  984  are coupled to their respective coil structures. 
     A lead frame is attached to transformer  400 , typically using an adhesive material. In this example, left lead frame  971  has a portion  173  that overlaps and is adhered to substrate  404 . Similarly, right lead frame  972  has a portion  974  that overlaps and is adhered to substrate  404 . 
     In this example, driver circuitry IC die  976  is attached to a die attach pad on left lead frame  971  and rectifier circuitry IC die  972  is attached to a die attach pad on right lead frame  972 . In this example, a wire bonding technique is used to interconnect transformer terminals  981 ,  982 ,  983 ,  984  with bond pads on IC die  972 ,  973  and leads on lead frame  971 ,  972 . 
       FIG.  10    is a cross-sectional view of the isolation device  1000  that includes symmetric air-core transformer  400 , illustrating thermal conductivity within device  1000 . Isolation device  1000  is encapsulated in a mold compound  1060  using a known integrated packaging technique. In this example, isolation device  1000  is mounted on a printed circuit board (PCB)  1080  on which additional components and/or integrated circuits are mounted (not shown). PCB  1080  includes metallic pads  1083 ,  1084  onto which the leads of lead frame  971 / 972  are soldered using known soldering techniques. Various metallic signal lines and power planes  1084 ,  1085 ,  1086 , etc. within PCB  1080  act as heat sinks for isolation device  1000 . 
     Heat is generated within coils  401 ,  402  due to resistive heating caused by the ohmic resistance (R) of the coils and the amount of current (I) being conducted by the coils. This is often referred to as “I 2 R heating”. Heat generated within the coils must be dissipated to keep the isolation device from overheating. Some heat is dissipated by infrared radiation away from device  1000 . Some heat may be dissipated by convection of the surrounding air around isolation device  1000 . However, most of the heat is dissipated by conduction from coils  401 ,  402  of transformer  400  through substrate  404  and then through lead frames  971 ,  972  to PCB  1080 , as illustrated by thermal conduction paths  1088 ,  1089 . In this example, thermal conduction path  1088  includes traveling through thermal path  451  ( FIG.  4   ). Likewise, thermal conduction path  1098  includes traveling through thermal path  452  ( FIG.  4   ). Thermal conduction paths  1088 ,  1089  have a higher thermal conductivity than the material of multilayer substrate  404  and thereby heat dissipation is improved. 
       FIG.  11    is a block diagram of example isolation device  1000  ( FIG.  10   ) that includes a laminate transformer  400  in which partial primary and secondary shields are provided to reduce EMI conduction through transformer  400 . Boundary region  1101  illustrates a galvanic isolation boundary that is provided by isolation device  1000  using laminate transformer  400 . 
     Circuitry  976  includes inverter switching circuitry and driver circuitry configured to invert a direct current (DC) voltage applied to terminal Vinp in a periodic manner so that a resultant oscillating voltage applied to primary coil  401  will induce a voltage in secondary coil  402 . Circuitry  978  rectifies and filters the induced voltage to provide a DC output signal on output terminal Viso. In this manner, a DC input signal is transferred across a galvanic isolation barrier to form an output DC signal. In this example, the isolation barrier is rated to provide an isolation voltage protection of 5 kv. In other example, the isolation barrier may be rated at 3 kv. In other examples, the isolation rating may be higher or lower than this, depending on the design of the isolation transformer. 
     Circuitry  978  is mounted on a die attach pad on a lead frame and is coupled to secondary coil  402  as described in more detail herein above. Circuitry  976  is mounted on a separate lead frame and is coupled to primary coil  401 . Circuitry  976  together with primary coil  401  reside in a primary voltage domain. Circuitry  978  together with secondary coil  402  reside in a secondary voltage domain. 
     Laminate transformer  711 , circuitry  731 ,  732  and the associated lead frames are all encapsulated together with a mold compound using a known or a later developed molding technique to form a packaged isolation device. 
     Other Embodiments 
     In described examples, a single isolation device is illustrated on a PCB, such as PCB  1080 , ( FIG.  10   ). In other examples, several isolation devices may be mounted on a single PCB to provide galvanic isolation to several signals that must communicate across an isolation barrier. 
     In described examples, the coils are illustrated as being octagonal. In another example, the coils may have a different symmetrical shape, such a circular, hexagonal, square, rectangular, etc. 
     In described examples, the transformer coils are formed for copper layers having a thickness of 12 um-30 um. In other examples, thinner copper may be used, depending on expected current flow. In other examples, thicker copper may be used to support higher current flow. 
     In described example, a three-loop primary coil and a four-loop secondary coil are illustrated. In another example, additional, or fewer, loops may be used for the primary and/or the secondary coil. In another example, the primary coil and the secondary coil may have the same number of loops. 
     In described examples, the lead frames are made from copper. In another example, the lead frames may be fabricated from another electrically conductive material, such as aluminum, etc. 
     In described examples, the coils and shields are made from copper. In another example, the coil and shields may be fabricated from another electrically conductive material, such as aluminum, etc. 
     In described examples, layers of the laminate substrate are laminate materials that include bismaleimide triazine (BT) and that have a high breakdown strength of 100-120V/um. Such material may be obtained from Mitsubishi Gas Chemical (MGC) as copper clad laminates and pre-pregs, for example. However, in other examples, different types of laminate material may be used, such as ABF (Ajinomoto Buildup Films) material. 
     In described examples, the separate circuit ICs are coupled to the lead frame leads using a wire bonding technique. In another example, other types of known or later developed techniques may be used to couple the ICs to the lead frame and/or transformer coils. 
     In described examples, separate left and right lead frame elements are illustrated. However, during fabrication, a large sheet or strip of lead frames is fabricated using etching, stamping or other known or later developed techniques. Multiple laminate transformers are then positioned on the lead frame sheet/strip using a pick and place machine and attached with laminate attach adhesive. The circuit IC die are also positioned ono the lead frame sheet/strip using a pick and place machine and attached with die attach adhesive. After a wire bonding process, the entire lead frame sheet/strip is then molded to form multiple isolation devices. The lead frame sheet/strip is then cut apart to separate the isolation devices. 
     In this description, the term “couple” and derivatives thereof mean an indirect, direct, optical, and/or wireless electrical connection. Thus, if a first device couples to a second device, that connection may be through a direct electrical connection, through an indirect electrical connection via other devices and connections, through an optical electrical connection, and/or through a wireless electrical connection. 
     Modifications are possible in the described embodiments, and other embodiments are possible, within the scope of the claims.