United States Patent
`US 6,888,438 B2
`(10) Patent No.:
`(12)
`Huiet al.
`(45) Date of Patent:
`May3, 2005
`
`
`US006888438B2
`
`(75)
`
`(54) PLANAR PRINTED CIRCUIT-BOARD
`TRANSFORMERSWITH EFFECTIVE
`ELECTROMAGNETIC INTERFERENCE
`(EMI) SHIELDING
`Inventors: Ron Shu Yuen Hui, Shatin (HK); Sai
`Chun Tang, Yuen Long (HK)
`;
`i,
`;
`(73) Assignee: City University of Hong Kong,
`Kowloon (HK)
`Subject to any disclaimer, the term ofthis
`:
`.
`patent is extended or adjusted under 35
`US.C. 154(b) by 190 days.
`
`(*) Notice:
`
`(21) Appl. No.: 10/282,335
`
`(22)
`
`(65)
`
`Filed:
`
`Oct. 28, 2002
`
`Prior Publication Data
`US 2003/0095027 Al May 22, 2003
`
`Related U.S. Application Data
`
`(63) Continuation-in-part of application No. 09/883,145,filed on
`Jun. 15, 2001, now Pat. No. 6,501,364.
`7
`(51)
`Tint. Cd eee ceeecteneteeeesenseneees HO1F 5/00
`(52)
`.. 336/200; 336/232; 336/223
`
`(58) Field of Search oe 336/200, 223,
`336/232; 29/602.1
`
`(56)
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`3,866,086 A *
`4,494,100 A *
`4,510,915 A *
`4,613,843 A *
`Eo
`4,748,532 A
`Eo
`4,890,083 A
`
`2/1975 Miyoshietal.
`1/1985 Stengel etal.
`4/1985 Ishikawaetal.
`9/1986 Esperet al.
`Commanderet al.
`5/1988
`Trenkleret al.
`12/1989
`
`8/1991 Ikeda
`5,039,964 A *
`7/1995 Ikeda
`5,431,987 A *
`1/1996 Walters... eee 29/602.1
`5,487,214 A *
`3/1996 Takahashietal.
`5,502,430 A *
`5,579,202 A * 11/1996 Tolfsen etal.
`eeraes A : oO1997 Danbyet al.
`7044,
`/1998 Murphy
`6,023,161 A *
`2/2000 Dantskeret al.
`6,420,953 Bl *
`7/2002 Dadafshar occ 336/200
`
`EP
`JP
`Ip
`JP
`JP
`
`FOREIGN PATENT DOCUMENTS
`0 147 499
`*
`7/1985
`54-110424
`*
`8/1979
`4-10680
`=
`1/1992
`6013247a
`*
`1/1994
`20011110651 A *
`4/2001
`
`OTHER PUBLICATIONS
`
`Tang et al., “Coreless planar printed—circuit-board (PCB)
`transformers—A fundamental conceptfor signal and energy
`transfer,” IEEE Transactions on PowerElectronics, vol. 15,
`No. 5, pp. 931941 (Sep. 2000).*
`Hui et al., “Coreless printed—circuit board transformers for
`signal and energy transfer,” Electronics Letters, vol. 34, No.
`11, pp. 1052-1054 (May 1998).*
`
`(Continued)
`
`Primary Examiner—Elvin Enad
`Assistant Examiner—Jennifer A. Poker
`(74) Attorney, Agent, or Firm—Merchant & Gould P.C.
`(7)
`ABSTRACT
`Novel designs for printed circuit board transformers, and in
`particular for coreless printed circuit board transformers
`designed for operation in powertransfer applications,
`in
`which shielding is provided by a combination of ferrite
`plates and thin conductive sheets.
`
`7 Claims, 8 Drawing Sheets
`
`EnheerShoes
`a ae
`
`_perrite Flete
`
`Thetnatty Benders
`Eosatsimg Laver
`
`Frismary Wining
`490 be dap aide if
`te HOR
`Polyorminane-tomtet
`fagabuied Copper Syee
`Frdocine Lenreite
`
`Law
`ss
`
`
`
`
`a, Thematly Cordura:
`Ineadating Leyes
`
`Pocrite Flee
`
`Copper Brvce
`
`1
`
`Exhibit 2002
`Momentum Dynamicsv. Witricity
`IPR2021-01116
`
`Exhibit 2002
`Momentum Dynamics v. Witricity
`IPR2021-01116
`
`1
`
`

`

`US 6,888,438 B2
`Page 2
`
`OTHER PUBLICATIONS
`
`Hui et al., “Some electromagnetic aspects of coreless PCB
`transformers,” IEEE Transactions on Power Electronics,
`vol. 15, No. 4, pp. 805-810 (Jul. 2000).*
`Ondaet al., “Thin type DC/DC converter using a coreless
`wire transformer,” IEEE Power Electronics Specialists Con-
`ference, pp. 1330-1334 (Jun. 1994).
`Coombs, C.F., “Printed Circuits Handbook,” 3rd Ed.
`McGraw-Hill, p. 6.32 (1998).
`Tang et al., “Characterization of coreless printed circuit
`board (PCB) transformers,” JEEE Transactions on Power
`Electronics, vol. 15, No. 6, pp. 1275-1282 (Nov. 2000).
`Paul, C.R., Introduction to Electromagnetic Compatibility,
`Chapter 11—Shielding, pp. 632-637 (1992).
`
`Tanget al., “A low-profile power converter using printed—
`circuit board (PCB) power transformer with ferrite polymer
`composite,” IEEE Transactions on Power Electronics, vol.
`16, No. 4, pp. 493-498 (Jul. 2001).
`
`Hui et al., “Coreless PCB based transformers for power
`MOSFETAGBTgate drive circuits,” IEEE Power Electron-
`ics Specialists Conference, vol. 2, 1171-1176 (1997).
`
`Bourgeois, J.M., “PCB Based Transformer for Power MOS-
`FETDrive,” IEEE, pp. 238-244 (1994).
`
`“High-Frequency Alalog Integral Circuit
`Goyal, R.,
`Design,” pp. 107-126 (1995).
`
`* cited by examiner
`
`2
`
`

`

`U.S. Patent
`
`May3, 2005
`
`Sheet 1 of 8
`
`US 6,888,438 B2
`
`Ferrite Flaic
`
`Thenwaily Qonductive
`Isiiulatimg Layer
`
`Primary indie
`feo the top side af de
`PCE
`
`Polyorethane-taaded
`“Wresatee Copper Wires
`
`Bichwise Lariat
`
`ri
`oom
`
`* Socondsry Winding
`fan ihe Britian subg
`af thie POS)
`
`
`
`
`
`
`Thenmally Cpcductive
`Jesulating Layer
`
`
`
`~Perrite Plate
`
`Fig. }.
`
`Covndinetis
`PRingy PIG
`|
`Faack Separtnin,
`Were, ue
`
`i !
`
`Primary Side
`
`Onwkcine
`Be] Phicknes, =
`
`
`
`SuliderMass,
`
`t
`!
`Secunda "Winding
`!
`
`Tranafeciuer Radsia, E
`
`
`TeraiCouductioe
`
`'
`i
`.
`|
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`|) weep Layer
`|
`L
`i
`Praiaoarcult Bosid Lanier
`my
`
`SeooruianrySide
`
`3
`
`

`

`U.S. Patent
`
`May3, 2005
`
`Sheet 2 of 8
`
`US 6,888,438 B2
`
`dan the beeragn side
`
`
`of Tiny Wind
`
` af
`(on bie tap gad
`the BITE]
`
`Coteer Shees
`
`Porrtte Plate
`
`Thermally Condacieve
`
`insulaoie Laver
`
`Pelyurethanc-comed
`Iya
`
`sechdary Winding
`
`4, |etnstly Conductive
`
`insulating Lever
`
`
`
`
`«Ferrite Flare
`
`per Shreve
`
`
`
`4
`
`

`

`U.S. Patent
`
`May3, 2005
`
`Sheet 3 of 8
`
`US 6,888,438 B2
`
`
`
`Cusducbsr
`‘Thickness, fe
`
`
`
`i
`'
`,
`Tljean ly Conduct,
`
`
`i Cesger Sheets
` Saeondary Wading
`|
`Forme Flutes
`ingvéniing Layer
`'
`!
`Prisiad lanl Board Laminate
`'
`Transiiemicr Fexdiad, #
`mt
`Senuncdiry Side
`
`Fig, 3h,
`
`Asis efSyienigtry
`z
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`FreeSpace
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`Fine Eales
`
`:
`
`Ietalotar Lage
`
`4am
`
`Primary Windings
`
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`|
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`
`ms
`z
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`Fig, 5
`
`5
`
`

`

`U.S. Patent
`
`May3, 2005
`
`Sheet 4 of 8
`
`US 6,888,438 B2
`
`Magnetic—=e Hnwinnab)
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`Lado
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`25~
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`

`

`U.S. Patent
`
`May3, 2005
`
`Sheet 5 of 8
`
`US 6,888,438 B2
`
`&fens}
`
`
`Fig. 11,
`
`7
`
`

`

`U.S. Patent
`
`May3, 2005
`
`Sheet 6 of 8
`
`US 6,888,438 B2
`
` tt}
`
`kt mm!
`
`8
`
`
`

`

`US 6,888,438 B2
`
`U.S. Patent
`
`May3, 2005
`
`Sheet 7 of 8 a atee
`
`X time)
`Fig. 16
`
`
`
`
`
`|——FerwsPlaces Only
`
`SeeConger Sheri Only
`+
`
`
`
`
`
`9
`
`

`

`U.S. Patent
`
`May3, 2005
`
`Sheet 8 of 8
`
`US 6,888,438 B2
`
`100%
`90%
`80%
`70%
`
`
`
`360%
`250%
`2 40%
`
`
`5 30%
`No Shielding
`20%
`—— Fernite Plaics Only
`
`
`10%
`ammaaa Copper Sheets Only
`
`‘e
`—
`—
`
`
`ove dLteres rese
`OE+0 1E+6 2E+6 3E+6 4E+6 5E+6 G6GE+6 7E+6 8E+6 9E+6 JE+7
`Frequency (Hz)
`
`
`Fig. 18
`
`10
`
`10
`
`

`

`US 6,888,438 B2
`
`1
`PLANAR PRINTED CIRCUIT-BOARD
`TRANSFORMERS WITH EFFECTIVE
`ELECTROMAGNETIC INTERFERENCE
`(EMD SHIELDING
`
`This application is a continuation in part of U.S. Utility
`application Ser. No. 09/883,145, filed Jun. 15, 2001, subse-
`quently issued as U.S. Pat. No. 6,501,364 on Dec. 21, 2002,
`entitled PLANAR PRINTED-CIRCUIT BOARD TRANS-
`FORMERS WITH EFFECTIVE ELECTROMAGNETIC
`
`INTERFERENCE (EMI) SHIELDING, which is in its
`entirety incorporated herewith by reference, and of which
`the present application is a continuation-in-part.
`
`FIELD OF THE INVENTION
`
`This invention relates to a novel planar printed-circuit-
`board (PCB) transformer structure with effective (EMI)
`shielding effects.
`
`BACKGROUND OF THE INVENTION
`
`Planar magnetic components are attractive in portable
`electronic equipment applications such as the power sup-
`plies and distributed power modules for notebook and
`handheld computers. As the switching frequency of power
`converter increases,
`the size of magnetic core can be
`reduced. When the switching frequency is high enough (e.g.
`a few Megahertz),
`the magnetic core can be eliminated.
`Low-cost coreless PCB transformers for signal and low-
`power(a few Watts) applications have been proposed by the
`present
`inventors in U.S. patent applications Ser. No.
`08/018,871 and U.S. Ser. No. 09/316,735 the contents of
`which are incorporated herein by reference.
`It has been shown that the use of coreless PCB trans-
`
`formerin signal and low-powerapplications does not cause
`a serious EMC problem. In powertransfer applications
`however,
`the PCB transformers have to be shielded to
`comply with EMC regulations.
`Investigations of planar
`transformer shielded with ferrite sheets have been reported
`and the energy efficiency of a PCB transformershielded with
`ferrite sheets can be higher than 90% in Megahertz operating
`frequency range. However, as will be discussed below, the
`present inventors have found that using only thin ferrite
`materials for EMI shielding is not effective and the EM
`fields can penetrate the thin ferrite sheets easily.
`
`PRIOR ART
`
`FIGS. 1 and 2 show respectively an exploded perspective
`and cross-sectional view of a PCB transformer shielded with
`
`ferrite plates in accordance with the prior art. The dimen-
`sions of the PCB transformer undertest are detailed in Table
`I. The primary and secondary windings are printed on the
`opposite sides of a PCB. The PCB laminate is made of FR4
`material. The dielectric breakdown voltage of typical FR4
`laminates range from 15 kV to 40 kV. Insulating layers
`between the copper windings and the ferrite plates should
`have high thermal conductivity in order to facilitate heat
`transfer from the transformer windings to the ferrite plates
`and the ambient. The insulating layer should also be a good
`electrical
`insulator to isolate the ferrite plates from the
`printed transformer windings. A thermally conductive sili-
`cone rubber compound coated onto a layer of woven glass
`fibre, which has breakdown voltage of 4.5 kV and thermal
`conductivity of 0.79 Wm1K~",
`is used to provide high
`dielectric strength and facilitate heat transfer. The ferrite
`plates placed on the insulating layers are made of 4F1
`material from Philips. The relative permeability, u,, and
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`2
`resistivity, 0,, of the 4F1 ferrite material are about 80 and
`10° Qm,respectively.
`
`SUMMARYOF THE INVENTION
`
`According to the present invention there is provided a
`planar printed circuit board transformer comprising at least
`one copper sheet for electromagnetic shielding.
`Viewed from another aspect the invention provides a
`planar printed circuit board transformer comprising,
`(a) a printed circuit board,
`(b) primary and secondary windings formed by coils
`deposited on opposedsides of said printed circuit board,
`(c) first and second ferrite plates located over said primary
`and secondary windings respectively, and
`(d) first and sccond conductive shects located over said
`first and second ferrite plates respectively.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`An embodimentof the invention will now be described by
`way of example and with reference to the accompanying
`drawings, in which:-
`FIG. 1 is an exploded perspective view of a PCBtrans-
`former in accordance with the prior art,
`FIG. 2 is a cross-sectional view of the prior art trans-
`former of FIG. 1,
`FIGS. 3(a) and (b) are exploded perspective and cross-
`sectional views respectively of a PCB transformerin accor-
`dance with an embodiment of the present invention,
`FIG. 4 shows the R-Z plane of a prior art PCB
`transformer,
`FIG. 5 is a plot ofthe field intensity vector of a conven-
`tional PCB transformer,
`FIG. 6 plots the tangential and normal components of
`magnetic field intensity near the boundary between the
`ferrite plate and free space in a PCB transformerofthe prior
`art,
`
`FIG. 7 is a plot of the field intensity vector of a PCB
`transformer according to the embodimentof FIGS. 3(a) and
`(b),
`FIG. 8 plots the tangential and normal components of
`magnetic field intensity near the copper sheet in a PCB
`transformer according to the embodiment of FIGS. 3(a) and
`(6),
`FIG. 9 is shows the simulated field intensity or a PCB
`transformer without shielding and in no load condition,
`FIG. 10 shows measured magnetic field intensity of a
`PCBtransformer without shielding and in no load condition,
`FIG. 11 shows simulated magnetic field intensity of a
`PCBtransformer with ferrite shielding in accordance with
`the prior art and in no load condition,
`FIG. 12 shows measured magnetic field intensity of a
`PCB transformer with ferrite shielding and in no load
`condition,
`FIG. 13 shows simulated magnetic field intensity of a
`PCBtransformer in accordance with an embodimentof the
`invention and in no load condition,
`FIG. 14 shows measured magnetic field intensity of a
`PCB transformer in accordance with an embodimentof the
`
`present invention and in no load condition,
`FIG. 15 shows simulated magnetic field intensity of a
`PCBtransformer in accordance with an embodimentof the
`
`
`
`present invention and in 20Q load condition,
`
`11
`
`11
`
`

`

`US 6,888,438 B2
`
`3
`FIG. 16 shows measured magnetic field intensity of a
`PCBtransformer in accordance with an embodimentof the
`
`present invention and in 20Q load condition,
`TIG. 17 plots the energy efficiency of various PCB
`transformers in 100 load condition, and
`FIG. 18 plots the energy efficiency of various PCB
`transformers in 100Q/100 pF load condition.
`
`DETAILED DESCRIPTION OF PREFERRED
`EMBODIMENTS
`
`the ferrite
`invention,
`In accordance with the present
`shielded transformer of the prior art shown in FIGS. 1 and
`2 can be modified to improve the magnetic field shielding
`effectiveness by providing a conductive sheet (for example
`of copper or aluminum) on the surface of each ferrite plate
`as shown in FIGS. 3(a) and (6). As an example, the modified
`transformer and the ferrite-shielded transformer are of the
`same dimensionsas shownin Table I. The area and thickness
`of the conductive sheets in the example arc 25 mmx25 mm
`and 70 um, respectively.
`The magnetic field intensity generated from the shielded
`PCBtransformers is simulated with a 2D field simulator
`using a finite-element-method (FEM). A cylindrical coordi-
`nates system is chosen in the magnetic field simulation. The
`drawing model, in R-Z plane, of the PCB transformer shown
`in FIG. 4 is applied in the field simulator. The z-axis is the
`axis of symmetry, which passes through the center of the
`transformer windings. In the 2D simulation, the spiral cir-
`cular conductive tracks are approximated as concentric
`circular track connected in series. The ferrite plates and the
`insulating layers adopted in the simulation model are in a
`circular shape, instead of in a square shape in the trans-
`former prototype. The ferrite plates and the insulating layers
`may be made of any conventional materials.
`A.Transformer Shielded with Ferrite Plates
`
`The use of the ferrite plates helps to confine the magnetic
`field generated from the transformer windings. The high
`relative permeability, u,, of the ferrite material guides the
`magnetic field along and inside the ferrite plates. In the
`transformer prototype, 4F1 ferrite material is used though
`any other conventional ferrite material cold also be used.
`The relative permeability of the 4F1 material is about 80.
`Based on the integral form of the Maxwell equation,
`2
`pias =0
`Cc
`
`)
`
`the normal component of the magnetic flux density is
`continuousacross the boundary betweentheferrite plate and
`free space. Thus, at the boundary,
`
`B,,=B2,
`
`(2)
`
`(in
`where B,, and B,, are the normal component
`z-direction) of the magnetic flux density in the ferrite plate
`and free space, respectively.
`From (2),
`
`HoHp=HoH2n
`
`Fa,=n
`
`(3)
`
`From (3), at the boundary between the ferrite plate and free
`space, the normal componentof the magnetic field intensity
`in free space can be muchhigherthan that in theferrite plate
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`40
`
`45
`
`55
`
`60
`
`65
`
`4
`whenthe relative permeability of the ferrite material is very
`high. Therefore, when the normal componentof the H-field
`inside the ferrite plate is not sufficiently suppressed (e.g.
`when the ferrite plate is not
`thick enough),
`the H-field
`emitted from the surface of the ferrite plates can be enor-
`mous. FIG. 5 showsthe magnetic field intensity vector plot
`of the transformer shielded with ferrite plates. The primary
`is excited with a 3A 3 MHz current source and the secondary
`is left open. The size of the arrows indicates the magnitude
`of the magneticfield intensity in dB A/m. FIG. 5 showsthat
`the normal component of the H-field inside the ferrite plate
`is not suppressed adequately and so the H-field emitted from
`the ferrite plate to the free space is very high.
`The tangential (H,) and normal (H,) components of
`magnetic field intensity near the boundary between the
`ferrite plate and free space, at R=1 mm,are plotted in FIG.
`6. The tangential H-field (H,) is about 23.2 dB and is
`continuous at the boundary. The normal componentof the
`H-field (H,) in the free space is about 31.5 dB andthatinside
`the ferrite plate is about 12.5 dB at the boundary. The normal
`component of the H-field is,
`therefore, about 8% of the
`resultant H-field inside the ferrite plate at the boundary.
`Thus, the ferrite plate alone cannot completely guide the
`H-field in the tangential direction. As described in (3), the
`normal componentof the H-field in the free space is 80 times
`larger than that in the ferrite plate at the boundary. From the
`simulated results in FIG. 6, the normal component of the
`magneticfield intensity in the free space is about 19 dB, Le.
`79.4 times, higher than that inside the ferrite plate. Thus,
`both simulated results and theory described in (3) show that
`the using ferrite plates only is not an effective way to shield
`the magnetic field generated from the planar transformer.
`
`TABLEI
`
`Geometric Parameters of the PCB Transformer
`
`Geometric Parameter
`
`Dimension
`
`Copper Track Width
`Copper Track Separation
`Copper Track Thickness
`Number of Primary Turns
`Number of Secondary
`Turns
`Dimensions of Ferrite
`Plates
`PCB Laminate Thickness
`Insulating Layer Thickness
`Transformer Radius
`
`0.25 mm
`1 mm
`70 ym (2 Oz/ft?)
`10
`10
`25 mm x 25 mm x 0.4 mm
`
`0.4 mm
`0.228 mm
`23.5 mm
`
`B. Transformer Shielded with Ferrite Plates and Copper
`Sheets
`
`A PCBtransformer using ferrite plates coated with con-
`ductive sheets formed of copper as a shielding (FIGS. 3(a)
`and (b)) has been fabricated. The size of the copper sheets
`is the same as that of the ferrite plate but its thickness is
`merely 70 wm. Thin copper sheets are required to minimize
`the eddy current flowing in the z-direction, which may
`diminish the tangential component of the H-field.
`Based on the integral form of the Maxwell equation,
`
`p
`
`4
`
`and assuming that the displacement current is zero and the
`current on the ferrite-copper boundary is very small and
`negligible, the tangential component of the magnetic field
`
`12
`
`12
`
`

`

`US 6,888,438 B2
`
`(8)
`
`eH
`
`p
`p
`,
`= =2x(|H,(n dp)|-| Hin ap)|)
`Hi
`
`
`
`
`
`eA
`
`;
`SE = 20log) 9 aaHi
`or
`
`
`
`SE = 2x10 logo]
`
`where H, is the incident magneticfield intensity and H, is the
`magnetic field intensity transmits through the barrier.
`Alternatively, the incident field ban be replaced with the
`magnetic field when the barrier is removed.
`Magnetic field intensity generated from the PCB trans-
`formers with and without shielding has been simulated with
`FEM 2D simulator and measured with a precision EMC
`scanner. In the field simulation,
`the primary side of the
`transformer is excited with a 3 MHz 3A current source.
`
`5
`intensity is continuous across the boundary between the
`ferrite plate and free space. Thus, at the boundary,
`
`Hy=H,
`
`(5)
`
`where H,, and H,, are the tangential component (in
`r-direction) of the magnetic field intensity in the ferrite plate
`and copper, respectively. Because the tangential H-field on
`the surfaces of the copper sheet and the ferrite plates are the
`same at the boundary, thin copper sheets have to be adopted
`to minimize eddy currentloss.
`Consider the differential form of the Maxwell equation at
`the ferrite-copper boundary,
`
`p
`
`6)
`
`10
`
`15
`
`
`
`4
`
`7)
`
`20
`
`25
`
`30
`
`40
`
`45
`
`50
`
`However, the output of the magnetic field transducer in the
`EMCscanner will be clipped when the amplitude of the
`high-frequencyfield intensity is too large. Thus, the 3 MHz
`3A current source is approximated as a small signal (0.1A)
`3 MHzsource superimposed into a 3A DC source because
`the field transducer cannot sense DC source. In the mea-
`where @, 4 and o are the angular frequency, permeability
`surement setup, a magnetic field transducer for detecting
`and conductivity of the medium, respectively. Because cop-
`vertical magnetic field is located at 5 mm below the PCB
`per is a good conductor (o=5.80x107 S/m) and the operating
`transformer.
`frequency of the PCB transformer is very high (a few
`A. PCB Transformer Without Shielding
`megahertz), from (7), the magnetic field intensity, H, inside
`The magnetic field intensity of the PCB transformer
`the copper sheet is extremely small. Accordingly, the normal
`without any form of shielding and loading has been simu-
`component of the H-field inside the copper sheet is also
`lated and its R-Z plane is shown in FIG. 9. From the
`small. Furthermore, from (3), at the ferrite-copper boundary,
`simulated result, the magneticfield intensity, at R=O mm and
`Z=5 mm,
`is about 30 dBA/m. The measured magnetic
`the normal componentof the H-field inside the ferrite plate
`35
`intensity,
`in z-direction, is shown in FIG. 10. The white
`is 80 timesless than that inside the copper sheet. Asaresult,
`square and the white parallel lines in FIG. 10 indicate the
`the normal componentof the H-field inside the ferrite plate
`positions of transformerand the current carrying leadsofthe
`can be suppressed drastically.
`transformer primary terminals, respectively. The output of
`the magnetic field
`By using finite element methods,
`the magnetic field transducer, at 5 mm beneath the centre of
`intensity vector plot of the PCB transformer shielded with
`the transformer, is about 130 dBuV.
`ferrite plates and conductive sheets has been simulated and
`B. PCB Transformer Shielded With Ferrite Plates
`is shown in FIG. 7. The tangential (H,) and normal (H,)
`components of magnetic field intensity near the conductive
`sheet, at R=1 mm,are plotted in FIG. 8. From FIG. 8, the
`tangential H-field (H,) is about 23 dB and approximately
`continuous at the boundary. The normal componentof the
`H-field (H,) in conductive sheet is suppressed to about 8 dB
`and that inside the ferrite plate is about -7.5 dB at
`the
`boundary. Therefore, the normal component of the H-field
`is, merely about 0.09% of the resultant H-field inside the
`ferrite plate at the boundary. Accordingly, at
`the ferrite-
`conductive sheet boundary, the H-field is nearly tangential
`and confined inside in the ferrite plate. Besides, the normal
`componentof the H-field emitted into the conductive sheet
`and the free space can be neglected in practical terms. Since
`the normal component of the H-field emitted into the con-
`ductive sheet is very small, the eddy current loss due to the
`H-field is also very small. This phenomenonis verified by
`the energy efficiency measurements of the [ferrite-shielded
`PCB transformers with and without conductive sheets
`described below.As a result, the use of ferrite plates covered
`with conductive sheets is an effective way to shield the
`magnetic field generated from the transformer windings
`without diminishing the transformer energy efficiency.
`The shielding effectiveness (SE) of barrier for magnetic
`field is defined as [10]
`
`The simulated magnetic field intensity of a PCB trans-
`former shielded with ferrite plates alone, under no load
`condition, is shown in FIG. 11. The simulated result shows
`that the magnetic field intensity, at R=-O mm and Z=5 mm,
`is about 28 dBA/m. The measured magnetic intensity,
`in
`z-direction, is shown in [IG. 12. The output of the magnetic
`field transducer, at 5 mm beneath the centre of the
`transformer, is about 128 dBuV. Therefore, with the use of
`4F1ferrite plates, the shielding effectiveness (SE), from the
`simulated result, is
`
`SE=2x(30-28)=4 dB
`
`55
`
`is
`
`The shielding effectiveness obtained from measurements
`
`SE=2x(130-128)=4 dB
`
`60
`
`65
`
`Both simulation and experimental results showsthat the
`use of the 4F1 ferrite plates can reduce the magnetic field
`emitted from the transformer by 4 dB (about 2.5 times).
`C. PCB Transformer Shielded With Ferrite Plates and Con-
`ductive Sheets
`
`FIG. 13 Showsthe simulated magnetic field intensity of
`a PCBtransformerin accordance with an embodimentof the
`invention shielded with ferrite plates and conductive sheets
`under no load condition. From the simulated result,
`the
`
`13
`
`13
`
`

`

`US 6,888,438 B2
`
`7
`magnetic field intensity, at R=-O mm and Z=5 mm,is about
`13 dBA/m. FIG. 14 shows the measured magnetic intensity
`in z-direction. The output of the magneticfield transducer,at
`5 mm beneath the center of the transformer, is about 116
`dBuV. With the use of 4F1 ferrite plates and conductive
`sheets, the shielding effectiveness (SE), from the simulated
`result, is
`
`SE=2x(30-13)=34 dB
`
`The shielding effectiveness obtained from measurements
`
`is
`
`SE=2x(130-116)=28 dB
`
`As a result, the use of ferrite plates covered with conduc-
`tive sheets is an effective way to shield magnetic field
`generated from PCBtransformer. The reduction of magnetic
`field is 34 dB (2512 times) from simulation result and 28 dB
`(631 times) from measurement. The SE obtained from the
`measurementis less than that obtained from the simulated
`result. The difference mainly comes from the magnetic field
`emitted from the current carrying leads of the transformer.
`From FIG. 14, the magnetic field intensity generated from
`the leads is about 118 dB, which is comparable with the
`magnetic field generated from the transformer. Therefore,
`the magnetic field transducer beneath the centre of the
`transformer also picks up the magnetic field generated from
`the lead wires.
`D. PCB Transformer in Loaded Condition
`
`Whena load resistor is connected across the secondary of
`the PCB transformer, the opposite magnetic held generated
`from secondary current cancels out part of the magneticfield
`setup from the primary. As a result, the resultant magnetic
`field emitted from the PCB transformer in loaded condition
`is less than that in no load condition. FIG. 15 showsthe
`
`simulated magnetic field intensity of the PCB transformer
`shielded with ferrite plates and conductive sheets in 20Q
`load condition. From the simulated result, the magneticfield
`intensity, at R=O mm and Z=5 mm, is about 4.8 dBA/m,
`which is much less than that
`in no load condition (13
`dBA/m). FIG. 16 shows the measured magnetic intensity in
`z-direction. The output of the magnetic field transducer, at 5
`mm beneath the centre of the transformer,
`is about 104
`dBzV and that in no load condition is 116 dBuV.
`Energy efficiency of PCB transformers shielded with (i)
`ferrite plates only,(ii) conductive sheets only and(iii) ferrite
`plates covered with conductive sheets may be measured and
`compared with that of a PCB transformer with no shielding.
`FIG. 17 shows the measured energy efficiency of the four
`PCB transformers with 100 resistive load. In the PCB
`
`transformer shielded with only conductive sheets, a layer of
`insulating sheet of 0.684 mm thicknessis used to isolate the
`transformer winding and the conductive sheets. From FIG.
`17, energy efficiency of the transformers increases with
`increasing frequency. The transformer shielded with copper
`sheets only has the lowest energy efficiency among the four
`transformers. The energy loss in the conductive-shielded
`transformer mainly comes from the eddy current, which is
`induced from the normal componentof the H-field generated
`from the transformer windings,circulating in the conductive
`sheets.
`
`The energyefficiency of the transformer with no shielding
`is lower than that of the transformers shielded with ferrite
`
`the input impedance of
`plates. Without ferrite shielding,
`coreless PCB transformeris relatively low. The energyloss
`of the coreless transformer is mainly due to its relatively
`high i°R loss (because ofits relatively high input current
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`8
`compared with the PCB transformer covered with ferrite
`plates). The inductive parameters of the transformers with
`and without ferrite shields are shown in Table II. However
`
`this shortcoming of the coreless PCB transformer can be
`overcome by connecting a resonant capacitor across the
`secondary of the transformer. The energyefficiency of the 4
`PCB transformers with 100@//1000 pF capacitive load is
`shownin FIG. 18. The energyefficiency of the coreless PCB
`transformer is comparable to that of the ferrite-shielded
`transformersat the maximum efficiency frequency (MEF) of
`the coreless PCB transformer.
`
`The ferrite-shielded PCB transformers have the highest
`energy efficiency amongthe four transformers, especially in
`low frequency range. The high efficiency characteristic of
`the ferrite-shielded transformers is attributed to their high
`input
`impedance. In the PCB transformer shielded with
`ferrite plates and conductive sheets, even though a layer of
`conductive sheet is provided on the surface of each ferrite
`plate,
`the eddy current
`loss in the conductive sheets is
`negligible as discussed above. The H-field generated from
`the transformer windings is confined in the ferrite plates.
`The use of thin conductive sheets is to direct the magnetic
`field in parallel
`to the ferrite plates so that
`the normal
`component of the magnetic field emitting into the conduc-
`tive sheet can be suppressed significantly. The energy effi-
`ciency measurements of the ferrite-shielded transformers
`with and without conductive sheets confirm that the addition
`of conductive sheets on the ferrite plates will not cause
`significant eddy current loss in the conductive sheets and
`diminish the transformer efficiency. From FIGS. 17 and 18,
`the energy efficiency of both ferrite-shielded transformers,
`with and without conductive sheets, can be higher than 90%
`at a few megahertz operating frequency.
`It will thus be seen that the present invention provides a
`simple and effective technique of magnetic field shielding
`for PCB transformers. Performance comparison, including
`shielding effectiveness and energy efficiency, of the PCB
`transformers shielded in accordance with embodiments of
`the invention, conductive sheets and ferrite plates has been
`accomplished. Both simulation and measurement results
`show that the use of ferrite plates covered with conductive
`sheets has the greatest shielding effectiveness (SE) of 34 dB
`(2512 times) and 28 dB (631 times) respectively, whereas
`the SE of using only ferrite plates is about 4 dB (2.5 times).
`Addition of the conductive sheets on the surfaces the ferrite
`plates does not significantly diminish the transformer energy
`efficiency. Experimental results show that the energy effi-
`ciency of both ferrite-shielded transformers can be higher
`than 90% at megahertz operating frequency. But the planar
`PCB transformer shielded with both thin ferrite plates and
`thin copper sheets has a muchbetter electromagnetic com-
`patibility (EMC) feature.
`The conductive sheets may preferably be copper sheets,
`but other conductive materials may be used such as alumi-
`num.
`
`It should also be understood that while the printed circuit
`board may be a single board with the two windings formed
`on opposite sides, it is also possible that the two windings
`may be formed on separate boards that are laminated
`together to form a composite structure. It is also possible that
`the two windings may be formed on separate printed circuit
`boards that may be incorporated in different devices.
`Another possibility is that the ferrite plus conductive mate-
`rial shielding could also be applied to a single winding
`forming a PCB inductor.
`
`14
`
`14
`
`

`

`9
`
`TABLEII
`
`Inductive Parameters of the PCB Transformers
`
`US 6,888,438 B2
`
`10
`(d) first and second conductive sheets located over said
`first and second ferrite plates respectively.
`3. A transformer as claimed in claim 2 wherein a ther-
`
`1.22 wH
`3.92 uH
`
`1.22 vuH
`3.92 wuH
`
`1.04 »wH
`3.74 uo
`
`0.18 uH
`0.18 uH
`
`mally conductive insulating layer is located between each
`Mutual-
`said winding and its associated said ferrite plate.
`inductance
`4. A transformer as claimed in claim 2 wherein said
`between
`Self
`printed circuit board is a laminate, comprising at least two
`Leakage-
`Primary
`inductance
`Self-
`layers.
`inductance
`and
`of
`inductance
`5. A planar printed circuit board transformer comprising:
`
`
`of Primary Secondary—of PrimarySecondary
`10
`Transformers Winding
`Winding
`Windings
`Winding
`primary and secondary winding,
`first and second ferrite plates located over said primary
`and secondary windings respectively,
`conductive sheets located oversaid first and secondferrite
`
`No Shielding
`Shielded
`with Ferrite
`Plates Only
`Shielded
`with Ferrite
`Plates and
`Copper
`Sheets
`
`3.80 uH
`
`3.80 “H
`
`3.62 uH
`
`0.18 uH
`
`Whatis claimedis:
`
`plates respectively for electromagnetic shielding.
`6. Aplanar printed circuit board transformer comprising.
`(a) a first printed circuit board,
`(b) a primary winding formed by a coil deposited on said
`first printed circuit board,
`(c) a second printed circuit board,
`(d) a secondary winding formed by a coil depos